MODIFIED MANGANESE OXIDE OCTAHEDRAL MOLECULAR SIEVES
FOR OXIDATION AND CONSECUTIVE OXIDATION-ACID
REACTIONS
FITRI HAYATI DARMALIS
UNIVERSITI TEKNOLOGI MALAYSIA
MODIFIED MANGANESE OXIDE OCTAHEDRAL MOLECULAR SIEVES
FOR OXIDATION AND CONSECUTIVE OXIDATION-ACID
REACTIONS
FITRI HAYATI DARMALIS
A thesis submitted in fulfilment of the
requirements for the award of the degree of
Doctor of Philosophy (Chemistry)
Faculty of Science
Universiti Teknologi Malaysia
DECEMBER 2009
iii
To my husband, Dr. Hendriyawan
To my mother, Raidas
To my father, Darmalis
To my sons: Faizan Munawwar Alfindri and Muhammad Husein Murtaza
For my Brothers: Yon Elfi, Joni Indra, Yalmasri and Khairul Arif
iv
ACKNOWLEDGEMENTS
Bismillahirrahmanirrahim
In the name of Allah, the Most Gracious, the Most Merciful
All praise to be Allah, the supreme Lord of the world. May peace and
blessings to Rasulullah Muhammad Shollallahu’ Alaihi Wassalam, all the prophets,
his families, his close friends and all Muslims.
Firstly, I wish to express my deep sincere appreciation to my supervisors Prof
Dr. Halimaton Hamdan and Assoc. Prof. Dr. Hadi Nur, for their gratitude,
encouragement, friendly advice, earnest guidance, and motivation. Without
continuous support and curiosity from my supervisors, this thesis would not have
been the same as presented here. I am also indebted to UTM and research grant from
the Ministry of Science Technology and Innovation Malaysia (MOSTI) for funding
my Ph.D. study under VOT number 74506. This support is gratefully acknowledged.
I also would like to express my gratitude to all lecturers and researchers in
Department of Chemistry for their support and Ibnu Sina Institute for Fundamental
Science Studies (IIS) for catalysts characterization. Special gratitude is addressed to
Dr. Lee and my colleagues at Zeolite and Porous Materials Group (ZPMG), namely
Mr. Izan, Mrs. Rozana, Mrs. Suryani and Miss. Hidayah and others for useful
discussion and help. My sincere appreciation also extends to all my friends who have
provided supports at various occasions especially Mrs. Rosyida Permatasari and Mrs.
Suryani Alifah.
Last but not least, my gratitude goes to my caring family, my husband, my
mother and father, my mother and father (Allah Yarham) -in law, my son and my
brothers. To Bapak Nur Anas Djamil, Ibu Sofiah Djamaris and Dr Hadi’s family. I
thank you for continuous support, prayer, love, understanding and encouragement.
v
ABSTRACT
The research was focused on improvement of the catalytic activity of
octahedral manganese oxide molecular sieve (OMS-2) in oxidation and consecutive
oxidation-acid reactions. For oxidation reaction, OMS-2 was modified through
incorporation of metals and ion-exchange. Sulphation was applied in order to create
the acid sites on Ti-OMS-2 sample which was proven to have the highest oxidative
properties and tested for consecutive oxidation-acidic reaction. Ti, Fe, Co and Cu
were incorporated into the OMS-2 framework by isomorphous substitution to form
Ti-OMS-2, Fe-OMS-2, Co-OMS-2 and Cu-OMS-2, respectively. A new method to
synthesize Ti-OMS-2 with high Ti/Mn ratio was applied. Titanium incorporated
OMS-2 was successfully synthesized without addition of manganese (II) solution
which was normally necessary to synthesize metal substituted OMS-2. Ion-exchange
was carried out in order to replace K+ ion in the tunnel structure of OMS-2
framework by H+ ions using concentrated HNO3. Sulphation was done by
impregnation of certain amount of H2SO4 in different solvent. The characterization
results show that Ti-OMS-2 exhibited a significantly higher Lewis acidity compared
to the un-incorporated one. The physicochemical properties-catalytic activity of the
modified OMS-2 catalyst was studied in the oxidation of cyclohexane, cylohexene
and styrene, and also in consecutive transformation of 1-octene to 1,2-octanediol.
Oxidation of cyclohexane with TBHP as oxidant on transition metal substituted
OMS-2 showed that Ti-OMS-2 with high titanium content gave the highest
conversion, which may be due to the presence Ti sites in the framework and nonframework. For metal incorporated in the framework, there is a correlation of ionic
radii of metal substituted with conversion of cyclohexane. An increase in ionic
radii of metal substituted OMS-2 increased the conversion of cyclohexane. This
correlation may be due to the increase in the Lewis acidity in the metal incorporated
OMS-2. The study on the catalytic activity of H-exchanged catalysts in oxidation of
cyclohexane showed an increase in conversion of cyclohexane after ion-exchange.
For further investigation, Ti-OMS-2 with high Ti/Mn ratio was used in oxidation of
cyclohexene and styrene using TBHP as oxidant. The results showed that both
titanium sites in framework and non-framework increased the activity of OMS-2 in
the oxidation of cyclohexene. However, it was observed that only non-framework
titanium species induced a synergetic effect that enhanced the oxidation of styrene.
There is a correlation between Ti site location in Ti-OMS-2 catalyst with activation
of C-H and C=C bonds. Ti sites in the framework only played role in C-H bond
activation whereas Ti site non-framework enhanced the catalytic activity for both
types of bond activation. SO42-/Ti-OMS-2 was proven to be active for consecutive
transformation of 1-octene to 1,2-octanediol. However, it was confirmed that
Brönsted acid sites did not exist in the sample. The success of the consecutive
reactions may be due to the generation of Brönsted acid from hydrolysis of water on
the Lewis acid sites of SO42-/Ti-OMS-2 sample.
vi
ABSTRAK
Penyelidikan difokuskan pada peningkatan aktiviti pemangkinan daripada
penapis molekul oksida mangan oktahedral (OMS-2) sebagai mangkin dalam tindak
balas pengoksidaan dan juga tindak balas berturutan pengoksidaan dan keasidan.
Untuk tindak balas pengoksidaan, OMS-2 diubah suai melalui pemasukan logam dan
pertukaran ion. Pensulfatan telah dilakukan untuk membina tapak asid pada
mangkin Ti-OMS-2 yang terbukti mempunyai aktiviti pemangkinan paling tinggi
dalam tindak balas pengoksidaan dan seterusnya diuji pada tindakbalas berturutan
pengoksidaan dan keasidan. Ti, Fe, Co dan Cu dimasukkan ke dalam bingkaian
OMS-2 melalui penukargantian isomorfus untuk membentuk Ti-OMS-2, Fe-OMS-2,
Co-OMS-2 dan Cu-OMS-2. Satu kaedah baru telah digunakan untuk mensintesis
Ti-OMS-2 pada nisbah Ti:Mn yang tinggi. Titanium yang digabungkan dengan
OMS-2 telah berjaya disintesis tanpa penambahan larutan mangan(II) yang biasanya
diperlukan untuk mensintesis penukargantian logam pada OMS-2 seperti yang
dilaporkan sebelum ini. Penukaran ion telah dilakukan untuk menggantikan ion K+
didalam struktur terowong dengan ion H+ menggunakan HNO3 pekat. Pensulfatan
telah dilakukan dengan pengisitepuan jumlah tertentu H2SO4 dalam pelarut yang
berbeza. Hasil kaedah pencirian didapati bahawa penggabungan titanium kepada
OMS-2 telah meningkatkan keasidan Lewis jika dibandingkan dengan OMS-2 sahaja.
Pengoksidaan terhadap sikloheksana dengan TBHP sebagai agen pengoksidaan pada
OMS-2 yang telah ditukarkan dengan logam peralihan memperlihatkan bahawa TiOMS-2 yang mempunyai kandungan titanium yang tinggi menghasilkan peratus
penukaran yang paling tinggi, mungkin disebabkan oleh wujudnya tapak titanium
dalam dan luar bingkaian. Untuk penggabungan logam di dalam bingkai, ada
hubungkait antara jejari ion logam yang menukarganti dengan peratus penukaran
sikloheksana.
Peningkatan jejari ionik daripada logam yang menukarganti
meningkatkan peratus penukaran sikloheksana. Hubungkait ini mungkin disebabkan
oleh peningkatan tapak asid Lewis pada logam yang digabungkan dengan OMS-2.
Kajian terhadap aktiviti pemangkinan ke atas mangkin tertukarganti H di dalam
pengoksidaan menunjukkan peningkatan peratusan penukaran daripada sikloheksana
setelah penukaran ion. Untuk kajian selanjutnya, Ti-OMS-2 dengan nisbah Ti:Mn
yang lebih tinggi telah digunakan dalam pengoksidaan sikloheksena dan stirena
dengan TBHP sebagai agen pengoksida. Didapati bahawa kedua-dua tapak titanium
dalam dan luar bingkai meningkatkan aktiviti OMS-2 dalam pengoksidaan
sikloheksena. Walau bagaimanapun, telah dibuktikan bahawa hanya spesies
titanium bukan-bingkai sahaja menghasilkan kesan sinergi yang meningkatkan
pengoksidaan stirena. Didapati bahawa wujud hubungkait antara lokasi tapak
titanium pada mangkin Ti-OMS-2 dengan pengaktifan ikatan C-H dan C=C. Tapak
titanium dalam bingkai memainkan peranan hanya untuk pengaktifan ikatan C-H
sahaja sedangkan tapak titanium luar bingkai meningkatkan aktiviti pemangkinan
pada kedua-dua jenis pengaktifan ikatan. Sampel SO42-/Ti-OMS-2 aktif untuk
pertukaran berterusan 1-oktena kepada 1,2 oktanadiol. Bagaimanapun, telah
dipastikan bahawa tidak ada tapak asid Brönsted pada sampel SO42-/Ti-OMS-2.
Kejayaan daripada tindak balas berterusan tersebut mungkin disebabkan oleh
pembentukan asid Brönsted daripada hidrolisis air yang terjadi pada tapak asid Lewis
yang terdapat pada sampel SO42-/Ti-OMS-2.
vii
TABLE OF CONTENTS
CHAPTER
TITLE
DECLARATION
ii
DEDICATION
iii
ACKNOWLEDGEMENTS
iv
ABSTRACT
v
ABSTRAK
vi
TABLE OF CONTENTS
vii
LIST OF TABLES
xii
LIST OF FIGURES
xv
LIST OF SYMBOLS/ABBREVIATIONS
xxi
LIST OF PUBLICATIONS
1
xxiv
INTRODUCTION
1
1.1 Research Background
5
1.1.1
Porous Manganese Oxide Materials
5
1.1.2
Manganese Oxide Octahedral Molecular Sieves (OMS-2)
7
1.1.3
Modification of OMS-2 Materials
9
1.1.4
Designing of Sulfated Ti-OMS-2 as Bifunctional
Oxidative and Acidic Catalyst
2
PAGE
12
1.2 Research Questions and Scope of the Research
13
1.3 Research Objectives
14
LITERATURE REVIEW
18
2.1
Introduction
18
2.2
Classification of Porous Manganese Oxide
18
viii
2.3
2.4
2.5
Material Synthesis
24
2.3.1 Synthesis of Porous Manganese Oxide
24
2.3.2 Synthesis of OMS-2 Materials
26
Modification of Manganese oxide
29
2.4.1
29
Overview of Modification of OMS-2 materials
2.4.2 Ion Exchange
37
2.4.3 Impregnation
38
2.4.4 Alkylsilylation
39
2.4.5 Sulphation
41
Catalytic Activity
42
2.5.1 Catalytic Activity of OMS-2 Materials
42
2.5.2 Catalytic Oxidation
45
2.5.3 Titanium Incorporated Materials
49
2.5.4
52
Sulphated Metal Oxides as a Solid Acid Catalyst
2.5.5 Synthesis of Diols
3
53
EXPERIMENTAL
55
3.1
Synthesis of OMS-2 Materials
55
3.1.1 Synthesis of OMS-2 without Buffer (OMS-2a)
57
3.1.2 Synthesis of OMS-2 with buffer (OMS-2b)
57
Modification of OMS-2 materials
57
3.2.1 Synthesis of Metal Substituted OMS-2 (M-OMS-2)
57
3.2.2 Ion Exchange of OMS-2 and M-OMS-2 Samples
58
3.2.3
Synthesis of Ti Incorporated OMS-2 (Ti-OMS-2)
59
3.2.4
Synthesis of Ti Impregnated OMS-2 [Ti-OMS-2 (imp)] 59
3.2
3.2.5 Preparation of TiO2-OMS-2 (mix)
3.2.6 Synthesis of Sulphated Ti-OMS-2
3.3
59
(SO42-/Ti-OMS-2)
60
3.2.7 Surface Modification by Alkylsilylation
60
Characterization Techniques
60
3.3.1 X-Ray Diffraction (XRD) Spectroscopy
61
3.3.2 Atomic Absorption Spectroscopy (AAS)
62
3.3.3 Fourier Transform Infrared (FTIR) Spectroscopy
63
3.3.4 Total Specific Surface Area (BET) and Pore Volume . ...
.....Analysis
65
ix
3.3.5
Thermal Gravimetry and Differential Thermal Analysis
......(TG-DTA)
67
3.3.6 Field Emission Electron Scanning Microscopy (FESEM) 68
3.3.7
Photoluminescence
68
3.3.8 X- Ray Photoelectron Spectroscopy (XPS)
70
3.3.9
71
Pyridine Adsorption
3.3.10 Adsorption Capacity of Adsorbed Water
74
3.3.11 Gas Chromatography (GC) Analysis
74
3.3.12 Gas Chromatography-Mass Spectrometry (GC-MS)
3.4
4
......Analysis
75
Catalytic Testing
75
3.4.1
Oxidation of Benzyl Alcohol
77
3.4.2
Oxidation of Cyclohexane
78
3.4.3
Oxidation of Cyclohexene
79
3.4.4
Oxidation of Styrene
80
3.4.5
Transformation of 1-octene to 1,2-octanediol
81
PHYSICOCHEMICAL PROPERTIES OF OMS-2 AND
MODIFIED OMS-2 CATALYSTS
83
4.1
Introduction
83
4.2
Physicochemical Properties of Prepared OMS-2 by Different
Methods
84
4.3
Physical Properties of Metal Substituted OMS-2 Material
92
4.3
Physical Properties of H-OMS-2 and H-M-OMS-2 Materials
102
4.4
Physicochemical Properties of Ti-OMS-2 Materials
106
4.4.1 Structural Properties of Ti Substituted OMS-2 Catalyst 106
4.4.2 Acidity Properties
113
4.4.3
114
Morphology, Surface Area and Textural Properties
4.4.4 Thermal Stability
4.4.5
119
More Evidence of the Location of Titanium on ...............
.....Ti-OMS-2 Materials
121
4.5
Alkylsilylated of OMS-2 and Ti-OMS-2(0.67)
127
4.6
Sulphated Ti-OMS-2
129
x
5
CATALYTIC ACTIVITY OF OMS-2 AND MODIFIED OMS-2
SAMPLE IN OXIDATION AND ACID REACTIONS
135
5.1
Introduction
135
5.2
Catalytic Activity and Selectivity of OMS-2 and Modified
OMS-2 Samples in Oxidation Reactions
5.2.1
135
Oxidation of Benzyl Alcohol over OMS-2 Prepared by
.....Different Method
5.3.2
135
Oxidation of Cyclohexane over Metals Substituted and
.....Ion.Exchanged OMS-2
5.4.3 Oxidation of Cyclohexene over Ti-OMS-2 Catalyst
140
150
5.4.2 Oxidation of Styrene over Different Location of
.....Titanium sites on Ti-OMS-2 Catalysts
5.3
The Effect of Lewis Acidity in Catalytic Oxidations
5.4
The Role of Different Location of Ti Sites in Ti-OMS-2 in
Oxidation Reactions
5.5
156
157
Catalytic Study on Consecutive Reaction of 1-octene to
1,2-octanediol
6
152
158
SUMMARY AND CONCLUSION
161
6.1
Summary
161
6.2
Conclusion
166
REFERENCES
171
APPENDIXES
189-199
xi
LIST OF TABLES
TABLE NO
1.1
TITLE
PAGE
The synthetic manganese oxides, their natural counterpart and
structures as reported by Suib [12, 20-22].
1.2
2.1
The cations were doped OMS-2 and their location in OMS-2
material.
2.3
2.4
templates.
26
Review of Metal Doped into OMS-2; its synthesis routes, location,
properties, and catalytic application.
30
Some catalytic applications of OMS-2 materials reported in the last
42
Characteristic features of Shell’s epoxidation catalyst compared
with Titanium Silicalite 1.
2.6
19
Synthesis of tunnel and layered manganese oxides with various
decade.
2.5
10
Classification of tunnel and layered manganese oxides and their
crystallographic data [13].
2.2
7
49
The comparisons of the common mild oxidant are used in oxidation
process.
50
3.1
Position of bands and classification for linkages of pyridine.
72
4.1
Effect of potassium concentration in reflux method on the synthesis
of OMS-2.
4.2
The relative intensity and ratio of I(110)/I(200) plane of OMS-2b
and calcined OMS-2b samples calculated by XRD.
4.3
85
88
The ionic radii of metals ion and average crystallite size of OMS-2
and M-OMS-2 samples.
94
xii
4.4
Lattice parameter (a and c) and cell volume (V) of OMS-2 and MOMS-2 samples.
4.5
95
The relative intensity and ratio of I(110)/I(200) plane of OMS-2 and
M-OMS-2 samples.
96
4.6
The physical properties of metal ions.
97
4.7
Source of metal, its charge and their effect to cryptomelane
structure.
98
4.8
AAS data of OMS-2 and M-OMS-2 samples.
99
4.9
The relative intensity and ratio of I(200)/I(211) plane of OMS-2
and M-OMS-2 samples calculated by XRD.
102
4.10
The percentage of potassium substituted by H+.
104
4.11
Chemical composition and physicochemical properties of OMS-2,
Ti-OMS-2 and TiO2–OMS-2.
4.12
107
The lattice parameters (a and c) and cell volume (V) of OMS-2 and
Ti-OMS-2 samples.
109
4.13
Vibrational spectroscopy feature of samples.
111
4.14
The relative intensity and ratio of I(111)/I(211) plane of samples
calculated by XRD.
4.15
116
Binding Energies (eV) of Mn 2p, Ti 2p, and its line separation
(BE) and difference of line separation from selected samples.
122
4.16
Assignments of as-observed IR bands on sulfated samples [154].
131
5.1
Conversion of benzyl alcohol and selectivity to benzaldehyde by
different catalysts.
5.2
136
The relation of amount of potassium exchanged by H+ with
enhancement of % conversion of cyclohexane on H-M-OMS-2
catalyst.
5.3
144
The correlation of Lewis acidity of samples to conversion of
cyclohexane, cyclohexene and styrene.
156
xiii
5.4
The possible role of Lewis acids of catalyst in oxidation of
cyclohexane, cyclohexene and styrene.
157
5.5
The role of Ti sites location in oxidation reaction.
158
6.1
The physicochemical properties-catalytic activity relationship of the
catalysts.
163
xiv
LIST OF FIGURES
FIGURE NO
1.1
TITLE
PAGE
Steps Potential energy diagram for a catalytic reaction (solid
line), i.e. reaction of A and B to form AB, compared with the noncatalytic reaction (dashed line). The presence of a catalyst
lowers the activation energy (Ea) considerably [7].
1.2
Process options in catalytic oxidation and some of their
disadvantages.
1.3
2
3
Crystal structure of cryptomelane-type OMS-2: potassium atoms
are shown as green spheres; MnO6 octahedra are shown in brown.
8
1.4
Mars van Krevelen mechanism.
9
1.5
Partial periodic table showing transition metals that have been
introduced into OMS-2 materials.
10
1.6
Transformation of alkenes to diols via two step reaction.
12
1.7
The schematic of the research approach and research questions.
15
1.8
The significant of the use Ti-OMS-2 in heterogeneous oxidation
and acid reactions.
2.1
Schematic structures of one-dimensional tunnel and layered
manganese oxides [13].
2.2
2.4
22
Intergrowth tunnels of (a) (1×1) and (1×2), and (b) (2×2) and
(2×3) in the tunnel manganese oxides.
2.3
17
23
Transformation reaction from birnessite to hollandite under
hydrothermal conditions.
28
The ion exchange reaction.
37
xv
2.5
Impregnation of porous catalyst.
38
2.6
Chemical structure of OTS.
39
2.7
Mechanism of complete reaction of OTS on the support surface
to form a well ordered layer.
40
2.8
Mechanism of incomplete reaction of OTS on the support surface. 40
2.9
Epoxidation of alkenes using (a) organic peracids, (b) chlorohyrins
route and (c) H2O2 or TBHP as the oxidant.
2.10
47
Epoxidation of propene on Shell catalyst using an organic
peroxide.
50
2.11
Oxidation reactions catalyzed by TS-1.
51
3.1
Materials preparation and their labelling.
56
3.2
The IUPAC classification for adsorption isotherms, where
nad = amount of adsorbed and P/Po= relative pressure.
3.3
The physical process following absorption of a photon by a
molecule.
3.4
69
The mechanism of interaction between pyridine molecules with
Lewis.
3.5
66
73
The interaction between pyridine molecules with Brönsted acid
sites.Catalytic reactions over various modified OMS-2.
73
3.6
Catalytic reactions over various modified OMS-2.
76
4.1
XRD patterns of OMS-2 materials and reference pattern of
Crypromelane, Q JCPDS 29, 1020.
4.2
86
Effect of calcination on XRD patterns of OMS-2b materials, (a)
before calcination, (b) calcination at 400oC, (c) at 500oC, and (d)
at 600oC.
88
4.3
FTIR spectra of (a) OMS-2a and (b) OMS-2b.
89
4.4
Defect on OMS-2 structure.
90
xvi
4.5
Amount of adsorbed water on the surface of OMS-2a and OMS-2b
samples.
90
4.6
TGA plots for OMS-2 materials in N2 atmosphere.
91
4.7
X-ray diffractograms of (a) OMS-2; (b) Ti-OMS-2(0.05); (c)
Fe-OMS-2(0.09); (d) Co-OMS-2(0.02); and (e) Cu-OMS-2(0.04). 93
4.8
Schematic incorporation of metals in M-OMS-2 materials.
4.9
XRD pattern of (a) cryptomelane and (b) Ti-, (c) Cu-, (d) Co- and
93
(e) Fe-OMS-2 was prepared by oxidation of its metal ions source
by potassium permanganate in acidic condition without the
addition of Mn2+ solution.
97
4.10
Morphology of OMS-2 and M-OMS-2 samples.
101
4.11
Schematic synthesis of H-OMS-2 and H-M-OMS-2.
102
4.12
XRD pattern of (a) OMS-2 and (b) H-OMS-2.
103
4.13
Morphology of H-OMS-2 and H-M-OMS-2 samples.
105
4.14
X-ray diffractograms of (a) cryptomelane (JCPDS 29, 102), (b)
OMS-2, (c) Ti-OMS-2 (0.18), (d) Ti-OMS-2 (0.43), (e) Ti-OMS-2
(0.67), (f) TiO2-OMS-2 (imp) and (g) Ti-OMS-2 (mix).
4.15
IR spectra at lower wavelength region of (a) OMS-2, (b)
Ti-OMS-2 (0.18), (c) Ti-OMS-2 (0.67).
4.16
111
Photoluminescence spectra of OMS-2, Ti-OMS-2 (0.43) and
TiO2-OMS-2 (mix). The excitation wavelength is 430 nm.
4.18
110
IR spectra at higher wavelength region of (a) OMS-2, (b)
Ti-OMS-2 (0.18), (c) Ti-OMS-2 (0.67).
4.17
108
113
FTIR spectra of (a) Ti-OMS-2 (0.67) and (b) OMS-2 after
evacuation under vacuum at 400 oC for 4 h followed by pyridine
adsorption at room temperature and evacuation at 150 oC for an
hour.
4.19
114
Morphology of (a) Ti-OMS-2 (0.18), (b) Ti-OMS-2 (0.18) and
(c) TiO2-OMS-2 (imp).
115
xvii
4.20
N2 adsorption isotherm for OMS-2 at 77 K.
4.21
N2 adsorption isotherm for (a) Ti-OMS-2(0.43) and (b)
Ti-OMS-2(0.67) at 77 K.
4.22
118
TGA profile (a) in original and (b) differential forms of OMS-2,
Ti-OMS-2 (0.18) and TiO2-OMS-2 (imp).
4.23
116
120
Detailed XPS spectra for the Mn 2p transition for (a) OMS-2, (b)
Ti-OMS-2(0.18), (c) Ti-OMS-2(0.67) and (d) TiO2-OMS-2(imp). 123
4.24
Bond strength on bridging oxygen atom.
4.25
Detailed XPS spectra for the Ti 2p transition for (a) Ti-OMS-2
(0.18), (b) Ti-OMS-2(0.67) and (c) TiO2-OMS-2(imp).
4.26
126
FTIR spectra of modified OTS samples (a) OTS/OMS-2 and (b)
OTS/Ti-OMS-2.
4.27
125
128
Percentage of adsorbed water on the sample (a) OMS-2, (b)
Ti-OMS-2(0.67), (c) OTS/Ti-OMS-2(0.67) and (d) OTS/OMS-2. 128
4.28
Proposed polymeric octadecylsiloxane on the surface of OMS-2
and Ti-OMS-2 samples.
4.29
129
XRD pattern of Ti-OMS-2(0.04) and sulphated Ti-OMS-2 (0.04).
# = TiOSO4.H2O,* = MnSO4 7H2O ¤ = MnSO4.
130
4.30
FTIR spectra of Ti-OMS-2 and sulphated Ti-OMS-2.
131
4.31
The bridging of bidentated structure of sulphated Ti-OMS-2.
132
4.32
Thermograms (TGA) of samples.
133
4.33
First derivative curves (DTGA) of samples.
133
4.34
FESEM micrograph of SW150-Ti-OMS-2(0.67) sample in
different magnitude.
134
5.1
The schematic reaction of benzyl alcohol to benzaldehyde.
136
5.2
Overall alcohol oxidation mechanism [24].
139
5.3
Resonance model of Mn-O-Mn bond structure: Resonance
structure in crystalline OMS-2 (Structure A); and non-resonance
structure in amorphous materials (Structure B).
140
xviii
5.4
Schematic reaction of cyclohexane.
5.5
The conversion and product selectivity of oxidation of
141
cyclohexane with tert-butyl hydroperoxide (TBHP) using OMS-2,
M-OMS-2 and H-M-OMS-2. All reactions were carried out at
60 ºC for 24 h with cyclohexane (26 mmol), 70% aqueous TBHP
(10 mmol), and catalyst (50 mg) under reflux condition.
5.6
142
The relationship of ionic radii of metals substituted OMS-2 to
conversion of cyclohexane.
144
5.7
Yield of products vs time on Ti-OMS-2.
146
5.8
Homolytic pathway to form radical from TBHP over catalyst.
148
5.9
Formation of cyclohexyl hydroperoxide.
148
5.10
The heterolytic pathway of the formation of cyclohexanone from
cyclohexyl hydroperoxide.
5.11
The homolytic pathway of the formation of cyclohexanol from
cyclohexyl hydroperoxide.
5.12
148
149
The heterolytic pathway of the formation of cyclohexanol and
cyclohexanone from 1,4-dicyclohexyltetraoxidane.
149
5.13
Reaction condition of cyclohexene and its products.
150
5.14
The conversion and product selectivity of oxidation of
cyclohexene with tert-butyl hydroperoxide (TBHP) using TiO2,
OMS-2, Ti-OMS-2(0.18), and Ti-OMS-2(0.67). {All reactions
were carried out at 70 ºC for 2 h with cyclohexene (5 mmol), 70%
aqueous TBHP (10 mmol), acetonitrile (15 ml) and catalyst (50
mg). The conversion and the amount of product obtained in
blank experimental have been subtracted}.
5.15
Oxidation of styrene and its product on catalysts using TBHP as
oxidant.
5.16
151
The conversion and product selectivity of oxidation styrene with
tert-butyl hydroperoxide (TBHP) using TiO2, TiO2-OMS-2,
Ti-OMS-2, OMS-2 and TS-1. All reactions were carried out at
153
xix
70 oC with styrene (5 mmol), 70% aqueous TBHP (10 mmol),
acetonitrile (15 ml) and catalyst (50 mg) with vigorous stirring.
5.17
154
Consecutive oxidation and acid reaction to form of 1,2 octane diol
from 1-octene on sulphated Ti-OMS-2 catalyst.
159
5.18
Yield of epoxyoctane and 1,2 octane diol after 24 h reaction.
159
6.1
Assignments of modified OMS-2 in oxidation of cyclohexane and
consecutive reaction of 1-octene to 1,2 octanediol consecutive
reaction of 1-octene to 1,2 octanediol
6.2
167
The role of the location of Ti sites in Ti-OMS-2 in oxidation of
cyclohexane, cyclohexene and styrene.
169
xx
LIST OF SYMBOLS/ABBREVIATIONS
%
-
percent
~
-
approximately
μ
-
micron (10-6)
2
-
Bragg angle
Å
-
angstrom (10-10)
a.u.
-
arbitrary unit
BET
-
Brunnauer, Emmett and Teller
c.a.
-
about (Latin:circa)
cm-1
-
per centimeter
2+
Co
-
Cobalt ion
Co-OMS-2
-
Cobalt substituted OMS-2
Cu K
-
X-ray diffraction from copper K energy levels
Cu2+
-
Copper ion
Cu-OMS-2
-
Copper substituted OMS-2
d
-
distance
DTG
-
Differential thermogravimetry
e.g.
-
example (Latin: exempli gratia)
EDAX
-
Energy dispersive analysis by X-ray
equilibrium pressure and vapour pressure Po of the
adsorbate at the temperature where the isotherm is
measured
et al.
-
and others (Latin: et alia)
eV
-
electrovolt
Fe
-
Iron ion
Fe-OMS-2
-
Iron substituted OMS-2
FESEM
-
Field Emission Scanning Electron Microscope
FID
-
Flame ionisation detector
FTIR
-
Fourier transform infrared
3+
xxi
g
-
grams
h
-
hour
H2 O2
-
Hydrogen peroxide
HF
-
Hydrofluoric acid
i.e.
-
that is (Latin : id est)
IUPAC
-
International Union of Pure and Applied Chemistry
K
-
degree Kelvin
KBr
-
Potassium bromide
M
-
Molar
mg
-
meter square per gram
mA
-
milliampere
min
-
minute
mL
-
millilitre
mol
-
mole
M-OMS-2
-
Metal substituted OMS-2
N2
-
Molecular nitrogen
nm
-
nanometer (10-9)
o
-
degree celcius
OL-1
-
OL-1 is manganese oxide Octahedral layered with
interlayer spacing of ~7 Å.
OMS-1
-
Manganese oxide octahedral molecular sieve with 3 x 3
tunnel structure.
OMS-2
-
Manganese oxide octahedral molecular sieve with 2 x 2
tunnel structure.
OMS-5
-
Manganese oxide octahedral molecular sieve with 2 x 4
tunnel structure.
OMS-6
-
Manganese oxide octahedral molecular sieve with 2 x 3
tunnel structure.
OMS-7
-
Manganese oxide octahedral molecular sieve with 1 x 1
tunnel structure.
OTS
-
Octadecyltrichlorosilane
P/Po
-
relative pressure; obtained by forming the ratio of the
equilibrium pressure and vapour pressure po of the
adsorbate at the temperature where the isotherm is
measured
SO42-/Ti-OMS-2
-
Sulphated Ti-OMS-2
ST150-Ti-OMS-2
-
Sulphated Ti-OMS-2 with 150 L concentrated H2SO4
using toluene as solvent
2 -1
C
xxii
ST200-Ti-OMS-2
-
Sulphated Ti-OMS-2 with 200 L concentrated H2SO4
using toluene as solvent
SW150-Ti-OMS-2
-
Sulphated Ti-OMS-2 with 150 L concentrated H2SO4
using water as solvent
SW200-Ti-OMS-2
-
Sulphated Ti-OMS-2 with 200 L concentrated H2SO4
using toluene as solvent
t
-
crystallite size
TBHP
-
tert-butyl hydroperoxide
TGA
-
Thermogravimetry analysis
Ti
-
Titanium ion
TiO2
-
Titanium dioxide
3+
TiO2-OMS-2 (imp) -
Impregnation of TiO2 on OMS-2 surface
TiO2-OMS-2 (mix) -
Physical mixture of rutile TiO2 and OMS-2
Ti-OMS-2
-
Titanium substituted OMS-2
TS-1
-
Titanium Silicate-1
wt %
-
weight percentage
-
wavelength
xxiii
LIST OF PUBLICATIONS AND PRESENTATIONS
1. H. Nur, F. Hayati, H. Hamdan, "On the location of different titanium sites in TiOMS-2 and their catalytic role in oxidation of styrene", Catalysis
Communications, 8 (2007) 2007-2011.
2. F. Hayati, H. Nur, H. Hamdan, "Titanium Doped Octahedral Manganese Oxide
Hybrid Catalyst in the Oxidation of Cyclohexene", Buletin Kimia, 21 (2005) 4954.
3. F. Hayati, H. Hamdan, H. Nur, "Synergetic effect of titanium and OMS-2 as TiOMS-2 hybrid catalyst in oxidation of cyclohexene", Book abstract of Annual
Fundamental Science Seminar 2005, 4-1 July 2005, Johor Bahru, Malaysia. p. 67.
4. F. Hayati, H. Nur and H. Hamdan, "Synthesis and characterization of octahedral
molecular sieves (OMS-2)", Book of abstract of Annual Fundamental Science
Seminar 2004, 14-15 June 2004, Johor Bahru, Malaysia. p. 73.
CHAPTER 1
INTRODUCTION
A catalyst is a substance which accelerates a chemical reaction. The basic
principle of how a catalyst works for a chemical reaction is shown by the example in
Figure 1.1. It does so by forming bonds with the reacting molecules (i.e. adsorption),
followed by breaking and weakening of the intramolecular bonds.
Next, the
adsorbed species react on the surface to a particular product, often in several
consecutive steps. Finally, it detaches itself from the catalyst (i.e. desorption) and
leaves the catalyst unaltered so that it is ready to interact with the next set of
molecules. A catalyst cannot alter the chemical equilibrium of a given reaction; it
only creates a favourable reaction pathway. This is done by decreasing the activation
barrier (Ea,cat) compared to non catalytic reaction (Ea,non) and thus increasing the
reaction rate. In general, a successful catalyst increases the yield of the desired
product while decreasing that of other products, which has advantages for both
economic and environmental reasons [1].
The first introduction of the word ‘catalysis’ was by Berzelius in 1836, while
Ostwald presented the first correct definition of a catalyst in 1895 [2]. He described
a catalyst as a substance that changes the rate of a chemical reaction without itself
appearing in the products. Today, catalysis lies at the heart of our quality of life: the
reduced emissions of modern cars, the abundance of fresh food at our stores, and the
new pharmaceuticals that improve our health are made possible by chemical
reactions controlled by catalysts [3]. It covers multidisciplinary science that serves a
broad range of chemical industries covering specialty, fine, intermediate, commodity,
2
and life science chemicals [4]. It played a major role in establishing the economic
strength of the chemical and related industries in the first half of the 20th century and
an estimated 90% of all of the chemical processes introduced since 1930 depend on
catalysis [5]. According to a report from Freedonia [6] the world catalyst demand
will rise by 3.6% per year to $12.3 billion in 2010.
Figure 1.1: Potential energy diagram for a catalytic reaction (solid line), i.e. reaction
of A and B to form AB, compared with the non-catalytic reaction (dashed line). The
presence of a catalyst lowers the activation energy (Ea) considerably [7].
Catalysis plays an important role in the green chemical processes, which is to
minimize environmental impact and to reduce costs of the process. It is crucial to
achieve the “ideal synthesis” which would be atom efficient, safe, one step, involving
no wasted reagents, based on renewable resources, and environmentally acceptable
to overcome some of the biggest problem areas in synthetic methodology such as in
oxidation and acid-catalyze reactions [8].
In catalytic process, for example catalytic oxidation, there are some options to
consider, i.e. phase (gas or liquid) and catalytic system as shown in Figure 1.2.
Firstly, the selection of either a gas or liquid phase depends largely on the boiling
3
point and thermal stability of the reactants, especially of a desired fine chemical
intermediate. However, catalytic gas phase is limited to relatively simple molecules.
More complicated molecules cannot be brought easily into the gas phase and many
of these molecules are unstable at elevated temperatures. In addition, the gas phase
needs special reactor and costly. Therefore, working in liquid phase is relatively
easier than in gas phase, as simpler equipments are required which can be performed
for more complicated molecules.
Catalytic oxidation
Gas phase
x
x
High temperature
x
Most compounds not
stable at elevated
temperature
Liquid phase
Homogeneous
Heterogeneous
x Low thermal stability
x Difficulty of recovery
Easy recovery and
regeneration of catalyst
Simple molecule only
and regeneration
Figure 1.2: Process options in catalytic oxidation and some of their disadvantages.
As also shown in Figure 1.2, another important choice to consider is whether
the oxidation should be performed using homogeneous or heterogeneous catalyst. In
homogeneous catalysis, the reaction mixture and the catalysts are all in the same
phase, usually the liquid phase. The catalyst may be a metal complex, which is
dissolved in a solvent together with the reagents.
Homogeneous catalysts are
characterized by high activity and selectivity. The main advantage of homogeneous
catalysts is the ease of accessibility of the active site, resulting in a high activity, no
mass transfer limitations and generally low temperature and pressure requirements.
The main disadvantages of this type of catalysts are low thermal stability, difficulty
of recovery and regeneration. In combination with high cost of catalyst, this makes
homogeneous catalysis less popular in industry. Homogeneous catalysts are mostly
found in batch processes where volumes are small and the added value is high, e.g.,
in pharmaceuticals.
4
The problems faced by homogeneous catalyst can be solved by using
heterogeneous catalyst [1, 2, 5]. In heterogeneous catalysis, the catalyst is in a
different phase than the reaction mixture. Usually, the catalyst is a solid substance
(e.g., a metal or a metal oxide) and the reaction mixture can be a gas or a liquid.
Heterogeneous catalyst may be easily recovered by filtration and recycle.
The
success of this type of catalyst is due to the ease at which it can be applied in all
types of reactions, carried out in both continuous and batch mode. It is relatively
easy to separate the catalyst from the reaction mixture and reuse it. The solid,
heterogeneous phase, however, may introduce mass transfer limitations, presumably
resulting in a lower activity or selectivity. Therefore, the selection of suitable active
site for heterogeneous catalyst is a challenge in academic and industry.
Many heterogeneous or solid catalysts are based on porous inorganic solids
[5]. The important physical properties of these materials are surface area (often very
large and measured in hundreds of m2/g), pore volume, pore size distribution (which
can be very narrow or very broad), the size and shape of the particles and their
strength.
The solid catalyst provides a surface, usually large internal, for the
substrates to adsorb and react on.
Thus the surface characteristics (roughness,
functional groups, organophilicity, hydrophobicity, etc.) are also vital to
performance.
Following the definition accepted by the International Union of Pure and
Applied Chemistry (IUPAC), porous materials can be grouped into three classes
based on their pore diameter (d): microporous, d < 2.0 nm; mesoporous, 2.0 < d < 50
nm; macroporous, d > 50 nm. In the class of microporous materials, zeolites and
related materials such as aluminophosphates have found for a long time applications
outside the traditional areas of acid and bifunctional catalysis [9-14]. With the
introduction of the ordered micelle-templated inorganic materials, the choice of
available supports has been considerably extended into the mesoporous domain.
Examples are M41S groups materials such as MCM-41, MCM-48, MSU, HMS,
FSM-16, and various SBA type materials [9, 10]. Macroporous metal oxides such
as titania, zirconia and alumina are example for macroporous material. However, all
of the above materials in general are insulating materials. In addition they are often
5
synthesized with charge compensation in mind. For example, Al3+ substituted for
Si4+ in zeolites lead to an inherent cation-exchanged capacity.
Another way to approach the generation of microporous material is to
generate element with mixed valencies in a structure which should also lead to cation
exchange capacity [11]. Porous manganese oxides with mixed valencies could be a
challenge due to wider application over aluminosilicate materials e.g. redox catalysis,
battery and sensor [12].
1.1
Research Background
1.1.1
Porous Manganese Oxide Materials
Porous manganese oxide is one of the largest families of porous materials.
These are two major structures of porous manganese oxides, tunneled and layered
materials with pore size from ultra-micropores to mesopores [13].
Natural
manganese oxides are found abundant as manganese nodules which exist in a wide
variety of locations such as the ocean floor, the beds of many fresh-water lakes,
rocks, and soil. Manganese oxide minerals have been used for thousands of years as
pigments and to clarify glass, and more recently as ores of Mn metal, catalysts and
battery material [14].
Porous manganese oxide materials have been described as useful catalysts in
the oxidation of carbon monoxide, methane and butane, the reduction of nitric oxide
with ammonia and demetallation of topped crude in the presence of hydrogen [15].
However, the structures and properties of the natural source are usually not uniform
and its activity is not reproducible.
The porous manganese oxides have been extensively investigated for their
economic value and their potential applications.
Due to their excellent cation-
exchange and molecule adsorptive properties, like the aluminosilicates, these
6
manganese oxides can be used as ion-sieves, molecular-sieves, and catalysts similar
to the aluminosilicates. In contrast to aluminosilicate based zeolite, these manganese
oxides are mixed valencies materials (mainly, 4+, 3+, or 2+). The mixed valency
materials are important in biology, chemistry, and physics which occur in manganese
redox enzymes, in natural manganese oxide nodules, chemiluminescence systems,
electron transfer and electrocatalysis, electrochromism, secondary nonaqueous
rechargeable batteries, magnetics, ceramics, and biological systems. In chemistry,
mixed valency in mixed metal oxides is important for electron transport.
For
example, the effectiveness of metal oxides as catalysts for redox reactions, as
electrode materials for electrochemical processes, and as chemical sensors for
reductive gases are usually governed by their ability and tendency to cycle between
different valence states of relevant cations and the mobility of oxygen ions. From
this standpoint, manganese oxide materials have distinct advantages over
aluminosilicate molecular sieve materials for applications in redox catalysis,
batteries, and chemical sensors [12].
The synthetic manganese oxides have been intensively studied by Suib and
his group since 1990’s[16-18]. The synthetic names, the natural minerals and unit
structures are listed in Table 1.1. In general manganese oxide materials can be
classified into two kinds i.e. tunnel and layered structures. The term of octahedral
molecular sieves (OMS) and octahedral layered (OL) materials are referred to the
synthetic manganese oxides with tunnel and layered structure, respectively [19].
OMS materials comprise infinite 3-D crystalline frameworks with molecule-sized
tunnels similar to the naturally occurring zeolites. The structural frameworks of the
manganese oxides consists of MnO6 octahedral units shared by corners and/or edges
in
comparison
with,
in
general,
SiO4-AlO6 frameworks
of
the
porous
aluminosilicates. OMS-1 and OMS-2 are two types of OMS materials which are
built of 3x3 and 2x2 MnO6 octahedral units, respectively. The other types are
pyrolusite-, ramsdellite-, and romanechite-type manganese oxides which have onedimensional (1×1), (1×2) and (2×3) tunnel structures, respectively. OL materials
have interlayer spacings that are similar to clay-type materials. OL-1 is synthetic
name of birnessite which has the interlayer spacing of ~7 Å and it contains
exchangeable cations and water molecules.
7
Table 1.1: The synthetic manganese oxides, their natural counterpart and structures
as reported by Suib [12, 20-22].
Synthetic Name
Natural counterpart
MnO6 octahedral units
OMS-1
todorokite
3x3
OMS-2
cryptomelane
2x2
OMS-5
-
2x4
OMS-6
romanechite
2x3
OMS-7
pyrolusite
1x1
OL-1
birnessite
2x~
Among most of the OMS and OL materials, OMS-1 and OMS-2 materials
have been used widely as catalysts, chemical sensors and batteries due to their
mixed-valency, high porosity, thermal stability, surface areas, and inexpensive
manufacturing cost. However, OMS-2 materials have been used as catalyst even
more widely than OMS-1 materials since they are more active and selective in
catalytic oxidation [20].
1.1.2
Manganese Oxide Octahedral Molecular Sieves (OMS-2)
As mentioned before OMS-2 materials are synthetic manganese oxide which
is built of 2x2 MnO6 octahedral and also known as synthetic cryptomelane [11]. The
tunnel size of this material is 4.6 Å x 4.6 Å and potassium ion exists in the tunnel to
balance the charge of the structure as shown in Figure 1.3. General composition of
OMS-2 materials is K0.8–1.5Mn8O16. These materials are not expensive and easy to
prepare compared to other manganese oxide type materials.
For its application in catalytic reaction, OMS-2 materials were reported to be
selective in oxidation of benzyl alcohol [23, 24]. Besides, OMS-2 materials are
potential electrocatalysts for the oxidation of methanol, for fuel cell applications
8
[25], are also active catalyst for the total oxidation of benzene and ethanol [26] and
epoxidation of olefins [27, 28]. The mild oxidants i.e. molecular oxygen and terbutyl hydrogen peroxide (TBHP) used as oxidizing agents on OMS-2 materials have
let to these materials being promising catalysts in fulfilling the environmental
concern and regulations for clean environment.
Figure 1.3: Crystal structure of cryptomelane-type OMS-2: potassium atoms are
shown as green spheres; MnO6 octahedra are shown in brown.
The high oxidation ability of OMS-2 materials has mainly been related to two
factors: the presence of Mn2+/Mn4+ or Mn3+/ Mn4+ redox couples and the ability of
active participation of the lattice oxygen in these systems in oxidation process
leading to a Mars van Krevelen type of oxidation mechanism [23, 24].
The
mechanism involves two steps as illustrated in Figure 1.4. Firstly, the lattice oxygen
oxidizes the substrate molecule, followed by a reoxidation of the partially reduced
catalyst by molecular oxygen in order to regenerate the catalyst.
To date, however, the report on liquid phase oxidation via Mars-van
Krevelen-type
mechanism
is
limited,
i.e.,
homogeneous
catalysis
by
phosphovanadomolybdate [29] and heterogeneous catalysis by OMS-2 catalyst [23,
9
24].
The high selectivity of product makes this mechanism interesting and
worthwhile for intensive study.
Figure 1.4: Mars van Krevelen mechanism.
1.1.3 Modification of OMS-2 Materials
In order to alter their structures and properties and generate better electronic
and catalytic performance, the modification was done by doping cation into OMS-2
materials. The cations doped into OMS-2 materials are listed in Table 1.2. These
cations can substitute potassium and/or manganese ions which exist in the tunnel and
framework structure, respectively [30]. Doping of alkali metals or NH4+ or H+ was
reported to substitute some potassium ions which exist in the tunnel structure of
OMS-2 materials.
Transition metal oxides were incorporated in the framework
and/or exist in the tunnel structure. The existence of metal oxides in the framework
indicated that those metal oxides were substituted for manganese. When metals exist
in a tunnel structure, it suggests that the metals have replaced potassium. The
location of doped metal depends on the preparation method [31]. The transition
metal cations are mostly situated in the framework positions of the OMS-2 structure
with a priori incorporation. In a posteriori incorporation, the cations are situated in
either the tunnel positions (if prepared by ion-exchange) or in extraframework
positions (if prepared by homogeneous precipitation).
10
Table 1.2: The cations were doped OMS-2 and their location in OMS-2 material.
Doping cations
Location
References
H+/NH4+
In tunnel structure
[23, 24, 32]
Alkali metals
In tunnel structure
[32-34]
Transition metals
In framework and/or
tunnel structure
[27, 30, 31, 35-46]
The summary on incorporation of transition metals into OMS-2 materials is
shown in Figure 1.5. Among these transition metals, titanium incorporated OMS-2
has not been reported and therefore is appropriate to be explored.
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Zr
Nb
Mo
Tc
Ru
Rh
Pd
Ag
Cd
Have been doped into OMS-2
Not studied
Figure 1.5: Partial periodic table showing transition metals that have been
incorporated into OMS-2 materials.
The physical and chemical properties of doped OMS-2 are greatly influenced
by the type, amount and location of the doping ions, where the properties of doped
materials are significantly different from those of the undoped ones [12]. In the
tunnel cations position, H+ doped OMS-2 exhibits the surprising activity in oxidation
of alcohols and acid-catalyze condensation of phenylhydroxylamine with aniline to
2-aminodiphenylamine. Among the transition metal doped OMS-2 materials, FeOMS-2 seems to be the best catalyst in some catalytic reactions such as oxidative
dehydrogenation of 1-butene [40], oxidation of toluene [41], decomposition of the
cyanine dye and pinacyanol chloride [44], and oxidative dehydrogenation of ethanol
[46]. However the activity of Fe-OMS-2 is lower than Co-OMS-2 in oxidation of
styrene [27]. It suggests that the activity of metal doped OMS-2 also depends on the
type of substrates involved in the reactions.
11
Titanium oxide attracts much attention in catalysis as well as photocatalysis
and it has been used in the synthesis of many chemical compounds. Titanium
incorporated material shows outstanding catalytic properties, particularly in liquid
phase oxidation process [47-49]. Shell catalyst (Ti(IV)/SiO2) is the basis of the
commercial
process
for
the
epoxidation
of
propene
with
ethylbenzene
hydroperoxide. Thus, the discovery of titanium silicate-1 (TS-1) by Taramasso et al.
[50] exhibited the remarkable catalytic activity, selective epoxidations with 30%
aqueous hydrogen peroxide under very mild conditions, constituted a milestone in
oxidation catalysis. The discovery of TS-1 led to the study on incorporation of
titanium into porous materials: microporous material such as silicoaluminophosphate
(SAPO-5) and aluminophosphates (AlPO-5, AlPO-11 and AlPO4-36) and
mesoporous materials such as MCM-41 and MCM-48.
The incorporation of titanium into manganese oxide molecular sieve
frameworks is feasible because of similar sizes, charges, and coordination tendencies
of manganese and titanium cations. Ionic radii of octahedral Mn3+, Mn4+, and Ti4+ of
0.65 Å, 0.53 Å and 0.61 Å, respectively [54-55] in crystals are close to one another,
hence Ti4+ can easily substitute either Mn3+ or Mn4+ without causing much structural
disorder and serious charge imbalance.
Due to the mixed-valence character of
manganese in OMS-2, this material has distinct advantages over silicate,
aluminosilicate and aluminophosphate molecular sieve materials in catalytic
applications [12, 30]. The incorporation of titanium into OMS-2 is expected to give
excellent performance in oxidation reaction.
The previous sections have demonstrated that OMS-2 and metal doped OMS2 materials are potential catalysts in oxidation reactions. The catalytic activity of
metal doped OMS-2 varies depending on the substrate. Further study of the metal
doped OMS-2 is required in order to understand the physicochemical propertiescatalytic activity of the catalysts. Besides that, the effect of titanium incorporated
OMS-2 has not been reported.
Therefore, the study on the effect of titanium
incorporated OMS-2 to the physicochemical properties and catalytic activity of
OMS-2 material in oxidation reaction is needed.
Further investigation in the
potential application of Ti incorporated OMS-2 in consecutive oxidation and acid
12
reaction should be done. The next section describes how Ti incorporated OMS-2 is
modified to bifunctional oxidative and acidic catalyst.
1.1.4 Design of Sulphated Ti-OMS-2 as Bifunctional Oxidative and Acidic
Catalyst
A bifunctional or multifunctional catalyst is the catalyst which has two or
more active sites. The catalysts have active sites which can catalyze two or more
transformations which are carried out as a “one-pot” process. They offer a number
of advantages to the organic chemist: in particular, they result in a reduced number of
operations, giving significant time-cost benefits, but they also often allow “difficult”
intermediate compounds (i.e., those that are volatile, toxic, or otherwise noxious) to
be prepared and elaborated in situ, thus preventing problems associated with their
isolation and handling [51].
A bifunctional oxidative and acidic catalyst catalyzes for oxidation and acidreaction, respectively. The catalyst effectively directs transformation of alkenes to
alcohols. The oxidative sites catalyze the oxidation of alkenes to epoxide followed
by transformation of epoxide to diols in the presence of Brönsted acid site in the
catalyst. At present, 1,2-diols are manufactured industrially by a two-step sequence
consisting of epoxidation of an olefin with a peracid followed by hydrolysis of the
resulting epoxide as shown in Figure 1.6.
Figure 1.6: Transformation of alkenes to diols via two step reaction.
Recently, Prasetyoko et al. [52-54] reported that titanium silicalite (TS-1)
loaded with sulphated zirconia or niobium oxide demonstrated bifunctional oxidative
and acidic properties. However, TS-1 is expensive and difficult to prepare. The
13
purpose of this research is to design a novel bifunctional oxidative and acidic
catalyst.
Combining oxidative OMS-2 and acidity, one can come up with the
bifunctional catalyst for consecutive liquid phase oxidation and acid reactions.
Sulphated titanium oxide is a solid superacid which exhibits both Lewis and
Brönsted acidity [55] which are considered as acid sites. The solid acid is used to
overcome the problems of using homogeneous acid.
One approach is to create acidity via incorporation of acidic sites in the
framework of OMS-2. It is expected that bifunctional oxidative and acidic catalyst
can be created by sulphation of TiO2 phase in titanium supported OMS-2 (Ti-OMS2) to form SO42--TiO2 superacid active site. Based on this consideration, a catalytic
model of consecutive oxidation and acid catalyst (sulphated Ti-OMS-2) to catalyze
consecutive reactions of alkenes to alcohols e.g. 1-octene to 1,2-octanediol could be
synthesized. MnO6 octahedra as a basic of OMS-2 unit acts as an oxidative site for
epoxidation of 1-octene to 1,2 epoxyoctane and SO42--TiO2 superacid acts as an
acidic site for transformation of 1,2 epoxyoctane to 1,2 octanediol. Hence the use of
OMS-2 materials which are relatively cheap and easily prepared and use a mild
oxidant for oxidation process and sulphated TiO2 environmentally friendly as acid
sites is expected to be the best solutions to overcome the related problems.
1.2
Research Questions and Scope of Research
Based on the above descriptions, one considers that OMS-2 is the potential
heterogeneous catalysts for liquid phase oxidation. Although modification of OMS-2
by doping of transition metal oxides have been reported but Ti incorporated OMS-2
has not been reported yet [30-46]. The effect of the location, amount and synthesis
method of titanium incorporated OMS-2 to the catalytic properties of OMS-2 in
some oxidation reactions have not been studied yet.
The relationship between
physicochemical properties and catalytic activity of titanium and other metals
incorporated system are not well understood. In addition, design of bifunctional
oxidative and acidic catalyst in order to form the more effective and efficient catalyst
14
for consecutive oxidation and acid reactions is a challenge that has less studied.
Modification of OMS-2 by enhancement of the oxidative ability and creation of the
acid site may together be beneficial and advantageous to create the bifunctional
catalyst for synthesis of diols from alkenes.
Figure 1.7 shows the research approach and questions of the research. The
research includes the study on effect of synthesis method and some modifications to
physicochemical properties of OMS-2 and their correlation to catalytic activity of the
catalysts. Generally, there were two categories of modifications of OMS-2; firstly,
metals substituted and H-exchanged, aimed to improve the catalytic oxidation of
OMS-2 catalyst.
The catalysts were characterized by several characterization
methods to find out their physicochemical properties. Thus, those were correlated to
the catalytic activity of the catalysts in oxidation of cyclohexane. However, this
study emphasized on titanium incorporated OMS-2. The effect of synthesis method,
amount and location of the titanium substituted to the physicochemical properties of
OMS-2 were evaluated. Characterizations were done by several methods such as
structural and acidity analysis, surface area, hydrophobicity-hydrophylicity, etc.
Thus, their physicochemical properties were correlated to catalytic activity of the
catalyst in several oxidation reactions such as cyclohexane, cyclohexene and styrene.
The second modification was sulphation of titanium supported OMS-2 aimed to
create acid sites on the oxidative catalyst. The catalysts were characterized and
tested in consecutive oxidation and acid reactions for direct synthesis of diols from
alkene. Briefly, the research attempted to answer some questions as depicted in
Figure 1.7.
1.3
Research Objectives
The research proposed is a fundamental study of heterogeneous catalytic
system for both oxidation process and consecutive reaction of alkenes to alcohols
under mild conditions. Since it is a one step, inexpensive and easy preparation of the
15
Synthesis
x OMS-2 synthesized in different
methods
x Synthesis of metals substituted
OMS-2 (M = Ti, Fe, Cu, Co)
x H-exchanged OMS-2
x Sulfation of Ti-OMS-2
Characterization
x
x
x
x
Structural analysis
Acidity analysis
Surface area
Hydrophobicityhydrophilicity
Research
Questions
Catalytic activity
x Oxidation reactions
x Consecutive
oxidation and acid
reactions
1. Which method gives the best performance of OMS-2 in oxidation of
benzyl alcohol.
2. What are the effects of metals substituted and H-exchanged to the
physicochemical properties of OMS-2 and its correlation to catalytic
activity of the catalyst in oxidation reaction?
3. How are the catalytic activities of Ti-OMS-2 compared to the other
metals incorporated OMS-2?
4. What are the effects of the synthesis method, amount and location of
titanium sites to physicochemical properties of Ti-OMS-2 and its
correlation to catalytic activity in oxidation reaction?
5. How is catalytic activity of Ti sites in different substrates in oxidation
reaction?
6. Is sulfation of Ti-OMS-2 catalyst successfully designed as a bifunctional
oxidative and acidic catalyst for synthesis of diols from alkene?
Figure 1.7: The schematic of the research approach and research questions.
16
catalyst using mild oxidant as oxidizing agent, these systems are more economical
and environmentally friendly (see Figure 1.8).
In light of the issues described previously, the research was conducted with
the following objectives:
1. To synthesize manganese oxide octahedral molecular sieve (OMS-2),
titanium supported OMS-2 (Ti-OMS-2), metal supported OMS-2 (MOMS-2) and H doped metal supported OMS-2 (H-M-OMS-2).
2. To modify the surface of Ti-OMS-2 by sulphation as a bifunctional
catalyst.
3. To study the physicochemical properties-catalytic activity relationship in
model reactions.
17
Multidisciplinary Science
Catalysis
x
x
x
x
Oxidation reaction
Importance
Success of the industry in 20th century
Greener industry in 21th century
The heart of quality of life
A healthy economy growth
Biggest
problem
areas
x OMS-2 as a promising heterogeneous
catalyst (active with mild oxidant,
inexpensive and easy preparation)
x TS-1 as a milestone in oxidation catalysis
Acid-catalyze reaction
Sulfated metal oxide as a strong
solid acid catalyst which more
environmentally friendly
Catalyst
Design
Catalyst
Design
Sulfated Ti-OMS-2
Ti-OMS-2
A new heterogeneous catalyst for
oxidation reaction under mild condition
Bifunctional oxidative and acidic
catalyst for consecutive reaction of
alkene to diols (one pot reaction)
Significant
More economic and environmentally
friendly catalysts
Figure 1.8: The significant of the use Ti-OMS-2 in heterogeneous oxidation and acid
reactions.
CHAPTER 2
LITERATURE REVIEW
2.1
Introduction
This chapter presents brief introduction of classification porous manganese
oxide, synthesis of porous manganese oxide and OMS-2 materials, an overview of
modification and catalytic activity of OMS-2 materials, oxidation reaction, titanium
incorported materials, sulphated metal oxides as solid acid and synthesis of diols.
The modification and characterization techniques and reactions which used as model
in this research are described.
2.2
Classification of Porous Manganese Oxide
The tunnel and layered manganese oxide minerals can generally be classified
into three groups i.e. the pyrolusite-ramsdellite family with (1×n) tunnel structure,
the hollandite-romanechite family with (2×n) tunnel structure, and the todorokite
family with (3×n) tunnel structure, respectively. All structures contain infinite chains
of edge-sharing MnO6 octahedral structural units, and the numbers 1, 2, 3, and n
correspond to the number of octahedra in the unit chain width. The chains are linked
by corner sharing to form a one dimensional tunnel structural network.
This
structure comprises of infinite 3-D crystalline frameworks with molecule-sized
tunnels similar to those found in zeolites. When n = , the network corresponds to a
19
layered structure which is similar to clay-type materials.
Table 2.1 shows the
nomenclature of major tunnel and layered manganese oxides proposed by Turner and
Buseck [13].
Table 2.1: Classification of tunnel and layered manganese oxides and their
crystallographic data [13].
Mineral or
compound
Approximate
formula
Crystal
system
(space
group)
Lattice
constants
(Å)
Structural
features
(1×n) family
Pyrolusite
(-MnO2)
MnO2
Tetragonal
(P42/mn2)
a=4.39;
c=2.87
(1×1) tunnel
Ramsdellite
MnO2
Orthorhombic a=4.53;
(Pbnm)
b=9.27;
c=2.87
(1×2) tunnel
Nsutite (MnO2)
[MnII, MnIII, MnIV]
(O,OH)2
Hexagonal
a=9.65;
c=4.43
(1×1)/(1×2)
complex
tunnel
LiMn2O4
Cubic (Fd3m)
a=8.25
(1×3) tunnel
Li1.09Mn0.91O2
Hexagonal
(R3m)
a=2.85;
c=14.26
(1×) layer
of 4.75 Å
wide
Vernadite (
MnO2)
MnO2
·H2O·R2O,RO, R2O3
(R=Na,Ca, Co, Fe,
Mn)
Hexagonal
a=2.86;
c=4.7
(1×) layer
of 4.75 Å
wide
(2×n) family
Hollandite
(R)2[Mn8]O16·xH2O
(R=Ba, K, Na, NH4)
Tetragonal
(I4/m) or
monoclinic
(I2/m)
a=9.96;
c=2.86
a=10.03;
b=5.76;
c=9.90;
=90.42
(2×2) tunnel
Continued in Page 20
20
Romanechite
or psilomelane
(R)2[Mn5]O10·xH2O
Monoclinic
(A2/m)
a=9.84;
b=2.88;
c=13.85
=92.30
(2×3) tunnel
RUB-7
(R=Ba, K, Na)
(R)2[Mn6]O12·xH2O
Monoclinic
(C2/m)
a=14.19;
b=2.85;
c=24.34;
=91.29
(2×4) tunnel
Rb0.27MnO2
(R=Rb, K, Na)
(Rb)4[Mn7]O14
Monoclinic
(A2/m)
a=15.04;
b=2.89;
c=14.64;
=92.4
(2×5) tunnel
Birnessite
Na4Mn14O27·9H2O
and RyMnO2·xH2O
(R=monovalent or
divalent metal ions)
Orthorhombic a=8.54;
b=15.39;
Hexagonal,
c=14.26
monoclinic,
orthorhombic,
or triclinic
(3×n) family
Todorokite
( R)[Mn6 ]O18·xH2O
(R=divalent metal
ions and Na, K)
Monoclinic
(P2/m)
a=9.76;
b=2.84;
c=9.55;
=94.1
(3×3 ) tunnel
Buserite
NayMnO2·xH2O and
RyMnO2·xH2O
(R=divalent metal
ions)
Hexagonal
a=8.41;
c=10.01
(3×) layer
of ~10 Å
wide
Lithiophorite
[MnIII2MnIV4O12]
[Li2Al4(OH)12]
Monoclinic
(C2/m)
a=5.06;
b=8.70;
c=9.61;
=100.7
Sandwich
layer of
~9.5 Å apart
Na0.44MnO2
Orthorhombic a=9.10;
(Pbam)
b=26.34;
c=2.82
MnO6/MnO5
complex
tunnel
Li0.44MnO2
Orthorhombic a=8.93;
(Pbam)
b=24.44;
c=2.83
MnO6/MnO5
complex
tunnel
(2×) layer
of ~7 Å wide
Other families
Continued in Page 21
21
Ba6Mn24O48
Tetragonal
(I4/m)
a=18.17;
c=2.824
(1×1)×(2×2)
complex
tunnel
-NaMnO2
Monoclinic
(C2/m)
a=5.63;
b=2.86;
c=5.77;
=112.9
Layer 5.3 Å
wide
-LiMnO2
Monoclinic
(C2/m)
a=5.44;
b=2.81;
c=5.39;
=116.0
Layer 4.8 Å
wide
-NaMnO2
a=2.85;
b=6.31;
Orthorhombic c=4.77
(Pmnm)
Layer 6.31 Å
wide
-LiMnO2
Orthorhombic a=2.81;
(Pmnm)
b=5.75;
c=4.57
Layer 5.75 Å
wide
Mesoporous
OMS-1
(MOMS-1)
Mn2O3/Mn3O4
Hexagonal
a=54;
c=
Mesopore
size about 30
Å
Mesoporous
OMS-2
(MOMS-2)
Mn2O3/Mn3O4
Cubic (Ia3d )
-a
Mesopore
-a not reported
The schematic structures of manganese oxide with one-dimensional tunnel
and layered structures are shown in Figure 2.1. Pyrolusite-, ramsdellite-, hollandite-,
romanechite- and todorokite-type manganese oxides, and synthetic Rb0.27MnO2 have
one-dimensional (1×1), (1×2), (2×2), (2×3), (3×3), and (2×5) tunnel structures,
respectively. On the other hand, birnessite- and buserite-type manganese oxides
have layered structures with basal spacings of about 7 and 10 Å, respectively.
The tunnels and the interlayer spaces of the manganese oxides materials can
be occupied by metal ions and crystal water molecules. The birnessite and buserite
structures contain a single crystal water sheet and a double crystal water sheet
between the MnO6 octahedral sheets, respectively. Most Mn are tetravalent in these
22
manganese oxides, but a part of Mn are trivalent in order to balance the charge of the
foreign ions in the tunnels and interlayers.
MnO6
octahedron
a. (1x1) tunnel
pyrolusite
d. (2x2) tunnel
hollandite
g. (2x5) tunnel
Rb0.27MnO2
b. (1x2) tunnel
ramsdellite
e. (2x3) tunnel
romanechite
h. (2x) layer
birnessite
c. (1x) layer
Li1.09Mn0.91O2
f. (2x4) tunnel Rub-7
i. (3x3) tunnel
todorokite
j. (3x) layer buserite
Figure 2.1: Schematic structures of one-dimensional tunnel and layered manganese
oxides [13].
23
The intergrowth of two or more tunnel phases occurs also in the manganese
oxides as shown in Figure 2.2 [13]. An irregular intergrowth of (1×1) tunnels
(pyrolusite) and (1×2) tunnels (ramsdellite) in the structure of -MnO2 (nsutite) is
well known to electrochemists.
Complex intergrowths of the (2×2) tunnels
(hollandite) and the (2×3) tunnel (romanechite) are found in fibrous manganese oxide
minerals. Almost all the intergrowths are random, so that regular periodicity or
superstructure may not be apparent.
a. (1x1)/(1x2) tunnel
intergrowth (nsutite)
b. (2x2)/(2x3) tunnel
intergrowth
Figure 2.2: Intergrowth tunnels of (a) (1×1) and (1×2), and (b) (2×2) and (2×3) in
the tunnel structure of manganese oxides.
The naturally occurring manganese oxides, e.g. manganese nodules are found
on the sea floor at depths of thousands of meters and occur as vast deposits, which
are estimated to be as much as 500 billion tons in all oceans [56]. Manganese
nodules are generally composed of very small particles of poorly crystalline
manganese (IV) oxide minerals, which are related to terrestrial minerals, such as
binnersite, todorokite, cryptomelane, and nsutite. The compositions of manganese
nodules are not homogeneous and vary considerably from place to place on the ocean
floor. The occurrence of different mineral phases in manganese nodules depends on
the location, marine environment, and sediment type.
Compositional variations
occur even among samples collected at the same place. Manganese nodules consist
of a variety of metal oxides, mainly transition metal oxides. The major elements in
manganese nodules are Mn and Fe (~50 wt% each, excluding O), and the minor
elements (less than 2 wt%) are Co, Ni, Cu, Ti, Si, Al, Ca, and Mg.
In addition to being potentially important mineral sources for useful metals
due to the vast deposits and varied compositions of metal oxides, manganese nodules
are also valuable as potential cheap natural sources of adsorbents and catalyst due to
24
the porous structure (porosity in the order of 60%) and large specific areas (90 to 400
m2/g) [56]. Several promising processes utilizing manganese nodules have already
been proposed and applied in the petroleum industry and pollution control plants.
Manganese nodules are described as powerful catalysts in oxidation of carbon
monoxide, methane and butane, the reduction of nitric oxide with ammonia and
demetallation of topped crude in the presence of hydrogen [15]. Although some
catalytic studies of manganese nodules have been conducted, there are a few
fundamental and systematic studies on their catalytic properties due to the
complexity and uncertainty of their chemical and mineralogical composition. Suib
and co-workers [11] prepared the synthetic manganese oxide materials with
octahedral molecular sieves and octahedral layer structures. They claimed that these
materials are pure and more crystalline, and have more uniform and homogeneous
structure than their natural counterparts. OMS materials normally have an open
framework tunnel structure with sizes ranging from a few angstroms to hundreds of
angstroms. This unique property makes them excellent shape selective catalysts and
ion exchange materials, which make the application of OMS materials more
consistent and reproducible results [57, 20].
2.3
Material Synthesis
2.2.1
Synthesis of Porous Manganese Oxide
Porous manganese oxides, as well as layered claylike manganese oxide
materials are prepared via a variety of routes. Many of the materials with similar
gross structural features nevertheless show a diversity of properties depending on the
specific synthetic route. These differences may be attributed to variations in particle
size and the type and amount of defects in the structures. Thus, small changes in
synthetic parameters can result in materials with novel catalytic, electrochemical, and
ion-exchange properties [12].
25
The tunnel and layered manganese oxides can be prepared by a variety of
processes [12, 13]. These processes are classified into:
1. Dry process
solid state reaction
melting salt flux processes
2. Wet process
redox precipitation / reflux method
hydrothermal
hydrothermal soft chemical processes
3. Wet–dry process
sol–gel processes.
Stable phases with small tunnel and narrow layered structures, such as spinel,
hollandite, romanechite, and birnessite can be synthesized by any of these processes.
However, metastable phases with large tunnel and wide layered structures, such as
Rb0.27MnO2, RUB-7, todorokite, buserite, and mesoporous manganese oxides, can
only be prepared by the wet processes [13]. Metal ions and organic surfactants are
usually used as templates (see Table 2.2). The wide tunnel and layered structures are
easily formed under high template concentrations.
Generally, the ion-sieve,
electrochemical, and catalytic properties of the manganese oxides depend on the
synthetic process.
When Ba2+ is used as template for 2x2 tunnel structure of manganese oxide
the materials are called hollandite. If the template is K+, Na+, Pb2+, the related
materials are named cryptomelane, manjiroite and coronadite, respectively.
Generally, all those materials are grouped in hollandite type materials. The synthetic
layered and tunnel manganese oxide materials are also known as octahedral layer
(OL) structure materials and octahedral molecular sieves (OMS) materials,
respectively [19].
Thus, the synthetic binnersite, hollandite and todorokite are
labelled as OL-1, OMS-2 and OMS-1 material, respectively. In this work, we
emphasised on OMS-2 (holandite type) materials due to their advantages over other
manganese oxide materials in catalytic oxidation.
26
Table 2.2: Synthesis of tunnel and layered manganese oxides with various templates
[13].
Template
Compound
Wet process
Dry or wet–dry process
Pyrolusite (-MnO2)
H+
No template
Ramsdellite
H+
-a
Spinel
Li+
Li+, Mg2+
A0.44MnO2
-a
Na+
-AMnO2
-a
Na+
-AMnO2
-a
Li+, Na+
Hollandite
K+, Rb+ , Pb2+, Ba2+, NH4+,
H3 O+
K+, Rb+ , Pb2+, Ba2+
Romanechite
Ba2+
Na+
RUB-7
Rb+, K+, Na+
-a
Rb0.27MnO2
Rb+
-a
Birnessite
Alkali metal ions
Na+, K+
Ba6Mn24O48
-a
Ba+
Buserita
Na+
-a
Todorokite
Mg+, divalent transition
metal ion
-a
MOMS (Mesoporous
material)
CH3(CH2)15(CH3)3N+,
[CH3(CH2)3]4N+
-a
- a Not reported
2.2.2
Synthesis of OMS-2 Materials
In the solid state reaction, OMS-2 may be synthesized by calcinations of MnO2 at 600 oC for 2 h using Na+ or K+ as template [58]. However, using Na+ as a
template for the hollandite is rare, because the size of Na+ is somewhat smaller than
the tunnel sizes of hollandite types.
27
The precipitation process or also known as reflux method involving oxidation
of Mn salts and/or reduction of MnO4 salts in solutions is a typical method for the
2+
production of birnessite and hollandite. Birnessite can be obtained in alkaline or
weak acidic solution, while the hollandite can be obtained only in acidic solution.
The hollandite-type manganese oxides may be prepared directly by oxidation of
Mn2+ salts or reduction of AMnO4 (A = alkali metal) in acidic aqueous solution. The
oxidants can be used for synthesis of hollandite type material by oxidation of Mn2+
salts are KMnO4, KClO3, O2, H2O2 and K2S2O8 [30, 37, 59-62]. K+ and NH4+ are
usually used as the template for the preparation of hollandites. K-hollandites are
prepared by reacting MnSO4 or Mn(NO3)2 solution with KMnO4 or KClO3 in a 1 M
H2SO4 solution at above 60 oC. NH4-hollandites are obtained by using NH4S2O8 as
the oxidant. The hollandites without template metal ions in the tunnel sites could be
directly prepared by reacting Mn(NO3)2 with LiMnO4 or NaMnO4, or oxidation of
MnSO4 with ozone (O3) gas in a H2SO4 solution. In these reactions, H3O+ may act as
the template for the hollandite structure. A higher reaction temperature (>70 °C) and
higher concentration of H2SO4 (>4 M) are necessary for the hollandite formation
with small diameter of template metal than when using a K+ or NH4+ template which
has bigger diameter [32].
Alternatively, hollandite is formed by hydrothermal decomposition of
KMnO4 at higher temperature and lower pressure as compared to the conditions for
birnessite formation.
A K2Mn4O9 phase is obtained in this hydrothermal
decomposition reaction. Ohzuku et al. [63] prepared hollandites with NH4+, K+, and
Rb+ templates by treating Mn2O3 in 0.5 M H2SO4 solutions containing these ions at
100 °C. The hollandite without the template ions in the tunnel can be obtained by
leaching Mn2O3 in a 4-8 M H2SO4 solution at 105 °C. The treatment of Mn2O3 in
acidic solution results in the disproportionation of Mn2O3 into soluble Mn2+ and
MnO2 with a hollandite structure. Rossouw et al. [64] have synthesized a highly
crystalline hollandite without metal ion or NH4+ in the tunnel by leaching Li2MnO3
in a 1-4 M H2SO4 solution at 90 °C. Similar to the redox precipitation reaction, the
H3O+ ions may act as the template in these reactions. Hollandite is also formed in
the hydrothermal reacting -MnO2 in 5 M RbOH at >400 °C [65]. The structure of
the manganese oxide strongly depends on the concentration of RbOH. The product
28
is hollandite of low RbOH concentration, while the product is (2×5) manganese
oxide or birnessite-type manganese oxide at high RbOH concentration.
This
suggests that large tunnel and layered structures are preferred in alkaline solution at
high template concentrations.
A hydrothermal soft chemical process is a useful and unique method for the
preparation of the tunnel manganese oxides. This process comprises of two steps:
the first step is the preparation of a framework precursor with layered or analogous
structure and insertion of template ions or molecules into the interlayer space by a
soft chemical reaction, and the second step is the transformation of the templateinserted precursor into a tunnel or other structure by hydrothermal treatment. The
dimension of the resulting tunnel can be designed and predicted easily from the
dimensions of the template. Hollandite can be obtained by using K+ templates. The
schematic procedure is shown in Figure 2.3.
Layered structure
Layered structure
Tunnel structure
K+
Hydrothermal
reaction
Ion-exchange
reaction
Na-binnersite
K-binnersite
Hollandite
Figure 2.3: Transformation reaction from birnessite to hollandite under
hydrothermal conditions.
The hydrothermal reaction, however, may be most effective for the
transformation reaction, because under hydrothermal conditions the transformation
reaction can be carried out at low temperature to prevent destruction of metastable
tunnel structures, and the template ion or molecule can be incorporated/liberated
into/from the precursor from/to the solution phase to reach a composition for the
desired structure during the transformation reaction.
29
The sol–gel synthesis hollandite has been prepared by the reaction of KMnO4
with organic reducing agent [19, 33]. Two different groups of organic species were
investigated as reducing agents: multifunctional carboxylic acids, such as fumaric
acid and maleic acids, and polyols, including sugars. The authors have investigated
the effect of cations (Na, K), ratio of reducing agent to permanganate, temperature of
calcinations, and effect of pH on the structure of the materials produced, along with
their ion-exchange properties.
In other ways, OMS materials were also prepared by microwave heating
method [66, 67]. The microwave heating method gave different catalytic activity to
the thermal heating. The comparison of preparation of two sets of materials, using
the same starting reagents and conditions, but one heated by microwaves and the
other thermally, showed the different catalytic activity in oxidation of benzene [20].
The samples prepared in microwave heating are more active for total oxidation.
Among the preparation method used in the synthesis of OMS-2 material, a
reflux method was one route mostly used to prepare bulk OMS-2 materials [60, 62].
Study on the effect of different media i.e. acidic, neutral and basic condition in the
synthesis of OMS-2 by Maknawa et al. [23] showed that OMS-2 prepared in acidic
medium resulted in material with the highest surface area and maximum activity in
oxidation of alcohol.
2.4
Modification of Manganese Oxide
2.4.1
Overview of Modification of OMS-2 materials
Modification of OMS-2 materials was aimed at achieving better electronic
and catalytic performance of OMS-2. Some authors reported the incorporation of
metal cations into OMS-2 materials as summarized in Table 2.3. It shows that metal
incorporated OMS-2 alters the structures and properties of OMS-2 materials. Other
metal cations can substitute potassium ion in the tunnel or manganese ions in the
Cu2+
A sol-gel and Framework
reflux
method
-a
Adding
Mg2+,
Ni2+, Cu2+, dopant prior
to refluxing
Co2+, and
Fe3+
Location
-a
Synthesis
route
Ni2+, Cu2+, Adding
Fe3+, Co2+, dopant prior
to refluxing
and Zn2+
Metal/Ion
Doping
Thermal stability
and resistivity of
Cu-OMS-2
decrease and
increase,
respectively, as
amount Cu
incorporated
increase.
-a
-a
Properties
Liquid
phase
Gas
phase
Type
reaction
Gas
oxidative
phase
dehydrogenation
of ethylbenzene to
styrene
Decomposition of
H2O2
Oxidative
dehydrogenation
of ethanol
Catalytic
Application
[68]
[45]
[46]
References
Continued in Page 31
Copper content influenced
styrene selectivity.
The trends is Ni- > Cu- > Fe> Co- > Mg-OMS-2 to
decompose H2O2.
Fe-OMS-2 had the highest
conversion and selectivity to
acetaldehyde
Results
Table 2.3 : Review of Metal Doped into OMS-2; its synthesis routes, location, properties and catalytic application.
30
Reflux method
Hydrothermal
method
Hydrothermal
method
Reflux method
Ag+,
Co2+,
Cu2+,
and
Fe3+
Fe3+
Cr3+
Cu2+,
Zn2+,
Ni2+,
Co2+,
Al3+, or
Mg2+
Metal
cations
probably in
framework
except Cu2+
(mostly in
the tunnel)
Tunnel and
framework
Framework
Framework
All M-OMS-2 materials have
similar acidities but varied strength
and amount of basic sites.
M-OMS-2 has slightly higher
surface areas and pore volumes
M-OMS-2 has similar thermal
stabilities but slightly lower than
undoped OMS-2.
-a
The thermal stability of
cryptomelane was improved by
iron doping. The strength of basic
sites increased with increasing
amounts of iron doping.
Doped OMS-2 catalysts are very
stable over long times on stream
during reaction.
-a
-a
-a
Oxidation
of carbon
monoxide
-a
-a
-a
Gas
phase
[30]
[37]
[36]
Continued in Page 32
Phiscochemical
properties of M-OMS-2
Synthesis of Cr-OMS-2,
from Na-binnersite and
ion exchange with Cr.
Fe was in the framework
structure of OMS-2
Catalytic activity
[31]
depended on oxidation
number of Mn and the
position and nature of the
doped cations. K–OMS2 shows negligible
activity.
31
Ion-exchange
Hydrothermal Tunnel
method
H+
Li+,
Na+,
K+,
Rb+,
and H+
Tunnel
Tunnel
Ion-exchange
H+
-a
Reflux
method
Cu2+,
Zn2+,
Ni2+,
Co2+,
Al3+, or
Mg2+
The larger cations led to better
crystallinity and more ordered
tunnel structure. Larger cations
lead to higher thermal stability.
A-OMS-2 had nanofibrous
morphologies with various
widths on the order of nm and
various lengths on the order of
μm.
The exchange of the tunnel
cation with the smaller H+ ions
leads to weakening of the Mn–O
bond
The catalyst has Lewis acid
sites
-a
Aerobic
oxidation of
cyclohexanol
to
cyclohexanone
Oxidation of
alcohol
Oxidation of
benzyl alcohol
with O2
Decomposition
of 2-Propanol,
product:
acetone and
propene.
Liquid
phase
Liquid
phase
Liquid
phase
Gas
phase
[32]
[23]
[24]
[69]
Contiuned in Page 33
Higher surface areas lead to
better catalytic performance.
Selectivity 100% to
corresponding aldehyde/keton,
the highest conversion for
oxidation benzylic compound.
High activity and 100%
selectivity to benzaldehyde and
proven via Mars van Krevelen
mechanism.
The highest: Cu-OMS-2 due to
phase transition during reaction
and hausmannite phase
responsible to the highest
activity which is not observed
on others metal doped.
32
Hydrothermal
method
Sol-gel
assisted solidstate method
Hydrothermal
method
V5+ and
Nb5+
Fe3+,
Co2+,
Ni2+,
and
Cu2+
Cu2+
Framework
Framework
and tunnel
Framework
-a
The thermal heating of
nanomaterials were as high as
800 °C.
Fe(III) cations were most
suitable to be associated into
the structure of nanoscale
OMS-2.
M-OMS-2 lowers the
manganese oxide average
oxidation state and increases
its electrical resistivity.
-a
Oxidation of
toluene to
produce benzyl
alcohol,
benzylaldehyde,
and benzoic acid
-a
-a
Liquid
phase
-a
[43]
[41]
[42]
Continued in Page 34
XAS (XANES and
EXAFS) showed the
presence of octahedral
Mn4+ and that Cu2+ ion
were supposed to
interact with lattice
oxygen and manganese
atoms which consistent
with copper being sited
within the channel of
the OMS-2 framework.
Compared with
onventional OMS-2
catalysts, the nanoscale
OMS-2 showed
exceptional catalytic
activity in the green
oxidation of toluene.
Physicochemical
properties M-OMS-2
33
Ion exchange
Ion exchange
and
impregnation.
Reflux,
hydrothermal,
ion-exchange
and
impregnation
method.
Soft chemical
method
H+
Ce3+
Ce3+
Rb+
Tunnel
Framework,
tunnel, and
extraframework
Tunnel and
nonframework
Tunnel
Synthesized OMS 3D
architectures with
tunable tunnel
structures.
The thermal stability of
the material varies
depending on
preparation method.
-a
Wet oxidation of
phenol compounds
-a
Liquid
phase
Gas
phase
Liquid
Acid-catalyzed
phase
condensation of
phenylhydroxylamin
e with aniline to
produce 2aminodiphenylamine.
BET surface area of Ce- Oxidation and
dehydrogenation of
OMS-2 is higher than
cyclohexanone
OMS-2.
The average oxidation
state (AOS) of Mn
decreases with an
increasing extent of K+
exchange by H+
Successive exchanges
with HNO3 resulted in
better H+-exchanged
material.
[72]
[71]
[34]
[70]
Contiuned in Page 35
Control of crystal forms,
morphologies, and
tunnel sizes.
Best performance: well
crystallized OMS-2
where all the accessible
potassium ions were
exchanged for cerium
cations.
The selective product
formation depended on
the nature of the acidic
and basic properties of
the catalyst.
H-K-OMS-2 with the
highest %H+ gave the
highest selectivity for the
ortho isomer.
High selectivity to ortho
isomer compared with
para isomer.
34
Reflux,
hydrothermal,
ion-exchange and
impregnation
methods
Zr4+
-a Not reported
framework
Hydrothermal
method
Cu2+
Framework,
tunnel, and
extraframework
-a
Fe3+, Ni2+, Reflux,
Cu2+, and Hydrothermal,
Solvent free
Co2+
technique Sol-gel
and combustion
method.
The morphology of
Zr-OMS-2 materials
depends on the
preparation method.
Synthesis of CuOMS-2, from Kbinnersite using
potassium
permanganate and
potassium
persulphate and ion
exchange with Cu
-a
Oxidation of
side chain
ethylbenzene,
benzyl alcohol
and
cyclohexanol.
-a
Oxidation of
styrene
Liquid
phase
-a
Liquid
phase
The Zr-K-OMS-2
synthesized by the
impregnation method
showed efficient conversion
for oxidation of
ethylbenzene, benzyl
alcohol and cyclohexanol.
Cu-OMS-2-permanganate
material shows fibrous and
needle shape particle
morphology, whereas CuOMS-2-persulphate shows
only the globular type
aggregated particle
morphology.
Transition metal doped
alters the acidity of OMS-2,
leading to higher selectivity
to styrene oxide.
[39]
[38]
[27]
35
36
framework structure of OMS-2 materials. The physical and chemical properties of
doping ions, and the properties of doped materials are significantly different from
doped OMS-2 are greatly influenced by the type, amount, and the location of the
those of the undoped ones [12]. Different metals doped OMS-2 materials have
catalytic properties in different reactions and different amounts of doping affect the
catalytic abilities of the materials and different doping locations such as framework
or tunnels can provide different active sites for catalytic reaction [31, 32, 45, 46].
Table 2.3 also shows that the position of ion doping depended on the
synthesis method. Substitution of manganese in framework sites of OMS-2 materials
by other metal ions is possible by doping the initial solution with doping ions [69] or
isothermal/framework substitutions [28, 36].
Ion exchange of OMS-2 via post
synthesis treatment could only substitute the potassium in the tunnel structure while
impregnation could create agglomeration of the non-framework metal oxide on
surface OMS-2 particle.
The catalytic activities of metal doped OMS-2 materials are varied,
depending on the physicochemical properties of metal doped materials.
The
materials are active for oxidation, dehydrogenation and acid catalyze condensation.
Extensive research is needed to explore the potential applications of metal doped
OMS-2 in the catalytic oxidation reaction.
Heterogeneously catalyzed partial
oxidation of organic compound is widely applied in numerous chemical, biological
and pharmaceutical industries.
Products of selective oxidation of olefins are
important starting materials towards the production of many other fine chemicals
[73].
The previous section shows that the modifications on OMS-2 materials
mainly are based on doped of metals (alkali, alkali earth and transition of metal). In
order to enhance the physicochemical properties and catalytic activity of OMS-2
materials some modification was used. The following sub-section describes several
modification techniques used in this research.
37
2.4.2
Ion Exchange
Ion exchange is a natural phenomenon occurring continually in inorganic
substances and in living bodies on the earth’s surface. The ion exchange reaction can
be described as the interchange of ions between a solid phase and a liquid
surrounding the solid. Initially, ion exchange was confined to surface reactions, but
these were gradually replaced by gel type structures where the exchanged sites were
available throughout the particle. The process is shown graphically in Figure 2.4:
Na+
Na+
+ Ca++
Ca++ + Na+
Figure 2.4: The ion exchange reaction.
The sites exhibit affinity for certain ions over others and this phenomenon is
very helpful in removing objectionable ionic materials from process streams. The
affinity relationship can also be expressed by equilibrium (selectivity) equations
based on the reversibility of ion exchange reactions and the law of mass action.
K+ ions in OMS-2 can be ion-exchanged by treatment with HNO3. All alkali
metal ions can enter the lattice of K+-removed OMS-2. The K+ extraction and metal
ion adsorption reactions are topotactic, preserving the hollandite structure. Feng et
al. [74] have proposed that hollandite-type manganese oxides can be expressed by a
general formula {An}[xMn8x]O16 (n2, x1), where {}, [], and A denote the (2×2)
tunnel sites, octahedral sites for Mn, octahedral vacant sites, and metal ions in the
tunnel, respectively. The hollandite-type manganese oxides can be classified also as
redox-type and ion-exchange-type similar to the spinel-type manganese oxides.
{K2}[Mn3+2Mn4+6]O16 is a redox-type OMS-2, and {K2}[0.5Mn4+7.5]O16 is an ion
exchange-type OMS-2. The redox and ion-exchange extraction/insertion reactions
for OMS-2 can be written as follows:
38
8{K2}[Mn3+2Mn4+6]O16 + 32H+ 7{ }[Mn4+8]O16 + 16K+ + 8Mn2+
+ 16H2O
(Equation 2.1)
7{ }[Mn4+8]O16 + 2KOH {K2}[Mn3+2Mn4+6]O16 + H2O + (1/2)O2 (Equation 2.2)
while for the redox reactions;
{K2}[0.5Mn4+7.5]O16 + 2H+
{H2}[0.5Mn4+7.5]O16 + 2K+
(Equation 2.3)
for the ion-exchange reactions. The redox-type extraction of one K+ is attended by
the disproportionation of one Mn3+ to 0.5Mn2+ and 0.5Mn4+. The number of ionexchange-type sites is four times the number of manganese defects at the octahedral
site, similar to the spinel system.
2.4.3
Impregnation
Impregnation as a means of supported catalyst preparation is achieved by
filling the pores of a support with a solution of the metal salt from which the solvent
is subsequently evaporated (See Figure 2.5). The catalyst is prepared either by
spraying the support with a solution of the metal compound or by adding the support
material to a solution of a suitable metal salt, so that the required amount of active
component is incorporated into the support without the use of excess solution. This
Figure 2.5: Impregnation of porous catalyst.
39
is then followed by drying and subsequent decomposition of the metal salt at an
elevated temperature, either by thermal decomposition or reduction. Impregnation is
an extremely versatile technique (although it is not applicable to insoluble reagents)
which can be controlled to give good dispersion and a known loading of reagent.
2.4.4
Alkylsilylation
Alkylsilylation is modification of external surface of the catalyst by
attachment of organofunctional silane or organosilane.
Octadecyltrichlorosilane
(OTS) which has a chemical formula of C18H37SiCl3 is a type of family of
organosilane. The alkyl groups of OTS possess a hydrophobic characteristic as
depicted in Figure 2.6.
It is widely used for surface modification and
functionalization. OTS is preferred over other organic compounds since the silanes
can form bonds via several mechanisms [75]. Organosilane deposition has additional
benefits over other preparation methods because of its fast preparation, stable finish
and applicability to a wide range of substrates.
Polar head
Cl
Nonpolar tail
Si
Cl
Cl
Figure 2.6: Chemical structure of OTS.
Nur et al. [76] demonstrated that amphiphilic Ti-loaded NaY zeolite
prepared by partial modification with OTS successfully catalyzed epoxidation of 1octene in this system. In this catalytic system, part of the external surface of which
was covered with hydrophobic alkyl groups and the rest being left hydrophilic. The
amphiphilic catalyst exhibited much higher catalytic activity than that of hydrophilic
titanium-loaded NaY, without modification by OTS, or of a hydrophobic catalyst
with almost full coverage by the alkyl groups [76, 77].
40
The assembly or coating occurs by hydrolysis reaction of free surface
hydroxyl group on the support and the organosilanes. Figures 2.7 and 2.8 show the
mechanisms of alkylsilane using OTS on the surface of support. In case of complete
formation of assembled monolayer using OTS (Figure 2.7), all chlorine species
hydrolyze to form bonds with other hydroxyl groups. OTS reacts with the substrate
surface through only one hydroxyl group, while two other groups produce uniform
surface coverage through cross polymerization.
Figure 2.8 represents incomplete mechanism of OTS. In case of incomplete
reaction, there are some unreacted chlorosilane groups remaining on the surface
which might be hydrolyzed later, especially by trace amount of water in the reaction
vassel. This type of reaction favours deposition of multilayer alkylsilane, instead of
a monolayer deposition.
CH3
Cl
H
O
+
n Cl
Si
(CH2)17
CH3
(CH2)17
- HCl
O
Cl
surface
CH3
Si
O
(CH2)17
Si
O
O
O
n
OTS
surface
Figure 2.7: Mechanism of complete reaction of OTS on the support surface to form
a well ordered layer.
CH3
Cl
H
O
surface
+ n Cl
Si
Cl
OTS
(CH2)17 CH3
- HCl
CH3
CH3
CH3
(CH2)17 (CH2)17 (CH2)17 (CH2)17
CH3
(CH2)17
O Si O Si O Si O Si OH OH Si
O
O
HO
HO
O
surface
Figure 2.8: Mechanism of incomplete reaction of OTS on the support surface.
41
2.4.5
Sulphation
Sulphated metal oxides are usually made by the precipitation-sulfation (PS)
method [78]; where the respective metal hydroxide is precipitated from a salt
solution by increasing the pH through addition of concentrated ammonium
hydroxide; the filtered, washed, and dried solid is reacted with a sulfate source, e.g.,
sulfuric acid, and then calcined at elevated temperatures (normally above 700 K).
The highly acidic (or “superacidic”) system is obtained at a narrow temperature
range, allowing a loose, metastable oxide structure to be formed, e.g., the tetragonal
phase in the case of ZrO2, with a few percent sulfate residing on its surface.
Another apparent way of making sulphated metal oxides is by controlled
thermal decomposition of a metal sulfate salt. Arata et al. [79] made sulphated
zirconia and titania by decomposing the respective sulfate salts and claimed that the
obtained systems were superacidic. The sulfate decomposition (SD) method avoids
the complications associated with the PS method (type of starting salt, concentration
and pH effects, various factors in the sulfation stage, etc.). As in the PS case, the
eventual
sulphated
metal
oxide
product
is
shaped
up
by
the
calcination/decomposition temperature. However, in the SD method the sulphated
metal oxide is an intermediate in the thermochemical transformation of a sulfate salt
to the corresponding oxide, through eliminating the sulfate, essentially as free SO3.
Thus, the SD-derived sulphated metal oxide product is created only when most of the
sulfate has been removed, at temperatures usually higher than those needed for
making sulphated metal oxide of similar sulfate level by the PS method. On the
other hand, an SD-derived sulphated metal oxide might be expected to be more
ordered and structurally uniform than the parallel sulphated metal oxide obtained by
the PS method.
42
2.5
Catalytic Activity
2.5.1 Catalytic Activity of OMS-2 Materials
Cryptomelane OMS-2 (K-OMS-2) materials have promising applications as
ion-exchange materials, battery materials, chemical sensors, electromagnetic
materials, and catalysts [20]. Recent studies have been reported in detail on the
catalytic application of OMS-2 materials in several reactions, such as low
temperature carbon monoxide oxidation, total oxidation of methanol, acetone and 2propanol, oxidative dehydrogenation of 1-butene, oxidation of ethanol to
acetaldehyde, oxidation or dehydration of 2-propanol to acetone or propylene
respectively, dehydrogenation of ethylbenzene to styrene, oxidative dehydrogenation
of cyclohexane, partial oxidation of cyclohexane, decomposition of H2O2, and
oxidation of CO to CO2 for fuel cells. The details of catalytic applications of OMS-2
materials are shown in Table 2.4.
Table 2.4: Some catalytic applications of OMS-2 materials reported in the last
decade.
Catalyst
Application
Results
Author (s)
MnO2, M-OMS-1,
M-OMS-2
(M=Mg2+, Ni2+,
Cu2+, Co2+, and
Decomposition of
H2O2
The catalytic activity Zhou et al.
in trend of: M-OMS- [45]
2>M-OMS-1>MnO2
M-OMS-1 and MOMS-2 (Ni, Cu, Fe,
Co, Mg)
Oxidative
dehydrogenation of
ethanol
Most better CoOMS-1
OMS-1, OMS-2 and
OL-1
Oxidative
Dehydrogenation
of 1-Butene
The materials are not Krishnan et
stable during the
al. [40]
reaction.
Fe3+)
Zhou et al.
[46]
Continued in Page 43
43
Cu-OMS-1 and CuOMS-2
Oxidative
dehydrogenation of
ethylbenzene to
styrene
Copper content
influenced styrene
selectivity.
Tolentino et
al. [68]
M-OMS-2 (M=Ag,
Co, Cu)
Oxidation of
carbon monoxide
Catalytic activity of
doped OMS-2
catalysts toward CO
oxidation shows a
correlation among
average oxidation
state of Mn ion and
the position and
nature of the doped
cation.
Xia et al.
[31]
Hydrophobic OMS2, synthesis in buffer
solution, OMS-1,
and OL-1
Total oxidation of
benzene
Luo et al.
These materials be
[26]
very active in total
oxidation catalysts
or selective
adsorbents for VOCs
M-OMS-2 (Cu, Zn,
Ni, Co, Al, or Mg)
Decomposition of
2-Propanol to
acetone and
propene.
The highest active is
Cu-OMS-2
K-OMS-2 and H-KOMS-2
Oxidation of
H-K-OMS-2 > Kbenzyl alcohol with OMS-2, via Mars
O2
van Krevelen
mechanism
A-OMS-2 (A=Li,
Na, K, Rb, H)
Aerobic oxidation
of cyclohexanol to
cyclohexanone
OMS-2 and H-OMS- Oxidation of
2
alcohol
Chen et al.
[69]
Makwana et
al. [24]
Higher surface areas
lead to better
catalytic
performance.
Liu et al. [32]
100% Selectivity to
aldehyde/keton and
the highest
conversion for
oxidation benzylic
compound.
Makwana et
al. [23]
Continued in Page 44
44
OMS-2
Ghosh et al.,
Oxidation of cyclic Cyclooctene
[28]
selective to
olefins and
cyclooctane epoxide.
benzylic double
bonds with tertiarybutyl
hydroperoxide
(TBHP) as the
oxidant.
OMS-2 and
H-OMS-2
Acid-catalyzed
condensation of
phenylhydroxylami
ne with aniline to
produce 2aminodiphenylamine.
High selectivity to
ortho isomer
compared with para
isomer.
Kumar et al.,
[70]
Nano-OMS-2,
Synthesized by
reduction of KMnO4
with H2O2.
Oxidation of
benzyl alcohol and
fluorene
Lower activity in
oxidation of benzyl
alcohol and higher to
fluorine compared to
OMS-2 conventional.
Villegas et al.
[61]
Ce-OMS-2
Oxidation and
dehydrogenation of
cyclohexanol
Catalytic activity of Jothiramamaterials depends on lingam et al,
synthetic method.
[34]
OMS-2, synthesized
by low temperature
solvent-free method
Oxidation of 2OMS-2solvent free >
thiophenemethanol, OMS-2reflux
furfuryl alcohol
and cyclohexanol
Ding et al.
[80]
Microwave synthesis Oxidation of 2of OMS-2
thiophenemethanol
The microwave
synthesis of OMS-2
more active than
OMS-2 synthesized
by conventional
heating. However,
OMS-2 precursor is
the most active
catalyst.
Malinger et
al. [81]
OMS-2 and MOMS-2 (Fe, Ni, Cu,
Co)
Transition metal
doped alters the
acidity of OMS-2,
leading to higher
selectivity to styrene
oxide.
Ghosh et al.
[27]
Oxidation of
styrene
Continued in Page 45
45
OMS-2
Total oxidation
formaldehyde
Catalytic activity is
closely related to the
morphology of the
catalysts.
Xingfu et al.
[82]
OL-1 and OMS-2,
with different Mn
precursor
Total oxidation of
acetone
OMS-2 with
manganese
carboxylates as
precursor has high
surface area and
were extremely
active for acetone
total oxidation
Frías et al.
[83]
Zr-OMS-2
Liquid phase
oxidation
ethylbenzene,
benzyl alcohol and
cyclohexanol.
The activity of
catalyst depends on
synthesis method
and material
prepared by
impregnation more
active than direct
synthesis.
Jothiramalingam et al.
[39]
OMS-2
Total oxidation of
ethyl acetate
OMS-2 is an active
ethyl acetate
complete oxidation
catalyst with 100%
selectivity to CO2 at
about 673 K.
Gandhe et al.
[84]
Ce-OMS-2
Wet oxidation of
phenol compounds
OMS-2 materials
containing pure
CeO2 phase and
excess active xygen
species has lower
catalytic activity.
-Wolfovich et
al. [71]
2.5.2
Catalytic Oxidation
As known, the scope of partial oxidation catalysis is wide, ranging from the
large-scale production of commodities to the synthesis of minute amounts of
pharmaceuticals and fine chemicals. Compared with other chemical processes, the
oxidation process is complex and difficult to be controlled or stopped at a certain
46
stage.
For these reasons, the selective catalytic oxidation is an active field of
research. The driving forces for the industrial and academic research are: (i) the
formulation of alternative or new catalysts, (ii) reduction of the number of process
steps, (iii) elimination of waste by-products and (iv) development of new processes.
Catalytic oxidation is an important method of transforming hydrocarbon
feedstock like alkanes, alkenes and aromatics into more sophisticated oxygenated
products. Million tons of the oxidation product such as alcohol, carbonyl compounds
and epoxides are annually produced worldwide [85]. These compounds are used in
all areas of chemical industries ranging from pharmaceuticals to large-scale
commodities [86-88]. Furthermore, catalytic oxidations can be used to eliminate a
series of pollutants, thus producing healthier environment.
Traditionally, an oxidation process is conducted by using a stoichiometric
process with classic oxidants such as dichromate/sulfuric acid, chromium oxides,
permanganates, periodates, and osmium oxide. The process produces large amount
of waste. For example, using a stoichiometric amount of potassium dichromate or
permanganate results in 5 to 100 times weight of waste (mostly inorganic salts) per
kilogram of product [89]. This also means that most of the reactants introduced at
the start of the reaction are not converted to the desired product, leading to very low
atom-selectivity. Consequently, the use of traditional stoichiometric process is not
environmentally friendly and inefficient as well.
In contrast, a clean synthetic
technology should proceed with a high atom-economy and the overall synthesis must
be accomplished with low E-factor (by waste per kg product), thereby minimizing
the cost of waste disposal [85, 90]. As a result, currently there is considerable
pressure to replace these outdated methods by cleaner catalytically driven
technologies.
Epoxidation is specific to oxidation of alkene to form epoxide. Traditionally
the main methods for performing epoxidations in organic synthesis have been
oxidation with organic peracids (See Figure 2.9a) or, to a lesser extent, the
chlorohyrins route as shown in Figure 2.9b. The latter method has several drawbacks
such as the use of corrosive condition and production of pollutants with a
47
considerable environment impact. However, the use of many organic peracids, e.g.
peracetic acid, is problematical owing to restrictions on their transport, storage, and
handling.
These shortcomings have focused attention on the development of
methods using hydrogen peroxide or tert-butyl hydroperoxide (TBHP) as the oxidant
(Figure 2.9c).
O
+
R'CO3H
+
OH
base
+
HOCl
(a)
R'CO2H
Cl
O
(b)
O
+
R'COOH
+
R'OH
(c)
R' = H, t-Bu
Figure 2.9: Epoxidation of alkenes using (a) organic peracids, (b) chlorohyrins route
and (c) H2O2 or TBHP as the oxidant.
Besides that, common products of partial oxidation of alkanes are alcohol and
carbonyl compounds. Carbonyl compounds are also formed by the oxidation of
alcohol compounds. This catalytic oxidation process of both alkanes and alcohol
were studied in homogeneous and heterogeneous catalyst [91-96]. Homogeneous
catalysts are single-molecule catalysts, which typically consist of a transition-metal
atom, surrounded by one or more ligands. Such well-defined catalysts allow for
good mechanistic understanding and consequently, for catalyst performance
enhancement by ligand or metal tuning. As a result, homogeneous catalysts are
characterized by high activity and selectivity, and a good reproducibility.
Additionally, mild operating conditions may be applied, which allows for the
production of complex and temperature-sensitive substances. The major drawback
of homogeneous catalysis is the recovery of the catalyst from the reaction products.
As a result, the industrial applications of homogeneous catalysis are limited, and
48
successful industrial application is mainly due to the high chemo-, regio-, and
enantio-selectivities attainable with homogeneous catalysts.
In heterogeneous catalysis, the catalytically active substance is deposited on
solid support material with a large surface area and a high porosity.
Catalyst
recovery is relatively simple, although macroscopic diffusion limitations can be a
serious problem. Heterogeneous catalysts are robust, with a long service life and
often operate at relatively harsh conditions. The disadvantages of homogeneous
catalyst are low thermal stability and the difficulty of recovery and regeneration
which may be solved with heterogeneous catalyst; which are easily recovered by
filtration and recycled [5].
Catalytic oxidations in the liquid phase generally employ soluble metal salts
or complexes in combination with clean, inexpensive oxidants such as O2, H2O2, or
RO2H [47]. However, heterogeneous catalysts have the advantage, compared to their
homogeneous counterparts, of facile recovering and recycling.
Moreover, site-
isolation of active metal ions or complexes in inorganic matrices precludes their
dimerization/oligomerization to less reactive μ-oxo species and therefore endow
them with unique activities.
Recently, focus has been given to the use of heterogeneous catalysts for the
liquid phase epoxidation of olefins with milk oxidant such as alkyl hydroperoxides,
hydrogen peroxide or molecular oxygen [97]. The use of heterogeneous catalysts
may avoid the difficulties concerning separation, recovery and recycling. As in
solution, the concept of “site isolation” should provide very active catalysts [98].
Early transition metal-substituted molecular sieves are of particular interest: (i) they
have very high surface areas and a great accessibility to the active sites, and (ii) the
high dispersion of the cations is usually obtained by incorporating low amounts of
metal directly in the preparation mixture.
The comparison of those mild oxidants is demonstrated in Table 2.5. Alkyl
hydroperoxide is generally more active than hydrogen peroxide, but it is more
expensive and the active oxygen content is rather low. This reaction generates
49
stoichiometric amounts of corresponding alcohols, which in most cases are quite
easily recycled via a reaction with hydrogen peroxide. Unfortunately, this process
requires at least two extra separations and one extra reaction step.
Hydrogen peroxide, with respect to active oxygen content (47%) and the
nature of by-product (only water), seems to be the oxidant of choice in catalytic
liquid phase oxidations. However, the inherent co-production of water poses some
serious difficulties. Most transition metal catalysts are very sensitive to water, and
are prone to leaching. In cases where the catalysts are stable, water has serious
retarding affect on the oxidation reaction, making the search for new
environmentally friendly, effective oxidation catalyst a challenge. Dioxygen which
contains 100% active oxygen and generates no by-products is a very attractive
choice. Therefore, the development of selective oxidation of hydrocarbon using
dioxygen or air in mild condition could be the best option [99, 100].
Table 2.5: The comparisons of the common mild oxidant are used in oxidation
process [47, 97].
Alkyl hydroperoxide
Hydrogen peroxide
Dioxygen
Activity
More active
Active
Less active
Cost
More expensive
Expensive
Less expensive
By-product
Corresponding alcohol Water
No
Active oxygen
content
Lower than others
100 %
2.5.3
47 %
Titanium Incorporated Materials
Shell catalyst titanium (IV) silicon dioxide [Ti(IV)SiO2] catalyst and titanium
silicalite 1 (TS-1) are titanium based catalysts.
Those catalysts were the most
successful heterogeneous catalysts in liquid phase epoxidation [47].
Table 2.6
provides a comparison of the characteristics of Shell’s epoxidation catalyst compared
with titanium silicalite 1. The Shell catalyst was patented in 1971 by Shell Oil and is
50
industrially used for the epoxidation of propene using an organic peroxide (Figure
2.10).
This was the first truly heterogeneous epoxidation catalyst useful for
continuous operation in the liquid phase [86].
The catalyst was prepared by
impregnating silica with TiCl4 or an organo-titanium compound, followed by
calcination.
TiIV/SiO2
+
O
+
PhCH(CH3)O2H
PhCH(CH3)OH
Figure 2.10: Epoxidation of propene on Shell catalyst using an organic peroxide.
Table 2.6: Characteristic features of Shell’s epoxidation catalyst compared with
titanium silicalite 1.
Shell catalyst
TS-1
Structure
Amorphous, silica based
Crystalline microporous
structure
Ti incooporation
Several Ti siloxy sites: from
monopodal to tetrapodal
Isomorphous replacement of
T-atom sites
Substrate size
No limitations with regard to
substrate size
Micropores of 5.6 Å diameter
impose severe limitations
Oxidant
Limited to organic peroxides
Aqueous hydrogen peroxide
The superior catalytic activity of Ti(IV)SiO2 was attributed to both an
increase in Lewis acidity of the Ti(IV), owing to electron withdrawal by silanol
ligand, and to site isolation of discrete Ti(IV) centers on the silica surface preventing
oligomerization to unreactive oxo species (which occurs readily with soluble Ti(IV)
compounds).
Interestingly the catalyst has been reported to leach catalytically
inactive titanium species during the initial stages of the epoxidation reaction after
which the catalyst becomes truly heterogeneous. The catalyst was quite unique in
that it is heterogeneous, unlike other supported metal oxides such as MoO3 and V2O5
supported on SiO2 or other inert carriers, which often owed their catalytic activity to
soluble metal species that may rapidly leach out of the support.
51
TS-1 is an ordered crystalline microporous structure and it has made
remarkable progress in the oxidation reactions [50, 101-103]. TS-1 contains Ti(IV)
isomorphously substituted for silicon in the framework of silicalite-1, a hydrophobic
molecular sieve possessing a three-dimensional system of intersecting elliptical pores
with diameters of 5.3×5.5 and 5.1×5.5 Å. The catalyst is especially known for its
ability to use aqueous hydrogen peroxide as the oxidant for the epoxidation of small
alkenes [104-107]. The other reactions such as oxidation of alcohols, hydroxylation
of aromatics, ammoximation of cyclohexanone, oxidation of alkanes to alcohols and
ketones, oxidation of amines, oxidation of sulfur containing compounds, and
oxidation of ethers, among others, also have been carried out selectively with TS-1.
A summary of the reactions catalyzed by TS-1 is given in Figure 2.11 [108].
OH
OH
OH
O
OH
ArOH
OH
ArH
OH
O
R
C
H
CH2
TS-1
30% H2O2
R-CH=CH2
OH
R C
H
R-CH2-OH
R-CH=O
O
R'
R2NH
N OH
O
R
C
R'
R2NOH
Figure 2.11: Oxidation reactions catalyzed by TS-1.
Although water is known to seriously retard epoxidation reactions, the
hydrophobicity of the pores ensures a very low water content around the catalytic
titanium centers, enabling this remarkable feature of the catalyst [91, 109]. The
small pore size of the zeolite structure (about 6 Å) allowed shape selective catalysis
[110, 111], but on the other hand, restricted the reaction to small alkenes only, which
may be seen as a major drawback of the catalyst. To overcome the later problem Ti
incorporated in larger porous materials were intensively studied of [103, 112].
52
2.5.4
Sulphated Metal Oxides as a Solid Acid Catalyst
An acid that is stronger than H0 = -12, which corresponds to the acid strength
of 100% H2SO4, is known as a superacid [113]. Such a superacidity has been made
up by mixing a fluorine containing Brönsted acid (HF, HSO3F, CF3SO3H, etc) and a
fluorinated Lewis acid (BF3, SbF5, TaF5, etc.).
These superacids have been
developed since 1960s and have been applied to various organic syntheses,
especially in the field of hydrocarbon chemistry. They are responsible for producing
more than 1 x 108 MTon/year of products [114, 115]. However, these conventional
industrial acid catalysts have unavoidable drawbacks because of their severe
corrosivity and high susceptibility to water. The search for environmentally benign
heterogeneous catalysts has driven the worldwide ongoing research of new materials
as substitutes for current liquid acids and halogen-based solid acids. Among them
sulphated oxides, such as sulphated zirconia, titania, and iron oxide, displaying high
thermostability, very strong acidity, and high catalytic activity, have aroused
increasing interest. Since the primary work of Arata and Hino [116], the superacids
by sulphated metal oxides have been attracting more and more attention for
investigations.
Sulfation of metal oxides introduces quite strong Brønsted acidity and, in
general, enhances the catalytic activity in acid catalyzed reactions. The superacid
property of those materials is due to both Lewis acid and Brønsted acid sites [55].
The strong acid properties are related to sulfate ions. It has been proposed that the
high electronegativity of sulfur could induce polarisation of the neighboring OH
groups. The structure of the sulfate bound to the metal oxide is also a subject of
many investigations. However, it is not clear until now. One of the proposed
structure is that sulfate is coordinated to the metal in a chelated form, in which two of
the oxygen atoms of the sulphate are bound to one metal atom. Brønsted acid is
generated by cleavage of one of these bonds by water and formation of OH bond
[117].
53
2.5.5
Synthesis of Diols
Preparative procedures in which two or more transformations are carried out
as a “one-pot” process offer a number of advantages to the organic chemist: in
particular, they result in a reduced number of operations, giving significant time-cost
benefits, but they also can often allow “difficult” intermediate compounds (i.e., those
that are volatile, toxic, or otherwise noxious) to be prepared and elaborated in situ,
thus preventing problems associated with their isolation and handling [51]. To this
end, the development of catalyst, which has bifunctional/multifunctional active sites
for multiple chemical transformations in a one pot, is considered.
Recently, there have been many reports of heterogeneous bifunctional
catalysts used for one pot synthesis. Bifunctional palladium/amberlyst catalysts have
been used to carry out dehydration of the tertiary alcohol and the hydrogenation of
the in situ formed alkene in a single vessel [118]. In another example, caprolactam, a
precursor for Nylon-6, which generally requires a two step synthesis, was
synthesized in high yields in a single-pot using an aluminophosphate bifunctional
heterogeneous catalyst [119].
Diols are important starting materials for polyurethane chemistry which are
the important biomedical polymers, and are used in implantable devices such as
artificial hearts, cardiovascular catheters, pacemaker lead insulation, etc. [120, 121].
Diols are very important intermediate materials for biologically active natural
products such as compactin, macrolides, conduritols, cyclitols and others, as well as
statins, an important class of pharmaceuticals [122, 123]. Diols also are useful
materials for synthesis of physiologically-compound having specific uses in the field
of the drugs and agrichemicals [124]. A number of 1,2-diols such as 2,3-dimethyl2,3-butanediol, 1,2-octanediol, 1,2-hexanediol, 1,2-pentanediol, 1,2- and 2,3butanediol are of interest to fine chemical industries. In addition chiral 1,2-diols are
employed as intermediates for pharmaceuticals and agrichemicals [125].
The clean catalytic technology for the synthesis of diols are described in the
literature; they can be obtained from the acid sites that catalyzes the hydrolysis of
54
epoxides [126-128], regio- and enantio-selective reduction of diketones using
biocatalyst [129], bioconversion of heterocyclic which are linked with penyl or
benzyl groups [130]. However, the direct transformation of alkenes which are found
in great abundance in the realm of organic molecules to diols using the
heterogeneous catalyst is rarely reported.
The direct transformation of alkenes to diols can be produced using a
bifunctional oxidative and acidic catalyst. The oxidative and Brönsted acid sites of
the catalyst acts for oxidation of alkenes to epoxide and to hydrolyze epoxide to
diols, respecticely. The heterogeneous catalytic process for one pot reaction of
alkenes to diols has been reported by Prasetyoko et al. [52-54]. The catalytic process
occurs with the existence of bifunctional oxidative and acidic catalysts. The system
is the titanium silicate-1 (TS-1) loaded with sulphated zirconia or niobium oxide.
They reported that sulphated zirconia or niobium loaded on TS-1 acted as acid site
which has Brönsted acid sites whereas TS-1 itself acted as oxidative sites. The
similar properties are also found in Ti-beta and Ti-Al-beta zeolites [131]. The
presence of tetrahedral Ti4+ in the framework of zeolite acts as oxidative sites, while
the incorporation of Al3+ into Ti-beta induces Brönsted acid sites. Ti-site itself has
Lewis acidic character that can coordinate an alcohol molecule to give Brönsted acid.
Incorporation of trivalent metal ions (Al3+, B3+, Ga3+ and Fe3+) and titanium ion
(Ti4+) together in the framework of silica based molecular sieves display bifunctional
oxidative and acid characteristic [132-134].
CHAPTER 3
EXPERIMENTAL
3.1
Synthesis of OMS-2 Materials
The integral part of the research is the synthesis of OMS-2 materials followed
by modification and characterization of the samples by the characterization
techniques described in Chapter 2. The materials were prepared accordingly to the
flow chart shown in Figure 3.1. Generally, there are two groups of materials: OMS-2
and modified OMS-2. The experimental procedures are described in the following
sections.
3.1.1 Synthesis of OMS-2 without Buffer (OMS-2a)
OMS-2 was prepared according to method reported by DeGuzman et al.
[62]. A 0.4 M solution of KMnO4 (13.3 g in 225 mL of distilled, deionized water,
DDW) was added to a mixture of a 1.75 M solution of MnSO4.H2O (19.8 g in 67.5
mL DDW) and 6.8 mL of concentrated HNO3. The resulting black precipitate was
stirred vigorously and refluxed at 100 oC for 24 h. The precipitate was filtered and
washed with DDW until neutral pH and dried at 110 oC. The sample was labelled
OMS-2a.
56
without
OMS-2
buffer
with
OMS-2a
OMS-2b
Ti-OMS-2(0.05)
without buffer
Fe-OMS-2(0.09)
Cu-OMS-2(0.02)
M-OMS-2
Direct synthesis
Co-OMS-2(0.04)
Ti-OMS-2(X)
without Mn2+
ion
Fe-OMS-2
Cu-OMS-2
Materials
Co-OMS-2
impregnation
Modified
OMS-2
TiO2(imp)-OMS-2
physical mixing
TiO2-OMS-2(mix)
H-OMS-2
H-Ti-OMS-2(Y)
Post synthesis
ion exchange
H-Fe-OMS-2(0.09)
H-Cu-OMS-2(0.02)
H-Co-OMS-2(0.04)
OTS/OMS-2
alkylsilylation
OTS/Ti-OMS-2(Z)
water
sulfation
SW100-Ti-OMS-2(Z)
SW150-Ti-OMS-2(Z)
solvent
toluene
ST100-Ti-OMS-2(Z)
ST150-Ti-OMS-2(Z)
Note: X refer to Ti/Mn ratio = 0.18, 0.43 and 0.67; Y refer to Ti/Mn ratio = 0.05
and 0.67 and Z refer to Ti/Mn ratio = 0.67
Figure 3.1: Materials preparation and their labelling.
57
3.1.2
Synthesis of OMS-2 with buffer (OMS-2b)
For comparison, OMS-2 was also prepared following the method reported by
Luo et al. [26]. A 5.5 g of Mn(Ac)2.4H2O (from Fluka) in 40 mL of DDW was
dissolved in a buffer solution consisting of 2.5 mL of glacial acetic acid and 2.5 g of
KAc in 20 mL of DDW. A solution of 3.75 g of KMnO4 in 75 mL of DDW was
added dropwise to the resulting solution, followed by solution reflux process at 100
o
C for 24 h. The resulting brown precipitate was filtered and washed with DDW
several times and dried at 120 oC overnight. The final product is labelled as OMS2b.
3.2
Modification of OMS-2 materials
The next section explains the modification of OMS-2 materials in this work.
The modification was done by direct and post synthesis. Metal substituted OMS-2
(M-OMS-2) was prepared via direct synthesis and the other modification such as ion
exchange, impregnation, physical mixing, alkylsilylation and sulfation were prepared
via post synthesis.
3.2.1 Synthesis of Metal Substituted OMS-2 (M-OMS-2)
M-OMS-2 (M = Ti, Fe, Cu, and Co) was prepared by two methods i.e.
following the work by Suib’s group [30] and a new method that is the only metal
solution in acidic condition was oxidized by KMnO4 without addition of any
manganese ion solution. Different metals were used to study the effect of metal
types to properties and catalytic activities of OMS-2 materials.
In the first method, metal (M) cations (Ti3+, Fe2+, Co2+ and Cu2+) was added
to the acidic manganese ion solution before being oxidized by KMnO4. Metal ions
58
were doped into the OMS-2 structure with an a priori incorporation method, that is,
adding aqueous solutions of dopant before refluxing to form the OMS-2 structure.
The precursor of Ti3+, Fe2+, Co2+ and Cu2+ cations were Ti2(SO4)3 in H2SO4,
FeSO4.7H2O, Co(CH3COO)2 and CuSO4.5H2O, respectively. The precursor was
added prior to reflux, with M/Mn atomic ratios of 1:10 except Ti/Mn with ratio of
1:5, under agitation at room temperature. The mixture was oxidized by potassium
permanganate and the resulting precipitate was stirred vigorously and refluxed
following the procedure in Section 3.1.1. The samples are labeled as M-OMS-2(X),
where X is the molar ratio of M/Mn is calculated with AAS analysis.
In the second method, M-OMS-2 was prepared without the addition of any
manganese ion solution.
The metal solution was oxidized by potassium
permanganate in acidic condition. Typically, a 0.4 M solution of KMnO4 (13.3 g in
225 mL of deionized water) was added to a mixture of 1.75 M solution of metal and
6.8 mL of concentrated HNO3, except for Ti-OMS-2, no concentrated HNO3 was
added. The resulting black precipitate was stirred vigorously and refluxed following
the procedure in section 3.1.1. The samples were labelled as M-OMS-2.
3.2.2
Ion Exchange of OMS-2 and M-OMS-2 Samples
H doped catalyst was prepared by ion exchange of potassium in the tunnel
structure with acid. About 50 mL of concentrated HNO3 was added to 2.0 g of
OMS-2 and M-OMS-2 samples. The slurry was stirred vigorously at 80 oC for 6 h.
The product was filtered and washed several times with DDW. This procedure was
repeated for successive ion exchanges to obtain greater H+ exchange in samples. The
product was dried at 120 oC for 12 h, and then calcined at 280 oC for 6 h. The
samples were labelled as H-OMS-2 and H-M-OMS-2.
59
3.2.3
Synthesis of Ti Incorporated OMS-2 (Ti-OMS-2)
Titanium incorporated OMS-2 (Ti-OMS-2) was prepared in several Ti/Mn
ratios. The samples were prepared by stepwise addition of solution KMnO4 (13.3 g
in 225 mL of deionized water) to different amount of Ti2(SO4)3 15 % v/v in H2SO4
(25, 50 and 75 mL). The ratios of Ti:Mn were 0.18; 0.43 and 0.67 (as analyzed by
atomic absorption spectrometer).
Upon completion, the mixture was stirred,
refluxed, filtered, washed, and dried following the procedure in section 3.1.1 and it is
labeled as Ti-OMS-2 (0.18), Ti-OMS-2 (0.43) and Ti-OMS-2 (0.67), where the
number in parenthesis is the molar ratio of Ti/Mn.
3.2.4 Synthesis of Ti Impregnated OMS-2 [Ti-OMS-2 (imp)]
Titanium(IV) tetra-2-propoxide [Ti(OPri)4] was impregnated into OMS-2
powder in order to prepare the extraframework titanium on OMS-2. 69.6 mg of
Ti(OPri4) was dissolved in 10 mL of toluene. About 1 g of OMS-2 sample was
added to the solution and stirred overnight. After the evaporation of the solvent, the
solid sample was calcined at 500 oC for 3 h. Here, this modified OMS-2 is labelled
as Ti-OMS-2 (imp). The molar ratio of Ti/Mn was 0.18.
3.2.5
Preparation of TiO2-OMS-2 (mix)
The physical mixture of OMS-2 and TiO2 rutile was prepared as comparison.
The sample was prepared by mixing of 1 g of OMS-2 and 72.8 mg of TiO2 rutile.
The mixture was grinded to homogenize the sample. The calculated molar ratio of
Ti/Mn ratio is 0.67. The molecular weight of cryptomelane OMS-2 material was
collected from JPSDS-29, 1020 that is 734.59 g/mol. It is labeled TiO2-OMS-2
(mix).
60
3.2.6 Synthesis of Sulphated Ti-OMS-2 (SO42-/Ti-OMS-2)
Sulfation was done in order to create the acid site on Ti-OMS-2(0.67) sample.
Sulfation was carried out by addition of 150 μL or 200 μL of H2SO4 (18 M) in 20 mL
of solvent (water and toluene) to 1 g of catalyst and then stirred for 1 h. The product
was filtered, washed with DDW several times, then dried at 120 oC overnight and
calcined at 450 oC for 2 h. The samples were labelled as SW150-Ti-OMS-2 and
SW200-Ti-OMS-2 for 150 μL and 200 μL of H2SO4, using water as solvent. Using
toluene as solvent, the samples are labeled as ST150-Ti-OMS-2 and ST200-Ti-OMS2 for 150 μL and 200 μL of H2SO4, respectively.
3.2.7
Surface Modification by Alkylsilylation
Alkylsilylation of n-octadecyltrichlorosilane (OTS) was done on selected
samples; OMS-2 and Ti-OMS-2(0.67). Typically, a powder sample was immersed in
5 ml toluene containing 500 mol of OTS and the suspension was shaken for 5 min
at room temperature. Then, the solid was collected by centrifugation and dried at
110 ºC for 5 h.
3.3
Characterization Techniques
Characterization is a central aspect of catalyst development [Richards, 2006].
The elucidation of structures, compositions and chemical properties of both the
solids used in heterogeneous catalysis and the adsorbates and intermediates present
on the surface of the catalysts during reaction is vital for a better understanding of the
relationship between catalyst properties and catalytic performance.
The
characterization techniques were in this work are described in the following subsection.
61
3.3.1
X-Ray Diffraction (XRD) Spectroscopy
3.3.1.1 Introduction
X-ray diffraction (XRD) is a powerful method to define the crystallographic
structure of crystalline materials whereby no other means is feasible or even possible.
Each of the crystal materials has their own specific pattern that can be used as
references for the determination of solid crystal phase and it is used as a fingerprint
for every materials. It also provides information on the long range order, phase
purity, change in lattice parameter with changing composition and enables one to
assess preferred orientation effects, index diffraction patterns as well as to evaluate
background and line broadening effects [135, 136].
The presence or absence of some peaks of the diffractogram indicates the
existence of other crystal phase or the sample was contaminated with other phases.
The crystal structure of the OMS-2 materials under investigation is considered
similar to the structure of the reference sample, if all of the observed peaks are
present with similar diffraction peaks as the reference sample. Extra or missing
peaks observed indicate the presence of other crystalline phases, impurities or
changes in the structure of the sample.
A powdered XRD pattern is a plot of intensity of the diffracted beams as a
function of 2 which satisfies the Braggs equation:
n= 2d sin (Equation 2.4)
where is the diffraction angle, d is the interplanar spacing, is the wavelength of
the beam and n is an integer number. Wavelength of the CuK radiation is 1.5418
Å. The spacing of planes (hkl) or Miller indices is related to the unit cell parameters
of lattice.
The position of the diffraction peaks changes with the composition of the
lattice. OMS-2 materials have the cryptomelane structure with tetragonal geometry.
62
In the tetragonal lattice, the unit cell parameter, ao and co can be calculated by using
the following equation:
1
d2
h2 k 2 l 2
2
a2
c
(Equation 2.5)
3.3.1.2 Experimental
The synthesized samples were characterized by two model XRD instruments.
First, powder XRD patterns were collected on a Bruker Advance D8 using Siemens
5000 diffractometer with Cu KD radiation (O = 1.5418Å,) operated at 40 kV and 40
mA. It was used to characterize the crytallinity, structure and phase of the samples.
Typically, powder samples were ground, spread into a sample holder, and finally
analyzed. The pattern was scanned in the ranges between 5° to 70° at a step of
0.020° and step time of 1s (scanning speed of 1.2°/min). As comparison, some
selected samples were also characterized using Shimadzu XRD 6000 diffractometer
with the Cu K ( = 1.5405 Å) radiation and the diffracted monochromatic beam at
30 kV and 30 mA at Universiti Putra Malaysia.
3.3.2
Atomic Absorption Spectroscopy (AAS)
3.3.2.1 Introduction
Atomic absorption spectroscopy is one of the important instrumental
techniques for both quantitative and qualitative analysis of metallic and nonmetallic
elements in inorganic or organic materials [137]. This method provides a total metal
content of sample and is almost independent of the molecular form of the metal in
the liquid. The absorption of energy by ground state atoms in the gaseous state
forms the basis of this technique. When a solution containing metallic species is
introduced into a flame, the vapour of metallic species will be obtained; some of the
metal atoms may be raised to a sufficiently high energy level to emit the
63
characteristic radiation of the metal. But a large percentage of the metal atoms will
remain in the non-emitting ground state. These ground state atoms of a particular
element are receptive of light radiation of their own specific resonance wavelength.
Thus, when a light of this wavelength is allowed to pass through a flame having
atoms of the metallic species, part of that light will be absorbed and proportioned to
the density of the atoms in the flame. Therefore, once the absorption is known, the
concentration of the metallic element can be determined [138].
3.3.2.2 Experimental
A Perkin-Elmer model AAnalyst 400 spectrophotometer was used to carry
out the analyses of the amount of metal ions in the samples. Prior to analysis, the
solid sample was diluted by decomposition using hydroflouric acid method.
Approximately, 50 r 0.01 mg of a prepared, representative sample (200 mesh) was
placed in a Teflon decomposition vessel with 0.5 mL aqua regia (HNO3 : HCl = 1 : 3
v/v). 3 mL of HF (48%) was added and the vessel was tightly sealed and placed in
an oven at 110oC for 1 h. After cooling, the dissolution products are quantitatively
transferred to a 50 mL plastic beaker containing 2.8 g of HBO3. 10 mL of distilled
water was added and the mixture stirred magnetically to dissolve any insoluble
fluorides. Finally, the clear solution was diluted to 100 mL and stored in a plastic
bottle ready for major elemental analysis.
3.3.3
Fourier Transform Infrared (FTIR) Spectroscopy
3.3.3.1 Introduction
Field emission scanning electron microscope (FESEM) is a scientific
instrument that uses a beam of highly energetic electrons to examine objects on a
very fine scale. The magnification produced by a scanning microscope is the ratio
between the dimension of the final image display and the field scanned on the
specimen. The FESEM does not produce a true image of the specimen. Instead, it
64
produces a point by point reconstruction of the specimen. The image of FESEM is
produced by the signal which comes from the interaction of an electron beam with
the specimen in the column of FESEM [139].
FTIR is a most widely applied analytical tool for identifying types of
chemical bonds in a molecule by producing an infrared absorption spectrum that is
like a molecular "fingerprint". By interpreting the infrared absorption spectrum, the
chemical bonds in a molecule can be determined. Small differences in structure may
result in significant changes in the spectra observed, and absorption in this region is
probably unique for every molecular species. The region is extremely useful for the
purpose of identifying a molecule. Molecular bonds vibrate at various frequencies
depending on the elements and the type of bonds. For any given bond, there are
several specific frequencies at which it can vibrate [140].
In general, the infrared region between 4000-200 cm-1 can be divided into
four regions [135]:
a. The X-H stretch region (4000-2500 cm-1), where strong contributions
from OH, NH, CH and SH stretch vibrations are observed.
b. The triple bond region (2500-2000), where the contributions of gas phase
CO (2143 cm-1) and linearly adsorbed CO (2000-2200) are seen.
c. The double bond region (2000-1500), where in catalytic studies bridge
bonded CO, as well as carbonyl groups in adsorbed molecules (around
1700 cm-1) absorbs.
d. The fingerprint region (1500-500), where all single bond between carbon
and elements such as nitrogen, oxygen, sulphur and halogens absorb.
e. The M-X or metal-adsorbate region (around 200-450 cm-1), where the
metal-carbon, metal-oxygen and metal-nitrogen stretch frequencies in the
spectra of adsorbed species are observed.
IR is employed to study the framework structure, hydroxyl group and also
molecules adsorbed in the heterogeneous catalyst.
It provides a meaningful
information in the mid-infrared region (1400-400 cm ) attributed to Mn-O vibration;
-1
the characteristic of cryptomelane peaks. This is analogous with the various T-O
65
vibrational modes associated with the zeolites or mesoporous materials dominating
most of the region between 1400-400 cm-1.
3.3.3.2 Experimental
FTIR spectroscopy analysis was applied to determine the structure of OMS-2,
M-OMS-2, H-OMS-2 and SO4/Ti-OMS-2. The infrared spectra were recorded on
Spectrum One FTIR Spectrometer with 4 cm-1 resolution. Approximately 1 mg of
sample was ground together with 100 mg of kalium bromide using pestle and mortar.
The fine powder was then transferred to the ‘dye’ and 10 tonne of pressure was
applied for 2 minutes. The resulting pellet was put in the sample holder and the
FTIR spectrum of the sample was recorded in the range of 400 cm-1 to 4000 cm-1.
3.3.4
Total Specific Surface Area (BET) and Pore Volume Analysis
3.3.4.1 Introduction
Adsorption isotherm is a unique and useful technique in measuring surface
area and pore structure of a solid. The principal method of measuring total surface
area of porous structures is by physical adsorption of a particular molecular species
from a gas (typically nitrogen) onto the surface, maintained at a constant temperature
(usually at liquid nitrogen temperature of 77K). One of the most commonly used
measurements in molecular sieves research is the specific surface area as measured
by Brunauer Emmet Teller (BET) method.
This method involves adsorbing a
monolayer of liquid nitrogen onto a surface of sample followed by measuring the
amount of nitrogen that is released when the monolayer is vaporized. Based on this
quantity, the surface area of the sample is calculated.
Given the complexity of the pore structure in high-surface area catalyst, six
types of adsorption isotherms (see Figure 2.12) have been identified according to a
66
classification advanced by IUPAC. However, only four type of adsorption isotherms
usually found in catalysis [136, 141]:
a. Type II, typical of macroporous solid where the prevailing adsorption
processes are the formation of a monolayer at low relative pressures,
followed by gradual and overlapping multilayer condensation as the
pressure is increased.
b. Type IV, often seen in mesoporous solids, where condensation occurs
sharply at a pressure determined by Kevin-type rules.
c. Type I, characteristic of microporous solids, where pore filling takes
place without capillary condensation, and is distinguishable from the
monolayer formation process.
d. Type VI, corresponding to uniform ultramicroporous solids, where the
pressure at which adsorption takes place depends on surface-adsorbate
interactions, and shows isotherm with various steps each corresponding to
adsorption on one group of energetically uniform site.
nad
P/Po
P/Po
P/Po
VI
nad
V
nad
IV
nad
III
nad
II
nad
I
P/Po
P/Po
P/Po
Figure 3.2: The IUPAC classification for adsorption isotherms, where nad = amount
of adsorbed and P/Po= relative pressure.
67
3.3.4.2 Experimental
In this work, the isothermal N2 adsorption/desorption experiments were
conducted on a Quantachrome Autosorb 1 series instrument.
Before analysis,
samples were outgassed at 200 °C under vacuum for 22 h. The relative pressure
P/Po (P and Po are the pressures of N2 vapor at adsorption and its saturation vapor
pressure at 77 K, respectively) used for the calculation is in the range of 0–0.3.
3.3.5
Thermal Gravimetry and Differential Thermal Analysis (TG-DTA)
3.3.5.1 Introduction
Thermal gravimetry and differential thermal analysis (TG-DTA) is a
technique whereby the weight of a sample and the phase transitions or chemical
reactions can be followed through observation of heat absorbed or released over a
period of time while its temperature is being raised linearly. Briefly, the sample is
placed on the balance and the furnace for sample heating is installed beneath the
balance. Then it will be heated and the electronic recording mechanism will plot a
graph of weight and energy adsorbed (endothermic) or released (exothermic) against
time, termed thermogram. TG-DTA can be used in studying: thermal degradation of
a sample, chemical reaction resulting in changes of mass such as absorption,
adsorption, desorption and also sample purity.
3.3.5.2 Experimental
In this work, TG-DTA was performed using a Mettler Toledo TGA/SDTA
851 instrument under N2 atmosphere with a flow rate of 20 mL min-1 using about 25
mg of sample. The sample was heated in the temperature range of 30 to 900 °C at a
heating rate of 10 °C min-1.
68
3.3.6
Field Emission Electron Scanning Microscopy (FESEM)
3.3.6.1 Introduction
Field emission scanning electron microscope (FESEM) is a scientific
instrument that uses a beam of highly energetic electrons to examine objects on a
very fine scale. The magnification produced by a scanning microscope is the ratio
between the dimension of the final image display and the field scanned on the
specimen. The FESEM does not produce a true image of the specimen. Instead, it
produces a point by point reconstruction of the specimen. The image of FESEM is
produced by the signal which comes from the interaction of an electron beam with
the specimen in the column of FESEM [139].
3.3.6.2 Experimental
FESEM experiments were carried out for the determination of particle size
and morphology of the samples. Prior to sample scanning, the sample in powder
form was attached to sample holder by using double sided tape. The samples were
then coated with gold using BIO-RAD Polaron Division SEM Coating System
machine. Samples were scanned using Zeiss Supra 35VP FESEM operating at 35 100 kV.
3.3.7
Photoluminescence
3.3.7.1 Introduction
Photoluminescence spectroscopy is a contactless, nondestructive method of
probing the electronic structure of materials. Photoluminescence (PL) is the
spontaneous emission of light from a material under optical excitation [142]. PL
spectroscopy provides electrical characterization, and it is a selective and extremely
sensitive probe of discrete electronic states. Features of the emission spectrum can
69
be used to identify surface, interface, and impurity levels and to gauge alloy disorder
and interface roughness. The intensity of the PL signal provides information on the
quality of surfaces and interfaces. It gives a measure of the relative rates of radiative
and nonradiative recombination.
All solids, including semiconductors, have so-called “energy gaps” for the
conducting electrons. In order to understand the concept of a gap in energy, first
consider that some of the electrons in a solid are not firmly attached to the atoms, as
they are for single atoms, but can hop from one atom to another. These loosely
attached electrons are bound in the solid by differing amounts and thus have many
different energies. Electrons having energies above a certain value are referred to as
conduction electrons, while electrons having energies below a certain value are
referred to as valence electrons and labeled as conduction and valence bands,
respectively (see Figure 2.13). Furthermore, there is an energy gap between the
conduction and valence electron states.
Under normal conditions electrons are
forbidden to have energies between the valence and conduction bands.
Figure 3.3: The physical process following absorption of a photon by a molecule.
70
If a light particle (photon) has an energy greater than the band gap energy,
then it can be absorbed and thereby raise an electron from the valence band up to the
conduction band across the forbidden energy gap (see Figure 2.13). In this process
of photoexcitation, the electron generally has excess energy which it loses before
coming to rest at the lowest energy in the conduction band. At this point the electron
eventually falls back down to the valence band. As it falls down, the energy it loses
is converted back into a luminescent photon which is emitted from the material.
Thus the energy of the emitted photon is a direct measure of the band gap energy,
Eg.
The process of photon excitation followed by photon mission is called
photoluminescence. A spectrometer is used for measuring the intensity of light as a
function of wavelength.
3.3.7.2 Experimental
In this work PL analysis was done to study the effect of titanium incorporated
OMS-2 to PL spectra of OMS-2 material. It should prove is it titanium exists in the
framework structure or as non-framework TiOx particles. PL spectra were recorded
in air at room temperature on Perkin-Elmer LS 55 spectrometer. The emission
spectra were observed at excitation wavelength was 430 nm.
3.3.8
X- Ray Photoelectron Spectroscopy (XPS)
3.3.8.1 Introduction
Surface analysis by XPS is accomplished by irradiating a sample with
monoenergetic soft X-rays and analyzing the energy of the detected electrons. The
photons have limited penetrating power in a solid on the order of 1-10 micrometers.
They interact with atoms in the surface region, causing electrons to be emitted by
photoelectric effect. The kinetic energy (KE) of the emitted electrons is given by
following equation:
71
KE
hv BE ) s
(Equation 2.6)
where hv is the energy of the photon, BE is the binding energy of the atomic orbital
from which the electron originates, and s is the spectrometer work function.
The binding energy may be regarded as the energy differences between the
initial and final states after the photoelectron has left the atom. Because there is
variety of possible final states of the ions from each type of atom, there is a
corresponding variety of kinetic energies of the emitted electron. Besides, each
element has a unique set of binding energies, XPS can be used to identify and
determine the concentration of the elements in the surface. Variations in the element
binding energies (the chemical shifts) arise from the difference in chemical potential
and polarizability of compounds. These chemical shifts can be used to identify the
chemical state of the materials being analyzed.
3.3.8.2 Experimental
In this study, X-ray photoelectron spectra (XPS) were recorded using a
Krastos XSAM–HIS (SAC) electron spectrometer fitted with a Mg K source. The
anode was operated at 120 W (12 kV, 10 mA) and the analyzer was operated at
constant pass energy of 40 eV. The binding energy shifts due to surface charging
were corrected using the C 1s level at 284.6 eV.
3.3.9 Pyridine Adsorption
3.3.9.1 Introduction
Another application of FTIR spectroscopy is that it has been combined and
used in the characterization of nature of the acid sites (Brönsted and Lewis). The
acidity of the solids characterized by absorbed base probe molecule i.e. pyridine.
The choice of pyridine is due to its strong basicity property and its ability to interact
72
with a wide scale of acid strength as well as it can differentiate between the Brönsted
and Lewis sites. A solid acid is capable of converting an adsorbed basic molecule
into its conjugated acid form. The acid site is able to transfer a proton from the solid
to the adsorbed molecule (Brönsted acid site) or an electron pair from the solid to the
adsorbed molecule (Lewis acid site) [143]. By using this method of analysis, the
Brönsted and Lewis acid sites could be differentiated.
Pyridine selectively interacts with Brönsted and Lewis acid sites, giving rise
to infrared absorptions in the region of 1400-1700 cm-1. The characteristic IR bands
of adsorbed pyridine protonated by Brönsted acid sites (pyridinium ions) appear at
1545 cm-1 while the bands from pyridine coordinated to Lewis acid sites appear at
1450 cm-1 as shown in Table 2.7.
Table 3.1: Position of bands for linkages of pyridine.
Position of band (cm-1)
Linkage of pyridine
1638
Brönsted acid sites
1620
Lewis acid sites
1580
Physisorbed
1545
Brönsted acid sites
1490
Brönsted and Lewis acid sites
1450
Lewis acid sites
1439
Physisorbed
Lewis acid sites occur at the electron deficient sites and can accept a pair of
electrons. Pyridine with its nitrogen lone pair of electrons acts as an electron donor
while the metal acts as an electron acceptor. This interaction results in a peak at
~1455 cm-1 and can be shown as in Figure 2.14.
The Brönsted acid is chemical species that is able to lose, or "donate" a
hydrogen ion (proton). The existence of hydroxyl groups will give a peak at ~3600
cm-1. If the Brönsted acid site exists in the samples, the peak disappears after
73
introducing the pyridine and at the same time, a new peak at ~1545 cm-1 appears.
This new peaks is due to pyridine bounded to Brönsted acid sites. The mechanism is
depicted in Figure 2.15.
O
+
M
O
Mn
M
OO
O
N
N
Mn
+
OO
O
Figure 3.4: The mechanism of interaction between pyridine molecules with Lewis.
H
O
+
N
O
Mn
M
O
OO
N
H
O
O
Mn
M
O
OO
Figure 3.5: The interaction between pyridine molecules with Brönsted acid sites.
3.3.9.2 Experimental
The wafer of the sample (10–12 mg) was locked in the cell equipped with
CaF2 windows, and evacuated at 400 oC under vacuum condition for 4 h. Pyridine as
probe molecule was introduced into the evacuated sample for a minute at room
temperature, followed by desorption of physisorbed pyridine at 150°C under vacuum
for 1 h. IR spectra of the pyridine vibration region were monitored in the range of
1700–1400 cm-1 at room temperature.
74
3.3.10 Adsorption Capacity of Adsorbed Water
A study on the adsorption capacity of adsorbed water was carried out in order
to determine hydrophilicity relative of the samples.
About 1 g of sample was
dehydrated under vacuum at 100 ºC overnight. After dehydration, the sample was
exposed to water vapour at room temperature, followed by the determination of the
percentage of adsorbed water as a function of time.
3.3.11 Gas Chromatography (GC) Analysis
Gas chromatography is widely used in most qualitative and quantitative
analysis and it is basically limited to organic compounds that are volatile and
thermally labile. The GC analysis technique was used to separate, identified and
quantify the amount of reaction products.
Gas chromatography is a physical separation method in which the
components in a mixture are selectively distributed between the mobile phase, which
is an inert carrier gas, and a stationary phase, which is present as a coating of either
column packing particles of the inert column wall. The chromatographic process
occurs as a result of repeated sorption/desorption step during the movement of the
analytes along the stationary phase by the carrier gas. The separation is due to the
differences in distribution coefficients of the individual components in the mixture.
The instrumentation for GC consists of a gas control unit, a sample
introduction system or injector, a column housed in the temperature-programmable
column oven, and a detector or transfer line and/or interface to mass spectrometer.
The gas control unit performs flow-rate or pressure control of the gas flows through
the injector, the column, and the detector of carrier gas and, if required auxiliary
gases. The carrier gas (hydrogen, helium, or nitrogen) typically is applied at a
pressure below 0.3 MPa.
75
In this study, the GC analysis technique was used to separate and quantify the
amount of reaction product. The analysis was performed using Thermo Finnigan
Trace-GC instrument with Equity-1 capillary column (30 m x 0.25 mm ID, 0.25 m
film thickness) and connected to FID detector.
3.3.12 Gas Chromatography-Mass Spectrometry (GC-MS) Analysis
Gas chromatography-mass spectrometry (GC-MS) is a combination of two
powerful analytical tools: gas chromatography for the highly efficient gas-phase
separation of component in complex mixture, and a mass spectrometry for the
confirmation of identity of these components as well as for the identification of
unknowns.
Compared to FID detector, this detector separates the compounds
according to their mass-to-charge ratio.
In this study, the GC-MS analysis technique was used to evaluate the reaction
products. GC-MS analysis technique was applied using Agilent model G1540N GCMS instrument with HP-5MS capillary column (30 m x 0.25 mm ID, 0.25 m film
thickness).
3.4
Catalytic Testing
The catalytic reactions were done to study the physicochemical propertiescatalytic activities relationship of samples in oxidation and acid reactions. The
reactions were carried out in oxidation of alcohol, alkanes and alkenes and also in
consecutive oxidation and acid reactions for direct transformation of alkenes to diols
as depicted in Figure 3.2. The figure also shows the detail of the catalyst used in
each reaction. The following sections are the description of the reactions carried out
to examine the catalytic properties of catalysts synthesized.
76
Benzyl alcohol
to
benzaldehyde
with air as
oxidant
OMS-2a
OMS-2b
OMS
Ti-OMS-2(X)
Oxidation
reaction
Fe-OMS-2(0.09)
Cyclohexane to
cyclohexanol and
cyclohexanone with
TBHP as oxidant
Cu-OMS-2(0.02)
Co-OMS-2(0.04)
H-OMS-2
H-Ti-OMS-2(X)
H-Fe-OMS-2(0.09)
H-Co-OMS-2(0.04)
H-Cu-OMS-2(0.02)
Cyclohexene to
2-cyclohexenone
and
2-cyclohexenol
Catalytic
activity
Alkenes
with TBHP
as oxidant
OMS-2
TiO2
Ti-OMS-2(Y)
TS-1
OMS-2
Ti-OMS-2(Z)
Styrene to styrene
oxide,
TiO2-OMS-2(imp)
benzaldehyde and
phenylacetaldehyde TiO2-OMS-2(mix)
OTS/OMS-2
OTS/Ti-OMS-2
SW150-Ti-OMS-2(Y)
Consecutive
oxidation and
acid reaction
1-octene to 1,2
octanediol with
TBHP as
oxidant
SW200-Ti-OMS-2(Y)
ST150-Ti-OMS-2(Y)
ST200-Ti-OMS-2(Y)
Note: X refer to Ti/Mn ratio = 0.05 and 0.67; Y refer to Ti/Mn ratio = 0.18 and 0.67
and Z refer to Ti/Mn ratio = 0.18; 0.43 and 0.67.
Figure 3.6: Catalytic reactions over various modified OMS-2.
77
3.4.1
Oxidation of Benzyl Alcohol
3.4.1.1 Introduction
Solid-gas catalyzed-liquid reactions are often encountered in the chemical
process industry, most frequently in hydroprocessing operations and in the oxidation
of organic liquid phase [144-147]. In respect of the latter reaction type, the system
has been applied for mineralization of toxic organics in wastewaters. However,
synthesis of useful organic materials with this system is also one of particular fields
of application.
In this work the model reaction is the oxidation of benzyl alcohol with air as
oxidant.
The use of air as oxidant is advantageous due to its environmentally
friendly nature and no increase in cost. Furthermore, oxidation of benzyl alcohol is
one of the important oxidation processes since benzaldehyde as product is an
important intermediate for the processing of perfume and flavouring compounds
(almond flavour) and also as starting materials for other organic compound ranging
from pharmaceutical to plastic additives. One of the active materials for oxidation of
benzyl alcohol with molecular oxygen is OMS-2 material. The material shows the
rare Mars-van Krevelen type mechanism for oxidation with molecular oxygen in the
liquid phase [24]. The process usually occurs in gas phase oxidation reaction using
metal oxide/mixed metal oxide as a catalyst [148-152]. However, the other liquid
phase oxidation in polyoxometalate was reported via that mechanism [29]. This
mechanism is interesting because by controlling the lattice oxygen diffusion (with a
proper selection of oxide), an effective catalyst for liquid-gas system could be
obtained (as catalyst/active site).
3.4.1.2 Experimental
Catalytic reaction of samples was studied for oxidation of benzyl alcohol. A
15 mL of toluene, 1 mmol of benzyl alcohol, and 0.5 mmol of methyl benzoate as an
internal standard were put in a round-bottomed flask. The catalyst (~50 mg (0.5 eq),
considering one manganese atom to be one active site) was added and a reflux
78
condenser was attached. This assembly was placed in a silicon oil bath. The
reaction was thus carried out under semibatch conditions for 4 h.
The reflux
conditions ensured a constant temperature equal to the boiling point of toluene at 110
o
C. Samples of the reaction mixture were collected and centrifuged to separate the
solid before they were analyzed by GC and/or GC-MSD.
3.4.2
Oxidation of Cyclohexane
3.4.2.1 Introduction
Functionalisation of alkanes is of great interest in both theory and practice
due to their abundance and extremely low activity. Petroleum and natural gas are the
main sources of alkanes and about of 5% of nearly a billion ton of petroleum
extracted in the world is chemically processed to alkanes [153]. The inertness of
alkanes is the main obstacle to the development of methods for their
functionalisation in reactions with these compounds occurs under drastic conditions.
The main pathway of alkanes functionalization is their oxidation. The oxidation of
alkanes takes place by two main paths: compounds containing oxygen (partial
oxidation) and CO2 and H2O (extensive oxidation). The partial oxidation of alkanes
is significant to the chemical industry because these oxidation reactions are used to
convert petroleum hydrocarbon feedstocks into chemicals important in the polymer
and petrochemical industries.
Among various alkanes oxidation, partial oxidation of cyclohexane is much
attractive because of its main products i.e. cyclohexanol and cyclohexanone are very
important chemical intermediates. They are as starting materials to produce adipic
acid and caprolactam which are used in the manufacture of nylon-66 and nylon-6
polymer, respectively. Currently industrial process of oxidation of cyclohexane is its
liquid-phase oxidation with air at 150 oC in the presence of soluble cobalt catalyst
[154]. The drawbacks of the process are the poor conversion of cyclohexane (about
79
4%) and selectivity for cyclohexanone and cyclohexane (K-A-oil) of just about 70–
85%, depending on the conversion of cyclohexane. The intensive study was done in
oxidation of cyclohexane in homogeneous and heterogeneous catalysts [155-159]. A
review of oxidation of cyclohexane by Schuchardt et al. [160] concluded that further
work for finding the active catalyst in the oxidation of cyclohexane is still a great
challenge.
3.4.2.2 Experimental
The oxidation reactions were carried out at atmospheric pressure, as follows:
100 mg of catalyst was suspended in a mixture of 27.8 mmol (3 mL) of cyclohexane
(Merck), 10 mmol of TBHP (70% in water). The reaction mixture was heated under
reflux with magnetic stirring in an oil bath at 60 oC. After reaction, the catalyst was
removed and gas chromatography coupled to mass spectrometry (GC–MS, Agilent
model G1540N, DB-1MS 20M capillary column) was used to identify the reaction
products, which were quantified by gas chromatography (Trace GC) coupled to a
flame ionization detector, using an internal standard (cyclooctane, Fluka) and
calibration curves.
3.4.3
Oxidation of Cyclohexene
3.4.3.1 Introduction
Alkenes, either derived from natural resources or generated as products of the
chemical industry, are found in great abundance in the realm of organic molecules.
Heterogeneously catalyzed oxidations of alkenes have been widely applied in
numerous chemical, biological and pharmaceutical industries.
The selective
oxidation products are important starting materials towards the production of several
fine chemicals and polymers [28, 161]. One of the most useful transformations of
alkenes is epoxidation.
The epoxides can be transformed into a variety of
functionalised products. For example, reductions, rearrangements or ring-opening
80
reactions with various nucleophiles give diols, aminoalcohols, allylicalcohols,
ketones, polyethers etc. However, the epoxidation process in oxidation of alkenes is
competed by C-H bond oxidation, thus making the process more complex and the
selectivity lower.
Selective oxidation of cyclohexene is one of the important alkenes oxidation.
The oxygenated products of cyclohexene and their derivatives are very important in
organic synthesis owing to the existence of a highly reactive ,-unsaturated
carbonyl group, which are extensively used in the preparation of a range of chemical
intermediates and products [162].
3.4.3.2 Experimental
The catalyst performances were tested in the oxidation of cyclohexene using
tert-butyl hydroperoxide (TBHP) as oxidant. Cyclohexene (5 mmol), 70% (wt. %)
TBHP in water (10 mmol), catalyst (50 mg), cyclooctane (0.5 mmol) as internal
standard and acetonitrile (15 ml) as solvent were placed in a round-bottomed flask
with a reflux condenser and the reaction was performed with stirring at 80 °C in an
oil bath for 24h. In order to evaluate qualitatively the reaction products, the GC-MS
analysis technique was applied using Agilent model G1540N GC-MS instrument
with HP-5MS capillary column (30 m x 0.25 mm ID, 0.25 m film thickness). The
analysis of products quantitatively is using Thermo Finnigan Trace-GC instrument
with Equity-1 capillary column (30 m x 0.25 mm ID, 0.25 m film thickness) and
connected to FID detector.
81
3.4.4 Oxidation of Styrene
3.4.4.1 Introduction
The other interesting alkenes oxidation is the liquid-phase oxidation of
styrene; from which produced high value products such as styrene oxide and
phenylacetaldehyde. The rearrangement of styrene oxide to benzaldehyde also yields
valuable compounds or intermediates for production of fragrances, pharmaceuticals,
insecticides, fungicides and herbicides [163-166].
3.4.4.2 Experimental
The oxidation of styrene was carried out using the above catalysts. Styrene
(5 mmol), 70% (wt. %) in water of ter-butyl hydroperoxide (TBHP) (10 mmol),
catalyst (50 mg) and acetonitrile (15 ml) as solvent were placed in a round-bottomed
flask with a reflux condenser and the reaction was performed with stirring at 70 oC in
an oil bath. The products were collected after 3 h of reaction and analyzed by GC
and GC-MS.
3.4.5 Transformation of 1-octene to 1,2-octanediol
3.4.5.1 Introduction
Diols are important starting materials for polyurethane chemistry, natural
products, drugs and agrichemicals [120, 121, 124, 167]. A number of 1,2-diols such
as 2,3-dimethyl-2,3-butanediol, 1,2-octanediol, 1,2-hexanediol, 1,2-pentanediol, 1,2and 2,3-butanediol are of interest to fine chemical industries. In addition chiral 1,2diols are employed as intermediates for pharmaceuticals and agrichemicals [125].
Industrial diols are synthesized by two step reaction that is epoxidation of
alkenes followed by hydrolysis of epoxide in the presence of Brönsted acid sites.
82
The direct transformation of alkenes to diols was reported by Prasetyoko et al. [5254] using a bifunctional oxidative and acidic catalyst using aqueous H2O2 as oxidant.
The catalyst is the titanium silicate-1 (TS-1) loaded with sulphated zirconia or
niobium oxide and it was proven that TS-1 acted as oxidative site and sulphated
zirconia or niobium loaded on TS-1 as acidic site, respectively. Therefore it is
interesting to determine whether the sulphated Ti-OMS-2 act similarly to the TS-1
catalyst.
3.4.5.2 Experimental
The oxidation of 1-octene was carried out using ter-butyl hydroperoxide
(TBHP) as an oxidant.
A typically reaction mixture was composed of 50 mg
catalysts, 1-octene substrate 1 mL, 1 mL of TBHP, 10 mL of aceton as solvent, and
0.5 mmol of cyclooctane as internal standard. The mixture placed in round-bottomed
flask with reflux condenser. This assembly was placed in a paraffin oil bath. The
reaction mixture was vigorously stirred at 60 °C. The products were collected after
24 h of reaction and analyzed by GC and GC-MS.
CHAPTER 4
PHYSICOCHEMICAL PROPERTIES OF OMS-2 AND
MODIFIED OMS-2 CATALYSTS
4.1
Introduction
This chapter describes the physicochemical properties of OMS-2 and
modified OMS-2 catalysts. The samples were characterized by XRD, FTIR, surface
area analyzer, FESEM, TG-DTA, XPS, pyridine adsorption and adsorption of vapour
water. These characterizations are very important for the study of the structurecatalytic activity relationship.
The physicochemical properties were studied
including structural analysis, thermal stability, surface area and acidity study.
Besides using the previous method for incorporation of metal into the
framework of cryptomelane OMS-2, a new method was attempted.
The only
solution of transition-metal ion (without any manganese ion solution) in acidic
condition was oxidized by solution of potassium permanganate. This method gives
higher M/Mn molar ratio in M-OMS-2. However, this method was only suitable for
Ti2(SO4)3 as source of metal. The other metal sources such as FeCl3, FeSO4.7H2O,
CoSO4, CuSO4.5H2O and TiCl4 can not retain the cryptomelane structure of OMS-2.
84
4.2
Physicochemical Properties of Prepared OMS-2 by Different Methods
As mentioned before in Section 2.2.2, OMS-2 prepared in acidic condition
gave the best catalytic activity in oxidation reaction. Based on that, OMS-2 was
prepared in acidic condition. However, there were only two methods to prepare
OMS-2 in acidic condition reported i.e. with and without buffer solution. Hence in
this work, OMS-2 was prepared with and without buffer solution in order to study
the physicochemical properties-catalytic activity relationship of both samples and
then to choice the best method to prepare OMS-2. The preparation methods are
described in Section 3.1.1 and 3.1.2. OMS-2 prepared without buffer solution was
labeled as OMS-2a and OMS-2 prepared with buffer solution as OMS-2b.
The synthesis of cryptomelane from KMnO4 and Mn2+ depended on pH,
temperature and the type of countercation.
The only variable is pH while
temperature and the type of countercation are the same for both prepared OMS-2
samples. The buffer solution was used to control the pH of the mixture during the
synthesis.
pH played an important role in determining the success of growing
cryptomelane structure during the formation of OMS-2 materials. The amount of
counteraction also influenced the synthesis of OMS-2.
The molar ratios of
KMnO4/[KMnO4] + [MnSO4]) for OMS-2 prepared with and without buffer were
about 0.52 and 0.41, respectively. This was based on the work by DeGuzman et al.
[62] who studied the effect of potassium concentration under reflux in acidic
condition as shown in Table 4.1. If the molar ratio of KMnO4/[KMnO4] + [MnSO4])
is 0.32 or lower, OMS-2 was formed with low crystallinity.
In the synthesis, when KMnO4 was added to Mn2+ solution in acidic
condition, amorphous MnO2 was formed. The color of the reaction mixture turned
immediately from light pink to light brown and finally a dark brownish or black
precipitate was obtained. The addition of KMnO4 by dropwise to mixture of Mn2+
solution increased the crystallinity of OMS-2 formed. After several hours of heating
(in this work 100 oC), the cryptomelane phase was formed. After washing and
drying, a black and brown solids for OMS-2a and OMS-2b were obtained.
85
Table 4.1: Effect of potassium concentration under reflux on the synthesis of
OMS-2 [62].
KMnO4/[KMnO4] + [MnSO4])
The phase formed
0.88
Cryptomelane
0.74
Cryptomelane
0.41
Cryptomelane
0.36
Cryptomelane
0.32
Cryptomelane (poor crystallinity)
0.28
Cryptomelane (poor crystallinity)
0.26
Cryptomelane (poor crystallinity)
The X-ray diffraction patterns of OMS-2a, OMS-2b and standard synthetic
cryptomelane (KMn8O16) [168] are shown in Figure 4.1. XRD patterns of both
synthesized OMS-2 samples are similar to the standard pattern of cryptomelane, Q.
This indicated that both synthesized OMS-2 materials were of cryptomelane type
structure. Cryptomelane consists of a well-defined 2 x 2 tunnel structure having a
pore size of about 4.6 Å and composed of double chains of edge-sharing MnO6
octahedra and corner-sharing double chains. The 2 x 2 tunnel structure needs some
large ions, such as K+ ion in the tunnels, to prevent collapse of the framework.
In addition, no other peaks are observed in both samples which indicate that
no other phase of MnO2 such as a pyrolusite (a one-dimensional tunnel manganese
oxide with 1×1 MnO6 units) was observed; an impurities phase that is normally
observed in highly acidic condition especially in pH < 1. The pH of the mixture
during synthesis of OMS-2a and OMS-2b sample were in the range of 1-2 and 4.5,
respectively. Therefore, both conditions were suitable for the synthesis of pure phase
cryptomelane.
86
Intensity / a.u.
(c)
5
10
20
50
(541)
(002)
(521)
(600)
(510)
40
30
(411)
(301)
(211)
(310)
(220)
(200)
(110)
(b)
(a)
60
70
2 /º
Figure 4.1: XRD patterns of (a) reference pattern of crypromelane, Q JCPDS 29,
1020, (b) OMS-2a and (c) OMS-2b.
There are no significant differences in the relative peaks heights of OMS-2b
sample and cryptomelane-Q; indicated that both samples have similar preferred
orientation of crystal structure. However, there is a difference of relative peaks
heights of OMS-2a compared to OMS-2b and cryptomelane Q, in which the relative
intensity of (200) to (310) plane (as indicated by the arrow in Figure 4.1) in OMS-2a
> 1 while in OMS-2b and Cryptomelane-Q are that is <1. This indicates that there is
a difference between preferred orientation of the crystal in OMS-2a sample
compared to OMS-2b and cryptomelane-Q.
It is also observed that the peak intensity of OMS-2b sample is higher than
OMS-2a sample. The higher peak intensity indicates the more crystalline is the
sample. Therefore, OMS-2b is more crystalline than OMS-2a. This may be caused
by the acidity (pH) of the mixture during synthesis.
As mentioned above, the
reaction mixture in the synthesis of OMS-2b with the buffer solution was maintained
at pH of around 4.5 and synthesis of OMS-2a without buffer had pH in the range of
1-2.
The high acidity of the mixture in the synthesis of OMS-2a rendered
87
crystallization of OMS-2a faster than crystallization of OMS-2b. The consequence is
lower crystallinity of OMS-2a compared to OMS-2b. From the figure it is also
observed that diffraction peaks of OMS-2a sample as boarder as OMS-2b sample
implying that particles size of both samples is similar.
The thermal stability of OMS-2 was studied by calcination of OMS-2 sample
in a series of temperatures. XRD pattern of the calcined OMS-2b sample is shown in
Figure 4.2. It shows that no extra line is observed for calcined sample until 500 oC
which indicates that the sample was not decomposed at this temperature. Extra lines
are observed when the sample was calcined at 600 oC for 5 h; indicating the
formation of the other phases. The appearance of these peaks indicate that a part the
samples have been transformed from cryptomelane structure to hausmannite
structure (Mn3O4).
This phase was also observed by Chen et al. [69] during
oxidation process of 2-propanol on OMS-2, where cryptomelane was reduced to
hausmannite by 2-proponal, which at the same time is oxidized to CO2. Compared to
the other synthetic manganese oxides such as OMS-1, generally the thermal stability
OMS-2 material is higher. The thermal stability of OMS-1 is less than 400 oC [56].
OMS-2 is more thermally stable than OMS-1 due to the smaller pore opening of
OMS-2 than OMS-1. OMS-2 utilizes two MnO6 octahedra on each side to form a
2x2 square tunnel with a pore size of about 4.6 Å and similarly, OMS-1 has a 3x3
square tunnel with a pore size of about 6.9 Å.
There are some significant differences in the relative peaks heights of OMSOMS-2b after calcination.
Table 4.2 shows the relative intensity and ratio of
I(110)/I(200) planes (as indicated by arrow in Figure 4.2) of OMS-2b and calcined
OMS-2b samples. The relative intensity of peaks of 200 to 110 planes decreased by
increasing of temperature up to 500 oC, indicating the existence of different preferred
orientation of crystal after calcinations until that temperature. However, the relative
intensity of (110)/(200) plane became higher after calcination at 600 oC, implying the
changing of the preferred orientation on of the crystal after that temperature. This
may be due to the transformation of cryptomelane to hausmanite.
88
110 200
* hausmannite
*
*
(d)
Intensity / a.u.
(c)
(b)
(a)
5
10
20
30
40
50
60
70
2 /º
Figure 4.2: Effect of calcination on XRD patterns of OMS-2b materials, (a) before
calcination, (b) calcination at 400oC, (c) at 500oC, and (d) at 600oC.
Table 4.2: The relative intensity and ratio of I(110)/I(200) plane of OMS-2b and
calcined OMS-2b samples calculated by XRD.
Relative intensity (hkl)
samples
I(110)/I(200)
110
200
OMS-2b
75
68
0.90
OMS-2b (400)
87
56
0.64
OMS-2b (500)
44
24
0.54
OMS-2b (600)
66
65
0.98
89
FTIR spectra of samples are shown in Figure 4.3. The bands in the range of
500-700 cm-1 and 400-500 cm-1 are assigned to the Mn-O stretching of MnO6
octahedra and the Mn-O-Mn bending vibration in the MnO2 octahedral lattice,
respectively [169].
Both samples have peaks at 600 and 520 cm-1 which are
characteristic of IR spectra for cryptomelane structure [170]. This is in agreement
with XRD data in which both samples have cryptomelane type framework structure
regardless of different synthesis conditions. The bands around 2800-3600 cm-1 and
1620 cm-1 were observed in OMS-2a sample and not observed in OMS-2b sample.
Relative strong of these bands are due to water adsorbed on the surface of OMS-2a
and some hydroxyl groups not from hydrates but those directly bound to metal ions
[171] due to the defect on the structure as shown in Figure 4.4. This is in agreement
to XRD analysis in which OMS-2a has the lower crystallinity of than OMS-2b.
These bands are not observed in OMS-2b samples indicating that there was no
adsorb water in OMS-2b and vibration of Mn-OH in OMS-2b. Consequently, OMS2b is more hydrophobic compared to OMS-2a. This is also supported by the capacity
of adsorbed water vapour calculated on both OMS-2 samples as shown in Figure 4.5.
600
520
Figure 4.3: FTIR spectra of (a) OMS-2a and (b) OMS-2b.
90
O
O
O
O
Mn
O
4+
Mn
4+
O
OO
4+
O
O
O
O
O
OO
O
4+
Mn
O
O
Mn
O
4+
O
Mn
O
O
O
OH
4+
4+
Mn3+
O
O
O
Mn4+
O
O
Mn
Mn4+
HO
O
Mn4+
Mn
O
Mn4+
OO
O
Mn4+
O
O
4+
O Mn O
Mn
O
O
O
O
4+
O
OO
4+
Mn
Mn
O
O
Mn3+
Mn4+
O
O
OO
O
4+
O
Mn4+
O
O
Figure 4.4: Defect on OMS-2 structure.
Figure 4.5: Amount of adsorbed water on the surface of OMS-2a and OMS-2b
samples.
91
As shown in the figure, unbuffered sample (OMS-2a) absorbed water vapour almost
four times more than buffered sample (OMS-2b), confirming that the surface of
OMS-2a is more hydrophilic than OMS-2b.
The thermal stability of the sample was tested by TG analysis. Figure 4.6
shows thermograms of OMS-2a and OMS-2b in nitrogen atmosphere. The weight
loss below 200 ºC is due to the removal of water molecules present on the surface of
manganese oxide. It shows that the adsorbed water on the surface of OMS-2a and
OMS-2b are about 2.5% and 0.9%, respectively. It indicates that the surface of
OMS-2 with buffer (OMS-2b) is more hydrophobic than without (OMS-2a), which is
in agreement with the IR analysis. This is also supported by the adsorbed water
vapour calculated on the surface of both OMS-2 samples as shown in Figure 4.5.
Figure 4.6: TGA plots for OMS-2 materials in N2 atmosphere.
The weight losses at higher temperatures (>200 ºC, stage II) are due to the
evolution of oxygen from the lattice, as confirmed by temperature programmed
decomposition with mass spectroscopic detection done by Luo et al. [26]. Some of
92
the oxygen in the OMS-2 framework could be easily dislodged from the framework
and evolved into air without destroying the overall framework structure when the
sample was heated in nitrogen flow at as high as 520°C. The evolution of oxygen
resulted in the formation of framework oxygen vacancies, which may be catalytically
active sites for the oxidation reactions. At this stage OMS-2a sample lost about 2.5%
of its weight
and OMS-2b only about 0.8%, implying that it was easier to release
oxygen lattice on OMS-2a than OMS-2b.
At 520 ºC (stage III) both materials were decomposed into a stable lower
valence manganese oxide like Mn3O4 (hausmannite) and lost of O2 due to the
collapse of tunnel structure. Both samples showed that their structure collapsed at
similar start points.
However, the different environments may cause different
stability of OMS-2 sample that decomposed at lower temperature in nitrogen or
helium compared to in air [26]. At higher temperature (>720 ºC, stage IV) this
material further decomposed to other phase i.e. bixbyite. OMS-2b sample started to
decompose at a lower temperature than OMS-2a.
The phase transformation of
manganese oxide during the heating is given below:
OMS-2 Mn3O4 Mn2O3
Base on the discussion above, it shows that OMS-2a and OMS-2b samples
which were prepared without and with buffer, have different physicochemical
properties. OMS-2a is less crystalline and less hydrophobic compared to OMS-2b.
The thermal stability of both OMS-2 samples is similar. However, it is suggested
that oxygen lattice of OMS-2a was easier to be released than OMS-2b.
4.3
Physical Properties of Metal Substituted OMS-2 Material
The XRD patterns of synthesized M-OMS-2(X) samples prepared using the
first method (the mixture of metal source and Mn2+ in acidic condition was oxidized
by potassium permanganate solution) are shown in Figure 4.7. It shows that all
93
Figure 4.7: X-ray diffractograms of (a) OMS-2; (b) Ti-OMS-2(0.05); (c) Fe-OMS2(0.09); (d) Co-OMS-2(0.02); and (e) Cu-OMS-2(0.04).
synthesized M-OMS-2(X) have similar XRD patterns to that of OMS-2 pattern
which indicates that all M-OMS-2(X) have cryptomelane-type structure. No new
peaks due to metal species were observed in M-OMS-2(X) confirming that metals
are well incorporated in the framework structure of OMS-2 as depicted in Figure 4.8.
Figure 4.8: Schematic incorporation of metals in M-OMS-2 materials.
94
The figure shows that the peaks intensity of M-OMS-2(X) except Ti-OMS2(0.05) are lower than that of pure OMS-2 which indicate that crystallinity of MOMS-2(X) samples are lower than OMS-2. Ti-OMS-2(0.05) has higher intensity
compared to OMS-2 which indicates that Ti-OMS-2(0.05) is more crystalline than
OMS-2. Ti-OMS-2(0.05) sample has narrower peaks compared to pure OMS-2
which implies a bigger crystallite size in Ti-OMS-2 than OMS-2 sample. Fe-, Cu-,
and Co-OMS-2 samples have broader peaks compared to pure OMS-2 implying the
smaller crystallite size of those M-OMS-2 than the pure one. The average crystallite
sizes of all materials were calculated from the Scherrer equation and are listed in
Table 4.3.
Table 4.3: The ionic radii of metals ion and average crystallite size of OMS-2 and
M-OMS-2 samples.
Samples
a
Average crystallite size
(Å)a
OMS-2
208
Ti-OMS-2(0.05)
270
Fe-OMS-2(0.09)
199
Co-OMS-2(0.04)
178
Cu-OMS-2(0.02)
186
average crystallite size was calculated from the Scherer equation
OMS-2 is the mixed valence of manganese oxide which has an average
oxidation state (AOS) of manganese at around 3.8-3.9. This means that large amount
of manganese exists as Mn4+. The amount of Mn3+ and or Mn2+ is less than 10% of
the total quantity of manganese ion. One expects that any metal ion could replace
manganese ion in the same oxidation state. The initial oxidation state of metal ions
are Ti3+, Fe2+, Co2+ and Cu2+. The synthesis using potassium permanganate as
oxidation agent faced the possibility of metal ions being oxidized to the higher
oxidation state (Table 4.4). All transition metals may be oxidized to higher oxidation
state except Cu since the maximum oxidation state of Cu is +2, consequently Cu
favoured to exist as Cu2+. XPS analysis confirms that titanium exists as Ti4+ and
there was no Ti3+ was observed in Ti-OMS-2 sample (discussed in Section 4.4.5).
95
Table 4.4 also shows the lattice parameters and cell volume of OMS-2 and
M-OMS-2.
The lattice parameters and cell volume of OMS-2 changed after
incorporation of metal indicating that metal were incorporated in the framework
structure of OMS-2 material. In Ti-OMS-2 sample, the possibility of titanium with
4+ charge to be substituted to Mn4+ caused Ti-OMS-2 to has bigger cell volume size,
since the ionic radii of Ti4+ was bigger than Mn4+. As shown in Table 4.4, for the
other metal ions, ionic radii of metals with the same charge with Mnn+ ions are
smaller than ionic radii of Mnn+. This may due to the cell volume of Fe, Co and CuOMS-2 being smaller than OMS-2 sample.
Table 4.4: Lattice parameter (a and c) and cell volume (V) of OMS-2 and M-OMS-2
samples.
a (Å)
OMS-2
9.81
c (Å)
2.82
V (Å3)
271
The
Ionic
Possibility
possibility
radii
of
of Mnn+
of Mn+ ion
octahedra
ion was
in the
n+
M (Å) substituted
framework
Mn2+
0.83
3+
Mn
0.65
Mn4+
0.53
-
0.61
Mn4+
4+
Ti-OMS-2
9.82
2.82
272
Ti
Fe-OMS-2
9.80
2.81
270
Fe2+/Fe3+
0.61/0.55 Mn2+/Mn3+
Co-OMS-2
9.81
2.81
270
Co2+/Co3+
0.65/0.55 Mn2+/Mn3+
Cu-OMS-2
9.81
2.79
268
Cu2+
0.73
Mn2+
The intensity of the (310) and (211) reflections as shown by the arrows in
Figure 4.7 are varied by incorporation of metals. The calculation of the relative
peaks heights of I(310)/I(211) plane of M-OMS-2(X) samples are shown in Table
4.5. The table shows that only Ti-OMS-2(0.5) has significantly different in ratio of
I(310)/I(211) plane and become higher indicated that Ti incorporated caused
preferred orientation in OMS-2 crystal. For the other metals such as Fe-OMS2(0.09) and Cu-OMS-2(0.02) have lower of ratio of I(310)/I(211) plane than OMS-2.
This indicated that preferred orientation of Ti-OMS-2(0.5) is opposite to Fe-OMS-
96
2(0.09) and Cu-OMS-2(0.04). While Co-OMS-2 has ratio of I(310)/I(211) plane
very close to OMS-2 (i.e. 0.74 and 0.73, respectively) indicated that no preferred
orientation in OMS-2 crystal in that plane after incorporation of Co.
Table 4.5: The relative intensity and ratio of I(310)/I(211) plane of OMS-2 and MOMS-2 samples.
Relative intensity of (hkl)
Samples
I(310)/I(211)
310
211
OMS-2
73
100
0.73
Ti-OMS-2(0.05)
100
93
1.07
Fe-OMS-2(0.09)
65
100
0.65
Co-OMS-2(0.02)
74
100
0.74
Cu-OMS-2(0.04)
56
100
0.56
The XRD patterns of M-OMS-2 prepared without any manganese ion
solution are shown in Figure 4.9. The preparation was done in order to get higher
M/Mn ratio. Apparently, the peaks due to cryptomelane structure collapse upon
substitution by the Cu, Co and Fe metal ions. Substitution by Cu caused a decrease
in the intensity of the main peaks of cryptomelane and is broaden indicating its
amorphous nature. Substitution of Co is truly amorphous as well. Fe- OMS-2
patterns have new peaks corresponding to MnO and Fe2O3. This demonstrates that
the high amount of Cu, Co and Fe renders the structure of cryptomelane amorphous.
It is shown that only Ti-OMS-2 has maintained its cryptomelane structure. This
suggests that OMS-2 structure is retained and confirms the ability of Ti to efficiently
substitute manganese in the framework structure. This may be due to the factors,
such as similar electronegativity, oxidation state of metal ions and the source of
metal ions.
97
Figure 4.9: XRD pattern of (a) cryptomelane (JCPDS 29, 102) and (b) Ti-, (c) Cu-,
(d) Co- and (e) Fe-OMS-2 was prepared by oxidation of its metal ions source by
potassium permanganate in acidic condition without the addition of Mn2+ solution.
Pauling electronegativity of elements is listed in Table 4.6. It shows that
electronegativity of Ti is very close to electronegativity of Mn. This explained the
fact that high content of titanium still retained in the cryptomelane structure of OMS2. Whereas the electronegativity of the other elements (Fe, Co and Cu) is very
different from Mn which could caused the collapse the of cryptomelane structure
after the incorporation of high content of these elements.
Table 4.6 also shows the oxidation state of Mn in OMS-2 and the possibility
of oxidation state of the substituted metal ions. It is clearly observed that only
titanium fully fill all oxidation state of Mn in OMS-2 materials, due to the ability of
Ti to efficiently substitute manganese in the framework structure.
98
Table 4.6: The physical properties of metal ions
Element
Electronegativity The possible of oxidation
state of metal ions
Mn
1.55
+2, +3, +4 (in OMS-2)
Ti
1.54
+2, +3, +4
Fe
1.83
+2, +3
Co
1.88
+2, +3
Cu
1.90
+1, +2
The effect of source of metal ions was studied by the different source of Ti used i.e.
15% w/v Ti2(SO4)3 in H2SO4, TiCl4 and Ti(SO4)2 as listed in Table 4.8. It shows that
cryptomelane structure collapse by using of TiCl4 and Ti(SO4)2 as metal ions source.
This may be due to Ti4+ of those metal sources could not be oxidized to higher
oxidation state by potassium permanganate. Since synthesis of OMS-2 was via
redox reaction where Mn2+ was oxidized and potassium permanganate was reduced.
The use of Ti3+ {source Ti2(SO4)3} could replace the position of Mn2+ and was
oxidized to Ti4+, while Ti4+ could not. Although Cu, Co and Fe has the same charge
with Mn2+, they could not retain the cryptomelane structure which may be caused by
Table 4.7: Source of metal, its charge and their effect to cryptomelane structure.
Metal source
Metal ion
Structure
MnSO4. H2O
Mn2+
OMS-2
15% w/v Ti2(SO4)3 in H2SO4
Ti3+
retain of OMS-2 structure
TiCl4
Ti4+
collapse
Ti(SO4)2
Ti4+
collapse
CuSO4. 5H2O
Cu2+
collapse
CoSO4. 7H2O
Co2+
collapse
FeSO4. 7H2O
Fe2+
collapse
FeCl3
Fe3+
collapse
99
the inability of those metal ion to be in oxidation state +4, which is the main
oxidation state in OMS-2. Similarly, the use of Fe3+ with the same charge with Ti3+
could not retain the structure of OMS-2 since it could not be oxidized to +4 oxidation
state.
The results of AAS elemental analyses for cations in all synthesized M-OMS2 are summarized in Table 4.8. Although the same initial molar ratio of metal cation
was used in the reactants (0.1) in the syntheses of different metal doped M-OMS-2
materials except for Ti substituted, the final molar ratios of M:Mn in prepared
materials are different from each other. For Ti substituted OMS-2 the initial Ti:Mn
ratio
(0.05)
remain
unchanged
due
to
their
physicoproperties
such
as
electronegativity and oxidation state of Ti very closed to Mn. Based on the initial
ratio
of
M:Mn,
the
doped
metal
content
decreases
in
the
order
of
Ti3+>Fe2+>Co2+>Cu2+. The trend corresponds to electronegativity of metal ion which
electronegativity of Ti very closed to Mn and followed by Fe, Co and Cu,
respectively (Table 4.6). The elemental analysis of Ti-OMS-2 was also determined.
The ratio of Ti:Mn in Ti-OMS-2 sample was 0.67 and labelled as Ti-OMS-2(0.67).
Table 4.8: AAS data of OMS-2 and M-OMS-2 samples.
Initial
M/Mn
ratio
M
(mol)a
Mn
(mol)a
M/Mn
ratio
K
(mol)a
K/(Mn+M)
OMS-2
0.0
0.0
552.2
0.0
38.8
0.07
Ti-OMS-2 (0.05)
0.05
26.81
505.97
0.05
38.7
0.07
Ti-OMS-2 (0.67)
-a
228.2
342.0
0.67
21.5
0.04
Fe-OMS-2(0.09)
0.10
42.2
457.9
0.09
40.6
0.08
Cu-OMS-2(0.02)
0.10
9.9
528.0
0.02
41.2
0.08
Co-OMS-2(0.04)
0.10
20.7
518.4
0.04
38.3
0.07
a
no MnSO4.H2O was added to the initial reaction, see Section 3.2.3, the amount of Ti2(SO4)3 solution
was 75 mL.
The morphology of M-OMS-2 samples was analyzed by FESEM and its
micrographs are shown in Figure 4.10. It shows the morphology of Ti-OMS-2(0.05);
100
Fe-OMS-2(0.09); Co-OMS-2(0.04) and Cu-OMS-2(0.02). The figures show that the
morphology of Ti- and Co-OMS-2 are similar to fibrous morphology of OMS-2.
The widths of Ti-OMS-2 particles are relatively the same with OMS-2 in the ranges
of 20-30 nm. For Co- and Cu-OMS-2, the widths of the particles increase to about
100 nm and the length remain relatively the same, which is about several hundred
nm. The different morphology was observed in Fe- and Cu-OMS-2 samples. FeOMS-2(0.09) has a dense globular morphology with size of 100-200 nm, whereas
Cu-OMS-2(0.04) has mixed fibrous and dense globular morphology.
The difference of morphology of OMS-2 and M-OMS-2 dissagree to
difference of relative intensity of the peaks after incorporation of metal ions as
shown by XRD pattern of sample in Figure 4.7 and ratio of I(310)/I(211) plane in
Table 4.5.
It showed that Ti incorporated gave significant higher ratio of
I(310)/I(211) while Fe and Cu incorporated gave lower ratio than OMS-2. Co has
similar ratio with OMS-2. However, the different values are observed in calculation
of ratio of I(200)/I(211) plane of OMS-2 and M-OMS-2 samples which listed in
Table 4.9. The table shows that Ti has ratio of I(200)/I(211) plane very close to
OMS-2. Conversely, ratio of I(200)/I(211) plane of Co-OMS-2 was significantly
lower than ratio of OMS-2. Whereas ratio of I(200)/I(211) plane of Fe-OMS-2 and
Cu-OMS-2 be in agreement to ratio of I(310)/I(211) plane which significantly lower
than the ratio of OMS-2. This could be due to the very different morphology
observed in both samples which both (200) and (310) plane relatively decreases
compare to (211) plane. However, it could concluded that there are differences in
ratio of intensity of the peaks of samples after incorporation of metal ions which
indicated that difference of preferred orientations in OMS-2 crystal after
incorporation of metal ions. Moreover, the exactly preferred plane which count their
different morphology could not defined.
101
OMS-2
Ti-OMS-2(0.05)
Cu-OMS-2(0.04)
Fe-OMS-2(0.09)
Co-OMS-2(0.02)
Figure 4.10: Morphology of OMS-2 and M-OMS-2 samples.
102
Table 4.9: The relative intensity and ratio of I(200)/I(211) plane of OMS-2 and MOMS-2 samples calculated by XRD.
4.3
Samples
200
211
I(200)/I(211)
OMS-2
88
100
0.88
Ti-OMS-2(0.05)
81
93
0.87
Fe-OMS-2(0.09)
54
100
0.54
Co-OMS-2(0.02)
76
100
0.76
Cu-OMS-2(0.04)
58
100
0.58
Physical Properties of H-OMS-2 and H-M-OMS-2 Materials
Hydrogen form of OMS-2 and M-OMS-2 (H-OMS-2 and H-M-OMS-2,
respectively) were prepared by ion exchange of potassium ion in the tunnel structure
with H+ ions using concentrated HNO3 at 60 oC. This is a short and simple method
to give a higher amount of substituted H+ compared to using aqueous HNO3. The
synthesis of H-OMS-2 and H-M-OMS-2 are schematically shown in Figure 4.11.
Figure 4.11: Schematic synthesis of H-OMS-2 and H-M-OMS-2.
103
The ion exchange was repeated twice in order to maximise the amount of K+ that
may be exchanged by H+ ion.
The XRD pattern of OMS-2 and H-OMS-2 are shown in Figure 4.12. These
XRD patterns of OMS-2 and H-OMS-2 are similar indicating that the catalyst retains
its framework structure after even successive ion exchange with H+. However, there
are differences in the XRD pattern of OMS-2 and H-OMS-2 in terns of the peaks
intensity, relative peaks height and the peaks width of the sample, reflecting the
changes in the crystallinity, preferred orientation and crystallite size of the
cryptomelane structure of OMS-2 after ion exchange.
The amount of K+ that was exchanged by H+ was calculated by EDAX and is
given in Table 4.10.
It is observed that after ion exchange, the percentage of
potassium substituted by H+ on H-OMS-2 is about 44%. This amount is less than
50% of that reported by Kumar et al. [70]. The percentage of potassium substituted
by H+ on M-OMS-2 varies. The highest percentage is observed in H-Ti-OMS2(0.67) sample and the lowest in H-Fe-OMS-2(0.09) sample. The highest potassium
exchanged by H+ is due to the small amount of K+ in Ti-OMS-2 (0.67) sample (see
Table 4.8).
Figure 4.12: XRD pattern of (a) OMS-2 and (b) H-OMS-2.
104
Table 4.10: The percentage of potassium substituted by H+.
Samples
%K was exchanged by H+ (a)
H-OMS-2
44
H-Ti-OMS-2(0.67)
93
H-Fe-OMS-2(0.09)
12
H-Co-OMS-2(0.04)
37
H-Cu-OMS-2(0.02)
(a)
+
62
+
%K exchanged with H was calculated based on the decrease
in the wt% of potassium in EDAX data assuming that the amount
of Mn does not change after the ion-exchange.
The morphology of H-OMS-2, H-Ti-OMS-2(0.67), H-Fe-OMS-2(0.09), HCo-OMS-2(0.04) and H-Cu-OMS-2(0.02) samples are depicted in Figure 4.13.
FESEM micrograph of H-OMS-2 and H-Co-OMS-2 reveal a fibrous needle-like
morphology; similar to the morphology of coresponding parent samples (before ion
exchanged). H-Fe-OMS-2(0.09) keeps a dense globular morphology of Fe-OMS2(0.09) and H-Cu-OMS-2(0.02) also has similar morphology to its unexchanged
sample [Cu-OMS-2(0.02)]. These show that metal substituted samples, prepared by
mixing of metals ion with manganese ion have similar morphology to their ionexchanged samples indicated that the tunnel cation substitution has no effect on the
morphology of those samples. This is in agreement with the previous report by
Kumar et al. [70]. The particle sizes of all H-M-OMS-2 samples are relatively
similar to its M-OMS-2 samples. However, the difference was observed on the
morphology of H-Ti-OMS-2(0.67) which shows dense globular morphology similar
to Fe-OMS-2. Its morphology changed after ion-exchanged by H+ ions.
105
H-OMS-2
H-Ti-OMS-2(0.67)
H-Fe-OMS-2(0.09)
H-Co-OMS-2(0.04)
H-Cu-OMS-2(0.02)
Figure 4.13: Morphology of H-OMS-2 and H-M-OMS-2 samples.
106
4.4 Physicochemical Properties of Ti-OMS-2 Materials
This section demonstrates in more detail the physicochemical properties of
Ti-OMS-2 prepared by oxidation of titanium (III) sulfate solution in 15 % sulphuric
acid by potassium permanganate. As a comparison, the extraframework TiO2-OMS2 was prepared by impregnation of titanium to the OMS-2. Physical mixing of
OMS-2 and TiO2 rutile was also prepared. Table 4.12 summarizes the preparation
method, the chemical composition and labeling of titanium containing OMS-2.
4.4.1 Structural Properties of Ti Substituted OMS-2 Catalyst
XRD patterns of OMS-2, Ti-OMS-2 (0.18) and Ti-OMS-2 (0.43) show that
the samples are pure and highly crystalline and match those of cryptomelane Q
[168]; the natural counterpart of OMS-2 material (see Figure 4.14). The results
confirm that OMS-2, Ti-OMS-2 (0.18) and Ti-OMS-2 (0.43) materials consist of the
cryptomelane structure: 2 x 2 tunnels with a pore size of 4.6 Å, composed of double
chains of edge-sharing and corner-sharing MnO6 octahedra [172]. The absence of
other peaks in XRD patterns except the cryptomelane peaks suggests that Ti was
successfully incorporated in the framework of Ti-OMS-2. In order to confirm the
successful incorporation of titanium, the XRD patterns of Ti-OMS-2 was compared
with mechanical mixture of TiO2 (rutile) and OMS-2, in which the mixture (TiO2OMS-2 (mix)) shows the presence of rutile phase (see Figure 4.13 and Table 4.11).
If Ti were successfully incorporated in the framework of OMS-2, one
expects that the bigger is the substituted atom, the bigger is the unit cell volume.
Calculations of the unit cell volume of OMS-2, Ti-OMS-2 (0.18) and Ti-OMS-2
(0.43) as listed in Table 4.12 show that the unit cell volumes increase by the
incorporation of Ti in the framework of OMS-2. Ti-OMS-2 (0.43) which has higher
titanium content than Ti-OMS-2 (0.18), also has bigger cell volume than Ti-OMS-2
(0.18). This indicates that the amount of manganese in the framework structure
which was substituted by titanium also increases. The lattice enlargement is due to
0.18
0.43
0.67
0.18
0.67
Ti-OMS-2
(0.18)
Ti-OMS-2
(0.43)
Ti-OMS-2
(0.67)
TiO2-OMS-2
(imp) c
TiO2-OMS-2
(mix) d
552.2
552.0
342.0
420.4
369.9
99.4
228.2
181.4
74.4
0.0
Molar amount
of Tib
mechanical mixing
impregnation
direct synthesis
direct synthesis
direct synthesis
-
Methods of
introduction of Ti
species
non-framework
non-framework
framework and
non-framework
framework
framework
-
Location of Ti
species
152
-
rutile
amorphous
rutile
b
n.d. e
n.d. e
152
149
155
-
-
Surface area /
m2g-1
Structure of
nonframework Ti
species
Analysis was carried out by Atomic absorption spectrometer.
The amount of OMS-2, Ti-OMS-2 and TiO2–OMS-2 was 50 mg.
c
Titanium(IV) tetra-2-propoxide (Ti(OPri4) was impregnated from its toluene solution into OMS-2 powder and calcined at 773 K for 3h.
d
Catalyst was prepared by addition of calculated amount of Ti from TiO2 powder to OMS-2.
e
Not determined.
a
552.2
0.00
OMS-2
412.5
Molar amount
of Mnb
Molar
ratio of
Ti/Mna
Samples
Table 4.11 : Chemical composition and physicochemical properties of OMS-2, Ti-OMS-2 and TiO2–OMS-2.
107
108
* = TiO 2 (rutile)
*
*
(g)
*
*
(f)
(e)
*
*
*
Relative intensity / a.u.
*
(d)
(c)
(b)
(a)
10
20
30
40
50
60
70
2T/ o Figure 4.14: X-ray diffractograms of (a) cryptomelane (JCPDS 29, 102), (b) OMS-2,
(c) Ti-OMS-2 (0.18), (d) Ti-OMS-2 (0.43), (e) Ti-OMS-2 (0.67), (f) TiO2-OMS-2
(imp) and (g) Ti-OMS-2 (mix).
109
the replacement of the smaller Mn4+ ions (ionic radius is 0.53Å) by the relatively
larger Ti4+ ions (ionic radius is 0.61 Å). It strongly indicates that isomorphous
substitution of Mn atoms by Ti into the framework of Ti-OMS-2 has occured.
Moreover, it is found that the incorporation of titanium caused a decrease to the full
width at half maximum, indicating an increase in the grain size.
It is observed that the peaks of rutile phase for TiO2 appeared at the ratio of
Ti:Mn higher than ca. 0.5. In contrast, it is not observed at the ratio of Ti:Mn less
than ca. 0.5 (see Figure 4.14). By considering the upper limit of the titanium that
could be incorporated into the framework, one would expect non-framework titanium
species to be formed when the Ti:Mn ratio reached 0.5. The amount of Ti located
in non-framework is 25% in Ti-OMS (0.67). This argument is supported by the
presence of rutile phase of TiO2 in Ti-OMS-2 (0.67) (see Figure 4.14). However, no
reflection for rutile phase of TiO2 was observed in TiO2-OMS-2 (imp) where the
catalyst was prepared by impregnation method. This result implies that the structure
of TiO2 in TiO2-OMS-2 (imp) is in the amorphous form.
Table 4.12: The lattice parameters (a and c) and cell volume (V) of OMS-2 and TiOMS-2 samples.
Samples
a (Å)
c (Å)
V (Å3)
OMS-2
9.74
2.86
271
Ti-OMS-2(0.18)
9.84
2.84
275
Ti-OMS-2(0.43)
9.83
2.87
277
Figures 4.15 and 4.16 show the FTIR spectra of the samples. The lower
wavenumber region the five bands between 400 cm-1 - 800 cm-1 are assigned to MnO and Ti-O vibrations that are clearly seen (Figure 4.15) and the details of these
peaks are summarised in Table 4.13. The characteristic cryptomelane peaks at 464,
522, 600 and 714 cm-1 are similar in OMS-2 and Ti-OMS-2(0.18) samples, which is
in agreement with XRD analysis, indicating that both samples have pure
cryptomelane structure. However, the intensity of vibration bands due to Mn-O
lattice at OMS-2 is stronger than that Ti-OMS-2(0.18); indicating that titanium was
110
successfully incorporated into the framework structure of OMS-2. Figure 4.15 also
shows that bands of Ti-OMS-2 (0.67) are significantly shifted to higher wavenumber
and its intensity become lower than that of Ti-OMS-2(0.18) and OMS-2. The new
band observed at around of 581 cm-1 is due to stretching of Ti-O-Ti [173] indicating
the existence of TiO2 peak in Ti-OMS-2 (0.67) which agrees with XRD data.
1100
600
714
464
522
Transmittance / a.u.
(c)
(b)
(a)
1200
1100
1000
900
800
700
600
500
400
Wavenumber / cm-1
Figure 4.15 : IR spectra at lower wavelength region of (a) OMS-2, (b) Ti-OMS-2
(0.18), (c) Ti-OMS-2 (0.67).
The band around 714 cm1 which is assigned to the characteristic band of
manganese oxides with tunnel structure was also observed. It was observed that in
Ti-OMS-2(0.67), the corresponding band diminished indicating that there was
111
Table 4.13: Vibrational spectroscopy feature of samples.
Wavelength (cm-1)
Assignment
464
(O-Mn-O)
522
(Mn-O)
581
(O-Ti-O)
714
(Mn-O)
1600
(c)
3400
Transmittance / a.u.
(b)
(a)
4000
3000
2000
1500
1200
Wavenumber / cm-1
Figure 4.16: IR spectra at higher wavelength region of (a) OMS-2, (b) Ti-OMS-2
(0.18), (c) Ti-OMS-2 (0.67).
112
change in the tunnel structure of OMS-2 due to insertion of the titanium rutile phase.
The bands at 1000- and 1150 cm1 were also observed for all samples, which were
assigned to the vibration of the Mn3+-O bond [174]. This suggests that the oxidation
state of Mn in the structural framework is not only four but also include Mn3+
occupying the Mn4+ sites in the crystal of the tunnel structure.
Figure 4.16 shows the FTIR spectra of OMS-2 and Ti-OMS-2 at high
wavenumber. The bands at 3400 cm-1 and 1600 cm-1 are due to vibration and
stretching OH, respectively, designated to the presence of OH groups that probably
belong to the M-OH as well as to the absorbed water in the sample. It shows that by
incorporating titanium, the band at 3400 cm-1 and 1600 cm-1 are stronger than the
original OMS-2 which indicates that more hydroxyl groups are in the titanium
incorporated sample and are responsible to the surface of being more hydrophilic.
The photoluminescence (PL) was also used to confirm the absence of nonframework TiO2, since non-framework TiOx with very small crystallite size cannot
be detected by XRD. The PL spectra are useful to disclose the efficiency of charge
carrier trapping, migration and transfer, and to understand the nature of electron-hole
pairs in TiO2 semiconductor particles since PL emission results from the
recombination of photo-excited free carriers [175]. In this study, the 430 nm excited
PL spectra of all powder samples pressed at room temperature were examined in the
range of 560–680 nm. The PL spectra of OMS-2, Ti-OMS-2(0.43) and TiO2-OMS-2
(mix) are shown in Figure 4.17. The results indicate that the photoluminescence
intensity of TiO2-OMS-2 (mix) (em(max) ~600 ± 10 nm; FWHM ~40 nm) was
substantially higher than that of OMS-2 and Ti-OMS-2 (0.43) (see Figure 4.17). The
relative intensity of Ti-OMS-2 (0.43) and TiO2-OMS-2 (mix) is similar suggesting
that there is no TiOx particle in Ti-OMS-2 (0.43) sample. This result further supports
our suggestion that Ti was incorporated in the framework of Ti-OMS-2.
The
incorporation of Ti in the framework of OMS-2 was further supported by the surface
area analysis (see Table 4.12), which revealed that the surface area of OMS-2 and TiOMS-2 is almost the same.
113
Ti-OMS-2 (0.43)
Intensity / a.u.
TiO2 -OMS-2 (mix)
OMS-2
560
580
600
620
640
660
680
Wavelength / nm
Figure 4.17: Photoluminescence spectra of OMS-2, Ti-OMS-2 (0.43) and TiO2OMS-2 (mix). The excitation wavelength is 430 nm.
4.4.2
Acidity Properties
IR spectrum of acidity study by pyridine adsorption after evacuation under
vacuum at 400 oC and 150 oC are shown in Figure 4.18. The figure shows that Lewis
acid sites are formed in Ti-OMS-2 (0.67) as indicated by the appearance of peaks at
1447 cm-1, 1489 cm-1 and 1604 cm-1. In contrast, no peaks are observed for OMS-2
sample in Figure 4.18(b). The absence of peaks at 1540 cm-1 confirms that there are
no Brønsted acid sites in both samples. The data indicates that the insertion of Ti
into the framework of OMS-2 created Lewis acids in the sample.
114
1447
1604
(a)
Absorbance / a.u.
1489
(b)
1700.0
1600.0
1500.0
Wavenumber / cm-1
1400.0
1300.0
Figure 4.18: FTIR spectra of (a) Ti-OMS-2 (0.67) and (b) OMS-2 after evacuation
under vacuum at 400 oC for 4 h followed by pyridine adsorption at room temperature
and evacuation at 150 oC for an hour.
4.4.3
Morphology, Surface Area and Textural Properties
Figure 4.19 reveals the morphology of Ti-OMS-2 (0.18), Ti-OMS-2 (0.67),
and TiO2-OMS-2 (imp) materials.
As shown by Figure 4.19 (a) and (b), the
incorporation of Ti changes the fibrous morphology of OMS-2 (see Figure 4.10) to
spherical morphology and its particle size was reduced. The particle sizes are in the
range of 50 - 80 nm in Ti-OMS-2 (0.18). The particle sizes of Ti-OMS-2 (0.67) are
bigger than Ti-OMS-2 (0.18) which are in the range of 120 - 200 nm. The difference
in morphology of OMS-2 and Ti-OMS-2 are due to preferred orientation of Ti-OMS2. The Ti-OMS-2 materials tend to lie on their (110) plane, as revealed by the XRD
data (see Table 4.14). The table shows the relative intensity and the ratio of (110)
planes to (211) planes of OMS-2, Ti-OMS-2 (0.18) and Ti-OMS-2 (0.67). The ratio
of I(110)/I(211) of Ti-OMS-2 that was higher than OMS-2 indicates that Ti-OMS-2
is preferred in (110) plane than OMS-2. The higher ratio of I(110)/I(211) of Ti-
115
(a)
(b)
(c)
Figure 4.19: Morphology of (a) Ti-OMS-2 (0.18), (b) Ti-OMS-2 (0.67) and (c)
TiO2-OMS-2 (imp).
OMS-2(0.18) than Ti-OMS-2(0.67) explaines the difference of the length of particle
of both materials {see Figure 4.19 (a) and (b)}. Thus, the agglomeration of TiO2
rutile particle as identified by XRD is also observed in Ti-OMS-2(0.67).
The
preferred orientation was not observed in titanium impregnated materials. As shown
by Figure 4.19 (c) the impregnation of Ti did not alter the fibrous morphology of
OMS-2 but only showed that titania particle was finely dispersed on the fibrous
morphology of OMS-2 materials.
116
Table 4.14: The relative intensity and ratio of I(111)/I(211) plane of samples
calculated by XRD.
Relative Intensity of (hkl)
I(110)/I(211)
(110)
(211)
OMS-2
56
100
0.56
Ti-OMS-2 (0.18)
88
100
0.88
Ti-OMS-2 (0.67)
84
100
0.84
The N2 adsorption isotherm of OMS-2 is shown in Figure 4.20. Type II
adsorption isotherms with micropore filling at low P/Po and capillary condensation at
high P/Po are observed on OMS-2 sample. At low P/Po, micropore filling of Type I
adsorption isotherm occurs, indicating the existence of micropores in OMS-2. A
hysteresis loop type H3 occurs at high P/Po close to saturation, suggesting the
600
adsorption
500
desorption
Volume / cc g-1
400
300
200
100
0
0.0
0.2
0.4
0.6
0.8
P/Po
Figure 4.20: N2 adsorption isotherm for OMS-2a at 77 K.
1.0
117
existence of slit-shaped mesopores with non-uniform sizes or shapes in OMS-2
[141]. In general the loop closes at P/Po0.8 indicating mesoporosity but probably
due to pores that are generated by the spaces between the nanofibers of OMS-2
materials.
The N2 adsorption isotherms of Ti-OMS-2(0.43) and Ti-OMS-2(0.67) are
shown in Figure 4.21 (a) and (b). Adsorption isotherms of both Ti-OMS-2 samples
are different from OMS-2. The type IV adsorption isotherm was observed in TiOMS-2 with micropore filling at low P/Po.
Similar to OMS-2, at low P/Po,
micropore filling of Type I adsorption isotherm occurs, indicating Ti-OMS-2 has
micropores. However, the hysteresis loop type H2 observed in Ti-OMS-2 indicates
the presence of agglomerates of spheroid particles with nonuniform sizes or shapes
[141].
The surface area of samples are shown in Table 4.11. It shows that surface
area of Ti-OMS-2 sample close to OMS-2 sample indicated that incorporation of Ti
not significantly changes the surface area of OMS-2.
118
210
180
Volume / ccg-1
150
120
90
60
30
0
0.0
0.2
0.4
0.6
0.8
1.0
0.60
0.80
1.00
P/Po
200
180
160
Volume / cc g-1
140
adsorption
desorption
120
100
80
60
40
20
0
0.00
0.20
0.40
P/Po
Figure 4.21: N2 adsorption isotherm for (a) Ti-OMS-2 (0.43) and (b) Ti-OMS-2
(0.67) at 77 K.
119
4.4.4
Thermal Stability
Figure 4.22a and 4.22b represent TGA profiles in original and differential
forms recorded from OMS-2, Ti-OMS-2 (0.18) and TiO2-OMS-2 (imp). The thermal
stability of the material changed after the incorporation of titanium. First weight loss
occurred in the range temperature of 30–200 ºC which is attributed to the elimination
of adsorbed water molecules from the samples. Ti-OMS-2 (0.18) has the highest
weight loss (about 4.5%) in this stage indicating that more adsorbed water was
present on surface, implying more hydrophilic properties of the sample. This is
followed by OMS-2 and TiO2-OMS-2 (imp) with their weight losses of about 2.4%
and 1.2%, respectively.
The weight loss at temperature range of 200-350 ºC due to the evolution of
oxygen from the lattice was observed in OMS-2 and Ti-OMS-2 (0.18) samples. The
weight loss of OMS-2 and Ti-OMS-2 (0.18) samples are 2.0 and 2.5 %, respectively.
It shows that the start point in OMS-2 sample was at lower temperature compared to
Ti-OMS-2 (0.18), indicating that incorporation of titanium caused evolution of
oxygen from the lattice more difficult. However, such weight loss was not clearly
observed in TiO2-OMS-2(imp) sample indicating that oxygen evolution from the
lattice was blocked by impreganation of titanium on the surface of OMS-2 sample.
Figure 4.22b also shows significant weight loss of 9.4% from OMS-2 was
observed in the temperature range of 520-800 ºC and contributed to the collapse of
OMS-2 structure. Weight loss of Ti-OMS-2(0.18) and TiO2-OMS-2(imp) in this
stage are 7.2% and 6.6%, respectively. It shows that in both samples, the start point
shifts to higher temperature indicating that the thermal stability of samples increased
either upon incorporation or impregnation of titanium. OMS-2 lost its weight at two
120
Figure 4.22: TGA profile (a) in original and (b) differential forms of OMS-2, TiOMS-2 (0.18) and TiO2-OMS-2(imp).
121
parts i.e. at around 585 and 780 ºC due to the decomposition of this material into
stable lower valent manganese oxides, Mn3O4 (hausmannite) and Mn2O3 (bixbyite),
respectively [30]. Ti-OMS-2(0.18) has about three parts of weight loss in this stage
which indicates that the sample decomposed into three phases. The additional phase
could be MnTiO3 (phyrophanite) which is also observed in Mn-TiO2 [176]. The
addition of decomposition process suggests the existence of titanium incorporated in
the framework since the presence of extraframework titanium in TiO2-OMS-2 (imp)
sample had no additional weight loss. The dispersion of TiO2 on TiO2-OMS-2 (imp)
also caused the slight increase of the thermal stability of OMS-2 in first
decomposition to hausmannite and significant increase in decomposition to bixbyite
but no other weight loss was observed.
4.4.5
More Evidence of the Location of Titanium on Ti-OMS-2 Materials
XPS analyses were carried out in order to obtain information on the local
chemical environments of manganese and titanium in cryptomelane from the
variations in binding energies or chemical shifts of the photoelectron lines. Because
chemical shifts are very uniform among the photoelectron lines of an element, line
separations rarely vary by more than 0.2 eV [36]. A change in the separation of
photoelectron lines suggests that a change in the local environment may has occured.
Line separation of binding energy of Mn 2p and Ti 2p and its differences between
selected samples are listed in Table 4.15. Mn 2p XPS spectra of OMS-2, Ti-OMS-2
(0.18), Ti-OMS-2 (0.67) and TiO2-OMS-2 (imp) are shown in Figure 4.24.
The binding energies for Mn 2p1/2 and Mn 2p3/2 peaks of all samples were
about 654 and 642 eV, which are very close to those of MnO2 Mn 2p peaks. The
line separation for Mn 2p1/2 and Mn 2p3/2 peaks BE of OMS-2, Ti-OMS-2 (0.18),
Ti-OMS-2 (0.67) and TiO2-OMS-2 (imp) were 11.83, 11.82, 11.91 and 11.70 eV,
respectively. Difference of line separation (BE) of Ti-OMS-2 (0.18) and BE of
OMS-2 was 0.01 eV, which very close to each other indicating that there is no
653.73
653.88
654.05
Ti-OMS2(0.18)
Ti-OMS2(0.67)
TiO2-OMS2(imp)
642.35
641.97
641.91
641.75
BE Mn
2p3/2
(eV)
0.01
0.08
0.13
11.91
11.70
Difference of
BE Mn 2p of X
and OMS-2 (eV)
11.82
11.83
BE Mn
2p (eV)
464.01
463.56
462.89
BE Ti 2p1/2
(eV)
X refers to Ti-OMS-2(0.18), Ti-OMS-2(0.67) and TiO2-OMS-2(imp), respectively.
Y refers to Ti-OMS-2(0.67) and TiO2-OMS-2(imp), respectively.
653.58
BE Mn
2p1/2
(eV)
OMS-2
samples.
458.17
457.92
457.53
BE Ti
2p3/2
(eV)
5.84
5.64
5.36
BE Ti
2p (eV)
0.48
0.28
Difference of BE
Ti 2p of Y and TiOMS-2 (0.18) (eV)
Table 4.15: Binding Energies (eV) of Mn 2p, Ti 2p, and its line separation (BE) and difference of line separation from selected
122
123
2p1/2
2p3/2
(d)
Intensity / a.u.
(c)
(b)
(a)
658
656
654
652
650
648
646
644
642
640
638
Binding Energy / eV
Figure 4.23: Detailed XPS spectra for the Mn 2p transition for (a) OMS-2, (b) TiOMS-2 (0.18), (c) Ti-OMS-2 (0.67) and (d) TiO2-OMS-2 (imp).
change of the local environment of OMS-2 after incorporation of titanium. For other
samples i.e Ti-OMS-2 (0.67) and Ti-OMS-2 (imp) the difference of line separation
was also lower than 0.2 indicating that the changing of local environment Mn 2p
could not observed upon the addition of titanium. This is in agreement with results
by Cai et al. [36] which reported that the local environment of Mn 2p of Fe-OMS-2
124
did not show the significant change although the hematite phase was observed in that
sample by using XRD analysis.
However, the binding energies of Mn 2p of OMS-2 are lower than Ti-OMS-2
(O.18) (see Figure 4.23). This confirmed the substitution of some manganese by
titanium in the framework of OMS-2. The assumption is supported by considering
the binding strength on bridging oxygen atom in OMS-2 and Ti-OMS-2 as shown in
Figure 4.24. Structures A, B, C is pure OMS-2, Ti incorporated OMS-2 and TiO2,
respectively. The binding strength of (A) which is lower than (B) indicates that (A)
is less stable than (B), and consequently the binding energy of Mn in pure OMS-2 is
lower than in Ti-OMS-2 sample. This result is supported by TG analysis (section
4.4.4) which showed that incorporation of titanium into the framework of OMS-2
material caused an increase in thermal stability of OMS-2 material.
As shown in Table 4.15 the separation of titanium 2p1/2 and 2p3/2 peaks BE
increased from 5.36 to 5.64 eV (BE = 0.28) when the titanium doping ratio was
increased from 0.18 to 0.67. This indicates that local environment of titanium in TiOMS-2(0.18) is different from Ti-OMS-2(0.67).
The binding energy Ti 2p of
samples increases in the order of: Ti-OMS-2(imp) < Ti-OMS-2(0.67) < TiO2-OMS2(0.18) (See Figure 4.25) due to the difference in the local environment of each
sample. The lowest binding energy of Ti-OMS-2(0.18) caused by incorporation of
titanium in the framework of OMS-2 (Structure B) which has total bond strength
lower than Structure C (see Figure 4.24). The highest binding energy calculated for
TiO2-OMS-2 (imp) is due to the Ti-O-Ti bond (Structure C) that has the highest total
bond strength, thus indicating that extraframework titanium exists in the sample.
The middle of binding energy of Ti-OMS-2(0.67) has a significant difference of BE
(more than 0.2 eV) indicating that it has both Structures B and C (titanium
framework-extraframework exists in the sample) which agrees with the previous
statements.
The oxidation state of the Ti species was also examined by X-ray
photoelectron spectroscopy. Figure 4.25 shows the XPS of the materials in the Ti
2p1/2 and Ti 2p3/2 binding energy (BE) regions. The Ti 2p3/2 peak is centered at
125
457.53-458.17 eV, and the Ti 2p1/2 peak is found at 462.89-464.01 eV, with a spin
energy separation of 5.36-5.84 eV. This is characteristic of Ti4+ [177, 178]. In
particular, the peak position of Ti4+ is significantly influenced by its coordination
environment, and the Ti 2p3/2 XPS peak of the octahedral coordinated Ti4+ generally
locates at lower BE (about 457.5-458.3 eV), while that of tetrahedral coordinated
Ti4+ at higher BE (about 458.5–463.4 eV) [179]. The observed BE Ti 2p3/2 values
approach to those observed for the octahedral coordinated Ti4+, indicating that the Ti
species in Ti-OMS-2 materials are located at the octahedral coordinated
environments.
Configuration
Total bond strength
(A)
1.27
(B)
1.30
(C)
1.33
Figure 4.24: Bond strength on bridging oxygen atom.
126
2p3/2
2p1/2
Intensity / a.u.
(c)
(b)
(a)
468
466
464
462
460
458
456
454
Binding Energy / eV
Figure 4.25: Detailed XPS spectra for the Ti 2p transition for (a) Ti-OMS-2(0.18),
(b) Ti-OMS-2(0.67) and (c) TiO2-OMS-2(imp).
127
4.5
Alkylsilylated of OMS-2 and Ti-OMS-2(0.67)
The effect of modification by alkylsilylation on the surface of catalysts and
their catalytic activity in styrene oxidation studied by attachment of OTS on the
surface of OMS-2 (OTS/OMS-2) and Ti-OMS-2(0.67) (OTS/Ti-OMS-2) is depicted
in Figure 4.26. The peaks at 2920 and 2850 cm-1 are due to anti-symmetric and
symmetric stretching of C-H, respectively. This indicates that OTS was successfully
attached on the surface of the catalysts.
In order to determine the relative hydrophobic and hydrophilic properties of
the samples, water vapour adsorption on the surface of samples was applied. As
shown in Figure 4.27, it is clearly observed that the amount of adsorbed water on TiOMS-2(0.67) was higher than OMS-2. This suggests that the surface properties of
Ti-OMS-2 is more hydrophilic than OMS-2. Contrary to the creation of the more
hydrophobic surface by attachment of OTS on the surface of catalyst [180-182], the
modification of OMS-2 and Ti-OMS-2 surfaces by OTS generate the more
hydrophilic surface.
Actually, attachment of bimodal amphiphilic OTS on the
surface of catalyst increased both hydrophilic and hydrophobic groups on the surface
of catalysts. However, in this case, adsorption of water vapour on the modified
surface was higher than unmodified one (see Figure 4.27). The proposed polymeric
model on the surface is demonstrated in Figure 4.28 [183]. As shown in Figure 4.27,
the higher absorbed water vapour of Ti-OMS-2 than OMS-2 indicates that there are
more OH groups on Ti-OMS-2 than on OMS-2. Higher amount of OH, is preferred
in cross-linked polymeric octadecylsiloxane resulted less OH in alkylsilylated TiOMS-2 sample. OMS-2 surface which has less OH group favoured to be in the form
of isolated polymeric rather than cross-linked octadecylsiloxane after the
alkysililation modification on the OMS-2 surface which could create more OH
groups and adsorb more water vapour.
128
Transmittance / a.u.
(b)
(a)
2850
2920
4000.0
3000
2000
1500
Wavenumber / cm-1
1000
400.0
Figure 4.26: FTIR spectra of modified OTS samples (a) OTS/OMS-2 and (b)
OTS/Ti-OMS-2.
14
12
adsorbed water / %
(d)
10
(c)
8
6
(b)
4
(a)
2
0
0
2
4
Time / h
6
Figure 4.27: Percentage of adsorbed water on the sample (a) OMS-2, (b)
Ti-OMS-2 (0.67), (c) OTS/Ti-OMS-2 (0.67) and (d) OTS/OMS-2.
8
129
Figure 4.28: Proposed polymeric octadecylsiloxane on the surface of OMS-2 and TiOMS-2 samples.
4.6
Sulphated Ti-OMS-2
XRD patterns of Ti-OMS-2 and sulphated Ti-OMS-2 catalysts prepared in
water and toluene as solvent are demonstrated in Figure 4.29. Samples prepared with
150 and 200 μL of H2SO4, using toluene as solvent are referred to ST150-Ti-OMS-2
and ST200-Ti-OMS-2. Similarly, SW150-Ti-OMS-2 and SW200-Ti-OMS-2 refer to
the samples that were prepared with 150 and 200 μL of H2SO4, respectively, but
using water as solvent. From the figure it is observed that ST150-Ti-OMS-2 and
SW150-Ti-OMS-2 samples have similar XRD patterns with Ti-OMS-2 sample and
no new peaks are observed in both samples. This indicates that the sulfate group was
isomorphously attached to cryptomelane structure of Ti-OMS-2 materials due to the
low concentration of sulfate. In contrary, ST200-Ti-OMS-2 and SW200-Ti-OMS-2
samples also have cryptomelane peaks but there are new peaks observed in those
samples. The new peaks matched those of TiOSO4.H2O, MnSO4 7H2O, MnSO4
peaks by XRD database patterns. The peaks intensity of sulphated sample decreased
with the increasing amount of H2SO4 added. It indicates that at higher sulphate
content caused a decrease in crystallinity of the sample. It could be due to phase
transition of OMS-2 at high sulphate content to the other phases such as
TiOSO4.H2O, MnSO4 7H2O and MnSO4 which was observed in Figure 4.29.
130
*
#
*
SW200-Ti-OMS-2
#
¤
¤
ST200-Ti-OMS-2
Intensity / a.u.
#
SW150-Ti-OMS-2
ST150-Ti-OMS-2
Ti-OMS-2
5
10
20
30
40
50
60
70
2T / º
Figure 4.29: XRD pattern of Ti-OMS-2(0.04) and sulphated Ti-OMS-2 (0.04). # =
TiOSO4.H2O,* = MnSO4 7H2O ¤ = MnSO4.
Ti-OMS-2 and sulphated Ti-OMS-2 were studied by IR spectroscopy (Figure
4.30). Sulphated Ti-OMS-2 shows characteristic adsorption bands at 980–1230 cm-1,
which are assigned to the bidentate sulfate coordinated to metal elements [113, 184,
185]. The details of the bands of sulphated samples are summarized in Table 4.16.
The broad bands around 2800–3600 cm-1 may be attributed to surface silanols and
adsorbed water molecules, while deformational vibrations of adsorbed molecules
caused the adsorption bands at 1620 cm-1 [186].
A weak band at 1230 cm-1
corresponding to symmetric vibration of S=O was observed, but the asymmetric
Transmittance / a.u.
131
986
1230
1180 1060
1120
700
4000.0
3000
2000
1500
1000
400.0
Wavenumber / cm-1
Figure 4.30: FTIR spectra of Ti-OMS-2 and sulphated Ti-OMS-2.
Table 4.16: Assignments of as-observed IR bands on sulphated samples [187].
IR bands (cm-1)
assignment
species
2800-3600, br
OH
H-bonded OH and H2O
1620, rw
HOH
H-bonded H2O
1400, s (-)
asS=O
(SO3)ads
1370, s (-)
asS=O
(H2SO4)ads
1322, sh (-)
asS=O
(HSO4)ads
1230, w
s S=O
(H2SO4)ads
1205, sh
S-OH
(HSO4)ads
1186, sh
S-OH
(HSO4)ads
1060, s
sS-O
(H2SO4)ads
980, s
asS-O
(SO3)ads
br = broad, rw = rather weak, s = strong, sh = shoulder, w = weak, (-) the peaks was not observed in
our samples, as = asymmetric, s = symmetric.
132
vibration of S=O between 1320-1400 cm-1 [187-190] was absent in our sulphated
samples. The S-OH bending vibration expected near 1180 cm-1 on sulphated
samples was observed due to the strong hydrogen bonding to the surface [187]. The
bands appearing at 1060 and 980 cm-1 are characteristic of S–O symmetric and
asymmetric vibrations, respectively [187, 191, 192]. All bands are clearly observed
in all sulphated samples except SW150-Ti-OMS-2 which may be caused by the
lower sulfate content in this sample. As seen in Figure 4.30 the bands of ST samples
are more intense than SW sample which indicates that the amount of sulfate
anchored in sample ST is higher than those of SW. The bands around 700 cm-1
which correspond to the tunnel structures of OMS-2 materials are decrease after
sulfation with 200 mL H2SO4, due to interference of the other phases in the tunnel.
Based on the FTIR data, the proposed structure of sulphated titanium in TiOMS-2 present in the bridging of bidentated structure is represented in Figure 4.31
[113]. According to this model, the strength of Lewis acid sites Ti4+ is increased by
the inductive effect of the sulfate groups in the complex.
The model also
demonstrates that it is possible to convert Lewis acid sites into Brønsted acid sites by
water adsorption [193].
Figure 4.31: The bridging of bidentated structure of sulphated Ti-OMS-2.
In Figures 4.32 and 4.33 the thermograms of TG-DTA analysis and the first
derivative curves, respectively can be seen for Ti-OMS-2 and sulphated Ti-OMS-2.
Below 200 ºC a loss of mass can be seen due to the removal of water (in hydration or
structural). In the temperature range of 280–340 °C, the oxidative decomposition of
residual organic compounds occured. A weight decrease at 625–900 ºC was caused
by the decomposition of sulfate on the surface to form SO2 [192]. The loss of sulfate
in SW sample occurred at a lower temperature (a peak in the first derivative curve at
133
660 ºC) than in ST sample (peak at >800 ºC). Peaks at 600 and 820 ºC are due to
decomposition of Ti-OMS-2 materials, also observed for Ti-OMS-2 sample.
However, no peak at around 820 was observed for SW sample may be caused the
decomposition of Ti-OMS-2 in SW sample occurs at higher temperature.
95
Ti-OMS-2
Weight / %
SW150-Ti-OMS-2
ST150-Ti-OMS-2
90
85
80
100
200
300
400
500
600
700
800
Temperature / ºC
Figure 4.32: Thermograms (TGA) of samples.
Ti-OMS-2
SW150-Ti-OMS-2
ST150-Ti-OMS-2
mgmin-1
0.2
0.0
-0.2
-0.2
10
20
30
40
50
60
500
600
80
-0.4
100
200
300
400
700
Temperature / ºC
Figure 4.33: First derivative curves (DTA) of samples.
800
min
134
The effect of sulfation on morphology of sample was also observed by FESEM. The
micrograph of FESEM of SW150-Ti-OMS-2(0.67) is shown Figure 4.34. It shows
that agglomeration occurred on the sample after sulphation. The morphology of
sample became globular with particle size of about 100 nm.
The morphology is
similar to the image observed for Ti-OMS-2 (0.67) after ion exchange by H+ {H-TiOMS-2(0.67) sample}.
Figure 4.34: FESEM micrograph of SW150-Ti-OMS-2(0.67).
CHAPTER 5
CATALYTIC ACTIVITY OF OMS-2 AND MODIFIED OMS-2 SAMPLE
IN OXIDATION AND ACID REACTIONS
5.1
Introduction
This chapter describes the relationship between catalytic behaviour and their
physicochemical properties of OMS-2 and modified sample OMS-2 in oxidation and
consecutive oxidative-acidic reactions. The reactions were carried out in oxidation
of alcohol, alkanes and alkenes and also consecutive oxidation and acid reactions for
direct transformation of alkenes to diols.
5.2
Catalytic Activity and Selectivity of OMS-2 and Modified OMS-2
Samples in Oxidation Reactions
5.2.1
Oxidation of Benzyl Alcohol over OMS-2 Prepared by Different Method
Oxidation of benzyl alcohol was used to study the effect of various
preparation of OMS-2 to their catalytic activity on oxidation of benzyl alcohol. In
this work, OMS-2 samples were prepared in acidic conditions.
However, the
different synthetic methods in acidic condition i.e. with and without buffer solution
was used to study the effect of different methods in the oxidation of benzyl alcohol in
order to obtained the best performance of OMS-2 material. The reaction of benzyl
136
alcohol to benzaldehyde was carried out at 110 oC with toluene and air as the solvent
and oxidant, respectively, as shown in Figure 5.1. The selection of acidic method to
preparation of OMS-2 based on report by Makwana et al. [23]. They studied the
catalytic activity of prepared OMS-2 in oxidation of alcohol in different preparation
conditions namely acidic, neutral and basic.
They found that prepared OMS-2
material in acidic conditions is the most active in oxidation of alcohol.
OH
O
Catalyst
Toluene, 110 oC in air
benzyl alcohol
benzaldehyde
Figure 5.1: The schematic reaction of benzyl alcohol to benzaldehyde.
Table 5.1 shows conversion and selectivity of OMS-2a, OMS-2b catalysts
and blank reaction. The blank reaction is reaction that was carried out without
catalyst where all reaction conditions are keeping the same as by using catalyst. No
product was observed using blank reaction as expected. The table also shows that
OMS-2a gave a significantly higher conversion (90%) compared to only 50% for
OMS-2b. Both catalysts gave 100% selectivity, indicating that the catalysts were
selective to benzaldehyde. There is no further oxidized product such as benzoic acid
was observed in the reaction, as normally observed in the other systems via radical
Table 5.1: Conversion of benzyl alcohol and selectivity to benzaldehyde by different
catalysts.
Catalysts
Conversion / %
Selectivity / %
Blank
0
0
OMS-2a
90
100
OMS-2b
50
100
OTS/OMS-2b
34
100
chain reaction.
The selective product may be due to the Mars van Krevelen
mechanism pathway of the reaction of benzyl alcohol with molecular oxygen on
OMS-2 catalyst.
137
Most of oxidation processes with oxygen as the oxidant were based on the
radical chain autooxidation reaction. Unstable peroxide by the reaction of dioxygen
with free radicals results in low selectivity of the products was formed. Another
catalytic oxygen activation procedure is to use a metal catalyst and an oxygen donor
to form an oxometal or a peroxometal intermediate. In the peroxometal route, the
oxidation state of the metal ion remains unchanged and the metal ion simply acts as
Lewis acid, increasing the oxidizing power of the peroxo group. The oxometal
pathway, on the other hand, involves a two-electron reduction of the metal ion,
which is subsequently reoxidized by the oxygen donor. The latter pathway is known
as the Mars–van Krevelen mechanism [194] and is more commonly observed in gasphase oxidations than in liquid phase. Investigation on oxidation of alcohol with
molecular oxygen on OMS-2 as catalyst by kinetic and isotope labelling study
proved that reaction occurred via Mars van Krevelen mechanism [24].
The Mars van Krevelen mechanism involves a two-step mechanism; firstly,
oxidation of benzyl alcohol and the metal undergoes a two-electron reduction. Mn4+
which exists in OMS-2 materials acts as an oxidant to oxidize benzyl alcohol to
benzaldehyde via abstraction of the secondary H atom point to an electron-deficient
carbon centre in the intermediate formed in the rate-determining step and itself was
reduced to Mn2+. TPR studies on OMS-2 [195] show a much higher susceptibility to
reduction, as a reduction of Mn4+ is involved. This leads to easy release of oxygen
species from the lattice. The next step is the fast step that is the delocalization of
hydrogen atom in the intermediate formed to produce benzaldehyde. This process is
summarized as:
PhCH2OH + Mn4+
slow
fast
+
PhCHOH + H+ + Mn2+
PhCHO + H+
The second step is the regeneration of catalyst; Mn2+ reoxidation by oxygen
from air. There are three possible ways for an oxygen molecule to exchange its
atoms on an oxide which can be investigated with the oxygen isotopic exchange
reaction [196]:
138
1. R0-mechanism: The oxygen molecule exchanges one of its atoms with
another oxygen molecule from the gas phase, without the participation of
the oxygen of the metal oxides.
18
O2(g) + 16O2(g) 2 18O16O(g)
2. R1-mechanism: The oxygen molecule exchanges one of its atoms with the
surface oxygen of the metal oxide.
18
O2(g) + 16O(s) 18O16O(g) + 18O(s)
3. R2-mechanism: the oxygen molecule exchanges both its atoms with the
surface oxygen of the metal oxide.
18
O2(g) + 2 16O(s) 16O2(g) + 2 18O(s)
The R0 mechanism is a pseudo-Langmuir-Hinshelwood-type mechanism in
which the lattice oxygen is not involved and the exchange takes place between two
adsorbed molecules.
The R1 and R2 mechanisms are Mars–van Krevelen-type
mechanisms, in which one or two oxygens, respectively, from the air are exchanged
with lattice oxygen. The oxygen exchange occurs as:
where
denotes a lattice vacancy and
denotes within the lattice.
The molecular oxygen undergoes a two-electron reduction to form H2O2,
which is known to decompose easily in another catalytic cycle over OMS-2 to form
water as the final product [45]. There is no possibility of H2O2 to act as oxidant for
this reaction.
The data of Doornkamp et al. [196] suggests that much higher
temperatures were required to achieve a state of the catalyst such that the R0
mechanism can prevail. The relatively low temperature employed to carry out the
oxidation (110 ºC) and the oxygen-labelling experiment by Makwana et al. [24]
conclusively pointed to a Mars–van Krevelen mechanism.
Since an oxygen
exchange was involved, the activity of the catalyst was inversely dependent on the
Mn–O bond strength in the OMS-2. The proposed reaction scheme incorporating
Mars–van Krevelen-mechanism is shown in Figure 5.2.
139
TGA data confirmed that oxygen species from the lattice OMS-2a were easier
to release from the lattice than OMS-2b which produced more active OMS-2a than
OMS-2b in the oxidation of benzyl alcohol using air as oxidant. This also suggests
that the strength of Mn-O bond in OMS-2a is weaker than OMS-2b.
Figure 5.2: Overall alcohol oxidation mechanism [24].
The effect of crystallinity on the oxygen release in oxidation of benzyl
alcohol is described by Figure 5.3. The crystalline sample implies the maximising of
the resonance energy [197]. As a consequence, the Mn-O bonds are strengthened
which caused the difficulty of oxygen release from the lattice. This situation is
depicted as Structure A.
In less crystalline structure, the resonance effects are
weaker which induces the easy breakage of Mn-O bonds to release oxygen from the
lattice as shown in Structure B. From the XRD pattern it shows that OMS-2b is
more crystalline than OMS-2a. This supports the fact that OMS-2a is more active
than OMS- 2b due to the easier oxygen lattice release in OMS-2a than OMS-2b
caused by the lower crystallinity of OMS-2a.
The interaction of substrate and catalyst is important in catalysis process.
The easier access of substrate to the surface/active site of the catalyst can affect the
catalytic activity of the catalyst. From FTIR analysis it is proven that OH group of
the surface of OMS-2a is greater than OMS-2b. Since the functional group of benzyl
alcohol is OH as a consequent this site is easier to approach the surface/active site of
140
OMS-2a than OMS-2b which could enhance the catalytic activity of OMS-2a
sample. The effect of more accessibility of benzyl alcohol to the surface is also
observed after attachment of OTS on the surface of OMS-2a material.
The
conversion of benzyl alcohol decreased dramatically from 90% to 34% after
alkylsilylation (see Table 5.1). This could be due to the blocking of active sites after
modification.
Figure 5.3: Resonance model of Mn-O-Mn bond structure: Resonance structure in
crystalline OMS-2 (Structure A); and non-resonance structure in amorphous
materials (Structure B).
5.3.2
Oxidation of Cyclohexane over Metals Substituted and Ion Exchanged
OMS-2
Oxidation of cyclohexane with TBHP under solvent free condition was
carried out by using OMS-2, M-OMS-2 H-OMS-2 and H-M-OMS-2 as catalysts.
The most active OMS-2 which was tested in the oxidation of benzyl alcohol that is
OMS-2a (without buffer) and further labelled for that catalyst was OMS-2 only.
Those catalysts were studied for the effect of metal and H-exchanged on the catalytic
activity of modified OMS-2 in oxidation of cyclohexane. The schematic of the
reaction and its products was detected on those catalysts as depicted in Figure 5.4.
Thus, the use of solventless condition during the reaction is the best choice to
minimize the step and cost of the process.
141
O
OH
+
Catalyst
TBHP, 60 oC
cyclohexane
cyclohexanone
+
cyclohexanol
OH
O
cyclohexyl hydroperoxide
Figure 5.4: Schematic reaction of cyclohexane.
Figure 5.5 demonstrates the conversion and selectivity of OMS-2, M-OMS-2, HOMS-2 and H-M-OMS catalysts in oxidation of cyclohexane. The main products
observed after the reaction; detected by GC and GC-MS are cyclohexanol and
cyclohexanone. Beside those products, the cyclohexyl hydroperoxide as by product
was also observed with low selectivity in the range of 8-11%.
Cyclohexyl
hydroperoxide was an intermediate which was converted to cyclohexanol and
cyclohexanone [160, 198, 199].
Figure 5.5 also shows that in blank reaction
condition only cyclohexyl hydroperoxide was detected as product. The absence of
cyclohexanol and cyclohexanone in blank reaction indicates that catalyst was needed
to decompose cyclohexyl hydroperoxide to cyclohexanol and cyclohexanone. As
shown in Figure 5.5, no overoxidation products such as n-hexanal and adipic acid or
formation of cyclohexene as reported were observed. This indicates that the catalysts
were selective to cyclohexanol and cyclohexanone. The conversion and selectivity
to cyclohexanol and cyclohexanone are in the range of 4.4 - 7.8 and 89 - 92%,
respectively which are higher than that of industrial process of about 4% and 7585%, respectively [154].
Conversion / %
0
1
2
3
4
5
6
7
8
34
54
11
35
55
11
34
57
9
36
56
9
32
61
8
32
57
11
34
57
9
32
59
9
37
55
8
32
60
8
aqueous TBHP (10 mmol), and catalyst (50 mg) under reflux condition.
OMS-2, M-OMS-2 and H-M-OMS-2. All reactions were carried out at 60 ºC for 24 h with cyclohexane (26 mmol), 70%
Figure 5.5: The conversion and product selectivity of oxidation of cyclohexane with tert-butyl hydroperoxide (TBHP) using
100
40
50
10
cyclohexanone
cyclohexanol
cyclohexyl hydroperoxide
Product selectivity (%):
142
143
As shown in Figure 5.5 conversion of cyclohexane on OMS-2 catalyst was
lower than both metal substituted and ion-exchanged catalysts. The figure shows
that the modification of OMS-2 by metal substitution (metals = Ti, Fe, Cu, Co) and
H-exchanged increased the activity of that material in oxidation of cyclohexane. The
conversion of cyclohexane on OMS-2, Ti-OMS-2(0.09), Fe-OMS-2, Cu-OMS-2, CoOMS-2 and Ti-OMS-2(0.67) are 4.4; 5.1; 5.9; 6.1 and 7.0 %, respectively. The
highest conversion of cyclohexane was obtained by Ti-OMS-2(0.67). It suggested
that the presence of non-framework TiO2 in the sample (as observed in XRD
analysis) increases conversion of cyclohexane.
In a series of metal incorporated framework of OMS-2 catalysts, the
conversion of cyclohexane increased in the following order: Ti- < Fe- <Cu- <CoOMS-2.
The increase of ionic radii of metal substituted OMS-2 increased the
conversion of cyclohexane (Figure 5.6). It was expected that larger ionic radii of
metals resulted in turn strain of M-O bond. The strain on the Mn–O bond caused
positively charged metal, increased Lewis acid site which was proven by pyridine
adsorption analysis on Ti-OMS-2 sample. This Lewis active site plays a role in the
oxidation reaction through three general ways as discussed in Section 5.3.
In a series of H-M-OMS-2 catalysts the conversion of cyclohexane increases
by the following order: H-OMS-2 < H-Ti-OMS-2(0.05) < H-Fe-OMS-2(0.09) < HCu-OMS-2(0.02) < H-Co-OMS-2(0.04) < H-Ti-OMS-2(0.67). The similar trend
demonstrated by M-OMS-2 catalysts, confirmed that H exchanged M-OMS-2
materials enhanced the catalytic activity of these catalysts in the oxidation of
cyclohexane. The increase in conversion of cyclohexane after ion exchange is in the
range of 0.5 – 1.1% (see Table 5.2).
144
Conversion of cyclohexane / %
7
Co
Cu
6
Fe
Ti
5
4
0.60
0.65
0.70
0.75
0.80
Ionic radii of metals in framework OMS-2/ Å
Figure 5.6: The relationship of ionic radii of metals substituted OMS-2 to
conversion of cyclohexane.
Table 5.2: The relation of amount of potassium exchanged by H+ with enhancement
of % conversion of cyclohexane on H-M-OMS-2 catalyst.
Catalysts
% K exchanged by H+ Enhancement of % conversion
after ion exchange
OMS-2
44
1.03
Fe-OMS-2(0.09)
12
1.11
Cu-OMS-2(0.02)
62
0.71
Co-OMS-2(0.04)
37
0.64
Ti-OMS-2(0.67)
93
0.56
There are two possibilities of the effect of H-exchanged on OMS-2 and MOMS-2 catalysts. The first possibility is the presence of Brönsted acid in H-OMS-2
catalysts as reported by Kumar et al. [70]. It was reported that Brönsted acid on HOMS-2 played a role in acid-catalyzed condensation of phenylhydroxylamine with
aniline to produce 2-aminodiphenylamine which did not occur on OMS-2 sample.
However, the existence of Brönsted acid site at H-OMS-2 sample was not verified by
145
suitable characterization method such as pyridine adsorption or H-NMR. H-NMR
study on H-OMS-2 and H-Ti-OMS-2 sample did not show the existence of Brönsted
acid site. It was also supported by the fact that there was no correlation between the
amounts of H+ in H-M-OMS-2 with the enhancement of conversion of cyclohexane
after ion exchange as shown in Table 5.2. This indicates that Brönsted acids did not
play a role in the enhancement of conversion of cycylohexane.
Secondly, substitution of potassium by H+ may affect the Mn-O bond
strength in the framework structure of OMS-2 and M-OMS-2 catalysts. Post et al.
[200] reported that tetragonal cryptomelane may distorted to a monoclinic geometry
when the ratio of the lattice Mn ionic radius over the tunnel cationic radius exceeded
a value of 0.48. Since the ionic radius of H+ is much smaller than that of K+ the
cryptomelane lattice is distorted, this in turn strains the Mn–O bonds. The tetragonal
to monoclinic transition was also observed in sample OMS-2 with the lower average
oxidation state (AOS), which indicated that the number of Mn3+ sites was higher
[23]. The larger ionic radius of Mn3+ compared with Mn4+, fit better in a monoclinic
environment. Kumar et al. [70] reported that increasing the amount %H+ in the
tunnel decreased the average oxidation state (AOS) of Mn which indicated increasing
number of Mn3+ sites on H-OMS-2 samples. Based on that, we concluded that the
effect of H-exchanged catalysts on the enhancement of cyclohexane conversion was
due to the reduction of bond strength of Mn-O. The strain on the Mn–O bond made
the oxygen more negatively charged while the metal more positively charged,
causing an increase to the strength of Lewis acid site; thus playing a role to enhance
the catalytic activity of catalysts.
In order to study the distribution of the product during the reaction process,
the sample was analyzed regularly in certain time interval. Figure 5.7 shows the
distribution of products vs time on Ti-OMS-2(0.67) catalyst. It was observed that at
the first hour of reaction the amount of cyclohexyl hydroperoxide was higher
compared to cyclohexanol and cyclohexanone.
The amount of cyclohexyl
hydroperoxide was relatively the same with cyclohexanol and cyclohexanone at 2 h
reaction. After that time the amount of cyclohexyl hydroperoxide was observed to
be lower than others and its amount was relatively constant during reaction. This
146
confirmed
that
cyclohexyl
cyclohexanol/cyclohexanone.
hydroperoxide
was
the
intermediate
for
The amount of cyclohexanol and cyclohexanone
gradually increased during reaction. From Figure 5.7 it is also observed that the
amount of cyclohexanone at the first hour until 7 h of reaction is higher than
cyclohexanol. The amount of cyclohexanone was equalized by cyclohexanol after 8
h of reaction time. At the end of the reaction (24 h), amount of cyclohexanol was
higher than cyclohexanone (see Figure 5.5). The mechanism of the reaction of
cyclohexane with TBHP on Ti-OMS-2 catalysts was proposed based on the
distribution of the products.
2.0
Amount of Product / mmol
cyclohexanone
1.5
cyclohexanol
cyclohexyl hydroperoxide
1.0
0.5
0.0
0
2
4
6
8
Time / h
Figure 5.7: Yield of products vs time on Ti-OMS-2 in the reaction of cyclohexane
with TBHP as oxidant.
Metal-catalyzed oxidation involving alkyl peroxides may proceed either
through a homolytic or heterolytic mechanism. Transition metal salts of Co, Mn, Fe,
Cu, or the metal oxides are normally involved in homolytic cleavage [86]. Sheldon
et al. [48] noted that strong (one-electron) oxidants, e.g., later and/or first row
transition elements such as Cr(VI), Mn(III), Co(III) and Fe(III), favour oxo-metal
pathways and/or homolytic decomposition of TBHP. Due to the variable oxidation
states of Mn (+2, +3, and +4) in OMS-2, it may be possible for the reaction to follow
both homolytic and heterolytic pathway.
However, the possibility of reaction
147
mechanism of oxidation of cyclohexane was studied.
Since the ratio of
cyclohexanone to cyclohexanol in all the catalysts was relatively the same and less
than 1, implying the mechanism reaction on all catalysts was the same.
The distribution of the product vs time was studied for Ti-OMS-2(0.67). The
existence of cyclohexyl hydroperoxide indicated that autooxidation process occurred
in the oxidation of cyclohexane on OMS-2 and modified OMS-2 samples using
TBHP as oxidant [201]. It was also observed in oxidation of cyclohexane on OMS-1
[202] and oxidation of olefins on OMS-2 [28] with TBHP as oxidant. The first step
of this reaction was homolytic reaction of TBHP with catalyst to form radical as
shown in Figure 5.8. The formation of cyclohexyl hydroperoxide was from the
reaction of cyclohexyl radical with oxygen is shown in Figure 5.9. The formation of
cyclohexanone from cyclohexyl hydroperoxide is via heterolytic pathway while the
formation of cyclohexanol is via homolytic pathway [203]. The heterolytic and
homolytic pathway in decomposition of cyclohexyl hydroperoxide are shown in
Figures 5.10 and 5.11, respectively. However, there was a possibility of reaction of 2
cyclohexyl hydroperoxide radical to form 1,4-dicyclohexyltetraoxidane and then
decompose to form cyclohexanol, cyclohexanone and O2 as shown in Figure 5.12.
Based on the product distribution at first 8 h reaction and after 24 h reaction
on Ti-OMS-2(0.67), the mechanism of the reaction is proposed. Cyclohexanone was
higher at beginning of the reaction. However, the higher amount of cyclohexanol
than cyclohexanone after 24 h reaction indicated that the reaction occurred given via
mechanism in Figures 5.9 and Figure 5.10. After 24 h reaction, the amount of
cyclohexanol was higher than cyclohexanone; strongly suggest that the reaction of
cyclohexyl radical with OH radical to form cyclohexanol occurred as shown in
Figure 5.11. The mechanism in Figure 5.12 could not be proven by the distribution
of the products. Since the homolytic and heterolytic pathways occur in the reaction
of cyclohexane on OMS-2 and modified OMS-2, therefore mechanism in Figure 5.12
is possible.
148
OH
O•
O
catalyst
+
•OH
CH•
Figure 5.8: Homolytic pathway to form radical from TBHP over catalyst.
O•
O2
O
CH•
OH
O
+
CH•
Figure 5.9: Formation of cyclohexyl hydroperoxide.
O
O
H
H
catalyst
O + H2O
Figure 5.10: The heterolytic pathway of the formation of cyclohexanone from
cyclohexyl hydroperoxide.
149
OH
catalyst
O
•OH
+
O•
OH +
C•
H
•OH
OH
Figure 5.11: The homolytic pathway of the formation of cyclohexanol from
cyclohexyl hydroperoxide.
O•
O
O•
O
+
H
O
O
O2
+
O
+
O
O
OH
Figure 5.12: The heterolytic pathway of the formation of cyclohexanol and
cyclohexanone from 1,4-dicyclohexyltetraoxidane.
150
5.4.3
Oxidation of Cyclohexene over Ti-OMS-2 Catalyst
Oxidation of cyclohexene with TBHP as oxidant in acetonitrile as solvent
was done over OMS-2, Ti-OMS-2(0.18; 0.67) and TiO2 rutile.
The reaction
performed in order to study the effect of titanium on OMS-2 to catalytic oxidation of
cyclohexene. TiO2 rutile in the rutile phase was used since this phase was observed
as impurity in Ti-OMS-2(0.67). Ti-OMS-2 was chosen for further study of oxidation
reaction because of its performance in oxidation of cyclohexane. Beside that, TiOMS-2 (0.67) material could retain the cryptomelane structure at higher molar ratio
of metal doping to manganese compared to other metal doped. The reaction was
carried out at 70 oC for 2h and the products observed were epoxy cyclohexane, 2
cyclohexen-1-one and 2 cyclohexen-1-ol as shown in Figure 5.13.
O
O
Catalyst, TBHP
+
Acetonitrile, 80 oC
cyclohexene
epoxy cyclohexane
+
2 cyclohexen-1-one
OH
2 cyclohexen-1-ol
Figure 5.13: Reaction condition of cyclohexene and its products.
The conversion of cyclohexene and selectivity towards 2 cyclohexen-1-one,
2 cyclohexen-1-ol and epoxycyclohexane as the reaction products are shown in
Figure 5.14. As demonstrated, the reaction catalyzed by all the catalysts produced
the highest yield of cyclohexanone and their selectivity towards the formation of
products are almost similar to each other except for TiO2 rutile sample. There was
no epoxy cyclohexane observed as product by using TiO2 rutile as the catalyst. From
the figure it is also seen that the conversion of cyclohexene on Ti-OMS-2(0.18) is
151
significantly higher than that of OMS-2 catalyst. It was well evidenced that titanium
incorporated OMS-2 enhanced the catalytic performance of OMS-2 catalyst which
indicates that there is a synergetic effect of Ti and OMS-2 in Ti-OMS-2 catalyst.
However, the most active was Ti-OMS-2(0.67) catalyst, where the effect nonframework titanium active sites on OMS-2 material in the enhancement of its
catalytic activity in the oxidation of cyclohexene were significant.
70
60
Product selectivity (%)
2-cyclohexen-1-one
2 cyclohexen-1-ol
Conversion / %
50
epoxycyclohexane
40
77
68
30
76
20
25
10
18
20
66
34
5
4
7
0
TiO2
OMS-2
Ti-OMS-2
(0.18)
Ti-OMS-2
(0.67)
Figure 5.14: The conversion and product selectivity of oxidation of cyclohexene
with tert-butyl hydroperoxide (TBHP) using TiO2, OMS-2, Ti-OMS-2(0.18), and TiOMS-2(0.67). {All reactions were carried out at 70 ºC for 2 h with cyclohexene (5
mmol), 70% aqueous TBHP (10 mmol), acetonitrile (15 ml) and catalyst (50 mg).
The conversion and the amount of product obtained in blank experimental have been
subtracted}.
The formation of the allylic oxidation products 2-cyclohexene-1-one and 2cyclohexene-1-ol which was more dominant than epoxy cyclohexane shows the
preferential attack of the activated C-H bond over the C=C bond [204]. TBHP as
oxidant promoted the allylic oxidation pathway and epoxidation was minimized.
152
This was also observed under alumina-supported with divalent and trivalent
transition metal ions and complexes [204-206].
5.4.2 Oxidation of Styrene over Different Location of Titanium sites on TiOMS-2 Catalysts
Oxidation of styrene was carried out over different locations of titanium on
OMS-2 material in order to relate it to the catalytic activity of that material. The
catalysts used in this reaction were titanium silicate-1 (TS-1), manganese oxide
octahedral molecular sieve (OMS-2), titanium incorporated OMS-2 material {TiOMS-2(0.18, 0.43, 0.67)}, titanium impregnated OMS-2 material {TiO2-OMS2(imp)} and physical mixture of TiO2 rutile and OMS-2 material {TiO2-OMS2(mix)}. TS-1 was supplied by Nur et al., [207] and used as the reference catalyst.
The location of titanium was incorporated in the framework and/or located on the
surface of Ti-OMS-2 and elucidated by spectroscopic techniques as discussed in
Chapter 4. For each Ti-OMS-2 materials are, the following assumptions are made:
ƒ
Ti-OMS-2(0.18; 0.43): All titanium are incorporated well in the
framework structure of OMS-2.
ƒ
Ti-OMS-2(0.67): Titanium exists in framework and non-framework sites.
ƒ
TiO2-OMS-2(imp): The only sample where non-framework titanium
exists in OMS-2, there is chemical interaction between titanium and
OMS-2.
ƒ
TiO2-OMS-2(mix): A mixture of rutile TiO2 and OMS-2 exist; there is no
chemical interaction between both materials.
The reaction condition of styrene oxidation was similar to oxidation of
cyclohexene. The reaction was carried out at 70 oC in oil bath using acetonitrile and
TBHP as solvent and oxidant, respectively. The products of this reaction are styrene
oxide, phenyl acetaldehyde and benzaldehyde as shown in Figure 5.15.
153
O
Catalyst, TBHP
+
o
acetonitrile, 70 C
styrene oxide
styrene
O
+
O
phenyl acetaldehyde
benzaldehyde
Figure 5.15: Oxidation of styrene and its product on catalysts using TBHP as
oxidant.
Reaction products of oxidation of styrene using TBHP as the oxidant
catalyzed by TiO2 (rutile phase), OMS-2, TiO2-OMS-2 and Ti-OMS-2 were analyzed
by GC. In this study, TS-1 was also used as a reference catalyst. TS-1 (2% of
titanium, mol %) was prepared according to a procedure described earlier [207]. The
selectivity towards benzaldehyde, styrene oxide and phenylacetaldehyde as the
reaction products is shown in Figure 5.16. The reaction catalyzed by all the catalysts
produced the highest yield of benzaldehyde and their selectivity towards the
formation of products are almost similar to each other. The high selectivity (52 %)
towards benzaldehyde over TS-1 was surprising since phenylacetaldehyde and
benzaldehyde were the major products from the oxidation of styrene catalyzed by the
TS-1 zeolite [175], while the speculated product, styrene oxide, was not detected.
Brönsted acid sites originating from framework titanium species catalyzed the
rearrangement of the intermediate, leading to the formation of phenylacetaldehyde
[208]. This argument is in agreement with our oxidation of styrene over TS-1, since
no Brönsted acid sites was detected in our TS-1 [207]. The acidity study by pyridine
adsorption shows that Lewis acid sites were formed in Ti-OMS-2 (0.67). In contrast,
no Lewis acids were observed for OMS-2 sample. The absence of peaks at 1540
cm-1 confirmed that there were no Brönsted acid sites in both OMS-2 and Ti-OMS-2
(0.67) samples (see Section 4.4.2). This is the possible reason why TiO2-OMS-2, TiOMS-2, TiO2 and OMS-2 catalysts are not selective towards phenylacetaldehyde. A
0
10
20
30
40
50
60
Blank
5
12
83
TiO2
76
8
16
Benzaldehyde
Styrene oxide
TS-1
Phenylacetaldehyde
Product selectivity (%):
52
15
33
OMS-2
74
8
18
76
8
Ti -OMS-2
71
9
20
16
Ti/Mn = 0.43
Ti/Mn = 0.18
72
10
18
Ti/Mn = 0.67
68
14
18
Ti/Mn = 0.18
73
OTS/
Ti-OMS-2
70
20
10
Ti/Mn = 0.67
18
9
TiO2 -OMS-2 TiO2 -OMS-2 OTS/
OMS-2
imp
mix
69
16
15
Ti/Mn = 0.67
(15 ml) and catalyst (50 mg) with vigorous stirring.
Ti-OMS-2, OMS-2 and TS-1. All reactions were carried out at 70 C with styrene (5 mmol), 70% aqueous TBHP (10 mmol), acetonitrile
Figure 5.16: The conversion and product selectivity of oxidation styrene with tert-butyl hydroperoxide (TBHP) using TiO2, TiO2-OMS-2,
Conversion / %
70
154
155
high selectivity towards benzaldehyde which may be due to OMS-2 and TiO2,
promoted the carbon–carbon bond cleavage, thus resulting in the formation of
benzaldehyde.
As shown in Figure 5.16, a considerable increase in conversion of styrene
over Ti-OMS-2, OMS-2, TiO2-OMS-2, TiO2 and TS-1 after 3 h of the reactions was
clearly observed when Ti-OMS-2 (0.67) and TiO2–OMS-2 (imp) were used as
catalysts. The increase in oxidation activity of Ti-OMS-2 (0.67) and TiO2–OMS-2
(imp) can be explained on the basis of the presence of non-framework titanium
species.
The superior performance of Ti-OMS-2 (imp) and Ti-OMS-2 (0.67)
strongly suggests the occurrence of synergetic effect of non-framework Ti with
OMS-2, since Ti-OMS-2 (mix), a mechanical mixture of TiO2 and OMS-2, gave
relatively lower conversion of styrene (see Figure 5.16). Leaching was a particular
problem of solid catalysts in liquid phase reaction. The catalysts were recycled three
times. The activities of the recovered and dried Ti-OMS-2, OMS-2 and TiO2-OMS2 showed insignificant change (ca. 3-5%) which correspond to experimental
observations within experimental error in their catalytic activity in the recycling test.
This suggests a good regenerability of the catalysts in the oxidation of styrene with
tert-butylhydroperoxide.
As shown in Figure 5.16 the catalytic activity of OMS-2 increased after
attachment of OTS. It may be caused by access of styrene and TBHP to reach the
surface of that catalyst after modification due to the increasing hydrophobic and
hydrophilic properties of modified OMS-2. However, appropriate amount of OTS
attached to the sample was necessary to enhance the catalytic properties of sample.
In case of OTS/Ti-OMS-2 the decreased activity which may be caused by blocking
the Ti active site after alkylsylilation modification since the amount of OTS attached
to its surface was higher than OMS-2 surface.
156
5.3
The Effect of Lewis Acidity in Catalytic Oxidations
The analysis of acidity of Ti-OMS-2(0.67) proved that the titanium site on
OMS-2 caused increasing Lewis acidity of the catalyst.
Table 5.3 shows the
correlation between Lewis acidity and conversion of substrates in oxidation of
cyclohexane, cyclohexene and styrene with TBHP as oxidant.
Table 5.3: The correlation of Lewis acidity of samples to conversion of cyclohexane,
cyclohexene and styrene.
Catalysts
Lewis
acidity
strengtha
Conversion of
cyclohexane
(%)
Conversion of
cyclohexene
(%)
Conversion of
styrene (%)
Ti-OMS-2
(0.67)
strong
7.4
65
70
OMS-2
weak
4.4
21
52
a
The strength of Lewis acidity is relative to each other.
It shows that Ti-OMS-2(0.67) which have strong Lewis acidity than OMS-2
gave higher conversion in oxidation of cyclohexane, cyclohexene and styrene. It
indicated that Lewis acid played a role in the enhancement of catalytic activity of TiOMS-2 material. This was in agreement with Corma and García [108]; that Lewis
acids can catalyze oxidation reactions. According to them, there are two possible
ways for the Lewis acids to catalyze oxidation reactions which depend on the
substrates i.e abstraction of electron and acid-base adduct. The possible role of the
Lewis acid sites in enhancing the activity of Ti-OMS-2 in oxidation reactions are
summarized in Table 5.4.
It shows that the main products for oxidation of
cyclohexane are C-H bond activation in which Lewis acidity enhances the catalytic
activity via abstraction of electron to form radical in homolytic pathway.
In
cyclohexene there are two ways in which Lewis acid sites play a role in oxidation
reaction that is, electron abstraction and acid-base adduct.
However, the selectivity of cyclohexene as shown in Section 5.4.3 indicates
that the catalyst is more selective toward 2 cyclohexen-1-one and 2 cyclohexe-1-ol
compared to epoxycyclohexane and C-H bond activation is preferred than C=C bond
157
activation. In the oxidation of styrene, Lewis acid sites can only promote acid-base
adduct since only C=C bond can be activated and it has no allylic C-H bond.
Table 5.4: The possible role of Lewis acids of catalyst in oxidation of cyclohexane,
cyclohexene and styrene.
Oxidation
reactions
Products
cyclohexane
Cyclohexanone and C-H
cyclohexanol
Electron abstraction to form
radical
Cyclohexene
2 cyclohexen-1one, 2 cyclohexe1-ol and
epoxycyclohexane
C-H and
C=C
Electron abstraction to form
radical and acid-base adduct
Styrene
Benzaldehyde,
phenyl
acetaldehyde and
styrene oxide
C=C
acid-base adduct
5.4
Site
activation
Role of Lewis acid sites
The Role of Different Location of Ti Sites in Ti-OMS-2 in Oxidation
Reactions
Table 5.5 summarizes the role of location of Ti sites to catalytic activity of
Ti-OMS-2 in oxidation of cyclohexane, cyclohexene and styrene with TBHP as
oxidant. Although OMS-2 itself is active for all reactions, incorporation of titanium
in the framework and non-framework of OMS-2 catalyst evidently affect the
catalytic activity of OMS-2 materials.
For oxidation of cyclohexane and
cyclohexene, both Ti sites in framework and non-framework enhanced the catalytic
activity of OMS-2 catalysts. However, for oxidation of styrene, only titanium nonframework played a role in the enhancement of catalytic activity of Ti-OMS-2. As
mentioned before, the main products of oxidation of cyclohexane and cyclohexene
was C-H bond activation whereas in oxidation of styrene was C=C activation. This
suggested that incorporation of Ti in the framework only enhances the catalytic
158
activity of Ti-OMS-2 for C-H bond activation and not C=C bond activation. In
contrast, Ti sites in non-framework were active for both bond activations.
Table 5.5: The role of Ti sites location in oxidation reaction.
Location of Ti in OMS-2a
Oxidation
reactions
OMS-2
Ti in
framework
Ti in
Ti in nonframework and framework
non-framework
cyclohexane
+
++
+++
not available
cyclohexene
+
++
+++
not available
styrene
+
not active
++
++
a
5.5
The location of Ti in OMS-2 has been elucidated by several spectroscopy
techniques (see Section 4.4)
The relative catalytic activity of: + < ++ < +++
Catalytic Study on Consecutive Reaction of 1-octene to 1,2-octanediol
A model reaction for consecutive oxidation and acid reaction was the
synthesis of 1,2 octanediol from 1-octene. Actually, there are two steps in this
reaction; first is oxidation of 1-octene to 1,2 epoxyoctane using TBHP as oxidant,
second is ring opening of epoxide by hydrolysis due the existence of Brönsted acid in
the reaction mixture to form 1,2 octane diol (see Figure 5.17). Sulphated Ti-OMS2(0.67) samples were synthesized by using different solvent as catalysts. The effect
of the different solvents used in preparation of sample and the amount of sulfate
loading was evaluated in this reaction.
Figure 5.18 shows the yield of epoxyoctane and 1,2-octanediol on Ti-OMS2 (0.67) and sulphated Ti-OMS-2(0.67). Sulphated samples were prepared in water
and toluene in order to study the effect of solvent in preparation of sulfate sample. In
order to study the effect of sulfate content the different amount of concentrated
H2SO4 i.e.150 μL and 200 μL was used. The sample labelled as SW150-Ti-OMS-2,
indicates that sulphated sample was prepared in water with an amount of
159
concentrated H2SO4 (150 μL). Sulphated sample prepared in toluene was labelled
using abbreviation ST. There was no diol detected on Ti-OMS-2(0.67) catalyst.
Diol was observed in all sulphated catalysts. It is known that sulphated metal oxide
increases both Brönsted and Lewis acid sites of the samples.
In contrary, in the
case
Figure 5.17: Consecutive oxidation and acid reaction to form of 1,2 octane diol from
1-octene on sulphated Ti-OMS-2 catalyst.
1,2-epoxyoctane
1,2-octanediol
Yield of products / μmol
450
400
350
300
250
200
150
100
50
~0
0
Figure 5.18: Yield of epoxyoctane and 1,2 octane diol after 24 h reaction.
160
of sulphated Ti-OMS-2 sample, there was no Brönsted acid site detected by H1-NMR
analysis. Since diol was formed in the reaction, it suggested that Brönsted acid may
be in the reaction mixture playing the role in hydrolysis of epoxide to diols. It was
also possible that the existence of Brönsted acid in the reaction mixture was due to
Brönsted acid formed from Lewis acid sites through hydrolysis in the presence of
water [193]. Water existed in the reaction mixture since the reaction used aqueous
TBHP 70% in water as oxidant. The generation of Brönsted acid by Lewis acid (LA)
sites is described in the following equation:
LA + H2O H2O+
---LA H+ + LAOH-, where = vacant sites of Lewis acid.
However, although Ti-OMS-2 itself has Lewis acid site but no diol observed in that
sample could be due to the Lewis acidity of Ti-OMS-2 was low. The Lewis acidity
increases after attachment of sulphate due to the strain in bonding of Ti-O in the
presence of sulphate as described in 4.31.
From Figure 5.18, it is observed that both sulphated Ti-OMS-2 catalysts in
toluene and water with the same amount of sulfate loaded gave relatively the same
amount of both epoxide and diols. This indicates that either water or toluene as
solvent in synthesis of sulphated Ti-OMS-2 gave the same effect to the catalytic
activity of the catalysts. The effect of sulphate content was also shown in Figure
5.13.
The amount of 1,2-octanediol produced by sulphated sample in toluene
(ST150-Ti-OMS-2 compared to ST200-Ti-OMS-2) and in water (SW150-Ti-OMS-2
compared to SW200-Ti-OMS-2) was higher at higher sulphate loading. This could
be due to the increasing amount of Brönsted acids in the reaction mixture; indicated
by the increasing Lewis acid sites on the samples.
CHAPTER 6
SUMMARY AND CONCLUSION
6.1
Summary
Manganese oxide octahedral molecular sieve (OMS-2) and modified OMS2 materials have been successfully synthesized.
OMS-2 synthesized in buffer
solution is higher in crystallinity and hydrophobicity compared to OMS-2 prepared
without buffer solution. TG analysis shows that oxygen lattice of OMS-2 prepared
without buffer is easier to release than OMS-2 prepared with buffer solution.
Metal substituted OMS-2 prepared by oxidation of the mixture of metal and
manganese ion in acidic condition was successfully incorporated to OMS-2. The
capability of metal ion incorporated decreased in the order of Ti>Fe>Co>Cu.
Incorporation of metals did not change the morphology of OMS-2 except for FeOMS-2.
It is also observed that potassium exchanged by H+ retained the
cryptomelane structure of OMS-2 and has similar morphology to their parents.
A new method for synthesis of metal substituted OMS-2 material without
addition of any manganese ion solution is reported. The method gave higher M/Mn
ratio compared to the previous method. Only titanium substituted sample and only
Ti2(SO4)3 as titanium source retained the cryptomelane structure of OMS-2 material.
Incorporation of titanium changes the fibrous morphology of OMS-2 to spherical
with the particle size of about 50-80 nm. Detailed study of Ti-OMS-2 showed that
162
the sample with molar ratio of Ti:Mn of about 0.43 has the pure cryptomelane
structure. This is the highest level of framework substituted OMS-2 material ever
reported. However, with the higher Ti:Mn molar ratio than ca. 0.5, produced rutile
phases of TiO2 growth with cryptomelane phase. Lewis acid sites were observed in
Ti-OMS-2 sample, may be due to the defect of structure after incorporation of
titanium. The location of titanium in the framework was confirmed by XPS. The
alkylsilylation on OMS-2 and Ti-OMS-2 materials did not successfully make the
surface of those materials more hydrophobic. Lastly, sulfation on Ti-OMS-2 was
also not successful in creating the Brönsted acid site in Ti-OMS-2 material.
The physicochemical properties-catalytic activity of modified OMS-2
catalysts in model oxidation reactions and consecutive oxidation-acid reaction are
summarized in Table 6.1. The method of synthesis affects the physicochemical
properties OMS-2 catalyst which also changes the catalytic activity of the catalyst.
OMS-2 prepared in buffer is less active than OMS-2 prepared without buffer.
Modification of OMS-2 material by metals substitution and ion exchange by H+
increase the catalytic activity of OMS-2 material in oxidation of cyclohexane. Both
modifications gave similar effect that is to increase the Lewis acidic sites in OMS-2
which plays a role in abstraction of electron in homolytic pathway mechanism of
oxidation of cyclohexane.
The activity of metal substituted sample (Ti-OMS-2) is maximum at the
highest Ti/Mn ratio. Ti sites in such catalyst evidently exist in both framework and
non-framework location. The role of framework and non-framework Ti sites was
evaluated in catalytic oxidation of cyclohexane, cyclohexene and styrene. Titanium
framework plays a role in oxidation reaction by C-H bond activation whereas nonframework titanium is active for both C-H and C=C bond activations.
Sulphated Ti-OMS-2 catalyst is active in the synthesis of diols from alkenes.
However, Brönsted acid sites were not detected. In the system, Brönsted acids which
play an important role in hydrolysis of epoxides to diols could have been generated
from the interaction of Lewis acid sites on the samples with water.
o H-exchanged of
OMS-2 and MOMS-2
C-H bond oxidation: Homolytic and
Oxidation of
o OMS-2
cyclohexane with o Metal substituted heterolytic
TBHP as oxidant
M-OMS-2
(M=Fe, Ti, Co
and Cu)
o OMS-2 prepared
without buffer
o OMS-2 prepared Abstraction of secondary H+ : Mars
van Krevelen Mechanism.
with buffer
Oxidation of
benzyl alcohol
with air as
oxidant
The possible reaction mechanism
from the main product
Catalyst
Reaction
Continued in Page 166
¾ The Lewis active site plays an important role in
enhancing catalytic oxidation of the catalyst in the
case of cyclohexane via abstraction of electron to
form radical chains.
¾ There is a correlation of ionic radii of metal
substituted with conversion of cyclohexane.
Increasing ionic radii increases conversion of
cyclohexane which could be due to weaker M-O
bond resulting more positive metal centre and
increases Lewis active site.
¾ Catalytic activity of M-OMS-2 increases in the
following order Ti-<Fe-<Cu-<Co-OMS-2,
however Ti-OMS-2 at highest titanium content is
the most active catalyst.
¾ Both metal substituted and H-exchanged enhance
the catalytic activity of OMS-2
¾ OMS-2 prepared without buffer is more active
than prepared with buffer solution which is due to
effective oxygen release from the catalyst via
Mars van Krevelen mechanism.
Physicochemical properties-catalytic activity
relationship
Table 6.1: The physicochemical properties-catalytic activity relationship of the catalysts.
163
o TiO2 rutile
o Ti-OMS-2 (0.18; 0.67)
o OMS-2
o OTS/Ti-OMS-2
o OTS/OMS-2
o TiO2-OMS-2(imp)
o TiO2-OMS-2(mix)
o Ti-OMS-2
o TS-1
Oxidation of styrene
with TBHP as oxidant o OMS-2
Oxidation of
cyclohexene with
TBHP as oxidant
C=C bond
oxidation:
heterolytic metal
pathway
C-H and C=C bond
oxidation:
Homolytic and
heterolytic.
Continued in Page 167
¾ Alkylsylilation enhance the activity of OMS-2 but decrese
the ativity of Ti-OMS-2 which may be caused by blocking
of Ti active site.
¾ The catalytic activity of OMS-2 is higher than TS-1.
¾ The framework of titanium species in Ti-OMS-2 has no
effect in enhancement of catalytic activity. The only Ti in
non-framework structure of OMS-2 induces a synergetic
effect that enhances the catalytic activity.
¾ Ti-OMS-2(0.67) has a higher conversion to styrene
compared to OMS-2.
¾ Ti-OMS-2 which has a higher Lewis acid site than OMS-2
gave similar effect with oxidation of cyclohexane since
the main product is from C-H bond activation.
¾ Both titanium on the framework and non-framework
enhance the conversion of cyclohexene.
¾ The main products of 2 cyclohexen-1-one and 2
cyclohexen-1-ol indicates that C-H bond oxidation is
dominant.
164
Consecutive oxidation and acid
reaction of 1-octene to 1,2
octane diol with TBHP as
oxidant.
Sulphated Ti-OMS-2(0.67) in
different solvent and different
sulphate content.
¾ Solvent has no effect in the synthesis of
sulphated samples with the amount of diol
produced.
¾ The higher amount of sulphate content
forms more diol.
¾ No diol is observed in non sulphated
sample which indicate that there is a need
for more strength in Lewis acid site in
order to interact with water to form
Brönsted acid.
¾ Diol was observed using sulphated TiOMS-2 as catalyst due to the generation of
Brönsted acid through hydrolysis by
interaction of Lewis acid site and water.
165
166
6.2
Conclusion
The results show that different methods of preparation resulted in different
properties of OMS-2. The physicochemical properties-catalytic activity relationship
of modified OMS-2 in oxidation and in consecutive oxidation and acid reactions
were studied and presented in Figure 6.1. The results from the study provedöö that
metals substitution and H-exchanged enhance the catalytic activity of OMS-2 in
oxidation of cyclohexane with TBHP as oxidant. Sulphated Ti-OMS-2 successfully
catalyzed the consecutive oxidation and acid reactions for synthesis of diols from
alkenes; despite the presence of Brönsted acid sites. The reaction was successful due
to the generation of Brönsted acid during the reaction that resulted from interaction
of Lewis acid sites and water.
Synthesis of OMS-2 with and without buffer solution in acidic condition
gave different physicochemical properties and catalytic activity in oxidation of
benzyl alcohol using air as the oxidant. It showed that the catalytic activity of OMS-2
prepared without buffer is better than with buffer solution. The catalytic activity is
closely related to the variety of physicochemical properties of the catalysts. The
study showed that the properties include availability of oxygen lattice to be released,
crystallinity and hydrophilicity-hydrophobicity of the samples.
The synthesis of metal substitution is either with or without manganese ion,
added to metal solution in acidic condition before being oxidized by potassium
permanganate. Only Ti substituted OMS-2 was successfully synthesized by the later
method (without addition of manganese ion). Characterization of the samples shows
that high amount titanium Ti:Mn ratio of 0.43 retained the cryptomelane structure of
OMS-2, while other metals caused collapse of the cryptomelane structure. It is the
highest amount of metal incorporated OMS-2 ever been reported; achievable by the
first method which is without addition of manganese (II) ion solution.
Among the metal substituted OMS-2 catalysts by addition of manganese
ion, Ti substituted has the less enhancement effect to the catalytic activity. The
enhancement of the catalytic activity of metal substituted OMS-2 in oxidation of
Increased followed:
Ti4+- <Fe2+- <Co2+- <Cu2+-
Framework
Tunnel
H-exchanged
Weaken Mn-O bond
caused Mn more
positively charged
1,2 octanediol
Brönsted acid generated
during reaction
Reaction of 1-octene with TBHP
as oxidant
No Brönsted acid site
Sulfated Ti-OMS-2
Figure 6.1: Assignments of modified OMS-2 in oxidation of cyclohexane and consecutive reaction of 1-octene to 1,2 octanediol
Theactivityinoxidationofcyclohexane
withTBHPasoxidantincreases
Play role in electron abstraction
Strengthen Lewis acidity
Increasing of ionic radii decreases strength
of M-O caused M more positively charged
Ti-OMS-2 at high Ti/Mn
ratio (most active)
Framework + non-framework
Metal incorporated
Manganese oxide Octahedral
Molecular Sieve (OMS-2)
167
168
cyclohexane with TBHP as oxidant without using any solvent in increasing order is:
Ti- < Fe- < Cu- < Co-OMS-2. The results also show that all H-exchanged samples
of either OMS-2 or M-OMS-2 enhance the conversion of cyclohexane compared to
the unchanged one. The enhancement in catalytic performance of metals substituted
and H-exchanged in oxidation of cyclohexane with TBHP as oxidant is presented in
Figure 6.1. There is a correlation of ionic radii of M-OMS-2 to conversion of
cyclohexane, which is related to the strength Lewis acid sites. Similar effect was
also observed in H-exchanged catalyst.
The much smaller ionic radii of H+
compared to K+ causes the strain in the cryptomelane lattice, which in turn strains the
Mn–O bonds and makes Mn more positively charged, consequently increases Lewis
acidity. Lewis acid sites may play a role in the oxidation reaction via abstraction of
electron for activation of C-H bond.
The catalyst with high titanium content [Ti-OMS-2(0.67)] with Ti sites
located in framework and non-framework position is found to be the most active
catalyst for the reaction of cyclohexane.
The activity of the catalyst was also
+
increased after ion exchanged with H . This suggests that the superior catalytic
activity of the catalyst is also due to of non-framework Ti site, besides the increasing
Lewis acidity by H-exchange and incorporation of Ti in framework.
The catalytic activity of Ti substituted OMS-2 in the oxidation of
cyclohexene and styrene with TBHP as oxidant, demonstrates that framework and
non-framework Ti sites play an important role (see Figure 6.2).
However, in
oxidation of styrene, only non-framework titanium contributes to the increasing of
conversion of styrene. From the main products of the oxidation of cyclohexane,
cyclohexene and styrene, it can be concluded that the framework Ti sites enable the
abstraction of electron and have no effect to acid-base adduct pathway, whereas nonframework Ti sites play a role for both pathways as shown in Figure 6.2. It is also
observed that the activity of OMS-2 and modified OMS-2 catalysts are higher than
TS-1 which makes those modified OMS-2 catalysts as promising catalyst for
oxidation reactions.
Main products: C-H bond
activation
Cyclohexene
Ti non-framework only enhance the catalytic
activity of OMS-2
Acid-base adduct
Main products: C=C bond
activation
Styrene
Figure 6.2 : The role of the location of Ti sites in Ti-OMS-2 in oxidation of cyclohexane, cyclohexene and styrene.
Ti framework and non-framework
enhance the catalytic activity of OMS-2
Electron abstraction
Main products: C-H bond
activation
Cyclohexane
On the location of different Ti sites in TiOMS-2 in oxidation reactions
169
170
Lastly, it is concluded that every reaction requires the specific active sites,
hydrophobicity-hydrophylicity and also depends on the crystallinity of the catalyst.
The selection of suitable synthetic methods, type, amount and location of metal
substituted which gave significant effect to the physicochemical properties of OMS-2
materials is needed to find the best catalyst for a selected reaction. As a result of the
research, a guideline in the selection of the active site for suitable reaction is
proposed.
171
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Available online at www.sciencedirect.com
Catalysis Communications 8 (2007) 2007–2011
www.elsevier.com/locate/catcom
On the location of different titanium sites in Ti–OMS-2 and
their catalytic role in oxidation of styrene
Hadi Nur *, Fitri Hayati, Halimaton Hamdan
Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia
Received 20 December 2006; received in revised form 30 March 2007; accepted 2 April 2007
Available online 10 April 2007
Abstract
Octahedral manganese oxide molecular sieves (OMS-2) modified by impregnation of TiO2 exhibit a higher catalytic activity for oxidation of styrene with tert-butylhydroperoxide in comparison to titanium-incorporated OMS-2, where the styrene conversions were ca.
70% and 45–50%, respectively. The framework of titanium species has no effect on the enhancement of catalytic activity, while the nonframework of titanium species induces a synergetic effect that enhances the oxidation of styrene with tert-butylhydroperoxide.
2007 Elsevier B.V. All rights reserved.
Keywords: Titanium-incorporated octahedral manganese oxide molecular sieves; Non-framework titanium; Oxidation of styrene
1. Introduction
Octahedral manganese oxide molecular sieves (OMS-2)
are currently considered as one of the potential catalysts
in oxidation of alcohols and olefins [1–3]. OMS-2 materials
are manganese oxide (MnOx; x = 1.85–2.00) with a framework structure consisting of 2 · 2 type tunnels, built up of
MnO6 octahedra with a pore diameter of 4.6 Å [4].
Although, recently, the oxidation of styrene has been
reported by using Me–OMS-2 (Me = Fe, Cu, Ni and Co)
[5], it is desirable to study the catalytic performance of
OMS-2 material with the addition or incorporation of titanium. An understanding of the synthetic methods of catalyst preparation is needed for the precise control of the
structure and location of catalytically active sites on
OMS-2. Here, we report on the study of the effect of the
location of titanium sites, either in the framework or
non-framework of OMS-2, and their catalytic role in the
oxidation of styrene with tert-butyl hydroperoxide (TBHP)
as the oxidant. X-ray powder diffraction (XRD), photolu*
Corresponding author. Tel.: +60 7 5536077; fax: +60 7 5536080.
E-mail address: hadi@kimia.fs.utm.my (H. Nur).
URL: http://www.ibnusina.utm.my/~hadi (H. Nur).
1566-7367/$ - see front matter 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2007.04.002
minescence, surface area and pyridine adsorption measurements were used to characterize these samples. The
catalytic activity of Ti containing OMS-2 was also compared with TS-1 as a reference catalyst.
2. Experimental
2.1. Synthesis
OMS-2 was prepared by a precipitation method according to [6]. A 0.4 M solution of KMnO4 (13.3 g in 225 ml of
deionized water) was added to a mixture of a 1.75 M solution of MnSO4 Æ H2O (19.8 g in 67.5 ml deionized water)
and 6.8 ml of concentrated HNO3. The resulting black precipitate was stirred vigorously and refluxed at 373 K for
24 h. The precipitate was filtered and washed with deionized water until neutral pH and dried at 393 K. This gave
OMS-2. Titanium incorporated OMS-2 (Ti–OMS-2) was
prepared by the stepwise addition of a KMnO4 solution
(13.3 g in 225 ml of deionized water) to different amounts
of Ti2SO4 (25–75 ml) (15% v/v in H2SO4) in order to produce Ti–OMS-2 in the Ti/Mn ratio of 0.18, 0.43 and 0.67
(as analyzed by using an atomic absorption spectrometer).
Upon completion, the mixture was stirred, refluxed,
2008
H. Nur et al. / Catalysis Communications 8 (2007) 2007–2011
filtered, washed, and dried following the above procedure
and it was labeled as Ti–OMS-2 (0.18), Ti–OMS-2 (0.43)
and Ti–OMS-2 (0.67), where the number in parentheses is
the molar ratio of Ti/Mn. Titanium(IV) tetra-2-propoxide
ðTiðOPri4 Þ was impregnated from its toluene solution into
OMS-2 powder and calcined at 773 K for 3 h. Here, this
modified OMS-2 is called Ti–OMS-2 (imp). The molar
amount of Ti calculated to give the molar ratio of Ti/Mn
was 0.18. For comparison, a mechanical mixture of TiO2
and OMS-2 was prepared by the addition of a calculated
amount of TiO2 (rutile) powder to OMS-2 with a Ti/Mn
molar ratio of 0.67. This catalyst was labeled as TiO2–
OMS-2 (mix). Table 1 summarizes the preparation method,
the chemical composition and labeling of titanium containing OMS-2. In this study, TS-1 was also used as a reference
catalyst. TS-1 (2 mol% of titanium) was prepared according to a procedure described earlier [7,8].
2.2. Characterization
All samples were characterized by powder XRD for the
crystallinity and phase content of the solid materials using
a Shimadzu XRD 6000 diffractometer with the CuKa
k = 1.5405 Å) radiation as the diffracted monochromatic
beam at 30 kV and 30 mA. Atomic absorption analysis
(AAS) was employed for elemental composition analyses
of manganese and titanium in the sample. A Perkin–Elmer
model Analyst 400 spectrophotometer was used to carry
out the analyses. The acidity of the solids was characterized
by an absorbed base probe molecule. A wafer of the sample
(10–12 mg) was locked in the cell equipped with CaF2 windows, and evacuated at 400 C under vacuum condition for
4 h. Pyridine as a probe molecule was introduced into the
evacuated sample at room temperature. The IR spectra
of the sample were monitored at room temperature after
desorption of pyridine at 150 C for 1 h. Photoluminescence
spectra were recorded in air at room temperature on a Perkin–Elmer LS 55 spectrometer. The emission spectra
observed at an excitation wavelength was 430 nm.
2.3. Catalytic testing
Oxidation of styrene was carried out using the above
catalysts. Styrene (5 mmol), 70% aqueous ter-butyl hydroperoxide (TBHP) (10 mmol), catalyst (50 mg) and acetonitrile (15 ml) as solvent were placed in a round-bottomed
flask with a reflux condenser and the reaction was performed with stirring at 70 C in an oil bath. The products
were collected in a period of time and analyzed by GC
and GC–MS.
3. Results and discussion
XRD patterns of OMS-2, Ti–OMS-2 (0.18) and Ti–
OMS-2 (0.43) show that the samples are pure and highly
crystalline and matched those of cryptomelane Q [9]; the
natural counterpart of OMS-2 material (see Fig. 1). The
results confirmed that OMS-2, Ti–OMS-2 (0.18) and Ti–
OMS-2 (0.43) materials consist of the cryptomelane structure: 2 · 2 tunnels with a pore size of 4.6 Å, composed of
double chains of edge sharing and corner sharing MnO6
octahedra [4]. The absence of other peaks in the XRD patterns except the cryptomelane peaks suggested that Ti was
successfully incorporated in the framework of Ti–OMS-2.
In order to confirm the successful incorporation of titanium, the XRD patterns of Ti–OMS-2 were compared with
the mechanical mixture of TiO2 (rutile) and OMS-2 where
the mixture (TiO2–OMS-2 (mix)) showed the presence
of the rutile phase (see Fig. 1 and Table 1). If Ti is successfully incorporated in the framework of OMS-2, one expects
that the bigger the substituted atom is, the bigger is the unit
cell volume. Calculation of the unit cell volume of OMS-2
Table 1
Chemical composition and physicochemical properties of OMS-2, Ti–OMS-2 and TiO2–OMS-2
Samples
Molar ratio
of Ti/Mna
Molar
amount of
Mnb
OMS-2
Ti–OMS-2
(0.18)
Ti–OMS-2
(0.43)
Ti–OMS-2
(0.67)
TiO2–OMS2 (imp)c
TiO2–OMS2 (mix)d
0.00
0.18
552.2
412.5
0.43
a
b
c
d
e
Molar
amount of
Tib
Methods of
introduction of Ti
species
Location of Ti
species
Structure of nonframework Ti species
Surface area
(m2 g1)
0.0
74.4
–
Direct synthesis
–
Framework
–
–
155
152
420.4
181.4
Direct synthesis
Framework
–
149
0.67
342.0
228.2
Direct synthesis
Rutile
152
0.18
552.0
99.4
Impregnation
Framework and
non-framework
Non-framework
Amorphous
N.d.e
0.67
552.2
369.9
Mechanical mixing
Non-framework
Rutile
N.d.e
Analysis was carried out by atomic absorption spectrometer.
The amount of OMS-2, Ti–OMS-2 and TiO2–OMS-2 were 50 mg.
Titanium(IV) tetra-2-propoxide ðTiðOPri4 Þ was impregnated from its toluene solution into OMS-2 powder and calcined at 773 K for 3 h.
Catalyst was prepared by addition of calculated amount of Ti from TiO2 powder to OMS-2.
Not determined.
H. Nur et al. / Catalysis Communications 8 (2007) 2007–2011
*
(g)
*
*
(f)
(e)
*
*
*
Relative intensity / a.u.
*
(d)
(c)
(b)
(a)
10
20
30
40
2θ / o
50
60
70
Fig. 1. X-ray diffractograms of (a) cryptomelane (JCPDS 29, 102), (b)
OMS-2, (c) Ti–OMS-2 (0.18), (d) Ti–OMS-2 (0.43), (e) Ti–OMS-2 (0.67),
(f) TiO2–OMS-2 (imp) and (g) Ti–OMS-2 (mix).
(271 Å3) and Ti–OMS-2 (0.43) (277 Å3) shows that the unit
cell volumes increase on incorporation of Ti in the framework of OMS-2. The lattice enlargement originates from
the replacement of the smaller ionic Mn4+ (ionic radius is
0.53 Å) by the relatively larger ionic Ti4+ (ionic radius is
0.61 Å). This shows a strong evidence of isomorphous substitution of Mn atoms by Ti into the framework of
Ti–OMS-2. Moreover, it can be found that with the incorporation of Ti, the full width at half maximum decreases
indicating an increase in the grain size. It is observed that
the peaks of the rutile phase of TiO2 appear at a Ti/Mn
ratio higher than ca. 0.5 and are not observed at a Ti/Mn
ratio less than ca. 0.5 (see Fig. 1). By considering the upper
limit of the titanium that can be incorporated into the
framework, one would expect non-framework titanium
species to be formed when the Ti/Mn ratio reached ca.
0.5. The amount of Ti located in the non-framework is
ca. 25% in Ti–OMS (0.67). This argument is supported by
the presence of the rutile phase of TiO2 in Ti–OMS-2
(0.67) (see Fig. 1). However, no reflection for the rutile
phase of TiO2 is observed in TiO2–OMS-2 (imp) where
the catalyst is prepared by the impregnation method. This
result implies that the structure of TiO2 in TiO2–OMS-2
(imp) is in the amorphous form.
The photoluminescence (PL) was also used to confirm
the absence of non-framework TiO2, because non-framework TiOx with a very small crystallite size cannot be
detected by XRD. The PL spectra are useful to disclose
the efficiency of charge carrier trapping, migration and
transfer, and to understand the nature of electron–hole
pairs in TiO2 semiconductor particles since PL emission
results from the recombination of photo-excited free carriers [10]. In this study, the 430 nm excited PL spectra of all
pressed-powder samples at room temperature were examined in the range of 560–680 nm. The PL spectra of
OMS-2, Ti–OMS-2 (0.43) and TiO2–OMS-2 (mix) are
shown in Fig. 2. The results indicated that the photoluminescence intensity of TiO2–OMS-2 (mix) (kem(max)
600 ± 10 nm; FWHM 40 nm) was substantially higher
than that of OMS-2 and Ti–OMS-2 (0.43) (see Fig. 2). The
relative intensity of Ti–OMS-2 (0.43) and TiO2–OMS-2
(mix) is similar suggesting that there are no TiOx particles
exist in Ti–OMS-2 (0.43) sample. This result reinforces our
suggestion that Ti is incorporated in the framework of Ti–
OMS-2. The incorporation of Ti in the framework of
OMS-2 was also further supported by the surface area
analysis (see Table 1). It is revealed that the surface area
of OMS-2 and Ti–OMS-2 is almost the same.
Reaction products of oxidation of styrene using TBHP
as the oxidant catalyzed by TiO2 (rutile phase), OMS-2,
TiO2–OMS-2 and Ti–OMS-2 were analyzed by GC. The
major products in this reaction proved to be benzaldehyde,
styrene oxide and phenylacetaldehyde. The selectivities
towards benzaldehyde, styrene oxide and phenylacetaldehyde as the reaction products are shown in Fig. 3. As
Ti-OMS-2 (0.43)
TiO2-OMS-2 (mix)
Intensity / a.u.
* = TiO2 (rutile)
*
2009
OMS-2
560
580
600
620
640
Wavelength / nm
660
680
Fig. 2. Photoluminescence spectra of OMS-2, Ti–OMS-2 (0.43) and TiO2–
OMS-2 (mix). The excitation wavelength is 430 nm.
2010
H. Nur et al. / Catalysis Communications 8 (2007) 2007–2011
70
Ti/Mn = 0.67
Product selectivity (%):
Ti/Mn = 0.18
Phenylacetaldehyde
60
Styrene oxide
Benzaldehyde
Conversion / %
15
18
16
14
Ti/Mn = 0.43
Ti/Mn = 0.18
50
18
8
40
16
8
20
Ti/Mn = 0.67
9
18
30
10
69
74
20
76
71
8
16
10
5
12
83
0
Blank
72
15
33
76
TiO2
68
52
TS-1
OMS-2
Ti-OMS-2
TiO 2 -OMS-2
TiO2 -OMS-2
Fig. 3. The conversion and product selectivity of oxidation styrene with tert-butyl hydroperoxide (TBHP) using TiO2, TiO2–OMS-2, Ti–OMS-2, OMS-2
and TS-1. All reactions were carried out at 70 C with styrene (5 mmol), 70% aqueous TBHP (10 mmol), acetonitrile (15 ml) and catalyst (50 mg) with
vigorous stirring.
(b)
Absorbance / a.u.
indicated in Fig. 3, the reaction catalyzed by all the catalysts produced the highest yield of benzaldehyde and their
selectivities towards the formation of products are almost
similar to each other. The high selectivity (52%) towards
benzaldehyde over TS-1 was surprising since that phenylacetaldehyde and benzaldehyde were the major products
from the oxidation of styrene catalyzed by the TS-1 zeolite
[10], while a speculated product, styrene oxide, was not
detected. Brønsted acid sites originating from framework
titanium species catalyze the rearrangement of the intermediate, leading to the formation of phenylacetaldehyde [11].
This argument is in agreement with our oxidation of styrene over TS-1, since no Brønsted acid sites have been
detected in our TS-1 which has been analyzed by the pyridine adsorption method [7]. IR spectrum of acidity study
by pyridine adsorption after evacuation under a vacuum
at 400 C and 150 C revealed in Fig. 4a shows that Lewis
acid sites are formed in Ti–OMS-2 (0.43) as indicated by
the appearance of peaks at 1447 cm1, 1489 cm1 and
1604 cm1. In contrast, no peaks are observed for the
OMS-2 sample in Fig. 4b. The absence of peaks at
1540 cm1 confirms that there are no Brønsted acid sites
in both samples. This is the possible reason why TiO2–
OMS-2, Ti–OMS-2, TiO2 and OMS-2 catalysts are not
selective towards phenylacetaldehyde. A high selectivity
towards benzaldehyde may be due to OMS-2 and TiO2
promoting the carbon–carbon bond cleavage, thus resulting in the formation of benzaldehyde.
As shown in Fig. 3, a considerable increase in the conversion of styrene over Ti–OMS-2, OMS-2, TiO2–OMS-2,
TiO2 and TS-1 after 3 h of the reactions is clearly observed
when Ti–OMS-2 (0.67) and TiO2–OMS-2 (imp) are used as
catalysts. The increase in oxidation activity of Ti–OMS-2
1447
1604
1489
(a)
1640
1600
1560
1520
1480
1440
1400
-1
Wavenumber / cm
Fig. 4. FTIR spectra of (a) Ti–OMS-2 (0.43) and (b) OMS-2 after
evacuation under vacuum at 400 C for 4 h followed by pyridine
adsorption at room temperature and evacuation at 150 C for 1 h.
(0.67) and TiO2–OMS-2 (imp) can be explained on the
basis of the presence of non-framework titanium species.
The superior performance of Ti–OMS-2 (imp) and Ti–
OMS-2 (0.67) strongly suggests the occurrence of a synergetic effect of non-framework Ti with OMS-2, since Ti–
OMS-2 (mix), a mechanical mixture of TiO2 and OMS-2,
gives a relatively lower conversion of styrene (see Fig. 3).
Leaching is a particular problem of solid catalysts in liquid
phase reactions. The catalysts were recycled three times.
The activities of the recovered and dried Ti–OMS-2,
OMS-2 and TiO2–OMS-2 showed an insignificant change
(ca. 3–5%) that corresponds to experimental observations
within experimental error in their catalytic activity in the
H. Nur et al. / Catalysis Communications 8 (2007) 2007–2011
recycling test. This suggests a good regenerability of the
catalysts in the oxidation of styrene with tertbutylhydroperoxide.
The results of this investigation lead to the conclusion
that the framework of titanium species in Ti–OMS-2 has
no effect on the enhancement of catalytic activity, while
the existence of Ti in the non-framework structure of
OMS-2 induces a synergetic effect that enhances the catalytic activity in the oxidation of styrene with tert-butylhydroperoxide. Ti–OMS-2 containing non-framework Ti
is highly active (ca. 70% conversion of styrene) and selective towards the oxidation of styrene to give benzaldehyde
(68%) as the main product. Further detailed studies are,
however, necessary to understand the interactions between
TiO2 and OMS-2 support and the reaction mechanism.
Acknowledgements
This research was supported by the Ministry of Science
Technology and Innovation Malaysia (MOSTI), under
IRPA grant and The Academy of Sciences for the Developing
2011
World, under the TWAS Grants in Basic Sciences no. 04462 RG/CHE/AS.
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