OXIDATIVE COUPLING OF ORTHO-AMINOPHENOL OVER MESOPOROUS
SILICA CONTAINING COPPER(II) DIETHYLAMINO-SUBSTITUTED
SALEN COMPLEX
CHIN TIAN KAE
UNIVERSITI TEKNOLOGI MALAYSIA
OXIDATIVE COUPLING OF ORTHO-AMINOPHENOL OVER MESOPOROUS
SILICA CONTAINING COPPER(II) DIETHYLAMINO-SUBSTITUTED
SALEN COMPLEX
CHIN TIAN KAE
A thesis submitted in fulfillment of the
requirements for the award of the degree of
Master of Science (Chemistry)
Faculty of Science
Universiti Teknologi Malaysia
JANUARY 2010
iii
Sincerely dedication to my beloved family
Especially my father and mother
iv
ACKNOWLEDGEMENT
First of all, I would like to express sincere appreciation to my supervisor,
Prof. Dr. Salasiah Endud, and co-supervisor, Dr. Shajarahtunnur Jamil, for their
supervision and thoughtful guidance that make me successfully complete this
research study. My appreciation also sends to all the lecturers and staffs from
Department of Chemistry in Faculty of Science. Besides that, a great gratitude also
sends to Ibnu Sina Institute for instrumentation and technician supports.
My special thanks also would like to go to my seniors, friends and lab-mates,
especially Mr. Wong Ka Lun, Ms. Lau Su Chien, Ms. Azlin Shafrina Hasim, Ms.
Rozaina Saleh, Ms. Sheela Chandren, Ms. Ng Yew Choo and Ms. Quek Hsiao Pei,
for their helps, advices and supports.
I’m here also would like to send my greatest gratitude to my beloved family
for their spirit supports and care.
Finally, I am also indebted to the Ministry of Science, Technology and
Innovation (MOSTI) for financial support in this study through Science Fund No. 0301-06-SF0107 (vot 79083).
v
PREFACE
This thesis is the result of my work carried out in Department of Chemistry,
Universiti Teknologi Malaysia, between December 2006 to June 2009 under
supervision of Prof. Dr. Salasiah Endud and Dr. Shajarahtunnur Jamil. Parts of my
work described in this thesis have been reported in the following publications or
presentations:
1.
Endud, S., Chin, T. K., Jamil, S. and Wong, K. L. Catalytic Properties of
Metallosalen Supported on MCM-41 in Oxidation of Benzene. Poster
presentation in the Annual Fundamental Science Seminar 2007. Ibnu Sina
Institute for Fundamental Science Studies, Universiti Teknologi Malaysia,
Skudai, Johor. 28th - 29th May, 2007.
2.
Endud, S., Chin, T. K., Lau, S. C. and Hasim, A. S. Nanostructured Materials
with Metallosalen Complexes as Catalysts for Fine Chemicals Synthesis.
Poster presentation in the Nanotech Malaysia 2007. Putra World Trade Centre,
Kuala Lumpur. 28th - 30th November, 2007.
3.
Endud, S. and Chin, T. K. and Jamil, S. Heterogeneous Oxidative Coupling of
o-Aminophenol Over MCM-48 Supported Copper(II) Substituted Salen
Complex. Oral presentation in the Annual Fundamental Science Seminar 2008.
Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi
Malaysia, Skudai, Johor. 27th - 29th May, 2008.
4.
Endud, S., Lau, S. C., Chin, T. K. and Mohd. Rawi, S. A. Organic-Inorganic
Hybrid Nanomaterials for Biomimetic Catalytic Process. Poster presentation in
the Nanotech Malaysia 2008. Putra World Trade Centre, Kuala Lumpur. 14th 16th October, 2008.
vi
ABSTRACT
Copper(II)
N,N’-bis[4-(N,N-diethylamino)salicylidene]ethylenediamine
(CAS) complex, which has the metal-ligand coordination “CuN2O2” mimicking the
active site of the enzyme galactose oxidase, has been synthesized by the reaction of
copper(II) acetate monohydrate and the prepared ligand, N,N’-bis[4-(N,Ndiethylamino)salicylidene]ethylenediamine (A-salen). The structure of A-salen was
confirmed by FTIR, 1H- and 13C-NMR spectra, while the CAS complex was
characterized using FTIR and DR UV-Vis spectroscopy. The CAS complex was
then immobilized on Si-MCM-48, NH2-MCM-48 and SO3H-MCM-48, respectively.
After that, MCM-48 containing CAS complex was characterized by using FTIR, DR
UV-Vis, powder XRD, N2 adsorption-desorption isotherm, AAS, TGA, FESEM and
TEM. XRD patterns showed that the structure of mesoporous MCM-48 was
preserved but the crystallinity of MCM-48 had decreased after modification and
immobilization of CAS. Besides that, N2 physisorption measurement and TEM
image showed that the pore channel of MCM-48 was well maintained in the meso
range after modification and immobilization of CAS. The decrease in pore diameter
of MCM-48 suggests that the modification had occurred in the pore channel. The
catalytic activity of the supported CAS catalyst was tested in the heterogeneous
oxidation of o-aminophenol (AP). The reaction was carried out in methanol
containing H2O2 as oxidant at 70 oC for 24 hours. All the catalytic reactions were
monitored using GC-FID. The supported CAS catalyst showed lower percentage
selectivity and yield of 2-amino-3H-phenoxazin-3-one (APX), but gave higher TON
than those obtained from homogeneous CAS. Oxidation of AP over supported CAS
catalyst is considered as “green” process due to the recoverability of the catalyst.
However, the leaching out of CAS catalyst from MCM-48 matrix has affected the
catalytic performance of supported CAS catalyst. The supported CAS catalyst is
proposed to have the catalytic activity mimicking the phenoxazinone enzyme, a
copper-containing enzyme which catalyze the peptides-substituted o-aminophenol to
phenoxazinone chromophore. This is because the supported CAS catalyst exhibits
the catalytic behavior similar to phenoxazinone enzyme, which favors the oxidation
of AP to APX via the formation of o-quinone imine (QI) intermediate. The effect of
reaction time, temperature, molar ratio substrate to oxidant, different type of oxidant
and different type of solvent to the oxidation of AP were also examined.
vii
ABSTRAK
Kompleks kuprum(II) N,N’-bis[4-(N,N-dietilamino)salisiliden]etilenadiamina
(CAS) yang mempunyai koordinatan logam-ligan “CuN2O2” yang mimik kepada
bahagian aktif enzim galaktosa, telah disintesis melalui tindakbalas kuprum(II) asetat
monohidrat dengan ligan yang telah disediakan, N,N’-bis[4-(N,N-dietilamino)salisiliden]etilenadiamina (A-salen). Struktur A-salen telah dibuktikan dengan
menggunakan spektroskopi FTIR, 1H- dan 13C-NMR, manakala kompleks CAS telah
dicirikan menggunakan spektroskopi FTIR dan DR UV-Vis. Kompleks CAS
seterusnya dipegunkan ke atas Si-MCM-48, NH2-MCM-48 dan SO3H-MCM-48
masing-masing. Selepas itu, MCM-48 yang mengandungi kompleks CAS telah
dicirikan dengan menggunakan FTIR, DR UV-Vis, serbuk XRD, analisis penjerapan
gas N2, AAS, TGA, FESEM dan TEM. Corak XRD menunjukkan struktur
mesoliang MCM-48 masih kekal tetapi tahap penghabluran MCM-48 telah menurun
selepas pengubahsuaian dan pemegunan CAS. Selain itu, analisis penjerapan gas N2
dan gambaran TEM menunjukan saluran liang bagi MCM-48 masih kekal dalam
julat meso selepas pengubahsuaian dan pemegunan CAS. Pengurangan diameter
liang MCM-48 mencadangkan pengubahsuaian telah berlaku di dalam saluran liang.
Aktiviti pemangkinan mangkin CAS tersokong telah diuji dalam pengoksidaan
heterogen bagi o-aminofenol (AP). Tindakbalas ini dijalankan di dalam metanol
yang mengandungi H2O2 sebagai agen pengoksida pada 70 oC selama 24 jam.
Semua tindakbalas mangkin dipantau menggunakan GC-FID. Mangkin CAS
tersokong menunjukan peratusan pemilihan dan hasil bagi 2-amino-3H-fenoksazin-3on (APX) yang lebih rendah, tetapi memberi TON yang lebih tinggi berbanding
dengan nilai yang diperolehi daripada CAS homogen. Pengoksidaan AP dengan
mangkin CAS tersokong boleh dianggap sebagai proses “hijau” disebabkan mangkin
ini boleh diperolehi semula. Bagaimanapun, mangkin CAS mudah larut-resap dari
matriks MCM-48 dan ini telah mempengaruhi aktiviti mangkin bagi mangkin CAS
tersokong. Mangkin CAS terpegun dicadangkan mempunyai aktiviti mangkin mimik
kepada fenoksazinon enzim yang mengandungi kuprum, yang memangkinkan
peptida-tertukarganti o-aminofenol kepada kromofor fenoksazinon. Ini kerana
mangkin CAS tersokong menunjukan tindakan mangkin yang sama seperti enzim
fenoksazinon yang lebih cenderung untuk mengoksidakan AP kepada APX melalui
pembentukan perantara o-kuinon imina (QI). Kesan masa tindakbalas, suhu, nisbah
molar substrat terhadap agen pengoksida, jenis agen pengoksida yang berbeza dan
jenis pelarut yang berbeza terhadap pengoksidaan bagi AP juga dikaji.
viii
TABLE OF CONTENTS
CHAPTER
1
2
TITLE
PAGE
TITLE
i
DECLARATION
ii
DEDICATION
iii
ACKNOWLEDGEMENT
iv
PREFACE
v
ABSTRACT
vi
ABSTRAK
vii
TABLE OF CONTENTS
viii
LIST OF TABLES
xii
LIST OF FIGURES
xiv
LIST OF SCHEMES
xviii
LIST OF ABBREVIATIONS
xix
LIST OF APPENDICES
xxi
INTRODUCTION
1.1 Research Background and Problem Statement
1
1.2 Objectives of Study
8
1.3 Scope of Study
8
LITERATURE REVIEW
2.1 Transition Metal as Catalyst
2.1.1 Catalytic Properties of Copper Compounds
2.2 Schiff Base Salen Ligand and Its Complexes
11
13
15
ix
2.2.1 Copper(II) Schiff Base Salen Complexes
in Organic Reaction
2.3 Heterogeneous Catalyst
21
23
2.3.1 Heterogenization of Homogeneous
Catalysts on the Solid Supports
25
2.3.2 M41S Molecular Sieves as Mesoporous
Catalyst Support
26
2.3.3 Immobilization of Homogeneous Catalyst
on Mesoporous Silica
28
2.3.4 Mesoporous Silica MCM-48 as Catalyst
Support
32
2.4 Oxidative Coupling – Synthesis of
Phenoxazinone Chromophore
3
35
SYNTHESIS OF COPPER(II) DIETHYLAMINOSUBSTITUTED SALEN COMPLEX
SUPPORTED ON MCM-48
3.1 Chemicals and Reagents
39
3.2 Experimental
40
3.2.1 Synthesis of Diethylamino-Substituted
Salen (A-Salen) Ligand
40
3.2.2 Synthesis of Copper(II) DiethylaminoSubstituted Salen (CAS) Complex
40
3.2.3 Synthesis of Purely Siliceous MCM-48
(Si-MCM-48)
41
3.2.4 Synthesis of Amino-Functionalized MCM48 (NH2-MCM-48)
41
3.2.5 Synthesis of Sulfonic Acid-Functionalized
MCM-48 (SO3H-MCM-48)
42
3.2.6 Copper(II) Diethylamino-Substituted Salen
Complex Supported on MCM-48
42
x
3.3 Characterization of Copper(II) DiethylaminoSubstituted Salen Complex Supported on MCM43
48
3.3.1 Fourier Transform Infrared (FTIR)
Spectroscopy
43
3.3.2 Proton and Carbon-13 Nuclear Magnetic
Resonance (1H- and 13C-NMR)
Spectroscopy
43
3.3.3 Diffuse Reflectance Ultraviolet-Visible
(DR UV-Vis) Spectroscopy
44
3.3.4 Powder X-Ray Diffraction (XRD)
45
3.3.5 Nitrogen Adsorption-Desorption Isotherm
Analysis
46
3.3.6 Atomic Absorption Spectroscopy (AAS)
46
3.3.7 Thermogravimetric Analysis (TGA)
47
3.3.8 Field Emission Scanning Electron
Microscopy (FESEM)
3.3.9 Transmission Electron Microscopy (TEM)
3.4 Results and Discussion
48
48
49
3.4.1 Physicochemical Properties of Copper(II)
Diethylamino-Substituted Salen (CAS)
Complex
49
3.4.2 Physicochemical Properties of MCM-48
Containing Copper(II) DiethylaminoSubstituted Salen (CAS) Complex
4
61
CATALYTIC ACTIVITY OF MCM-48
CONTAINING COPPER(II) DIETHYLAMINOSUBSTITUTED SALEN COMPLEX IN THE
OXIDATION OF O-AMINOPHENOL
4.1 Catalytic Testing – Oxidative Coupling of oAminophenol
78
xi
4.2 Oxidation of Phenol and Its Derivatives
79
4.3 Preparation of 2-Amino-3H-phenoxazin-3-one
(APX) as Standard
4.4 Analysis of Catalytic Reaction
80
81
4.4.1 Gas Chromatography – Flame Ionization
Detector (GC-FID)
81
4.4.2 Gas Chromatography - Mass Spectrometry
(GC-MS)
82
4.5 Leaching Test
83
4.6 Results and Discussion
83
4.6.1 Effect of Reaction Time
89
4.6.2 Effect of Reaction Temperature
90
4.6.3 Effect of Molar Ratio of Substrate to
Oxidant
5
94
4.6.4 Effect of Different Oxidant
95
4.6.5 Effect of Different Solvent
97
CONCLUSION AND RECOMMENDATIONS
5.1 Conclusion
100
5.2 Recommendations
104
REFERENCES
106
Appendices A-F
126-131
xii
LIST OF TABLES
TABLE NO.
2.1
TITLE
Comparison of homogeneous and heterogeneous
catalysts (Hagen, 2006)
3.1
PAGE
23
FTIR stretching bands of EDA, A-Sal and A-Salen
ligand
51
3.2
1
54
3.3
13
3.4
DR UV-Vis data of A-Salen ligand and CAS complex
59
3.5
TGA data of neat CAS complex
60
3.6
XRD data of as-synthesized and calcined Si-MCM-48
63
3.7
FTIR data of OF-MCM-48 and CAS complex that
H-NMR data of A-Sal and A-Salen
C-NMR data of A-Sal and A-Salen
supported on MCM-48
3.8
67
XRD data of OF-MCM-48 and MCM-48 containing
CAS complex
3.9
55
69
Nitrogen adsorption-desorption isotherm data of SiMCM-48, OF-MCM-48 and MCM-48 containing CAS
complex
3.10
Percentage weight loss of MCM-48 containing CAS
complex
3.11
73
Copper content of CAS complex incorporated on
MCM-48
4.1
72
74
Catalytic activity of neat and supported CAS catalyst
in the oxidation of AP to APX
84
4.2
Leaching test and reusability of the supported catalyst
85
4.3
Oxidation of phenol and its derivative over supported
CAS catalyst
88
xiii
5.1
Comparison of catalytic oxidation of AP over
supported CAS catalyst, Co(salen) and copper
compound
102
xiv
LIST OF FIGURES
FIGURE NO.
1.1
TITLE
PAGE
The structure of the active site in common
metalloenzyme; alcohol dehydrogenase (1), nitrous
oxide reductase (2) and horseradish peroxidase (3)
1.2
3
The oxidative coupling of o-aminophenol (AP) to 2amino-3H-phenoxazin-3-one (APX)
6
1.3
The structure of 3H-phenoxazin-3-one (4)
7
2.1
Synthesis of Schiff bases compound by the
condensation of an amine group compound and a
carbonyl group substance
2.2
16
The structure of Schiff base tetradentate salen-type
ligand (5), salen ligand (6), Jacobson’s ligand (7) and
Katsuki’s ligand (8) (Canali and Sherrington, 1999)
2.3
17
Synthesis of salen and its derivatives (11) by the
condensation of salicylaldehyde derivatives (9) and
diamine compounds (10)
2.4
18
Synthesis of metal salen complex and its derivatives
(12) by the reaction of salen ligand and its derivatives
(11) with metal ion
2.5
Possible coordination geometries of metal Schiff base
salen complexes
2.6
19
Conformation of salen complex that mimic to
porphyrin structure of Cytochrome P-450 enzyme
2.7
18
20
Metal-ligand coordination of copper(II) salen
complex mimic to active site of galactose oxidase
22
xv
2.8
The possible chemical interaction between
mesoporous silica and the supported catalysts
prepared by immobilization via physical adsorption
method
2.9
Ionic interaction of cationic rhodium(I) diphosphine
complex and anionic host framework of Al-MCM-41
2.10
29
30
Electrostatic attractions between aminofunctionalized mesophase silica and iron
tetrasulfophthalocynine
31
2.11
Impregnation of tin on MCM-48
31
2.12
Immobilization of catalysts on mesoporous silica via
covalent bonding that occurred between spacer ligand
and (a) metal (Lee et al., 2003) or (b) ligand of metal
complex (Yu, et al., 2009)
2.13
Model of gyroid minimal surface of MCM-48
(Armatas and Kanatzidis, 2006)
2.14
32
33
Oxidative coupling of peptide-substituted oaminophenol (13) to actinomycin chromophore (14)
by phenoxazinone synthase in soil bacteria
35
3.1
The synthetic route to A-Salen ligand
49
3.2
FTIR spectra of EDA, A-Sal and A-Salen ligand
50
3.3
Intramolecular hydrogen bonding between proton of
phenolic and the electrons lone pair of atom nitrogen
in imine group
51
3.4
1
53
3.5
Delocalization of proton between phenolic and
H-NMR spectra of A-Sal and A-Salen ligand
nitrogen atom of imine group of A-Salen compound
54
3.6
13
56
3.7
The synthetic route of CAS complex
57
3.8
FTIR spectrum of CAS complex
58
3.9
DR UV-Vis spectra of A-Salen ligand and CAS
3.10
C-NMR spectra of A-Sal and A-Salen ligand
complex
59
TGA curve of neat CAS complex
60
xvi
3.11
FTIR spectra of as-synthesized and calcined SiMCM-48
3.12
62
XRD patterns of as-synthesized and calcined SiMCM-48
63
3.13
FESEM image of calcined Si-MCM-48
64
3.14
TEM image of calcined Si-MCM-48
64
3.15
FTIR spectra of OF-MCM-48 and MCM-48
containing CAS complex
3.16
XRD patterns of OF-MCM-48 and MCM-48
containing CAS complex
3.17
66
68
Nitrogen adsorption-desorption isotherm of SiMCM-48, OF-MCM-48 and MCM-48 containing
CAS complex
3.18
70
Illustration of pore system of Si-MCM-48 and
modified MCM-48 before and after functionalization
of OFA or CAS complex
3.19
TGA thermograms of MCM-48 containing CAS
complex
3.20
71
73
DR UV-Vis spectra of neat CAS complex and MCM48 supported CAS complex
76
3.21
FESEM image of MCM-48 supported CAS complex
77
3.22
TEM image of MCM-48 supported CAS complex
77
4.1
Catalytic oxidation of o-aminophenol (AP) to 2amino-3H-phenoxazin-3-one (APX)
4.2
78
Effect of reaction time on the conversion of AP and
selectivity towards APX by various types of MCM48 containing CAS catalyst
4.3
Effect of reaction time on the formation of APX by
various types of MCM-48 supported CAS catalyst
4.4
89
90
Effect of reaction temperature on the conversion of
AP and selectivity towards APX by various types of
MCM-48 supported CAS catalyst
91
xvii
4.5
Effect of reaction temperature on the formation of
APX by various types of MCM-48 containing CAS
complex
4.6
91
Effect of molar ratio substrate to oxidant on the
conversion of AP and selectivity towards APX by
various types of MCM-48 containing CAS catalyst
4.7
94
Effect of molar ratio substrate to oxidant in the
formation of APX by various types of MCM-48
containing CAS catalyst
4.8
95
Effect of different type of oxidant on the conversion
of AP and selectivity in the formation of APX by
various types of MCM-48 containing CAS catalyst
4.9
96
Effect of different type of oxidant on the formation of
APX by various types of MCM-48 supported CAS
complex
4.10
96
Effect of solvent on the conversion of AP and
selectivity towards APX by various types of MCM48 supported CAS catalyst
4.11
Effect of solvent in the formation of APX by various
types of MCM-48 containing CAS catalyst
4.12
98
98
Effect of different solvent on the stability of MCM48 supported CAS catalyst
99
xviii
LIST OF SCHEMES
SCHEME NO.
TITLE
1.1
Outline of study
2.1
The flow in the synthesis of purely siliceous
mesoporous materials
2.2
PAGE
10
27
Two different ways in the synthesis of
phenoxazinone chromophore
37
3.1
Modification of Si-MCM-48 with OFA
65
3.2
Diagrammatic representative of multilayer
adsorption, pore condensation and hysteresis in
pore channel
3.3
Proposed chemical interactions between CAS
complex and MCM-48 matrix
4.1
75
Proposed mechanism path for alcohol oxidation
over galactose oxidase (Chaudhuri et al., 1999)
4.2
70
86
The proposed mechanism for the oxidative
coupling of AP over CAS complex supported on
MCM-48
4.3
The possibility reaction pathway of AP to the
formation of APX
4.4
88
Reaction diagrammatic that showing the possibility
of products formed in the oxidation of AP
5.1
87
93
Synthesis of immobilized chiral Mn(III) salen
complex from OFMS (Yu et al., 2006)
104
xix
LIST OF ABBREVIATIONS
δ
Chemical shift
ao
Unit cell parameter
Å
Angstrom
% wt
Percentage weight
2θ
Bragg angle
13
Carbon-13 nuclear magnetic resonance
1
C-NMR
H-NMR
Proton nuclear magnetic resonance
AAS
Atomic absorption spectroscopy
Al2O3
Aluminium oxide
AP
o-Aminophenol
APX
2-Amino-3H-phenoxazin-3-one
A-Salen
N,N’-Bis[4-(N,N-diethylamino)salicylidene]ethylenediamine
CAS
Copper(II) N,N’-bis[4-(N,N-diethylamino)salicylidene]ethylenediamine
CAS-MCM-48
CAS supported on purely siliceous MCM-48
CAS-N-MCM-48
CAS supported on amino-functionalized MCM-48
CAS-S-MCM-48
CAS supported on sulfonic acid-functionalized MCM-48
Cu Kα
X-ray diffraction from copper Kα energy levels
DMF
N,N-Dimethylformamide
DR UV-Vis
Diffuse reflectance ultraviolet-visible
FESEM
Field emission scanning electron microscopy
FTIR
Fourier transform infrared
GC-FID
Gas chromatography - flame ionization detector
GC-MS
Gas chromatography - mass spectrometry
Hz
Hertz
IUPAC
International Union of Pure and Applied Chemistry
xx
J
Coupling constant
m
Meta
MCM
Mobil crystalline materials
N2
Nitrogen
Na2O
Sodium oxide
NH2-MCM-48
Amino-functionalized MCM-48
o
Ortho
OFA
Organo-functionalized agent
OFMS
Organo-functionalized mesoporous silica
OF-MCM-48
Organo-functionalized MCM-48
p
Para
PI
Polarity index
Rh-BPPM
Rhodium(I) (2S,4S)-N-tertbutyloxycarbonyl-4-diphenylphosphino-2-diphenylphosphinometylpyrrolidin
Ru-BINAP
Ruthenium(II) 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl
Si-MCM-48
Purely siliceous MCM-48
SiO2
Silicon dioxide
SO3H-MCM-48
Sulfonic acid-functionalized MCM-48
TEM
Transmission electron microscopy
TGA
Thermogravimetric analysis
TON
Turnover number
XRD
X-ray diffraction
xxi
LIST OF APPENDICES
APPENDIX
A
TITLE
Calculation on the percentage weight loss of water
molecules in CAS complex
B
1
127
H-NMR spectrum of 2-amino-3H-phenoxazin-3-one
(APX)
D
126
FTIR spectrum of 2-amino-3H-phenoxazin-3-one
(APX)
C
PAGE
13
128
C-NMR spectrum of 2-amino-3H-phenoxazin-3-
one (APX)
129
E
MS pattern of 2-amino-3H-phenoxazin-3-one (APX)
130
F
GC chromatograms of oxidation of AP (a) before
reaction and (b) after reaction
131
CHAPTER 1
INTRODUCTION
1.1
Research Background and Problem Statement
Catalyst is the substance that can speed up the reaction rate without being
substantially consumed in the process (Satterfield, 1991). In nature, all organic
substances are catalyzed by enzymes which are well-recognized as biocatalysts.
Enzymes or biocatalysts are macromolecules which consist of globular proteins.
While the protein moieties that bind with one or more metal ions are categorized as
metalloenzymes (Bugg, 2004). These naturally occurring biocatalysts have been
known to have novelty features, especially in giving high chemoselectivity,
regioselectivity, diastereoselectivity, as well as enantioselectivity of products which
typically cannot be easily achieved in the simple chemical system (Bommarius and
Riebel, 2004; Aehle, 2004; Buchholz et al., 2005).
However, until nowadays, the applications of biocatalysts for the commercial
production of high-value fine chemicals are still limited, while only a small fraction
of the known biocatalysts have been applied on a commercial scale (Hagen, 2006).
This is because the biocatalysts are hard to handle due to the protein properties of
enzymes which are not stable at high temperature, extreme pH-value and also easily
inhibited by some metal ion or peptidases (Hagen, 2006; Bommarius and Riebel,
2004; Buchholz et al., 2005). Moreover, biocatalysts have a low specific activity and
some of the biocatalysts require complicated co-substrates.
For examples,
dehydrogenases require nicotinamide-containing compounds as co-factor and
2
oxidases require flavin co-factor (Hagen, 2006; Bommarius and Riebel, 2004). From
the economic point of view, biocatalysts are not the best selection as they are
expensive and highly unaffordable. In addition, it may take a long development
times for new enzymes and processes (Hagen, 2006; Bommarius and Riebel, 2004).
Over the past few years, much effort has been put in the development of
catalysts based on transition metals and their complexes (Moutet and Ourari, 1997;
Salomão et al., 2007). This is because of the transition metals have vacant d-orbitals
which may hybrid to bound with various types of organic molecules (Hagen, 2006;
Masters, 1981). The transition metals can exist in various oxidation states which
contribute to their ability to interchange their oxidation numbers during the catalytic
process (Hagen, 2006).
Besides that, the scientist also found that most of the
metalloenzyme consists of transition metal as an active site. For example (Figure
1.1), alcohol dehydrogenase (1) contains a single zinc(II) ion at their active site,
nitrous oxide reductase (2) with copper atom in active center, horseradish peroxidase
(3) contains the protoheme group with an iron(III) atom in the active center and so on
(Bugg, 2004; Koval et al., 2008; Veitch, 2004). Furthermore, the easier handling of
such chemically synthesized catalyst is the preferred choice of manufacturers.
Therefore, the catalytic reaction based on transition metal as catalyst is an attractive
field for study.
3
Figure 1.1
The structure of the active site in common metalloenzyme; alcohol
dehydrogenase (1), nitrous oxide reductase (2) and horseradish
peroxidase (3)
Copper is one of the transition metals which have been recognized as an
important biological element since the identification of copper-containing active sites
in numerous oxidases, oxygenases and other metalloenzymes (Horváth et al., 2004).
These copper-containing proteins typically play a role as redox catalysts in a range of
biological processes, such as electron transfer or oxidation of various organic
substances (Koval et al., 2008). In general, copper is known to exist in the 0, +1, +2
and +3 oxidation states. Copper in zero oxidation state that is present in metal form,
typically is not a reactive element. On the other hand, copper(III) which is not
commonly found has been suggested as an intermediate in certain reactions involving
catalytic amounts of copper(II) ion. Based on the previous research trend, copper(II)
is preferably used for research more than copper(I).
This may be due to the
instability of copper(I) which is easily oxidized to copper(II).
Furthermore,
4
copper(II) has long been found to be a good catalyst to oxidize a wide range of
organic substances with high selectivity due to its mild oxidizing power and
compatibility with a variety of solvent systems (Jacob et al., 1998a).
Recently, copper(II) coordinated with organic ligands has been widely
synthesized for oxygen uptake and catalytic studies with the aim to understand the
mechanism of action of copper-containing enzyme. Schiff bases are the organic
ligand which has been widely used for research purpose due to their ability to
stabilize the different types and also the various oxidation states of metals (Ribeiro
da Silva et al., 2004; Caselli et al., 2005). The previous studies showed that the
coordination geometries adopted by copper ions vary with the oxidation state, where
copper(II) ion prefers square planar, trigonal bipyramidal and tetragonal or
octahedral geometries (Koval et al., 2008).
Thus, tetradentate and octadentate
ligands were compatible to coordinate with copper(II) ion.
N,N,O,O-tetradentate Schiff base ligands possess many attractive features
including facile approach, readily adjusted ancillary ligands, and tuneable steric and
electronic coordination environments on the metal center (Wang et al., 2003). On
the other hand, numerous known copper-containing proteins, such as galactose
oxidase has the “CuN2O2” group as its active site (Jacob et al., 1998b). Salen or
N,N’-bis(salicylidene)ethylenediamine is one of the members of N,N,O,Otetradentate Schiff base, which typically can be obtained by the condensation of
salicylaldehyde and ethylenediamine.
Copper(II) coordinated with salen and its
derivatives have been the subject of intensive study due to their potential as catalysts
in various oxidation reactions, including epoxidation of olefins, oxidation of phenol,
etc (Jacob et al., 1998a; Jacob et al., 1998b). Flexibility of the ethylenediamine
backbone in that ligand as observed in a number of transition metal complexes with
bidentate oxygen ligands is responsible for the complex to mimic the biological
function of enzymes (Karandikar et al., 2004; Lloret et al., 1989).
5
The increasing stringent environmental constraints have brought to the trend
in designing the heterogeneous catalytic system. Direct use of the solid catalysts in
the reaction is the easiest and conventional way to generate heterogeneous catalytic
system. Recently, heterogenization of homogeneous catalysts has been the subject of
intensive study.
Heterogenization of homogeneous catalysts is a technique that
involves the immobilization of homogeneous catalyst on a solid matrix (Chaube et
al., 2005; Bahramian et al., 2006a). Heterogenization of homogeneous catalysts is
an ideal method which combines the advantageous of homogeneous catalyst, such as
high catalytic activity and product yield, with the engineering advantages of
heterogeneous catalysts, including easily to recover, prolong catalytic life cycle,
increase thermal stability and reusability of catalyst (Mac Leod et al., 2007; Chaube
et al., 2005; Bahramian et al., 2006a; Kozlov et al., 1998; Leadbeater and Marco,
2002).
Among the solid supports, ordered mesoporous silica, which is discovered by
Mobil researchers, has been widely used for research studies due to its favorable
features including possessing high specific surface areas, tunable pore size from 16
to 100 Å, high chemical and thermal stability as well as provides a modifiable
surface (Taguchi and Schüth, 2005; Kresge et al., 1992). Mesoporous silica has been
reported as a good solid support, because the porous silica does not destroy or hinder
the active site of catalyst.
Moreover, it can enhance the catalytic activity and
selectivity of the catalyst. Over the past few years, MCM-41 has attracted a lot of
attention and more preferred used as support material if compared with other
mesoporous materials. This is because MCM-41 is easy to synthesize and obtainable
in highly pure phase, reproducible and the framework structure is more stable. On
the other hand, there are only a few of studies reported on heterogeneous catalytic
reaction based on MCM-48 as support material.
This three-dimensional cubic
mesoporous MCM-48 which possesses narrow pore size distribution, interwoven,
branched, regular cubic pore structure will offer the advantages of catalyst support,
especially towards resistance to pore blockage (Ryoo et al., 1999; Xu et al., 1998).
Thus, MCM-48 has been selected as solid support in this study.
6
Therefore, this study is conducted in order to investigate the catalytic
properties of copper(II) diethylamino-substituted salen complex supported on MCM48 in oxidation of o-aminophenol (AP) to 2-amino-3H-phenoxazin-3-one (APX) by
using aqueous peroxide as oxidant under mild conditions (Figure 1.2). The reaction
was carried out in organic solvent. To the best of our knowledge, this is the first
study to investigate the heterogeneous oxidation of o-aminophenol using MCM-48
modified copper(II) diethylamino-substituted salen complex.
Figure 1.2
The oxidative coupling of o-aminophenol (AP) to 2-amino-3Hphenoxazin-3-one (APX)
APX
is
one
of
the
derivatives
of
phenoxazinone
chromophore.
Phenoxazinone or 3H-phenoxazin-3-one (4) is a heterocyclic compound, which
consists of tricyclic iminoquinone skeleton as illustrated in Figure 1.3 (Bolognese et
al., 2002a; Hasegawa and Ueno, 1985). Phenoxazinone is found as a chromophore
part of the actinomycins, especially actinomycin D, which exerts intensive anticancer activity by inhibiting DNA dependent RNA polymerase (Hasegawa and
Ueno, 1985; Jain and Sobell, 1972; Toader et al., 2006). Thus, it is conceivable that
phenoxazinone compounds ， especially APX, might also have the anticancer
properties (Toader et al., 2006). A series of polycyclic iminoquinonic
phenoxazinone, including APX compound, has been subjected for anticancer
evaluation, and the corresponding compounds was exhibited activity against
leukemia and solid tumor cell lines at submicromolar concentrations (Bolognese et
al., 2002a).
7
9
8
7
6
10
9a
N
10a
5a
O
4a
5
1
2
4
3
O
(4)
Figure 1.3
The structure of 3H-phenoxazin-3-one (4)
Actinomycins or derivatives of phenoxazinone are synthesized in nature
through the oxidative coupling of two molecules of a substituted o-aminophenol in
the presence of oxygen catalyzed by phenoxazinone synthase, a copper-containing
enzyme in soil bacteria (Barry et al., 1989, Simándi et al., 2004; Simándi et al.,
1996; Szihyártó et al., 2006). Because of the limitation of enzymatic reaction system
as mentioned before, the usage of stoichiometric oxidants have been applied in the
final step of actinomycins synthesis. Potassium ferricyanide is the most frequently
used oxidizing agent in stoichiometric oxidation of peptide-substituted oaminophenol to actinomycin derivatives (Simándi et al., 1996; Meienhofer, 1970).
However, this classical stoichiometric oxidation reaction has generated large amount
of harmful inorganic waste.
Work by Barry and co-workers (1989) found that o-quinone imine (QI) is an
important intermediate in the formation of phenoxazinone chromophore when the oaminophenol or its derivatives are catalyzed by phenoxazinone synthase.
After
investigation and understanding the final step biosynthesis route of phenoxazinone
chromophore, much effort had been focused in the synthesis of the corresponding
compounds by using chemical catalytic reaction method, with the purpose to replace
the enzymatic and also the stoichiometric reaction system. As mentioned before, QI
is the intermediate product in the oxidation of o-aminophenol, which suggests
phenoxazinone synthase may have an activity that mimics catechol oxidase.
Catechol oxidase is a copper-containing enzyme that carries out the oxidation of
phenols to catechol or o-quinone. Previous studies showed that oxidation of oaminophenol and its derivatives were catalyzed by other types of copper containing
enzyme.
For example, conversion of 4-methyl-3-hydroxyanthranilic acid to
actinocin chromophore over Trametes versicolor laccase (multicopper oxidase) has
8
been demonstrated by Osiadacz and co-workers (1999). In this regard, copper is an
ideal selection as catalyst active site for this study.
1.2
Objectives of Study
The objectives of the research are:
i.
To synthesize copper(II) N,N’-bis[4-(N,N-diethylamino)salicylidene]ethylenediamine (CAS) by the reaction of copper(II) acetate
monohydrate
and
the
prepared
N,N’-bis[4-(N,N-diethylamino)-
salicylidene]ethylenediamine (A-Salen);
ii.
To incorporate CAS on purely siliceous MCM-48 (Si-MCM-48) and
organo-functionalized MCM-48 (OF-MCM-48);
iii.
To characterize the physicochemical properties of neat CAS and
MCM-48 containing CAS;
iv.
To investigate the catalytic activities of MCM-48 containing CAS in
the heterogeneous oxidation of o-aminophenol by using aqueous
peroxide as oxidant in organic solvent at mild temperature; and
v.
To study the effect of various parameters in the oxidation of oaminophenol such as effect of reaction time, temperature, molar ratio
substrate to oxidant, different type of oxidant and different type of
solvent.
1.3
Scope of Study
The research focuses on the synthesis of MCM-48 containing transition metal
complex as catalyst, copper(II) N,N’-bis[4-(N,N-diethylamino)salicylidene]ethylenediamine or copper(II) diethylamino-substituted salen complex (CAS), by using postsynthesis modification method and then the resulting supported catalyst will be tested
in the oxidation of o-aminophenol.
9
Firstly,
N,N’-bis[4-(N,N-diethylamino)salicylidene]ethylenediamine
(A-
Salen) was synthesized by the reaction of 4-(N,N-diethylamino)salicylaldehyde (ASal) and ethylenediamine (EDA).
The corresponding organic compound was
characterized using FTIR, DR UV-Vis, 1H- and 13C-NMR spectroscopy. After that,
copper(II) acetate monohydrate as metal source was coordinated with the prepared
CAS ligand to afford CAS complex. The metal complex was characterized using
FTIR and DR UV-Vis spectroscopy.
On the other hand, Si-MCM-48 was synthesized according to the procedure
as described by Lau (2005). Rice husk ash (RHA) obtained from open burning site
was used as silica source, while cetyltrimethylammonium bromide (CTABr) was
used as cationic surfactant. The calcined purely siliceous MCM-48 was further
modified by organo-functionalized agent (OFA) to afford amino- and sulfonic acidfunctionalized MCM-48. All the mesoporous silica was characterized using FTIR,
powder XRD and N2 adsorption-desorption isotherm.
Next, the prepared CAS complex was incorporated on Si-MCM-48 and OFMCM-48. The resulting MCM-48 containing CAS complex was characterized using
FTIR, powder XRD, DR UV-Vis, N2 absorption-desorption isotherm, AAS, TGA,
FESEM and TEM.
Finally, the corresponding MCM-48 containing CAS complex was tested in
the oxidation of o-aminophenol. The catalytic reaction was conducted in methanol at
70 oC, while aqueous hydrogen peroxide was used as oxidant. The catalytic reaction
was monitored by using GC-FID and GC-MS.
The retention of 2-amino-3H-
phenoxazin-3-one (the expected product) was determined using the standard that has
been prepared by simply oxidizing o-aminophenol at room temperature with open air
system. For comparison purpose, neat CAS complex and Si-MCM-48 were used in
catalytic testing.
Reusability, leaching test, effect of reaction parameters and
reaction mechanism were also studied. The outline of the study is shown in Scheme
1.1.
10
Synthesis of substituted salen
(4DEA-Sal) ligand and copper(II)
diethylamino-substituted salen
(CAS) complex
Synthesis of Si-MCM-48 and
organo-f unctionalized MCM-48
(NH2-MCM-48 and
SO3H-MCM-48
Characterization:
FTIR, DR UV-Vis and 1H- and
13
C-NMR spectroscopy
Characterization:
FTIR, XRD and N2 adsorptiondesorption measurement
Incorporation of CAS complex on MCM-48
CAS + Si-MCM-48
CAS-MCM-48
CAS + NH2-MCM-48
CAS-N-MCM-48
CAS + SO3H-MCM-48
CAS-S-MCM-48
Characterization of supported catalyst:
FTIR, XRD, DR UV-Vis, N2 adsorption-desorption
measurement, AAS, TGA, FESEM and TEM
Catalytic testing on the oxidation of o-aminophenol
Reusability and leaching
test study
Eff ect of reaction parameters study:
Reaction time
Temperature
Molar ratio substrate to oxidant
Dif ferent type of oxidant
Diff erent type of solvent
All the reactions was monitored by using GC-FID, while the reaction products
and prepared APX standard were analyzed using GC-MS
Scheme 1.1
Outline of study
CHAPTER 2
LITERATURE REVIEW
2.1
Transition Metal as Catalyst
Transition metals have been driven by their potential applications as
industrial catalysts for the last few decades. This is because the transition metals
consist of special novelty features, including (Masters, 1981):
(a)
The transition metals have high bonding ability or due to the exist of orbitals
d which provide much more valence shell orbitals to generate hybrid
molecular orbitals to bond with other moieties in the form of sigma (σ-) or pi
(π-) bonds;
(b)
Readily form chemical bonding with variety and also different types of
ligands, either the organic substances or inorganic compounds;
(c)
The bonded ligands can influence the behavior of transition-metal catalyst by
modifying its steric or electronic environment;
(d)
The transition elements have the ability to catalyze reaction between
coordinated substances due to the corresponding materials have high
coordination number; and
(e)
Ability of transition-metal readily to interchange their oxidation states during
catalytic reaction because of the materials have variability of oxidation state.
12
A great variety of chemical reactions have been carried out by utilizing
transition
metals
as
catalyst,
for
example:
hydrogenation,
isomerization,
dehydrogenation, asymmetric synthesis, oxidation, hydrosilylation as well as carboncarbon coupling reaction (Keim, 2004; Masters, 1981). Transition metals behave as
catalysts by itself or its complexes, whereas the transition metal complex are
typically referred to the metal that coordinated with ligand either the organic
substances such as Schiff bases or the inorganic compounds including halide group,
hydride, phosphine and etc (Wan Ibrahim, 2008).
In general, chemical industrial processes especially oxidation and reduction
involving transition metals or complexes can be carried out by using stoichiometric
or catalytic reaction method (Keim, 2004). The term “stoichiometric” reaction is
referred to the amount of catalyst (also function as oxidizing or reducing agents) use
should be equivalent to the reaction equation in order to gain a complete reaction
since the catalyst is consumed after reaction.
Thus, traditional stoichiometric
reaction method is known to generate high amount of harmful inorganic or organic
wastes, which is prohibited by the site of environmental point of view. On the other
hand, “catalytic” reaction is referred to the process that only requires small amount
of catalyst due to the catalyst itself is not being consumed. However, this catalytic
reaction typically requires promoters like oxidants or reducing agent in order to
generate the complete catalytic cycle. In this respect, clean promoters should be
chosen for the catalytic reaction with the purpose to reduce pollution. By contrast,
catalytic reaction method is deemed more beneficial and should be developed for
replacing stoichiometric reaction method.
13
2.1.1 Catalytic Properties of Copper Compounds
Copper is one of the transition elements that occupy Group 11 of the periodic
table with atomic number of 29. In general, copper commonly exists in the 0, +1, +2
and +3 oxidation states (Jacob et al., 1998a). Copper in zero oxidation state and
present in atomic form, typically is not reactive. Thus, atomic copper is normally not
directly used as catalyst but transformed to another form of mediates, such as
copper(I) or copper(II) ion, which are more active states.
On the other hand,
copper(III) that is not very stable has been suggested as an intermediate in certain
reactions involving catalytic amounts of copper(I) and copper(II) ion (Jacob et al.,
1998a; Kotschy and Timári, 2005).
Copper has been recognized as an important biological element since the
identification of numerous copper-containing active sites in numerous oxidase,
oxygenase and other metalloenzyme (Horváth et al., 2004). These copper-containing
proteins or enzymes play a role as redox catalysts in a range of biological processes,
such as electron transfer or oxidation of various organic substances. Actually, these
copper-containing proteins can be differentiated based on their function as metal ion
uptake, storage and transport; electron transfer; dioxygen uptake, storage and
transport; and catalysis (Koval et al., 2008). The coordination geometries adopted by
copper ions vary with the oxidation state, whereas copper(I) ions prefer linear,
trigonal and tetragonal geometries, while copper(II) ion prefer square planar, trigonal
bipyramidal and tetragonal or octahedral geometries.
In recent years, scientists have been able to identify seven different types of
copper active site in proteins by using crystallographic and spectroscopic techniques,
including copper with type 1 until type 4, CuA, CuB and CuZ (Koval et al., 2008).
Herein, characteristic of copper-containing proteins with type 1 until type 4 will be
only briefed as shown below:
14
(a)
Copper proteins with type 1 active site are recognized as “blue copper
proteins” due to their intense blue color in copper(II) state, which generally
participates in electron transfer processes. The copper center is surrounded
by two nitrogen donor atoms, a sulfur atom and a weak coordinated donor
atom, from protein moieties.
(b)
Copper proteins containing type 2 active site are recognized as “normal”
copper proteins due to their spectroscopic features similar to those of
common copper(II) ion coordination compounds.
These copper ions are
coordinated by four nitrogen atoms and/or oxygen atoms in either square
planar or distorted tetrahedral geometry (Solomon et al. 1996; Solomon et al.
1992).
These copper proteins are often involved in catalysis, such as
disproportionation of superoxide anion, C-H activation of benzylic substrates,
selective hydroxylation of aromatic substrate and oxidation of primary
alcohol.
(c)
Copper proteins with type 3 active site consist of dinuclear copper core
features, whereas each ion being bonded by three nitrogen atoms from
proteins (Solomon et al. 1996).
The two copper ions are strongly
antiferromagnetically coupled and these types of proteins have ability to
reversibly bind dioxygen under ambient conditions.
(d)
Copper proteins with type 4 active site also multicopper oxidases which are
usually composed of type 2 and type 3 active sites to form a trinuclear cluster
(Solomon et al. 1996). In some cases, such proteins also contain at least one
type 1 active site. These copper-containing proteins are able to catalyze a
wide range of organic oxidation reactions.
Over the last few years, the uses of copper as active element have been
widely applied in catalysis. According to Punniyamurthy and Rout (2008), copper
compounds have been the subject of intensive study due to its ability to catalyze
various oxidation processes. Their survey shows that copper compounds are able to
catalyze epoxidation, oxidation of alkane, benzylic oxidation, aromatic C-H
oxidation, Glaser-Hay acetylenic coupling reactions, alcohol oxidation, BaeyerVilliger oxidation, sulfoxidation and allylic oxidation by using dioxygen or peroxide
as oxidant.
Gichinga and Striegler (2008) reported that dinuclear copper(II)
15
pentadentate Schiff base complex was very active for the aerobic oxidation of 3,5-ditert-butylcatechol to 3,5-di-tert-butylquinone in methanol-aqueous phase. On the
other hand, copper compounds have also been employed as catalyst in carbon-carbon
bond formation reaction. For example, Kobayashi et al. (1999) has successfully
carried out the Mukaiyama-aldol reaction with moderate to excellent yields by using
chiral copper(II)-bis(oxazoline) complex as catalyst in ethanol-water phase at -15 to
0 oC. Meanwhile, Reetz and co-workers (2007) also reported that Cu-phthalocyanine
successfully catalyzed and improved the Diels-Alder reactions of azachalcones with
cyclopentadiene in aqueous media in the presence of serum albumins up to 98% of
endo-product.
Based on the reviews by Punniyamurthy and Rout (2008), it was shown that
copper(II) is more preferably used for catalytic study than copper(I). This is so
because the copper(I) compounds are air-sensitive and easily oxidized to copper(II).
Furthermore, copper(II) has long been found to be a good catalyst to oxidize a wide
range of organic substances with high selectivity due to its mild oxidizing power and
compatibility with a variety of solvent systems (Jacob et al., 1998a).
2.2
Schiff Base Salen Ligand and Its Complexes
Over the past few years, much effort has been focused on the synthesis and
investigation of various substituted metal complexes in order to develop an effective
and efficient catalyst models that mimic the activity of certain enzymes. Schiff base
transition metal complexes are more extensively studied because of their potential
applicability as catalysts for wide range of organic reactions. Transition metals are
used as catalysts active site due to certain advantages as mentioned before. While,
Schiff bases which have been recognized as “privileged ligand” is the popular
ligands for research study due to the several reasons, including:
16
(a)
Cheap production and ease of synthesis have attributed to the ability of bulk
production;
(b)
The multidentate characteristic of Schiff base ligands can stabilize different
type of metals in various oxidation states (Ribeiro da Silva et al., 2004;
Caselli et al., 2005);
(c)
Modifiable aromatic ring enable substitution of different functional group and
the substitution at the aromatic ring can change the electronic and steric
properties of the metal complexes; and
(d)
Controllable and modifiable stereochemistry of the metal complexes which
contribute to the numerous examples of unusual geometries about the central
metal ion (Marchetti et al., 1999).
The term “Schiff base” is referred to organic compounds that consist of
azomethine or imine or known as carbon-nitrogen double bond functional group.
The general formula for Schiff bases are R1R2C=NR3, where either R1 or R2 is
hydrogen atom or both R1 and R2 are substituted alkyl or aryl group and R3 is an aryl
or alkyl group. Schiff bases are generally synthesized by the condensation of an
amine group compound and a carbonyl group substance as shown in Figure 2.1
Figure 2.1
Synthesis of Schiff bases compound by the condensation of an amine
group compound and a carbonyl group substance
Tetradentate Schiff base is an azomethine functionalized chelating compound
containing four species of electron donor. N,N,O,O-tetradentate donor is one of the
Schiff base ligands that possesses numerous features such as facile approach, relative
tolerance, readily adjustable ancillary ligands as well as provides tunable steric and
electronic coordination environments on the chelated substances (Wang et al., 2003).
Salen and its derivatives are among the member of N,N,O,O-tetradentate Schiff-bases
17
ligands which have been received more attention recently. The structure of Schiff
base tetradentate salen-type ligand (5), salen ligand (6) and its derivatives are
illustrated in Figure 2.2. Schiff base tetradentate salen-type ligand (5) and its metal
complexes were firstly discovered in 1889 by Combes while studying the effect of
diamines on diketones (Canali and Sherrington, 1999). After that, salen derivatives
and their metal complexes have been gradually synthesized and characterized.
N
N
N
N
OH HO
OH HO
(5)
(6)
N
N
N
N
OH HO
Ph Ph
R
R
OH HO
R
(7)
Figure 2.2
R
(8)
The structure of Schiff base tetradentate salen-type ligand (5), salen
ligand (6), Jacobson’s ligand (7) and Katsuki’s ligand (8) (Canali and
Sherrington, 1999)
Salen is the abbreviation of ligand which typically called as N,N’ethylenebis(salicylidenaminato) or N,N’-bis(salicylidene)ethylenediamine, whereas
the term “sal” is referred as salicylaldehyde and the term “en” is due to
ethylenediamine as the ligand backbone which joins up the two molecules of
salicylaldehyde compound. The ligands abbreviation is changed based on the type of
salicylaldehyde and diamine compound are used. Salen ligand and its derivatives
(11) can be easily synthesized by condensation of 2 molecules of salicylaldehyde
derivatives (9) with 1 molecule of diamine compounds (10) as shown in Figure 2.3
(Cevik et al., 2005). Salen ligand and its derivatives (11) can bind with metal ion to
18
form metal complexes (12) through the coordination bonding from two atoms
nitrogen in imine groups and two atoms oxygen of phenolic groups (Cevik et al.,
2005; Venkataramanan et al., 2005).
Complexation of salen ligand and its
derivatives (11) with metal ion is illustrated in Figure 2.4.
CH2
O
H
R1
OH +
2 R2
R3
H2N
NH2
CH2
x
R4
(9)
R2
x
N
R1
OH HO
R3
(10)
N
R1
R4
R2
R4
R3
(11)
R1, R2, R3, R4 = organic or inorganic substituents
x = 2, 3, 4, 5 and etc.
Figure 2.3
Synthesis of salen and its derivatives (11) by the condensation of
salicylaldehyde derivatives (9) and diamine compounds (10)
Figure 2.4
Synthesis of metal salen complex and its derivatives (12) by the
reaction of salen ligand and its derivatives (11) with metal ion
19
Previous studies have shown that metal Schiff base salen complexes are
conformationally flexible and adopt a variety of geometries such as planar, umbrellatype and stepped conformations (Figure 2.5) that create various active site
environments for different chemical reaction (Deshpande et al., 1999; Bhadbhade
and Srinivas, 1993; Bhadbhade and Srinivas, 1998).
The characteristic of
tetradentate-binding motif characteristic of Schiff base salen mimics the porphyrin
framework in the heme-based oxidative enzyme, which has shown coordination of
metals that leave the two axial sites open for ancillary ligands as illustrated in Figure
2.6 (Venkataramanan et al., 2005; Fujii, 2002; Solomon et al., 1992; Cevik et al.,
2005).
Nonetheless, salen and its derivatives are more easily synthesized and
manipulated than porphyrins (Venkataramanan et al., 2005; Cevik et al., 2005).
Thus, various models of metal complexes containing Schiff base salen ligands have
been synthesized in order to study their catalytic activity as well as to develop the
catalyst systems that have catalytic activity mimicking natural enzyme.
X
N
O
M
N
O
N
O
Square planar conformation
X
Umbrella-type conf ormation
X
N M N
O
X
O
O
M
N
N
O
Stepped conformation
Figure 2.5
N
O
X
X
X
M
X
M = metal ion
Possible coordination geometries of metal Schiff base salen
complexes
20
Ancillary ligands
adducted
H3C
H3C
CH3 CH2
H2C
N
N
Fe
N
N
CH3
O OH HO
O
Ancillary ligands
adducted
N
O
M
N
O
Ancillary ligands
adducted
Ancillary ligands
adducted
Iron porphyrin active site
of Cytochrome P-450
Metal salen complex
Figure 2.6
Two axial
open site
Conformation of salen complex that mimic to porphyrin structure of
Cytochrome P-450 enzyme
According to the previous studies, salen complexes based transition metals as
active site are considered as a synthetic oxygen carries that mimic to the oxygen
carrying metalloenzymes and oxygenases, such as cytochrome P-450 and
hemoglobin, which play important roles in the catalytic oxidation of various organic
reactions (Bahramian et al., 2006a; Katsuki, 1996; Mac Leod et al., 2007; Mirkhani
et al., 2006, Meunier, 1992; Mansuy, 1993). Some of the common salen complexes
such as Mn(salen), Fe(salen), Cu(salen), Co(salen) and Ni(salen) are considered as
artificial metalloenzyme that mimic to the activity of Cytochrome P-450
(Venkataramanan et al., 2005; Poltowicz et al., 2006; McMorn and Hutchings,
2004). In this respect, various transition metal Schiff base salen complexes have
been synthesized for oxygen uptake study.
Venkataramanan and co-workers (2005) have mentioned that transition metal
salen complexes of Mn, Cr, Fe, Ru, Co, V and Ti, are widely used as efficient
catalysts for the selective oxygenation of organic sulfides, sulfoxides and aromatic
amines by using terminal oxidants as oxygen source. Besides, transition metal Schiff
base salen complexes are also active in epoxidation, oxidation of alcohol, BaeyerVilliger oxidation, oxidation of olefins and etc. Certain metal-containing Schiff base
salen complexes have also been used for carbon-carbon formation process. Recently,
Wan Ibrahim (2008) has reported on the Heck and Suzuki reaction over neat and
supported palladium(II) coordinated with N,N’-bis(4-methyl-α-methylsalicylidene)-
21
2,2-dimethylpropane-1,3-diamine, N,N’-bis(salicylic-dene)-2,2-dimethylpropane-1,3diamine
and
N,N’-bis(5-bromosalicyli-dene)-2,2-dimethylpropane-1,3-diamine,
respectively.
2.2.1 Copper(II) Schiff Base Salen Complexes in Organic Reaction
On the basis of literature reviews on the Schiff base salen complexes
containing copper, it can be inferred that most of the copper type that coordinated
with Schiff base ligand in single nuclear is copper(II) ion. As mentioned before, the
tetradentate-binding motif of Schiff base salen is typically in planar form which is
excellently adapted to the preferable coordination geometries of copper(II) ion in
square planar form. Thus, copper(II) is the best candidate for the synthesis of
mononuclear copper(II) Schiff base salen complexes.
Copper(II) Schiff base salen complexes have the metal-ligand coordination
mimic of galactose oxidase, which consists of “CuN2O2” group as shown in Figure
2.7. Galactose oxidase ia a mononuclear copper-containing enzyme, which catalyzed
the stereospecific oxidation of D-isomers of a range of primary alcohol substrates,
such as D-galactose and polysaccharides with D-galactose at their non-reducing end,
to the production of aldehyde (Baron et al., 1994, Wilkinson et al., 2004,
Punniyamurthy and Rout, 2008; Chaudhuri et al., 1999; and Wang and Stack, 1996).
22
Figure 2.7
Metal-ligand coordination of copper(II) salen complex mimic to
active site of galactose oxidase
Recently, chiral as well as achiral copper(II) Schiff base salen complexes
were employed in the oxidation of methyl aryl sulfide at ambient temperature by
using aqueous hydrogen peroxide as oxidant (Punniyamurthy and Rout, 2008; Zhu et
al., 2004; Velusamy et al., 2005). On the other hand, the same reaction with high
conversion and selectivity was also catalyzed by chiral copper(II) salen complex
immobilized on MCM-41 in the presence of tert-butyl hydrogen peroxide as oxidant
(Punniyamurthy and Rout, 2008; Ayalaa et al., 2004). A series of reusable Schiff
base complexes supported on SiO2 and Al2O3 have been applied in the oxidation of
cyclohexene in the presence of peroxide oxidant and the reactions gave a mixture of
cyclohexenone and cyclohexenol as major products (Punniyamurthy and Rout, 2008;
Mukherjee et al., 2006; Salavati-Niasari et al., 2005).
Jacob and co-workers (1998b) reported that the decomposition of hydrogen
peroxide over encapsulated substituted copper salens approach those of natural
catalase enzymes and correlate well with the peroxidative oxidation of phenol to
dihydroxy benzenes. A study by Deshpande et al. (1999) showed that the zeoliteencapsulated copper(II) salen and copper(II) chloro-salen complexes were active in
the oxidation of p-xylene to p-toluic acid as well as selective in the oxidation of
phenol to catechol by using hydrogen peroxide and tert-butyl hydrogen peroxide as
oxidants. Based on the reviews, it can be established that copper(II) Schiff base
23
salen complexes are also active in the decomposition of peroxides, instead of
dioxygen, to afford oxygen atom that is important in the oxidation process.
2.3
Heterogeneous Catalyst
Catalysts can be classified into two large groups according to the state of
aggregation in which they act: homogeneous and heterogeneous (Hagen, 2006).
Homogeneous catalysts are defined as the catalysts that are present in the same phase
as the reactants and products, while heterogeneous catalysts are the catalysts present
in different phase to the reaction medium. Overview comparisons of homogeneous
and heterogeneous catalysts are summarized in Table 2.1.
Table 2.1
Comparison of homogeneous and heterogeneous catalysts (Hagen,
2006)
Homogeneous
Heterogeneous
Active center
All metal atoms
Only surface atoms
Concentration
Low
high
Selectivity
High
Lower
Diffusion problems
Practically absent
Present
Reaction conditions
Mild (50-200 oC)
Severe (> 250 oC)
Applicability
Limited
Wide
Activity loss
Irreversible reaction and
Sintering of metal
poisoning
crystalline; poisoning
Defined
Undefined
Modification possibilities
High
Low
Thermal stability
Low
High
Complex
Easy by only filtration
Cannot or difficult
Can
high
Low
Structure/stoichiometry
Catalyst separation
Reusability
Cost of catalyst losses
24
Nowadays, applications of homogeneous catalytic systems are still being
driven far away in front of heterogeneous catalytic systems in industrial sector. This
may be due to the heterogeneous catalysts have lower catalytic activity if compared
to the homogeneous catalysts. However, these homogeneous catalysis processes
have generated a lot of inorganic and organic wastes that promote environmental
pollution. In addition, some of the industrial wastes are toxic and harmful to our
health, especially the waste containing heavy metal elements, chlorinated organic
compounds and radical substances.
Besides that, the increasing stringent
environmental constraints have promoted industrial sectors to find alternatives to
reduce or eliminate the use or generation of hazardous wastes. In this respect,
heterogeneous catalysts which are separable and reusable can play a key role in
replacing the conventional homogeneous catalysis processes.
Indeed, there are many ways of performing the heterogeneous catalytic
reaction. Practically, the easiest approach is to employ solid catalysts directly in
“single-pot reaction”.
However, only a few solid catalysts are commercially
available and their applications are very specific for certain reactions only. Although
recent technologies have successfully produced solid catalysts in nanosize, purposely
designed to increase the surface area of active sites that are expected to increase
again the reaction rate, but the active site not dissolvable in reaction medium causes
the solid catalysts cannot be performed as well as homogeneous catalysts. By the
way to gain the advantageous of homogeneous catalyst, such as high catalytic
activity and product yield, meanwhile consist of heterogeneous properties, including
recoverable, long catalytic life cycle, high thermal stability and reusable,
heterogenization of homogeneous catalysts on the solid supports will be an ideal
method to generate heterogeneous catalysts (Chaube et al., 2005; Bahramian et al.,
2006a; Mac Leod et al., 2007; Kozlov et al., 1998; Leadbeater and Marco, 2002).
25
2.3.1 Heterogenization of Homogeneous Catalysts on the Solid Supports
As mentioned before, homogeneous catalytic system meets with several
shortcoming, especially cannot be reused and generate a lot wastes. Besides that,
oxidative catalytic reactions that catalyze by homogeneous transition metal and its
complexes are often suffered with deactivation problem due to the homogeneous
catalysts are being easily self-oxidized and form the μ-oxo dimers or polymeric
species which are not active (Mirkhani et al., 2006; Collman et al., 1995; Srinivasan
et al., 1986; Poltowicz et al., 2006; Varkey et al., 1998). Therefore, the development
of heterogeneous catalysts by immobilizing the homogeneous catalysts on support
materials is considered as novel approach to create a “green chemistry” reaction,
meanwhile to overcome the shortcomings of homogeneous catalytic system.
There are many possible strategies and pathways to support homogeneous
catalysts on the solid matrix, including encapsulation, ion exchange, covalent
grafting or anchoring, surface coating, and electrostatic interaction (Taguchi and
Schüth, 2005; Bahramian et al., 2006a; Gupta et al., 2009). On the other hand, there
are many materials which can be used as solid supports, such as clays, polymers,
electrode graphite, carbon nanotubes, metal oxide, zeolites and ordered mesoporous
silica (Mirkhani et al., 2006; Bahramian et al., 2006a; Huang et al., 2008; Liu et al.,
2009; Knoll and Swavey, 2009; Taguchi and Schüth, 2005). An ideal solid support
should be resistance to thermal and chemical effect as well as does not hinder or
destroy the active elements. In this respect, ordered mesoporous silica will be an
excellent choice as support matrix due to their favorable properties that may achieve
the requirement as mentioned before.
26
2.3.2 M41S Molecular Sieves as Mesoporous Catalyst Support
Porous solid materials have attracted much attention because of their
potential application in chemical separation, heterogeneous catalyst and catalyst
supports. In the last few years, many open-framework inorganic materials with well
defined geometry and pore shapes have been synthesized, such as zeolites and
MCM-41.
Zeolites have been widely used in petroleum refining and petrochemical
industries since 1960s. This is because zeolites have excellent thermal and chemical
resistance as well as provide acidity which makes its preferable to be used as solid
acid catalyst. Unfortunately, the applications of zeolites in catalysis are limited due
to the mass transfer problem when applied in the chemical reactions that involve the
large or bulky reactant molecules. This problem promoted the trend to develop
larger pore size molecular sieves with well-defined pore channel. In 1992, Mobil Oil
Corporation successfully discovered a new family of silica-based molecular sieves
designated as M41S, which possesses regular, well-defined and ordered meso range
porous channel (Kresge et al., 1992; Beck et al., 1992).
M41S family is one of the ordered mesoporous silicate members, which can
be obtained via liquid crystal templating mechanism under basic conditions (Kresge
et al., 1992; Beck et al., 1992). M41S comprises three main mesophases including,
MCM-41 which is a hexagonal array of uniform pore channel; MCM-48 which is a
three dimensional cubic pore system; and MCM-50 which is an unstable lamellar
framework structure (Vartuli et al., 1994). These mesoporous molecular sieves are
generated through the coorperative and self-assembly of surfactant in certain shape,
followed by the migration and polymerization of anionic silicate and finally form the
desired pore structure of materials. Calcination or removing the organic surfactant
by burning process affords purely siliceous mesoporous molecular sieves. The flow
in the synthesis of purely siliceous mesoporous materials by using liquid crystal
templating mechanism is shown in Scheme 2.1.
27
Calcination
Calcined MCM-41
As-synthesized MCM-41
surfactant
Aggregation of
surfactant and
silica
Calcination
Calcined MCM-48
As-synthesized MCM-48
Silica
Calcination
As-synthesized MCM-50
Scheme 2.1
Calcined MCM-50
The flow in the synthesis of purely siliceous mesoporous materials
The unique features of such amorphous mesoporous silica are long ordered
framework with uniform pore channel; have a narrow pore size distribution with
tunable pore diameter in the range between 1.5 to 20 nm, possess large surface area
that can be up to 1000 m2/g and high pore volume, as well as possess modifiable
silanol surface (Kresge et al., 1992, Taguchi and Schüth, 2005; Chandrasekar et al.,
2008). This purely siliceous mesoporous silica does not have sufficient catalytic
properties due to absence of catalytically active sites in the silica framework. Thus,
M41S is often modified by introducing metals during the synthesis of mesoporous
silica or impregnation of active substances on the surface of as-prepared mesoporous
silica.
Among the M41S family, MCM-50 is totally not applicable for use as
material support due to the silica framework of the lamellar phase MCM-50 easily
collapses upon calcination.
Recently, ordered mesoporous silica has received considerable attention as
solid support in catalysis field due to their favorable properties as mentioned before.
A variety of metal salts and metal complexes supported on mesoporous matrix has
been reported successfully applied in wide range of liquid phase reactions, which
including epoxidation, dehydrogenation, carbon-carbon formation and esterification
28
(Yu et al., 2009, Jana et al., 2008; Diaz et al., 2000; Li and Rudolph, 2008). It has
been reported that these supported catalysts can be separated easily from the reaction
medium and the recovered catalysts can be reused again without the loss of their
catalytic performance. On the other hand, mesoporous silica supported catalyst also
has been successfully applied in gas phase reaction. For example, Chen and coworkers (2002) reported the gas phase isomerization of butane over aluminapromoted sulfated zirconia supported on mesoporous silica and propene
hydrogenation over molybdenum carbides immobilized on MCM-41 with reaction
carried out in flow reactor at atmospheric pressure was reported by Piquemal et al.
(2004).
Because of the pore size of mesophase silica can be enlarged up to 20 nm,
scientists thus showed interest to investigate the physiochemical and catalytic
properties of mesoporous silica encapsulated bulky catalysts, especially enzyme
molecules. Based on the reviews by Lee et al. (2009) and Zhao et al. (2006), they
showed that mesoporous silica has been successfully used as catalyst support for the
immobilization of variety type of enzymes. Subsequently, the enzyme confinements
in the pore channel of mesoporous silica generated synergistic effects that enhance
enzyme stability, improve product selectivity and facilitate separation and reusability
of enzymes.
2.3.3 Immobilization of Homogeneous Catalyst on Mesoporous Silica
Lee et al. (2009) and Zhao et al. (2006) have introduced a few commonly
methods for the immobilization of homogeneous catalysts based on the interaction
between the catalyst and the support matrix. Indeed, the immobilization of the
homogeneous catalysts on mesoporous silica can be categorized into three main
groups, which including physical adsorption, electrostatic interaction and covalent
bonding.
29
Physical adsorption is one of the simplest methods to immobilize
homogeneous catalysts, especially metal complexes and enzymes. This supporting
approach usually involved the formation of weak interaction such as hydrogen
bonding, hydrophobic and van der Waals attraction, which is considered do not
directly affect to the active site of catalysts. An example of physical adsorption of
chiral organometallic catalysts, Rh-BPPM and Ru-BINAP, into mesoporous silica
matrix for enantioselective hydrogenation was done by Jamis and co-workers (2000).
The corresponding catalysts have successfully catalyzed the hydrogenation of
cinnamic acid derivatives with 40-60% ee, and the recovered catalysts could be
reused. The possible chemical interactions between the supported catalyst and the
mesoporous silica matrix are illustrated in Figure 2.8.
Hydrogen
bonding
OH
X
OH
Catalyst
OH
Catalyst
van der Waals
attraction
OH
OH
OH
OH
OH
Mesoporous silica
Mesoporous silica
X = -OH or -NH2
Catalyst
Hydrophobic
attraction
R
R
(CH2) (H2C)
Si OMe
MeO Si
O
O
O
O
Mesoporous silica
R = non-polar functional group
Figure 2.8
The possible chemical interaction between mesoporous silica and the
supported catalysts prepared by immobilization via physical
adsorption method
30
Electrostatic attraction is one of the favorable methods that can be used in the
immobilization of catalysts on solid support via ionic interaction. Immobilizing via
electrostatic attraction is a facile method for supporting ionic catalysts or those
catalysts that can ionize under the immobilization condition (Zhao et al., 2006). The
purely siliceous mesophase silica is normally a neutral material.
Thus, some
modifications have to proceed in order to generate the ionic charge molecular sieves.
In general, ionic charges of mesophase silica pore wall can be varied to provide
appropriate condition for immobilization of catalyst. To create the negative charge
of mesoporous silica, aluminium atoms or negatively charge producible OFA can be
incorporated into the silica framework or on the silica wall (Lee et al., 2009).
Wagner et al. (2001) has reported work on the immobilization of chiral cationic
rhodium(I) diphosphine complex on Al-MCM-41 via ionic attraction, and the
resulting catalyst performed well in asymmetric hydrogenation of dimethyllitaconate
with high reusability.
The ionic interaction of cationic rhodium(I) diphosphine
complex and anionic host framework of Al-MCM-41 is shown in Figure 2.9.
P
Rh
P
O
O
Si
O
Figure 2.9
O
Al
O
O
Si
O
O
Ionic interaction of cationic rhodium(I) diphosphine complex and
anionic host framework of Al-MCM-41
On the other hand, mesoporous silica modified with functional group that can
generate positively charge, such as amino group, which is able to bind with
negatively charge catalysts. An example of ionic attraction of metal complex has
been reported by Pirouzmand and co-workers (2008).
They have successfully
immobilized iron tetrasulfophthalocynine on NH2-MCM-41 and NH2-MCM-48
respectively via electrostatic bonding (Figure 2.10) and the supported catalyst was
tested in oxidation of styrene.
31
Electrostatic interaction
Mesoporous silica
Figure 2.10
SO3H
O
O S
H
N
O
H H
OMe
O
Si
O
OH
OH
O
O Si
O
N
N
Fe
N
N
H
NO S
O
H H
O
SO3H
Electrostatic attractions between amino-functionalized mesophase
silica and iron tetrasulfophthalocynine
Covalent bonding is the most frequently used method for immobilization of
homogeneous catalysts, because the corresponding technique generally gives better
attachment and high stability against leaching, which can be allowed for repeating
use (Lee et al., 2009). Grafting or impregnation and tethering are the common
covalent attachment of catalyst on solid support, either directly or through a spacer
ligand, also called as organo-functionalization agent (Srinivas and Sivasanker, 2003).
Impregnation or grafting is typically referred to the direct covalent attachment of
metal salts to the unmodified silanol group of mesoporous silica. MCM-48 with
different ratio of Si/Sn was prepared by wet impregnation of SnCl2 on Si-MCM-48
has been reported by Endud and Wong (2007). They found that the tin-containing
MCM-48 performed well in the oxidation of benzyl alcohol to benzaldehyde and
successfully in retaining its catalytic activity and selectivity for up to 3 successive
runs. Figure 2.11 shows the covalent attachment of tin to silanol group of MCM-48.
O
Si
OH
OH
Sn
Sn
O
Si
O
Si
O
O
Si
O
Si
Covalent
bonding
O
Si
MCM-48
Figure 2.11
Impregnation of tin on MCM-48
32
Tethering is generally referred to covalent attachment of metal salts and metal
complexes to the solid support through a spacer ligand. In this respect, the silanol
surface of mesoporous silica has to be modified with OFA before immobilization of
catalysts. For efficient immobilization of catalysts, the mesoporous silica should be
modified either with nucleophilic functionalization agent, such as amino and thiol
groups, or with electrophilic functionalization agent, including carboxylic acid and
alkyl halide groups (Lee et al., 2009). Covalent attachment can occur either between
spacer ligand and metal or between spacer ligand and ligand of metal complex as
shown in Figure 2.12.
Covalent
bonding
NH2
OH
NH2
Spacer
ligand
Si
Si
Si
O O O O O O O O O
Mesoporous silica
H2N
O
V
O
O
Spacer
ligand
Si
Covalent
bonding
S
R1
H
N
O
R2
H
N
Mn
tBu
O
tBu
O
Mesoporous silica
(a)
Figure 2.12
(b)
Immobilization of catalysts on mesoporous silica via covalent
bonding that occurred between spacer ligand and (a) metal (Lee et al.,
2003) or (b) ligand of metal complex (Yu, et al., 2009)
2.3.4 Mesoporous Silica MCM-48 as Catalyst Support
The structure characterization by using powder X-ray diffraction and
transmission electron microscopy have shown that mesoporous silica MCM-48 exists
as a three dimensional cubic Ia3d space group and possesses bicontinuous structure
with amorphous silica walls follow the gyroid minimal surface that divides available
pore space into two non-intersecting subvolumes (Beck et al., 1992; Monnier et al.,
1993; Alfredsson and Anderson, 1996; Solovyov et al., 2005). Figure 2.13 shows
33
the model of gyroid minimal surface of MCM-48.
This enantiomeric pair of
independently interpenetrating three dimensional networks of pore channels is
believed beneficial for mass transfer kinetics, whereas the cubic MCM-48 has much
more resistant to pore blockage (Ryoo et al., 1999; Xu et al., 1998). From this point
of view, such highly interwoven structure of MCM-48 is supposed to be more
attractive and applicable as catalyst support than MCM-41. However, it can be
observed that catalytic studies based on MCM-48 as supported material are relatively
few. This may be because of the difficulty to achieve highly pure phase of MCM-48
and the framework is prone to collapse due to hydrolysis when explored to moisture
(Ryoo et al., 1999; Jun et al., 2000).
Figure 2.13
Model of gyroid minimal surface of MCM-48 (Armatas and
Kanatzidis, 2006)
Recently, many efforts have been focused on the production of highly
ordered, pure phase and hydrothermal stability of MCM-48 molecular sieve. For
example, the mixed cationic-neutral surfactant method that discovered by Ryoo et al.
(1999) was reported produce highly ordered mesophase MCM-48 by using low
molar ratio of surfactant to silica. On the other hand, the work done by Jun et al.
(2000), Kim et al. (2002) and Wang et al. (2007) have been successively improved
the hydrothermal stability of MCM-48.
Wang et al. (2007) reported that
hydrothermal stability of MCM-48 can be enhancing by introducing fluoride salts,
34
which was expected can improve the condensation of silicate, during the synthesis of
MCM-48. While, Jun et al. (2000) and Kim et al. (2002) reported the hydrothermal
stability of MCM-48 can be improved by post-synthesis restructuring of assynthesized MCM-48 in fluoride solution.
Besides that, the studies were also
focused on the optimization of the production of larger pore diameter of MCM-48.
The successive improvement of the physiochemical properties of mesophase
MCM-48 has promoted the utilization of MCM-48 as catalyst support. As observed,
the mesoporous silica MCM-48 has been successfully used to immobilize a wide
variety of metal salts and metal complexes. These MCM-48 supported catalysts also
has been well applied in various liquid phase reaction, including epoxidation of
olefin, dehydrogenation of alcohol, acylation of aromatic compound and
esterification of fatty acid and alcohol. Besides that, MCM-48 supported catalyst
also applicable in gas phase reactions.
For the examples, oxidation of carbon
monoxide over MCM-48 containing Au/TiO2 has been reported by Narkhede et al.
(2009) and isomerization of butane in vapor form over Al-MCM-48 was done by
Russo and co-workers (2008). More recently, comparison of catalytic property of
catalyst supported on MCM-41 and MCM-48 was done by Pirouzmand et al. (2008),
which showed that the functionalized MCM-48 adsorbed twice more iron
tetrasulfophthalocynine than functionalized MCM-41.
They also found that the
catalyst anchored on MCM-48 showed higher activity and durability than MCM-41
in oxidation of styrene.
Recently, the research based on MCM-48 as catalyst support is still being at
the fundamental state. Therefore, the heterogeneous catalytic reaction catalyzed by
MCM-48 modified with homogeneous catalyst is an interesting topic for research
study. In this study, the ordered mesophase MCM-48, thus was chosen as a catalyst
support and tested in the oxidation reaction.
35
2.4
Oxidative Coupling – Synthesis of Phenoxazinone Chromophore
Oxidative coupling is an oxidation reaction that undergoes two states of
process, whereas the organic reactants are initially oxidized to afford intermediates
that generally are not stable and then further attach to another reactants or
intermediates to form the final products. In the other words, oxidative coupling is
the process that involves dimerization or combination of the small organic molecules
to form larger molecular weight organic compounds. Oxidative coupling can occur
between C-C bonding as well as heteroatom bonding such as C-O and C-N.
Oxidative coupling in an important reaction in the synthesis of naturally
occurring bioactive organic compound, actinomycins (10), from the starting material
peptide substituted o-aminophenol (9) catalyzed by phenoxazinone synthase (a
copper-containing enzyme) in soil bacteria (Barry et al., 1989, Simándi et al., 2004;
Simándi et al., 1996; Szihyártó et al., 2006). The typical reaction is illustrated in
Figure 2.14.
Figure 2.14
Oxidative coupling of peptide-substituted o-aminophenol (13) to
actinomycin chromophore (14) by phenoxazinone synthase in soil
bacteria
36
Actinomycins or normally referred to actinomycin D is a naturally occurring
organic compound which consists of a substituted phenoxazinone ring bonded to two
identical cyclical pentapeptides (Mauger and Lackner, 2005; Jones, 1986).
Actinomycins are a powerful tumor-inhibiting antibiotic due to its ability as potent
inhibitors of DNA-dependent RNA synthesis (Mauger and Lackner, 2005; Nogrady
and Weaver, 2005; Sehgal et al., 1987). However, these bioactive compounds are
particularly limited in the treatment of Wilm’s tumor, choriocarcinoma, adult
Ewing’s and Kaposi’s sarcoma due to their high toxicity properties (Gringauz, 19;
Barry et al., 1989). With respect to their high potential tumor-inhibiting ability,
many researches have been carried out in order to modify their cytotoxicity as well
as to improve or broaden their antitumor activities (Meienhofer, 1970). Previous
studies showed that actinomycin binds to DNA by intercalation of the phenoxazinone
chromophore, while the cyclic peptide confers sequence specificity to adjacent GC
base pairs (Barry et al., 1989; Bolognese et al., 2002a). Thus, it is conceivable that
phenoxazinone compounds individually may be also having the anticancer properties
(Toader et al., 2006).
Phenoxazinone or 3H-phenoxazin-3-one (4) as shown in Figure 1.3 is one of
the members of quinonic compounds, which consists of tricyclic iminoquinone
skeleton as shown in Figure 1.1 (Bolognese et al., 2002b; Hasegawa and Ueno,
1985). Most of the organic compounds that consist of quinonic nucleus are typically
have biological active properties. This may be due to their ability to blind strongly
between base pairs of DNA through hydrogen bonds and π-stacking interaction
(Bolognese et al., 2002b). Thus, it has promoted the studies in the designing and
synthesis the various type of phenoxazinone chromophore in order to investigate
their potential biological activities. Previous studies showed that phenoxazinone
chromphore and its derivatives can be synthesized by two main approaches, which
are including: (i) oxidative coupling of the two similar molecules of o-aminophenol
compounds or by the use of only one reactant in reaction (method I); and (ii)
oxidative coupling of one molecule of o-aminophenol compounds and one molecule
of p-quinone compounds (method II) (Barry et al., 1989; Bolognese et al., 2002a).
Such reaction processes are illustrated in Scheme 2.2.
37
(Method I)
R1
R1
R1
NH2
N
NH2
OH
O
O
2
R2
R2
R2
Phenoxazinone chromophore
o-Aminophenol
(Method II)
R31
R7
R3
R4
NH2
R5
OH
O
R7
R8
R4
N
R8
O
R5
O
O
+
R9
R6
o-Aminophenol
p -Aminoquinone
R6
R9
Cross-coupled
phenoxazinone chromophore
R = inorganic or organic substitution
Scheme 2.2
Two different ways in the synthesis of phenoxazinone chromophore
The second method seems more useful for the purpose of synthesizing a wide
variety of phenoxazinone chromophores. However, Barry et al. (1989) found that
phenoxazinone synthase is reactive in catalyzing reaction method I, but cannot
catalyze the cross-coupled reaction as shown in method II. They suggested that
phenoxazinone synthase was catalyzed the peptide-substituted o-aminophenol in
final step biosynthesis of actinomycin to o-quinone imine (QI) as intermediate, and
the reactive QI intermediate would then be coupled by the second molecule of
peptide-substituted o-aminophenol to afford phenoxazinone chromophore. To our
best of knowledge, most of the catalytic studies on oxidation of o-aminophenol to
phenoxazinone compound are referred to the process of the first method, with the
aim to produce the artificial catalysts that mimic to phenoxazinone synthase activity.
38
Generally, the catalytic testing in the oxidation of o-aminophenol as model
reaction is intensively carried out by using transition metal salt or its complexes as
catalyst. In the earlier model reaction testing, copper metal and copper salts are used
to catalyze oxidation of o-aminophenol to 2-amino-3H-phenoxazin-3-one in the
presence of dioxygen, while the m-aminophenol and p-aminophenol gave the
different products (Szihyártó et al., 2006; Kaizer et al., 2002; Prati and Rossi, 1992).
o-Aminophenol catalyzed by cobalt(II) phthalocyanine derivative have been reported
(Szeverényi et al., 1991).
High conversion of o-aminophenol with selective
formation of 2-amino-3H-phenoxazin-3-one has been catalyzed by Co(salen) in the
methanol solvent by using dioxygen at room temperature (Maruyama et al., 1996).
Previous studies show that cobaloxime(II) and ferroxime(II) exhibit both catecholase
and phenoxazinone synthase activity (Szeverényi et al., 1991; Simándi et al., 1993;
Simándi et al., 1996; Simándi et al., 2004; Simándi and Simándi, 1999). Szihyártó et
al. (2006) reported kinetics and mechanism of catalytic oxidation of 2-aminophenol
in the presence of dioximatomanganese(II) by using dioxygen as oxidant. Beside
that, Kaizer et al. (2002) also reported the used of organic oxidant TEMPO-initiated
in the oxidation of o-aminophenol to 2-amino-3H-phenoxazin-3-one.
Of significance, all the catalytic oxidative coupling of o-aminophenol model
reaction reported in previous research typically was carried out in the homogeneous
liquid phase. There is only several works has been reported on the heterogeneous
catalytic oxidative coupling of o-aminophenol to 2-amino-3H-phenoxazin-3-one.
For examples, heterogeneous oxidative coupling of AP to APX over bis(2-[αhydroxyethyl]benzimidazolato)copper(II)
anchored
onto
chloro-methylated
polystyrene has done by Maurya et al. (2005); while El-Safty and co-workers (2002)
have reported on the kinetics and mechanism of AP oxidation by the supported
hexagonal mesoporous silica in the binary system with Amberlite resin.
CHAPTER 3
SYNTHESIS OF COPPER(II) DIETHYLAMINO-SUBSTITUTED SALEN
COMPLEX SUPPORTED ON MCM-48
3.1
Chemicals and Reagents
Rice husk ash (RHA) which is obtained from an open burning of rice husk,
containing 93 % of SiO2, was used as silica source. Hexadecyltrimethylammonium
bromide (CTABr) and triton-X 100 (TX) was used as organic template, while sodium
hydroxide (NaOH) were used as base. The prepared acetic acid (AcOH, 30% wt)
was used as acid solution for mix-gel pH adjustment. (3-Aminopropyl)trimethoxysilane (3APTMS) and (3-mercaptopropyl)trimethoxysilane (3MPTMS) was used as
silylating agent in order to prepare organo-functionalized MCM-48 (OF-MCM-48).
tert-Butyl hydrogen peroxide (TBHP, 70 % in water) is used to oxidize mercapto to
sulfonic acid.
4-(N,N-Diethylamino)salicylaldehyde (A-Sal) and ethylenediamine (EDA)
were used as the starting materials to synthesize ligand, namely N,N’-bis[4-(N,Ndiethylamino)salicylidene]ethylenediamine (A-Salen). On the other hand, copper(II)
acetate monohydrate (Cu(Ac)2.H2O) was used as metal source which was then
reacted with the prepared ligand to afford copper(II) diethylamino-substituted salen
complex.
40
3.2
Experimental
3.2.1
Synthesis of Diethylamino-Substituted Salen (A-Salen) Ligand
Schiff base selen ligand was prepared according to the molar ratio; A-Sal :
EDA = 2 : 1. Ethylenediamine (15 mmol) in ethanol (40 mL) was slowly mixed to
4-(N,N-diethylamino)salicylaldehyde (30 mmol) in ethanol (60 mL). The resulting
mixture was refluxed at 95 oC for 2 hours. After that, the solution was concentrated
and then recrystallized in ice-bath.
filtration.
The precipitate formed was recovered by
The resulting solid was recrystallized again in methanol and then
recovered by filtration to afford N,N’-bis[4-(N,N-diethylamino)salicylidene]ethylenediamine or A-Salen as a yellow solid (4.97 g, 80.64 %) with melting point
136.2-138.6 oC.
3.2.2 Synthesis of Copper(II) Diethylamino-Substituted Salen (CAS) Complex
Copper(II) Schiff base salen complex was synthesized according to the molar
ratio; Cu(II) : A-Salen = 1 : 1. Acetonitrile (60 mL) containing copper(II) acetate
monohydrate (4 mmol) and N,N’-bis[4-(N,N-diethylamino)salicylidene]ethylenediamine (4 mmol) was stirred at room temperature for 2 hours. After that, the
mixture was left in the fume-cupboard until precipitate formed. The precipitate was
then recovered by filtration and the solid was recrystallized in the mixture of hexane
and acetone. The precipitate was recovered by filtration to afford copper(II) N,N’bis[4-(N,N-diethylamino)salicylidene]ethylenediamine or CAS complex as a
brownish black solid (1.79 g, 91.33%).
41
3.2.3 Synthesis of Purely Siliceous MCM-48 (Si-MCM-48)
The synthesis of purely siliceous MCM-48 host matrix was carried out by
using hydrothermal method based on the molar composition of 5 SiO2 : 1.25 Na2O :
0.85 CTABr : 0.15 TX : 400 H2O (Lau, 2005). A a mixture of sodium silicate was
prepared by dissolving rice husk ash (8.08 g) and sodium hydroxide (2.55 g) in
distilled water (72.11 g). The mixture was stirred vigrously at 80 oC for 2 hours.
Meanwhile, a mixture of surfactant was prepared by adding CTABr (7.84 g), triton-x
100 (2.38 g) in distilled water (108.66 g) with vigrous stirring at 80 oC for 1 hours.
After that, the surfactant solution was quickly added to the sodium silicate solution
when the corresponding mixtures were cooled down to ambient temperature. A mixgel form material was then shaken and stirred vigorously for 15 minutes. The
resulting mix-gel was aged in the oven for 2 days at 100 oC. After aging for 2 days,
the pH of mix-gel was adjusted to around 10.2 by addition of acetic acid solution (30
%wt) when the mixture was cooled to an ambient temperature. Next, aging was
continued in the oven at 100 oC for another 2 days. After that, the mix-gel was
filtered and washed with distilled water, followed by dring in the oven at 100 oC.
Finally, calcination of the as-synthesized pure silica MCM-48 mesophase at 550 oC
for 6 hours afforded mesoporous Si-MCM-48.
3.2.4 Synthesis of Amino-Functionalized MCM-48 (NH2-MCM-48)
Dry toluene (30 mL) containing (3-aminopropyl)trimethoxysilane (9 mmol)
was added slowly into a suspension of calcined and dehydrated Si-MCM-48 (3.00 g)
in dry toluene (60 mL). The resulting mixture was refluxed at 110 oC for 24 hours.
After that, the suspension was recovered and then washed with acetonitrile for 3
hours using soxhlet extraction technique. Finally, the corresponding residue was
dried in the oven to afford the amino-functionalized MCM-48, NH2-MCM-48.
42
3.2.5 Synthesis of Sulfonic Acid-Functionalized MCM-48 (SO3H-MCM-48)
Dry toluene (30 mL) containing (3-mercaptopropyl)trimethoxysilane (9
mmol) was added slowly into a suspension of calcined and dehydrated Si-MCM-48
(3.00 g) in dry toluene (60 mL). The resulting mixture was refluxed at 110 oC for 24
hours. After that, the suspension was recovered and washed with acetonitrile for 3
hours using soxhlet extraction technique. Further, the dried mercapto-functionalized
MCM-48 (3.00 g) was suspended in acetonitrile (90 mL), followed by slowly
addition of TBHP (11 mL).
The corresponding mixture was stirred at room
temperature for 2 days. After that, the precipitate was recovered and washed with
acetonitrile.
Finally, the residue was dried in oven to afford sulfonic acid-
functionalized MCM-48, SO3H-MCM-48.
3.2.6 Copper(II) Diethylamino-Substituted Salen Complex Supported on
MCM-48
Dehydrated Si-MCM-48 (2.00 g) were added into a round bottom flask
containing CAS complex (8 mmol) dissolved in dry toluene (60 mL). Then, the
mixture was refluxed for 24 hours. After that, the precipitates were recovered via
filtration and then soxhlet-extracted with acetonitrile for 6 hours. The solids were
dried in oven to afford CAS complex supported on Si-MCM-48, CAS-MCM-48.
The corresponding CAS supported on amino-functionalized MCM-48 (CAS-NMCM-48) and CAS supported on sulfonic acid-functionalized MCM-48 (CAS-SMCM-48) were prepared similiarly by replacing Si-MCM-48 with NH2-MCM-48
and SO3H-MCM-48, respectively.
43
3.3
Characterization of Copper(II) Diethylamino-Substituted Salen Complex
Supported on MCM-48
3.3.1 Fourier Transform Infrared (FTIR) Spectroscopy
FTIR spectroscopy is a rapid and simplest characterization technique for
obtaining preliminary information regarding to the identity of a compound through
the frequencies of the normal modes of vibration of the specialty functional group in
molecule (Lambert et al., 1998). Thus, FTIR is the best technique to determine the
local structure of organic or inorganic compounds. In this study, FTIR was used to
analyze the synthesized ligand, metal complex and mesoporous materials.
Procedure
The infrared spectra of samples were recorded using Perkin Elmer Spectrum
One FTIR Spectrometer with resolution 4 cm-1. For solid compounds, the samples
were prepared by using pressed-disc technique with KBr salt used as a matrix
material. First, a few milligrams of dried sample was ground together with of KBr
salt in the ratio of 1 : 100. The fine powder was then pressed in hydraulic press for a
few minutes with the purpose to gain a thin transparent disc. After that, the thin
pellet was carefully put in the sample holder and the spectra of the sample were
recorded in the mid IR region from 4000 cm-1 to 400 cm-1 with 5 times scanning.
3.3.2 Proton and Carbon-13 Nuclear Magnetic Resonance (1H- and 13C-NMR)
Spectroscopy
NMR spectroscopy is a technique which exploits the magnetic properties of
NMR active nuclei such as 1H,
13
C, and etc.
An NMR spectrum provides
information on the number and type of chemical entities in molecule. The type of
chemical entities in molecule are shown in term of chemical shift due to the different
44
nuclei in molecule resonate at different frequencies. J-coupling or scalar coupling,
which arises from the interaction of different spin state through the chemical bonds
of molecule and resulting splitting of NMR signals, is one of the most useful
information in molecular structure determination. Therefore, the NMR spectroscopy
is a powerful tool in the determination of local structure of pure compound. In this
study, 1H- and
13
C-NMR spectra were recorded in order to analyze the prepared
ligand and the product.
Procedure
The 1H- and
13
C-NMR spectra of pure organic compounds were recorded
using a Bruker Avance 400 spectrometer. A small quantity of solid sample was
firstly dissolved with deuterated acetone. Next, the mixture was filtered and the
filtrate was pipetted into NMR narrow tube. For 1H-NMR analysis, the sample was
recorded at 400 MHz magnetic field with 16 times scanning cycle. After that, the
13
C-NMR signal was then collected in 100 MHz magnetic field over 24 hours.
3.3.3 Diffuse Reflectance Ultraviolet-Visible (DR UV-Vis) Spectroscopy
DR UV-Vis spectroscopy is a useful technique for the qualitative and
quantitative analysis through the determination of the amount of spectrum absorbed
by specific type of functional groups or coordination metal and type of bonding exist
in solid samples or molecules supported on the solid surfaces. The resulting analysis
is performed in the ultraviolet and visible regions of the spectrum. The amount of
light reflected from the non-penetration sample is reported as a percent of reflectance
(%R) which will be converted to Kubelka Munk (KM) unit. Therefore, DR UV-Vis
was used to determine the coordination of prepared metal complex as well as to
monitor metal complex that has been incorporated on mesoporous silica.
45
Procedure
The DR UV-Vis spectra of neat and supported copper(II) diethylaminosubstituted salen complexes were recorded using a Perkin Elmer Lambda 900 UVVIS-NIR Spectrometer respectively.
The dried and fine grounded sample was
dispersed homogeneously on the sample holder and the spectra of sample were
scanned in the wavelength between 190 to 800 nm. After that, all the samples data
that recorded as a percent of reflectance was then transferred to Kubelka Munk (KM)
unit.
3.3.4 Powder X-Ray Diffraction (XRD)
Powder XRD is a non-destructive and qualitative analysis method, which is
typically applied in solid-state chemistry and material science.
Powder XRD
patterns provide much useful information, including to determine the type of phases
and crystallographic texture of sample as well as to calculate unit cell and crystalline
size of material (Nur et al., 2004). Thus, the Si-MCM-48, modified MCM-48 and
MCM-48 containing copper(II) complex were characterized using powder XRD in
order to evaluate the crystallinity as well as to monitor the mesoporous materials
structure are well preserved after modification.
Procedure
The XRD pattern of the prepared porous materials were recorded using a
Bruker Advance D8 using Siemens 5000 diffractometer with Cu Kα radiation (λ =
1.5418 Å, kV = 40, mA = 40). First, the powder sample was spread equally on the
sample holder to form a thin and smooth layer. The sample was scanned in the 2θ
scale of 1.5o to 10° with step size 0.02° per second.
46
3.3.5 Nitrogen Adsorption-Desorption Isotherm Analysis
The principal operation of nitrogen adsorption-desorption analysis is
regarding to the amount of nitrogen gas absorbed by absorbent at temperature 77 K
and atmospheric pressure. The amount of nitrogen gas absorbed by absorbent is
calculated as a function of the equilibrium partial pressure of material.
Thus,
nitrogen adsorption desorption isotherm is a useful technique for determining the
surface area, pore volume and pore size distribution as well as providing the
information about the pore type, shape and texture of porous material (Satterfield,
1991; Sing et al. 1985; Leofanti, 1998). In this research, Si-MCM-48 and MCM-48
containing copper(II) complex were analyzed using nitrogen absorption desorption
isotherm in order to monitor and determine the porosity or pore texture of the
mesoporous silica.
Procedure
The nitrogen adsorption-desorption isotherms of the porous materials were
conducted at 77 K using a Micromeritics ASAP 2010. Samples around 0.2 g in a
tube were firstly outgassed at 473 K under a pressure below 10-5 atm. After that, the
sample was transferred for adsorption isotherm measurement when the system was
cooled down to ambient temperature
3.3.6 Atomic Absorption Spectroscopy (AAS)
AAS is an instrument that used to determine the concentration of trace
element in part per million, through the measurement amount of radiation at
particular wavelength absorbed by ground-state atoms that created in a flame
(Christian, 2004). Thus, the concentration of the prepared MCM-48 containing
copper(II) complex was quantitatively analyzed using AAS analysis technique.
47
Procedure
Approximately 0.05 g of MCM-48 containing copper(II) diethylaminosubstituted salen complex was dissolved with 0.5 mL of aqua regia (nitric acid :
hydrochloric acid = 1 : 3 ) and 3 mL of hydrofluoric acid in a Teflon bottle. Then,
the samples were heated in oven at 110 oC for 1 h. After cooling to the ambient
temperature, 10 mL deionized distilled water and 2.8 g of boric acid were added
respectively to the corresponding solution, followed by the magnetically stirring for a
few minutes. Finally, the solution was diluted to 100 mL by using deionized distilled
water and the solution was shaken until the solution is mixed homogeneously.
Content of copper in the solution was quantitatively analyzed using a GBC-Avanta
atomic absorption spectrophotometer at 248.33 nm.
3.3.7 Thermogravimetric Analysis (TGA)
The principal operation of thermogravimetric analysis is typically used for
monitoring the weight loss of the sample in a chosen atmosphere (usually nitrogen or
air) as a function of temperature. TGA provides quantitative measurement on the
mass change in materials associated with transition and thermal degradation. TGA
was used to analyze the decomposition pattern of neat CAS complex and also to
study the hydrophobicity of OF-MCM-48 and decomposition pattern of catalyst
within mesoporous silica.
Procedure
A small quantity of neat and supported copper(II) diethylamino-substituted
salen complex were put in an aluminum oxide crucible respectively. After that, the
thermogravimetric analysis of the samples were respectively performed by using
Mettler Toledo TGA/SDTA851 Thermal analyzer in the presence of nitrogen flow
with programming heat rate of 15 oC / min from 25 to 900 oC.
48
3.3.8 Field Emission Scanning Electron Microscopy (FESEM)
Field emission scanning electron microscopy is an electron microscope,
which capable to produce high resolution images with characteristic of threedimensional appearance and are useful for determining the surface topography,
morphology and texture of the sample. The principle operation of FESEM is based
on the generation of images through the conversion of signals that gain from the
detection of electrons that bombarded and then ejected from the sample surface
(Blake, 1990). Thus, the topography and morphology of Si-MCM-48 and MCM-48
containing CAS complex were studies using FESEM.
Procedure
First, a small amount of porous silica was attached to the sample holder.
Next, the sample was coated with a small amount of gold particle using BIO-RAD
Polaron Division SEM Coating System machine. The morphology of the samples
were analyzed using FESEM series JSM-6701F which operating at 15 kV.
3.3.9 Transmission Electron Microscopy (TEM)
TEM is a very powerful microscopy which capable to catch the image of
materials even though in nano size. The image of sample was generated from the
electrons transmitted through the thin specimen. Therefore, the pore structure and
size of Si-MCM-48 and MCM-48 containing CAS complex were analyzed by using
TEM.
49
Procedure
First, a small quantity of sample was mixed in acetone and following by the
ultrasonification of the mixture. A small drop of suspension was placed on Formvar
film-coated copper grids. The TEM micrographs of the dried samples were scanned
using JEM-2100 Electron Microscope JEOL with acceleration voltage 160 kV.
3.4
Results and Discussion
3.4.1
Physicochemical Properties of Copper(II) Diethylamino-Substituted
Salen (CAS) Complex
Condensation of 4-(N,N-diethylamino)salicylaldehyde (A-Sal) and ethylenediamine (EDA) has successfully afforded ligand of the complex as yellow solid (4.97
g, 80.64 %) with melting point 136.2-138.6 oC. The synthetic route to A-salen is
illustrated in Figure 3.1. The collected compound was analyzed by using FTIR, 1H
and 13C- NMR spectroscopy.
Figure 3.1
The synthetic route to A-Salen ligand
By comparing the FTIR spectra of A-Salen, EDA and A-Sal in Figure 3.2, the
absence of the stretching band of amine (primary N-H) and aldehyde (HC=O)
functional groups in the FTIR spectrum of A-Salen suggested the reaction between
A-Sal and EDA were occurred. On the other hand, a band observed at 1611 cm-1
(FTIR spectrum of A-salen) was assigned to the stretching mode of imine group
50
(HC=N). The above observations show that the aldehyde functional group of A-Sal
was reacted with amine functional group of EDA to afford an imine substituted
compound, namely N,N’-bis[4-(N,N-diethylamino)salicylidene]ethylenediamine (A-
Transmittance / %
Salen) ligand.
A-Salen
A-Sal
EDA
4000
3000
2000
1500
1000
400
-1
Wavenumber / cm
Figure 3.2 FTIR spectra of EDA, A-Sal and A-Salen ligand
According to the FTIR spectrum of A-Salen, a medium intensity broad
absorption band occurred in the range 2400-3200 cm-1, which overlaps with the
stretching bands of aromatic C-H at 3081 cm-1 and alkane C-H at 2912 cm-1 was
attributed to the stretching mode of hydroxyl group. This broad band is due to the
presence of intramolecular hydrogen bonding between proton of phenolic group and
the electrons lone pair of atom nitrogen in imine functional group, as illustrated in
Figure 3.3 (Aranha et al., 2007; Ueno and Martel, 1956; Freedman, 1961; Kasumov
and Köksal, 2005). Two stretching bands at 1560 and 1521 cm-1 were assigned to
the vibration modes of aromatic C=C. A strong absorption band at 1342 cm-1 was
attributed to the stretching mode of C-N, while the vibration mode of C-O was
51
observed at 1241 cm-1. The comparison FTIR data of EDA, A-Sal and A-Salen are
showed in Table 3.1.
Figure 3.3
Intramolecular hydrogen bonding between proton of phenolic and the
electrons lone pair of atom nitrogen in imine group
Table 3.1
FTIR stretching bands of EDA, A-Sal and A-Salen ligand
Wavenumber (cm-1)
Functional
groups
A-Sal
EDA
A-Salen
N-H
-
3361 and 3282
-
=C-H sp2
2977 and 2840
(aldehyde)
-
2967 and 2839 (imine),
3081 (aromatic)
C-H sp3
2977
2929 and 2856
O-H
3400-3200 (Unresolved)
-
C=O
1634
-
-
C=N
-
-
1611
1560 and 1522
-
1560 and 1521
C-N
1340
1356
1342
C-O
1248
C=C
(aromatic)
2912
2400-3200 (Unresolved
broad)
1241
52
The structure of the synthesized organic compound was further analyzed by
using NMR spectroscopy. By comparing the 1H-NMR spectra of A-Salen and A-Sal
as shown in Figure 3.4, the absence of a singlet at δ 9.53 and the presence of two
new signals respectively at δ 3.79 and δ 8.25 (1H-NMR spectra of A-Salen),
indicated that a new compound has been produced between the reaction of A-Sal and
EDA. Singlet at δ 3.79 integrated for four protons was assigned to those of H-8 and
H-8’, which are typically found as methylene protons of EDA backbone. On the
other hand, a singlet at δ 8.25 was assigned to those of imine protons, which are H7b and H-7b’. The absence of H-7a signal at δ 9.53 was suggested the aldehyde
group of A-Sal has been substituted to the imine group in the reaction of A-Sal and
EDA.
Nitrogen atom, which is less electronegative compared to oxygen atom,
causes the lower inductive-withdrawing effect in imine group. Thus, the proton of
imine group is more shielded than the proton of aldehyde group.
Based on the 1H-NMR spectrum of A-Salen, a triplet at δ 1.15 (12H, J = 7.2
Hz) was assigned to those of H-9 and H-9’, while a signal at δ 3.40 (8H, q, J = 7.2
Hz) was attributed to those of H-8 and H-8’. On the other hand, a doublet at δ 6.05
(2H, J = 2.4 Hz) was assigned to those of aromatic protons, H-6 and H-6’. A signal
at δ 6.22 (2H, dd, J = 8.8 Hz, J = 2.4 Hz) was attributed to those of aromatic protons,
H-4 and H-4’, while a doublet at δ 7.08 (2H, d, J = 8.8 Hz) was assigned to those of
H-3 and H-3’. The J-coupling 2.4 Hz indicated that those of aromatic protons for H6 and H-4 (H-6’ and H-4’) are located in meta position, while the J-coupling 8.8 Hz
indicated that those of H-4 and H-4’ are ortho to H-3 and H-3’, respectively. A
singlet resonated at low field δ 13.42 was assigned to the phenolic protons which are
highly deshielded due to their position located in the center of two benzene rings as
well as affected by the presence of intramolecular hydrogen bonding.
The
broadening and decreasing intensity of the signal at δ 13.42 was due to the proton of
phenolic which can be delocalized between the phenolic and nitrogen atom of imine
group through the intramolecular hydrogen bonding as shown in Figure 3.5
(Silverstein et al. 1991; Wojciechowski et al., 2001). Thus, a low intensity broad
singlet at δ 2.87 thus was attributed to the proton that relocated to the nitrogen atom
of imine group. The comparison 1H-NMR data of A-Sal and A-Salen are shown in
Table 3.2.
H
6
N
7a
1
5
2
4
-CH2-
O
-CH2-
3
-OH
H-7a
H-4
H-3
8'
7b'
6'
2'
7b
N
N
H-3 and
H-3’
8
OH HO
2
H-6 and
H-6’
-CH3
5
3
3'
H-4 and H4’
H-6
6
1
1'
5'
4'
-CH3
OH
A-Sal
N
-CH3
N
-CH2-
4
A-Salen
H-8 and
H-8’
H-7b and
H7b’
-C=NH-
-OH
13
12
11
10
Figure 3.4
9
8
1
7
6
5
4
3
2
1
ppm
H-NMR spectra of A-Sal and A-Salen ligand
53
54
Figure 3.5
Delocalization of proton between phenolic and nitrogen atom of imine
group of A-Salen compound
1
Table 3.2
H-NMR data of A-Sal and A-Salen
Chemical shift (ppm)
Protons
A-Sal
A-Salen
7.41 (1H, d, J = 8.8 Hz)
7.08 (2H, d, J = 8.8 Hz)
6.41 (1H, dd, J = 8.8 Hz and
6.22 (2H, dd, J = 8.8 Hz,
J = 2.4 Hz)
J = 2.4 Hz)
H-6 (H-6’)
6.06 (1H, d, J = 2.4 Hz)
6.05 (2H, d, J = 2.4 Hz)
H-7a
9.53 (1H, s)
-
H-7b and H-7b’
-
8.25 (2H, s)
H-8 (H-8’)
3.50 (4H, q, J = 7.2 Hz)
3.40 (8H, q, J = 7.2 Hz)
H-9 (H-9’)
1.21 (6H, t, J = 7.2 Hz)
1.15 (12H, t, J = 7.2 Hz)
H-10 (H-10’)
-
3.79 (4H, s)
-HC=NH-
-
2.87 (bs)
-OH
11.71 (1H, s)
13.42 (bs)
H-3 (H-3’)
H-4 (H-4’)
By comparing the
13
C-NMR spectra of A-Salen and A-Sal as illustrated in
Figure 3.6, the absence of a aldehyde carbon signal at δ 192.14 and the presence of a
imine carbon signals at δ 165.26 (13C-NMR spectra of A-Salen), suggested the
aldehyde functional group of A-Sal was reacted with amine functional group of EDA
to afford an imine functionalized compound. On the other hand, a new signal at δ
59.51 was attributed to those of methylene carbons from the EDA backbone, C-8 and
55
C-8’. From the
13
C-NMR spectra of A-Salen, there were 10 types of carbons has
been detected by NMR spectroscopy analysis, which totally matched to the expected
structure of A-Salen. The comparison data of 13C-NMR A-Sal and A-Salen are listed
in Table 3.3.
Table 3.3
13
C-NMR data of A-Sal and A-Salen
Chemical shift (ppm)
Carbons
A-Sal
A-Salen
C-1 (C-1’)
111.37
108.55
C-2 (C-2’)
164.45
163.56
C-3 (C-3’)
135.39
132.81
C-4 (C-4’)
104.48
102.91
C-5 (C-5’)
154.34
151.05
C-6 (C-6’)
96.17
97.48
C-7a
192.14
-
C-7b and C-7b’
-
165.26
C-8 and C-8’
44.39
44.04
C-9 and C-9’
11.92
12.06
C-10 and C-10’
-
59.51
Based on the information obtained from the 1H- and
13
C-NMR spectra, A-
Salen has been successfully synthesized via the condensation of A-Sal and EDA and
the prepared A-salen ligand was suggested to exist in the symmetrical form.
H
6
N
7a
1
5
2
O
OH
-CH3
3
4
A-Sal
C-4
C-3
C-7a
C-2
C-5
C-6
C-1
8'
7b'
6'
N
C-2 and C-2’
200
190
180
170
150
7b
N
2'
6
1
OH HO
2
5
3
3'
-CH2-
N
-CH3
4
A-Salen
C-3 and C-3’
C-5 and
C-5’
160
N
8
1'
5'
4'
C-4 and C-4’
C-7b and C-7b’
-CH2-
140
130
120
13
110
100
90
80
C-8 and C-8’
70
60
C-NMR spectra of A-Sal and A-Salen ligand
50
40
30
20
10
ppm
56
Figure 3.6
C-6 and C-6’
C-1 and C-1’
57
Complexation of A-Salen and copper(II) acetate monohydrate (Figure 3.7)
had afforded a brownish black solid (1.79 g, 91.33 %). The obtained solid complex
was characterized using FTIR spectroscopy, DR UV-Vis spectroscopy and TGA.
Figure 3.7
The synthetic route of CAS complex
As observed from the FTIR spectra of CAS complex in Figure 3.8, the
absence of the broad stretching band in the range 2400-3200 cm-1 and the slight shift
of C=N absorption band to lower frequency at 1591 cm-1, suggested the copper(II)
N,N’-bis[4-(N,N-diethylamino)salicylidene]ethylenediamine (CAS) complex has
been successfully synthesized.
The absence of broad band suggested the
disappearance of intramolecular hydrogen bonding due to the phenolic proton has
been substituted by copper(II) ion (Aranha et al., 2007; Kasumov and Köksal, 2005;
Bahramian et al., 2006b; Pui et al., 2007). While, a low intensity band at 3447 cm-1
was attributed to the stretching mode of OH group of molecular water in CAS
complex. C=N vibration band shifted to lower frequency because of the presence of
the coordination bond between copper(II) ion and the imine nitrogen lone pair which
has decreased the bonding strength of C=N (Aranha et al., 2007; Kasumov and
Köksal, 2005; Bahramian et al., 2006b; Pui et al., 2007). A band at 3085 cm-1 was
assigned to the stretching mode of C-H of aromatic ring. While, two bands at 1591
cm-1 (overlapped with C=N absorption band) and 1514 cm-1 were attributed to
aromatic C=C vibrations. The C-N and C-O stretching bands were observed at 1354
cm-1 and 1248 cm-1, respectively.
Transmittance / %
58
Figure 3.8
FTIR spectrum of CAS complex
The prepared complex was further investigated using DR UV-Vis
spectroscopy and the corresponding spectra are given in Figure 3.9. By comparing
the DR UV-Vis spectra of A-Salen and CAS, new broad band observed at 557 nm
which is assigned to the d-d transition of copper(II) ion, indicated that N,N’-bis[4(N,N-diethylamino)salicylidene]ethylenediamine was coordinated with copper(II)
ion. According to the DR UV-Vis spectrum of CAS, a band observed at 416 nm was
attributed to ligand-to-metal charge transfer transitions, in which an electron is
excited from a predominantly ligand-centred orbital to a predominantly metalcentred orbital (Jacob et al., 1998a). This band also was found in overlapping with
the π-π* transition of C=N. A band observed at 312 nm was assigned to the n-π*
transition of C=N. A shoulder presence in the 280 nm was assigned to π-π* type
transitions of aromatic ring. Table 3.4 shows the DR UV-Vis data of A-Salen and
CAS.
59
312 nm
416 nm
557 nm
283 nm
385 nm
CAS
A-Salen
190
400
600
800
Wavelength / nm
Figure 3.9
Table 3.4
DR UV-Vis spectra of A-Salen ligand and CAS complex
DR UV-Vis data of A-Salen ligand and CAS complex
Wavelength (nm)
Type of transitions
A-Salen
CAS
π-π* (aromatic)
270 (shoulder)
280 (shoulder)
n-π* (C=N)
283
312
π-π* (C=N)
385
416
Metal-ligand charge transfer
-
416
d-d transition
-
557
60
Thermogravimetric analysis (TGA) curve of neat CAS complex in nitrogen
atmosphere is shown in Figure 3.10, while the TGA data of neat CAS complex is
listed in Table 3.5. The weight loss up to 260 oC was attributed to the dehydration
process (Nagar and Sharma, 1990; Sabio-Reva et al., 1999). From the TG curve, it
can be observed that the dehydration process occurred in two stages with the
temperature intervals of 110-150 oC and 170-260 oC, respectively. The first stage
was attributed to the elimination of one water molecule (Table 3.5) not coordinated
with CAS complex, while the following weight loss was attributed to the removal of
two water molecules (Table 3.5) coordinated with CAS complex. After dehydration,
the anhydrous neat CAS complex was decomposed at around 280 oC.
150 oC
o
170 oC 260 C
100 oC
280 oC
Weight loss / %
100
80
60
40
30
50
200
400
600
800
900
o
Temperature / C
Figure 3.10
TGA curve of neat CAS complex
Table 3.5
TGA data of neat CAS complex
Temperature (oC)
Process
Weight loss (%)
100 - 150
Dehydration (1 mol H2O)
2.92 (3.41)a
170 - 260
Dehydration (2 mol H2O)
7.17 (6.82)a
280 - 560
Pyrolysis
53.09
Combustion of residue
-
> 560
a
Percentage weight loss obtained through the calculation (Appendix A)
61
3.4.2
Physicochemical
Properties
of
MCM-48
Containing
Copper(II)
Diethylamino-Substituted Salen (CAS) Complex
The mesoporous silica, which has been synthesized using hydrothermal
liquid-crystal template mechanism, was analyzed by FTIR spectroscopy. The FTIR
spectrum of as-synthesized Si-MCM-48 showed some specific bands at 1228 and
1066, 797 and 578 cm-1 and 452 cm-1, were attributed respectively to the asymmetric
stretching, symmetric stretching and bending vibration of Si-O-Si (Ng, 2006; Wong,
2007; Wan Ibrahim, 2008). These vibration bands indicate that polymerization of
silicate ion has been achieved by the liquid-crystal templating mechanism. A broad
band at 3450 cm-1 was assigned to the stretching band of O-H group and its
corresponding bending mode was observed at 1651 cm-1 (Ng, 2006). A band at 961
cm-1 was attributed to defective Si-O stretching mode (Ng, 2006; Wong, 2007; Wan
Ibrahim, 2008).
Three absorption bands at 2920, 2851 and 1488 cm-1 were observed in the
FTIR spectrum of uncalcined Si-MCM-48, which was assigned to asymmetric
stretching, symmetric stretching and bending mode respectively of C-H group from
organic surfactant molecule. However, these absorption bands disappeared in the
FTIR spectrum of calcined Si-MCM-48, which suggests that the organic surfactant
has been removed completely from the pore channel of Si-MCM-48. On the other
hand, the spectrum of calcined Si-MCM-48 are equally similar to as-synthesized SiMCM-48 suggesting that the characteristic of the amorphous silica framework is well
maintained after thermal treatment. The FTIR spectra of as-synthesized and calcined
MCM-48 are displayed in Figure 3.11.
62
Transmittance / %
Calcined MCM-48
4000
As-synthesized MCM-48
3000
2000
1500
1000
400
-1
Wavenumber / cm
Figure 3.11
FTIR spectra of as-synthesized and calcined Si-MCM-48
The powder XRD patterns of as-synthesized and calcined Si-MCM-48 are
shown in Figure 3.12. The XRD pattern of as-synthesized Si-MCM-48 showed a
high intensity of d211 Bragg reflection, d220 Bragg reflection shoulder and two
unresolved peaks, d420 and d332, between 2θ range 2o and 5o, which indicate the
successful formation of Ia3d bicontinuous cubic phase of Si-MCM-48 (Endud and
Wong, 2007; Kresge et al., 1992; Beck et al., 1992). After removing the organic
template from the pore channel, the intensity of the corresponding signals were
numerously increased due to the increasing long-range order of Si-MCM-48.
However, the loss of surfactant as pore channel support causes the slight shift of d211
signal towards higher 2θ degree due to shrinkage effect, and directly decreases the
interplanar distance of pore channel. The 2θ degree, intensity, lattice parameter (dspacing) and unit cell parameter (ao) of d211 signal of as-synthesized and calcined SiMCM-48 are listed in Table 3.6.
Intensity / a.u.
63
Figure 3.12
Table 3.6
Si-MCM-48
XRD patterns of as-synthesized and calcined Si-MCM-48
XRD data of as-synthesized and calcined Si-MCM-48
2θ (o)
Intensity of
d-spacing (Å)
ao* (Å)
d211 (counts)
As-synthesized
2.22
3405
39.72
45.86
Calcined
2.28
6885
38.76
44.76
*
ao = 2d100 / √3 (Kresge et al., 1992; Sun et al., 1997).
The FESEM image of the pure silica Si-MCM-48 shown in Figure 3.13
confirmed that the Si-MCM-48 consisted of fine spherical particles with diameter not
larger than 300 nm. Subsequently, TEM image of Si-MCM-48 illustrated in Figure
3.14 showed that the long-range order of Si-MCM-48 pore structure was retained
successfully after removal of the organic template via calcination at 550 oC.
64
Figure 3.13
Figure 3.14
FESEM image of calcined Si-MCM-48
TEM image of calcined Si-MCM-48
65
After calcination, the presence of silanol group on the surface of Si-MCM-48
causes the porous material to have lower hydrophobic property. This may affects the
absorbability of the porous material towards the hydrophobic catalyst or organic
compounds. In this study, Si-MCM-48 was modified with amino and sulfonic acidfunctionalized agents, respectively.
The chemical reaction of Si-MCM-48 and
organo-functionalized agent (OFA) is illustrated in Scheme 3.1. The MCM-48 that
has been modified with OFA was then incorporated with CAS complex.
Both
modified forms of MCM-48 were characterized using FTIR, DR UV-Vis and AAS
spectroscopy, XRD, N2 adsorption-desorption measurement, TGA, FESEM and
TEM
MCM-48
MCM-48
MCM-48
MCM-48
MCM-48
Scheme 3.1
Modification of Si-MCM-48 with OFA
The FTIR spectra of modified MCM-48 are shown in Figure 3.15. The
presence of bands in 3000-2800 cm-1 region were attributed to the vibration mode of
C-H group, while the bands in the range between 1610-1400 cm-1 were assigned to
the vibration modes of C-H, C=N and C=C (aromatic). The presence of these bands
suggests that the Si-MCM-48 has been successfully modified with the OFA and the
CAS complex is supported on MCM-48 matrix. By contrast, the FTIR spectra in
Figure 3.15 are equally similar to the FTIR spectrum of calcined Si-MCM-48 in
Figure 3.11, which suggests the structure of mesoporous silica is well preserved after
66
modification. On the other hand, the decreasing intensity of Si-O stretching mode,
which can be observed from the FTIR spectra of OF-MCM-48, might be due to the
substitution of silanol group with OFA. The absorption bands and the representative
functional groups of OFA and CAS complex that were modified on MCM-48 matrix
are listed in Table 3.7.
Si-O-Si stretching and
bending region
CAS-S-MCM-48
OH stretching
region
SO3H-MCM-48
SH-MCM-48
Transmittance / %
CAS-N-MCM-48
NH2-MCM-48
CAS-MCM-48
OH bending
band
C-H stretching
region
C=C (aromatic),
C=N stretching
and C-H
bending region
4000
3000
2000
Si-O stretching
1500
1000
-1
Wavenumber / cm
400
67
Figure 3.15
FTIR spectra of OF-MCM-48 and MCM-48 containing CAS complex
Table 3.7
FTIR data of OF-MCM-48 and CAS complex that supported on
MCM-48
Samples
Wavenumber (cm-1)
CAS-MCM-48
1523 (C=C aromatic)
NH2-MCM-48
2941 (C-H), 1560 (bending N-H),
1497 and 1446 (bending C-H)
CAS-N-MCM-48
2941 (C-H), 1606 (C=N), 1559 and 1519 (C=C aromatic),
1559 (bending N-H), 1471 and 1412 (bending C-H)
SH-MCM-48
2936 and 2851 (C-H)
SO3H-MCM-48
2941 (C-H), 1412 (bending C-H)
CAS-S-MCM-48
1563 and 1527 (C=C aromatic),
1471 and 1412 (bending C-H)
Due to the lack of FTIR technique in determination of the formation of
sulfonic acid from the oxidation of mercapto-functionalized MCM-48, SO3H-MCM48 and SH-MCM-48 thus were subjected respectively for acidity analysis. The
respective samples in distilled water showed pH 3 and 6 [pH of distilled water is 6].
The changing of pH was indicated that the mercapto functional group, which is
neutral, has been successfully oxidized to sulfonic acid functional group.
The quality of the organo-functionalized MCM-48 and MCM-48 supported
CAS complex were then evaluated by using XRD. Based on the XRD patterns in
Figure 3.16, the MCM-48 framework is much affected by the amino-functionalized
agent. This can be determined through the representative XRD signals of NH2MCM-48, with d220, d420 and d332 were not clearly observed and the d211 peak was
also highly decreased. However, the framework has been slightly improved after
incorporation of CAS complex, whereas the intensity of d211 peak was increased and
the signal d220 was observed as a small shoulder. On the other hand, the bicontinuous
cubic phase Ia3d of MCM-48 is well maintained for Si-MCM-48 containing CAS
complex (CAS-MCM-48) and mercapto-functionalized MCM-48 (SH-MCM-48)
even after their oxidation to sulfonic acid-functionalized MCM-48 (SO3H-MCM-48),
68
followed by incorporation of CAS complex. However, the mesoporous structure of
MCM-48 becomes less ordered as indicated by the reduced relative intensity of the
representative signals.
In general, all the MCM-48 representative peaks were
observed to be shifted to higher 2θ angle due to reduction of the pore diameter of
MCM-48 after modification with OFA and incorporation of CAS complex. The 2θ
degree, intensity, lattice parameter (d-spacing) and unit cell parameter (ao) of d211
signal of the modified MCM-48 are shown in Table 3.8.
Intensity / a.u.
CAS-MCM-48
NH2-MCM-48
CAS-N-MCM-48
SH-MCM-48
SO3H-MCM-48
CAS-S-MCM-48
5.0
20 / degree
1.5
Figure 3.16
10.0
XRD patterns of OF-MCM-48 and MCM-48 containing CAS
complex
69
Table 3.8
XRD data of OF-MCM-48 and MCM-48 containing CAS complex
Samples
2θ (o)
Intensity of
d-spacing (Å)
ao* (Å)
d211 (counts)
*
CAS-MCM-48
2.34
3236
37.77
43.61
NH2-MCM-48
2.36
1094
37.35
43.13
CAS-N-MCM-48
2.32
1965
38.13
44.03
SH-MCM-48
2.33
5806
37.90
43.76
SO3H-MCM-48
2.31
2270
38.21
44.12
CAS-S-MCM-48
2.28
3867
38.69
44.68
ao = 2d100 / √3 (Kresge et al., 1992; Sun et al., 1997).
The pore properties of the organo-functionalized and CAS complex loaded on
MCM-48 was evaluated using nitrogen adsorption-desorption isotherm. According
to the isotherms of Si-MCM-48, OF-MCM-48 and MCM-48 containing CAS
complex as shown in Figure 3.17, both of which are type IV in IUPAC classification,
with characteristic of capillary condensation in mesoporous channels (Sing et al.,
1985). The adsorption behavior of isotherm type IV is briefly described by using
isotherm of Si-MCM-48 as representative and the typical diagrammatic of multilayer
adsorption is illustrated in Scheme 3.2. At the initial state (at lower relative pressure),
the nitrogen adsorption mechanism in mesopores is comparable to that on planar
surfaces. After completion of monolayer (A), multilayer adsorption is starting to
take place (B). After achieving a critical film thickness (C), capillary condensation
occurs in the core of the pore and followed by the pore completely filled with liquid
nitrogen (D). At the pressure, which is less than the pore condensation pressure, pore
evaporation thus occurs by a receding meniscus (E). Finally, in the relative pressure
range between (F) and (A), adsorption and desorption are reversible (Lowell et al.,
2006). The presence of the second hysteresis loop (region G) was attributed to the
presence of external pores, which are generally formed due to the aggregation of the
particles.
70
CAS-S-MCM-48
SO3H-MCM-48
SH-MCM-48
CAS-N-MCM-48
NH2-MCM-48
CAS-MCM-48
Si-MCM-48
0.0
Figure 3.17
0.2
0.4
0.6
0.8
Relative pressure / P/Po
1.0
Nitrogen adsorption-desorption isotherm of Si-MCM-48, OF-MCM-
Volume adsorbed /
cm3g-1 STP
48 and MCM-48 containing CAS complex
G
D
E
A
B
F
C
0.0
1.0
Relative pressure / P/Po
Pore channel
A
Scheme 3.2
N2 molecules
Pore wall
B
C
D
E
Diagrammatic representative of multilayer adsorption, pore
condensation and hysteresis in pore channel
F
71
Based on the IUPAC classification, isotherms of Si-MCM-48, CAS-MCM-48,
SH-MCM-48, SO3H-MCM-48 and CAS-S-MCM-48 have the characteristic of
hysteresis loops type H2. This type of hysteresis loops indicates that the resulting
porous materials have nonuniform size or shape, which are associated with capillary
condensation in pores with narrow necks and wide bodies or typically referred as
‘ink bottle’ pores (Endud and Wong, 2007; Leofanti et al., 1998; Sing et al., 1985).
Meanwhile, the isotherms of NH2-MCM-48 and CAS-N-MCM-48 are found to
exhibit hysteresis loops characteristic of type H3.
These porous materials are
suggested to have a nonuniform size or shape, which are associated with capillary
condensation in slit shape pores (Endud and Wong, 2007; Leofanti et al., 1998; Sing
et al., 1985). The isotherms show that the modification of Si-MCM-48 with OFA
and incorporated with CAS complex have affected much to the regularity of the pore
channel.
In general, the surface area, pore diameter and pore volume of the modified
MCM-48 were smaller if compared to the calcined Si-MCM-48. This indicated that
the pore channel of MCM-48 is being occupied by the OFA and CAS complex, as
illustrated in Figure 3.18 (Zhang et al., 2006; Park and Komarneni, 1998). The
nitrogen adsorption-desorption isotherm data of Si-MCM-48, OF-MCM-48 and
MCM-48 containing CAS complex are summarized in Table 3.9.
Figure 3.18
Illustration of pore system of Si-MCM-48 and modified MCM-48
before and after functionalization of OFA or CAS complex
72
Table 3.9
Nitrogen adsorption-desorption isotherm data of Si-MCM-48, OFMCM-48 and MCM-48 containing CAS complex
Surface area, SBET
Pore diameter, Dp
Pore volume, Vp
(m2 g-1)
(nm)
(cm3 g-1)
Si-MCM-48
867
4.04
1.01
CAS-MCM-48
457
4.43
0.72
NH2-MCM-48
672
2.66
0.57
CAS-N-MCM-48
634
2.75
0.62
SH-MCM-48
670
3.31
0.73
SO3H-MCM-48
684
3.12
0.69
CAS-S-MCM-48
734
3.26
0.80
Samples
Figure 3.19 shows the TG curves of neat CAS complex and MCM-48
supported CAS complex. For the supported CAS complex, exothermic weight losses
observed at temperatures below 150 oC were attributed to the loss of physisorbed
water.
The percentage weight losses for CAS-MCM-48, CAS-N-MCM-48 and
CAS-S-MCM-48 at temperatures below 150 oC were 10.61 %, 10.34 % and 9.21 %,
respectively.
This indicated that CAS-S-MCM-48 was the most hydrophobic,
followed by CAS-N-MCM-48 and CAS-MCM-48, respectively. In the temperature
range between 150-330 oC, all supported CAS complex showed minimal weight
losses associated with the removal of water molecule from CAS complex and
decomposition of OFA. The organic ligand of CAS complex and OFA were largely
decomposed at temperatures above 330 oC. The weight loss continuously dropped
after 750 oC, which might be attributed to the dehydroxylation of silica matrix. Table
3.10 shows the percentage weight loss of MCM-48 supported CAS complex.
Weight loss / %
73
Figure 3.19
Table 3.10
TGA thermograms of MCM-48 containing CAS complex
Percentage weight loss of MCM-48 containing CAS complex
Weight loss (%)
Samples
Below 150 oC
150-330 oC
330-750 oC
CAS-MCM-48
10.61
1.93
4.29
CAS-N-MCM-48
10.34
4.35
8.48
CAS-S-MCM-48
9.21
4.45
9.35
The quantity of CAS complex supported on the matrix of MCM-48 was
elementally analyzed by using atomic absorption spectroscopy and the typical results
are shown in Table 3.11. The results showed that the MCM-48 modified with
sulfonic acid functionalized group has the highest content of CAS complex, which is
0.1501 mmol g-1, followed by CAS-MCM-48 (0.0592 mmol g-1) and CAS-N-MCM48 (0.0568 mmol g-1). The CAS complex with hydrophobic characteristic seems to
be preferred the hydrophobic organo-modified MCM-48 than the hydrophilic silanol
surface of Si-MCM-48. Thus, it is reasonable for CAS-S-MCM-48, which showed
the highest hydrophobic property also consisted of the highest content of CAS
74
complex. However, this did not occur in NH2-MCM-48 because the NH2-MCM-48
has narrow slit pore channel (Dp = 2.66 nm) whichaffected diffusion and attachment
of CAS complex on the MCM-48 pore channel (estimated CAS molecular dimension
= 1.65 nm, simulated using ChemBio3D Ultra 11.0 of Cambridge Software ChemBio
Office Ultra 2008 v11).
Table 3.11
a
Copper content of CAS complex incorporated on MCM-48
Samples
Cu content (mmol g-1)a
CAS-MCM-48
0.0592
CAS-N-MCM-48
0.0568
CAS-S-MCM-48
0.1501
Copper content of CAS complex over 1 g of MCM-48.
On the other hand, CAS-S-MCM-48 has the highest content of CAS complex,
because of the sulfonic acid functionalized agent can retain the complex in MCM-48
matrix through the formation of stronger chemical interaction, electrostatic ionic
bonding, with CAS complex.
For CAS-N-MCM-48, it is proposed that CAS
complex was tethered on amino functional group of MCM-48. On the other hand,
CAS complex was retained in Si-MCM-48 through the formation of hydrogen
bonding with the silanol group of silica matrix.
The possibility of chemical
interactions formed between CAS complex and MCM-48 matrix are illustrated in
Scheme 3.3.
75
Hydrogen bonding
Hydrogen bonding
N
N
H
O
O
H
O
H
O
N
Cu
O
H
O
N
O
H H
O
H
O
Si-MCM-48
Hydrogen bonding between CAS
and silanol group of Si-MCM-48
N
N
N
N
O
Cu
O
H
N
NH2
H2N
Si
O
O
O
OH
O
O
O
O
O
N
HO3S
Si
Si
OMe
O
OH
N
O
O
S O
N
Cu
Si
O
OH
O
O
OMe
OH
NH2-MCM-48
SO3H-MCM-48
Thethering of CAS on
NH2-MCM-48
Ionic bonding between CAS and
sulf onic acid of SO3H-MCM-48
Scheme 3.3 Proposed chemical interactions between CAS complex and MCM-48
matrix
By comparing the DR UV-Vis spectra of neat CAS complex and MCM-48
supported CAS complex, the presence of the d-d transitions band, which is the main
characteristic of transition metal complex, in the range between 450 nm and 600 nm
indicated that the CAS complex has been successfully incorporated on MCM-48
matrix. The shifting of the d-d transitions band to lower or higher wavelength
suggests the presence of specific interaction between the CAS complex and silica
mesophase. On the other hand, a broad and high intensity of band in the 300-430 nm
region was attributed to ligand-to-metal charge transfer transitions, π-π* and n-π*
transitions of C=N. The few bands presented in the range of 190-300 nm were
assigned to π-π* type transitions of aromatic ring. The DR UV-Vis spectra of neat
CAS complex and are shown in Figure 3.20.
76
416 nm
557 nm
K-M / a.u.
CAS
K-M / a.u.
312 nm
d-d transitions
of Cu(II) ion
CAS-MCM-48
CAS-N-MCM-48
400
CAS-S-MCM-48
400
190
600
500
600
Wavelength / nm
800
Wavelength / nm
Figure 3.20
DR UV-Vis spectra of neat CAS complex and MCM-48 supported
CAS complex
The FESEM image of MCM-48 containing CAS complex (Figure 3.21)
showed that the fine spherical particles of MCM-48 with diameter not larger than
300 nm are still retained. The incorporation of CAS complex in the matrix of MCM48 was attributed to agglomeration of particles. On the other hand, the TEM image
of MCM-48 containing CAS complex is shown in Figure 3.22. By comparing with
the TEM image of Si-MCM-48 in Figure 3.13, it can be observed that the long range
ordered of the MCM-48 pore system was still maintained well in the meso size range
after modification and incorporation of CAS complex.
77
Figure 3.21
Figure 3.22
FESEM image of MCM-48 supported CAS complex
TEM image of MCM-48 supported CAS complex
CHAPTER 4
CATALYTIC ACTIVITY OF MCM-48 CONTAINING COPPER(II)
DIETHYLAMINO-SUBSTITUTED SALEN COMPLEX IN THE
OXIDATION OF O-AMINOPHENOL
4.1
Catalytic Testing – Oxidative Coupling of o-Aminophenol
The catalytic activity of the prepared MCM-48 containing CAS catalyst was
analyzed in the oxidation of o-aminophenol (AP) by using peroxide as oxidant in
organic solvent at the mild temperature as shown in Figure 4.1.
Figure 4.1
Catalytic oxidation of o-aminophenol (AP) to 2-amino-3Hphenoxazin-3-one (APX)
In the previous work, catalytic oxidation of o-aminophenol and its derivatives
to phenoxazinone chromophores by using homogeneous catalysts have been
successfully achieved in methanol. Thus, methanol is chosen as the solvent for this
catalytic study. From both economic and environmental point of view, hydrogen
peroxide is the best choice among the terminal peroxides because of its high oxygen
content as well as is a cheap mild oxidizing agent with only water being formed as
waste product (Kureshy et al., 2006; Louloudi, et al., 2002; Pietikäinen, 1998).
79
According to Kureshy et al. (2006), hydrogen peroxide is particularly useful for
liquid phase oxidation for the synthesis of fine chemicals, pharmaceuticals,
agrochemicals and electronic materials owing to its characteristic as mentioned
before.
Procedure
Methanol (30 mL) containing o-aminophenol (1.50 mmol), 30-32 % aqueous
hydrogen peroxide (2.250 mmol), N,N-dimethylformamide (0.600 mmol) as internal
standard and MCM-48 containing CAS complex or Si-MCM-48 (80 mg) or neat
CAS complex (0.013 mmol) were refluxed at 70 oC for 24 hours. Meanwhile, the
reaction conducted without presence of catalyst was used as blank. All catalytic
reactions testing were monitored by using GC-FID and the data obtained were used
to evaluate the performance of catalysts in the oxidation of AP through the
calculation of the percentage conversion and turn over number (TON) of AP as well
as the yield and percentage selectivity towards formation of 2-amino-3Hphenoxazin-3-one (APX). The retention time of APX was determined by comparing
with the retention time of prepared pure APX standard.
The effect of reaction parameters, such as reaction temperature, reaction time,
molar ratio of substrate to oxidant, type of oxidant and type of solvent, towards the
oxidation of AP was also studied.
4.2
Oxidation of Phenol and Its Derivatives
Previous study has reported that phenol and its derivatives can be oxidized to
hydroquinone and benzoquinone substances as major products.
Normally,
hydroquinone substances are the reactive compounds that could be further oxidized
to quinonic substances, as well as coupling to afford polymeric compounds (Maurya
and Sikarwar, 2007).
Recently, the oxidation of 2,3,6-trimethylphenol over
biomimetic catalyst based on iron(III) porphyrin dendrimer supported on MCM-41
80
has been reported by Lau (2009). The corresponding supported iron(III) porphyrin
dendrimer catalyst was successfully catalyzed 2,3,6-trimethylphenol to 2,3,5trimethylbenzoquinone with high selectivity. Therefore, a few phenolic compounds,
such as phenol, 2,3,6-trimethylphenol, p-hydroquinone and m-aminophenol, have
been used as the reaction model, with the aim to understand the catalytic behavior of
supported CAS catalyst towards oxidation of phenolic compounds.
Procedure
Phenol (1.5 mmol) was refluxed in methanol (30 mL) containing CAS
catalyst supported on MCM-48 (80 mg) and 30-32 % aqueous hydrogen peroxide
(2.25 mmol) at 70 oC for 24 hours. Then, the catalytic reactions were monitored by
using GC-FID.
After that, the same catalytic reaction was repeated for 2,3,6-
trimethylphenol, p-hydroquinone and m-aminophenol.
4.3
Preparation of 2-Amino-3H-phenoxazin-3-one (APX) as Standard
2-Amino-3H-phenoxazin-3-one was synthesized with slight modification
according to the procedures described by Gents et al. (2005) and Gagliardo and
Chilton, (1992).
Procedure
o-Aminophenol (0.50 g) is oxidized simply by stirring in methanol with open
air at room temperature for a month. Solid that precipitated out from the solution
was filtered and the residue was recrystallized from methanol to afford 2-amino-3Hphenoxazin-3-one (0.08 g, 16.45%) as reddish black solid with melting point 253258 oC [252-258 oC (Gagliardo and Chilton, 1992)]. IR (Appendix A) υmax (KBr)
cm-1: 3410 and 3309 (primary N-H), 1657 (C=N), 1587 (C=O, unsaturated), 1573
(C=C, unsaturated), 1600-1460 (C=C aromatic), 1293 and 1273 (C-N), 1204 and
81
1175 (C-O); 1H NMR (CDCl3) (Appendix B) δ ppm: 5.13 (2H, broad, s, NH2), 6.43
(1H, s, H-4), 6.50 (1H, s, H-1), 7.41 (3H, m, H-6, H-7 and H-9), 7.77 (1H, d, J = 8.0
Hz, H-8); 13C NMR (CDCl3) (Appendix C) δ ppm: 100.87 (1C, C-4), 104.15 (1C, C1), 116.04 (1C, C-6), 125.29 (1C, C-9), 128.81 (1C, C-7), 129.59 (1C, C-8), 133.97
(1C, C-9a), 142.79 (1C, C-5a), 145.72 (1C, C-4a), 148.72 (1C, C-10a), 149.43 (1C,
C-2), 180.35 (1C, C-3); EIMS (Appendix D) m/z (%): 212 (100, M+), 185 (49), 184
(22).
4.4
Analysis of Catalytic Reaction
In this research, all the catalytic reactions were analyzed by using gas
chromatography (GC). GC is one of the separation equipments that has been widely
used for the determination of such small molecular or vaporizable organic
compounds.
GC separates the organic substances in gaseous form based on
absorption on or partitioning in stationary phase from a gas phase (Christian, 2004).
The eluted solutes will be detected and then converted to signal by the mean of
retention time. Thus, the catalytic reaction mixture can be simply analyzed by using
this technique.
4.4.1
Gas Chromatography – Flame Ionization Detector (GC-FID)
Flame ionization detector (FID) is recognized as a highly sensitive detector,
which is best to detect all types of organic substances and other easily flammable
components. This detector is responded to the ionic substances and the measurement
is performed by counting the number of ions being formed.
Therefore, gas
chromatography equipped with flame ionization detector is preferable used for
quantitative analysis.
The quantitative measurement of eluted solutes can be
determined by referring to the peak area of signals since the concentration of solutes
are proportional to the peak area of signals. Furthermore, GC-FID also can be used
82
for qualitative analysis by comparing the retention time of eluted solutes with the
retention time of a standard (Christian, 2004).
Method
The chromatography analysis of the catalytic reaction was carried out by
using Agilent Model 6890N GC instrument equipped with Thermo Finnigan, HP-5
30 m x 0.32 mm 0.25 μm column. Pure helium gas was used as carrier gas. Sample
(1 μL) was injected to GC and the mixture was separated using temperature
programming: 70-170 oC (14 oC / min), 170-260 oC (22 oC / min), 260-285 oC (8 oC /
min) and 285 oC (hold for 2 minutes).
4.4.2
Gas Chromatography - Mass Spectrometry (GC-MS)
Mass spectrometry (MS) is a powerful instrumental technique that can
provide the information about the structural and molecular weight of substances in
the form of mass fragmentation spectrum.
When coupling with GC, the
corresponding instrumentation technique is capable to identify and quantify complex
mixtures of trace substances (Christian, 2004). Hence, GC-MS is a good, highly
sensitive and selective instrument that can be used for qualitative as well as
quantitative analysis. However, in this research GC-MS is only used for qualitative
analysis purpose.
Method
The reaction mixtures and prepared standard were analyzed using of Agilent
model 5973/6890N GC-MS instrument equipped with 30 m x 0.25 mm x 0.2 μm
non-polar column. Pure helium gas was used as carrier gas. Sample (0.5 μL) was
injected to GC and the mixture was separated using temperature-programming
method.
83
4.5
Leaching Test
Both MCM-48 supported CAS catalyst (80 mg) was refluxed respectively in
methanol (30 mL) containing 30-32 % aqueous hydrogen peroxide (2.250 mmol) and
N,N-dimethylformamide (0.600 mmol) at 70 oC for 24 hours. After that, MCM-48
supported CAS catalysts were recovered and the solids were respectively subjected
for elemental analysis according to the procedure as shown in section 3.3.6. The
same leaching test procedure also was repeated for both MCM-48 supported CAS
catalysts in reaction solvent medium chloroform.
4.6
Results and Discussion
o-Aminophenol (AP) is a reactive compound because it can be easily
oxidized in the presence of oxidant. As shown in Table 4.1, 16.93 % or 0.2540
mmol of AP has been self-oxidized to afford 0.0616 mmol of 2-amino-3Hphenoxazin-3-one (APX). Si-MCM-48 that does not possess any active element was
not catalytically active in the oxidation of AP, whereas the percentage conversion of
AP and the formation of APX are only slightly higher than the blank. On the other
hand, the catalytic activities were increased 3 to 6 times faster when the catalysts
were introduced to the reaction. This indicated that the corresponding catalysts are
active in the oxidation of AP. Based on the data obtained, the neat CAS catalyst
(homogeneous catalyst) gave the percentage conversion of AP comparable to the
CAS catalyst supported on MCM-48 matrix (heterogeneous catalyst). On the other
hand, the neat CAS catalyst showed the lower TON value if compared to the
supported CAS catalyst. This may because of the application of oxidative catalyst
based on complexes of transition metal as active site in homogeneous catalytic
system might be deactivated by the formation of μ-oxo dimer or bimolecular selfoxidation and directly affects to the performance of the homogeneous catalyst (Jacob
et al., 1998a; Karandikar et al., 2004; Lou et al., 2007). However, the neat CAS
catalyst gave higher yield and selectivity than the supported CAS catalyst in the
formation of APX.
84
Among the supported CAS catalysts, CAS-S-MCM-48 gave the highest
conversion of AP because the CAS-S-MCM-48 consists of the highest content of
CAS.
On the other hand, CAS-N-MCM-48 shows the highest of TON value,
indicating that the catalyst is highly active in the oxidation of AP.
Table 4.1
Catalytic activity of neat and supported CAS catalyst in the oxidation
of AP to APX
Cu
Conversion
Yield of APX
(mmol)
of AP (%)
(mmol)
No catalyst
-
16.93
0.0616
-
100.00
Si-MCM-48
-
23.32
0.0925
-
100.00
CAS
0.0130
78.96
0.4114
91.11
81.75
CAS-MCM-48
0.0047
65.49
0.2425
209.01
73.75
CAS-N-MCM-48
0.0045
70.09
0.2709
233.63
74.98
CAS-S-MCM-48
0.0120
85.30
0.4042
106.63
67.87
Samples
a
TONa
Selectivity
(%)
Conversion of AP (mmol) / content of Cu (mmol).
According to previous studies, leaching-out of the active site from the
supporting material and blocking of the pore channel by the substrates or products
have been identified as major problems for the heterogeneous catalyst that prepared
by supporting the homogeneous catalyst on solid material. According to the leaching
test results as shown in Table 4.2, the highest percentage of catalyst leached out from
MCM-48 matrix is CAS-MCM-48 (54.07 %), followed by CAS-N-MCM-48 (48.83
%) and CAS-S-MCM-48 (29.32 %). This suggests that CAS complex has poor
chemical interaction with the material support, which attributes to leaching out of
active site from the MCM-48 matrix.
In the reusability study, the percentage
conversion of AP for CAS-MCM-48 and CAS-N-MCM-48 catalysts were decreased
within 5-20% for each cycle. On the other hand, the CAS-S-MCM-48 did not show
any significant loss of its catalytic activity for each cycle. The above findings suggest
the active site leaching-out from MCM-48 is a major problem that affect to the
catalytic performance of supported catalyst. Besides that, the blocking of MCM-48
85
pore channel by substrates or products also was seem to be contributed to
deactivation of the supported CAS catalyst.
Table 4.2
Cycle
1
2
3
Leaching test and reusability of the supported catalyst
Sample
Cu leach out (%)
Conversion of
AP (%)
Selectivity (%)
CAS-MCM-48
54.07
61.09
69.35
CAS-N-MCM-48
48.83
69.76
70.72
CAS-S-MCM-48
29.32
80.98
69.42
CAS-MCM-48
-
50.78
70.20
CAS-N-MCM-48
-
57.19
77.58
CAS-S-MCM-48
-
76.81
73.11
CAS-MCM-48
-
46.81
85.26
CAS-N-MCM-48
-
46.68
82.99
CAS-S-MCM-48
-
78.87
77.39
CAS is a complex that consists of a copper(II) ion binding with a N,N,O,Otetradentate ligand, which is considered mimic to the molecular structure of galactose
oxidase active site as shown in Figure 2.7. In this respect, CAS might have the
catalytic functional behavior similar to the galactose oxidase. The previous studies
have successfully identified the functional roles for the structural features of
galactose oxidase in the oxidation of alcohol (Whittaker and Whittaker, 1993;
Chaudhuri et al., 1999). The corresponding studies have provided the basis of
catalytic mechanism in the oxidation of alcohol. The proposed mechanism path for
alcohol oxidation over galactose oxidase is shown in Scheme 4.1.
86
(Tyr495)
O
(His581) N
S
OH
(Tyr272)
N
O
CuII
(His495) N
O
Cu
H
O C R
H
H
(Alcohol)
S
II
N
H
O C R
H
H-abstraction and
electron stransfer
H2O2
RCH2OH
OH
N
S
OH
N
O
O O
HO
CuII
CuII
N
S
H
R
O
C
N
O
O2
C R
H
H
Scheme 4.1
Proposed mechanism path for alcohol oxidation over galactose
oxidase (Chaudhuri et al., 1999)
Based on the proposed mechanism in the oxidation alcohol over galactose
oxidase as shown in Scheme 4.1, a similar reaction mechanism is proposed for the
oxidation of AP to APX with the presence of MCM-48 containing CAS and aqueous
hydrogen peroxide as oxidant. Scheme 4.2 showed the proposed mechanism for the
oxidative coupling of AP over CAS complex. The mechanism showed that the
oxidative dimerization of AP required three equivalents of hydrogen peroxide to
afford APX. By simulating the catalytic behavior of galactose oxidase, the reaction
initially proceeds with the dehydrogenation of phenolic group.
Then the
rearrangement of aromatic ring and deprotonation of amino group affords o-quinone
imine (QI) as reaction intermediate (Szihyártó et al., 2006, Simándi et al., 2004;
Simándi et al., 1996). However, QI could not be detected in the reaction mixture due
to the corresponding compound is very reactive and could react further to form
another products. Thus, a molecule of AP is then coupled with the QI to afford APX
compound.
87
H
O O
H2N
CuII
O
O
H
H
CuII
O
H
O
N
O
O
O
O
H
NH2
N
N
MCM-48
N
MCM-48
MCM-48
N
N
CuII
O
H
O
O
H
NH2
AP
H
O O
N
CuII
O
H
O
NH2
O
H H
N H
NH
+
OH
Scheme 4.2
N
N
CuII
O
HN
+
+
2 H2O
O
O
QI
CAS
supported on
MCM-48
O
QI
AP
MCM-48
MCM-48
N
2 H2O2
4 H2O
N
NH2
O
O
APX
The proposed mechanism for the oxidative coupling of AP over CAS
complex supported on MCM-48
On the other hand, oxidation of phenol and its derivatives were carried out in
order to investigate the ability of CAS catalyst supported on MCM-48 to oxidize the
phenolic compound to respective quinonic products as mentioned before. The results
of the respective reactions were summarized in Table 4.3. According to the GC
analysis, there were no products detected for both oxidation reaction of phenol and
2,3,6-trimethylphenol. These suggested that the CAS catalyst supported on MCM-48
cannot catalyze the mono-hydroxy substitution aromatic compound to quinonicsubstitution product via hydroxylation process. However, p-benzoquinone has been
detected in the oxidation of p-hydroquinone over supported CAS catalyst. The
corresponding catalyst is also not active in catalyzing the meta substituted
aminophenol; while the catalytic reaction was occurred for ortho-substituted
aminophenol as shown in Table 4.1. This indicated that the phenolic compound with
88
nucloephilic amino at ortho position is very reactive and easily to be oxidized in the
presence of catalyst and oxidant. The overall findings proposed that CAS supported
MCM-48 catalyst could catalyze the dehydrogenation reaction, but cannot function
well in hydroxylation reaction. In this respect, AP can be successfully converted to
APX, thus is conceivable via the formation of QI as intermediate. Scheme 4.3 shows
the possible reaction pathway of AP to the formation of APX.
Table 4.3
Oxidation of phenol and its derivative over supported CAS catalyst
Reactant
Products
Phenol
-
2,3,6-Trimethylphenol
-
p-Hydroquinone
p-Quinone
m-Aminophenol
-
Amino-p -hydroquinone
HO
NH2
Amino-p -quinone
O
NH2
O
OH
NH2 CAS supported
on MCM-48
OH
AP
[O]
NH
N
NH2
O
O
O
APX
QI
NH2
NH2
OH
O
OH
Amino-o-hydroquinone
Scheme 4.3
O
Amino-o-quinone
The possibility reaction pathway of AP to the formation of APX
89
4.6.1
Effect of Reaction Time
The effect of reaction time on the oxidation of AP was carried out in
methanol containing aqueous H2O2 (reactant : oxidant = 1 : 1.5) at 70 oC for 24
hours. Based on the graph shown in Figure 4.2, the catalytic conversion of AP was
highly accelerated at reaction times below 3 hours for all the supported catalysts.
Afterwards, the conversion of AP was almost increased with low percentage and the
catalytic conversion of AP was continuously increased up to 24 hours. On the other
hand, CAS-MCM-48 and CAS-N-MCM-48 gave 100 % selectivity of reaction for
the first 2 hours, while for CAS-S-MCM-48 the selectivity began to drop after 1 hour
Conversion and selectivity
(%)
of reaction.
100
CAS-M CM -48
80
CAS-N-M CM -48
CAS-S-M CM -48
60
CAS-M CM -48
40
CAS-N-M CM -48
20
CAS-S-M CM -48
Black marker - Conversion
Blank marker - Selectivity
0
0
5
10
15
20
25
Reaction time (hours)
Figure 4.2
Effect of reaction time on the conversion of AP and selectivity
towards APX by various types of MCM-48 containing CAS catalyst
According to Figure 4.3, the formation of APX was observed to be
exponentially increased at low reaction times over all MCM-48 supported CAS
catalysts. Afterward, the production of APX was increased linearly over 24 hours.
Typically, at the beginning of reaction, the higher concentration of AP is attributed to
higher opportunity of collisions between AP and the supported CAS catalyst, which
is then converted to products. Thus, the percentage conversion of AP and yield of
APX will be higher increase at the beginning of reaction. After a few hours of
90
reaction, the concentration of AP decreased and lead to the catalytic reaction to slow
down.
0.5
Yield (mmol)
0.4
0.3
CAS-MCM-48
0.2
CAS-S-MCM-48
CAS-N-MCM-48
0.1
0
0
Figure 4.3
5
10
15
20
Reaction time (hours)
25
Effect of reaction time on the formation of APX by various types of
MCM-48 supported CAS catalyst
4.6.2
Effect of Reaction Temperature
The effect of reaction temperature on the oxidation of AP was studied at
room temperature (RT), 40 oC, 55 oC and 70 oC. The corresponding reactions were
performed in methanol containing aqueous H2O2 (reactant : oxidant = 1 : 1.5) for 24
hours. According to Figures 4.4 and 4.5, the reaction carried out at 70 oC gave the
highest percentage conversion of AP and the yield of APX, followed by reactions
done at 55 oC, 40 oC and RT. External heat energy can promote the reaction rate by
increasing the diffusion rate and directly increase the collision between substrate and
catalyst.
Thus, the higher the reaction temperature, the higher the substrate is
converted to product. As observed that the percentage conversion of AP and the
yield was increased almost linearly over both supported CAS catalysts.
91
100
80
80
60
60
40
40
20
20
0
0
RT
40
55
70
CAS-MCM-48
Selectivity (%)
Conversion (%)
100
CAS-N-MCM-48
CAS-S-MCM-48
CAS-MCM-48
CAS-N-MCM-48
CAS-S-MCM-48
Bar chart - Conversion
Line chart - Selectivity
o
Reaction temperature ( C)
Figure 4.4
Effect of reaction temperature on the conversion of AP and selectivity
towards APX by various types of MCM-48 supported CAS catalyst
Yield (mmol)
0.50
0.40
0.30
CAS-M CM -48
CAS-N-M CM -48
0.20
CAS-S-M CM -48
0.10
0.00
RT
40
55
70
o
Reaction temperature ( C)
Figure 4.5
Effect of reaction temperature on the formation of APX by various
types of MCM-48 containing CAS complex
On the other hand, CAS-MCM-48 and CAS-N-MCM-48 showed 100 %
selectivity at the reaction temperature below 40 oC, while CAS-S-MCM-48 only
gave within 93-96 % of selectivity towards formation of APX. At the reaction
temperature above 40 oC, the percentage selectivity was decreased within 5-30 %
over all supported CAS catalyst. It can be found that the decreasing of selectivity
92
towards formation of APX was due to the increasing formation of by-products. The
overall findings show that the catalytic conversion of AP is proportional to the
reaction temperature, while the reaction selectivity is inversely proportional to
reaction temperature.
In this research, the by-products formed in the oxidation of AP could not be
clearly identified. However, the previous study on the oxidation of aminophenol in
methanol over copper compounds was reported by Prati and Rossi (1992), in which
compound (15) as shown in Scheme 4.3 was formed as a minor product.
Furthermore, Simándi and co-workers (1996) detected two free-radical by product
intermediates when investigating the kinetic and mechanism of the cobaloxime(II)catalyzed oxidation of 2-aminophenol by oxygen. These free-radical intermediates
expected
could
be
2-amino-p-hydroquinone
and
2-amino-4-(2-
aminophenoxy)phenol. On the other hand, a previous study on the oxidation of
aniline and phenol showed that aniline could be oxidized to benzylhydroxyl amine,
nitrobenzene and diazo substances, while phenol could be oxidized to ortho- and
para-hydroquinone or quinine (Castillejos-López, et al., 2009).
Reaction
diagrammatic that showing the possible products formed in the oxidation of AP is
summarized in Scheme 4.4.
93
O
NH2
N
NH2
O
O
O
2-Amino-p -quinone
HO
APX
(Identified major product)
NH2
H
N
OH
O
2-Amino-p -hydroquinone
10H -Phenoxazine
Low
possibility
NHOH
NH2
NH
O
NH2
OH
OH
O
NH2
OH
AP
Phenylhydroxyl
amine
2-amino-4-(2aminophenoxy)phenol
QI
O
NO2
OH
OH
2-Nitrophenol
Phenol
OH
OMe O
(15)
Me
O
NH2
Me
N
H
O
3-Amino-4a,10-dihydro-2H phenoxazin-2-one
or
OH
HO
N N
2,2'-(Diazene-1,2-diyl)diphenol
Scheme 4.4
O
NH2
N
OH
3-amino-4aH -phenoxazin-2-ol
Reaction diagrammatic that showing the possibility of products
formed in the oxidation of AP
94
4.6.3 Effect of Molar Ratio of Substrate to Oxidant
The effect of molar ratio of substrate to oxidant on the oxidation of AP was
studied at substrate to oxidant = 1.0 to x, whereas x = 1.0, 1.5, 2.0 and 2.5. The
corresponding reactions were carried out at 70 oC in methanol containing aqueous
H2O2 as oxidant for 24 hours. According to Figure 4.6, the molar ratio substrate to
oxidant of 1.0 to 2.5 showed the highest conversion of AP, but gave the lowest
selectivity in the formation of APX. The more quantity of oxidant is introduced, the
larger the opportunity of oxidant to oxidize the reactant to products. However,
excess introducing the quantity of oxidant in the reaction will be led to over
oxidation problem. This can be observed clearly in the case of CAS-S-MCM-48
catalyzed AP in methanol containing H2O2 with substrate to oxidant = 1.0 to 2.5,
whereas the yield of APX was not increase when the molar ratio of oxidant was
increased; but decreased significantly up to 22 % if compared to the reaction
condition with substrate to oxidant = 1.0 to 2.0. Besides that, the excess of oxidant
also promoted the high yield of by products, which has been attributed to decrease of
selectivity to APX. From the Figure 4.7, it can be observed that both MCM-48
supported CAS catalyst was seemed to be achieved optimum reaction level for the
100
100
80
80
60
60
40
40
20
20
0
0
1.0:1.0 1.0:1.5 1.0:2.0 1.0:2.5
Molar ratio substrate to oxidant
Figure 4.6
Selectivity (%)
Conversion (%)
molar ratio substrate to oxidant 1.0 : 2.0.
CAS-M CM -48
CAS-N-M CM -48
CAS-S-M CM -48
CAS-M CM -48
CAS-N-M CM -48
CAS-S-M CM -48
Bar chart - Conversion
Line chart - Selectivity
Effect of molar ratio substrate to oxidant on the conversion of AP and
selectivity towards APX by various types of MCM-48 containing
CAS catalyst
95
Yield (mmol)
0.5
0.4
0.3
0.2
CAS-MCM-48
CAS-N-MCM-48
0.1
CAS-S-MCM-48
0
x=
1.01.0
x =1.5
1.5
x =2.0
2.0
x =2.5
2.5
Molar ratio substrate to oxidant (1:x)
Figure 4.7
Effect of molar ratio substrate to oxidant in the formation of APX by
various types of MCM-48 containing CAS catalyst
4.6.4
Effect of Different Oxidant
Oxidant is an important component that converts the substrate to products.
An ideal oxidant is expected to promote the selectivity in formation of desired
products, and itself reduced to non-polluting substances.
Aqueous hydrogen
peroxide (H2O2) with only water as waste, is a ‘green’ oxidant. Organic peroxide,
tert-butyl hydrogen peroxide (TBHP) is known as a strong oxidizing agent, with only
methanol as waste, also is the best selection for ‘green’ reaction process. Therefore,
the effect of different oxidants on the oxidation of AP was carried out in methanol
containing H2O2 and TBHP at 70 oC for 24 hours, respectively. According to
Figures 4.8 and 4.9, the percentage conversion of AP and yield of APX over both
supported CAS catalyst were significantly increased, when the TBHP was used to
replace H2O2 as oxidant. This may due to the higher thermal stability of TBHP
compared to H2O2 (Zhang et al., 2009; Wang et al., 2008). Besides that, TBHP,
which is more hydrophobic than H2O2, has the higher affinity for supported CAS
catalyst, hence increases the activity of the catalyst (Pires et al., 2000). Moreover,
the supported CAS catalyst is more selective in the formation of APX in the presence
96
of TBHP than in H2O2. Thus, this study showed that TBHP performed better than
100
100
80
80
60
60
40
40
20
20
0
0
H2 O2
H2O2
TBHP
Selectivity (%)
Conversion (%)
H2O2 as oxidant in promoting the catalytic oxidation of AP to APX.
CAS-MCM-48
CAS-N-MCM-48
CAS-S-MCM-48
CAS-MCM-48
CAS-N-MCM-48
CAS-S-MCM-48
Bar chart - Conversion
Line chart - Selectivity
Oxidant
Effect of different type of oxidant on the conversion of AP and
Figure 4.8
selectivity in the formation of APX by various types of MCM-48
containing CAS catalyst
0.6
Yield (mmol)
0.5
0.4
CAS-M CM -48
0.3
CAS-N-M CM -48
CAS-S-M CM -48
0.2
0.1
0
H2O2
H2O2
TBHP
Oxidant
Figure 4.9
Effect of different type of oxidant on the formation of APX by
various types of MCM-48 supported CAS complex
97
4.6.5
Effect of Different Solvent
Solvent is also one of the important part in catalytic study due to the solvent
is the medium that reaction occurs. Solvent is a medium that can influence the
catalytic reaction due to the solvent can stabilize intermediates and transition states
and thereby modify the intrinsic barrier to reactions as well as act as efficient means
for energy transfer (Masel, 2001). In this respect, the effect of organic solvent in the
oxidation of AP was studied by comparing the catalytic reaction conducted in
methanol, acetonitrile and chloroform. Methanol (PI = 5.1) and acetonitrile (PI =
5.8) are known as polar solvent, while chloroform (PI = 4.1) is a medium polar
solvent.
According to Figures 4.10 and 4.11, almost both of catalytic reaction
performed in methanol and acetonitrile gave the higher conversion of AP and better
yield of APX. This might be due to the typical polar solvents have higher solubility
of AP compound that enable the substrate to convert to products easily. On the other
hand, the use of chloroform gave the lowest conversion of AP and yield of APX.
This was because of the reactant not easily dissolved in chloroform at normal
condition, which has been affected to the catalytic reaction process. However, both
catalytic reaction carried out in methanol are not selective in the formation of APX
due to the ability of polar protic solvent (methanol) to stabilize the intermediates
through the hydrogen bonding or dipole moment interaction and promote the
formation of undesired products. Chloroform and polar aprotic acetonitrile, which
have low ability to stabilize the reaction intermediates was attributed to low
opportunity of reactant converts to undesired intermediates, and this might be a
reason why these solvents showed the higher selectivity in the formation of APX.
98
100
100
Conversion (%)
80
60
60
40
40
20
20
0
0
Methanol
Acetonitrile
Chloroform
Methanol
AcetonitrileChloroform
Selectivity (%)
80
CAS-MCM-48
CAS-N-MCM-48
CAS-S-MCM-48
CAS-MCM-48
CAS-N-MCM-48
CAS-S-MCM-48
Bar chart - Conversion
Line chart - Selectivity
Solvent
Effect of solvent on the conversion of AP and selectivity towards
Figure 4.10
APX by various types of MCM-48 supported CAS catalyst
Yield (mmol)
0.5
0.4
CAS-MCM-48
0.3
CAS-N-MCM-48
CAS-S-MCM-48
0.2
0.1
0
Methanol
Figure 4.11
Acetonitrile
Solvent
Chloroform
Effect of solvent in the formation of APX by various types of MCM48 containing CAS catalyst
On the other hand, the leaching-out problem was considered as one of the
major factors that will affects the catalytic performance of the MCM-48 supported
CAS catalyst. In this respect, the leaching test regarding to different type of solvent
was carried out in methanol (polar solvent) and chloroform (medium polar solvent).
From the leaching test results as shown in Figure 4.12, it can be observed that both
99
MCM-48 supported CAS catalysts are unstable in methanol, whereas the CAS
catalysts easily leach-out from MCM-48 matrix.
While, both supported CAS
catalysts was observed more stable in chloroform. Thus, both MCM-48 supported
CAS catalysts were performed well as heterogeneous catalyst and successfully
catalyzed AP to APX with almostly achieves 100 % selectivity in chloroform. Based
on the analysis, it can be concluded that both CAS catalyst supported on MCM-48 is
performed well as heterogeneous catalyst, meanwhile gave the high selectivity in the
oxidation of AP to APX if the reaction is carried out in low polarity solvent.
Cu leached-out (%)
100
80
60
CAS-MCM-48
40
CAS-S-MCM-48
CAS-N-MCM-48
20
0
Methanol
Chloroform
Solvent
Figure 4.12
Effect of different solvent on the stability of MCM-48 supported CAS
catalyst
CHAPTER 5
CONCLUSION AND RECOMMENDATIONS
5.1
Conclusion
A series of MCM-48 containing copper(II) diethylamino-substituted salen
(CAS) complex has been successfully synthesized by wet incorporation of the CAS
complex onto purely siliceous, amino and sulfonic acid-functionalized MCM-48,
respectively. CAS complex was synthesized by the reaction of copper(II) acetate
monohydrate and the prepared diethylamino-substituted salen (A-Salen) ligand. ASalen ligand, or namely N,N’-bis[4-(N,N-diethylamino)salicylidene]ethylenediamine,
was prepared by the reaction of ethylenediamine (EDA) and 4-(N,N-diethylamino)salicylaldehyde (A-Sal).
On the other hand, MCM-48 as catalyst support was
synthesized by using hydrothermal method, with rice husk ash (RHA) used as silica
source.
The prepared A-Salen liagnd was characterized using Fourier transformed
infrared (FTIR), proton and carbon-13 nuclear magnetic resonance (1H and
13
C-
NMR) spectroscopy. CAS complex was characterized using FTIR, DR UV-Vis
spectroscopy and thermogravimetric analysis (TGA). On the other hand, the MCM48 containing CAS complex was successful characterized by using FTIR, DR UVVis spectroscopy, powder X-ray diffraction (XRD), N2 physisorption measurement,
TGA, atomic absorption spectroscopy (AAS), field emission scanning electron
microscopy (FESEM) and transmission electron microscopy (TEM).
101
FTIR, 1H and 13C-NMR spectroscopy suggested the reaction of EDA and ASal afforded A-Salen, through the identification of imine functional group in
molecular structure of product. FTIR and DR UV-Vis spectroscopy confirmed that
CAS complex was successfully synthesized.
On the other hand, powder XRD
showed that the framework of mesoporous MCM-48 was preserved after organofunctionalized modification and incorporation of CAS complex. While, FTIR and
DR UV-Vis spectroscopy confirmed that CAS complex was supported on MCM-48.
Nitrogen physisorption measurement showed that the surface area, pore diameter and
pore volume of MCM-48 was decreased after the Si-MCM-48 was being modified
with OFA and CAS complex. FESEM images showed that the MCM-48 synthesized
by using RHA was present as fine spherical particles with size not larger than 300
nm. Consequently, TEM images showed that the long range ordered pore channel of
MCM-48 was successfully preserved in meso range after the modification.
The catalytic activity of the prepared catalysts was tested in the oxidative
coupling of o-aminophenol (AP) by using aqueous hydrogen peroxide as oxidant.
All catalysts are active in the oxidation of AP and 2-amino-3H-phenoxazin-3-one
was the reaction major product. The neat CAS (homogeneous catalyst) gave the
better conversion of AP, yield of APX and also more selective in the formation of
APX than the supported CAS catalyst (heterogeneous catalyst).
However, the
supported CAS catalyst showed the better TON than the neat CAS catalyst. Based
on the leaching test study, all the supported catalysts are not stable in methanol,
whereas within 29-55% of CAS catalyst leached-out from MCM-48 matrix. The
leaching problem has highly affected to the catalytic performance of MCM-48
supported CAS catalyst, especially gave the low selectivity in the formation of APX
product. The leaching-out of active site from MCM-48 matrix was believed highly
influence by the type of solvent used in the reaction.
In this work, the prepared CAS catalyst, which has the metal-ligand
coordination “CuN2O2” that mimic to the active site of galactose oxidase as shown in
Figure 2.6, was considered has the potential function as biomimetic catalyst.
Furthermore, the CAS catalyst also being proposed to has the catalytic activity that
mimic to the biosynthesis mechanism of phenoxazinone synthase, which oxidize AP
102
to APX via the formation of o-quinone imine (QI) as reaction intermediate.
However, the supported CAS catalyst is not superior to phenoxazinone enzyme due
to the prepared catalyst cannot catalyze AP to APX at ambient temperature with high
conversion and yield.
Furthermore, the catalytic oxidation of AP over CAS
supported on MCM-48 required the long reaction time to achieve optimize reaction
process.
The catalytic reaction testing showed that the supported CAS catalyst is
active in the oxidation of AP with using aqueous peroxide as oxidant. Besides that,
the supported CAS catalyst is almost selective in the formation of APX, with
percentage selectivity higher than 70% under normal condition.
However, the
supported CAS catalyst gave the lower catalytic performance if compared to the
catalytic oxygenation of AP over homogeneous Co(salen) and copper compounds,
which have been respectively reported by Maruyama et al. (1996) and Horváth et al.
(2004). Thus, some modifications should be carried out in order to improve the
catalytic activity of supported CAS catalyst in the oxidation of AP. The comparison
of catalytic reaction over supported CAS catalyst, neat Co(salen) and neat copper
compound were summarized in Table 5.1.
Comparison of catalytic oxidation of AP over supported CAS catalyst,
Table 5.1
Co(salen) and copper compound
Category
CAS-S-MCM-48
Co(salen)
CuCl-phen
Methanol
Methanol
DMF
H2O2
O2
O2
Temperature ( C)
70
Ambient temperature
60
Reaction time (hrs)
24
2.5
2
Conversion (%)
85
100
93
Yield (%)
54
94
81
Selectivity (%)
68
-
-
Solvent
Oxidant
o
103
Finally, the overall catalytic findings show that oxidation of AP to APX can
be catalyzed by using the CAS catalyst supported MCM-48. However, the leaching
problem has highly affected their selectivity in the formation of APX. Thus, the
research works on enhancing the stability of CAS catalyst supported on mesophase
MCM-48 should be carried out in the future study.
104
5.2
Recommendations
For future study, some surface modifications are required in order to solve the
leaching-out of copper(II) Schiff base salen complex from the mesoporous matrix.
One of the solutions is to use the salen type ligand or other Schiff bases ligand which
contains functional group that can form covalent bonding with specific OFA. For
example, synthesis of chiral Mn(III) salen complexes immobilized on MCM-48 via
the post-growing of salen ligand from MCM-48 matrix (in Scheme 5.1) has been
reported by Yu and co-workers (2006). The anchored Mn(III) salen catalysts were
stable and could be recycled without loss of its activity in the epoxidation of olefin.
Scheme 5.1
Synthesis of immobilized chiral Mn(III) salen complex from OFMS
(Yu et al., 2006)
According to the Table 5.1, it can be found that the oxidation of AP by using
oxygen gas as oxidant gave higher conversion and higher yield of APX than the
reaction using peroxide as oxidant. Thus, oxygen gas should be used to replaced
H2O2 as oxidant in the catalytic oxidation of AP to APX over MCM-48 supported
CAS catalyst.
105
Oxidation of o-aminophenol substance to phenoxazinone chromophore is a
complicated organic synthesis reaction. Barry and co-workers (1989) has proposed
that the pathway for the formation of phenoxazinone chromophore is a complex
multistep sequence, which involving three steps of two-electron oxidations and two
steps of conjugated additions. This multistep sequence organic process is seemed
cannot be easily carried out by using simple mononuclear copper of CAS catalyst.
On the other hand, the oxidative coupling of o-aminophenol substances to
phnoxazinone chromophores is catalyzed in nature by phenoxazinone synthase,
which has been identified as multicopper oxidase. Therefore, an ideal by simulating
the property or structure of phenoxazinone synthase is a way to prepare an effective
and efficiency biomimetic catalyst.
For example, a tetracopper(II)-tetraradical
cubiodal core complex has been successfully synthesis by Mukherjee and co-workers
(2007), and the corresponding catalyst was successfully catalyzed the oxidation of
AP using aerial oxygen to afford APX with 100% yield under mild condition. In
future work, the multicopper complexes should be synthesized in order to replace
CAS complex as catalyst for the heterogeneous oxidative coupling of AP.
Oxidative coupling of o-aminophenol derivatives is the final step in the
synthesis of actinomycin or phenoxazinone chromophore.
The heterogeneous
catalytic reaction over copper(II) coordinated with Schiff bases ligand can be tested
in the synthesis of actinomycin substances. Consideration of this bulky molecule
synthesis, the larger pore size of mesoporous based silica is required. Thus, SBA-15
with pore size tunable up to 30 nm should be used to replace the MCM-48. On the
other hand, these biological active compounds consist of protein property which is
very sensitive. Thus, peroxide which is known as strong oxidizing agent should be
replaced by oxygen gas as oxidant.
106
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Appendix A
Calculation on the percentage weight loss of water molecules in CAS complex
Formula weight of CAS complex, [Cu(A-Salen)(H2O)2].H2O = 528.100 gmol-1
The percentage weight loss of one water molecule in CAS complex
= (formula weight of water molecule / formula weight of CAS complex) x 100 %
= (18.016 gmol-1 / 528.100 gmol-1) x 100 %
= 3.41%
The percentage weight loss of two water molecules in CAS complex
= ((2 x formula weight of water molecule) / formula weight of CAS complex)
x 100 %
= (2 x 18.016 gmol-1 / 528.100 gmol-1) x 100 %
= 6.82 %
O
O
C-N
C=N
C=C
(aromatic)
C-O
N-H
C=O
4000
3000
2000
C=C (unsaturated)
1500
Wavenumber / cm
1000
400
127
-1
Appendix B
% Transmittance / a.u.
NH2
FTIR spectrum of 2-amino-3H-phenoxazin-3-one (APX)
N
N
10a
6
5a
O
4a
H-6, H-7
and H-8
H-1
7.7
7.6
7.5
NH2
3
O
7.4 ppm
H-4
NH2
H-9
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5 ppm
128
8.0
4
2
Appendix C
7.8
5
1
H-NMR spectrum of 2-amino-3H-phenoxazin-3-one (APX)
10
9a
8
7
1
9
9
10
10a
5a
O
4a
8
7
C-9
6
C-6
5
1
4
2
NH2
3
O
C-7
C-8
C-1
C-4
C-4a
C-3
C-10a
C-5a
C-9a
C-2
180
170
160
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
ppm
129
190
Appendix D
C-NMR spectrum of 2-amino-3H-phenoxazin-3-one (APX)
N
13
9a
130
Appendix E
MS pattern of 2-amino-3H-phenoxazin-3-one (APX)
9
10
9a
N
10a
5a
O
4a
8
7
6
5
1
4
2
NH2
3
O
2-amino-3H-phenoxazin-3-one (APX)
m/z = 212
131
APPENDIX F
GC chromatograms of the oxidation of AP (a) before reaction and (b) after reaction
MeOH
Intensity
AP
DMF
0
2
4
6
8
10
Retention time / min
12
14
(a)
MeOH
APX
AP
DMF
0
2
4
6
8
10
Retention time / min
(b)
12
14