MESOPOROUS MCM-48 SYNTHESIZED FROM RICE HUSK ASH SILICA:
PHYSICOCHEMICAL PROPERTIES AND ITS CATALYTIC ACTIVITY IN
ACYLATION REACTION
LAU CHIN GUAN
UNIVERSITI TEKNOLOGI MALAYSIA
PSZ 19:16 (Pind. 1/97)
UNIVERSITI TEKNOLOGI MALAYSIA
BORANG PENGESAHAN STATUS TESISi
JUDUL :
MESOPOROUS MCM-48 SYNTHESIZED FROM RICE HUSK
ASH SILICA: PHYSICOCHEMICAL PROPERTIES AND ITS
CATALYTIC ACTIVITY IN ACYLATION REACTION
SESI PENGAJIAN: 2004/2005
Saya :
LAU CHIN GUAN
(HURUF BESAR)
mengaku membenarkan tesis ( PSM / Sarjana / Doktor Falsafah )* ini disimpan di
Perpustakaan Universiti Teknologi Malaysia dengan syarat-syarat kegunaan seperti
berikut:
1. Tesis adalah hakmilik Universiti Teknologi Malaysia.
2. Perpustakaan Universiti Teknologi Malaysia dibenarkan membuat salinan untuk
tujuan pengajian sahaja.
3. Perpustakaan dibenarkan membuat salinan tesis ini sebagai bahan pertukaran antara
institusi pengajian tinggi.
4. **Sila tandakan ( — )
—
SULIT
(Mengandungi maklumat yang berdarjah keselamatan atau
kepentingan Malaysia seperti yang termaktub di dalam
AKTA RAHSIA RASMI 1972)
TERHAD
(Mengandungi maklumat TERHAD yang telah ditentukan
oleh organisasi/badan di mana penyelidikan dijalankan)
TIDAK TERHAD
Disahkan oleh
______________________________
(TANDATANGAN PENULIS)
________________________________
(TANDATANGAN PENYELIA)
Alamat Tetap:
13-1A, JALAN SUNGAI ABONG,
84000 MUAR,
_____________
JOHOR, MALAYSIA.
______
ASSOC. PROF. DR. SALASIAH
ENDUD
Tarikh:
Tarikh: 8 MARCH 2005______
8 MARCH 2005 ___
Nama Penyelia
CATATAN: * Potong yang tidak berkenaan.
** Jika tesis ini SULIT atau TERHAD, sila lampirkan surat daripada pihak
berkuasa/organisasi berkenaan dengan menyatakan sekali sebab dan tempoh
tesis ini perlu dikelaskan sebagai SULIT atau TERHAD.
i Tesis dimaksudkan sebagai tesis bagi Ijazah Doktor Falsafah dan Sarjana secara
penyelidikan, atau disertasi bagi pengajian secara kerja kursus dan penyelidikan,
atau Laporan Projek Sarjana Muda (PSM).
“I hereby declare that I have read this thesis and in
my opinion this thesis is sufficient in terms of scope and
quality for the award of the degree of Master of Science (Chemistry)”.
Signature
:
Name of Supervisor :
Assoc. Prof. Dr. Salasiah Endud
Date
8 MARCH 2005
:
BAHAGIAN A  Pengesahan Kerjasama*
Adalah disahkan bahawa projek penyelidikan tesis ini telah dilaksanalan melalui
kerjasama antara _______________________ dengan ________________________
Disahkan oleh:
Tandatangan
: _________________________________
Nama
: _________________________________
Jawatan
: _________________________________
Tarikh: ______________
(Cop rasmi)
* Jika penyelikan tesis/projek melibatkan kerjasama.
BAHAGIAN B  Untuk Kegunaan Pejabat Sekolah Pengajian Siswajah
Tesis ini telah diperiksa dan diakui oleh:
Nama dan Alamat Pemeriksa Luar
: Assoc. Prof. Dr. Irmawati Binti Ramli
Department of Chemistry
Faculty of Science & Environmental Studies
Universiti Putra Malaysia
43400 UPM, Serdang, Selangor.
Nama dan Alamat Pemeriksa Dalam
: Assoc. Prof. Dr. Abdul Rahim B. Yaakob
Department of Chemistry
Faculty of Science
Universiti Teknologi Malaysia, Skudai.
Nama Penyelia Lain (jika ada)
: _____________________________________
_____________________________________
_____________________________________
Disahkan oleh Penolong Pendaftar di SPS:
Tandatangan
: _________________________________
Nama
: Ganesan A/L Andimuthu
Tarikh: ______________
MESOPOROUS MCM-48 SYNTHESIZED FROM RICE HUSK ASH SILICA:
PHYSICOCHEMICAL PROPERTIES AND ITS CATALYTIC ACTIVITY IN
ACYLATION REACTION
LAU CHIN GUAN
A thesis submitted in fulfilment of the
requirements for the award of the degree of
Master of Science (Chemistry)
Faculty of Science
Universiti Teknologi Malaysia
MARCH 2005
ii
“I declared that this thesis entitle “MESOPOROUS MCM-48 SYNTHESIZED
FROM RICE HUSK ASH SILICA: PHYSICOCHEMICAL PROPERTIES
AND ITS CATALYTIC ACTIVITY IN ACYLATION REACTION” is the
results of my own research except as in cited references. The thesis has not been
accepted for any degree and is not concurrently submitted in candidature of any
degree.
Signature
: ____________________
Name
: LAU CHIN GUAN
Date
:8 MARCH 2005
iii
For the Lord Almighty,
my beloved family
and
specially for Yang Eik Hien
iv
ACKNOWLEDGEMENTS
Halleluyah! All praise, glory and thanks to almighty God for His amazing
grace that led me throughout the whole process of completing this research.
Heartfelt thanks to my project supervisor, Assoc. Prof. Dr. Salasiah Endud,
who introduced me to the field of mesomorphous materials. Her patience,
understanding, supervision and thoughtful guidance throughout this study is greatly
appreciated. I am particularly grateful to MOSTI for financial support in this study
through IRPA funding 09-02-06-0057-SR0005/09-04.
I wish to express my special appreciation to Dr. Hadi Nur, the lecturer of
Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia,
for giving me worthy advices, valuable suggestions and constructive discussions
particularly in the vision of scientific ethics for conducting a research. I would also
like to thank Lim Kheng Wei for helping me to carry out the
27
Al MAS NMR
measurements. My appreciation is also extended to Norizah bt. Abdul Rahman who
help me recorded field emission SEM images in University of Yamagata, Yonezawa,
Japan. Not forgetting all of the laboratory staffs of Faculty Science and the member
of Zeolite and Porous Materials Group (ZPMG), who have in many ways contributed
and gave me moral support to the success of my study.
I am grateful for my family’s ceaseless love and support whenever I need.
My heartfelt thanks and gratitude to the church members of Inter-Vasity Christian
Centre (IVCC) for their endless prayer and encouragement throughout the whole
duration of this research.
The holistic life during this research is unable to be achieved without the
companion of everyone mentioned. Kudos to all of you!!
v
PREFACE
This thesis is the result of my work carried out in the Department of Chemistry,
Universiti Teknologi Malaysia between Jun 2002 to September 2004 under supervision
of Assoc. Prof. Dr. Salasiah Endud. Part of my work described in this thesis has been
reported in the following publications:
1. Lau, C.G. and Endud, S. (2002). “Sintesis Bahan Mesoliang MCM-48
Menggunakan Campuran Templat Surfaktan Kationik dan Neutral.” Proceedings
of the Fifth UKM-ITB Joint Seminar on Chemistry. Bandar Hilir, Melaka. 16-17
July. 425-432.
2. Lim, K.W., Lau, C.G. and Endud, S. (2002). “High Surface Area Catalysts for
th
Alkylation and Oxidation Reactions”. Poster presentation at the 15 Simposium
Kimia Analisis Kebangsaan (SKAM-15). Universiti Sains Malaysia, Minden,
Pulau Pinang. 10-12 September. P 90.
3. Lau, C.G. and Endud, S. (2003). “Optimization of Synthesis of Mesoporous
Materials from Carbonaceous Rice Husk Ash”. Report for Post-Graduate Study
1st Assessment. Pusat Pengajian Siswazah, Universiti Teknologi Malaysia.
4. Lau, C.G. and Endud, S. (2003). “Synthesis of Mesoporous Materials from
Carbonaceous Rice Husk Ash (RHA) and Its Application As Catalyst In FriedelCrafts Reaction” Annual Meeting of Zeolite and Porous Materials Group. A
Farmosa, Melaka.
vi
5. Lau, C.G. and Endud, S. (2003). “Hydrothermal Stability of MCM-48
Mesoporous Molecular Sieves: Effect of Aluminium Content”. Proceedings of
Annual Fundamental Science Studies. Johor Bahru, Johor. 20-21 May. 115-120.
6. Lau, C.G. and Endud, S. (2003). “Phase Transformation of Mesoporous
th
Molecular Sieves: Effect of Sodium Hydroxide.” Oral presentation at the 16
Simposium Kimia Analisis Kebangsaan (SKAM-16), Universiti Malaysia
Sarawak, Kuching, Sarawak. 9-11 September 2003. 2C-01.
7. Nur, H., Lau, C.G., Endud, S. and Hamdan, H. (2004). “Quantitative
Measurement of A Mixture of Mesophases Cubic MCM-48 and Hexagonal
MCM-41 by 13 C CP/MAS NMR” Materials Letters. 58. 1971-1974.
8. Lau, C.G., Nur, H. and Endud, S. (2004). “Preparation of MCM-48 with A
Bimodal Pore Size Structure by Post-Synthesis Alumination”. Oral presentation
at the Regional Symposium on Membrane Science & Technology 2004. Johor
Bahru, Johor. 21-25 April.
9. Lau, C.G., Nur, H. and Endud, S. (2004). “Highly Effective Cubic Aluminated
Mesoporous Catalyst in Friedel-Crafts Acylation”. Proceedings of 2004 Annual
Fundamental Science Seminar 2004. Skudai, Johor. 14-15 June.
10. Lau, C. G., Nur, H. and Endud S. (2005). “Bimodal Pore Size Mesoporous
MCM-48 Materials Prepared by Post-Synthesis Alumination”. J. Phys. Sci.
(accepted ).
vii
ABSTRACT
The cubic structural mesoporous molecular sieves Si-MCM-48 has been successfully
controlled by optimizing the gel compositions via a mixed surfactant templating
route using cationic cetyltrimethylammonium bromide (CTABr) and neutral Triton
X-100 (TX-100) surfactants. Rice husk ash, an agricultural waste obtained from an
open burning site with high silica content (93 % SiO2) has been utilized as active
silica reagent in the synthesis process. The Si-MCM-48 mesoporous materials were
structurally characterized by X-Ray Powder Diffraction (XRD), and Fourier
Transform Infrared Spectroscopy (FTIR). The results show that the crystallinity and
phases of the products depend on the compositions of Na2O, surfactants, H2O and pH
values. Moreover, 13C CP/MAS NMR technique had been developed to quantify a
mixture of cubic MCM-48 and hexagonal MCM-41 mesophases by means of
interpretation of their surfactant organization, which cannot be determined by XRD
technique. In order to generate active sites for catalytic applications, aluminomesoporous materials Al-MCM-48 were prepared by post-synthesis alumination of
mesoporous Si-MCM-48 and post-synthesis alumination of Si-MCM-48 mesophase
using sodium aluminate as the aluminium reagent. The aluminated MCM-48
materials were characterized using XRD, 27Al MAS NMR, FTIR and nitrogen
adsorption-desorption measurements. The results reveal that unimodal Al-MCM-48,
which possesses narrow pore size distribution around 26Å, had been synthesized
from post-synthesis alumination of mesoporous Si-MCM-48. Whereas, bimodal AlMCM-48, which possesses dual narrow pore size distributions around 26 Å and 38 Å
had been generated by post-synthesis alumination of uncalcined Si-MCM-48
mesophase. 27Al MAS NMR results depict that aluminium had been tetrahedrally
incorporated into the framework structure of MCM-48. The nature and the
concentration of acid sites of Al-MCM-48 materials have been monitored by IR
spectroscopy using pyridine as the probe molecule and temperature-programmed
desorption of ammonia (TPDA). Acidity studies on the samples demonstrated that
the acidity strength of samples prepared via post-synthesis alumination of
mesoporous Si-MCM-48 is greater than samples prepared via post-synthesis
alumination of Si-MCM-48 mesophase. Aluminated MCM-48 materials have been
employed in the acylation of bulky aromatic compound, 2-methoxynaphthalene with
acetic chloride to produce 2-acetyl-6-methoxynaphthalene, which is intermediate for
preparing naproxen, a non-steroidal anti inflammation drug. Catalytic activities have
been investigated in solvents with different polarity and the results illustrate that the
conversion and selectivities of products rely on the polarity of solvent. The
conversion of the 2-methoxynaphthalene can be as high as 42 % with 86 %
selectivities towards the desired 2-acetyl-6-methoxynaphthalene in polar solvent,
nitrobenzene. Whereas, the conversion of the 2-methoxynaphthalene is 30 % with 56
% selectivity of 2-acetyl-6-methoxynaphthalene in non-polar solvent, cyclohexane.
viii
ABSTRAK
Penapis molekul mesoliang Si-MCM-48 yang berbentuk kiub telah berjaya disintesis
dengan mengoptimumkan komposisi gel melalui kaedah campuran surfaktan kationik
setiltrimetilammonium bromida (CTABr) dan surfaktan neutral Triton X-100 (TX100). Abu sekam padi yang mempunyai kandungan silika yang tinggi (93 % SiO2), di
mana ia diambil daripada kawasan pembakaran terbuka telah digunakan sebagai
sumber silika yang aktif dalam proses sintesis ini. Struktur penapis molekul mesoliang
Si-MCM-48 ini dicirikan dengan kaedah pembelauan sinar-X (XRD) dan spektroskopi
inframerah transformasi Fourier (FTIR). Keputusan menunjukkan darjah kehabluran
dan ketulenan fasa bahan mesoliang adalah bergantung kepada komposisi Na2O,
surfaktan, H2O dan nilai pH. Di samping itu, teknik 13C CP/PSA RMN telah
digunakan untuk mengkaji ketulenan fasa campuran MCM-48 yang berfasa kiub dan
MCM-41 yang berfasa heksagon secara kuantitatif, di mana ia tidak dapat dilakukan
dengan menggunakan kaedah XRD. Aluminium MCM-48 (Al-MCM-48) telah
disintesis dengan menggunakan dua kaedah pasca-sintesis untuk menghasilkan tapak
aktif pemangkinan, iaitu, penyelitan aluminium ke dalam bingkaian Si-MCM-48 dan
penyelitan aluminium ke dalam fasa meso Si-MCM-48 dengan menggunakan natrium
aluminat sebagai sumber aluminium. Sampel Al-MCM-48 dicirikan dengan kaedah
XRD, 27Al PSA RMN, FTIR dan penjerapan dan nyahjerapan nitrogen. Analisis
penjerapan nitrogen menunjukkan liang Al-MCM-48 yang bersebar secara seragam
dengan purata liang disekitar taburan 26 Å telah dihasilkan dengan menggunakan
kaedah penyelitan aluminium ke dalam bingkaian bahan mesoliang Si-MCM-48. Di
samping itu, Al-MCM-48 yang memiliki taburan dua jenis mesoliang yang tertabur di
antara 26 Å dan 38 Å telah dihasilkan melalui kaedah penyelitan aluminium ke dalam
fasa meso Si-MCM-48 tanpa kalsin. Keputusan 27Al PSA RMN menunjukkan
aluminium bingkaian berkordinatan tetrahedral telah dihasilkan melalui kedua-dua
kaedah tersebut. Jenis dan kepekatan tapak asid yang terdapat pada permukaan
Al-MCM-48 telah ditentukan dengan menggunakan kaedah spektroskopi inframerah
menggunakan piridina sebagai molekul prob dan kaedah nyahjerapan ammonia pada
suhu terkawal (TPDA). Hasil kajian keasidan menunjukkan sampel yang disediakan
melalui penyelitan aluminium ke dalam bingkaian bahan mesoliang Si-MCM-48
adalah lebih kuat daripada sampel yang disediakan melalui penyelitan aluminium ke
dalam fasa meso Si-MCM-48 tanpa kalsin. Potensi bahan Al-MCM-48 sebagai
mangkin Friedel-Crafts telah diuji dengan menggunakan tindak balas pengasilan
sebatian 2-metoksinaftalena dengan asetil klorida untuk menghasilkan 2-asetil-6metoksinaftalena, bahantara untuk menyediakan naproxen, ubat anti-keradangan yang
non-steroid. Kajian aktiviti pemangkinan dengan menggunakan pelarut yang berlainan
kepolaran menunjukkan peratus pertukaran dan kepilihan produk adalah bergantung
kepada kepolaran pelarut. Peratus pertukaran 2-metoksinaftalena boleh mencapai
setinggi 42 % dengan 86 % kepilihan kepada 2-asetil-6-metoksinaftalena di dalam
pelarut polar (nitrobenzena). Di samping itu, peratus pertukaran 2-metoksinaftalena
hanya sebanyak 30 % dengan 56 % kepilihan kepada 2-asetil-6-metoksinaftalena di
dalam pelarut yang tidak polar (sikloheksana).
ix
TABLE OF CONTENTS
CHAPTER
1
TITLE
PAGE
TITLE
i
STATEMENT
ii
DEDICATION
iii
ACKNOWLEDGEMENTS
iv
PREFACE
v
ABSTRACT
vii
ABSTRAK
viii
TABLE OF CONTENTS
ix
LIST OF TABLES
xiv
LIST OF FIGURES
xvi
LIST OF SYMBOLS
xxiii
LIST OF APPENDICES
xxv
INTRODUCTION
1.1
Green Chemistry for Sustainable Development
1
1.2
Heterogeneous Catalysts
2
1.3
Research Background and Problem Statement
4
1.4
Research Objectives
6
1.5
Scope of the Study
6
1.6
Outline of the Thesis
7
x
2
EXPERIMENTAL
2.1
2.2
Chemical Analysis of Rice Husk Ash (RHA)
9
2.1.1
Determination of LOI (Loss of Ignition)
9
2.1.2
Determination of Silica
9
Synthesis
of
Materials
Via
Purely
A
Siliceous
Mixed
Si-MCM-48
10
Cationic-Neutral
Templating Route
2.3
Removal of Organic Templates
2.4
Synthesis
of
Aluminated
11
Cubic
Mesoporous
12
Preparation of Protonated Al-MCM-48 (H-Al-
14
Materials (Al-MCM-48)
2.5
MCM-48)
2.6
Charaterization of MCM-48 Molecular Sieves
15
2.6.1
15
Powder X-Ray Diffraction (XRD)
2.6.2 Fourier Transform Infrared Spectroscopy
16
(FTIR)
2.6.3 Magic Angle Spinning Nuclear Magnetic
16
Resonance (MAS NMR)
2.6.4
Nitrogen Adsorption Measurements
2.6.5 Field
Emission
Scanning
17
Electron
18
Microscopy (FESEM)
2.7
2.6.6 Acidity Measurement
18
Catalytic Testing
19
2.7.1 Activation of H-Al-MCM-48
19
2.7.2 Acylation
of
2-Methoxynapthalene
with
21
Acetyl Chloride over H-Al-MCM-48
3
OPTIMIZATION
OF
SYNTHESIS
AND
CHARACTERIZATION OF PURELY SILICEOUS
MESOPOROUS MOLECULAR SIEVES MCM-48
3.1
Introduction
23
xi
3.2
Proposed
Formation
Mechanisms
and
the
26
Evolution of Synthesis Routes for the M41S
Mesoporous Materials
3.3
Synthesis of MCM-48 Materials
29
3.4
Results and Discussion
32
3.4.1 Characterization of Rice Husk Ash (RHA)
32
3.4.2 Synthesis of Purely Siliceous Mesoporous
33
Materials
35
3.4.2.1 Effect of pH Value
Oxide/Silica
40
Surfactant/Silica
44
3.4.2.4 Effect of Water/Silica (H2 O/SiO 2 )
46
3.4.2.2 Effect
of
Sodium
(Na2 O/SiO 2 ) Ratio
3.4.2.3 Effect
of
(Sur/SiO 2 ) Ratio
Ratio
3.5
4
Conclusion
49
QUANTITATIVE
MEASUREMENT
COMPOSITION
OF
CUBIC
OF
MCM-48
PHASE
AND
HEXAGONAL MCM-41 PHASE MIXTURES BY
USING 13 C CP/MAS NMR
5
4.1
Introduction
50
4.2
Results and Discussion
52
4.3
Conclusion
59
TAILORING
THE
ALUMINOSILICATE
Al-MCM-48 MESOPOROUS MOLECULAR SIEVES
AS
CATALYSTS
FOR
FRIEDEL-CRAFTS
REACTION
5.1
Introduction
60
xii
5.2
61
Post-Synthesis Route to Mesoporous Al-MCM-48
Materials
5.3
Results and discussion
62
5.3.1 Post-Synthesis Alumination of Mesoporous
62
Si-MCM-48
5.3.2 Post-Synthesis Alumination of Si-MCM-48
73
Mesophase
5.3.3
5.4
6
Proposed Mechanism
84
Conclusion
CATALYTIC
87
ACTIVITY
OF
ALUMINATED
MCM-48 MOLECULAR SIEVES IN THE FRIEDELCRAFTS
ACYLATION
2-METHOXYNAPHTHALENE
OF
WITH
ACETYL
CHLORIDE
6.1 Introduction
88
6.2 Generation of Active Sites in Al-MCM-48
89
6.3 Characterizationof Acidity
93
6.4 Friedel-Crafts Acylation
94
6.5 Results and Discussion
97
6.5.1
Characterization of Acidity of Al-MCM-48
97
6.5.1.1 Temperature-Programmed
97
Desorption
of
Ammonia
(NH3 -
TPD)
6.5.1.2 Infrared
Spectroscopy
(IR)
of
100
with
109
Adsorbed Pyridine
6.5.2 Acylation
of
2-Methoxynapthalene
Acetyl Chloride
6.6
6.5.2.1 The Effect of Various Catalysts
111
6.5.2.2 The Effect of Solvent
114
Conclusion
120
xiii
7
GENERAL CONCLUSION AND
RECOMMENDATIONS
7.1
Main Results
121
7.2
Recommendations
124
REFERENCES
125
APPENDICES
141
xiv
LIST OF TABLES
TABLE NO.
TITLE
PAGE
2.1
Summary of synthesis conditions for Si-MCM-48 materials.
12
2.2
Sample codes for Al-MCM-48 with different concentration
13
of sodium aluminate prepared by post-synthesis alumination
of mesoporous Si-MCM-48.
2.3
Sample codes for Al-MCM-48 with different Si/Al ratio
14
prepared by post synthesis alumination of Si-MCM-48
mesophase.
3.1
Surfactant packing parameter g, expected structure and
31
examples for such structures.
5.1
Unit cell parameters of Al-MCM-48 prepared from the
66
purely siliceous Si-MCM-48 with different concentrations of
NaAlO 2 aqueous solution. The unit cell parameter has been
calculated from the interplanar spacing using the formula
ao =d211 √6.
5.2
Sorption properties of the parent Si-MCM-48 and the
aluminated samples prepared via post-synthesis alumination
of mesoporous Si-MCM-48.
73
xv
5.3
Unit cell parameters of Si-MCM-48 and Al-MCM-48
prepared from post-synthesis alumination.
75
The unit cell
parameter has been calculated from the interplanar spacing
using the formula ao =d211 √6.
5.4
Sorption properties of Si-MCM-48 and Al-MCM-48.
82
6.1
Total acid amount of H-Al-MCM-48 materials determined by
100
NH3 -TPD.
6.2
Number of Brönsted and Lewis acid sites in the samples.
110
6.3
GC Data for the Acylation Products.
111
6.4
Catalytic Activities of Various Catalysts for the Acylation of
112
2-Methoxynaphthalene with Acetyl Chloride.
xvi
LIST OF FIGURES
TITLE
FIGURE NO.
1.1
Development
of
publications
PAGE
on
ordered
mesoporous
3
materials since 1990.
1.2
Flowchart of the research design.
8
2.1
Experimental setup for acidity study .
20
2.2
GC-FID and GC-MS oven-programme setup.
21
3.1
Schematic structural illustrations of M41S family (a)
24
hexagonal MCM-41; (b) cubic MCM-48; (c) lamellar
MCM-50.
3.2
Schematic
of
possible
mechanistic
pathways
for
the
26
formation of MCM-41: (1) Liquid crystal phase initiated
and (2) silicate anion initiated.
3.3
Schematic diagram of the transformation mechanism from
27
lamellar to hexagonal mesophase.
3.4
Schematic showing of interfacial interactions for surfactant
28
micelles in cooperatively assembly.
3.5
Illustration of the regions of generic surfactant.
30
xvii
3.6
XRD diffractogram of RHA obtained from an open burning
34
site.
3.7
FTIR spectrum of RHA obtained from open burning site.
34
3.8
XRD diffractograms of (a) as-synthesized mesoporous
37
materials; (b) calcined mesoporous materials with various
pH value.
3.9
XRD diffractograms of (a) as-synthesized mesoporous
41
materials; (b) calcined mesoporous materials with various
Na2 O/SiO 2 ratios.
3.10
XRD diffractograms of (a) as-synthesized mesoporous
45
materials; (b) calcined mesoporous materials with various
Sur/SiO 2 ratios.
3.11
XRD diffractograms of (a) as-synthesized mesoporous
47
materials; (b) calcined mesoporous materials with various
H2 O/SiO 2 ratios.
4.1
X-ray diffraction (XRD) patterns of mesophases MCM-48
53
and/or MCM-41 prepared by difference of the Na2 O/SiO 2
ratio; (a) 0.20, (b) 0.25, (c) 0.30, (d) 0.35 and (e) 0.40.
XRD pattern (f) was obtained by mixing samples (a) and
(e) with the composition of 50:50.
4.2
13
C CP/MAS NMR spectra of mesophases MCM-48 and/or
MCM-41 prepared with various Na2 O/SiO 2 ratios; (a) 0.20,
(b) 0.25, (c) 0.30, (d) 0.35 and (e) 0.40. A contact time of 1
ms was applied.
55
xviii
4.3
Integrated intensity ratio of the C5 –C14 and C1 peaks
56
(normalized to percentage of mesophases MCM-48 and
MCM-41), calculated from Figure 4.2, of MCM-48 and/or
MCM-41 prepared with various Na2 O/SiO 2 ratios; (a) 0.20,
(b) 0.25, (c) 0.30, (d) 0.35 and (e) 0.40.
4.4
X-Ray diffraction (XRD) patterns of mesoporous (a)
57
MCM-48 and (b) MCM-41 after reinsertion of CTABr.
4.5
13
C CP/MAS NMR spectra of mesoporous (a) MCM-48
58
and (b) MCM-41 after reinsertion of CTABr. A contact
time of 1 ms was applied.
5.1
XRD patterns of the parent Si-MCM-48 and its aluminated
samples
through
secondary
synthesis
with
63
different
concentrations of NaAlO 2.
5.2
Mechanism of post-synthesis alumination of mesoporous
65
MCM-41.
5.3
FTIR spectra of the parent and samples prepared via postsynthesis
of
mesoporous
Si-MCM-48
with
67
different
concentrations of NaAlO 2 ; (a) purely siliceous Si-MCM-48
(MP-5), (b) 0.10 M, (c) 0.25 M, and (d) 0.50 M.
5.4
27
Al MAS NMR spectra of Al-MCM-48 prepared via post-
68
synthesis alumination of mesoporous Si-MCM-48 with
different concentrations of NaAlO 2.
5.5
N2 adsorption-desorption isotherms and its BJH pore size
distribution curve (inset) of the parent Si-MCM-48 and its
aluminated samples through post-synthesis alumination of
mesoporous Si-MCM-48 with different concentrations of
NaAlO 2 ; (a) purely siliceous Si-MCM-48 (MP-5),
70
xix
(b) 0.10M, (c) 0.25 M, and (d) 0.50 M.
5.6
The α s plots of (a) Si-MCM-48, (b) 010Al-MCM-48, (c)
72
025Al-MCM-48, and (d) 050Al-MCM-48.
5.7
XRD patterns of the (a) as-synthesized and (b) calcined Al-
74
MCM-48 via post-synthesis alumination.
5.8
FTIR spectra of aluminosilicates Al-MCM-48 samples
77
prepared by post-synthesis alumination with various Si/Al
ratios; (a) 20, (b) 30, (c) 50, and (d) 100.
5.9
27
Al MAS NMR spectra of the calcined aluminosilicate Al-
78
MCM-48 samples prepared by post-synthesis alumination
with various Si/Al ratios; (a) 20, (b) 30, (c) 50, and (d) 100.
5.10
N2 adsorption-desorption isotherms and their corresponding
79
pore size distribution curve (inset) of aluminosilicate AlMCM-48 samples prepared by post-synthesis alumination
with various Si/Al ratios; (a) 20, (b) 30, (c) 50, and (d) 100.
5.11
The α s plots of aluminosilicate Al-MCM-48 samples
81
prepared through post-synthesis alumination with different
Si/Al ratios; (a) 20, (b) 30, (c) 50, and (d) 100.
5.12
FESEM micrograph of Si-MCM-48.
83
5.13
FESEM micrograph of Al-MCM-48-50.
83
5.14
Proposed
85
mechanism
of
post-synthesis alumination of
Si-MCM-48 mesophase.
5.15
Schematic
illustration
secondary mesopores.
of
formation
mechanism
of
86
xx
6.1
(a) Framework of Si-MCM-48, and (b) Framework of
90
Al-MCM-48.
6.2
Generation of Brönsted acid sites.
91
6.3
Generation of Lewis acid sites; (a) Lewis acidity due to
92
framework tricoordinated aluminium, and (b) Lewis acidity
associated with both octahedral and tetrahedral EFAL.
6.4
Mechanism of acylation of aromatics in the presence of
95
aluminium chloride.
6.5
The active positions of 2-methoxynaphthalene.
97
6.6
NH3 -TPD spectra of samples prepared through post-
98
synthesis alumination of Si-MCM-48 mesoporous materials
with different concentration of NaAlO 2 .
6.7
NH3 -TPD spectra of samples prepared through post-
99
synthesis alumination of Si-MCM-48 mesophase with
different Si/Al gel ratios.
6.8
FTIR spectra in the hydroxyl region of (a) Si-MCM-48, (b)
102
010AlMCM-48, (c) 025AlMCM-48, and (d) 050AlMCM48 recorded at 400 o C under 10-5 mbar pressure.
6.9
FTIR spectra in the hydroxyl region of (a) Si-MCM-48, (b)
103
Al-MCM-48-20, (c) Al-MCM-48-30, (d) Al-MCM-48-50,
and (e) Al-MCM-48-100 recorded at 400 o C under 10-5
mbar pressure.
6.10
FTIR spectra in the hydroxyl region after pyridine
desorption at 150 o C; (a) Si-MCM-48, (b) 010Al-MCM-48,
(c) 025Al-MCM-48, and (d) 050Al-MCM-48.
104
xxi
6.11
FTIR
spectra
in
the
hydroxyl region after pyridine
105
desorption at 150 o C (a) Si-MCM-48, (b) Al-MCM-48-20,
(c)
Al-MCM-48-30,
(d)
Al-MCM-48-50,
and
(e) Al-MCM-48-100.
6.12
Structures showing interaction of (a) silanols with Lewis
106
acid sites and (b) Al-OH groups by H-bonding (represented
by arrows).
6.13
FTIR spectra of adsorbed pyridine on Si-MCM-48 and
samples
prepared
through
post-synthesis
107
alumination
mesoporous Si-MCM-48 evacuated at 25 o C, 150 o C, 250
o
C, and 400
o
C. (H, Hydrogen bonded pyridine; B,
Brönsted bound pyridine; L, Lewis bound pyridine).
6.14
FTIR spectra of adsorbed pyridine on samples prepared
108
through post-synthesis alumination Si-MCM-48 mesophase
evacuated at 25 o C, 150 o C, 250 o C, and 400 o C. (H,
Hydrogen bonded pyridine; B, Brönsted bound pyridine; L,
Lewis bound pyridine).
6.15
Effect of solvents on conversion of 2-methoxynaphthalene
116
over Al-MCM-48-20.
6.16
Effect of solvents on selectivity of the products from
116
acylation of 2-methoxynaphthalene over Al-MCM-48-20
catalyst.
6.17
Products of acylation of 2-methoxynaphthalene catalysed
117
by H-Al-MCM-48.
6.18
Proposed
mechanism
of
the
Acylation
of
2-methoxynaphthalene with acetyl chloride over Brönsted
acid sites in Al-MCM-48.
118
xxii
6.19
Proposed
mechanism
of
the
acylation
of
2-methoxynaphthalene with acetyl chloride over Lewis
acid sites in Al-MCM-48.
119
xxiii
LIST OF SYMBOLS
RHA
rice husk ash
LOI
loss of ignition
L
litre
mL
millilitre
o
Celsius
C
K
Kelvin
g
gram
min
minute
h
hour
MCM
Mobile Composition Material
M
molar
Si/Al
silicon-to-aluminium ratio
Si-MCM-48
purely siliceous MCM-48
Al-MCM-48
aluminosilicate MCM-48
XRD
X-Ray diffraction
FTIR
Fourier Transform Infrared
27
Al MAS NMR
27
Al magic-angle-spinning nuclear-magnetic-resonance
C CP/MAS NMR
13
C cross-polarization magic-angle-spinning nuclear-
13
magnetic-resonance
TPD
temperature-programmed desorption
FESEM
field emission scanning electron microscopy
d
inter-planar spacing
Cu-Kα
X-ray diffraction from copper Kα energy levels
λ
wavelength
kV
kilovolt
mA
milliampere
2θ
Braggs angle
xxiv
KBr
kalium bromide
TMS
tetramethylsilane
s
second
Hz
Hertz
CaF2
calcium fluoride
I.D.
internal diameter
GC
gas chromatrography
GC-MS
gas chromatrograpy-mass spectrometry
FID
flame ionization detector
IUPAC
International Union of Pure and Applied Chemistry
AlO 4
aluminate, framework aluminium in zeolite
SiO 4
siliceous; framework silicon in zeolite
TEM
transmission electron microscopy
LCT
liquid crystal templating
g
surfactant packing parameter
BET
Brunauer, Emmett and Teller
Si-OH
silanol group
EFAL
extra framework aluminium
ao
unit cell parameter
N2
nitrogen
P/P o
relative pressure; obtained by forming the ratio of the
equilibrium pressure and the vapor pressure Po of the
adsorbate at the temperature where the isotherm is
measured.
PSD
pore size distribution
BJH
Barrett, Joyner and Halenda
αs
alfa-S
Rt
retention time
TON
turnover number
ms
millisecond
xxv
LIST OF APPENDICES
APPENDIX
TITLE
PAGE
A
Calculation of the amount of pyridine adsorbed on
141
the sample in the acidity study of secondary
aluminated Al-MCM-48 samples.
B
Quantitative gas chromatography calibration plot
142
of 2-methoxynaphthalene by using naphthalene as
internal standard.
C
Calculation of % conversion, % selectivity, and
143
turnover number (TON).
D
An example of chromatogram for liquid products
of
conversion
of
2-metoxynaphthalene
144
in
nitrobenzene.
E
An example of chromatogram for liquid products
of
conversion
of
2-metoxynaphthalene
145
in
dichloroethane.
F
An example of chromatogram for liquid products
of
conversion
cyclohexane.
of
2-metoxynaphthalene
in
146
xxvi
G
Mass
spectra
of
methoxynaphthalene
and
(a)
(b)
2-acetyl-6-
147
1-acetyl-7-
methoxynaphthalene.
H
Quantitative calculation of phase composition via
integrated intensity ratio of the C5 –C14 and C1
peaks
of
mesophases.
13
C
CP/MAS
NMR
spectra
of
148
CHAPTER 1
INTRODUCTION
1.1
Green Chemistry for Sustainable Development
In the 21st century, building a sustainable future has been the greatest
challenge of the global society.
The development of science and technology has
become the crucial role in order to fulfilling current need and to preserve a well
living environment for the future generations.
Therefore, the public, legislative, and
environmentalists are urging the development of cleaner technologies to serve
mankind.
Hence, it has stimulated the exciting opportunities for catalysis and
catalytic processes.
Catalyst is a substance, which accelerates the rate of a chemical reaction
whilst it may be recovered chemically unchanged at the end of the reaction [1]. The
presence of the catalyst is essential for (i) obtaining new structures, (ii) increasing the
productivity, (iii) decreasing the raw materials and energy consumption, (iv)
minimizing the waste production and getting a better environment [2].
Catalysis is a privileged way to a clean and powerful chemistry.
Today,
catalysts play a vital role in the chemicals industry, with a total contribution of ca.
20% of the world GNP in the 20th century [3]. In addition, 80% of the industrial
reactions use catalysts.
The British agency Frost and Sullivan, which published a
study in 1998 [4] evaluated the catalysts European market to $ 3.7 billions turnover
in 1998. With about 4% growth per year, it should increase to $ 5 billions in 2005.
2
1.2
Heterogeneous Catalysts
Catalysts
heterogeneous.
can
be
classified
into
two
categories,
homogenous
and
Homogenous catalyst is the catalyst, which presents in the same
phase as the reagents. Sulfuric acid has been widely used as homogenous catalyst in
the alkylations or isomerizations of hydrocarbon [5].
Whereas, catalysts are
heterogeneous if they are present in a different phase from the reactants. One of the
prominent heterogeneous catalysts is zeolites which are extensively employed in
petroleum refinery processes [6].
Heterogeneous catalysis is the backbone of the modern chemical industry,
because of the necessity to achieve environmental benign processes in the industry.
In addition, heterogeneous catalysts offer numerous potential advantages over
homogenous catalysts, such as easier working up procedures, easy catalyst separation
from the reaction mixture, reduction of environmental pollutant, avoidance of salt
formation and waste disposals [7].
Since 1960s, zeolites catalysts have conquered the petroleum refining and
petrochemical industries.
chemical stability.
This is due to the zeolites that have excellent thermal and
Moreover, zeolites provide great acid strength, which are
comparable to homogeneous acid catalysts [7].
processes
are
hydrocracking
of
heavy
The most important of these
petroleum distillates, octane number
enhancement of light gasoline by isomerization, the isomerization of xylenes (to
produce para-xylene, the precursor chemical for terephthalic acid), and etc [6].
However, the utilization of zeolites in the areas of specialty and fine chemicals
synthesis is still limited, even though their potential is considered to be very high in
this area as well. The small pore opening of zeolites, in the range of ca. 0.2-1 nm, is
a major restriction for it to utilize in organic reactions [6]. The reactants with sizes
exceeding the dimensions of the pore are not able to process via zeolites. Therefore,
numerous attempts have been devoted to increasing the pore size of crystalline
molecular sieves [8].
In 1992, a novel family of ordered silicate mesoporous molecular sieves,
designated as M41S has been discovered by researchers at Mobil R & D Corporation
3
[9]. The most important member among these materials is MCM-41 and MCM-48,
which possesses hexagonal and cubic symmetry, respectively.
The uniqueness of
M41S mesoporous materials are the pore size are uniform and tunable in the range
between 1.6-10 nm.
Furthermore, these mesoporous materials also possess high
thermal stability and have extremely high surface areas.
This innovative discovery
has greatly expanded the area of microporous molecular sieves (zeolites) into the
mesopore range and has created new opportunities beyond catalysis.
The ordered
mesoporous materials have been found as promising materials in optics and
electronics, as nano size template, and as adsorbents for heavy metals [10].
The
rapid growing of publications in mesoporous materials since 1990 is shown in Figure
1.1 [10].
Figure 1.1
1990 [10].
Development of publications on ordered mesoporous materials since
4
1.3
Research Background and Problem Statement
Malaysia has been found as one of the major rice production country,
whereby 425,080 hectares (3.21 % of the total land in peninsular Malayisia) of the
land have been used for paddy plantation [11]. Therefore, it generates abundance of
waste namely rice husk, a thin but abrasive skin in nature covering the edible rice
kernel. It has been reported that Malaysia produces a ca. 18 million tons of paddy in
which about one fifth of it is the husk [12]. This means that the annual production of
rice leaves behind about 3.6 million tons of husk as waste product, usually disposed
by combustion.
Unfortunately, the 20 % of the rice husk ash (RHA) residues left
after the combustion constitute environmental problems due to severe air and water
pollution problems. However, RHA can be considered as a potential feature of the
rice husk, which the RHA residues can be employed as raw materials in a variety of
applications.
Previous research had shown that the rice husk ash containing 96-99% SiO 2
can exist either in amorphous phase or in crystalline phases such as, α-cristobalite
and tridymite [13-14].
In fact, the amorphous silica is the most active silica
precursor in the synthesis of zeolites.
Hence, the large amount of silica freely
obtained from this source provides abundant and cheap alternatives of silica for
many industrial uses.
From the previous report, MCM-41 has been successfully
synthesized by using the silica extracted from RHA [15].
However, no report has
been found on the synthesis of MCM-48 directly from RHA, since the synthesis of
MCM-48 mesoporous materials seems to be more challenging than the synthesis of
MCM-41. By using rice husk ash as the silica source in the synthesis of MCM-48,
the production costs can be reduced subsequently besides helping to overcome
environmental pollution.
slightly impure silica.
Indeed, it should be noticed that RHA is considered
The content of silica and all impurities in RHA vary
depending on the variety, climate and geographic location [16]. Therefore, in order
to transform the RHA to valuable mesoporous materials, modification and
optimization of the synthesis condition should be carried out.
The three-dimensional cubic porous system of MCM-48 mesoporous
materials has more advantageous than one-dimensional hexagonal porous system.
5
However, it is particularly difficult to synthesize MCM-48 mesoporous materials,
since cubic MCM-48 mesophase are obtained as an intermediate between the
transformation from a hexagonal or disordered mesophase to a more stable lamellar
mesophase [17]. Hence, instead of pure phase, the mixtures of different phases are
frequently obtained during the synthesis.
It is a great challenge to characterize the
mesoporous materials, since the XRD is not capable to distinguish the phases in the
state of mixtures of different ordered mesophases.
Friedel–Crafts acylation of aromatic compounds is one of the prominent
processess in the synthesis of aromatic ketones that has been widely used as an
intermediate to obtain fine, specialty and pharmaceutical chemicals.
However, the
majority of these manufacturing processes still rely on homogeneous reagents and
catalysts. Many of these processes are developed simply to maximize product yield,
disregarding the environmental impact of inorganic waste and toxic byproducts
formed during the reaction.
Among the Lewis acid catalysts, anhydrous aluminium
chloride was the most widely employed reagent to trigger the Friedel-Crafts reaction
in the liquid phase in the laboratory as well as in the industry. However, the use of
standard Lewis acid catalyst is faced with several problems, such as non-regenerable,
requires further treatment after reaction, produces large amounts of hazardous
corrosive waste, catalyzes undesirable reaction, and also uses more than the
stoichiometric amount.
Therefore, the demand for less pollutant and more effective
chemical processes has become the current concern. Zeolites have been found to be
less useful in these chemicals processes due to the limitation of its pore opening for
bulky organic molecules.
Conversely, the emergence of mesoporous materials has
breakthrough the restriction of zeolites, since the larger pore size of these materials
allows bulky organic molecules to diffuse through the pores to reach the active sites.
Moreover, utilization of mesoporous materials such as MCM-48 in the production of
fine chemicals is still being studied and developed but is yet to be available
commercially.
6
1.4
Research Objectives
The objectives of this research are:
(1)
to establish and optimize a new synthetic mesoporous Si-MCM-48
molecular sieves by using rice husk ash as silica source.
(2)
to develop a novel characterization technique for measurement of
mesophases composition.
(3)
to synthesis and tailor the unimodal and bimodal of Al-MCM-48 via
post synthesis route.
(4)
to characterize the physicochemical properties of Al-MCM-48.
(5)
to investigate the catalytic properties of the Al-MCM-48 in the
Friedel-Crafts
acylation
of
2-methoxynaphthalene
with
acetyl
chloride.
1.5
Scope of the Study
In this research, syntheses of purely siliceous mesoporous Si-MCM-48
molecular sieves via mixed cationic-neutral templating route have been optimized by
means of varying the initial condition of original gel compositions proposed by Ryoo
et al. [17], such as pH value, Na2 O/ SiO 2 , surfactant/SiO 2 , and H2 O/SiO 2 .
Cetyltrimethylammonium bromide (CTABr) has been used as cationic surfactant,
whereas Triton X-100 (TX-100) as neutral surfactants. Rice husk ash (RHA) which
were obtained from open burning site will be used as silica source.
novel approach for quantification of mesophase purity by using
13
Moreover, a
C CP/MAS NMR
has been developed in order to verify the mesophases compositions from hexagonal
MCM-41 to cubic MCM-48.
Modification of MCM-48 is devoted by introducing the aluminium into the
Si-MCM-48 by two post synthesis approaches. Both mesoporous and mesophase of
Si-MCM-48 will be employed as parent materials in post-synthesis alumination.
Acidity of the samples are investigated by using temperature-programmed desorption
(TPD) of ammonia and pyridine adsorption methods.
7
Appropriate techniques are utilized to characterize the physicochemical
properties of the mesoporous materials which include powder X-ray diffraction
(XRD), Fourier Transform Infrared Spectroscopy (FTIR), nitrogen adsorptiondesorption measurement,
27
Al magic angle spinning nuclear magnetic resonance
spectroscopy (27 Al MAS NMR), and field emission scanning electron microscopy
(FESEM).
Finally, the mesoporous catalysts will be tested as potential catalysts in
laboratory scale.
Investigation of its catalytic activity will be conducted using
Friedel-Crafts acylation of bulky aromatic compound, 2-methoxynaphthalene with
acetyl
chloride.
Solvents
with
various
polarities
dichloroethane, and nitrobenzene will be used in this study.
such
as
cyclohexane,
The research design is
schematically illustrated in Figure 1.2.
1.6
Outline of the Thesis
This
thesis
illustrates
the
information
concerning
the
synthesis,
characterization and the potential catalytic application of mesoporous MCM-48
molecular sieves. Chapter 1 elucidates the research background and the strategies to
respond the current issue.
Chapter 2 describes the experimental methodology.
Whereas, Chapter 3 covers the chemistry and fundamental aspects of mesoporous
MCM-48 molecular sieves.
The results of the optimization of synthesis of
mesoporous MCM-48 molecular sieves are also present in this chapter. Chapter 4
explains the novel technique for quantification of mesophases compositions by using
13
C CP/MAS NMR.
Chapter 5 contains the studies in tailoring the unimodal and
bimodal of Al-MCM-48 by using different post-synthesis alumination approaches.
Chapter 6 reveals the discussion of the acidity studies of Al-MCM-48 by using NH3 TPD and pyridine adsorption.
In addition, the catalytic activity of Al-MCM-48
catalysts, which is tested by Friedel-Crafts acylation is presented in this chapter too.
Finally, Chapter 7 summarizes the results obtained with recommendation for future
work.
8
Optimization of Synthesis of Purely
Develop A Novel Characterization
Siliceous Mesoporous Materials
Technique for Measurement of
Mesophases Composition by Using
from Rice Husk Ash
13
C CP/ MAS NMR
Modification of Si-MCM-48
Post-Synthesis Alumination of
Post-Synthesis Alumination of
Mesoporous Si-MCM-48
Si-MCM-48 Mesophase
Characterization of Its Physicochemical Properties
XRD, FTIR, nitrogen adsorption-desorption measurement, 27 Al MAS NMR,
FESEM, NH3 -TPD, and pyridine adsorption.
Catalytic Testing
Friedel-Crafts acylation
Product
Analysis
Gas Chromatrography (GC)
Gas Chromatrography-Mass Spectrometry (GC-MS)
Figure 1.2
Flowchart of the research design.
CHAPTER 2
EXPERIMENTAL
2.1
Chemical Analysis of Rice Husk Ash (RHA)
2.1.1
Determination of LOI (Loss of Ignition)
Loss of Ignition (LOI) test was carried out to analyze the content of volatile
organic compound inside the rice husk ash (RHA).
The LOI of RHA was
determined based on the SIRIM procedure (ISO 3262-1975). 1 g of dried sample
was placed in a platinum crucible and ignited in the muffled furnace at 1000o C for 30
minutes to achieve constant mass, followed by cooling in a desiccator. The loss of
ignition, as a percentage by mass, is given by the formula:
% LOI = (mo -m1 ) × 100
mo
(Eq.2.1)
where mo is the mass of the sample and m1 is the mass of sample after ignition.
2.1.2
Determination of Silica Content
Silica content in RHA was measured based on the SIRIM method (ISO 32621975). 1 mL of 50% sulfuric acid had been slowly added into the residue obtained
from LOI analysis. The crucible was heated gently until the fuming ceased and the
10
heating was continued at 900o C for 30 minutes in the muffled furnace, the residue
was then removed from the furnace, cooled in the desiccator and weighed (m2 ). The
residue was dissolved in 1:5 mixture of H2 SO4 :HF solution, and evaporated on a hot
plate until no further white fumes evolved.
The crucible was ignited in the muffled
furnace at 900o C for 30 minutes, removed, cooled in the desiccator, and weighed
(m3 ). The silica content was calculated using following equation:
% SiO 2 = (m2 -m3 ) × 100
m1
2.2
(Eq. 2.2)
Synthesis of Purely Siliceous Si-MCM-48 Materials Via A Mixed
Cationic-Neutral Templating Route
Siliceous Si-MCM-48 was synthesized based on a procedure proposed by
Ryoo R. et al. [17]. Rice husk ash (93% SiO 2 ) obtained from an open burning site
was used as silica source in this study.
The synthesis procedure consists of three parts. The first part (PART A) was
to partially dissolve carbonaceous rice hush ash (RHA) to obtain sodium silicate or
water glass. Sodium silicate (Na2 SiO 3 ) was prepared by combining 4.0 g of RHA (4
g, 93% SiO 2 ) with NaOH pellet (1.25g, Merck 99%) and H2 O (35g, double distilled).
The resulting gel mixture was heated and stirred for 2 hours at 353 K. The mixture
was then cooled to room temperature and used in the second part of the synthesis.
The second part (PART B) was the preparation of the mixed surfactant
solution.
The
surfactant
mixture
was
prepared
by
dissolving
cetyltrimethylammonium bromide (3.87 g, Fluka 99%) and Triton X-100 (1.17 g,
Mallinckrodt 97%) simultaneously with heating in 55 g H2 O. The surfactant solution
was then cooled to room temperature.
Third part (PART C) of the synthesis procedure was the preparation of the
gel. The silica source prepared from PART A and the surfactant solution prepared
11
from PART B were quickly poured into a 125 mL polypropylene bottle. The bottle
was capped and shaken rapidly and vigorously. The gel mixture thus obtained was
heated for the formation of the surfactant-silica mesophases under static conditions at
97 o C for two days. The resulting gel mixture in the bottle had a molar composition
of:
5 SiO 2 : 1.25 Na2 O : 0.15 TX-100 : 0.85 CTABr : 400 H2 O
The gel mixture was then cooled to room temperature. The initial pH of the
gel mixture is ca. 12.1. Subsequently, the reaction mixture was adjusted to pH 10.2
by drop wise addition of 30 wt% acetic acid (Merck) with vigorous stirring. The
reaction mixture after the pH adjustment was heated again to 97 o C for another 2
days.
The precipitated products, Si-MCM-48 from the reaction mixture were then
filtered, washed with 2 L of distilled water, and dried in an oven at 97 o C overnight.
In order to obtain highly ordered Si-MCM-48 mesophase, the molar
compositions had been varied from the original molar ratio to accomplish optimal
condition.
The synthesis experiments were repeated systematically many times by
changing the pH value of pH adjustment, Na2 O/SiO 2 , surfactant/SiO 2 , and H2 O/SiO 2
ratios. The reaction ratios and the sample codes are tabulated in Table 2.1.
2.3
Removal of Organic Templates
To remove the surfactant, the oven-dried as-synthesized samples were
calcined in air under static conditions using muffled furnace at 540 o C for 6 hours,
with a linear temperature ramp of 1 o C /min and two plateaus of 60 minutes each at
150 and 350 o C.
12
Table 2.1: Summary of synthesis conditions for Si-MCM-48 materials.
Sample code
Reaction gel composition
MP-1
5 SiO 2 : 0.85 CTABr : 0.15 TX-100 : 1.25 Na2 O : 400 H2 O (pH=11)
MP-2
5 SiO 2 : 0.85 CTABr : 0.15 TX-100 : 1.25 Na2 O : 400 H2 O (pH=10.2)
MP-3
5 SiO 2 : 0.85 CTABr : 0.15 TX-100 : 1.25 Na2 O : 400 H2 O (pH=9)
MP-4
5 SiO 2 : 0.85 CTABr : 0.15 TX-100 : 1.25 Na2 O : 400 H2 O (pH=7.5)
MP-5
5 SiO 2 : 0.85 CTABr : 0.15 TX-100 : 1.00 Na2 O : 400 H2 O
MP-6
5 SiO 2 : 0.85 CTABr : 0.15 TX-100 : 1.50 Na2 O : 400 H2 O
MP-7
5 SiO 2 : 0.85 CTABr : 0.15 TX-100 : 1.75 Na2 O : 400 H2 O
MP-8
5 SiO 2 : 0.85 CTABr : 0.15 TX-100 : 2.00 Na2 O : 400 H2 O
MP-9
5 SiO 2 : 0.75 ( 0.85 CTABr : 0.15 TX-100) : 1.25 Na2 O : 400 H2 O
MP-10
5 SiO 2 : 1.25 ( 0.85 CTABr : 0.15 TX-100) : 1.25 Na2 O : 400 H2 O
MP-11
5 SiO 2 : 1.50 ( 0.85 CTABr : 0.15 TX-100) : 1.25 Na2 O : 400 H2 O
MP-12
5 SiO 2 : 2.00 ( 0.85 CTABr : 0.15 TX-100) : 1.25 Na2 O : 400 H2 O
MP-13
5 SiO 2 : 0.85 CTABr : 0.15 TX-100 : 1.25 Na2 O : 250 H2 O
MP-14
5 SiO 2 : 0.85 CTABr : 0.15 TX-100 : 1.25 Na2 O : 300 H2 O
MP-15
5 SiO 2 : 0.85 CTABr : 0.15 TX-100 : 1.00 Na2 O : 350 H2 O
MP-16
5 SiO 2 : 0.85 CTABr : 0.15 TX-100 : 1.50 Na2 O : 450 H2 O
* A parallel series of experiment (as blank experiment) had been carried out, with the
conditions similar as listed in Table 2.1 but the gel mixtures did not undergo the pH
adjustment process.
2.4
Synthesis of Aluminated Cubic Mesoporous Materials (Al-MCM-48)
Modifications of catalytically inactive Si-MCM-48 were carried out after
successfully acquiring the optimal parameter to synthesis Si-MCM-48.
In order to
generate active sites for catalytic applications, aluminium was introduced into the
Si-MCM-48 framework.
investigated intensively.
In this study, two post-synthesis approaches had been
The molar composition of the parent Si-MCM-48 stated
13
below would be used in order to prepare aluminated mesoporous materials
Al-MCM-48:
5 SiO 2 : 0.85 CTABr : 0.15 TX-100 : 1.00 Na2 O : 400 H2 O
The sodium aluminate (NaAlO 2 ) used in the alumination process was of
technical grade, obtained from Riedel-de Haën and used without further purification.
(a)
Post-Synthesis Alumination of Mesoporous Si-MCM-48
Post-synthesis alumination of mesoporous Si-MCM-48 was conducted
according to the method which was proposed by Hamdan et al. [18]. Aluminium
was incorporated into the calcined silica framework by isomorphous substitution.
Calcined parent Si-MCM-48 was aluminated by stirring a 1 g portion of the
sample in 50 ml of 0.1 M, 0.25 M, or 0.5 M aqueous solution of NaAlO 2 in a tightly
closed polyethylene bottle. The mixture was immersed in an oil bath at 60 o C and
stirred vigorously for 3 hours.
The samples were filtered, thoroughly washed with
double distilled water, dried overnight at 97
o
C and calcined in air at 550 o C for 2
hours. Table 2.2 lists the sample codes for Al-MCM-48 with various concentrations
of NaAlO 2 .
Table 2.2: Sample codes for Al-MCM-48 with different concentration of sodium
aluminate prepared by post-synthesis alumination of mesoporous Si-MCM-48.
Code
Concentration of Sodium Aluminate (M)
010Al-MCM-48
0.1
025Al-MCM-48
0.25
050Al-MCM-48
0.50
14
(b)
Post-Synthesis Alumination of Si-MCM-48 Mesophase
The preparation of parent Si-MCM-48 mesophase was similar to those stated
in Section 2.2. The pH adjusted gel mixtures were cooled to ambient temperature
after 2 days aging at 97 o C. Subsequently, appropriate amounts of 5 wt% aqueous
solutions of sodium aluminate were slowly added into the cold gel mixture and
heated further at 97 o C for 7 days.
The aluminated products were then filtered,
washed with 2 L of distilled water, and dried in an oven at 97 o C overnight.
Furthermore, removal of template from as-synthesized samples was carried out as
mentioned in section 2.3. The amount of sodium aluminate that was added depends
on the Si/Al ratio. The sample codes for different Si/Al ratio are tabulated in Table
2.3.
Table 2.3: Sample codes for Al-MCM-48 with different Si/Al ratio prepared by post
synthesis alumination of Si-MCM-48 mesophase.
Code
Si/Al ratio*
Al-MCM-48-20
20
Al-MCM-48-30
30
Al-MCM-48-50
50
Al-MCM-48-100
100
*Si/Al ratio mentioned above is calculated from compositions of starting gel
mixtures
2.5
Preparation of Protonated Al-MCM-48 (H-Al-MCM-48)
In order to obtain the acidic form H-Al-MCM-48, the Al-MCM-48 samples
must be ion exchanged with aqueous solution of ammonium nitrate (Merck 99%).
1 g of calcined Al-MCM-48 powder was stirred vigorously in 50 mL of 1 M aqueous
solution of NH4 NO3 for 16 h at 60 o C. Subsequently, the samples were filtered,
washed with distilled water, and dried in oven at 97 o C overnight. The NH4 +-Al-
15
MCM-48 samples were calcined at 550 o C in air for 4 h and maintained 2 h to
acquire the acidic H-Al-MCM-48.
2.6
Charaterization of MCM-48 Molecular Sieves
Comprehensive characterization techniques have been utilized in order to
elucidate and provide unambiguous structural information and properties of MCM48.
These structure and properties elucidation methods embrace X-ray powder
diffraction (XRD), nitrogen adsorption measurements, Fourier Transform Infrared
Spectroscopy (FTIR),
27
( Al MAS NMR) and
27
Al Magic Angle Spinning Nuclear Magnetic Resonance
13
C Cross-Polarization MAS NMR (13 C CP/MAS NMR),
temperature-programmed desorption of ammonia (NH3 -TPD), infared spectroscopy
of pyridine adsorption, and field emission scanning electron microscopy (FESEM).
2.6.1
Powder X-Ray Diffraction (XRD)
Powder XRD is a powerful technique for the qualitative and quantitative
characterization of zeolite materials.
XRD measurements can signify whether the
catalyst is amorphous, crystalline, or quasi-crystalline, yield an estimate of average
crystallite size, and yield d-spacing and lattice parameters, allowing identification of
the present phases [19].
X-Ray diffractogram were acquired using Bruker D8 Advance powder
diffractometer with Cu-Kα as the radiation source with λ = 1.5418 Å at 40 kV and 40
mA.
The sample was ground to a fine powder using a mortar and then lightly
pressed to form a thin layer on sample holder. Samples were measured in the range
of 2θ = 1.5o -10o with 0.02o step size and 1 second step time. The data analysis
program automatically calculated the reflection position and d spacing.
16
2.6.2
Fourier Transform Infrared Spectroscopy (FTIR)
Infrared spectroscopy is a method for characterization of long range and
short-range bond order caused by lattice coupling, electrostatic and other effects.
Normally FTIR provides meaningful information in the mid-infrared region (1400400 cm-1 ) which attributed to the framework vibrations of zeolite which tetrahedral
linked of SiO 4 or AlO 4 [20].
Infrared spectra were obtained on Perkin Elmer (1600 series) spectrometer
using the KBr wafer technique. The sample was mixed with KBr in the ratio of 1 mg
sample to 300 mg KBr. The mixture was ground to a finely divided powder, loaded
between two 13 mm diameter die and pressed under 10 tons of pressure for 1 minutes
to obtained self-supporting pellet.
This technique avoids excessive grinding which
might cause structural degradation. The spectra were recorded at room temperature
with 4 cm-1 resolutions between 1400 - 400cm-1 .
2.6.3
Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR)
Nuclear
magnetic
resonance
(NMR)
spectroscopy
provides
structural
information of zeolites that is more complete than that available from any of the
techniques described [21].
NMR technique reflects the short-range ordering of
structure and is very sensitive to the local environment of nuclei.
Magic angle
spinning (MAS) is a useful line-narrowing technique in solid state NMR
spectroscopy, in which the sample is mechanically rotated rapidly on an axis that
makes an angle of 54.7o relative to the direction of the static magnetic field.
Sufficiently rapid MAS bring about the coherent averaging of inhomogeneous linebroadening effects, such as the chemical shift anisotropy and inhomogeneous
magnetic dipole-dipole interactions.
27
study.
Al MAS NMR and
27
13
CP/MAS NMR spectroscopy had been used in this
Al MAS NMR has been greatly employed to distinguish between
tetrahedrally and octahedrally coordinated in the framework (at approx. 50 and 0
17
ppm, respectively). Whereas,
13
C CP/MAS NMR is a valuable tool to interpret the
surfactant organization in Si-MCM-48 [22]. In the cross-polarization (CP) approach,
spin polarization from a more abundant spin set that has a larger nuclear magnetic
moment (1 H) is transferred via a double-resonance method to a less-abundant spin set
that has a smaller nuclear magnetic moment (13 C). Therefore, the sensitivity of
13
C
will increase.
The MAS NMR experiments were performed using Bruker Avance 400
MHz 9.4 T spectrometer. The
27
Al MAS NMR spectra were measured at 104.2
MHz, spinning at 7 kHz, 1.9 µsec pulses and 2 s relaxation time delays.
Each
spectrum was obtained with 6000 scans. Chemical shifts were quoted in ppm from
external Al(H2 O)6 3+.
Whereas, the
13
C CP/MAS NMR experiments were performed using a 3.8 µs
90o pulse with a delay time of 5 s, a contact time of 1 ms and spinning rate of 7 kHz
and 2000 transients. Chemical shifts for 13 C were referred to TMS.
2.6.4
Nitrogen Adsorption Measurements
Nitrogen adsorption technique enables the measurement of the surface area,
pore volume, pore size distribution, and pore texture of porous materials [23]. The
most common technique for measuring the surface area is by static volumetric
determination, whereby a known quantity of an inert gas (usually nitrogen) is
adsorbed onto the material under test, maintained at a constant temperature (usually
liquid nitrogen temperature 77 K) and the surface area determined by application of
the Brunauer-Emmett-Teller (BET) Theory.
The nitrogen adsorption-desorption measurements of calcined samples were
performed using Micromeritics ASAP 2010 volumetric adsorption analyzer using
nitrogen as the adsorbate at 77 K. Samples weighing between 0.1-0.2 g were placed
in the sample bulbs, attached to the out gas station and dehydrated at 473 K under
18
vacuum overnight. After cooling to room temperature, the sample bulb was quickly
transferred to the sample station for adsorption measurement.
The data were
acquired automatically by the computer program.
2.6.5
Field Emission Scanning Electron Microscopy (FESEM)
Prior to sample scanning, the samples were coated with titanium on titanium
sputter. The samples were scanned using JOEL JSM-6330F field emission scanning
electron microscope operating at 15 kV.
2.6.6
Acidity Measurement
Acidity of the MCM-48 samples were performed by using two types of
techniques; temperature-programmed desorption (TPD) of ammonia and infrared
spectroscopy of pyridine adsorption. Each method used basic compounds to probe
the acid sites of the samples. TPD measurement will provide information related to
acid strength and total amount of acid sites.
Whereas, infrared spectroscopy of
adsorbed pyridine are capable to distinguish Brönsted and Lewis type acid sites in
the samples.
(a)
Temperature-Programmed Desorption (TPD) of Ammonia
Temperature-programmed desorption (TPD) analysis were conducted on
TPDRO 1100 of Thermoquest.
Ammonia (NH3 ) had been employed as probe
molecule. 0.2 g of samples were purged at 723 K in a nitrogen stream (20cc/min) for
3 h.
Afterward, NH3 (10cc/min) was adsorbed at 353 K for 30 minutes. Next,
desorption of NH3 were initiated by continuous heating of the sample in a flow
(20cc/min) of nitrogen with linear temperature ramp 10 K/min up to 873 K. The
desorbed amount of NH3 was acquired continuously using thermal conductivity
detector.
19
(b)
Infrared Spectroscopy of Pyridine Adsorption
The acidified samples were ground to a fine powder and pressed into very
The optical thickness was approximately 10 mg cm-2 .
thin self-supporting wafers.
The sample disks were mounted on an IR cell equipped with CaF2 windows. The IR
cell was connected into a vacuum system and the samples were in situ calcined at
400o C for about 16 h.
After pre-treatment, the samples were cooled to room
temperature under evacuation and the IR spectra were recorded in the range 40001300 cm-1 .
The experimental setup for acidity study is schematically displayed in
Figure 2.1.
The above samples were then exposed to 10 Torr of pyridine at room
temperature for 10 minutes.
step by step for 1 h at 25
Desorption of pyridine was carried out by evacuation
o
C, 150
o
C, 250 o C and 400 o C. Temperatures were
measured by a thermocouple positioned just below the disk and the infrared spectra
recorded using Perkin Elmer 1600 spectrometer with a resolution of 2 cm-1 . The
integrated intensities of the bands at 1545 cm-1 and at 1450 cm-1 were determined
and used as a measure of the amounts of Brönsted-bound pyridine (Bpy) and Lewisbound pyridine (Lpy), respectively.
2.7
Catalytic Testing
2.7.1
Activation of H-Al-MCM-48
The H-Al-MCM-48 catalysts (0.2 g) were pre-treated in a muffled furnace at
673 K for 2 h before underwent for catalytic testing. Subsequently, the samples were
cooled to approximate 473 K and poured into the reaction mixtures.
Figure 2.1: Experimental setup for acidity study.
20
21
2.7.2
Acylation of 2-Methoxynaphthalene with Acetyl Chloride over H-AlMCM-48
The Friedel-Crafts acylation reaction of 2-methoxynaphthalene with acetyl
chloride was performed in the liquid phase in a batch reactor. The reactions were
carried out with mixing 200 mg of catalyst, 475 mg of 2-methoxynaphthalene, and
471 mg of acetyl chloride together in 10 mL nitrobenzene. The reactors were tightly
close and put into the oven at 393 K for 20 h. Liquid samples were taken out before
and after the reactions and kept in closed vials.
For optimization purposes, this
reaction was also carried out in dichloroethane and cyclohexane.
The withdrawn liquid samples were analyzed by HP-5890 Series II gas
chromatography equipped with non-polar capillary column ULTRA 1 (cross linked
methylsilicone, 25 x 0.20 mm I.D.) and a FID detector.
The setup of oven
temperature programme was illustrated in Figure 2.2. Naphthalene had been used as
internal standard to quantify the results.
In the other hand, authentic sample,
6-acetyl-2-methoxynaphthalene, had been applied as reference compound for
identification of target compound.
Temperature / oC
1 min
280
10 oC/min
50
1 min
Time / min
Figure 2.2
GC-FID and GC-MS oven-programme setup.
Besides, products of the reaction were also determined by GC-MS (Agilent
6890N-5973 Network Mass Selective Detector) equipped with HP-5MS column
(30m x 0.251 mmx 0.25 µm). Sample was analyzed on splitless mod with helium as
22
the carrier gas. The oven temperature programme setup was similar as illustrated in
Figure 2.2.
CHAPTER 3
OPTIMIZATION OF SYNTHESIS AND CHARACTERIZATION OF
PURELY SILICEOUS MESOPOROUS MOLECULAR SIEVES MCM-48
3.1
Introduction
Molecular sieves defined as porous materials which are competent for
selective adsorption and are capable to separate molecule with different sizes and
shapes in a mixture [24]. According to International Union of Pure and Applied
Chemistry (IUPAC), porous materials can be classified into three groups depending
on their pore size [25].
Pore size distributions less than 20 Å are related to
microporous materials. Materials having pores between 20 Å to 50 Å represent
mesoporous materials. Whereas, materials that have pores larger than 500 Å are
considered as macroporous materials.
Pore textures of the porous materials are
important to determine the realm of applications. The most well known and widely
used of molecular sieves are zeolites since it occupies uniqueness of shape
selectivities, adsorption abilities and high ion exchange capacities [26].
Zeolites are hydrated aluminosilicates constructed of AlO 4 and SiO 4
tetrahedra. Each AlO 4 - ion contributes one negative charge to the framework. The
negative charge is balanced by cations typically of the type Na+, H+, Ca2+, and R4 N+.
Zeolites are composed of channels and cages that are connected and extended in
three dimensions. Upon dehydration, cations and small organics are able to penetrate
and remain within the cages. These properties make zeolites useful as molecular
sieves. However, the dimension of their micropores has limited its applicability in
transformation of bulky molecules.
Therefore, numerous rational attempts and
24
approaches have been carried out to extend the pore sizes of zeolites into the
mesoporous regime.
In 1992, a novel family of mesoporous molecular sieves designated as M41S
has been discovered by scientists at Mobil R & D Corporation [9, 27]. The M41S
family comprises of 3 main phases: MCM-41, having a hexagonal array of
unidimensional pores; MCM-48, having a cubic pore system; MCM-50 displaying an
unstable lamellar phase. Figure 3.1 shows schematic structural illustrations of M41S
materials. MCM mention here is the acronym for Mobil of Composition of Material.
Mesoporous materials can be formed by a cooperative, self-assembly of surfactant/
silicate ionic pairs leading to a network structure in 3 dimensions upon silicate
condensation [28].
Calcinations and removal of surfactant “template” leave an
ordered array of pores with amorphous walls and a mono-dispersion of diameters.
The resultant high surface area solids can be further modified by reacting surface
silanols with a variety of compounds such as trimethylchlorosilane and AlCl3 [2931].
(a)
(b)
Figure 3.1
(c)
Schematic structural illustrations of M41S family (a) hexagonal
MCM-41; (b) cubic MCM-48; (c) lamellar MCM-50.
Mesoporous materials from the M41S family possess several unique
properties including; (i) pores which are tunable from 15-100Å; (ii) uniform pore
diameter in the mesopores range; (iii) large surface areas above 700 m2 /g, while the
pore volume in the range of 0.7-1.2 cm3 g-1 .
It was reported that mesoporous
materials MCM-41 could be heated to 1123 K in dry air or 1073 K in air with 8 torr
of water vapour before structural collapse began [32]. The high thermal stability of
the materials is advantageous for application as industrial catalysts.
25
MCM-48 has a three-dimensional cubic pore system, which is indexed in the
cubic space group symmetry Ia3d. On the basis of X-ray diffraction (XRD) and
transmission electron microscopy (TEM) studies, MCM-48 material has been found
to possess a pair of disconnected, enantiomeric networks of three-directional
mesoporous channels, which are centered on the gyroid minimal surface (Figure 3.1)
[33]. The MCM-48 channels are constructed with atomically disordered inorganic
silica walls around surfactant molecules. Therefore this material shows no X-ray
reflections above 2θ = 10o . However, due to long-range ordering of the uniform
sized pores, X-ray reflections appear at very low reflection angles in the range of 2θ
= 1.5-8o [34]. In general, the pore wall thickness of MCM-48 was found to be in the
range of 8 to 10 Å [35].
In the beginning of discovery of M41S materials, intensive researches have
been focused on MCM-41 since it is easier to synthesis, reproducible, and can be
obtained in pure phase. In comparison to MCM-41, only few work on MCM-48 in
the beginning, since the synthesis of MCM-48 by using conventional hydrothermal
technique is more difficult and not reproducible [9, 36]. By using conventional
technique to synthesize MCM-48, instead of pure cubic phase, it is favored
contaminated with mixture of phases. This is due to the cubic MCM-48 which is
present as intermediate during phase transformations from hexagonal to lamellar.
The interest in MCM-48 materials has, indeed, increased during the past few
years. This may be attributed to a growing realization that three dimensional pore
systems have several advantages in catalytic and separation processing, compared
with one dimensional systems [37]. The interior of the particles is more readily
accessible because the pore openings are not restricted to one direction. Such a
structure is expected to be less prone to the pore blocking than one-dimentional
channels such as those in MCM-41 silica. Furthermore, the pore curvature entails
more agitated flow in the system. This will increase the diffusion rate of reactants
and products through a tri-directional channel structure.
26
3.2
Proposed Formation Mechanisms and the Evolution of Synthesis Routes
for the M41S Mesoporous Materials
Up to presence, several models have been proposed to explain the formation
of mesoporous MCM materials in the presence of surfactants.
Beck et al. had
proposed liquid crystal templating (LCT) mechanism for the formation of the M41S
materials due to the similarities in the microscopy and diffraction results for MCM41 to those of surfactant/water liquid crystal or micellar phases [9, 27]. The LCT
mechanism exploits the continuous solvent (water) region to create inorganic walls
between the surfactant liquid crystal structures. They proposed that the structure is
defined by the organization of the surfactant molecules into liquid crystals which
serve as templates around which condensation of the inorganic species occurred.
Two possible pathways were proposed and are shown in Figure 3.2. Firstly, the
liquid crystal mesophase is intact prior to the addition of silicate species (Pathway 1),
or the introduction of the silicate species that mediates the ordering of the subsequent
silicate encased surfactant micelles (Pathway 2).
(1)
(2)
Figure 3.2
Schematic of possible mechanistic pathways for the formation of
MCM-41: (1) Liquid crystal phase initiated and (2) silicate anion initiated [9].
27
Alternatively, another formation mechanism proposed by Stucky and coworkers is the transformation mechanism from lamellar to hexagonal phase and is
schematically drawn in Figure 3.3 [28, 38-39]. The central tenet of their concept is
that the mesophase formed is governed by charge density, coordination state, and
steric requirements of the inorganic and organic species at the interface and not
necessarily by a preformed liquid crystal structure. In contrast, a micellar assembly
of organic molecules will be broken up and rearranged upon addition of inorganic
species to form a new phase often with lamellar morphologies.
As reported
previously, the initial reaction mixture was layered but gradually transformed into
the hexagonal phase. This transition was governed by the electrostatic interaction
between the positive surfactant head groups and the negative silicates. In the early
stage of the process, the presence of highly charged silicate species permits a
lamellar surfactant configuration.
As rearrangement and polymerization of the
silicate species proceed, the charge density reduces and to balance the electrostatic
interactions, curvature has to be introduced into the layers. Consequently, lamellar
structure transforms into hexagonal mesophase.
Figure 3.3
Schematic diagram of the transformation mechanism from lamellar to
hexagonal mesophase. The arrow indicates the reaction coordinate [28].
28
In essence, ordered mesoporous molecular sieves can be synthesized in
varieties of synthetic pathways as schematically illustrates below in Figure 3.4. The
synthetic routes have been devoted in the synthesis of ordered mesoporous materials
in order to develop a simple, high product yield, and cost efficient method [17, 40].
In other hand, the evolutions of improvement in the synthetic route are provoked by
the efforts to achieve reproducible method to obtain the mesophase with high purity.
Figure 3.4
Schematic showing of interfacial interactions for surfactant micelles
in cooperatively assembly.
The discovery of the first ordered M41S mesoporous materials were based on
the inspiration of pioneering work of Yanagisawa et al. in 1990 [41]. Yanagisawa et
al. had discovered structurally related mesoporous silica with pore diameter of 2-4
nm through ion exchange of long chain alkyltrimethylammonium (C 16 ) cations with
interlayer Na+ ions of the layered polysilicate kanemite. Consequently, the insight of
Yanagisawa et al. had lead Beck et al. to develop the more versatile synthesis
pathway based on the supramolecular assembly of cationic surfactants (S+) and
cationic inorganic precursors (I-) [27]. Later, the S+I- electrostatic pathway of Mobil
had been greatly extended by Stucky and his co-workers [38-39] to a whole series of
other electrostatic assembly mechanisms.
The extended complimentary routes
include a charge-reversed S-I+ assembly mechanism, as well as counterion- mediated
29
S+X-I+ and S-M+I- pathways, where X- = Cl-, Br- and M+ = Na+, K+ [38-39]. In the
synthetic routes mentioned above, the surfactant was not reusable since it could only
be removed by thermal treatment. On the other hand, another neutral pathway was
introduced by the group of Pinnavaia based on the hydrolysis of an electrically
neutral inorganic precursor (Io ) in the presence of a neutral amine (So ) [42] or
polyethylene oxide (N o Io ) [43] as the predominate structure directing agent. In this
approach, the interactions at surfactant-inorganic precursor interfaces are based on
hydrogen bonding. Thus, the surfactants can easily extracted by using ethanol and
was therefore reusable [44].
More recently, Stucky and co-workers had introduced a new synthesis route
involving amphiphilic di and tri-block copolymers as organic structure directing
agents [45].
Besides that, Ryoo and co-workers also succeeded in producing
mesoporous silica by utilizing triblock copolymer (EO 20 PO70 EO20 ) [46]. In general,
the produced materials have larger mesopores and thicker wall if compared to the
electrostatic assembly pathways.
3.3
Synthesis of MCM-48 Materials
Since there is increasing awareness of diffusion advantages of cubic
mesoporous system compared to mono-dimension of hexagonal MCM-41, intensive
investigations towards synthesis routes were tremendously developed in order to
overcome the synthetic shortcomings and the difficulties in its preparation [36, 4754]. The results of the studies clarify that the crystallinity of the MCM-48 products
strongly relies on the gel composition, crystallization time and temperature. It was
showed that MCM-48 was an intermediate phase between the transformation from
hexagonal or disordered surfactant-silica mesophase to a more stable lamellar
mesophase.
However, transition of the MCM-48 mesophase to lamellar can be
quenched by adjusting the pH of the reaction mixture. In addition, it was found that
the ethanol which is formed by the hydrolysis of tetraethylorthosilicate (TEOS)
during the synthesis plays an important role in formation of MCM-48. The ethanol
will cause a systematic rearrangement in the structure of micelles by penetrating the
30
micelles surfaces. Moreover, the phase transitions also depends on the nature of
anions, their concentration, and the presence of the cations in the synthesis gel [55].
These play a mediation role in the electrical balance and in slight charge density
mismatch that control both the interface curvature and the mesophase characteristics.
Since the liquid-crystal structures of the surfactant serve as organic template,
the behavior of the surfactant in binary surfactant/water systems is the key for the
controlled preparation of silica mesophases [56]. According to a microscopic model
introduced by Israelachvili et al. [57], the geometry of mesophase structures can be
qualitatively predicted from dimensionless packing parameter g, which is defined in
Eq. 3.1 :
g≡ V
ao lc
(Eq. 3.1)
where V is the effective volume of the hydrophobic chain, ao is the mean aggregate
surface area per hydrophilic head group and lc is the critical hydrophobic chain
length. The illustration of the regions of generic surfactant is showed in Figure 3.5.
Figure 3.5
Illustration of the regions of generic surfactant.
31
In classical micelle chemistry, mesophase transition occurs when the g value
is increased above critical value as illustrated in Table 3.1. In addition, the phase
transitions also reflect a decrease in surface curvature from the cubic (Pm3n) over the
hexagonal to the lamellar phase.
Table 3.1: Surfactant packing parameter g, expected structure and examples for such
structures.
g
Expected Structure
Example
1/3
Cubic (Pm3n)
SBA-1
1/2
Hexagonal (p6)
MCM-41, FSM-16, SBA-3
1/2-2/3
Cubic (Ia3d)
MCM-48
1
Lamellar
MCM-50
There are many modifications of original Beck et al. [9] recipe in order to
improve the stability, and long range structure of MCM-48 mesophase were
suggested [40, 54, 58,]. Sayari A. [54] had reported a simple and reproducible
method for the high yield synthesis of exceptionally good quality MCM-48 using
fumed silica and CTABr with no organic additives. Whereas, Peña M.L et al. [58]
reported that the formation of cubic structure can be finely controlled in system
SiO 2 :CTAOH/Br:H2 O. On the other hand, the yields and the stability of MCM-48
products had been reported to be greatly enhanced by reducing the pH of the
solution.
Besides, a novel synthesis route to MCM-48 in room temperature as well as
in hydrothermal synthesis route had been reported [59].
Compared to the
hydrothermal pathway, the novel room temperature pathway is faster than the
traditional ones and convenient in producing MCM-48 spheres with controlled
porosity.
Recently, mixed surfactant and gemini surfactant methods have facilitated the
effective avenue of preparing MCM-48 [17, 60-63]. The advantages of the desired
approaches are using low surfactant to silica molar ratio and surfactant concentration,
32
capable of providing highly ordered MCM-48 with a wide range of average pore
sizes and unit cell sizes, and the most important feature is that MCM-48 mesophase
becomes energetically favorable.
Usually, MCM-48 mesoporous materials can be synthesized by using various
types of commercial available silica sources such as fume silica Cab-O-Sil [54],
TEOS [59], colloidal silica (Ludox) [17], and many more. In this study, rice husk
ash (RHA) obtained from open burning site will be employed directly as a cheaper
alternative silica source for preparing the Si- MCM-48 mesophase since there are no
reports on it until this moment.
Si- MCM-48 will be synthesized using mixed
cationic-neutral templating route which was proposed by Ryoo R.
et al. [17].
Modification of the gel compositions proposed by Ryoo et al. should be carried out
since the nature of the RHA is totally different from the common commercial
available silica sources. Moreover, untreated RHA consists of a variety of impurities
[64], which will greatly affect formation of mesophase. Therefore, optimization
experiments should be carried out intensively in order to obtain pure phase and high
quality of MCM-48 materials in optimizes condition. The optimization experiments
will focus on the pH value, Na2 O/SiO 2 , Sur/SiO 2 and H2 O/SiO 2 of the initial gel
compositions. The resulting mesoporous materials will be structurally characterized
by using powder X-ray diffraction technique.
3.4
Results and Discussion
3.4.1
Characterization of Rice Husk Ash (RHA)
Sample of rice husk ash (RHA) obtained from open burning site was light
brown in color. The silica content in RHA is approximately 93.0 %. Whereas, the %
LOI of RHA is about 4.8 %, which represents the loss of vo latile organic compounds
and adsorbed water on the ash. In addition, the RHA had a surface area of ca. 30
m2 g-1 . Nevertheless, several researchers had reported that RHA also consists of trace
elements like Fe2 O3 , CaO, MgO, Na2 O, K2 O, and MnO [64-65].
33
Figure 3.6 shows the XRD diffractogram of RHA in the range of 2θ = 5o -70o .
The broad hump around 2θ = 23o indicates that the RHA is in amorphous phase.
Whereas, the FTIR spectrum of sample RHA in Figure 3.7 shows a typical
amorphous silica spectrum with intense asymmetric, symmetric stretching and
bending vibration for Si- O-Si bonds at wave numbers 1103, 800, and 468 cm-1
respectively [20]. Silanol groups not detected in the RHA reveals that the RHA had
been underwent high thermal combustion.
The amorphous RHA is the ideal
candidate in the synthesis of mesoporous materials in the temperature below 100 o C.
Indeed, the amorphous phase silica is easily dissolved in basic solution. Thus, it
potentially function as active silica reagent and suitable utilize in the synthesis of
mesoporous materials. According to previous studies [14], crystalline RHA also
could be used in the synthesis of zeolite. However, it required longer aging time and
temperature above 150 o C. Therefore, crystalline RHA is not suitable employed in
the synthesis of mesoporous materials.
3.4.2
Synthesis of Purely Siliceous Mesoporous Materials
The multi- step calcination temperature programme are beneficial to the
ordered long-range structure and surface acidity of mesoporous materials upon
thermal treatment [66].
Decomposition of the surfactants in the mesoporous
materials by Hofmann degradation began at 150o C [67].
Prolonged thermal
treatment time at current temperature will help eliminate the evolved product
properly. Whereas, the organic residues will be converted to carbon dioxide and
water by oxidation at 350 o C. The oxidation of CxHy fragments will be completed by
prolonged treatment time at this temperature. However, the template removal is not
complete at 350 o C and requires additional heating to 540 o C for several hours. The
polymeric species or coke which generated during the template elimination processes
are more stable and require higher temperatures and longer treatment times to be
removed.
The temperature and the ramping rate of the template elimination
processes should be well programmed and slowed because the relative intensities of
the low angle reflections are very dependent on the distribution of matter in the
pores.
34
Intensity
5
10
Figure 3.6
20
30
2θ (o )
40
50
60
XRD diffractogram of RHA obtained from an open burning site.
Transmittance (%)
800.4
468.7
1103.2
1500 1400 1300 1200 1100 1000 900 800
wavenumber (cm-1 )
Figure 3.7
700
600
500
FTIR spectrum of RHA obtained from an open burning site.
400
35
In general, the structure of ordered mesophases, which did not underwent the
pH adjustment process are unstable upon the template elimination processes by
thermal treatment. Their XRD diffractograms (not shown) indicate the structure of
the mesophases collapse and transform to amorphous after calcination. Analyses of
the mother liquor after hydrothermal aging shows it reached a pH value of ca. 12.1.
At extremely high value of pH, the dissolved silicates ions are mainly present in the
gel mixture [68]. Therefore, it is no doubt that the degree of polymerization of the
silicate species in the mesophase frameworks is low.
Loosely polymerized
frameworks apparently are unstable and inevitably collapse upon thermal treatment.
On the other hand, it is believed that a few units of silicate polyanions are complexed
with the head group of the surfactant by the electrostatic interaction [69]. Therefore,
the mesophase that was constructed by the weakly interacts composite (silicate +
surfactant) are not stable towards thermal treatment.
Hence, pH adjustment is
essentially needed in order to obtain high degree of polymerized mesophase
frameworks.
Moreover, coacervation has also been observed during the experiment. This
phenomenon is rarely observed during hydrothermal synthesis of MCM-41 or MCM48. Coacervation is a type of aggregation [68], in which the silica particles are
surrounded by an adsorbed layer of material that makes the particles less hydrophilic,
but does not form bridges between particles.
The particles aggregate as a
concentrated liquid phase immiscible with the aqueous phase.
Therefore, the
adsorption of finely dispersed carbon in the particles will form a very viscous
intermediate, which will inhibits the process of polymerization of silicates ions.
3.4.2.1 Effect of pH Value
XRD diffractograms in Figure 3.8 are consisted of one intense reflection peak
at 2θ = 2.2o , a weak reflection shoulder peak at 2θ = 2.5o , and several unresolved
peaks between 2θ = 3.5o -5.5o . The most intense reflection peak is indexed as (211),
the shoulder reflection peak is indexed as (220), whereas the several unresolved
peaks are indexed as (321), (400), (420), (332), (422) and (431). The XRD pattern is
36
characteristic of the Ia3d bicontinuous cubic lattice structure of mesoporous
molecular sieves MCM-48. In general, the formation of bicontinuous cubic phase is
favorably after subsequent pH adjustment.
According to the XRD diffractogram, the most intense reflection peak d211 is
designated by the mesopores system in the mesoporous materials. It can be seen that
calcined samples presented more intense mesopores reflection peak than the
corresponding as-synthesized ones. Similar trends were also observed in previous
studies and this type of behavior is normally explained by an increase of scattering
contrast between the siliceous walls of the structure and the pores space [67]. On the
other hand, the presence of the surfactants within the channels in as-synthesized
samples attribute to the reduction of the intensity of mesopores reflection peak.
Marler et al. [70] reported that by comparing XRD diffractograms from MCM-48
samples containing different kinds of adsorbate, there are several order of electron
density.
They observed high intensity diffraction peaks for samples having no
adsorbate molecules into their pores.
The intensity of XRD reflection peaks
decreased when electron density of the adsorbate approached that of the silica wall.
When pH of the gel mixture adjusted to pH 11, the d211 /d220 reflection peak
ratio of as-synthesized sample is increased noticeably after calcination.
This
indicates the sample MP-1 still consists some amount of unstable ordered cubic
mesophase. It was observed that the pH of the resultant gel mixture was ca. 11.7.
At this high pH stage, nucleation of the particles are very slow and not favored, but
the dissolution of the silica frameworks is more dominated [68]. Therefore, some of
the mesophase frameworks are constructed by a tiny of ≡Si-O-Si≡ units, and readily
collapse upon thermal treatment.
However, the well definite, narrow and high
intensity of reflection peaks of calcined MP-1 reveal that the rema ining cubic
mesoporous materials have high degree of crystallinity. Hence, it can be concluded
that MP-1 is only contaminated by a minute amount of unstable cubic mesophase.
At pH 10.2, the d211 /d220 reflection peak ratio seems not to have significant
changes. Thus, it reveals that no contamination of unstable mesophase and other
phases in this stage. According to previous studies [68], nucleation and particles are
grown gradually in this stage. Therefore, the mesophase frameworks are built
37
Intensity
d 211
d 211
d 211
d 211
d 220
MP-3 (pH 7.5)
d 220
d 220
MP-3 (pH 9)
d 220
MP-2 (pH 10.2)
MP-1 (pH 11)
2
3
4
d 211
Intensity
5
6
o
2θ ( )
(a)
d 211
7
8
9
10
d 211
d 211
d 220
d 220
MP-4 (pH 7.5)
d 220
MP-3 (pH 9)
d 220
MP-2 (pH 10.2)
MP-1 (pH 11)
2
3
4
6
5
7
8
9
10
2θ (o )
(b)
Figure 3.8
XRD diffractograms of (a) as-synthesized mesoporous materials; (b)
calcined mesoporous materials with various pH value.
38
by satisfactory amount of ≡Si-O-Si≡ units, which are stable upon thermal treatment.
It is noticed that the sample MP-2 possesses higher crsytallinity of ordered cubic
structure among the samples. Nonetheless, the d220 peak is partially covered by the
broadening of d211 reflections peak. The broadening of the reflection peak is due to
smaller particle sizes of sample MP-2 if compared to sample MP-1.
According to XRD diffractograms in Figure 3.8, bicontinuous cubic
mesophase can still be notified until pH 7.5.
However, the structural ordering
deteriorated progressively from pH 9 unto pH 7.5, since the intensity of d211
reflection peak decreased visibly starting from pH 9. At pH 7.5, the d220 reflection
peak of as-synthesized MP-4 is remarkably higher than d211 reflection peak. The d220
reflection peak is drastically decreased after undergoing the thermal treatment.
Apparently, the d220 reflection peak is overlapped by the reflection of lamellar phase.
The lamellar arrangement of surfactants and silica layers will collapse and become
amorphous material after the elimination of the surfactants [51].
The drastic
decrease of the d220 reflection peak after calcinations, that is due to the reflection
counts that attribute to lamellar structures are not longer interfered the d211 reflection
peak. Therefore, it is suggested that in pH 7.5, the sample MP-4 is significantly
contaminated by the lamellar phase.
The simultaneous presence of the cubic and lamellar phases indicates the
occurrence of phase transitions of cubic phase to lamellar phase in pH 7.5.
Apparently, pH 7.5 is the critical point that the cubic mesophase reorganizes into the
lamellar mesophase.
By lowering the pH unto pH 7.5, the silicate polyanions
condense gradually to form oligomers at the silicate-surfactant interfaces, eventually
reducing the average area per surfactant headgroup (ao ) sufficiently to induce the
formation of layer structures [49].
The structural order and thermal stability of bicontinuous cubic mesoporous
materials are decreased in following order:
MP-2 (pH 10.2) > MP-1 (pH 11) > MP-3 (pH 9) > MP-4 (pH-7.5)
39
Based on the above discussions, it is shown that by lowering the pH, the
structure of the silica-surfactant mesophases in this mixed cationic- neutral templating
route changes continuously from cubic to lamellar, as the following sequence:
Unstable Ia3d cubic mesophase (pH 12.1) as mention previously→ unstable Ia3d
cubic mesophase + stable Ia3d cubic mesophase (pH 11)→ stable Ia3d cubic
mesophase (pH 10.2)→ ill-defined Ia3d cubic mesophase (pH 9)→ ill-defined Ia3d
cubic mesophase + lamellar (pH 7.5)
The changes are consistent with the general tendency of the cubic to lamellar
phase transitions observed with decreasing the surface curvature around the
surfactant micelles. The changes may be explained by the concentration of the
silicate anions on the surface of the surfactant micelles. By lowering the pH, the
concentration of the silicate oligomers on the interfaces increases gradually,
eventually it may lead to a significant contraction of the micellar surface, resulting in
the phase transition from cubic to lamellar. The results of the experiments show that
pH 10.2 is the optimal pH value to obtain highly ordered MCM-48 and has been
chosen to further investigate the influences of other parameters.
By reducing the pH value of the gel mixture, the predominance silicate
polyanions which dissolve in the solution, are tended to precipitate, because of the
decrease of the solubility of silicate ions [68]. In between, when decreasing the pH
value, the silanol groups are formed according to the following equilibrium [69]:
≡Si-O- + H3 O+
≡Si-OH + H2 O
(Eq. 3.2)
The consequential silanol groups have a strong tendency to oligomerise and
subsequently polymerize in such way in order to generate ≡Si-O-Si≡ units.
Polymerization of the silica frameworks will proceed by the following equilibrium:
≡Si-O- + HO-Si≡
≡Si-O-Si≡ + OH-
(Eq. 3.3)
40
It was observed in this study that the pH value of the subsequent
hydrothermal aged gel mixture, would slightly increased ca. 0.7 if compared to preadjusted pH value. The increases of the pH value are due to the formation of the
mesophase frameworks, which is composed of ≡Si-O-Si≡ units, by condensation
process (Eq. 3.3) during the nucleation period. The OH-, which was released when
the condensation reaction takes place, had increased the pH of the gel mixture.
In the other hand, the formation of the ≡Si-O-Si≡ units will proceed largely
via subsequent water condensation reaction, as following equilibrilium:
≡Si-OH + HO-Si≡
≡Si-O-Si≡ + H2 O
(Eq. 3.4)
The polymerization of mesophase frameworks by condensation occurs in
such way that it will facilitate to maximize the number of siloxane bonds (Si- O-Si)
and minimize the number of terminal hydroxyl groups through internal condensation
[71]. Therefore, these particles will condense internally to the most compact state
with SiOH groups remaining on the outside.
3.4.2.2 Effect of Sodium Oxide/Silica (Na2 O/SiO2 ) Ratio
The XRD diffractograms of as-synthesized and calcined mesoporous
materials, which were varied in different Na2 O/SiO 2 ratios (Na2 O/SiO 2 =0.20-0.40)
have been shown in Figure 3.9 (a) and (b), respectively. Different types of XRD
diffraction patterns have been found in Figure 3.9, which indicate that the structure
of the mesophase obtained from the experiments are different. It suggests that the
structure of mesophase is very sensitive and strongly depends on the amount of
sodium hydroxide.
It can be seen that, while Na2 O/SiO 2 = 0.20, XRD diffractogram of sample
MP-5 shows characteristic XRD reflection peaks of typical Ia3d bicontinuous cubic
MCM-48 mesophase as mentioned in earlier discussions. As the Na2 O/SiO 2 ratio is
41
Intensity
d100
d100
d110 d200
d110 d200
d211
d211
d210 MP-8 (Na2 O/SiO 2 =0.40)
d210 MP-7 (Na2 O/SiO 2 =0.35)
MP-6 (Na2 O/SiO 2 =0.30)
d220
MP-2 (Na2 O/SiO 2 =0.25)
d220
MP-5 (Na2 O/SiO 2 =0.20)
2
3
4
5
6
o
2θ ( )
7
8
9
10
(a)
Intensity
d100
d100
d110 d200
d211
d211
d110 d200
d210 MP-8 (Na2 O/SiO 2 = 0.40)
d210 MP-7 (Na2 O/SiO 2 = 0.35)
MP-6 (Na2 O/SiO 2 =0.30)
d220
MP-2 (Na2 O/SiO 2 = 0.25)
d220
MP-5 (Na2 O/SiO 2 =0.20)
2
3
4
5
6
2θ (o )
7
8
9
10
(b)
Figure 3.9
XRD diffractograms of (a) as-synthesized mesoporous materials; (b)
calcined mesoporous materials with various Na2 O/SiO 2 ratios.
42
increased to 0.25, the XRD diffractogram of sample MP-2 still corresponds to Ia3d
bicontinuous cubic MCM-48 symmetry. However the d211 peak is broader and the
intensity is decreased if compared to sample MP-5. The result reveals that the degree
of crystallinity and cubic array of MP-2 mesophase is lower than MP-5. Therefore, it
suggests that the ordered Ia3d bicontinuous cubic array initiates to deteriorate with
the increase of Na2 O/SiO 2 ratio. However, the symmetry of Ia3d bicontinuous cubic
still clearly determined by the XRD and no other contamination by other mesophases
had been detected.
When Na2 O/SiO 2 ratio were increased to 0.35 (MP-7), the XRD pattern
exhibits of a intense reflection peak, 2 weak reflection peaks, and a very weak
reflection peak. These four peaks in the diffractogram are corresponded to (100),
(110), (200), and (210) Miller-Bravais indices, indicative of a p6 hexagonal lattice
array of mesoporous molecular sieves MCM-41.
On the other hand, hexagonal
mesophase was still acquired when Na2 O/SiO 2 ratio had been further increased to
0.40 (MP-8). However, the intensity of the reflection peaks of sample MP-8 is much
higher and distinct than sample MP-7. This reveals that the increase of Na2 O/SiO 2
ratio will enhance the crystallinity of hexagonal array.
However, when Na2 O/SiO 2 ratio reached 0.30 (MP-6), Ia3d bicontinuous
cubic MCM-48 mesophase and p6 he xagonal MCM-41 mesophase mixture has
suspected within the accuracy of the X-ray experiment. It is observed that the XRD
pattern for MP-6 appeared to be superposition of XRD pattern for the cubic MCM-48
(MP-5) and hexagonal MCM-41 (MP-8) mesophases. Thus, it reveals that these two
highly order phases have unit-cell dimensions very similar to those of the
components of the mixed phases.
In MP-6, the most intense reflection peak is
corresponded to d211 of cubic MCM-48 lattice reflection and d100 of hexagona l
MCM-41 lattice reflection. The d220 reflection peak of MCM-48 began to disappear
and masked with the d100 reflection peak of MCM-41. In addition, the d321 , d400 , d420 ,
d332 , d422, and d431 reflection peaks of MCM-48 between 2θ values of 3.5o -5.5o are
overlapped with d110 and d200 reflection peaks of hexagonal MCM-41.
It can be concluded that by increasing the Na2 O/SiO 2 ratio, the structure of
the silica-surfactant mesophases in this mixed cationic-neutral templating route
43
changes continuously from cubic to hexagonal mesophase. Apparently, Na2 O/SiO 2 =
0.30 (MP-6) is the critical Na2 O/SiO 2 ratio in which transition of the Ia3d
bicontinuous cubic mesophase to p6 hexagonal mesophase take place. The sequence
of transition are arranged in following order:
Ia3d cubic (MP-5 and MP-2) → Ia3d cubic + p6 hexagonal (MP-6) → p6 hexagonal
(MP-7 and MP-8)
The XRD results mentioned above clearly illustrates that the Na2 O/SiO 2 ratio,
or more correctly NaOH, plays a crucial role in the phase transition of Ia3d cubic
mesophase to p6 hexagonal mesophase.
Therefore, it suggests that the anionic
counterions, either in free or bound states, which are present in the micelles
environment will determine the geometry of the mesophase.
In the high
concentration of anions, which in this case is OH-, the electrostatic repulsion forces
in the head group region of micelles has been screened out by the constituent anions.
By increasing the OH-, the repulsion among the surfactant head-groups will decrease
consequently; therefore, reduce the ao of the surfactant head- groups. Theoretically,
the increasing of OH- anions will lead the phase transitions of hexagonal to cubic
mesophase. Conversely the results of the experiment are totally different from the
theory. In this circumstance, the mesophase transition cannot be understood in terms
of increasing the concentration of OH- ions.
However, in this complex system, the geometry of the mesophase not only
depends on the OH- ions, because other anions like silanolate groups, ≡Si-O-, are also
simultaneously present in the system. When increasing NaOH, not only OH- has
been increased in the system, but the Na+ also increases accordingly. Previous
studies had pronounced, the increasing concentration of Na+ ions has known to
inhibit the polycondensation of the silica in the basic media by forming {SiO -, Na+}
pair. Apparently, the free silanolate groups that potentially bound in the interface
had been decreased drastically. In fact, the ion binding of silanolate groups are more
tightly held by the micelle interfaces than OH- ions. Therefore, the screening effect
of silanolate groups towards the repulsion of polar head- groups (ao ) is more
influential than OH- ions.
Rationally, increasing of Na+ will diminish the free
silanolate groups, and eventually enlarge the repulsion of ao . Hexagonal mesophase
44
are more favored in the high ao of the surfactant head group. Therefore, increasing of
the Na2 O/SiO 2 ratio will lead the phase transitions from cubic to hexagonal
mesophases.
3.4.2.3 Effect of Surfactant/Silica (Sur/SiO2 ) Ratio
To investigate the effect of the Sur/SiO 2 ratio, syntheses were performed with
varying the Sur/SiO 2 ratios from 0.15-0.40.
The XRD diffractograms of as-
synthesized and calcined mesoporous materials in this series of investigation are
presented in Figure 3.10, respectively.
For Sur/SiO 2 ratios from 0.15-0.20, the
typical Ia3d bicontinuous cubic MCM-48 is obtained, as evidenced by XRD pattern
of both as-synthesized and calcined samples. The XRD diffractograms demonstrate
no significant differences in the crystallinity and long-range ordering of MCM-48
system among these ratios.
When the Sur/SiO 2 ratio was increased up to 0.25, the crystallinity and the
long-range ordering of MCM-48 has decreased gradually. However, the distinct
reflection peaks of XRD indicate that the long-range ordering of MCM-48 system is
still preserve entirely, although it possess lower crystallinity.
At Sur/SiO 2 = 0.40, the intensity of d211 reflection peak decreases drastically after the
sample underwent the thermal treatment. In high Sur/SiO 2 ratio, the frameworks of
mesophase partially collapse upon thermal treatment. However, the broad reflection
peak of d211 after calcination reveals that the sample system is able to retain
mesoporous system upon thermal treatment.
As reported previously by Beck et al. [27], Sur/SiO 2 ratio plays an important
role in determination of the resulting mesophases.
In mixed cationic-neutral
templating route, the Ia3d bicontinuous cubic mesophase is easily achieved in
extremely low Sur/ SiO 2 ratio, as low as Sur/SiO 2 = 0.15; compared to single cationic
surfactant route, which required Sur/SiO 2 ratio greater than 1. Conceivably, mixed
surfactants system exhibit complex phase behaviors in aqueous solution [72]. In this
45
Intensity
d211
d211
d211
d211
d220
d211
MP-12 (Sur/SiO 2 =0.40)
d220
MP-11 (Sur/SiO 2 =0.30)
d220
d220
MP-10 (Sur/SiO 2 =0.25)
MP-2 (Sur/SiO 2 =0.20)
d220
MP-9 (Sur/SiO 2 =0.15)
2
3
4
5
6
2θ ( )
(a)
7
8
9
10
o
d211
Intensity
d211
d211
d211
d211
d220
MP-12 (Sur/SiO 2 =0.40)
d220
MP-11 (Sur/SiO 2 =0.30)
d220
MP-10 (Sur/SiO 2 =0.25)
d220
d220
MP-2 (Sur/SiO 2 =0.20)
MP-9 (Sur/SiO 2 =0.15)
2
3
Figure 3.10
4
5
6
2θ (o )
(b)
7
8
9
10
XRD diffractograms of (a) as-synthesized mesoporous materials; (b)
calcined mesoporous materials with various Sur/SiO 2 ratios.
46
system, cationic surfactant, CTABr, and neutral surfactant, TX-100, are completely
miscible and form liquid-crystalline micellar mesophases cooperatively. Therefore,
the Ia3d bicontinuous cubic mesophase can be simply acquired in relatively low Sur/
SiO 2 ratio.
In this entire Sur/SiO 2 ratios investiga tion, there are no obvious differences in
mesophase transitions. Solitary Ia3d bicontinuous cubic MCM-48 pore system was
detected in the whole series of this study. This may be due to the range of study not
in the boundary of the transitions. However, the rapid drop of the d220 reflection
peak with the increasing of Sur/SiO 2 ratio and the partially collapse of mesophase
frameworks in Sur/SiO 2 = 0.40, indicates the Ia3d bicontinuous cubic mesophase
gradually deteriorate and transform into unstable mesophase.
3.4.2.4 Effect of Water/Silica (H2 O/SiO2 ) Ratio
Water has been used as an aqueous solvent in the synthesis of mesoporous
materials, as well as in zeolites synthesis. The H2 O/SiO 2 ratio plays a key role in the
ordering of the resulting mesophases. The XRD diffractograms of as-synthesized
and calcined mesoporous materials with various H2 O/SiO 2 ratios are illustrated in
Figure 3.11. While H2 O/SiO 2 = 50 (MP-13), XRD diffractogram of as-synthesized
sample shows that the disordered mesophase is obtained.
Whereas, Ia3d
bicontinuous cubic mesophase has been obtained in H2 O/SiO 2 = 60 (MP-14).
However, the intensity of mesopores reflection peak in both samples, MP-13 and
MP-14, has significantly decreased and the peak width becomes broader after
calcination.
The broad peak observed in MP-13 and MP-14 after calcination
indicates low degree of long-range order. In addition, the ordered mesophase has
transformed to disordered mesoporous materials upon the thermal treatment.
As the H2 O/SiO 2 ratios are between 70-80 (MP-15 and MP-2), a more
ordered Ia3d bicontinuous cubic MCM-48 mesophase are formed, as verified by the
more intense XRD pattern. After undergoing the calcinations process, the intensity
47
d211
d211
Intensity
d211
d220
d211
d220
d211
MP-16 (H2 O/SiO 2 =90)
d220
MP-2 (H2 O/SiO 2 =80)
d220
MP-15 (H2 O/SiO 2 =70)
MP-14 (H2 O/SiO 2 =60)
MP-13 (H2 O/SiO 2 =50)
2
3
4
5
6
o
2θ ( )
7
8
9
10
(a)
d211
d211
Intensity
d211
MP-16 (H2 O/SiO 2 =90)
d220
d220
d211
d211
MP-2 (H2 O/SiO 2 =80)
MP-15 (H2 O/SiO 2 =70)
MP-14 (H2 O/SiO 2 =60)
MP-13 (H2 O/SiO 2 =50)
2
3
4
5
6
2θ ( )
o
7
8
9
10
(b)
Figure 3.11
XRD diffractograms of (a) as-synthesized mesoporous materials; (b)
calcined mesoporous materials with various H2 O/SiO 2 ratios.
48
of mesopores reflection peak increases and peak width becomes more narrower. It
suggests that the mesophase is well polymerized and stable upon thermal treatment.
In addition, the mesopores system possesses high degree of long-range ordering.
When the H2 O/SiO 2 ratio increases up to 90, the well-defined Ia3d
bicontinuous cubic MCM-48 mesophase can still be achieved.
However, after
undergoing the thermal treatment, the mesopores reflection peak has drastically
dropped and all the characteristic reflection peaks for Ia3d bicontinuous peaks have
vanished instantly. Only a weak mesopores reflection peak has been detected from
the sample MP-16. Indeed, long-range mesopores ordering of sample MP-16 have
been destroyed by the thermal treatment and only a tiny number of mesopores are
left.
In low H2 O/SiO 2 ratios (H2 O/SiO 2 = 50 and 60), the surfactants concentration
is very high. Basically, deposition of silicate ions to the micelles in the solution will
induce growth and elongation of the micelles, but do not have significant
entanglement and restriction in mobility of the micellar rods [49]. In concentrated
solution, the mobility of the micellar rods is restricted. Thus, the assembly of the
micellar rods into Ia3d bicontinuous cubic array is hindered during the reaction. In
addition, the cross- linking and entanglement of the micellar rods also inhibit the
ordered Ia3d bicontinuous cubic arrangement or result in local disruptions in the
assembled structure. Therefore, disordered or low degree of ordering mesoporous
materials have been generated in this region.
The significant decrease of the
intensity of mesopores reflection peak after calcination, reveals that the restriction of
the mobility and the entanglement of the micellar rod reduce the degree of
frameworks polimerization.
In high H2 O/SiO 2 ratio (H2 O/SiO 2 = 90), the surfactants concentration is
relatively low. In low concentration solution, the isotropic surfactant molecules or
micelles may be dissociated or reorganized upon the adding of silicate species [39].
In addition, the interaction of the micelles interface and the inorganic silicate ion are
very weak due to the dilute solution, thus eventually the polymerizations of silicate
frameworks proceed relatively slow.
frameworks had been generated.
Therefore, loosely and unstable silicate
49
Conversely, H2 O/SiO 2 ratios between 70 and 80 are the optimal H2 O/SiO 2
ratio in order to obtain well-defined Ia3d bicontinuous cubic mesopores system and
well-polymerized silicate frameworks. In this condition, the micelles grow freely
with minimal entanglement. In addition, they have enough mobility to assemble in
proper Ia3d bicontinuous cubic arrays through the intermicellar silicate condensation
during the reaction [49].
3.5
Conclusion
Optimal condition to obtain highly crystalline and well-defined of purely
siliceous hexagonal MCM-41 and Ia3d bicontinuous cubic MCM-48 mesoporous
materials have been successfully achieved via mixed cationic-neutral templating
route using the cationic cetyltrimethylammonium bromide (CTABr) and neutral
Triton X-100 (TX-100) surfactants.
The amorphous rice husk ash has been
profitably utilized as an active silica source for synthesizing the ordered mesoporous
materials.
Mesoporous materials with high thermal stability were obtained for
samples which underwent pH adjustment process. Each parameter like pH value,
Na2 O/SiO 2 , Sur/SiO 2 and H2 O/SiO2 ratios, are strongly influences and interplay the
types and quality of the resulting mesophases. Indeed, it had been found that most of
the mesoporous materials obtained from the optimization experiments existed in
mixture of phases.
CHAPTER 4
QUANTITATIVE MEASUREMENT OF PHASE COMPOSITION OF CUBIC
MCM-48 AND HEXAGONAL MCM-41 PHASE MIXTURES BY USING
13
4.1
C CP/MAS NMR
Introduction
In Chapter 3, it has been demonstrated that the synthesis of mesoporous
materials MCM-48 involved various phase transitions, and thus mixtures of different
ordered phases were obtained instead of pure phases. This is due to the fact that the
properties of the resulting materials are strongly depended on the nature of the phase.
Therefore, determination of the phase purity has become a great challenge in the
characterization of mesoporous materials.
Usually, comprehensive techniques are conducted in order to provide
unambiguous structural information of mesoporous materials.
Powder X-ray
diffraction (XRD) can readily provide direct information of the pore architecture and
symmetry of mesoporous materials.
Thus, XRD analysis becomes the primary
methodology in identifying different mesophases, because the different mesophases
exhibit distinct “finger print” XRD diffraction patterns [9].
However, reflection
peaks of the XRD patterns of mesoporous materials only exhibit in low-angle range,
i.e. 2θ less than 10o , and no reflection peaks are detected at higher angles. It has
been concluded that the pore walls are mainly amorphous. In the case of mixtures of
different ordered mesophases, XRD patterns become ambiguous and not reliable to
distinguish the phase of the mesoporous materials.
Thus, XRD is in general not
51
particularly suitable for the quantitative phase composition determination for most
ordered
surfactant-templated
materials
because
of
noncrystallinity
of
their
frameworks [73].
On the other hand, phase compositions of mesoporous materials also have
been visualized by a very powerful tool, transmission electron microscopy (TEM).
TEM micrograph enables one to estimate the pore size and the pore wall thickness.
However, the interpretation of TEM data is not unambiguous because of the many
possible alignments of the ordered mesoporous specimens with respect to the
direction of the electron beam [74].
For instance, the TEM image of hexagonal
MCM-41 phase may be identical to those of a lamellar phase when imaged
perpendicular to the channels [9].
Moreover, scanning electron microscopy (SEM)
only visualizes the morphology image for the mesoporous materials [75].
Hence,
microscopy methodology are not applicable in determining the phase composition of
mesoporous materials.
Very recently, self-consistent approaches for quantification of the phase
composition of ordered mesoporous materials are proposed on the basis of gas
adsorption and thermogravimetry [76-77].
The principle is based on the
thermogravimetry weight change patterns of as-synthesized samples and by fitting
the nitrogen adsorption isotherm for the calcined mixed phase with a linear
combination of nitrogen adsorption isotherms for calcined pure mesophases.
This
integrated method is only suitable for the determination of the phase composition of
MCM-41/lamellar and MCM-48/lamellar phase mixtures.
Complicated calculations
are involved in order to validate the phase purity. However, this approach is unable
to determine the phase composition of hexagonal and cubic phase mixtures, due to it
not having distinct differences in adsorption and thermogravimetry weight change
properties.
Magic-Angle-Spinning Nuclear Magnetic Resonance (MAS NMR) has been
widely employed to investigate the structural properties of zeolites and other porous
materials.
MAS NMR function as a powerful technique to elucidate the structural
properties of amorphous materials, as well as crystalline materials, since the NMR
technique is capable of probing local atomic environments in unrivalled detail.
52
Nowadays, solid state NMR has great potentials to be the powerful characterization
tool especially for studying amorphous solids, due to the problem of sensitivity that
had been solved by the advent of super-conducting magnets and on-line computers to
accumulate the NMR signals in routine work.
Usually,
29
Si MAS NMR has been used to assign the local environments of
the Si atoms averaged throughout the lattice in the related porous materials.
Furthermore, it can extend to calculate the Si/Al ratio for the lattice, which only
accounts for the framework Al atoms [21]. On the other hand, 1 H MAS NMR has
been applied in order to study quantitatively the acidity of zeolites and related
catalysts [78].
The concentrations of non-acidic OH groups and OH groups with
different strengths of acidity in the related catalysts are able to be distinguished by
using this technique.
Recently, high-resolution
13
C and
29
Si solid state NMR spectroscopy have
been utilized to study the surfactant organization in MCM-41 mesoporous materials
[22]. Thus, in this chapter we will demonstrate a novel approach for quantification
of phase purity of MCM-48 by using
13
C CP MAS NMR spectroscopy. A series of
phases from hexagonal MCM-41 unto cubic MCM-48 was synthesized as described
in Section 3.4.2.2 and used in this study.
4.2
Results and Discussion
Figure 4.1 shows the diffraction patterns of mesophases MCM-48 and/or
MCM-41 which were synthesized by adjusting the Na2 O/SiO 2 ratio ranging from
0.20 to 0.40. While the Na2 O/SiO 2 ratios are 0.25, 0.30 and 0.35, the low angle
reflections between 2θ values of 1.5o and 10o showed a change in intensities as the
new, hexagonal MCM-41 phase started to appear. The peaks at 2θ=3.8o , 4.5o and
5.8o appear in sample (e) confirming that the sample was pure hexagonal MCM-41,
whereas the product pattern of sample (a) matched that of the pure cubic MCM-48
phase. The low-angle peaks due to the silica matrix between 2θ values of 1.5o and
53
Intensity
d211
d100
d100
d211
d220
(a)
d220
(b)
(c)
d110
d200
d210
(d)
d110d
200
d210
(e)
(f)
2
3
Figure 4.1
4
5
6
2θ (o )
7
8
9
10
X-ray diffraction (XRD) patterns of mesophases MCM-48 and/or
MCM-41 prepared by difference of the Na2 O/SiO 2 ratio; (a) 0.20, (b) 0.25, (c) 0.30,
(d) 0.35 and (e) 0.40. XRD pattern (f) was obtained by mixing samples (a) and (e)
with the composition of 50:50.
54
10o clearly show that major peaks corresponding to the Ia3d symmetry of the
MCM-48 structure are observed in sample (a) [79]. Although the peaks at 2θ=3.8o ,
4.5o and 5.8o from the long-range ordering of mesophase MCM-41 are still clearly
observed in the XRD pattern of sample (c), we cannot conclude that the sample is
pure hexagonal MCM-41, since the peaks at 2θ=1.6o and 2.7o for cubic MCM-48 are
overlapped by the XRD pattern of hexagonal MCM-41. When samples (a) and (e)
were mixed with a composition ratio of 50:50, the specific peaks for cubic MCM-48
are no longer observed (Figure 4.1 (f)). This implies that in a mixture of hexagonalcubic phase the presence of MCM-48 at a certain level could not be identified by
XRD due to the very weak reflections.
Figure 4.2 shows a series of the
13
C CP/MAS spectra; with increasing
Na2 O/SiO 2 ratios. The corresponding peaks are assigned according to the study of
Simonutti R. et al. [22]. Interestingly, it is observed that the higher the intensity of
C5 –C14 peak the lower the Na2 O/SiO 2 ratio, suggesting that the organization of the
surfactant is affected by the Na2 O/SiO 2 ratio. The interaction of the surfactant and
the MCM-48 resulted in considerable increase of the methylene chain (C5 –C14 ) peak;
which is not observed for MCM-41.
This is acceptable due to the fact that the
surfactant packing parameters g of MCM-48 and MCM-41 are different [57, 80]. In
fact, the higher and narrower the methylene chain (C5 –C14 ) peak in cubic MCM-48,
the more it indicates the mobility of the corresponding methylene chain in cubic
MCM-48 are higher than hexagonal MCM-41. The less curvature of cubic assembly
of micelles allows more spaces for the mobility of methylene chain.
should be noted that
13
However, it
C NMR peaks in the mesophase are strongly dependent upon
the contact time used for the
13
C CP/MAS NMR measurement [22]. In order to
eliminate this possibility, in this work, all
13
C CP/MAS spectra were collected using
the same contact time. Based on this phenomenon, the amount of cubic MCM-48
and hexagonal MCM-41 can be quantified by comparing the intensities of the C5 –C14
peaks. As shown in Figure 4.3, the integrated intensity ratio of C5 –C14 and C1 peaks
is then normalized to the percentage of cubic MCM-48 and hexagonal MCM-41. A
reasonably good correlation is obtained.
This calculation was made with the
assumption that the purity of samples (a) and (e) in Figure 4.1 and 4.2 was 100%.
55
+
(a)
(b)
(c)
(d)
(e)
55
Figure 4.2
50
45
13
40
35
30
25
20
15
10
ppm
C CP/MAS NMR spectra of mesophases MCM-48 and/or MCM-41
prepared with various Na2 O/SiO 2 ratios; (a) 0.20, (b) 0.25, (c) 0.30, (d) 0.35 and (e)
0.40. A contact time of 1 ms was applied.
56
MCM-41 / %
MCM-48 / %
Integrated intensity ratio of the
C5 -C14 and C1 peaks/ a.u.
Integrated intensity ratio of the C5 –C14 and C1 peaks (normalized to
Figure 4.3
percentage of mesophases MCM-48 and MCM-41), calculated from Figure 4.2, of
MCM-48 and/or MCM-41 prepared with various Na2 O/SiO 2 ratios; (a) 0.20, (b) 0.25,
(c) 0.30, (d) 0.35 and (e) 0.40.
It is thought that the CTABr can be used as a probe molecule to determine the
long-range order structure of mesoporous MCM-48 and MCM-41, after removal of
template by calcination at 823 K for 6 h. It was observed that the BET surface area
was ca. 1000 m2 g-1 , and there was no significant difference in the shape of the
nitrogen adsorption among the samples. The incorporation of CTABr into the pore
of the samples was attempted by mixing the solid sample (0.1 g), CTABr (3.87 g)
and H2 O (45 ml) at 373 K overnight and dried in an oven at 370 K. As shown in
Figure 4.4, after reinsertion of CTABr, the siliceous mesoporous samples partially
collapsed after the hydrothermal treatment by surfactant solution.
analyzed by
13
However, as
C CP/MAS NMR, as shown in Figure 4.5, the intensities of the C5 –C14
peaks of MCM-48 and MCM-41 are different. This suggests that the characteristic
of a highly interwoven and branched pore structure of MCM-48 and hexagonal array
of MCM- 41 is still maintained after reinsertion of CTABr, although the sample was
no longer well ordered.
57
d211
d100
(a)
(b)
2
3
4
5
6
7
8
9
10
o
2θ ( )
Figure 4.4
X-Ray diffraction (XRD) patterns of mesoporous (a) MCM-48 and (b)
MCM-41 after reinsertion of CTABr.
58
(a)
(b)
55
Figure 4.5
50
45
13
40
35
30
25
20
15
10
ppm
C CP/MAS NMR spectra of mesoporous (a) MCM-48 and (b)
MCM-41 after reinsertion of CTABr. A contact time of 1 ms was applied.
59
4.3
Conclusion
This work has demonstrated that quantification of a mixture of cubic MCM48 and hexagonal MCM-41 mesophases is possible by the interpretation of their
13
C
CP/MAS NMR spectra, which cannot be determined by X-ray diffraction techniques.
However, the availability of pure phases of MCM-48 and MCM-41 greatly facilitates
the quantitative analysis.
Although, at present, this method is only for semi-
quantitative measurement of mesophase samples, we expect that this technique is
applicable to a wide range of mesoporous structures.
CHAPTER 5
TAILORING THE ALUMINOSILICATE Al-MCM-48 MESOPOROUS
MOLECULAR SIEVES AS CATALYSTS FOR FRIEDEL-CRAFTS
REACTION
5.1
Introduction
Purely siliceous MCM-48 mesoporous materials have limited practice,
because it lacks of intrinsic acidity and ion exchange capacity.
electrically neutral structure.
This is due to its
In order to utilize this material for catalysis, it is
necessary to generate the appropriate catalytically active sites to the silicate
framework regarding the application.
Therefore, suitable amount of metallic
elements have to be incorporated in the wall structure.
At present, mesostructures
containing aluminium [18, 60, 81-87], titanium [88-89], iron [90-91], zinc [92],
vanadium [93-94], zirconium [95], nickel [91], cobalt [91], ruthenium [96], thorium
[97] and uranium [97] have been synthesized to endow the mesoporous materials
with desirable properties.
In this study, aluminium had been incorporated into Si-MCM-48 in order to
generate Brönsted sites because the presence of this active sites have been shown to
catalyze the Friedel-Crafts reactions in heterogenous catalysis, as proposed by
Gauthier et al. [98].
61
5.2
Post-Synthesis Route to Mesoporous Al-MCM-48 Materials
Tremendous variety of materials has been prepared using all the possibilities
of inclusion chemistry to introduce catalytically active species in mesoporous silica
guest materials.
These include two major approaches, either by direct synthesis
method or post-synthesis methods such as ion-exchange, impregnation, adsorption,
grafting of reactive metal complexes and deposition of clusters or layers of metal
oxides or of metal clusters [99]. In can be seen that, the mesostructured pore lattice
yields a clear advantage by providing enormous space for insertion of active sites.
For direct synthesis method, aluminium source was added into the gel mixture during
the synthesis process. Whereas, in the post-synthesis method, aluminium source was
added after the completion of synthesis of the parent materials.
Both approaches have its advantages and disadvantages. For instance, direct
synthesis method provides simple working up procedures.
However, the
incorporation of aluminium into framework often leads to irreproducible results [32].
The results also indicate that the degree of ordering in the aluminated mesoporous
materials deteriorates even at relatively low levels of aluminium incorporation.
Furthermore, a significant number of the aluminium (active sites) has migrated inside
the pore walls region through direct synthetic route [100]. Eventually, it affects the
efficiency of the catalysts.
Conversely, the virtue of post-synthesis method is that the aluminated
mesoporous materials can be easily prepared with all aluminium incorporated
tetrahedrally in the framework and a framework Si/Al ratio as low as 1.9 [18]. The
resulting active sites are accessible for interaction with organic substrate [100].
However, the siliceous parent materials are hydrothermally unstable, which will lead
to severe destroying of mesostructure ordering upon exposure to aqueous
environment during hydrothermal synthesis.
Furthermore, in some post synthesis
approaches, the active sites are readily leached from the framework after undergoing
the reaction [31].
62
In this chapter, we will demonstrate two different post-synthesis approaches
to prepare aluminosilicate MCM-48 catalysts, that are; (1) isomorphous substitution
of aluminium into mesoporous Si-MCM-48 molecular sieves (in calcined form) via
treatment with sodium aluminate; (2) addition of an aqueous solution of sodium
aluminate into the reactant mixture after completing the formation of surfactantsilicate mesostructure of MCM-48. Herein, in order to clarify the synthesis method,
the former method will be denoted as post-synthesis alumination of mesoporous
Si-MCM-48 and the later will be denoted as post-synthesis alumination of
Si-MCM-48 mesophase. The former method was first proposed by Hamdan et al.
[18], where Al-MCM-41 with Si/Al as low as 1.9 has been prepared. The two
different approaches of post-synthesis are expected to contribute considerable
differences in pore structure and active sites of cubic MCM-48. The samples will be
characterized by using powder X-ray diffraction (XRD), Fourier Transform Infrared
(FTIR), 27 Al MAS NMR spectroscopy, and nitrogen adsorption measurement.
5.3
Results and discussion
5.3.1
Post-Synthesis Alumination of Mesoporous Si-MCM-48
(a)
X-ray Diffraction (XRD)
Figure 5.1 presents the XRD patterns of the parent Si-MCM-48 and the
aluminated samples through post-synthesis alumination of mesoporous Si-MCM-48
with different concentrations of NaAlO 2 .
Highly ordered purely siliceous cubic
Si-MCM-48 mesoporous molecular sieves (MP-5), which have been discussed
previously in Chapter 3, were employed as parent materials for the post-synthesis
alumination.
As reported previously, the optimal conditions for post-synthesis
alumination of mesoporous Si-MCM-48 are in 0.10 M, 0.25 M, and 0.50 M of
aqueous solutions of NaAlO 2 at 60 o C for 3 h [18].
63
Intensity
050Al-MCM-48
025Al-MCM-48
010Al-MCM-48
Si-MCM-48
1.5
2
3
4
5
6
7
8
9
2θ (o )
Figure 5.1
XRD patterns of the parent Si-MCM-48 and its aluminated samples
through secondary synthesis with different concentrations of NaAlO 2 .
10
64
According to the XRD patterns display in Figure 5.1, the intensities of the
d211 reflection peak have decreased gradually with the increase of the concentrations
of NaAlO 2 aqueous solutions.
Endud S.
Based on the proposed alumination mechanism by
[101] as shown in Figure 5.2, the excessive dissolution of siliceous
frameworks during alumination would deteriorate the long-range structural ordering
of the parent Si-MCM-48. In addition, the several unresolved peaks between 2θ =
3.5o -5.5o have disappeared after alumination.
This gives the evidence that high
degree of long-range structural integrity has been distorted upon alumination.
The
distortion of the structural ordering was critical when the concentration of NaAlO 2
aqueous solution achieved 0.50 M.
However, the presence of significantly intense
d211 reflection peak indicates the cubic mesoporous system of MCM-48 is maintained
during the secondary synthesis.
However, the d211 reflection peak has been visibly shifted towards higher 2θ
angles with an increase of concentrations of NaAlO 2 solutions.
This suggests the
contraction of the unit cell parameter (refer to Table 5.1) during alumination.
No
significant difference of contraction of unit cell parameter was observed between the
010Al-MCM-48 and 025Al-MCM-48 samples.
At the same time, the intensity of
d211 reflection peak of these two samples (Figure 5.1) also does not show obvious
difference. However, the unit cell parameter of 050Al-MCM-48 has been detected
slightly lower than 010Al-MCM-48 and 025Al-MCM-48 samples. It was observed
that the intensity of d211 reflection peak of 050Al-MCM-48 (Figure 5.1) is
significantly lower than 010Al-MCM-48 and 025Al-MCM-48. Therefore, it can be
suggested that the contraction of unit cell parameter was related to the degree of
crystallinity of the framework lattice.
Figure 5.2
Mechanism of post-synthesis alumination of mesoporous MCM-41 [101].
65
66
Table 5.1: Unit cell parameters of Al-MCM-48 prepared from the purely siliceous
Si-MCM-48 with different concentrations of NaAlO 2 aqueous solution. The unit cell
parameter has been calculated from the interplanar spacing using the formula
ao =d211 √6.
Sample/Code
Molar concentration of NaAlO 2
Unit cell parameter (Å)*
Si-MCM-48 (MP-5)
-
95.4
010Al-MCM-48
0.10
89.4
025Al-MCM-48
0.25
89.0
050Al-MCM-48
0.50
86.2
*Calculated from calcined sample
(b)
Fourier Transform Infrared Spectroscopy (FTIR)
Figure 5.3 shows the FTIR spectra of parent Si-MCM-48 and the aluminated
samples via post-synthesis alumination of mesoporous Si-MCM-48 with different
concentrations of NaAlO 2 . The FTIR spectra demonstrate that all of the Al-MCM-48
samples exhibit similar framework vibrations with its parent materials Si-MCM-48.
However, compared to the parent Si-MCM-48, the lattice vibration bands of
Al-MCM-48 samples were shifted to lower wavenumbers after the incorporation of
aluminium.
These shifts are due to the increase of the mean T-O distances in the
walls caused by the substitution of the small silicon atom (rSi4+ = 0.26Å) by the larger
aluminium atom (rAl3+ = 0.39Å) [34].
27
(c)
Al MAS NMR Spectroscopy
The species of aluminium in the aluminated samples is further characterized
by
27
Al MAS NMR spectroscopy.
Figure 5.4 demonstrates the
27
Al MAS NMR
spectra of the calcined aluminated Al-MCM-48 samples. The 27 Al MAS NMR
67
(d)
(c)
(b)
Transmittance (%)
1027
1032
1052
797
967
(a)
461
1239
1084
1500
1400
Figure 5.3
1300
1200
1100 1000 900
wavenumber (cm-1 )
800
700
600
500
400
FTIR spectra of the parent and samples prepared via post-synthesis of
mesoporous Si-MCM-48 with different concentrations of NaAlO 2 ; (a) purely
siliceous Si-MCM-48 (MP-5), (b) 0.10 M, (c) 0.25 M, and (d) 0.50 M.
68
Tetrahedral Al
Intensity / a.u.
(c)
(b)
(a)
100
Figure 5.4
27
50
0
ppm
Al MAS NMR spectra of Al-MCM-48 prepared via post-synthesis
alumination of mesoporous Si-MCM-48 with different concentrations of NaAlO 2 ; (a)
0.10 M, (b) 0.25 M, and (c) 0.50 M.
spectra exclusively exhibit solitary intense peak at 50 ppm.
This peak reveals that
aluminium species in all of the samples is in tetrahedral environment, i.e., in the
framework position.
The similar intense peak at 50 ppm for all of the samples
suggests that the aluminium can easily be incorporated into the framework of parent
Si-MCM-48 on the NaAlO 2 concentrations from 0.10 M to 0.50 M. Interestingly,
Endud S. [101] had reported that using similar method with MCM-41 gave both
69
octahedral and tetrahedral species. Hence, it can be suggested that aluminium can be
easily substituted into framework of 3-dimentional cubic MCM-48 compared to
mono-dimensional MCM-41.
It has been observed that the intensity of the peak
increase with the increasing of the concentration of NaAlO 2 aqueous solutions.
It
reveals that the incorporation of Al into the Si-MCM-48 framework has increased
with the increase of the concentration of NaAlO 2 solutions.
However, this is
accompanied by the structural distortions as indicated by the XRD patterns shown in
Figure 5.1.
Thus, it is suggested that the post-synthesis alumination of mesoporous
Si-MCM-48 can be achieved using low concentration of NaAlO 2 solutions, since the
aluminium can be incorporated into the framework as effective as using high NaAlO 2
concentrations.
(c)
Nitrogen Adsorption Measurement
The surface properties of the samples were measured by nitrogen adsorption
experiment at 77 K.
Figure 5.5 presents the nitrogen adsorption-desorption
isotherms and its BJH pore size distribution curve of the calcined samples.
The
nitrogen adsorption-desorption isotherm of purely siliceous Si-MCM-48 shows the
sample possesses the typical irreversible type IV adsorption isotherm with type H4
hysteresis loop in accordance with IUPAC recommendations [23]. This indicates the
resultant Si-MCM-48 materials possess uniform slit-shaped mesopores.
The
adsorption branch of isotherm exhibits a sharp inflection at relative pressures of
0.30<P/P o <0.36, which corresponds to capillary condensation of nitrogen inside the
mesopores. The position within the inflection point is related to the diameter of the
pore. Based on the desorption branch of the N2 isotherm, the pore size distribution
(PSD) of the materials was calculated using the method proposed by Barrett-JoynerHalenda (BJH) [102]. From the PSD curve, it is demonstrated that the Si-MCM-48
materials possess a remarkably narrow pore size distribution centred on 26 Å.
However, it is noteworthy that the BJH method underestimates the calculated pore
size typical of M41S mesoporous materials by ca. 10 Å [35]. By taking into account
the underestimated value of the calculated pore size, the pore size diameters of
Si-MCM-48 are closed to that of the CTABr surfactant micelle (39.7 Å) [103].
600
100
1000
Pore Diameter, (Å)
500
400
300
200
100
0
0
0.2
0.4
0.6
0.8
Relative Pressure (P/P o )
450
400
350
300
250
200
150
100
50
0
1
Pore Area, (m2 /g)
700
Volume Adsorbed (cm3 /g, STP)
800
26 Å
Pore Area, (m2 /g)
Volume Adsorbed (cm3 /g, STP)
70
1000
100
Pore Diameter, (Å)
0
300
1000
100
Pore Diameter, (Å)
250
200
150
100
180
160
140
120
100
20
0
0
0.2
0.4
0.6
0.8
Relative Pressure (P/P o )
(c)
Figure 5.5
0.2
0.4
0.6
0.8
Relative Pressure (P/P o )
(b)
1
N2 adsorption-desorption isotherms (
38 Å
360 Å
1000
100
Pore Diameter, (Å)
0
0.2
0.4
0.6
0.8
Relative Pressure (P/P o )
(d)
adsorption,
desorption)
and its BJH pore size distribution curve (inset) of the parent Si-MCM-48 and its
aluminated samples through post-synthesis alumination of mesoporous Si-MCM-48
with different concentrations of NaAlO 2 ; (a) purely siliceous Si-MCM-48 (MP-5),
(b) 0.10 M, (c) 0.25 M, and (d) 0.50 M.
1
80
60
40
50
0
Pore Area, (m2 /g)
350
Volume Adsorbed (cm3 /g, STP)
400
Pore Area, (m2 /g)
Volume Adsorbed (cm3 /g, STP)
(a)
24 Å
23 Å
1
71
For aluminated samples 010Al-MCM-48 and 025Al-MCM-48, their nitrogen
adsorption-desorption
isotherms
still
displayed
adsorption isotherm with type H4 hysteresis loop.
typical
irreversible
type
IV
However, the sharp vertical
inflection around 0.30<P/P o <0.36 is absent for both samples, which may be
attributed to the pore size heterogeneity of the samples [104]. The heterogeneity of
the pore size distribution is further evidenced by the PSD curves, which are broader
in both samples compared to the parent Si-MCM-48. The heterogeneity of the pore
size may be due to the presence of aluminium species on the walls of the pore
channels.
On the other hand, the nitrogen adsorption-desorption isotherm of 050AlMCM-48 is classified as type II adsorption isotherm with type H3 hysteresis loop,
which is characteristic of non-uniform macroporous texture.
Other than a narrow
pore size distribution curve centred around 38 Å, a noticeable broad pore size
distribution curve centred around 360 Å has also been detected. It was possible that
during the alumination in 0.50 M NaAlO 2 aqueous solution, the ordered mesopore
systems of parent Si-MCM-48 materials was damaged severely as indicated by its
XRD pattern (Figure 5.1).
As a result, the ordered mesopores had been partially
transformed to macroporous amorphous materials.
mesopore system was maintained after alumination.
Only a small portion of the
Besides, the broad pore size
distribution curve centred on 360 Å may be attributed to the secondary mesopores
that had been created by aggregation or agglomeration of large particles.
Besides, the porosity of the materials had been investigated through the α splot method proposed by Sing et al. [105]. Non-porous silica TK-800 was served as
reference material to construct the α s-plot. Figure 5.6 presents the α s-plot of parent
Si-MCM-48 and the its aluminated samples through secondary synthesis with
different concentrations of NaAlO 2 .
The α s-plot of samples Si-MCM-48, 010Al-
MCM-48, and 025Al-MCM-48, show a pronounced upward deviation from linearity
at α s~0.85, indication of the capillary condensation, which is the characteristic of
mesopores materials.
Therefore, the α s-plots further confirmed that these samples
consist of mesopores.
On the contrary, a mere straight line through the origin is
observed from the α s-plot of 050Al-MCM-48.
72
250
Volume Adsorbed (cm3 /g, STP)
Volume Adsorbed (cm3 /g, STP)
500
400
300
200
100
200
150
100
0
0
0
0.5
1
0
1.5
0.5
αs
αs
(a)
(b)
1
1.5
1
1.5
35
200
Volume Adsorbed (cm3 /g, STP)
Volume Adsorbed (cm3 /g, STP)
50
150
100
50
30
25
20
15
10
5
0
0
0
0.5
αs
1
(c)
Figure 5.6
1.5
0
0.5
αs
(d)
The α s plots of (a) Si-MCM-48, (b) 010Al-MCM-48, (c) 025Al-
MCM-48, and (d) 050Al-MCM-48.
73
Table 5.2 shows the sorption properties of the parent Si-MCM-48 and the
aluminated samples prepared through post-synthesis alumination of mesoporous
Si-MCM-48. The parent Si-MCM-48 shows high BET surface area of 1058 m2 /g
and pore volume of 1.17 cm3 /g. For the other aluminated samples, the BET surface
areas and the pore volume are progressively decreased with the increasing of the
concentrations of NaAlO 2 . These results are in the agreement with the XRD data in
Figure 5.1, which revealed that the framework of parent Si-MCM-48 has been
destroyed gradually with the increase of the concentrations of NaAlO 2 .
The
mesoporous framework and pore system of the parent Si-MCM-48 materials have
been damaged severely in 0.5 M NaAlO 2 as evidenced by the XRD and nitrogen
adsorption analysis.
However, it is noteworthy that the unimodal pore system of
010Al-MCM-48 and 025Al-MCM-48 is still retained.
Table 5.2: Sorption properties of the parent Si-MCM-48 and the aluminated samples
prepared via post-synthesis alumination of mesoporous Si-MCM-48.
BET surface
BJH desorption pore
BJH desorption pore
area (m2 /g)
volume (cm3 /g)
diameter (Å)
Si-MCM-48
1058
1.17
26
010Al-MCM-48
577
0.68
23
025Al-MCM-48
435
0.58
24
050Al-MCM-48
300
0.24
38 and 360
Sample
5.3.2
Post-Synthesis Alumination of Si-MCM-48 Mesophase
(a)
X-ray Diffraction (XRD)
Figures 5.7 (a) and
(b) show the XRD patterns of as-synthesized and
calcined samples of Al-MCM-48 with various Si/Al ratios prepared via postsynthesis alumination of Si-MCM-48 mesophase.
It is noted that the Si/Al ratio
mentioned here refers to the Si/Al ratio in the gel compositions. The XRD patterns
2
d220
d211
Intensity
3
10
2
(a)
4
5
(b)
6
2θ (o )
7
8
XRD patterns of the (a) as-synthesized and (b) calcined Al-MCM-48 via post-synthesis alumination.
3
Al-MCM-48-20
Al-MCM-48-20
9
Al-MCM-48-30
Al-MCM-48-30
8
Al-MCM-48-50
Al-MCM-48-50
7
Al-MCM-48-100
d220
d211
Al-MCM-48-100
5
6
o
2θ ( )
Figure 5.7
4
Intensity
9
10
74
75
of entire samples exhibit a sharp d211 Bragg reflection, a weak d220 Bragg reflection
shoulder, and several unresolved peaks between 2θ = 3.5o -5.5o . The XRD patterns of
all Al-MCM-48 samples are analogous to purely siliceous Si-MCM-48 (Figure 5.1),
which can be classified as Ia3d bicontinuous cubic phase (refer Chapter 3 for the
detail Miller-Bravais index) [9].
The similarity of the XRD patterns between
Al-MCM-48 and Si-MCM-48 (Figure 5.1) suggests that no massive structural change
occurs after introduction of aluminium source during the post-synthesis alumination
of the Si-MCM-48 mesophase. In addition, ti is observed that the intensities of the
calcined samples increase rapidly in comparison with the as-synthesized samples.
This implies that the samples possess high thermal stability and structural distortions
of the samples are negligible.
The results of the XRD analysis of the samples are listed in Table 5.3. The
data in Table 5.3 reveal that calcinations of the samples leads to a contraction of the
unit cell parameter, due to the condensation of Si-OH groups upon thermal treatment
[83]. The contractions of the unit cell parameter of all samples are relatively small,
which further proves the high thermal stability of the samples.
It is noted that the
cubic unit cell parameter of Al-MCM-48 samples are slightly similar to those of
Si-MCM-48 sample, revealing that incorporation of aluminium into the
Table 5.3
Unit cell parameters of Si-MCM-48 and Al-MCM-48 prepared from
post-synthesis alumination.
The unit cell parameter has been calculated from the
interplanar spacing using the formula ao =d211 √6.
Samples
d211 spacing (Å)
Unit cell parameter (Å)
As-
Calcin
As-
Calcined
Contraction
synthesized
ed
synthesized
Si-MCM-48
40.59
38.77
99.4
95.0
4.4
Al-MCM-48-20
39.98
37.18
97.9
91.1
6.9
Al-MCM-48-30
39.70
37.38
97.3
91.2
6.3
Al-MCM-48-50
39.94
37.49
97.8
91.8
6.1
Al-MCM-48-100
39.90
37.86
97.7
92.7
5.1
(%)
76
of MCM-48 through post synthesis alumination of Si-MCM-48 mesophase does not
affect the structural stability of the mesophases.
(b)
Fourier Transform Infrared Spectroscopy (FTIR)
Figure 5.8 shows the FTIR spectra of aluminosilicates Al-MCM-48 prepared
by post-synthesis alumination of Si-MCM-48 mesophase with various Si/Al ratios.
The FTIR spectra of the aluminosilicate Al-MCM-48 samples present similar
framework vibrations with purely siliceous Si-MCM-48, as discussed in Chapter 3.
Moreover, the lattice vibration bands of Al-MCM-48 samples were shifted to lower
wave numbers in comparison with purely siliceous Si-MCM-48.
The shifts of
wavenumbers increase with the decrease of Si/Al ratios, as a result of the increment
of the mean T-O distances.
(c)
27
Al MAS NMR Spectroscopy
Figure 5.9 depicts the
27
Al MAS NMR spectra of the calcined aluminosilicate
Al-MCM-48 samples. The Al-MCM-48 samples with various Si/Al ratios show an
absolute intense peak at 50 ppm in the
27
Al MAS NMR spectra. This proves that the
aluminium species in all of the samples is tetrahedrally coordinated to the
framework. This reveals that aluminium can be incorporated into the framework of
MCM-48 via both post-synthesis alumination approaches.
approaches
have
successfully
framework of MCM-48.
incorporated
aluminium
Both post-synthesis
tetrahedrally
into
the
The framework aluminium of MCM-48 provides the
Brönsted acid sites which are active as acid catalyst. Therefore, it is suitable to be
used as catalyst in Friedel-Craft reactions.
77
(a)
(b)
Transmittance (%)
(c)
1072
(d)
1076
805
965
460
1079
1080
1500 1400
1300 1200 1100 1000 900 800
wavenumber (cm-1 )
700
600
500
Figure 5.8
FTIR spectra of aluminosilicates Al-MCM-48 samples prepared by
post-synthesis alumination with various Si/Al ratios; (a) 20, (b) 30, (c) 50, and
(d) 100.
400
78
Tetrahedral Al
Intensity / a.u.
(d)
(c)
(b)
(a)
100
Figure 5.9
27
50
0
ppm
Al MAS NMR spectra of the calcined aluminosilicate Al-MCM-48
samples prepared by post-synthesis alumination with various Si/Al ratios; (a) 20, (b)
30, (c) 50, and (d) 100.
(d)
Nitrogen Adsorption Measurement
Figure 5.10 shows the nitrogen adsorption-desorption isotherms for calcined
samples and their corresponding BJH pore size distribution (PSD) curves.
The
obtained isotherms demonstrated that all of the samples display the typical type IV
adsorption isotherm with H3 hysteresis loop, as identified by IUPAC [23]. These
results reveal that the samples possess slit-shaped mesopores. However, it noted that
the hysteresis loops of aluminium containing MCM-48 are larger and more definite
than Si-MCM-48 (Figure 5.5 (a)).
This suggests that the mesopore volume of
Al- MCM-48 samples from post-synthesis alumination are higher than Si-MCM-48.
Pore Area, (m2 /g)
1000
38 Å
Volume Adsorbed (cm3 /g, STP)
1200
26 Å
Pore Area, (m2 /g)
Volume Adsorbed (cm3 /g, STP)
79
1200
1000
800
100
1000
Pore Diameter, (Å)
600
400
200
800
26 Å
38 Å
1000
100
Pore Diameter, (Å)
600
400
200
0
0
0
0.2
0.4
0.6
0.8
Relative Pressure (P/P o )
1
0
0.2
0.4
0.6
0.8
Relative Pressure (P/P o )
38 Å
1000
800
100
1000
Pore Diameter, (Å)
600
400
200
Pore Area, (m2 /g)
1000
Volume Adsorbed (cm3 /g, STP)
1200
(b)
26 Å
Pore Area, (m2 /g)
Volume Adsorbed (cm3 /g, STP)
(a)
1
800
26 Å
38 Å
100
1000
Pore Diameter, (Å)
600
400
200
0
0
0
0.2
0.4
0.6
0.8
Relative Pressure (P/P o )
1
0
0.2
0.4
0.6
0.8
Relative Pressure (P/P o )
(c)
Figure 5.10
N2 adsorption-desorption isotherms (
(d)
adsorption,
desorption)
and their corresponding pore size distribution curve (inset) of aluminosilicate AlMCM-48 samples prepared by post-synthesis alumination with various Si/Al ratios;
(a) 20, (b) 30, (c) 50, and (d) 100.
1
80
Further investigation shows that the desorption isotherms branch of the
aluminosilicate
Al-MCM-48
samples
exhibit
two
sharp
vertical
inflections
characteristic of capillary condensation at relative pressures of 0.30<P/P o <0.36 and
0.47<P/P o <0.50.
The multi-step of vertical inflection illustrates that uniform pore
filling occurs within multi-level of narrow pore size distributions as reported
previously [106].
Therefore, two sharp vertical inflections in desorption branch of
aluminosilicate Al-MCM-48 samples indicate its pore size distributions are clustered
around two areas.
From the plot of BJH pore size distribution curves, it is
comfirmed that aluminosilicate Al-MCM-48 samples possess two distinguishable
pore size distributions centered around ca. 26 Å and 38 Å. It is noted that the pore
volume of the larger mesopore (38 Å) increases with the increase of aluminium
content, indicating that the amount of aluminium plays an important role in the
formation of these mesopores.
Therefore, the narrow peak of PSD curve in
mesopore region gives the evidence that dual well-ordered and uniform pore
structure of Al-MCM-48 mesoporous materials have been created during the post
synthesis alumination.
The mesoporosity of the aluminosilicate Al-MCM-48 samples are further
supported by the α s-plot.
Figure 5.11 shows the α s-plot of the aluminosilicate
Al-MCM-48 samples through post-synthesis alumination of the Si-MCM-48
mesophase.
The α s-plots of all Al-MCM-48 samples present a prominent upward
deviation from linearity at α s~0.85 which reflects that the capillary condensation has
occurred within the uniform pore size.
Undoubtedly, this result suggests that the
Al-MCM-48 samples are characterized by mesopores texture, as well as purely
siliceous Si-MCM-48 texture.
The sorption properties for Si-MCM-48 and Al-MCM-48 prepared through
post-synthesis alumination of Si-MCM-48 mesophase are listed in Table 5.4. It can
be seen that all of the samples possess high BET surface areas and the values are not
significantly different from each other.
Therefore, it can be concluded that the
addition of aluminium did not influence the surface areas.
However, the pore
volume of aluminium containing MCM-48 mesoporous materials is significantly
higher than Si-MCM-48. The average pore volume increases around 29%-39% after
81
Volume Adsorbed (cm3 /g, STP)
Volume Adsorbed (cm3 /g, STP)
600
500
400
300
200
100
0
600
500
400
300
200
100
0
0
0.5
1
1.5
0
0.5
αs
(a)
1.5
1
1.5
(b)
700
Volume Adsorbed (cm3 /g, STP)
600
Volume Adsorbed (cm3 /g, STP)
αs
1
500
400
300
200
100
0
600
500
400
300
200
100
0
0
0.5
1
αs
(c)
Figure 5.11
1.5
0
0.5
αs
(d)
The α s plots of aluminosilicate Al-MCM-48 samples prepared
through post-synthesis alumination with different Si/Al ratios; (a) 20, (b) 30, (c) 50,
and (d) 100.
82
Table 5.4: Sorption properties of Si-MCM-48 and Al-MCM-48.
Total Pore
% Increase of
3
volume (cm /g)
pore volume*
26
1.17
-
991
26 and 38
1.58
35
Al-MCM-48-30
983
26 and 38
1.63
39
Al-MCM-48-50
975
26 and 38
1.56
33
Al-MCM-48-100
1043
26 and 38
1.51
29
Samples
BET surface
Pore diameter
2
area (m /g)
(Å)
Si-MCM-48
1058
Al-MCM-48-20
*The % increase of pore volume is calculated based on pore volume of siliceous
MCM-48
addition of aluminium. The increase of pore volume may be attributed to the larger
pore (pore centred at ca. 38 Å) that was created by the incorporation of aluminium.
Based on the above results, it can be proposed that aluminosilicate
Al-MCM-48 samples prepared via post-synthesis alumination of Si-MCM-48
mesophase possessing hierarchical mesoporosity, are made up of two types of
narrow porous systems, ordered Ia3d bicontinuous cubic and narrow but disordered
pore system.
The nitrogen adsorption-desorption analyses show that the entire
Al-MCM-48 samples exhibit dual narrow pore size distributions.
Nevertheless, the
XRD diffractograms only show Ia3d bicontinuous cubic diffraction pattern and no
overlapping of Bragg diffraction peaks have been detected.
It is known that only
ordered arrangement of mesoporous pore system will give the Bragg diffraction
peak, and hence a disordered pore system will not contribute to the diffraction peak
at low 2θ angle.
From the experimental design, it is known that well-defined and
ordered Ia3d bicontinuous cubic pore system (Si-MCM-48) had formed completely
prior to the addition of the aluminium source. Thus, it can be inferred that the pore
size distribution centered at ca. 26 Å is due to the bicontinuous Ia3d cubic pore
83
system, because of the similar pore size distribution to Si-MCM-48.
This is
supported by the fact that the contraction of the unit cell of Al-MCM-48 samples is
similar to that of Si-MCM-48 (see Table 5.3).
(e)
Field Emission Scanning Electron Microscopy (FESEM)
Figure 5.12 and 5.13 displays the FESEM micrograph of Si-MCM-48 and
Al-MCM-48-50, respectively.
The FESEM micrograph reveals that the particle
morphology of both samples consists of agglomerated uniform sphere.
Figure 5.12
Figure 5.13
FESEM micrograph of Si-MCM-48.
FESEM micrograph of Al-MCM-48-50.
84
5.3.3
Proposed Mechanism
Although a novel bimodal pore structure MCM-48 has been successfully
synthesized through post synthesis alumination of Si-MCM-48 mesophase, the
mechanism of the formation of secondary mesopores is not well understood.
However, Ryoo et al. [107] had reported that the aluminium would be incorporated
in two different regions simultaneous, in the surfactant-silicate mesostructure and the
outer-region of MCM-48 particles, if the aluminium source is added after completing
the formation of surfactant-silicate mesostructures.
The diffusion of aluminium into the surfactant-silicate mesostructure was
reported to be time consuming. Therefore in this experiment, a diffusion time of 7
days has been undertaken in order to allow the aluminium to diffuse greatly into the
pre-formed surfactant-silicate mesostructure of MCM-48.
It was expected that
framework aluminium inside the pore would be generated after the substitution of
CTA+ with Al3+.
It has been shown that the outer region of the MCM-48 particles consists of
significant amount of silanol groups.
The aluminium can be inserted into the
framework of MCM-48 in tetrahedral coordination through interaction of silanol
groups with the aluminate ions.
In this order, the proposed mechanism of post-
synthesis alumination is demonstrated in Figure 5.14.
It is proposed that the larger mesopores centred at ca. 38 Å are probably
created by adjoining of the primary particles by framework aluminium on outer
surface of the particles and followed by random cross-linking between these
particles. Based on the BJH pore size distribution curve, the degree of cross-linking
can be correlated to the number of aluminate ions. In fact the number of framework
aluminium in the outer region of MCM-48 particles is strongly dependent on the
concentration of aluminate ions. Therefore, the secondary pore volume (pore centred
at 38 Å) is increased by increasing the number of aluminate ions, since the quantity
of framework aluminium in the outer region of the MCM-48 particles determines the
degree of cross-linking of the particles. Schematic illustration of the formation of the
secondary mesopore has been illustrated in Figure 5.15.
O
O
O
O
Si
Si
Si
Si
Si
Si
Si
+ 2 H2 O
Si
Si
Si
Si
O
O
O
O
O
O
O
Si
Si
Si
Si
Si
O
Si
O
O
Si
O
O
O
OH
OH2 +
+
+
Si
Si
Si
Si
Al(OH)4-
O- +
O-
O- +
O-
+ Al(OH)4 -
OH2 +
OH
Si
Si
Si
Si
O
O
O
O
O
O
O
Si
Si
Si
Si
Surfactant Rod
Proposed mechanism of post-synthesis alumination of Si-MCM-48 mesophase.
OH
OH
OH
OH
Outer Surfaces
Figure 5.14
O
O
O
Si
Silica Walls
O
Si
Silica Walls
Al(OH)4-
Al
OH
O
O
OH
OH2
+
OH2 +
+ 2 H2 O
85
85
Si-MCM-48
+
Figure 5.15
Al3+
Schematic illustration of formation mechanism of secondary mesopores.
Narrow but disordered mesopore
86
86
87
5.4
Conclusion
Aluminium
has
been
successfully
incorporated
tetrahedrally
into
the
framework structure of MCM-48 via post-synthesis alumination of mesoporous
Si-MCM-48 using sodium aluminate as the aluminium source.
The cubic pore
system of its parent Si-MCM-48 is well retained in 0.10 M and 0.25 M solutions of
sodium aluminate at 60o C for 3 h. The BJH pore size distribution shows the resultant
Al-MCM-48 possesses narrow pore size distribution, which is identical to its parent
Si-MCM-48.
However, bimodal mesoporous Al-MCM-48 with interconnected hierarchical
structure has been synthesized via post synthesis alumination of Si-MCM-48
mesophase. Two types of pore systems; ordered bicontinuous Ia3d cubic MCM-48
pore system and narrow but disordered pore system centered at 26 Å and 38 Å have
been generated, respectively.
The structure of ordered bicontinuous Ia3d cubic Al-
MCM-48 is well resolved in the Si/Al ratios ranking from 20 unto 100 (gel ratio).
The
27
Al MAS NMR spectra demonstrate that the aluminium is incorporated
tetrahedrally into the framework of MCM-48.
CHAPTER 6
CATALYTIC ACTIVITY OF ALUMINATED MCM-48 MOLECULAR
SIEVES IN THE FRIEDEL-CRAFTS ACYLATION OF
2-METHOXYNAPHTHALENE WITH ACETYL CHLORIDE
6.1
Introduction
Acidity plays an important role as active sites in catalysts. The nature of the
acidity of the catalysts frequently determines the function of the catalysts. Generally,
the surface silanol groups of purely siliceous mesoporous materials are isolated and
non- interacting, and hence are catalytically inactive [108]. However, the framework
substitution of aluminium for silicon during hydrothermal synthesis was performed
to pave the way for the generation of Brönsted acid sites, which are accepted to be
the active site for many important reactions.
Endowing the acidic properties to the large uniform pores in mesoporous
molecular sieves have undoubtedly opened up a new era in solid acid catalysis. It
has greatly extended the application of zeolites to deal with large molecules whose
diffusion are strongly impeded in the micropores of zeolites [109].
More
importantly, the acidity of ordered mesoporous aluminosilicates was reported to be
mild, which are lower than that of zeolites and similar to that of amorphous silicaalumina [110]. It appears that the best possibilities for these materials in catalysis
will be in processes needing moderate acidity and involving bulky size molecules.
Thus, processes that can take advantage of the unique properties of the mesoporous
materials is the catalytic transformations of oxygen-containing compounds and
hydrocarbons with emphasis on dehydration and derivatization reactions of
89
monohydroxy compounds, rearrangements of aliphatic and diaryl diols, ring opening
and isomerization of epoxides and Friedel-Crafts alkylation/acylation reaction of
hydrocarbons.
Also, the mild acidity of mesoporous aluminosilicates can reduce the
formation of carbonaceous deposits (“coke”) and prevent the unwanted side reactions
during the transformation of organic compounds over acid catalysts.
Subsequent to the successful synthesis of mesoporous aluminosilicate
Al-MCM-48 as described in Chapter 5, it is of interest to investigate the nature of the
acidity of the Al-MCM-48 in order to develop a better understanding for prediction
of the catalytic activity and selectivity of Al-MCM-48. Also, it might be interesting
to use this material as catalyst in Friedel-Crafts acylation reactions.
6.2
Generation of Active Sites in Al-MCM-48
In purely siliceous mesoporous materials, the SiO 4 unit of the framework is
electronically neutral due to the +4 charge of Si and the four –1 charges from the
oxygen atom, as illustrated in Figure 6.1 (a). Hence, purely siliceous mesoporous
materials do not exhibit acidity.
However, the framework of purely siliceous
mesoporous materials loses neutrality when lattice Si4+ cations are replaced by lattice
Al3+ cations. The negatively charged framework of the mesoporous materials is
illustrated in Figure 6.1 (b).
According to Lowenstein’s -Al-O-Al- avoidance rule, the linking of two
negatively charged AlO 4 tetrahedral in zeolite framework are energetically
unfavourable and less stable than isolated [111].
Therefore, similar to zeolite,
Al-MCM-48 material is composed of alternating silicon and aluminium atoms that
has an overall negative charge caused by framework aluminium. Subsequently, the
negative charge that has been introduced by the Al substitution will be compensated
for by a cation, usually Na+ ions, which are present in the starting materials.
90
(a)
O
0
O
O
Si
O
4+
Si
O
O
O
Si
O
Si
O
0
Si
O
OO
OO
OO
O
O
O
O
O
O
(b)
-1
O
Al
O
Figure 6.1
3+
Si
O
O
O
Al
OO
Si
Si
OO
-1
OO
O
(a) Framework of Si-MCM-48, and (b) framework of Al-MCM-48.
Typically, Al-MCM-48 mesoporous material is synthesized in the sodium
form. Hence, the Na+ ions have to be replaced by H+ in order to generate the
Brönsted acid sites. It can readily be accomplished by ion exchanging Na+ with
NH4 + using well-established techniques followed by thermal decomposition of the
NH4 + cations into proton and ammonia [112]. The generation of Brönsted acid sites
is displayed schematically as shown in Figure 6.2.
Until recently, the formation of Lewis acid sites in Al-MCM-48 mesoporous
materials is not clearly understood. However, it appears that these sites originate
from extra framework Al species (EFAL) present in the form of Al3+, AlO +,
Al(OH)2+ or charged Alx Oy n+ clusters within the sample [113]. Based on the earlier
findings, there are two possible explanations for the generation of Lewis acid sites.
One, Lercher et al. postulated by infrared studies [114] that Lewis acidity is due to
framework tricoordinated aluminium formed upon dehydroxylation. A second was
postulated by MAS NMR spectroscopy [115], which shows that the presence of
Lewis acid sites is associated with both octahedral and tetrahedral extra framework
Al (EFAL) species, created by dehydroxylation of the hydrogen forms of zeolites, as
demonstrated in Figure 6.3.
91
As-synthesized
Al-MCM-48
+
O Na O
O
-
Al
Si
Si
OO
OO
O
+
O Na O
O
-
Si
OO
Al
O
Si
OO
OO
O
Ammonium exchange (NH4 +)
+
O NH4 O
O
-
Al
Si
Si
OO
OO
O
+
O NH4 O
O
Si
OO
> 300 o C
O
O
O
Al
OO
Figure 6.2
O
Si
OO
O
- NH3
H
O
Si
Al
OO
H
Brönsted acid from
of Al-MCM-48
-
O
Si
OO
O
Si
OO
O
Al
OO
O
Si
OO
Generation of Brönsted acid sites.
O
O
O
O
O
Al
OO
O
OO
Si
Si
OO
O
H
OO
O
Al
Al
O
OO
O
OO
O
O
O
-H2O
-H2O
Lewis acidity associated with both octahedral and tetrahedral EFAL.
Si
Si
O
(b)
Generation of Lewis acid sites:
OO
O
H
OO
Al
O
H
Lewis acidity due to framework tricoordinated aluminium.
Si
Si
O
H
(a)
Figure 6.3
(b)
(a)
O
O
O
O
Si
Si
OO
O
OO
O
Al
-
Al
+
Si
Si
AlO+
OO
O
OO
O
OO
O
OO
Si
Al
O
O
OO
O
Si
O
O
92
92
93
6.3
Characterization of Acidity
Bases usually have been employed as a probe molecule to characterize the
acidity of solid catalysts. By using adsorption and desorption of bases, the total
acidity and acid strength distribution on solid catalysts can be monitored effectively.
The adsorbed basic molecule will be converted into its conjugated acid form by the
solid acid. The acid site is able to transfer a proton from the solid to the adsorbed
molecule (Brönsted acid site) or an electron pair from the solid to the adsorbed
molecule (Lewis acid site). In order to evaluate the amount and type of acid sites in
the aluminated MCM-48 molecular sieves, two methods are used which employ the
diagnostic bases pyridine and ammonia.
(a)
Temperature-Programmed Desorption (TPD) of Ammonia
Temperature-Programmed desorption (TPD) has been used extensively to
analyse the desorption of various sorbates from porous and non-porous adsorbents.
TPD of ammonia usually is employed to investigate the strength of the acidity, since
the information from the NH3 -TPD profile is not able to differentiate the type of the
acid sites. The acid strength of the solid catalysts is determined by the temperature
of the desorption peak. Furthermore, it also provides the information of total amount
of desorbed ammonia, which is dissociated with the acid sites during the adsorption
process. Prior to the measurement, the samp le will be treated with excess of the
ammonia, and any physically adsorbed base molecules are then removed by
prolonged evacuation in the flowing of inert gas. Afterward, whatever is left on the
surface is accounted for as chemically adsorbed species and its concentration are
considered as the total amount of acid sites. The ammonia (pKb = 4.74), the stronger
base than pyridine, is able to interact with both weak and strong acid sites, while
pyridine interacts only with relatively strong acid sites. Since the TPD of ammonia
is not able to distinguish both Brönsted and Lewis sites, infrared spectroscopy study
of adsorbed pyridine should be carried out in order to distinguish the acid sites.
94
(b)
Infrared Spectroscopy (IR) of Adsorbed Pyridine
Pyridine (pKb = 8.8) is selective probe molecule, which is capable to
distinguish the Brönsted as well as Lewis acid sites. It will give rise to infrared
absorptions in the range of 1400-1700 cm-1 . The characteristic bands of pyridine
protonated by Brönsted acid sites (pyridinium ions) appear at ~1546 cm-1 and 1640
cm-1 , while the bands from pyridine coordinated to Lewis acid sites appear at ~ 1455
cm-1 and 1620 cm-1 . By measuring the intensity of those bands and from the values
of the extinction coefficients [116], the number of Brönsted and Lewis acid sites of
retaining pyridine at certain desorption temperature can be calculated.
6.4
Friedel-Crafts Acylation
Friedel-Crafts reactions were first studied in 1877 by the French alkaloid
chemist Charles Friedel and his American partner, James Crafts [117]. FriedelCrafts reactions have generated continued and sustained interest due to their
importance and versatility in laboratory and industry scales.
The Friedel-Crafts reaction is commonly considered as a process of uniting
two or more organic molecules through the formation of carbon to carbon bonds
under the influence of certain strongly acidic metal halides catalysts such as
aluminium chloride, boron trichloride, ferric chloride, and zinc chloride [117]. In
other words, the Friedel-Crafts type reactions involve any substitution, isomerization,
elimination, cracking, polymerization or addition reactions taking place under the
catalytic effect of Lewis acid type acidic halides (with or without co-catalyst) or
proton acids.
The Friedel-Crafts reaction of aromatic acylation to afford aromatic ketones
has been pursued with renewed vigour since aromatic ketones are largely used as the
essential intermediates for various fine, pharmaceutical, and fragrances industries.
For example, they are components in the synthesis of ibuprofen, S-naproxen and
95
R
C
O
C
R
AlCl3
Cl
O
AlCl3
Cl
+
C
R
+
O
AlCl4
O
R’
R
.
+
R
+
C
C
O
H
+
R’
O
C
R
+
(+ o, m)
R’
Figure 6.4
chloride.
Mechanism of acylation of aromatics in the presence of aluminium
H+
96
raspberry ketone [118-119]. In the Friedel-Crafts acylation of aromatic compounds,
a hydrogen atom (or other substituent group) of an aromatic nucleus is replaced by
an aryl group through the interaction of an acylating agent in the presence of a
Friedel-Crafts catalyst. The mechanisms of acylation of aromatics in the presence of
aluminium chloride are illustrated in Figure 6.4.
The current use of conventional Lewis acid catalysts such as aluminium
chloride implies a number of problems related to the fact that a greater than
stoichiometric amount of the catalysts are needed due to the complex formation with
the acylating agent as well as the carbonyl product. The intermediate complex is
usually hydrolysed with water and consequently produces a large amount of waste
products that cause serious technological and environmental problems [120]. In
addition, the inherent disadvantage of the use of conventional homogenous Lewis
acid catalysts is non-regenerable and generated hazardous corrosive waste products.
Therefore, there is a long- felt need and demand to substitute these reagents with noncorrosive, environment friendly, and reusable catalysts in the Friedel-Crafts
reactions.
The use of zeolites as catalysts for organic reactions began in the early 1960s
[121]. Cur rently, the use of zeolites and mesoporous materials has been widely
studied for their application in the synthesis of specialty and fine chemicals. Indeed,
the key opportunities for the use of zeolites and mesoporous materials as catalysts
rely on their unique pores, which can control the selectivity of the reaction.
In this chapter, we will demonstrate the catalytic capability of Al-MCM-48
mesoporous materials in liquid phase acylation of 2-methoxynaphthalene with acetyl
chloride. Previous studies reported that acylation of 2-methoxynaphthalene could
yield
three
kinds
of
isomers;
1-acetyl-2- methoxynaphthalene,
methoxynaphthalene, and 1-acetyl-7- methoxynaphthalene [122].
2-acetyl-6The active
positions of 2- methoxynaphthalene are displayed in Figure 6.5. The goal of this
work is to achieve the product of the acylation in 6-position, which is the particular
interest for the production of (S)-naproxen, an important non-steroidal anti
inflammation drug. The acidity study of the samples will be characterized via TPD
of ammonia and the infrared spectroscopy of adsorbed pyridine, whereas the product
97
of the catalytic activity testing will be quantified and qualified by using GC-FID and
GC-MSD.
1**
8*
OCH3
7
6
3•
5
** Very activated position
* activated position
• slightly activated position
4
Figure 6.5
The active positions of 2-methoxynaphthalene.
6.5
Results and Discussion
6.5.1
Characterization of Acidity of Al-MCM-48
6.5.1.1 Temperature-Programmed Desorption of Ammonia (NH3 -TPD)
Figure 6.6 shows the NH3 -TPD profiles of samples prepared by postsynthesis alumination of mesoporous Si- MCM-48 with different concentrations of
NaAlO 2 solutions.
The NH3 -TPD profiles of these materials are very similar,
indicating that the acidity is similar in nature. Among the samples prepared by postsynthesis alumination of mesoporous Si- MCM-48, the H-010Al-MCM-48 sample
shows the higher temperature desorption peak at ca. 220 o C. However, the NH3 -TPD
profiles indicate clearly that the temperature desorption peak is shifted to lower
temperatures as the Al content was increased.
As described in Chapter 5, the
aluminium content in the samples is increased in the following order, H-010AlMCM-48 < H-025Al-MCM-48 < H-050Al-MCM-48. From the Tmax of NH3 -TPD
profiles in Figure 6.6, it is suggested that H-010Al-MCM-48 sample contain acid
sites with higher acidic strength than that seen in H-025Al-MCM-48 and H-050AlMCM-48 samples. Nevertheless, the acid sites in all of the Al-MCM-48 samples are
98
600
200
500
100
400
300
200
0
Temperature ( °C )
Signal ( mV )
Temperature profile
H-010Al-MCM-48
H-025Al-MCM-48
H-050Al-MCM-48
100
0
10
20
30
40
0
50
Time ( min )
Figure 6.6
NH3 -TPD profiles of samples prepared by post-synthesis alumination
of Si-MCM-48 mesoporous materials with different concentrations of NaAlO 2 .
weaker than those usually present in zeolites with strong acid sites such as ZSM-5
which shows a high temperature peak at 470 o C [123].
The NH3 -TPD profiles of samples prepared by post-synthesis alumination of
the Si-MCM-48 mesophase with different Si/Al gel ratios are displayed in Figure
6.7. The intensity of desorption peak is distinctly increased with the decrease of the
Si/Al gel ratios. With increasing aluminium content, the total amount of desorbed
ammonia reaches a maximum value of 0.35 mmol/g when Si/Al gel ratio is 20.
However, it is noteworthy to point out that the maxima of the NH3 -TPD desorption
peak of the entire samples is exhibited below 180 o C. It suggests that the acidic
strength of the samples prepared from alumination of the Si- MCM-48 mesophase is
weaker than the samples prepared from the alumination of Si- MCM-48 mesoporous
materials.
600
80
500
Signal ( mV )
100
Temperature profile
H-Al-MCM-48-20
400
H-Al-MCM-48-30
H-Al-MCM-48-50
H-Al-MCM-48-100 300
60
40
20
200
0
100
0
0
10
Figure 6.7
20
30
40
Time ( min )
50
60
NH3 -TPD spectra of samples prepared by post-synthesis alumination
of Si-MCM-48 mesophase with different Si/Al gel ratios.
Comparison of the acidic strength of the samples prepared by post-synthesis
alumination of mesoporous MCM-48 with the samples prepared by post-synthesis
alumination of mesophase MCM-48 show that the nature of the acidic sites generated
from both methods is different.
The actual reason for the difference in acidic
strength of the samples is not yet understood. However, based on the XRD data
described in Chapter 5, the mesostructure of Al-MCM-48 prepared from the postsynthesis alumination of mesoporous Si- MCM-48 has been distorted after
undergoing the alumination process.
However, the long-range ordering of the
Al-MCM-48 samples prepared from alumination of the Si- MCM-48 mesophase is
still well retained even after 7 days of undergoing the alumination process.
Therefore, the acidic strength of the samples prepared from post-synthesis
alumination of the mesoporous Si- MCM-48 was possibly enhanced by the
disintegration of the mesostructure during the process of synthesis.
Total acid amount of all the H-Al-MCM-48 samples determined by NH3 -TPD
are tabulated in Table 6.1. From the data in Table 6.1, it can be seen that the total
acid amount increases as the aluminium content in Al-MCM-48 increases, for both
Temperature ( °C )
99
100
samples prepared by post-synthesis alumination of mesoporous Si- MCM-48 and
samples prepared by post-synthesis alumination of the Si- MCM-48 mesophase. The
total amount of acid sites of samples prepared by post-synthesis alumination of
mesoporous materials is higher than that prepared by post-synthesis alumination of
the Si-MCM-48 mesophase, indicating that the acid sites are readily formed by
incorporation of aluminium with mesoporous Si-MCM-48.
Table 6.1: Total acid amount of H-Al-MCM-48 materials determined by NH3 -TPD.
Sample
Total amount of desorbed
Tmax (o C)
ammonia (mmol/g)
H-010Al-MCM-48
0.69
220
H-025Al-MCM-48
0.87
205
H-050Al-MCM-48
1.03
200
H-Al-MCM-48-20
0.35
165
H-Al-MCM-48-30
0.28
165
H-Al-MCM-48-50
0.16
155
H-Al-MCM-48-100
0.09
180
6.5.1.2 Infrared Spectroscopy (IR) of Adsorbed Pyridine
Prior to the exposure of H-Al-MCM-48 samples to pyridine, the samples
were subjected to vacuum at 400 o C under 10-5 mbar for 16 h in order to activate the
samples and to release the interference [124]. The FTIR spectra of hydroxyl groups
of the purely siliceous Si- MCM-48 and samples prepared through post-synthesis
alumination of mesoporous Si- MCM-48 are presented in Figure 6.8 in the region of
stretching vibrations of O-H bands. The spectra in Figure 6.8 show a very intense
band at 3742 cm-1 , which is attributed to terminal silanol groups (Si- OH). The FTIR
results demonstrate that the Si- MCM-48 sample possesses a very large amount of
silanol groups. It can be seen that the increase of aluminium content leads to a
decrease in the intensity of the band assigned to terminal hydroxyl groups. This may
101
be due to the structural redistributions resulting from the insertion of Al species into
the mesoporous structure during alumination.
On the other hand, the FTIR spectra of hydroxyl groups of purely siliceous
Si-MCM-48 and the samples prepared through post-synthesis alumination of
Si-MCM-48 mesophase are displayed in Figure 6.9. The FTIR spectra in all samples
show a single sharp band at 3742 cm-1 , assigned to the non-acidic terminal silanol
groups. Here, the aluminium content in the samples does not affect the intensity of
the band assigned to the terminal hydroxyl groups as shown by the similar intensity
of all peaks.
The
27
Al MAS NMR data (Figure 5.4 and 5.9) show that tetrahedral
aluminium is present in all of the aluminated samples. However, there are no IR
bands corresponding to the Brönsted site, i.e. the ‘bridging hydroxyls’ (Si- OH-Al
groups), which are reported for zeolites like H-ZSM-5 and H-Y. Normally, this band
appears in the range of 3600-3680 cm-1 and its intensity increases with increasing
aluminium incorporation.
Thus, the absence of the –OH band corresponding to
Brönsted site is rather unexpected for Al-MCM-48 materials. However, Weglarski J.
et al. [108] also did not observe any band corresponding to the Brönsted acid site in
the mesoporous MCM-41 material.
Figures 6.10 and 6.11 show the FTIR spectra of –OH stretching regions after
pyridine desorption of the samples at 150 o C. Sorption of the pyridine at 150 o C
leads the small decrease in intensity of the –OH band in Si-MCM-48 sample. This
effect may be caused by the adsorption of pyridine on non-acidic Si-OH groups
[108]. On the same observations, the O-H stretching band also shows a severe loss
in intensity and broadens in all of the aluminated samples.
This is presumably
caused by the interaction of free silanol groups with Lewis acid sites of EFAL of AlOH groups of the framework by hydrogen bonding as depicted in Figure 6.12.
After adsorption of pyridine, bands attributed to the pyridine ring vibrations
appeared at v = 1400 cm-1 – 1650 cm-1 region. Figure 6.13 and 6.14 depict the FTIR
spectra of the pyridine adsorbed on the purely siliceous Si-MCM-48 and aluminated
102
3742
(d)
Absorbance
(c)
(b)
(a)
4000
3900
Figure 6.8
3800
3700
3600
3500
3400
3300
3200
3100
3000
1/cm
FTIR spectra in the hydroxyl region of (a) Si-MCM-48, (b) H-
010AlMCM-48, (c) H-025AlMCM-48, and (d) H-050AlMCM-48 recorded at 400 o C
under 10-5 mbar pressure.
103
3742
(e)
Absorbance
(d)
(c)
(b)
(a)
4000
3900
Figure 6.9
3800
3700
3600
3500
3400
3300
3200
3100
3000
1/cm
FTIR spectra in the hydroxyl region of (a) Si-MCM-48, (b) H-Al-
MCM-48-20, (c) H-Al-MCM-48-30, (d) H-Al-MCM-48-50, and (e) H-Al-MCM-48100 recorded at 400 o C under 10-5 mbar pressure.
104
(d)
(c)
Absorbance
(b)
(a)
4000
3900
3800 3700
3600
3500
3400
3300 3200
3100
3000
1/cm
Figure 6.10
o
FTIR spectra in the hydroxyl region after pyridine desorption at 150
C; (a) Si-MCM-48, (b) H-010Al-MCM-48, (c) H-025Al-MCM-48, and (d) H-
050Al-MCM-48.
105
(e)
Absorbance
(d)
(c)
(b)
(a)
4000
3900
Figure 6.11
o
3800
3700
3600
3500
3400
3300
3200
3100
3000
1/cm
FTIR spectra in the hydroxyl region after pyridine desorption at 150
C (a) Si-MCM-48, (b) H-Al-MCM-48-20, (c) H-Al-MCM-48-30, (d) H-Al-MCM-
48-50, and (e) Al-MCM-48-100.
106
(a)
(b)
H
O
O
Al
O
O
Figure 6.12
O
O
O
O
O
O
O
H
O
Al
Si
H
Si
O
O
O
O
Structures showing interaction of (a) silanols with Lewis acid sites
and (b) Al-OH groups by H-bonding (represented by arrows).
H-Al-MCM-48 samples in the region 1400 cm-1 – 1650 cm-1 at 10-5 mbar pressure
after desorption at 25 o C, 150 o C, 250 o C, and 400 o C. The FTIR spectrum of purely
siliceous Si-MCM-48 only exhibits bands at 1445 cm-1 and 1596 cm-1 . These bands
disappeared after desorption of pyridine at above 150 o C. Hence, it suggests that
pyridines formed hydrogen bonds with silanol groups. In addition, no Brönsted
acidity and Lewis acidity were observed in Si-MCM-48.
All of the spectra of aluminated H-Al-MCM-48 samples have been found to
exhibit bands due to hydrogen bonded pyridine (1445 and 1596 cm-1 ), Lewis bound
pyridine (1623 and 1455 cm-1 ), pyridinium ion ring vibration due to pyridine bound
to Brönsted acid sites (1546 and 1639 cm-1 ) and a band at 1492 cm-1 which can be
assigned to pyridine associated with both Brönsted and Lewis sites. Thus, the results
indicate that both approaches of post-synthesis alumination have successfully
generated both Brönsted and Lewis acid sites in the mesoporous H-Al-MCM-48
materials.
The Lewis acid sites from the aluminated H-Al-MCM-48 samples may
originate from three coordinated framework aluminium [115], since no octahedral
aluminium sites were observed by
27
Al MAS NMR measurements. It is noted that
the samples prepared from post-synthesis alumination of mesoporous Si-MCM-48
107
(a) Si-MCM-48
(b) H-010Al-MCM-48
400 o C
400 o C
250 o C
250 o C
150 o C
150 o C
H
L
H
H
B+L
25 o C
1700
B
25 o C
1600
1500
1400
(c) H-025Al-MCM-48
1300
1/cm
1700
1600
1500
1400
1300
1/cm
(d) H-050Al-MCM-48
400 o C
o
400 C
o
250 o C
o
150 o C
250 C
150 C
L
L
H
H
B+L
B+L
25 o C
B
o
25 C
1700
1600
Figure 6.13
1500
1400
1300
1/cm
1700
B
1600
1500
1400
1300
1/cm
FTIR spectra of adsorbed pyridine on Si-MCM-48 and samples
prepared via post-synthesis alumination mesoporous Si-MCM-48 evacuated at 25 o C,
150 o C, 250 o C, and 400 o C. (H, hydrogen bonded pyridine; B, Brönsted bound
pyridine; L, Lewis bound pyridine)
108
(a) H-Al-MCM-48-20
(b) H-Al-MCM-48-30
400 o C
400 o C
250 o C
250 o C
150 o C
o
150 C
L
L
H
H
25 o C
1700
B
B+L
1600
1500
B+L
25 o C
1400
1300
1/cm
1700
(c) H-Al-MCM-48-50
B
1600
1500
1300
1/cm
(d) H-Al-MCM-48-100
400 o C
400 o C
250 o C
250 o C
150 o C
150 o C
L
H
L
H
B+L
o
25 C
1700
1400
25 o C
B
1600
Figure 6.14
1500
1400
1300
1/cm
1700
B+L
B
1600
1500
1400
FTIR spectra of adsorbed pyridine on samples prepared via post-
synthesis alumination Si-MCM-48 mesophase evacuated at 25 o C, 150 o C, 250 o C,
and 400 o C. (H, hydrogen bonded pyridine; B, Brönsted bound pyridine; L, Lewis
bound pyridine)
1300
1/cm
109
possess strong acidity, since the band corresponding to Lewis acid sites can be
retained even after evacuation at 400 o C. These results are consistent with the results
obtained from the NH3 -TPD measurements.
Post-synthesis alumination of
mesoporous Si- MCM-48 introduce the Al3+ ions into the tetrahedral environment of
the silica surface of mesoporous Si- MCM-48, which will generate unsaturation or
Lewis acid sites. This saturation can have an inductive effect on the ne ighboring
silanol group rendering it more acidic.
Table 6.2 represents the relative concentration (µmol pyridine/ g sample) of
Brönsted and Lewis acid sites calculated from the integrated band intensities at v =
1546 cm-1 and v = 1455 cm-1 respectively after pyridine desorption at 25 o C, 150 oC,
250 o C, and 400 o C, by using the absorption coefficients from the literature [116].
An assumption has been made that each pyridine molecule interacts with an acid site
[123].
Based on the data in Table 6.2, it can be seen that by increasing the
evacuation temperature, the interaction of the pyridine with both Brönsted and Lewis
acid sites has been reduced. Furthermore, the amount of both Brönsted and Lewis
acid sites of samples prepared through post-synthesis alumination of mesoporous
Si-MCM-48 is higher than the samples prepared through post-synthesis alumination
of Si-MCM-48 mesophase.
Hence, post-synthesis alumination of mesoporous
Si-MCM-48 is more effective in incorporating aluminium in the framework than the
post-synthesis treatment of the Si-MCM-48 mesophase.
6.5.2
Acylation of 2-Methoxynapthalene with Acetyl Chloride
Acylation of 2-methoxynaphthalene with acetyl chloride had
been chosen to evaluate the catalytic activity of the H-Al-MCM-48 catalysts.
Previous studies had demonstrated that the selectivity of the acylation of 2methoxynaphthalene is strongly dependent on the temperature of reaction.
Therefore, 120 o C had been chosen since it is the optimum temperature to obtained
the product of the acylation in 6-position [122].
0.25
0.50
-
-
-
-
H-025Al-MCM-48
H-050Al-MCM-48
H-Al-MCM-48-20
H-Al-MCM-48-30
H-Al-MCM-48-50
H-Al-MCM-48-100
100
50
30
20
-
-
-
∞
Si/Al*
10
18
18
26
30
51
45
-
25 o C
10
17
18
23
29
38
42
-
150 o C
9
18
16
22
21
26
26
-
250 o C
Brönsted
10
13
7
9
13
9
10
-
400 o C
355
356
461
563
256
269
304
-
25 o C
µmol pyridine g-1
30
43
48
89
93
108
98
-
150 o C
21
20
41
28
84
78
53
-
250 o C
Lewis
*Si/Al ratio mentioned above is defined as silicon to aluminium ratio calculated from compositions of starting gel mixtures
0.10
-
mol L-1
[NaAlO 2 ]
H-010Al-MCM-48
Si-MCM-48
Sample
Table 6.2: Number of Brönsted and Lewis acid sites in the samples.
18
19
40
20
75
108
38
400 o C
110
110
111
Gas chromatography (GC) had been used to characterize the products of the
reactions. Internal standard approach had been applied to quantify the resultant
compound precisely.
In this case, naphthalene had been chosen as the internal
standard and it possesses a good correlation in the standard calibration plot.
Furthermore, qualitative verification was carried out by comparing the retention time
of the resultant compounds with the authentic samples. Based on the chromatogram,
2 types of products were identified as tabulated in Table 6.3. The products were
further confirmed by using gas chromatography-mass spectrometry (GC-MS)
Table 6.3: GC Data for the acylation products.
Product
Rt (min)
Compound
1
18.232
1-acetyl-7-methoxynaphthalene (1,7-AMN)
2
19.463
2-acetyl-6-methoxynaphthalene (2,6-AMN)
6.5.2.1 The Effect of Various Catalysts
The data of the catalytic activity are shown in Table 6.4. Turnover numbers
of the catalytic sites are calculated based on the total number of both Brönsted and
Lewis acid sites at 150 o C, since the reactions are carried out below 150 o C and the
assumption that each pyridine molecule interacts with an acid site.
Basically,
acylation of 2- methoxynaphthalene with acetyl chloride over H-Al-MCM-48 only
formed 2 types of products, 1-acetyl-7-methoxynaphthalene and 2-acetyl-6methoxynaphthalene. Most of the aluminated H-Al-MCM-48 catalysts function as
selective catalysts, which are able to produce the desired product, 2-acetyl-6methoxynaphthalene (above 83 %) as the major product.
As expected, Si-MCM-48 did not show any activity in the acylation of
2-methoxynaphthalene. This is in agreement with the acidity study, which indicates
that Si-MCM-48 does not possess either Brönsted or Lewis acid sites.
comparison, the aluminated Al-MCM-48 catalysts are active in the acylation of
By
112
Table 6.4: Catalytic Activities of Various Catalysts for the Acylation of 2Methoxynaphthalene with Acetyl Chloride.
Catalyst#
Conversion
Selectivity (%)
Turnover Numbers
(%)
1,7-AMN
2,6-AMN
(TON)*
None
-
-
-
H-010Al-MCM-48
34
15
85
36
H-025Al-MCM-48
35
12
88
36
H-050Al-MCM-48
22
17
83
24
H-Al-MCM-48-20
42
14
86
56
H-Al-MCM-48-30
39
14
86
89
H-Al-MCM-48-50
36
15
85
90
H-Al-MCM-48-100
32
13
87
128
Si-MCM-48
Reaction conditions: temperature = 120 o C; time = 20 h; autogenous pressure; weight
of catalyst = 0.2 g; 2- methoxynaphthalene:acetyl chloride = 1:2 molar ratio; solvent
= nitrobenzene
# All of the catalysts had been modified into H-form.
*TONs are calculated based on the total number of acid sites, determined by IR
spectra of absorbed pyridine at 150 o C.
2-methoxynaphthalene, giving 1-acetyl-7-methoxynaphthalene and 2-acetyl-6methoxynaphthalene.
However, the product of the most kinetically active position (1-acetyl-2methoxynaphthalene) is not observed (Figure 6.5). At the high temperature of 120
o
C and in the presence of the Brönsted acid sites, 1-acetyl-2- methoxynaphthalene is
subject to protiodeacylation.
This phenomenon is not involved in the
thermodynamically more stable and the sterically unhindered 6-, or less hindered
8-substituted aromatic ketones.
For samples prepared via post-synthesis alumination of mesoporous SiMCM-48 materials, the conversion of 2-methoxynaphthalene increases with the
decrease of the aluminium content. The conversion of 2-methoxynaphthalene via H-
113
010Al-MCM-48 and H-025Al-MCM-48 catalysts are similar, which is around 34-35
%. Whereas, H-050Al-MCM-48 only gave the conversion of 2- methoxynaphthalene
around 22 %. The loss of the BET surface area and the crystallinity of H-050AlMCM-48 catalyst are the main reasons of the lower conversion of 2methoxynaphthalene.
However, conversion of 2- methoxynaphthalene by using samples prepared
via post-synthesis alumination of Si- MCM-48 mesophase is increased with decrease
of the aluminium content.
By using H-Al-MCM-48-20, the conversion of
2-methoxynaphthalene can be achieved unto 42 %.
The conversion of
2-methoxynaphthalene in this case is affected by the acidity of the samples, since the
BET surface area and the crystallinity of the samples are similar as described in
Chapter 5. However, the TON of the catalytic sites of the catalysts prepared via
post-synthesis alumination of Si- MCM-48 mesophase is increased with the
decreasing total number of acid sites (determine by adsorbed pyridine at 150 o C).
The decrease of the TON is in the following order: H-Al-MCM-48-20 > H-AlMCM-48-30 ≈ H-Al-MCM-48-50 > H-Al-MCM-48-100, which indicates that the
reusability of each active site is higher in the catalyst containing less active site.
The turn over numbers (TON) are greater than 1 showing that H-Al-MCM-48
has catalyzed the reaction. It was found that the conversions and the TON of the
acylation of 2- methoxynaphthalene over catalysts prepared by post-synthesis
alumination of Si-MCM-48 mesophase are higher than the catalysts prepared by
post-synthesis alumination of mesoporous Si- MCM-48. However, the selectivity of
the products through both catalysts is similar. Indeed, the strength and amount of
acidity of the catalysts prepared by post-synthesis alumination of mesoporous
Si-MCM-48 are higher than the catalysts prepared by post-synthesis alumination of
Si-MCM-48 mesophase. Thus, it suggests that the selectivity of the products is
independent of the strength of the acidity and the concentration of acid sites. Other
factors seem to be more influential in the conversion of 2-methoxynapthalene.
As mentioned in Chapter 5, the BET surface areas of the samples prepared
via post-synthesis alumination of Si-MCM-48 mesophase are higher than the
samples prepared via post-synthesis alumination of mesoporous Si- MCM-48.
114
Furthermore, the crystallinity of cubic MCM-48 is higher and highly ordered in the
samples prepared via post-synthesis alumination of Si-MCM-48 mesophase. Hence,
high BET surface areas and high degrees of crystallinity are probably leading to
higher conversion in acylation of 2- methoxynaphthalene by using samples prepared
via post-synthesis alumination of Si-MCM-48 mesophase.
Nevertheless, the pore systems of the catalysts prepared from the two
different approaches varied from each other. The bimodal and hierarchical pore
systems of the catalysts prepared via post-synthesis alumination of the Si-MCM-48
mesophase may have contributed to the high conversion of the acylation of
2-methoxynaphthalene because they offer advantages such as easier diffusion and
greatly enhance the mass transfers of the reactants and products in the catalysts.
6.5.2.2 The Effect of Solvent
Nitrobenzene, dichloroethane, and cyclohexane, which are different in
polarity, have been used as solvents towards acylation of 2- methoxynaphthalene with
acetyl chloride over H-Al-MCM-48-20 catalyst. H-Al-MCM-48-20 had been chosen
in the study of the effect of solvents since it gave the higher conversion of
2-methoxynaphthalene and possess high BET surface area and high crystallinity.
The polarity of the solvents increases in the following order: cyclohexane <
dichloroethane < nitrobenzene.
The solvents employed for acylation of
2-methoxynaphthalene exert a significant effect on the activity and selectivity over
H-Al-MCM-48-20 catalyst.
The effect of the solvents on the conversion of
2-methoxynaphthalene over H-Al-MCM-48-20 catalyst is presented in Figure 6.15.
A non-polar solvent (cyclohexane) shows lowest conversion of 2- methoxynaphthalene, ca. 30 %. However, only slightly increase has been observed in the
conversion of 2-methoxynaphthalene in nitrobenzene and dichloroethane. This may
be due to the polarity value of both solvents which are very close to each other.
Hence, it can be proposed that the acylation of 2- methoxynapthalene over H-AlMCM-48 catalyst is more active in polar solvents than in non-polar solvents.
115
Figure 6.16 depicts the effect of solvents on the selectivity of the products
from the acylation of 2- methoxynaphthalene over H-Al-MCM-48-20 catalyst. The
results
reveal
that
only
1-acetyl-7- methoxynaphthalene
and
2-acetyl-6-
methoxynaphthalene have been formed from the reaction in the presence of
nitrobenzene, dichloroethane, and cyclohexane.
The solvent s do not affect the
variety of the products, but they have a significant role in determining the selectivity
of the products. The polar nitrobenzene shows higher selectivity towards 2-acetyl-6methoxynaphthalene (86 %) than non-polar cyclohexane (56 %). The selectivity of
2-acetyl-6- methoxynaphthalene in different solvents increases in the order:
cyclohexane < dichloroetha ne < nitrobenzene, suggesting that the 2-acetyl-6meththoxynaphthalene, is readily formed in polar solvent. However, the reason is
not yet understood.
Even though both Brönsted and Lewis acid sites are present in all the
aluminated Al-MCM-48 catalysts; it is difficult to verify which type of acid sites
plays a more important role in the acylation of 2-methoxynaphthalene. Both of the
acid sites may have contributed to the acylation of 2- methoxynaphthalene. Figure
6.17 illustrates the products of acylation of 2- methoxynaphthalene catalysed by
H-Al-MCM-48.
The proposed mechanisms for the acylation of 2- methoxy-
naphthalene with acetyl chloride by Brönsted and Lewis acid sites are depicted in
Figure 6.18 and 6.19, respectively. Both of the active sites will attack the acetyl
chloride in order to generate active intermediate, acylium ion, which functions as
electrophile species. Consequence, 2- methoxynaphthalene will be substituted by
acylium ion via the electrophilic process in the position of ortho, para and meta.
Nevertheless, future work should be done to support the proposed mechanisms using
both active sites on the H-Al-MCM-48 since it is not well understood up to the
moment.
116
50
47
42
Conversion (%)
40
30
30
20
10
0
Nitrobenzene
Figure 6.15
Dichloroethane
Cyclohexane
Effect of solvents on conversion of 2-methoxynaphthalene over
H-Al-MCM-48-20.
100
86
76
Selectivity (%)
80
56
60
44
40
24
20
14
0
Nitrobenzene
Dichloroethane
Cyclohexane
1-Acetyl-7-Methoxynaphthalene
2-Acetyl-6-Methoxynaphthalene
Figure 6.16
Effect of solvents on selectivity of the products from acylation of 2-
methoxynaphthalene over H-Al-MCM-48-20 catalyst.
117
O
OCH3
2-Methoxynaphthalene
CH3
OCH3
O
+ H3C
C
Acylation
Cl
C
Protiodeacylation
Acetyl Chloride
1-Acetyl-2-Methoxynaphthalene
Acylation
CH3
O
C
OCH3
OCH3
+
H3C
C
O
2-Acetyl-6-Methoxynaphthalene
Figure 6.17
MCM-48.
1-Acetyl-7-Methoxynaphthalene
Products of acylation of 2-methoxynaphthalene catalysed by H-Al-
118
OH+
O
H3C
Cl + H-Al-MCM-48
C
OCH3
+
H3C
C
Cl Al-MCM-48
OH+
H3C
C
Cl Al-MCM -48
O
H3C
C
H
OCH3
+
O
C
+ HCl
Al-MCM-48
CH3
OCH3
+
Figure 6.18
H-Al-MCM-48
Proposed mechanism of the acylation of 2-methoxynaphthalene with
acetyl chloride over Brönsted acid sites in H-Al-MCM-48.
119
O
H3C
O
+
Cl + AlO /Al-MCM-48
C
H3C
OCH3
O
H3C
C+
+
O
H3C
+ C+ + AlO Cl /Al-MCM-48
AlO+ Cl- /Al-MCM-48
C
H
OCH3
+
O
C
CH3
OCH3
+
Figure 6.19
AlO+/Al-MCM-48
+
HCl
Proposed mechanism of the acylation of 2-methoxynaphthalene with
acetyl chloride over Lewis acid sites in H-Al-MCM-48.
120
6.6
Conclusion
In the acidity study, NH3 -TPD measurement reveals that all of the aluminated
samples exhibit mild acidity. The numbers of the acid sites are increased with the
increasing of aluminium content. However, the acidity of samples prepared through
post-synthesis alumination of mesoporous Si- MCM-48 is stronger (in term of acidity
strength) and higher (in term of number of acidity) than the samples prepared
through post-synthesis alumination of Si- MCM-48 mesophase. In addition, studies
of the adsorption of pyridine on aluminated H-Al-MCM-48 samples have revealed
that the samples possess both Brönsted and Lewis acid sites. The results of the
pyridine adsorption are consistent with the results of NH3 -TPD, which demonstrated
the stronger and higher number of acidity in the samples prepared via post-synthesis
alumination of Si-MCM-48 mesophase.
All of the aluminated H-Al-MCM-48 catalysts have been proven active to
catalyze the acylation of 2- methoxynaphthalene with acetyl chloride by using
nitrobenzene as a solvent. It has been found that there were no differences in the
selectivity of the resultants products in all type of catalysts. However, samples
prepared via post-synthesis alumination of Si- MCM-48 mesophase possess higher
catalytic activity if compared to the samples prepared via post-synthesis alumination
of mesoporous Si- MCM-48.
The conversion and the selectivity of 2- methoxy-
naphthalene seem to be independent upon the acidity strength. The BET surface
area, degree of crystallinity, and the porous system of the catalysts play the
significant roles in the catalytic activity. Additionally, the selectivity of the desired
2-acetyl-6- methoxynaphthalene is influenced by the polarity of the solvents. The
selectivity of 2-acetyl-6-methoxynaphthalene increases in the following order:
cyclohexane < dichloroethane < nitrobenzene.
CHAPTER 7
GENERAL CONCLUSION AND RECOMMENDATIONS
This thesis deals with the attempts to synthesize purely siliceous mesoporous
Si-MCM-48 and aluminosilicate Al-MCM-48 from rice husk ash.
Two post-
synthesis alumination approaches have been devoted to prepared mesoporous
Al-MCM-48.
A novel technique of mesophases quantification has developed to
determine the purity of mesophases.
Furthermore, the physicochemical, chemical,
and catalytic properties of these materials have been investigated.
These have
allowed us to reach the following conclusions.
7.1
Main Results
Rice husk ash has been profitably utilized as an active silica source for
synthesizing the ordered mesoporous materials.
highly
crystalline
and
well-defined
purely
Optimal condition to synthesize
siliceous
Ia3d
bicontinuous
cubic
MCM-48 mesoporous materials via mixed cationic-neutral templating route using the
cationic cetyltrimethylammonium bromide (CTABr) and neutral Triton X-100 (TX100) surfactants were successfully achieved.
Si-MCM-48 is 10.2.
The optimal pH value for synthesize
The optimal compositions of the reactant mixture are stated
below:
5 SiO 2 : 1.00 Na2 O : 0.15 TX-100 : 0.85 CTABr : 400 H2 O
122
By adjusting the pH during the process of aging, this has greatly increased the
thermal stability of mesoporous materials.
It has been found that the mesophase is
very sensitive towards the parameters like pH, Na2 O/SiO 2 , Sur/SiO 2 and H2 O/SiO 2
ratios.
Instead of pure Ia3d bicontinuous cubic Si-MCM-48 and hexagonal
Si-MCM-41 mesophases, contamination of phases has been detected during the
optimization processes.
13
C CP/MAS NMR characterization technique has been developed to quantify
the mesophases compositions on the basis that the geometry of mesophase structure
is determined by the arrangement of surfactants. Quantification of a mixture of cubic
MCM-48 and hexagonal MCM-41 mesophases is possible by the interpretation of
their spectra, which cannot be determined by X-ray diffraction techniques.
Modifications of Si-MCM-48 by incorporation of aluminium into the
framework have been carried out by using two post-synthesis alumination
approaches.
Different approaches of post synthesis alumination have generated
different porous systems in the materials.
Post-synthesis alumination of mesoporous
Si-MCM-48 using sodium aluminate as the aluminium source has produced
mesoporous Al-MCM-48 with unimodal pore system as well as pore system in its
parent materials, ordered cubic mesoporous Si-MCM-48. The cubic pore system of
its parent Si-MCM-48 is well retained in 0.10 M and 0.25 M solutions of sodium
aluminate at 60o C for 3 h.
The BJH pore size distribution shows the resultant
Al-MCM-48 possesses narrow pore size distribution, which is identical to its parent
Si-MCM-48.
In addition, the
27
Al MAS NMR spectra depict the aluminium is
tetrahedrally coordinated with the structure of MCM-48.
However, bimodal mesoporous Al-MCM-48 with interconnected hierarchical
structure has been synthesized via post synthesis alumination of Si-MCM-48
mesophase.
Nitrogen adsorption-desorption measurement and XRD analysis reveal
that these materials are constructed by two types of pore systems; ordered
bicontinuous Ia3d cubic MCM-48 pore system and narrow but disordered pore
system centered at 26 Å and 38 Å, respectively.
The structure of ordered
bicontinuous Ia3d cubic Al-MCM-48 is well resolved in the Si/Al ratios ranking
123
from 20 to 100 (gel ratio).
The
27
Al MAS NMR spectra demonstrate that the
aluminium is incorporated tetrahedrally into the framework of MCM-48.
Studies of the NH3 -TPD on aluminated Al-MCM-48 reveal that all of the
aluminated samples exhibit mild acidity, in which the Tmaks of the ammonia
desorption are in the range between 155 – 220 o C. The numbers of the acid sites are
increased with the increasing of aluminium content. However, the acidity of samples
prepared through post-synthesis alumination of mesoporous Si-MCM-48 is stronger
(in terms of acidity strength) and higher (in terms of number of acidity) than the
samples prepared through post-synthesis alumination of Si-MCM-48 mesophase. In
addition, studies of the adsorption of pyridine on aluminated Al-MCM-48 samples
have shown that the samples possess both Brönsted and Lewis acid sites. The results
of the pyridine adsorption are consistent with the results of NH3 -TPD, which
demonstrated the stronger and higher number of acidity in the samples prepared via
post-synthesis alumination of Si-MCM-48 mesophase.
All of the aluminated Al-MCM-48 catalysts have been proven active to
catalyze the acylation of 2-methoxynaphthalene with acetyl chloride by using
nitrobenzene as a solvent.
It is demonstrated that ca. 42 % of conversion of
2-methoxynaphthalene with ca. 86% of selectivity towards desirable 6-acetyl-2methoxynaphthalene product has been achieved by using Al-MCM-48 as catalyst. It
has been found that there are no differences in the selectivity of the resultants
products in all type of catalysts. Samples prepared via post-synthesis alumination of
Si-MCM-48
mesophase
exhibit
higher
catalytic
activity
in
acylation
of
2-methoxynaphthalene if compared to the samples prepared via post-synthesis
alumination of mesoporous Si-MCM-48.
The conversion and the selectivity of
2-methoxynaphthalene seem to be independent from the strength and number of
acidity.
Nevertheless, the BET surface area, degree of crystallinity, and the porous
system of the catalysts are correlated well in its catalytic activity. Furthermore, the
selectivity of the desired 2-acetyl-6-methoxynaphthalene is strongly influenced by
the polarity of the solvents.
The selectivity of 2-acetyl-6-methoxynaphthalene
increases in the following order: cyclohexane < dichloroethane < nitrobenzene.
124
7.2
Recommendations
In this research, the novel technique of mesophases quantification by
13
C
CP/MAS NMR had been well demonstrated. It is recommended that further work on
other families of surfactant base mesomorphous materials can be carried out in order
to expand its applicability to a wide range of mesoporous structures.
The bimodal mesoporous Al-MCM-48 molecular sieves with interconnected
hierarchical pore system seem to have great diffusion advantages.
It apparently
emerges as excellent candidates for membrane and packing materials in separation
processes. Therefore, further work to employ the bimodal Al-MCM-48 as packing
materials in chromatography should be developed.
In the present work, the catalytic experiment is not enough to justify the
effectiveness of the catalyst in Friedel Crafts acylation. Further improvements in the
reaction parameters should be investigated in order to maximize the catalytic
potential of Al-MCM-48 as well as to enhance the product selectivity in FriedelCrafts acylation.
125
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141
APPENDIX A
Calculation of the amount of pyridine adsorbed on the sample in the acidity study of
secondary aluminated Al-MCM-48 samples.
The general formula to obtain the amount of pyridine in µmole per gram sample:
B (cm-1) . sample surface (cm2)
Adsorbed pyridine (µmole) =
Adsorption Coef. (cm. µmole). Weight (g)
Where B (band area) = Imax uǻ½
Where Imax is the intensity of the band (in absorbance unit) and ǻ½ half width at half
height.
For a pellet that is 13 mm in diameter and 10 mg in weight, the sample surface that is
transversed by the radiation is 0.7857 cm (from only 10 mm exposed to the IR
radiation) on a 5.92 mg sample. The adsorption coefficient values are taken from the
literature [116] where
Brönsted = 3.03 r 0.01
Lewis
= 3.80 r 0.01
Therefore, for the samples used in this study which were prepared from 10 mm
diameter pellet and 10 mg sample in which the area transversed by the radiation and
its respective weight are 0.7857 cm2 and 5.92 u 10-3 g respectively, the amount of
pyridine adsorbed (in µmole) is calculated according to the following:
Brönsted acidity = B (cm-1) u 43.80 (cm. Pmole g-1)
Lewis acidity
= B (cm-1) u 34.92 (cm. Pmole g-1)
The area of the band is determined by means of the computer program of the FTIR
instrument.
142
APPENDIX B
Quantitative gas chromatography calibration plot of 2-methoxynaphthalene by using
naphthalene as internal standard.
1.6
1.2
Peak area of naphthalene
Peak area of 2-methoxynaphthalene
1.4
1
0.8
0.6
0.4
0.2
y = 5.0042x - 0.0133
R2 = 0.999
0
0
0.1
0.2
0.3
2-Methoxynaphthelene / Pmole
0.4
143
APPENDIX C
Calculation of % conversion, % selectivity, and turnover number (TON)
Conversion (%) = Amount of 2-methoxynaphthalene reacted
Amount of 2-methoxynaphthalene input
Selectivity (%) = Peak area of desired product
Total peak area of all products
X 100%
X 100%
Turnover number (TON) = Amount of 2-methoxynaphthalene reacted (Pmole)
Amount of acid sites (Pmole)
2-Acetyl-6-methoxynaphthalene
1-Acetyl-7-methoxynaphthalene
2-Methoxynaphthalene
Naphthalene
Nitrobenzene
Acetyl chloride
144
APPENDIX D
An example of chromatogram for liquid products of conversion of 2-
metoxynaphthalene in nitrobenzene
2-Acetyl-6-methoxynaphthalene
1-Acetyl-7-methoxynaphthalene
2-Methoxynaphthalene
Naphthalene
Dichloroethane
Acetyl chloride
145
APPENDIX E
An example of chromatogram for liquid products of conversion of 2-
metoxynaphthalene in dichloroethane.
2-Acetyl-6-methoxynaphthalene
1-Acetyl-7-methoxynaphthalene
2-Methoxynaphthalene
Naphthalene
Cyclohexane
Acetyl chloride
146
APPENDIX F
An example of chromatogram for liquid products of conversion of 2-
metoxynaphthalene in cyclohexane.
147
APPENDIX G
Mass
spectra
of
(a)
2-acetyl-6-methoxynaphthalene
methoxynaphthalene
(a) 2-Acetyl-6-methoxynaphthalene
(b) 1-Acetyl-7-methoxynaphthalene
and
(b)
1-acetyl-7-
148
APPENDIX H
Quantitative calculation of phase composition via integrated intensity ratio of the C5 –
C14 and C1 peaks of 13 C CP/MAS NMR spectra of mesophases.
+
Na2O/SiO2=0.20
Na2O/SiO2=0.25
Na2O/SiO2=0.30
Na2O/SiO2=0.35
Na2O/SiO2=0.40
Sample
Intensity of
Intensity of
(Na2 O/SiO 2 )
C5 –C14 peak
C1 peak
C5 –C14 /C1
Normalized to
percentage of
mesophases
0.20 (MCM-48)
1102
182
6.1
100 % MCM-48
0.25
866
162
5.4
63.2 % MCM-48
0.30
837
176
4.8
31.6 % MCM-48
0.35
885
198
4.5
15.8 % MCM-48
0.40(MCM-41)
631
149
4.2
0 % MCM-48