SYNTHESIS, CHARACTERIZATION AND ACTIVITY OF Al-MCM-41
CATALYST FOR HYDROXYALKYLATION OF EPOXIDES
AZMI BIN MOHAMED
UNIVERSITI TEKNOLOGI MALAYSIA
PSZ 19:16 (Pind. 1/97)
UNIVERSITI TEKNOLOGI MALAYSIA
BORANG PENGESAHAN STATUS TESIS
JUDUL: SYNTHESIS, CHARACTERIZATION AND ACTIVITY OF
Al-MCM-41 CATALYST FOR HYDROXYALKYLATION OF EPOXIDES
SESI PENGAJIAN: 2004 / 2005
AZMI BIN MOHAMED
Saya
_____
(HURUF BESAR)
mengaku membenarkan tesis ini disimpan di Perpustakaan Universiti Teknologi Malaysia dengan
syarat-syarat kegunaan seperti berikut :
1.
Hakmilik tesis adalah dibawah nama penulis melainkan penulisan sebagai projek bersama dan
dibiayai oleh UTM, hakmiliknya adalah kepunyaan UTM.
Naskah salinan di dalam bentuk kertas atau mikro hanya boleh dibuat dengan kebenaran bertulis
daripada penulis.
Perpustakaan Universiti Teknologi Malaysia dibenarkan membuat salinan untuk tujuan pengajian
sahaja.
Tesis hanya boleh diterbitkan dengan kebenaran penulis. Bayaran royalti adalah mengikut kadar
yang dipersetujui kelak.
*Saya membenarkan/tidak membenarkan Perpustakaan membuat salinan tesis ini sebagai bahan
pertukaran antara institusi pengajian tinggi.
**Sila tandakan (√)
2.
3.
4.
5.
6.
√
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)
Alamat Tetap: LOT 344, KOK LANAS,
BATU 15.5 JLN KUALA KRAI 16450
KOTA BHARU KELANTAN DARUL
NAIM
Tarikh: 18 APRIL 2005
_____________________________
(TANDATANGAN PENYELIA)
PROF. DR. HALIMATON HAMDAN
(NAMA PENYELIA)
Tarikh: 18 APRIL 2005
CATATAN: * Potong yang tidak berkenaan.
** Jika Tesis ini SULIT atau TERHAD, sila lampirkan surat daripada pihak berkuasa/
organisasi berkenaan dengan menyatakan sekali tempoh tesis ini perlu dikelaskan
sebagai SULIT atau TERHAD.
“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
: Prof. Dr. Halimaton Hamdan
Date
: 18 April 2005
BAHAGIAN A – Pengesahan Kerjasama *
Adalah disahkan bahawa projek penyelidikan tesis ini telah dilaksanakan melalui
kerjasama antara _____________________ dengan _________________________
Disahkan oleh:
Tandatangan : ..........................................................
Nama
: ..........................................................
Jawatan
:...........................................................
Tarikh : ..........................
(Cop rasmi)
* Jika penyediaan tesis/projek melibatkan kerjasama.
BAHAGIAN B – Untuk Kegunaan Pejabat Sekolah Pengajian Siswazah
Tesis ini telah diperiksa dan diakui oleh:
Nama dan Alamat Pemeriksa Luar
: Prof. Madya Dr. Yeap Guan Yeow
School of Chemical Sciences
Universiti Sains Malaysia
11800 Minden Pulau Pinang
Nama dan Alamat Pemeriksa Dalam : Prof. Madya Dr. Nor Aishah Saidina Amin
Fakulti Kejuruteraan Kimia &
Kejuruteraan Sumber Asli
Universiti Teknologi Malaysia
81310 Skudai Johor
Nama Penyelia Lain (jika ada)
:
Disahkan oleh Penolong Pendaftar di SPS:
Tandatangan : ................................................................
Nama
: GANESAN A/L ANDIMUTHU
Tarikh : ........................
SYNTHESIS, CHARACTERIZATION AND ACTIVITY OF Al-MCM-41
CATALYST FOR HYDROXYALKYLATION OF EPOXIDES
AZMI BIN MOHAMED
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
APRIL 2005
ii
I declare that this thesis entitled “Synthesis, Characterization and Activity of
Al-MCM-41 Catalyst for Hydroxyalkylation of Epoxides” is the result of my own
research except as cited in references. The thesis has not been accepted for any
degree and is not concurrently submitted in candidature of any other degree.
Signature
:
…………….………………
Name
:
Azmi bin Mohamed
Date
:
18 April 2005
iii
Dedication to my beloved father, mother, family and friends…
iv
ACKNOWLEDGEMENT
First of all, in a humble way I wish to give all the Praise to Allah, the
Almighty God for with His mercy has given me the strength, keredhaanNya and time
to complete this work.
I am deeply indebted to Prof. Dr Halimaton Hamdan, my Supervisor, for her
patience, supervision, encouragement and thoughtful guidance towards the
completion of this thesis. I wish to express special appreciation to Assoc. Prof. Dr
Salasiah Endud, Assoc. Prof Dr. Zainab Ramli, Assoc. Prof. Mohd Nazlan Mohd
Muhid and Dr. Hadi Nur for their kindness and support. I also want to thank to all
my colleagues in Zeolite and Porous Material Group for their help, support, interest
and valuable hints.
I am particularly grateful to Ibnu Sina Institute For Fundamental Science
Studies, Department of Chemistry, Faculty of Science Universiti Teknologi Malaysia
and Majlis Amanah Rakyat (MARA) for all facilities, study leave and financial
support .
Lastly, I would like to acknowledge my family; whose patient love enabled
me to complete this research. Thank you.
v
ABSTRACT
Perfumery chemicals and intermediates are produced on a large scale by
Friedel Crafts alkylation or acylation of aromatic compounds in the presence of
Lewis acid catalyst. However, problem in the industrial process of perfumery
chemical and intermediate manufacture like toxity, corrosivity and production of
pollutants, make convenient to change the conventional Lewis acid AlCl3 or FeCl3
catalysts by acid solid catalyst. Thus, Al-MCM-41 catalysts were prepared with
various SiO2:Al2O3 ratios via direct and secondary syntheses using sodium aluminate
as the aluminium source. Al-MCM-41 was characterized by X-ray Diffraction
(XRD), Surface Area Analyzer Instrument and Fourier Transform Infrared
Spectroscopy (FTIR). The results indicate that Al-MCM-41 sample with a uniform
hexagonal pore structure and high surface area was synthesized. Structural studies by
27
Al and 29Si MAS NMR spectroscopy indicated that Al are in the tetrahedral form
and located in the framework. The presence of distorted framework aluminium was
also observed, more significantly in the secondary aluminated samples. Maximum
amount of Al was incorporated by direct synthesis with SiO2:Al2O3 ratio of 10 and a
calculated Si/Al ratio of 15.2. Acidity studies using Pyridine Desorption
Measurement and Temperature Programmed Desorption of Ammonia (TPD-NH3)
show that the acidity of Al-MCM-41 increases with increase in Al incorporation into
the MCM-41 framework. The potential of H-Al-MCM-41; as a heterogeneous
catalyst was studied in the hydroxyalkylation of benzene with propylene oxide as a
model reaction. Favourable reaction conditions such as SiO2:Al2O3 ratios,
temperature, time on stream, the reactant mole ratio and solvent have significant
influence on the distribution of products. Gas chromatography analysis indicates that
H-Al-MCM-41 with SiO2:Al2O3 ratio of 10 demonstrates the highest catalytic
activity with a conversion of benzene and selectivity of 92.3% and 87.5%
respectively. The formation of 2-phenyl-1-propanol was favourable occurred at a
temperature of 393 K after 24 hours with propylene oxide to benzene mole ratio of
0.5 using nitrobenzene as the solvent. The activity enhancement for catalyst is
associated with the presence of distorted tricoordinated aluminium as Lewis acid
sites. The strength of Lewis acid sites was correlated to appropriate aluminium
content, temperature, B/L ratio, crystallinity and surface area of sample which played
a role in order to improve catalytic activity of Al-MCM-41. Aprotic dipolar solvent
such as nitrobenzene stabilized the unstable intermediate of propoxy cations to
prevent propylene oxides oligomerisation. The results indicate that instead of
aluminium content, solvent and reactant mole ratio also play a role to give high
conversion and selectivity of 2-phenyl-1-propanol.
vi
ABSTRAK
Bahan kimia dan perantaraan pewangi biasanya dihasilkan pada skala yang
besar melalui tindak balas pengalkilan dan pengasilan sebatian aromatik dengan
mangkin asid Lewis. Masalah yang timbul dalam proses industri pengeluaran bahan
kimia dan perantaraan pewangi seperti ketoksikan, kakisan dan penghasilan sisa
adalah bertepatan dengan menggantikan mangkin asid Lewis konvensional AlCl3
atau FeCl3 kepada mangkin pepejal berasid. Maka, mangkin Al-MCM-41 disediakan
dengan pelbagai nisbah SiO2:Al2O3 melalui sintesis terus dan sekunder menggunakan
natrium aluminat sebagai sumber aluminium. Al-MCM-41 telah dicirikan
menggunakan teknik Pembelauan Sinar-X (XRD), Analisis Luas Permukaan dan
Spektroskopi Inframerah. Keputusan menunjukkan Al-MCM-41 mempamerkan
struktur liang heksagon yang seragam dengan luas permukaan yang tinggi. Kajian
struktur oleh Spektroskopi 27Al dan 29Si Putaran Sudut Ajaib-Resonans Magnet
Nukleus (PSI-RMN) menunjukkan aluminium hadir dalam bentuk tetrahedral dan
terletak dalam rangka struktur. Kehadiran rangka struktur aluminium terherot juga
dapat diperhatikan lebih signifikan dalam sampel sintesis secara sekunder.
Kandungan maksimum aluminium memasuki bingkaian dipamerkan oleh sampel
dengan nisbah SiO2:Al2O3 bersamaan 10 dan Si/Al dihitung bersamaan 15.2. Kajian
keasidan dijalankan menggunakan Penjerapan Piridina dan Penyahjerapan Ammonia
Suhu Teraturcara (TPD-NH3) menunjukkan keasidan Al-MCM-41 meningkat dengan
penambahan aluminium ke dalam bingkaian MCM-41. Maka, potensi mangkin
H-Al-MCM-41 dalam tindak balas Friedel-Crafts diuji ke atas tindak balas
penghidroksialkilan benzena dengan propilena oksida sebagai tindak balas model.
Taburan hasil tindak balas didapati bergantung kepada keadaan terbaik tindak balas
seperti nisbah SiO2:Al2O3, suhu tindak balas, masa tindak balas, nisbah mol reaktan
dan pelarut. Analisis kromatografi gas menunjukkan H-Al-MCM-41 dengan nisbah
SiO2:Al2O3 bersamaan 10 mempamerkan aktiviti permangkinan yang tinggi dengan
darjah penukaran benzena dan kepilihan masing-masing 92.3% dan 87.5%. 2-fenil-1propanol terhasil pada kadar terbaik pada suhu 393 K selepas 24 jam dengan nisbah
mol propilena oksida kepada benzena bersamaan 0.5 dengan nitrobenzena sebagai
pelarut. Peningkatan aktiviti permangkinan sampel ini dikaitkan dengan kehadiran
aluminium trikoordinatan terherot sebagai tapak asid Lewis. Kekuatan tapak asid
Lewis dikaitkan dengan kandungan aluminium, suhu, nisbah B/L, kehabluran dan
luas permukaan sampel yang berperanan meningkatkan aktiviti permangkinan AlMCM-41. Pelarut dwipolar aprotik seperti nitrobenzena dapat menstabilkan bahan
perantaraan ion propoksi bagi mengelakkan pengoligomeran propilena oksida.
Keputusan menunjukan selain daripada kandungan aluminium dalam sampel, pelarut
dan nisbah reaktan juga memainkan peranan dalam meningkatkan darjah penukaran
dan kepilihan 2-fenil-1-propanol.
vii
TABLE OF CONTENTS
CHAPTER
1
2
TITLE
PAGE
DECLARATION
ii
ACKNOWLEDGEMENT
iv
ABSTRACT
v
TABLE OF CONTENTS
vii
LIST OF TABLES
xi
LIST OF FIGURES
xiii
LIST OF SYMBOLS
xvii
LIST OF APPENDICES
xviii
INTRODUCTION
1.1
General Introduction
1
1.2
Research Background
2
1.3
Problem Statement
3
1.4
Research Objectives
5
1.5
Scope of Study
5
LITERATURE REVIEW
2.1
Friedel Crafts Alkylation of Aromatic Compounds
6
2.2
Mesoporous Materials
8
2.2.1
Mesoporous MCM-41 Molecular Sieves
8
2.2.2
Mechanisms of Formation for MCM-41
11
2.3
Incorporation of Aluminium into MCM-41
12
viii
2.4
Catalytic Applications of MCM-41
2.5
Characterization Techniques
2.5.1
Fourier Transform Infrared Spectroscopy
16
2.5.2
Powdered X-ray Diffraction Measurement
17
2.5.3
Magic Angle Spinning-Nuclear Magnetic
Resonance (MAS NMR) Spectroscopy
2.5.4
Temperature Programmed Desorption of
Ammonia (TPD-NH3)
2.6.2
22
Pyridine Adsorption – Fourier Transformed
Infrared Spectroscopy Measurement
23
Quantitative Analysis of Hydroxyalkylation of
Benzene with Propylene Oxide
3
21
Surface Acidity Measurement
2.6.1
2.7
19
Nitrogen Adsorption and Desorption
Measurement
2.6
15
25
EXPERIMENTAL
3.1
Synthesis of Mesoporous MCM-41
3.1.1
Synthesis of Purely Siliceous MCM-41
(Si-MCM-41)
3.1.2
3.1.3 Preparation of H-Al-MCM-41
28
29
Characterization of Mesoporous MCM-41
3.2.1
X-ray Diffraction Measurement
30
3.2.2
Surface Area Measurement
30
3.2.3
Fourier Transform Infrared Spectroscopy
30
3.2.4
Solid State Nuclear Magnetic Resonance
Spectroscopy
3.3
27
Synthesis of Aluminated MCM-41
(Al-MCM-41)
3.2
27
31
Acidity Studies of Mesoporous MCM-41
3.3.1
Temperature Programmed Desorption of
Ammonia
3.3.2
Pyridine Adsorption – Fourier Transformed
31
ix
Infrared Spectroscopy Measurement
3.4
32
Catalytic Activity of Mesoporous MCM-41 in
Hydroxyalkylation of Benzene with Propylene Oxide
3.5
3.4.1 Activation of Catalysts
32
3.4.2
33
Catalytic Reaction Procedure
Most Favourable Condition of Model Reaction
34
3.5.1
SiO2:Al2O3 Ratio
34
3.5.2
Temperature
34
3.5.3
Time on Stream
34
3.5.4
Reactant Mole Ratio Composition
34
3.5.5 Solvent
36
3.5.6
36
Autoclave Reactor
3.6
Reusability of Catalysts
36
3.7
Characterization of Hydroxyalkylation of Benzene
36
with Propylene Oxide Reaction
4
RESULTS AND DISCUSSION
4.1
X-ray Diffraction Analysis
38
4.2
Fourier Transformed Infrared Spectrum Analysis
43
4.3
Magic Angle Spinning Nuclear Magnetic Resonance
(MAS NMR)
4.3.1.
27
Al MAS NMR
47
4.3.2
29
Si MAS NMR
49
4.4
Nitrogen Adsorption and Desorption Analysis
4.5
Pyridine Adsorption – Fourier Transformed Infrared
Spectroscopy Measurement
4.6
4.9
61
Catalytic Activity of Mesoporous MCM-41 in
Friedel Crafts Reaction
4.8
54
Temperature Programmed Desorption of Ammonia
(TPD-NH3)
4.7
53
63
Determination of Amount of Desired Product
(2-phenyl-1-propanol)
65
Effect of SiO2 : Al2O3 Ratios
66
x
4.10
Effect of Temperature
68
4.11
Effect of Propylene Oxide : Benzene Mole Ratio
70
4.12
Effect of Reaction Time
71
4.13
Effect of Solvent
73
4.14
Effect of Autogenous Pressure
74
4.15
Reusability of Al-MCM-41
75
4.16
Proposed Mechanism of Hydroxyalkylation
of Benzene with Propylene Oxide Catalyzed
Al-MCM-41
5
78
CONCLUSIONS AND RECOMMENDATIONS
5.1
Conclusions
81
5.2
Recommendations
83
REFERENCES
84
APPENDICES
91
xi
LIST OF TABLES
TABLE NO.
TITLE
2.1
Type of M41S mesoporous material
3.1
Sample Codes for Al-MCM-41 with different
SiO2: Al2O3 ratios
3.2
Operation Parameters for Gas Chromatography Flame
4.3
40
Some properties of Si-MCM-41 and Al-MCM-41
Wave number (cm ) of IR spectra of Al-MCM-41
Si MAS NMR
54
Number of acid sites (µmol pyridine g-1) in
H-Al-MCM-41 samples
4.8
51
Surface properties of Al-MCM-41 with various
SiO2:Al2O3 ratios
4.7
49
Calculated peak distribution and Si/Al ratio from
29
4.6
45
Quantitative peak intensities of 27Al MAS NMR of
Al-MCM-41 samples
4.5
42
-1
samples with various SiO2:Al2O3 ratios
4.4
37
The degree of crystallinity of samples with various
SiO2:Al2O3 ratios
4.2
37
Operation Parameters for Gas Chromatography-Mass
Spectroscopy (GC-MS)
4.1
9
29
Ionization Detector (GC-FID)
3.3
PAGE
60
Ratio of Brønsted (B) to Lewis (L) acidity in the
H-Al-MCM-41samples at different desorption temperatures 60
4.9
Amount gas adsorbed of various H-Al-MCM-41
4.10
Gas Chromatography data for hydroxyalkylation of
61
xii
propylene oxide with benzene
4.11
63
The effect of temperature on the conversion and yield of
desired product using Dir-Al-MCM-41 (10) at constant
parameter (Reactant Mole Ratio: 0.5; Time: 24 hours;
Solvent: Nitrobenzene)
4.12
69
Effect of propylene oxide : benzene mole ratio on
hydroxyalkylation of benzene with propylene oxides over
Dir-Al-MCM-41 (10) at constant parameter (Temperature:
393 K; Time: 24 hours; Solvent: Nitrobenzene)
4.13
70
Effect of solvent on hydroxyalkylation of benzene with
propylene oxides over Dir-Al-MCM-41 (10) at constant
parameter (Temperature: 393 K; Reactant Mole Ratio:
0.5; Time: 24 hours)
4.14
74
Effect of autogenous pressure on hydroxylakylation of
benzene with propylene oxide at 393 K over
Dir-Al-MCM-41(10) at constant parameter
(Temperature: 393 K; Reactant Mole Ratio: 0.5; Time:
24 hours; Solvent: Nitrobenzene)
4.15
75
Reusability of Dir-Al-MCM-41 (10) at 393 K at
constant parameter (Temperature: 393 K; Reactant Mole
Ratio: 0.5; Time: 24 hours; Solvent: Nitrobenzene)
76
xiii
LIST OF FIGURES
FIGURE NO.
TITLE
PAGE
2.1
The mesoporous M41S family [30]
10
2.2
The structure of mesoporous MCM-41 material [33]
10
2.3
(1) Liquid crystal phase initiated and (2) silicate anion
initiated [34]
2.4
11
Schematic presentation on the generation of Brønsted and
Lewis acid sites with (a) Distorted framework
Tricoordinated Aluminium (b) Extraframework Aluminum
(EFAL) in Al-MCM-41
14
2.5
FTIR spectrum for purely siliceous Si-MCM-41 [53]
18
2.6
Graphical representation of the Bragg equation. The
diffraction of x-rays is interpreted as the reflection on
a set of planes ( h k l ) [51]
18
2.7
29
20
2.8
The most frequently found types of gas physisorption
n
Si chemical shifts of Q units in solid silicates [54]
isotherms: na = amount adsorbed, ms = mass of solid
adsorbent, p = equilibrium pressure, p0 = saturation
vapour pressure [56]
22
2.9
Structure of (a) Pyridine and (b) Pyridinium Ion
23
3.1
Experiment set up for catalytic testing
33
3.2
Flow chart of determining the most favourable procedure
for propylene oxide and benzene
4.1
XRD powder pattern of (a) calcined at 823 K (b)
as synthesized Si-MCM-41
4.2
35
XRD powder pattern of Dir-Al-MCM41 (10) of
39
xiv
(a) calcined (b) as synthesized samples
4.3
XRD powder pattern of Al-MCM-41 with various
(SiO2:Al2O3) ratios
4.4
27
29
50
Nitrogen adsorption isotherm of (a) Dir-Al-MCM41 (10)
(b) Sec-Al-MCM41(0.25 M)
4.11
48
Si MAS NMR spectra of Si-MCM-41 and Al-MCM-41
with various SiO2:Al2O3 ratio
4.10
46
Al MAS NMR spectra of Zeolite A and Al-MCM-41
samples with various SiO2:Al2O3 ratio
4.9
44
FTIR spectra of calcined Al-MCM-41 with various
SiO2:Al2O3 ratios : (a) 10 (b) 20 (c) 40 (d) 80
4.8
44
FTIR spectra of Sec-Al-MCM-41 (a) after (b) before
calcination at 823 K
4.7
42
FTIR spectra of Si-MCM-41 (a) after (b) before
calcination at 823 K
4.6
41
XRD powder pattern of Sec-Al-MCM41 (0.25M)
(a) calcined at 823 K (b) before calcination at 823 K
4.5
39
52
FTIR spectra of hydroxyl region of Dir-Al-MCM41
with SiO2:Al2O3 (a) 10 (b) 20 (c) 40 (d) 80 dehydrated
at 673K under 10-5 mbar pressure
4.12
FTIR spectra of hydroxyl region of Sec-Al-MCM41(0.25 M)
with dehydrated at 673K under 10-5 mbar pressure
4.13
55
55
FTIR spectra of pyridine desorbed on Dir-Al-MCM41 (10)
under vacuum 10-5 mbar pressure at (a) 298K (b) 423K
(c) 523K (d) 673K for every 1 hour
4.14
56
FTIR spectra of pyridine desorbed on Dir-Al-MCM41 (20)
under vacuum 10-5 mbar pressure at (a) 298K (b) 423K
(c) 523K (d) 673K for every 1 hour
4.15
56
FTIR spectra of pyridine desorbed on Dir-Al-MCM41 (40)
under vacuum 10-5 mbar pressure at (a) 298K (b) 423K
(c) 523K (d) 673K for every 1 hour
4.16
57
FTIR spectra of pyridine desorbed on Dir-Al-MCM41 (80)
under vacuum 10-5 mbar pressure at (a) 298K (b) 423K
(c) 523K (d) 673K for every 1 hour
57
xv
4.17
FTIR spectra of pyridine desorbed on Sec-Al-MCM-41
(0.25M) under vacuum 10-5 mbar pressure at (a) 298K
(b) 423K (c) 523K (d) 673K for every 1 hour
4.18
58
Temperature Programmed Desorption of Ammonia
(TPD-NH3) spectra of H-Al-MCM-41 of Dir-Al-MCM41
of (a) 10 (b) 20 (c) 40 (e) 80 (d) Sec-Al-MCM-41(0.25M)
4.19
Chromatogram of liquid product hydroxyalkylation of
benzene with propylene oxide without catalyst at 363 K
4.20
62
64
Chromatogram of liquid product hydroxyalkylation of
benzene with propylene oxide using Dir Al-MCM41 (10)
at 363 K after 3 hours
4.21
Calibration Curve of Standard with Internal Standard
(Toluene 2.0 M in Nitrobenzene)
4.22
64
65
Conversion of benzene and Selectivity of product (%)
with various SiO2:Al2O3 ratio at constant parameter
(Temperature: 363 K; Reactant Mole Ratio: 0.5; Time:
24 hours; Solvent: Nitrobenzene)
4.23
67
Amount of 2-phenyl-1-propanol (desired product) (mmol)
with various SiO2:Al2O3 ratio at constant parameter
(Temperature: 363 K; Reactant Mole Ratio: 0.5; Time:
24 hours; Solvent: Nitrobenzene)
4.24
68
Effect of temperature on the conversion and selectivity
of product using Dir-Al-MCM41 (10) at constant
parameter (Reactant Mole Ratio: 0.5; Time: 24 hours;
Solvent: Nitrobenzene)
4.25
69
The effect of reaction time on benzene conversion over
Dir-Al-MCM-41 (10) at constant parameter (Temperature:
393 K; Reactant Mole Ratio: 0.5; Solvent: Nitrobenzene)
4.26
72
The effect of reaction time on selectivity and oligomerisation
of propylene oxides over Dir-Al-MCM-41 (10) at constant
parameter (Temperature: 393 K; Reactant Mole Ratio: 0.5;
Solvent: Nitrobenzene)
4.27
72
X-ray diffractogram patterns of H-Dir-Al-MCM41 (10)
during three recycles
77
xvi
4.28
Proposed mechanism for Al-MCM-41 catalyzed
hydroxyalkylation of aromatics with propylene oxides
79
xvii
LIST OF SYMBOLS
B/L ratio
-
Brønsted acid sites to Lewis acid sites ratio
BET
-
Brunnauer, Emmett and Teller
Cu Kα
-
X-ray diffraction from Copper K energy levels
FTIR
-
Fourier Transform Infrared Spectroscopy
GC-MS
-
Gas Chromatography- Mass Spectroscopy
h
-
Hour
IS
-
Internal Standard
IUPAC
-
International Union of Pure and Applied Chemistry
KBr
-
Potassium Bromide
MAS NMR
-
Magic-Angle-Spinning Nuclear Magnetic Resonance
N2
-
Nitrogen
OH
-
Hydroxyl
P/Po
-
Relative pressure; obtained by forming the ratio of the
equilibrium pressure and vapour pressure Po of the adsorbate
at the temperature where the isotherm is measured
SiO2: Al2O3
-
Silica to Alumina ratio
SiO4
-
Siliceous; framework silicon in zeolite
T
-
Reaction Temperature
TO4
-
Tetrahedral unit where T= Al or Si
XRD
-
X-ray Diffraction technique
λ
-
Wavelength
2θ
-
Bragg Angle
xviii
LIST OF APPENDICES
APPENDIX
TITLE
A
Mass spectra of 2-phenyl-1-propanol
B
Chromatograms of reactant (a) benzene (b) propylene
PAGE
91
oxide (c) nitrobenzene as a solvent
92
C
Nitrogen Adsorption Isotherm
94
D
Calculation method of Conversion, Selectivity, Yield
and Percentage of Oligomerisation
96
CHAPTER 1
INTRODUCTION
1.1
General Introduction
The concept of catalysis was first discovered by Berzelius in 1836. The word
catalysis came from combination of two Greek words, κατα (kata) and λυδειν
(lysein) which was defined as ‘loosening down’ [1]. The phenomenon of catalysts
has been extensively studied since the early decades of the 19th century, and used
unconsciously for a much larger period. Nowadays, the catalyst market is seeing
moderate growth especially in fine chemicals and environmental markets sectors
which posesses higher perfoming. Meanwhile polymerization catalysts are growing
at more moderate rate, whereas refining and petrochemical catalysts are experiencing
low to flat growth. According to Comyns [2], the global catalyst market had a
volume of $10.5 billion in 2001 and is expected to grow to almost $13.5 billion or
4.6% per year by 2007. Environmental catalysts are the biggest segment in the
merchant, accounting for 27% of 2001 market. Polymerization catalysts are second
with nearly 22%, followed by refining (21%), petrochemical (20%) and fine
chemical and intermediates (10%). However, fine chemical and environmental
sectors are expected to grow at near or above 8% per year for the next 6 years. The
use of combinatorial catalyst for discovery and optimisation of catalytic performance
is expected to have a significant effect on the rate at which new catalysts are
2
developed [3, 4]. Basically catalyst can be classified into two types which are
homogeneous and heterogeneous catalyst. Homogeneous catalyst particularly Lewis
acid catalyst is well known and has been applied in Friedel-Crafts alkylation and
acylation reactions. However, new policies were introduced involving the
applications of homogeneous catalysts as a result of the problems caused by them;
such as corrosion, loss of catalyst and disrupting the environment [5]. The policies
focused on environment protection and avoidance of unfriendly reactants and
catalysts with better selectivity in order to minimize product waste and expensive
separations and recycling [6]. Meanwhile, heterogeneous catalysts such as molecular
sieves, zeolites and porous materials for liquid phase organic synthesis reactions can
give a lot of benefits such as clean reaction product solution after filtration, ease of
recovery and avoidance of corrosion. Therefore, development of efficient
heterogeneous catalysts is interesting and useful especially in the production of fine
chemical and intermediates.
1.2
Research Background
Recently, many perfume chemical and intermediates are produced on a large
scale by Friedel Crafts reactions. The reaction usually involves the alkylation or
acylation of an aromatic compound in the presence of Lewis acid catalyst. For
example, the Friedel Crafts alkylation of benzene with ethylene oxide is a
commercial route to produce β-phenethyl alcohol or 2-phenyl-ethanol. 2-phenylethanol is an important intermediate which is used because of its exquisite odour of
natural rose petal [7]. On the other hand, the alkylation of 2-methoxynapthalene with
propylene oxide is the preferred method to produce a precursor for non-steroidal,
anti-inflammatory agent naproxen [8, 9]. Basically, aluminium chloride is the most
common catalyst in the Friedel Crafts alkylation instead of sulphuric acid,
phosphoric acid, ferric chloride and boron trifluoride. The common alkylation agents
are olefin, alkyl halide, alcohol and epoxides [10].
3
The reaction of benzene or alkylbenzene with epoxides in the presence of
some homogeneous Lewis acid was first reported by Hata et al. [11]. Next, Nakajima
et al. [12] studied stereospecific Friedel-Crafts alkylation of benzene with propylene
oxide by aluminium chloride as Lewis acid catalyst and stereochemistry of ring
opening of epoxides. In 1970s, asymmetric induction in the Friedel-Crafts reaction of
benzene with (+)-1, 2-epoxybutane was studied by Nakajima et al. [13]. Meanwhile,
Inoue et al. [14] examined the reaction of toluene and anisole with 2-methoxyoxirane
and 2, 3-dimethyloxirane in the presence of aluminum chloride as Lewis acid. Later,
in the 80s, SnCl4 as catalyst on stereoselective Friedel-Crafts alkylation via epoxide
transannular and cycloalkylation reactions were studied [15, 16].
1.3
Problem Statement
Basically, introduction of hydroxyl group into an aromatic compound using
ethylene or propylene oxides are relatively well established in the presence of Lewis
acid catalysts. However, the selectivity of hydroxyalkylated products were affected
by side reactions such as epoxide oligomerisation or further reaction of the
hydroxyalkylated intermediate with the starting reactant to yield bisarylalkane
derivates.
Hence, a cleaner alternative process which is truly catalytic is needed due to
serious effluent problem associated with the use of a stoichiometric amount of AlCl3
and the corrosive reaction conditions. A lot of current processes in the production of
fine chemicals and intermediates are using homogeneous catalyst. The manufacture
of fine chemicals and intermediates involving the batch processes, are associated
with the production of large quantities of toxic waste [17]. Homogeneous catalysts
such as mineral acid, strong base and toxic metal reagent impose many drawback
including handling difficulties, inorganic contamination of organic products, the
formation of large volume of toxic waste and poor reaction selectivity leading to
unwanted isomers and side products [18].
4
In the hydroxyalkylation of aromatic with epoxides, the epoxides was added
into a suspension of anhydrous AlCl3 in the aromatic subtracts [19]. The postulated
mechanism of hydroxyalkylation proposed that aluminium chloride form an addition
compound with the epoxide which preferably opens at the most substituted carbon
atom [12]. As a result, a very reactive intermediate forms and reacts rapidly with the
aromatic and another molecule of epoxide. In this reaction, the tendency of epoxide
oligomerisation decreases due to dilution of epoxide [11]. The attack of the aromatic
gives rise to the formation of an alcohol-AlCl3 complex [20]. The complex is
generally soluble in the aromatic and is therefore more available than the unreacted
suspended AlCl3. The alcohol-AlCl3 complex becomes an increasingly important
negative factor since it is a polymerization catalyst for the epoxide [7]. Therefore, a
stoichiometric amount or excess of aluminium chloride and large excess of the
aromatic are needed to prevent oligomerisation of the epoxide. The complex has to
be decomposed with water in order to obtain the desired product. The reaction and
work-up also should occur below 25ºC; otherwise the alcohol-AlCl3 complex will
react further with another aromatic molecule to afford 1, 2-diaryalkanes [21].
Furthermore, AlCl3 catalyzed hydroxyalkylation requires a hydrolysis step resulting
in a hydrated AlCl3 waste stream. As a result, the catalyst is not reusable [14].
Extensive studies were conducted on alkylation and hydroxyalkylation of aromatic
using heterogeneous catalyst as a model reaction [22-26]. However, zeolites such as
H-ZSM-5,
modernite,
H-Beta
and
ZnNaY
catalysed
intermolecular
hydroxyalkylation of epoxides are very difficult because of competing epoxide
oligomerisation and rearrangement [21]. The main limitation of zeolites is the range
of pore sizes available. The small pore size of zeolites prevent it from being useful in
new applications with bulky and large molecule such as polymerization. Besides, the
cations present inside the structure may in some cases obstruct the pore apertures and
limit the rate of reactions [27]. In some cases for example, the formation of coke
which is deposited inside the pore of zeolite can hinder the normal diffusion of
reactants and products in and out of the catalyst [28, 29]. For this reason, in the past,
efforts were directed towards the synthesis of similar structures which led to the
discovery of MCM-41 [30]. Thus, the recent synthesis of mesoporous molecular
sieves MCM-41 has expanded the capabilities of heterogeneous catalyst. Compared
to zeolites, mesoporous MCM-41 materials is a useful candidate.
5
1.4 Research Objectives
The objectives of this research are:
1. To synthesize and characterize mesoporous Al-MCM-41 with different
SiO2:Al2O3 ratios through direct and secondary synthesis.
2. To study acidity properties of aluminium containing MCM-41.
3. To investigate the catalytic activity of hydroxyalkylation of benzene with
propylene oxide as a model reaction.
1.5
Scope of Study
In this research, Al-MCM-41 was synthesized by direct and secondary means
using sodium aluminate as the source of aluminium. Through both methods of
syntheses, aluminium was substituted for silicon in the framework and on the
surface. The insertion of aluminium into the framework of MCM-41 creates acid
sites. The structure and physical properties of catalyst were studied by X-ray
Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR),
Magic Angle Spinning Nuclear Magnetic Resonance, (27Al and
29
27
Al and
29
Si
Si MAS NMR),
Nitrogen Adsorption and Surface Area Analyzer. The acid properties of catalyst were
characterized using Temperature Programmed Desorption of Ammonia (TPD-NH3)
and Pyridine Adsorption Measurement. Al-MCM-41 was tested to catalyse the
hydroxyalkylation of benzene and propylene oxide; chosen as a model reaction, to
produce
2-phenyl-1-propanol.
The
testing
of
desired
catalyst
on
the
hydroxyalkylation of ethylene oxide and benzene could not be carried out due to the
current strict regulation on the import of ethylene oxide. The Friedel-Crafts reactions
were carried out which include six main parameters, namely SiO2:Al2O3 ratios,
temperature, time on stream, reactant mole ratio composition, solvent and autoclave
reactor effect. The products will be characterized by Gas Chromatography and Mass
Spectroscopy techniques.
CHAPTER 2
LITERATURE REVIEW
2.1
Friedel Crafts Alkylation of Aromatic Compounds
Friedel-Crafts reaction was discovered by French chemist, Charles Friedel
and his co-worker, James Crafts in the late 1800. According to Neckers and Doyle
[31], Friedel-Crafts alkylation reaction is the only substitution reaction that directly
introduces an activating group to the benzene ring. Through that reaction, a hydrogen
atom (or other substituted group) of an aromatic nucleus is replaced by an alkyl
group through the interaction of an alkylating agent in the presence of a FriedelCrafts catalyst. Lewis acid type catalysts for aromatic alkylation include aluminium
chloride, ferric chloride, boron trifluoride, antimony pentachloride, zinc chloride and
titanium chloride. Instead of Lewis acid, Brønsted acid type also plays important role
as the catalyst such as HF, H2SO4 and H3PO4. Friedel-Crafts reactions are usually
rapid at room temperature and in most cases, must be controlled by a cooling bath.
The most frequently used alkylating agents are alkyl halides, alkenes, ethers,
epoxides and alcohols. Certain types of ether and epoxides can be used as an
alkylating agent for aromatic compounds in the presence of Friedel-Crafts catalyst.
Examples are diethyl ether, diisopropyl, and di-n-butyl ethers which react with
benzene and its homologues to give alkylated products [10]. The condensation of
7
ethyl benzene ether with benzene and aluminium chloride gives a mixture of ethyl
benzene, biphenyl methane and m- and p-dibenzylbenzene [20]. Ethylene oxide, an
epoxide, normally condenses with aromatic compounds to yield β-phenethyl alcohol.
The condensation of ethylene oxide and benzene to β-phenethyl alcohol, a synthetic
ingredient of rose perfume, is catalysed by aluminium chloride. Instead of producing
β-phenethyl alcohol, ethylene oxide is also partly converted into dibenzyl [7].
Ethylene, propylene and butylene oxides also react with aromatic hydrocarbons in
the presence of BF3 to form aromatic alcohols [31]. The possibility of alkylating
aromatic hydrocarbons with ethers in the presence of boron fluoride is attributed to
the property of these ethers to form coordination compounds with BF3. Upon
heating, ethers split up into olefins and alcohol. The alkylations of benzene with
diethyl, diisopropyl, diisoamyl, isopropyl, phenyl, ethyl, n-propyl, benzyl, and
dibenzyl ethers have been studied.
The overall reactions of alkyl halides, alcohols, alkenes and epoxides acting as
alkylating agents in the presence of aluminium chloride can be summarised as
follows:
AlCl3
Ar-H
+
RX
Ar-R
+
HX
(2.1)
AlCl3
Ar-H
+
ROH
Ar-R
+ H2O
(2.2)
R-CH-CH3
(2.3)
AlCl3
Ar-H + R-CH=CH2
Ar
Ar-H + CH2CH2O
AlCl3
Ar-CH2-CH2-OH
(2.4)
8
2.2
Mesoporous Materials
International Union of Pure and Applied Chemistry (IUPAC) classifies three
categories for pore sizes in solids. Pore size distributions larger than 500 Å are
macroporous. Materials having pores between 20 Å to 500 Å represent mesoporous
material. Materials with pore size distribution less than 20 Å are related to
microporous materials. Mesoporous materials belong to a new family of material
with sizes intermediate to those usually studied by chemists and material scientist,
and therefore mesoporous materials pose new challenge in their synthesis and
characterization.
The first report on the formation of ‘low bulk density silica’ was reported by
Chiola et al. [32]. The findings were filed as a patent for Sylvania Electric Product
Inc. According to their research, the silica is obtained through the reaction of
cetyltrimethylammonium bromide and tetraethylorthosilicates. Nevertheless, the
discovery did not emphasize on the silica characterization. In 1992, researchers at the
Mobil Oil Company reported a novel family of materials called M41S [30, 33-34].
The breakthrough assures a bright future due to their properties with a well-defined
pore size between 15-100Å. With the discovery of this new type of material, the pore
size constraint of microporous materials, with pore diameter smaller than 15Å, was
overcome.
2.2.1 Mesoporous MCM-41 Molecular Sieves
The family of mesoporous M41S material consists of three types, as summarized in
Table 2.1:
9
Table 2.1: Type of M41S mesoporous material
Family Type
Structure
MCM-41
Hexagonal
MCM-48
Cubic
MCM-50
Lamellar
MCM-41 is one of the most studied and promising member of the M41S
family. MCM-41 is the abbreviation for Mobil Crystalline Material. These
mesoporous material presents regular arrays of uniform channels. By choosing
adequate reactants and reaction conditions, it is possible to tailor the channel
dimension in the range of 15-100Å or even larger. However, as the pore size
increases, the regularity of the structure is affected. The BET (Brunauer-EmmetTeller) surface area is typically over 1000 m2/g [33, 34]. The pore is usually between
0.7 and 1.2 cm3/g. The adsorption capacity is exceptionally high (more than 50 wt%
for cyclohexane at 40 Torr, 67 wt% for benzene at 50 Torr).
MCM-41 possesses excellent thermal and hydrothermal stability (up to
800°C). It is relatively stable in acidic medium (pH 2) [35]. However, it is destroyed
in a basic medium (pH12). MCM-41 is composed of silica framework, which is
almost catalytically inactive. However, the isomorphous substitution of silicon by a
variety of metals gives rise to acidic properties (Al, Ga, Fe) [36]. The possibility of
using the pore channels of MCM-41 as a support for existing catalysts has also been
considered [3, 4].
10
Figure 2.1
Figure 2.2
The mesoporous M41S family [30]
The structure of mesoporous MCM-41 material [33]
11
2.2.2
Mechanisms of Formation for MCM-41
According to Tanev et al. [37], the synthesis of MCM-41 consists of four
complementary routes:
i.
S+I-: direct co-condensation of anionic inorganic silicate species (I-) with
a cationic surfactant (S+)
ii.
S-I+: direct co-condensation of cationic inorganic silicate species (I+) with
an anionic surfactant (S-)
iii.
S+X-I+: counter-ion mediated assembly where X-= Cl- or Br-
iv.
S-M+I-: counter-ion mediated assembly where M+= Na+ or K+
The routes are based on ion pairing between ionic silicon species and surfactants.
There is also a neutral route, which is based on hydrogen bonding between neutral
silicates species and neutral surfactant (S0I0). Basically the synthesis of MCM-41
always involves a liquid template mechanism which contains two-steps. The
mechanism is summarised in Figure 2.3.
Surfactant
Micelle
Hexagonal
Array
Micellar Rod
Silicate
Calcination
1
2
Figure 2.3
Silicate
MCM-41
(1) Liquid crystal phase initiated and (2) silicate anion initiated [34]
12
The first step is the co-condensation of inorganic silicon species with organic
surfactant.
In this early step, there are three possible mechanisms. In the first
mechanism, hexagonal arrangements of micellar rods exist prior to the
polymerisation of the silicate species at the surface of the rods. Then, micellar rods
are encapsulated into 2-3 monolayers of silica. Subsequently, these rods interact to
form hexagonal arrangements. In the third mechanism, the hexagonal arrangement is
formed through the interaction of the surfactants with the silicate species. The silicate
species screen the charge of the surfactants, which renders possible the
agglomeration of micellar rods. Nevertheless, the real mechanism depends on the
reaction conditions. Finally, mesoporous MCM-41 is obtained through the removal
of surfactant from the structure. This may proceed via calcination or via solventextraction.
2.3
Incorporation of Aluminium into MCM-41
Purely siliceous Si-MCM-41 does not possess acidity. Thus, it is difficult to
introduce and apply it as a solid acid. Incorporation of metal such as aluminium [38],
titanium and zirconium [39] into the mesoporous structure have been investigated
and it was found to possess acidity. Basically, the incorporation of aluminium into
mesoporous materials is particularly important since it forms solid acid catalyst
possessing acid sites. The acidity generated is associated with the presence of
aluminium in the framework. The first detailed report on synthesis and
characterization of aluminium incorporated mesoporous materials was studied by
Corma et al. [38]. The aluminium containing MCM-41 can be synthesized by both
direct and secondary synthesis using a wide range of Si: Al ratios, depending on the
surfactant and synthetic conditions [40]. The typical characteristic of Al-MCM-41
with highly ordered mesoporosity, large surface area, high thermal stability and some
acidity, allude to the possibility of applying these materials as catalyst in the
synthesis and conversion of large molecules.
13
Basically, the catalytic activity of protonic aluminium containing MCM-41 is
attributed to the presence of acidic sites arising from the AlO4 tetrahedral units in the
framework. These acid sites may be Brønsted or Lewis in character. A purely
siliceous framework is electronically neutral due to +4 charge of Si and four -1
charges from oxygen atoms. However, the substitution of another element such as
aluminium atom affects the charge density of the framework. As a result, purely
siliceous MCM-41 loses neutrality when lattice Si4+ cations are replaced by Al3+
cations. This requires the Al atoms to be tetracoordinated and consequently becomes
negatively charged. The distribution of tetrahedral Si and Al atoms in the framework
is generally, governed by Loewenstein’s rule [41]. The rule suggests that AlO4
tetrahedral in aluminosilicate networks do not share oxygen atoms. Thus, according
to that (AlOAl) avoidance principle, Al-MCM-41 is composed of alternating silicon
and aluminium atoms and imposes an overall negative charge. The negatively
charged framework is balanced by Na+ ions present in the system.
Thus, in order to form acidic mesoporous materials, ion exchange with
ammonium nitrate is carried out followed by thermal decomposition of the NH4+
cations into protons and ammonia. The Brønsted acid sites are protons loosely
attached to lattice oxygen atoms in the vicinity of aluminium. A scheme for the
formation of these sites is shown in Figure 2.4. Further heating removes water from
the Brønsted site, exposing a distorted tricoordinated Al ion. The sites have an
electron-pair acceptor property which is identified as a Lewis acid site (Figure 2.4a).
According to Uytteroeen et al. [42], the presence of Lewis acid sites is associated
with both octahedral and tetrahedral extra framework Al (EFAL) species, created by
dehydroxylation of hydrogen forms of mesoporous materials (Figure 2.4b). The
surface of aluminium containing MCM-41 can thus display either Brønsted or Lewis
acid sites, or both depending on how the sample is prepared.
14
O
O
Si
Na+
_
O
Al
O
O
Si
O O
O O
Na+
O
Si
O O
O
_
Al
O O
O
Si
O O
O
Ion exchange with NH4NO3
solution at 363 K
O
O
Si
NH4+
_
O
Al
O
O
Si
O O
O O
O
NH4+
Si
O O
O
_
Al
O O
O
Si
O O
O
Calcination at 673 K for 4 hours
O
O
Si
H
Brønsted acid form
of Al-MCM-41
H
O
O
O
Al
O
Si
O O
O O
O
Si
O O
Al
O O
O
Si
O O
O
-H2O
(a)
O
O
Si
O
_
O
Al
O O
O
+
Si
O O
O
Al
O O
Si
O
O
O
Lewis acid form of
Al-MCM-41
or
(b)
O
O
Si
O
AlO+
_
O
Al
O O
O
Si
O O
O
Si
O O
O
Figure 2.4
Scheme for the generation of Brønsted and Lewis acid sites with (a) Distorted
Framework Tricoordinated Aluminium (b) Extra framework Aluminum (EFAL) in Al-MCM-41
15
2.4
Catalytic Applications of MCM-41
The discovery of mesoporous MCM-41 family having a regular pore
distribution between 20-100Å has opened up new possibilities in the field of
heterogeneous catalysis. Al-MCM-41 is a potential heterogeneous catalyst in organic
reactions such as Friedel-Crafts reaction, epoxidation and Diels-Alder reaction.
Friedel Crafts reaction which involves acid catalysed reaction such as alkylation or
acylation, rearrangement and isomerization cracking are important processes in
organic synthesis, fine chemical production as well as in petrochemical industry [43].
In recent years, zeolite and molecular sieve play important role as solid acid catalysts
and are replacing the conventional homogeneous mineral acid catalyst [44]. Since
siliceous MCM-41 does not possess acidity, it is difficult to use as synthesized
mesoporous material as a catalyst. Therefore, incorporation of metals such as
aluminium, titanium and iron into mesoporous structure have been investigated in
order to enable MCM-41 to be used as solid acid catalysts [3, 4, 38-39].
MCM-41 incorporated with various metals has been studied in alkylation; as
a replacement to AlCl3, HF and H2SO4, the environmentally hazardous Friedel Craft
catalysts. The combinations of large pores and mild acidity in catalyst have shown
positive results in Friedel Craft alkylation and acylation. For example, the acylation
of 2-metoxynaphthalene could produce different positional isomers with different
ratios, depending on the reaction conditions. Most desirable product is 2-acetyl-6methoxynaphthalene, used in the synthesis of anti-inflammatory drug 2-(6-methoxy2-naphthyl) propionic acid (Naproxen). Through acylation of naphthalene with acetic
anhydride by using mesoporous acidic Al-MCM-41, almost exclusive acylation of 1position occurred. Meanwhile, the replacement of acetyl chloride as acylating agent
and
Lewis
acid
with
Zn-MCM-41
achieved
the
desired
2-acetyl-6-
methoxynaphthalene [45]. Recently, aromatic alkylation on mesoporous Al-MCM-41
has been the subject of several studies leading to industrially relevant results such as
the production of ethylbenzene and cumene. The most recent example is the
synthesis of 2, 6-dialkyllnaphthalene (2-DIPN) via selective transalkylation of
alcohol and polymethylated aromatics catalysed by Al-MCM-41 [46, 47].
16
Next, the synthesis potential of carbonyl compounds has been explored over
acidic Al-MCM-41 under extremely different experimental conditions. The synthesis
of jasminaldehyde (α-η-amylcinnamaldehyde) and alkyl glucosides are revealing
examples of the roles of Al-MCM-41 as catalyst in reactions involving carbonyl
compounds. In the traditional aldol condensation of 1-heptanal with benzaldehyde on
acid catalyst, the yield of jasminaldehyde was often lowered by side reactions of both
aldehydes. However, Al-MCM-41 has been proved to be preferable for the reaction.
Through the reaction, the acetal gradually released 1-heptanal on the surface of the
catalyst, allowing the aldolic condensation with benzaldehyde to occur. At the same
time, self condensation favoured by high local concentration of n-heptanal was
minimized, enhancing the yields with selectivity of up to 90% at 80% conversion;
comparable to those produced using homogeneous catalysts [48].
2.5
Characterization Techniques
2.5.1
Fourier Transform Infrared Spectroscopy
Infrared Spectroscopy is an important method of structure characterization. In
this research, information about the samples is given by lattices coupling,
electrostatic and other effects. The KBr pellet technique is frequently used for
investigation of vibrations of the framework. The frequencies between 1500 cm-1 and
400 cm-1 provide structural information on the composition and the manner in which
the individual SiO4 tetrahedra are linked. Assignments of the spectra are classified
into two classes of absorption [49]:
1. Internal vibration of the TO4, (T= Si or Al) tetrahedra which are the primary
units of the structure. These are not sensitive to structural variations.
Vibration observed in this class are the asymmetric O-T-O stretch (1250-900
cm-1), symmetric O-T-O stretch (720-650 cm-1) and T-O bend
(500-420 cm-1).
17
2. External linkages between tetrahedra, and so are sensitive to framework
composition and topology. In this group are the double ring vibration (580610 cm-1) due to external vibrations of double 4 and 6 membered rings, pore
opening vibration (300-400 cm-1) due to breathing motion of large rings,
symmetric stretch (750-820 cm-1) and asymmetric stretch (1150-1050 cm-1).
2.5.2
Powdered X-ray Diffraction Measurement
Powdered X-ray Diffraction technique is most commonly used as a
“fingerprint” in the identification of a crystalline material. It provides a lot of
informations about unit cell dimensions, phase impurity and crystal structure [50].
All the informations are in a pattern which is a plot of the intensity of the diffraction
beams as a function of 2θ. The X-ray wavelength commonly employed is the
characteristic Kα radiation, λ = 1.5418 Å, emitted by copper. When samples diffract
X-rays, the atoms or ions, which act as secondary point sources, scatter the X-rays.
According to Bragg, crystals are built up in layers or planes such that each acts as a
semi-transparent-mirror. Some of the X-rays are reflected off a plane with the angle
of reflection equal to the angle of incidence, but the rest are transmitted to be
subsequently reflected by succeeding planes [51]. All characteristics must obey the
Bragg law which is
nλ = 2 d sin θ
(2.5)
where λ is the wavelength of the X-rays, d is the distance between planes, and the θ
is the angle of incidence of the X-ray beam to the plane [52]. The spacing of planes
(hkl) or Miller indices is related to the unit cell parameters of lattices. The Miller
indices are reflections contributing to composite peak scattered by each set of lattice
planes of randomly distributed crystallites at the appropriate 2θ angle. During the
measurement, the XRD technique presents only an averaged view of the structure.
Changes in the distance of the framework affect the position of the peaks in the
diffractogram. For example, replacement of Al-O bonds (1.69Å) by the shorter Si-O
% Transmitance
18
2
1
1. not sensitive
2. sensitive
2
1200
2
1
1
1300.0
1
1100
1000
900
800
-1
700
600
500
400.0
Wave number cm
Figure 2.5
FTIR spectrum for purely siliceous Si-MCM-41 [53]
θ
θ
θ
θ
d
Figure 2.6
Graphical representation of the Bragg equation. The diffraction of
x-rays is interpreted as the reflection on a set of planes (h k l) [51]
19
bonds (1.61Å) causes the unit cell to contract, decrease in the d-spacing and shifts of
the diffraction peaks towards higher 2θ values. The unit cell parameters of MCM-41
can be calculated from interplanar spacing using the formula:
ao = 2d100 / √3
(2.6)
The measured widths of diffraction peaks carry informations on the dimensions of
the crystallite size of samples. For amorphous phase and small particles, either broad
and weak diffraction lines or no diffraction at all is observed. Consequently, if a
catalyst contains particles with a distribution of size, XRD may only detect the larger
ones.
2.5.3
Magic-Angle-Spinning Nuclear Magnetic Resonance (MAS NMR)
Spectroscopy
29
Si MAS NMR
29
Si MAS NMR structural studies on mesoporous material rely on the local
environment of SiO4 which can be affected by the chemical shift of the central Si
atom. In order to determine the shift of SiO4 group, two features namely the number
of SiOT bridges formed by the given SiO4 tetrahedron (degree of polymerisation)
and the number of Si or Al atoms in the second coordination sphere of the central
silicon with a given number of SiOT bridges (degree of tetrahedral Al substituted)
are used [54]. Qn notation is commonly adopted in order to present the structure of
building units or silicate anion. In this notation, Q represents a silicon atom bonded
to four oxygen atoms forming a tetrahedron. The superscript n indicates the
connectivity, i.e. the number of other Q units attached to the SiO4 tetrahedron under
study. Thus, Q0 denotes the monomeric orthosilicates anion SiO44-, Q1 end-groups of
chains, Q2 middle groups in chains or cycles, Q3 chain branching sites and Q4 three
dimensionally cross-linked groups.
20
-
OOSi OO-
OOSiOSi
O-
OSiOSiOSi
O-
-
Q0
Q1
Si
O
SiOSiOSi
O-
Q2
Si
O
SiOSiOSi
O
Si
Q3
Q4
Q0
Q1
Q1 (<SiOSi = 180º)
Q2
Q3
Q4
-60
Figure 2.7
-70
29
-80
-90
δ, ppm
-100
-110
-120
Si chemical shifts of Qn units in solid silicates [54]
Figure 2.7 shows the range of
29
Si chemical shifts of Qn units in solid
silicates. The replacement of one or more Si atoms by Al atoms in the outer
coordination sphere of a Qn unit results in significant low field shifts which is of less
negative δ values. In general, each Si-O-Si Æ Si-O-Al substitution brings about a
deshielding of the chemical shift anisotropy to 5 ppm for the central silicon atom.
27
Al MAS NMR
27
Al nucleus is favourable in NMR due to the 100% natural abundance of the
nuclei and the fast relaxation which is generally observed for quadrupolar nuclei.
27
Al has a nuclear spin I = 5 / 2 and therefore a nuclear quadrupole moment, which
raises additional complications at the experimental and theoretical level. The
technique conducted at high magnetic field, produces good quality spectra with
relatively high signal-to-noise ratios and can normally be obtained within
comparatively short measurement times. Basically
27
Al MAS NMR chemical shifts
are dominated by the following structural features [54]:
i.
the coordination number of the T atom in the TOn polyhedra
21
ii.
the number and kind of atoms connected directly with the basic TO4
tetrahedra in the silicate or aluminosilicates framework
iii.
the bonding geometry around the T atom (TOT bond angles, TO bond
length and the type and location of non framework cations)
The
27
Al MAS NMR technique is able to distinguish between tetrahedrally
(framework Al) and octahedrally (non-framework Al) coordinated aluminium by
clearly separated shifts ranges of about +50 to +80 ppm for AlO4 and about -10 to
+20 ppm for AlO6.
2.5.4 Nitrogen Adsorption and Desorption Measurement
Gas adsorption measurements are widely used for determining the surface
area and pore size distribution of a variety of different solid material such as
industrial adsorbents, catalyst and pigments. Sorption capacity measurement
provides one of the simplest and most direct ways of characterizing zeolitic materials
[55]. According to Sing et al. [56], adsorption is the enrichment of one or more
components in an interfacial layer. The relationship between the amount adsorbed
and the equilibrium pressure (or relative pressure) at a known temperature are
defined as adsorption isotherm. The isotherm is usually illustrated in graphical form
of na plotted against p or p/p0. According to IUPAC isotherm shape can be classified
to four types which is shown in Figure 2.8.
Based on the figure, reversible isotherm of type II is the normal form given
by a nonporous or macroporous adsorbent. The shape is indicative of unrestricted
monolayer-multilayer adsorption up to high p/p0. Meanwhile, type I isotherm is also
reversible, but exhibits a distinctive plateau so that na approaches a limiting value as
p/p0 become 1. Type I isotherms are given by microporous catalysts such as
molecular sieve zeolites and activated carbon. The limiting uptake is governed by the
accessible pore volume. The most striking feature of the type IV isotherm are the
hysteresis loop and the plateau at high p/p0. Isotherms of this type are typical of
mesoporous adsorbents such as silica gels, M41S family and some other oxide
catalysts.
22
Figure 2.8
The most frequently found types of gas physisorption isotherms:
a
n = amount adsorbed, ms = mass of solid adsorbent, p = equilibrium pressure,
po = saturation vapour pressure [56]
2.6
Surface Acidity Measurement
2.6.1
Temperature Programmed Desorption of Ammonia (TPD-NH3)
Temperature programmed desorption of simple bases are widely used in order to
assess the total number and strength of acid sites [57]. For example TPD-NH3 has
been employed to characterize the acidity of ZSM-5 [58], modernite [59] and zeolite
Y [60]. In a typical TPD experiment, catalyst is retained in a reactor that can be
heated at linear rate. After pre-treatment, the catalyst is saturated with a probe
molecule under well defined adsorption conditions. After the excess gas is flushed
out of the reactor, the sample is heated in a flowing inert gas stream. Temperature-
23
programmed reaction studies are conducted by replacing the inert gas with a reactive
gas feed to the reactor. A thermocouple inserted in the catalyst measures the
temperature and a detector downstream measures the effluent gas composition. The
concentration of the desorbing gas in the effluent gas is monitored by thermal
conductivity, flame ionization or mass spectroscopy.
2.6.2
Pyridine Adsorption – Fourier Transformed Infrared Spectroscopy
Measurement
Numerous authors studied the acidity of mesoporous Al-MCM-41
aluminosilicates via Fourier Transformed Infrared Spectroscopy [61-63]. The nature
of acid sites was investigated using pyridine as the probe molecule. Pyridine (pKb =
8.8) is chosen as probe molecule since it enables a clear distinction between Lewis
and Brønsted type acid sites [64]. In addition, demonstrates that pyridine is more
easily protonated than ammonia and that the pyridinium ion is thermally more stable
than the ammonium ion. In any case, pyridine is to be classified as a relatively hard
base. The molecular size of molecule may give rise to steric hindrance in
intermolecular interactions.
a)
..
N
N
H
Figure 2.9
b)
N
H+
Structure of (a) Pyridine and (b) Pyridinium Ion
24
The pyridine molecule can undergo coordination to aprotic sites, in which it can be
protonated to form the pyridinium ion, PyH+ on acidic OH and undergo H-bonding
with less acidic groups. Corma et al. [38] studied the acidity and the catalytic activity
of Al-MCM-41 respectively. The following bands are present in the FTIR spectra of
pyridine-adsorbed samples:
i.
1620 cm-1 and 1455 cm-1: (Pyridine coordinated to Lewis acid sites) L
ii.
1570 cm-1 : (Hydrogen bonded pyridine) H
iii.
1640 cm-1 and 1456 cm-1 : (Pyridine protonated by Brønsted acid sites Pyridinium ions) B
iv.
1490 cm-1: (Pyridine associated with both Brønsted and Lewis acid sites)
B+L
According to Hughes and White [65], the measurement provides information
about the number of Brønsted and Lewis acid sites which can be calculated by
measuring the intensity of those bands and from the values of the extinction
coefficient. The amount of pyridine in µmole per g sample is calculated using the
following Equation 2.7:
Adsorbed pyridine (µmol)
=
B (cm-1) x sample surface area (cm2)
( 2.7 )
Adsorption Coefficient (cm µmole-1) x weight (g)
B (band area) = Imax x ∆1/2
Imax is the intensity of the band (in absorbance unit)
∆1/2, half width at half height.
In this measurement, the band area is determined using computer program of the
FTIR instrument. The pellet is of 13 mm in diameter and 10 mg in weight. The
sample surface area which is transferred by the radiation is 0.7857 cm-2 (from only
10 mm exposed to the FTIR radiation) on a 5.92 mg sample weight. The adsorption
coefficient values which are involved:
Brønsted = 3.03 ± 0.01
(2.8)
Lewis = 3.80 ± 0.01
(2.9)
25
Therefore, the amount of pyridine adsorbed is calculated as follows:
Brønsted acidity = B (cm-1) x 43.80 (cm µmol g-1)
(2.10)
Lewis acidity = B (cm-1) x 34.93 (cm µmol g-1)
(2.11)
2.7
Quantitative Analysis of Hydroxyalkylation of Benzene with Propylene
Oxide
Basically, the quantitative analysis estimates of the mass of particular solute
present in a sample are obtained from either peak height or peak area of Gas
Chromatography. Then, the values obtained are compared with peak height or area of
reference solute present in the sample at a known concentration or mass. In this
research, the main task is to determine the concentration of desired component which
2-phenyl-1-propanol in a mixture of sample. 2-phenyl-1-propanol was quantitatively
measured through Internal-Standardization technique Calibration-Curve Method. The
use of an internal standard probably gives the most accurate quantitative results [66].
The requirements for a suitable internal standard are as follows:
i.
The internal standard must be a compound which is well separated from
all components in the mixture being analyzed under the existing operation
conditions.
ii.
The internal standard shall not react with any component of the sample,
nor should it influence the physical properties of the other samples e.g.
their volatility.
iii.
The amount of internal standard to be added should be comparable to the
content of the sample component to be determined.
Internal-Standardization technique Calibration-Curve Method is based on the
preparation of a series of known solutions varying in concentration of the component
to be determined with known amounts of standard added. Hence, calibration curve
was plotted between the ratio of standard and internal standard (peak area) against
the concentration of standard. The calibration curve should be linear with slopes
26
equal to the terms (Standard/Internal Standard) peak area ratio and concentration of
standard. The curve also should cross the origin of the coordinate system. The
concentrations of the component are determined through the ratio of the amounts of
internal standard and sample injected. In principle, this ratio can be determined prior
to injection into the instrument, so that the absolute amounts injected need not be
measured. This is the principle in which the technique of internal standardization is
based [67]. Thus, internal standardization technique plays a role to give accurate
concentration of the desired product.
The main advantage of the internal standard technique is the ability to determine
the component in question without knowing the absolute amount of sample injected.
Another major advantage of the internal standard technique is that both the
components to be determined and the standard are introduced into the instrument by
a single injection. Under such conditions, it is easily ensured that the bands of the
component analyzed and of the standard are eluted and recorded under the same
conditions. However, the necessity of adding to the sample analyzed an extraneous
admixture in these technique may become a source of difficulties. Thus, the
technique can not be used if there is no free space in the chromatogram for the peak
of standard. The standard must be a compound which is well separated from all
components of the mixture analyzed under existing operating conditions. Lastly, the
standard also shall not react with any component of the sample, nor should it
influence the physical properties of other compound.
CHAPTER 3
EXPERIMENTAL
3.1
Synthesis of Mesoporous MCM-41
In this research, purely siliceous MCM-41 (Si-MCM-41) and aluminosilicate
MCM-41 (Al-MCM-41) have been synthesized using Ludox as a silica source [68].
Al-MCM-41 with various SiO2:Al2O3 ratios were synthesized by direct synthesis
(Dir) and secondary (Sec) syntheses.
3.1.1
Synthesis of Purely Siliceous MCM-41 (Si-MCM-41)
A clear solution of sodium silicate with a Na / Si ratio of 0.5 was prepared by
combining aqueous NaOH (23.5 g, 1.0 M) solution with a colloidal silica (9.415 g),
Ludox (SiO2 Aldrich 30 wt%) and heating the resulting gel mixture with stirring for
2 hours at 353 K. The sodium silicate solution was added dropwise into a
polypropylene bottle containing a mixture of 0.16 g of 25 wt% aqueous NH3, 2.86 g
cetyltrimethylammonium bromide (CTAB Fluka 99%) and 6.0 g H2O, with vigorous
stirring at room temperature. The resulting gel mixture in the bottle has a molar
composition:
28
5.9 SiO2 : 1 CTAB : 1.5 Na2O : 0.15 (NH4)2O : 250 H2O
After stirring for 2 hours, the gel mixture was heated to 370 K for 1 day. The
mixture was then cooled to room temperature. Subsequently, pH of the reaction
mixture was adjusted to 10.2 by dropwise addition of 25 wt% acetic acid with
vigorous stirring. After the pH adjustment the reaction mixture was heated again to
370 K for 1 day. The pH adjustment to 10.2 and subsequent heating for 1 day was
repeated twice. The precipitated product, Si-MCM-41 with CTAB template was
filtered, washed with doubly distilled water and dried in an oven at 370 K. The SiMCM-41 was calcined in air under static conditions using a furnace Nabertherm
model L5/S27. The calcination temperature was increased from room temperature to
823 K over 10 hours and maintained at 823 K for 4 hours.
3.1.2
Synthesis of Aluminated MCM-41 (Al-MCM-41)
Synthesis of Al-MCM-41 was carried out following two techniques:
a) Direct synthesis (Dir)
b) Secondary synthesis or post-synthesis alumination (Sec)
(a)
Direct synthesis
The procedure is almost the same as the synthesis of Si-MCM-41 except in
the second part where aluminium source was added to the mixture. The aluminium
source used in this study is sodium aluminate, NaAlO2 (Riedel-de Häer Al2O3 5056%). Different amount of sodium aluminate was added by varying the SiO2:Al2O3
ratio of the solution as listed in Table 3.1. The molar composition of Al-MCM-41 is
5.9 SiO2: (0.07-0.59) Al2O3 :1 CTAB : 1.5 Na2O : 0.15 (NH4)2O : 250 H2O
29
Table 3.1: Sample Codes for Al-MCM-41 with different SiO2:Al2O3 ratios
Al-MCM-41 Direct Synthesis
SiO2:Al2O3 ratio
Sample Code
(b)
10
Dir-Al-MCM-41(10)
20
Dir-Al-MCM-41(20)
40
Dir-Al-MCM-41(40)
80
Dir-Al-MCM-41(80)
Secondary Synthesis
Calcined sample of Si-MCM-41 (1 g) was stirred with 50 mL 0.25 M sodium
aluminate solution, in a tightly closed polyethylene bottle at 333 K for 3 hours. The
sample was filtered and washed with distilled water. Secondary Al-MCM-41 sample
was obtained after calcination in air at 823 K for 3 hours. The sample is labeled as
Sec-Al-MCM41-(0.25M)
3.1.3
Preparation of H-Al-MCM-41
H-Al-MCM-41 samples were prepared by using ion exchange technique.
Calcined Al-MCM-41 (1 g) sample was ion exchanged with 50 mL ammonium
nitrate solution (NH4NO3 Riedel-de Häer 99%) 1.0 M with vigorous stirring at 363 K
overnight. The protonic form of Al-MCM-41 was obtained by deammoniating the
NH4-AlMCM-41 sample at 673 K for 2 hours.
30
3.2
Characterization of Mesoporous MCM-41
3.2.1
X-ray Diffraction Measurement
The sample was carefully ground to a fine powder using the mortar and pestle
before mounting it on the sample holder. Then, sample was lightly pressed between
two glass slides to get a thin layer. X-ray diffraction were acquired using Bruker D8
Advance with Cu Kα radiation with λ = 1.5418 Å at 40 kV and 40 mA. Samples
were measured in the range of 2θ = 1.5-10º, with step interval of 0.02º step size and 1
second step time.
3.2.2
Surface Area Measurement
Surface area and pore volume of Dir-Al-MCM-41(10) and Sec-Al-MCM41-
(0.25M) samples were analysed using ASAP 2000 Micromeritics apparatus with
nitrogen as adsorbate at 77K, whereas Dir-Al-MCM41-(20), Dir-Al-MCM41-(40)
and Dir-Al-MCM41-(80) samples were measured using Surface Area Analyser
instrument (Thermo Finnigan Qsurf Analyser) by single point BET technique. The
technique assumes that intercept of single point equation is zero since the slope is
always so much larger than the intercept.
3.2.3
Fourier Transform Infrared Spectroscopy
25 mg of the sample was diluted with 300 mg of dry KBr and ground to a finely
divided powder, loaded into a 13 mm die, and pressed under 10 tons pressure for 5
minutes to obtain a self supporting pellet. This technique avoids excessive grinding
which might cause structural degradation. All measurements were performed at
ambient temperature to keep the hydration state of the samples constant and to
minimize any structural changes. The spectra were recorded in the range of
1500 cm-1 – 400 cm-1 with 4 cm-1 resolution using a Shidmadzu Fourier-Transform
Infrared FTIR-8300 Spectrometer.
31
3.2.4
Solid State Nuclear Magnetic Resonance Spectroscopy
Solid state NMR spetra were carried out using a Bruker Ultrashield 400 NMR
Spectrometer. 29Si MAS NMR spectra were measured at spectral frequency of 79
MHz using 4 mm zirconia double bearing rotor with recycle time delay of 600 s
and spinning rate of 10 kHz with 45º pulses. Chemical shifts were given in ppm
from external tetramethylsilane (TMS).
27
Al MAS NMR spectra were measured
at 104.2 MHz, spinning rate of 7000Hz, 1.9 µsec pulses and 2 s recycle time
delay. Each spectrum was obtained with 6000 scans. The chemical shift of
27
Al
were reported relative to Al(H2O)63+ as the reference.
3.3
Acidity Studies of Mesoporous MCM-41
3.3.1
Temperature Programmed Desorption of Ammonia
Acidity measurement of mesoporous MCM-41 was carried out using
Temperature-Programmed Desorption (TPD) with ammonia (NH3) as an
adsorbed molecule. All samples were measured on TPDRO 1100 of
Thermoquest. For pre-treatment, samples were purged at 383 K in a nitrogen
stream for 2.5 hours. Then, ammonia was adsorbed at 353 K for 0.5 hour. In the
analysis step, decomposition of the NH4+ form or desorption of NH3 were
initiated by continuous heating of sample in a 30 ccm/min flow of helium up to
873 K at a heating rate of 15 K/min. The desorbed amount of NH3 was recorded
continuously using a thermal conductivity cell.
32
3.3.2
Pyridine Adsorption – Fourier Transformed Infrared Spectroscopy
Measurement
The system consists of an IR cell, vacuum line, and pyridine as an adsorbate.
Self supporting thin wafers of 13 mm diameter was made by pressing about 10 mg of
sample between 2 steel die under 5 tons of pressure for 10 seconds. The wafer was
placed in a ring type sample holder and transferred into IR cell equipped with CaF2
window. The IR cell containing the sample wafer was attached to the vacuum line
and dehydrated at 673 K and 10-5 m bar for 8 hours. The IR spectrum of dehydrated
sample was then measured at room temperature. Pyridine was adsorbed into sample
at room temperature and allowed to interact for a few seconds. Weakly adsorbed
pyridine species were removed by evacuation under vacuum at 10-5 m bar at room
temperature, 423 K, 523 K and 673 K for every 1 hour.
3.4
Catalytic Activity of Mesoporous MCM-41 in Hydroxyalkylation of
Benzene with Propylene Oxide
3.4.1
Activation of Catalysts
250 mg of H-Al-MCM-41 sample was activated in a tube furnace before
adding into the reaction mixture. The catalyst was placed in a sintered glass tube in
nitrogen gas flow and heated at 673 K for 2 hours. Subsequently, H-Al-MCM-41
sample was kept in a desiccator filled with silica gel.
33
3.4.2
Catalytic Reaction Procedure
0.25 g catalyst was added to a reaction vessel containing (5.59mL) 80 mmol
of propylene oxide (Merck, 99%) and (14.5mL) 160 mmol of benzene (Merck,99%)
in the presence of 10 mL of nitrobenzene (J.T. Baker, 99%) as solvent. The reaction
was magnetically stirred for 24 hours at 363 K. Blank runs were also carried out for
this system. The effluents were collected in closed vials after 24 hours. A sample of
the liquid product was taken manually and analyzed by Gas Chromatography (HP
5890 Series II: Carbowax20M-FID)
water out
water in
Temperature controller
Benzene +
Propylene Oxide
+ Solvent
H-Al-MCM-41
Silicone Oil bath
Power Supply
Magnetic Stirrer
Figure 3.1
Experiment set up for catalytic testing
34
3.5
Most Favourable Condition of Model Reaction
As a model reaction, propylene oxide was reacted with benzene using various
parameters. In this reaction, six parameters were chosen in order to investigate the
effect on the distribution of products. The procedure for favourable condition of
hydroxyalkylation of benzene with propylene oxide is summarized in Figure 3.2.
3.5.1
SiO2:Al2O3 Ratio
In catalytic testing, catalysts with different SiO2:Al2O3 ratios were applied.
The reactions were carried out at 363 K for 24 hours. The performance of catalyst
was measured according to the conversion of benzene and selectivity of the desired
product.
3.5.2
Temperature
Hydroxyalkylation of benzene with propylene oxides were carried out at five
different temperatures which are 333 K, 363 K, 393 K, 423 K and 453 K for 24
hours. The catalyst showing the best efficiency in part 3.5.1 was used.
3.5.3
Time on Stream
In order to observe the trend of conversion and selectivity of desired product,
effluent samples were collected in closed vials at 1, 2, 3, 4, 24 and 28 hours.
3.5.4
Reactant Mole Ratio Composition
Reactant mole ratio composition was carried in order to investigate the
influence of the amount of propylene oxide on the yield of desired product.
35
PROPYLENE OXIDE + BENZENE
MOST FAVORABLE
CONDITION
SiO2 : Al2O3 RATIO EFFECT
TEMPERATURE EFFECT
REACTANT MOLE RATIO
TIME ON STREAM EFFECT
SOLVENT EFFECT
AUTOCLAVE REACTOR TECHNIQUE
THE MOST FAVORABLE
CONDITION
Figure 3.2 Flow chart of determining the most favorable procedure for propylene
oxide and benzene
36
3.5.5
Solvent
The effect of solvent in hydroxyalkylation reaction was studied using three
different solvents: nitrobenzene, cycloctene and dichloromethane.
3.5.6
Autoclave Reactor
Instead of using the reflux technique, hydroxyakylation of benzene with
propylene oxides were carried out in two types of autoclave: Teflon line autoclave
and stainless steel autoclave.
3.6
Reusability of Catalysts
In order to investigate the reusability of aluminium containing MCM-41, the
catalyst with SiO2:Al2O3 ratio of 10 was reused three times. After each run, the
catalyst was thoroughly washed with dichloromethane and dried in oven at 370 K for
2 hours. The catalyst was calcined at 823 K for 6 hours.
3.7
Characterization of Hydroxyalkylation of Benzene with Propylene Oxide
Reaction
The residue was characterized by using Gas Chromatography- Flame
Ionization Detector (GC-FID) model Hewlett Packard series 5890II and Gas
Chromatography -Mass Spectroscopy (GC-MS) (Agilent 6890N-5973 Network Mass
Selective Detector) technique. Operation parameters for GC-FID ad GC-MS are
shown in Tables 3.2 and 3.3.
37
Table 3.2: Operation Parameters for Gas Chromatography Flame Ionization
Detector (GC-FID)
Parameter
Properties
Column
Carbowax-2M
Carrier Gas
Hydrogen
Temperature Programming
313-473 K
Initial Time
5 min
Hold Time
5 min
Temperature Rate
10 K / min
Sample Injection Volume
1.0 µL
Table 3.3: Operation Parameters for Gas Chromatography-Mass Spectroscopy
(GC-MS)
Parameter
Properties
Column
HP-5-MS
Carrier Gas
Helium
Temperature Programming
313-473 K
Initial Time
5 min
Hold Time
5 min
Temperature Rate
10 K / min
Sample Injection Volume
1.0 µL
CHAPTER 4
RESULTS AND DISCUSSION
4.1
X-ray Diffraction Analysis
Figure 4.1 shows the powder X-ray Diffraction pattern of Si-MCM-41
sample. The XRD pattern of the as-synthesized Si-MCM-41 exhibits an intense peak
and two additional peaks showing d spacings corresponding to hexagonal lattice.
Upon calcination at room temperature to 823 K for over 10 hours and maintained at
823 K for 4 hours, the intensity of the XRD peaks increased by about 2.7 times
(Figure 4.1a). These results reflect that the degree of ordering was dramatically
improved by removal of surfactant. The result indicates that Si-MCM-41 with a high
degree of long range ordering and well formed hexagonal structure was prepared.
Meanwhile, the X-ray diffraction data of Dir-Al-MCM-41(10) is shown in Figure
4.2. The calcined sample (Figure 4.2a) has four peaks that can be indexed on a
hexagonal lattice as (100), (110), (200) and (210). XRD patterns of as-synthesized
Dir-Al-MCM-41(10) samples (Figure 4.2b) also show the characteristic peaks. After
calcination, the intensity of the peak increases and 2θ shifts to a higher value
indicating contraction of the lattice. Contraction of the lattice is caused by the
removal of template and subsequent condensation of silanol (Si-OH) groups.
Relative Intensity
39
Calcined Si-MCM-41
hkl
d(Å)
100
40.01
110
22.45
200
18.68
210
14.21
(100)
As synthesized Si-MCM-41
hkl
d(Å)
100
41.60
110
23.56
200
18.68
(a)
(110) (200)
(210)
(b)
1.5 2
3
4
5
6
7
8
9
2-Theta - Scale
Figure 4.1 XRD powder pattern of (a) calcined at 823 K (b) as-synthesized Si-MCM-41
Calcined Al-MCM-41
hkl
d(Å)
100
37.07
110
21.26
200
18.45
210
13.88
Relative Intensity
(100)
(a)
(110) (200)
(210)
As synthesized Al-MCM-41
hkl
d(Å)
100
41.29
110
24.07
200
20.87
210
15.73
(b)
1.6 2
3
4
5
6
7
8
9
2-Theta-Scale
Figure 4.2
XRD powder pattern of Dir-Al-MCM41(10) of (a) calcined (b) as
synthesized samples
40
Figure 4.3 shows the XRD powder patterns of Al-MCM-41 samples with
various SiO2:Al2O3 ratios prepared by direct and secondary synthesis. The intensity
of the 3 main peaks which are (100), (110) and (200) increased gradually with
increasing aluminium content. In general, the main peak which is narrow and most
intense, indicates higher crystallinity of the sample. In order to investigate the
correlation of XRD data to crystallinity of sample, the crystallinity of samples were
calculated by comparing the intensity of the strong XRD peak (100) with Si-MCM41 sample. The degree of crystallinity data of Al-MCM-41 samples is summarized in
Table 4.1. The result demonstrates that the crystallinity of samples increased with
increasing aluminium content in the MCM-41 framework.
Dir-Al-MCM-41(10)
sample possesses the highest crystallinity of 83.6%; meanwhile Dir-Al-MCM41-(80)
shows the lowest degree of crystallinity of 18.1%. It is expected that the decrease in
crystallinity is due to the destruction of pore walls related to the presence of
unreacted colloidal silica and some amorphous solid in the Al-MCM-41 framework.
It is also clearly observed that Sec-Al-MCM-41(0.25M) possesses a higher degree of
crystallinity compared to Dir-Al-MCM-41(80). The results indicate a distortion of
the long range ordering of the Dir-Al-MCM-41(80) structure as a consequence of
imperfectly built hexagonal arrays. As a result, structural irregularity of the silicate
wall occurred after secondary incorporation of aluminium.
Table 4.1: The degree of crystallinity of samples with various SiO2:Al2O3 ratios
Sample
Degree of Crystallinity (a) (%)
Dir-Al-MCM-41(10)
83.6
Dir-Al-MCM-41(20)
74.5
Dir-Al-MCM-41(40)
58.1
Dir-Al-MCM-41(80)
18.1
Sec-Al-MCM-41(0.25M)
49.0
(a) Comparison to Si-MCM-41 at (100) peak
41
Relative Intensity
Dir-Al-MCM-41(10)
Dir-Al-MCM-41(20)
Dir-Al-MCM-41(40)
Sec-Al-MCM-41(0.25M)
Dir-Al-MCM-41(80)
0
1.6 2
3
4
5
6
7
8
9
2-Theta- Scale
Figure 4.3 XRD powder pattern of Al-MCM-41 with various (SiO2:Al2O3) ratios
Relative Intensity
42
(a)
(b)
0
1.5
2
3
4
5
6
7
8
9
10
2-Theta - Scale
Figure 4.4 XRD powder pattern of Sec-AlMCM-41(0.25M) (a) calcined at 823 K
(b) before calcination at 823 K
Table 4.2: Some properties of Si-MCM-41 and Al-MCM-41
Sample
Si-MCM-41
As Synthesized
d100(Å)
a0(Å)
41.60
48.03
Calcined
d100(Å)
a0(Å)
40.01
46.20
Dir- Al-MCM-41(10)
41.29
47.68
37.07
42.80
Dir- Al-MCM-41(20)
41.72
48.17
38.18
44.08
Dir- Al-MCM-41(40)
42.13
48.65
38.95
44.97
Dir- Al-MCM-41(80)
42.29
48.83
40.33
46.56
Sec-Al-MCM-41(0.25M)
41.76
48.22
39.60
45.72
43
Table 4.2 shows the d spacings of (100) peak and lattice parameters a0 for as
synthesized and calcined Si-MCM-41 and Al-MCM-41 samples with various
SiO2:Al2O3 ratios. Accordingly, all calcined samples exhibit peak with lower d
spacings compared to as-synthesized samples; indicating a lattice contraction of
about 10 %. This lattice contraction is due to condensation of Si-OH group in the
meso structure. Figure 4.4 shows the powder XRD pattern of secondary synthesized
Al-MCM-41(0.25 M) before and after calcination. The XRD pattern of secondary
Al-MCM-41 prepared with 0.25 M sodium aluminate (NaAlO2) consists of three
peaks; with the strongest peak at low 2θ (100) and two weak peaks at higher 2θ
((110) and (200)). The intensity of the (100) peak also increased after calcination.
4.2
Fourier Transformed Infrared Spectrum Analysis
Figure 4.5 shows the FTIR spectra of Si-MCM-41 before and after
calcination at 823 K in the wavenumber region of 1300-400cm-1. Based on the
spectrum of calcined samples, there are two intense bands (ν1 and v2) at 1236 and
1094 cm-1 which are assigned to the asymmetric T-O-T (T= Si or Al) stretching
vibration. The band at 799 cm-1 (ν4) is associated with symmetric T-O-T (T= Si or
Al) stretching and the band at 468 cm-1 (ν5) is assigned to a TO4 bending mode. The
silanol group stretching vibration (ν3) occurs at 965 cm-1. For as synthesized SiMCM-41 sample, a similar vibration band with low intensities are observed. The
bands at 720.4 cm-1 and 673.2 cm-1 are associated with vibrations by the organic
template.
Infrared spectrum of Sec-Al-MCM-41 (0.25 M) before and after calcination
at 823 K in the wavenumber region of 1300-400 cm-1 is illustrated in Figure 4.6. The
spectrum clearly shows a low intense peak at 959 cm-1, assigned to Si-O-H or Si-OH
vibrations. The result is due to the generation of extra framework aluminium species
(EFAL). The AlO+ was generated through interaction between silanol group and
44
Relative Transmittance (%)
(a)
ν4
ν3
(b)
ν1
ν5
ν2
1300
1200
1100
1000
900
800
700
600
500
400
Wave number cm-1
Figure 4.5 FTIR spectra of Si-MCM-41 (a) after (b) before calcination at 823 K
(a)
ν4
ν3
Relative Transmitance (%T)
ν1
ν5
ν2
(b)
1300.0
1200
1100
1000
900
800
700
Wavenumber cm-1
600
500
Figure 4.6 FTIR spectra of Sec-Al-MCM-41(0.25M) (a) after (b) before
calcination at 823 K
45
extra framework aluminium. The spectrum also shows a decrease in wavenumber
from 1099 cm-1 to 1089 cm-1 due to the rearrangement of amorphous silicate wall in
the precursor species.
Figure 4.7 shows the FTIR spectra of the calcined Al-MCM-41 samples with
various SiO2:Al2O3 ratios. The IR data of the sample (wave number) are summarized
in Table 4.3. The FTIR spectra of all Al-MCM-41 samples are quite similar to
Si-MCM-41. The substitution of silicon by aluminium causes a shift of the lattice
vibration band to a lower wave number. These shifts are due to the increase of the
mean T-O distances in the wall. In this case, it is caused by the substitution of the
small silicon atom (r Si4+ = 0.26 Å) by the larger aluminium atom (r Al3+ =0.39Å).
Table 4.3: Wave number (cm-1) of IR spectra of Al-MCM-41 samples with various
SiO2:Al2O3 ratios
Samples
Wave number (cm-1)
ν1
ν2
ν3
ν4
ν5
Si-MCM-41
1236
1094
965
799
468
Dir-Al-MCM-41(10)
1218
1055
912
768
457
Dir-Al-MCM-41(20)
1229
1064
960
787
455
Dir-Al-MCM-41(40)
1225
1074
949
790
458
Dir-Al-MCM-41(80)
1233
1088
956
801
465
Sec-Al-MCM-41(0.25M)
1234
1089
959
804
463
46
(d)
Relative Transmittance (%T)
(c)
(b)
(a)
ν3
ν4
ν1
ν5
ν2
1300
1200
1100
1000
900
800
700
600
500
400
Wave number cm-1
Figure 4.7 FTIR spectra of calcined Al-MCM-41 with various SiO2:Al2O3 ratios :
(a) 10 (b) 20 (c) 40 (d) 80
47
4.3
Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR)
4.3.1
27
Al MAS NMR
27
Al MAS NMR is able to distinguish between tetrahedrally and octahedrally
coordinated aluminium. Therefore it makes it possible to establish the degree of
aluminium substitution in the silica framework; since framework Al is associated
with tetrahedral sites. Figure 4.8 shows 27Al MAS NMR spectra of zeolite A and AlMCM-41 samples with various SiO2:Al2O3 ratios by direct and secondary syntheses.
It is observed that among Al-MCM-41 samples, Dir-Al-MCM-41(10) possesses the
highest intensity at - 56 ppm which is characteristic of tetrahedrally coordinated
framework aluminium sites [69]. Meanwhile Dir-Al-MCM-41(80) exhibits the
lowest intensity at - 50 ppm. The result is in agreement with XRD data which was
mentioned previously. Thus, Dir-Al-MCM-41 (10) with highly crystallinity and well
ordered properties exhibits well resolved peak, meanwhile Dir-Al-MCM41 (80)
sample which is partially amorphous was characterized by low intense and broad
peak. It can be expected that the amorphous phase in Al-MCM-41 framework is due
to the destruction of pore walls and the presence of unreacted colloidal silica.
As can be seen from Figure 4.8, all samples retain aluminium in the
tetrahedral framework position even after calcination. In other words, the aluminium
does not dislodge from the framework during template removal. However, the main
signals of Dir-Al-MCM-41(40), Dir-Al-MCM-41(80) and Sec-Al-MCM-41(0.25M)
samples are shifted about 2-6 ppm towards high fields, indicating a distortion of the
framework aluminium. This could also possibly be due to the fact that during
aluminium incorporation in the tetrahedral silicate matrix, some of the trivalent
aluminium are anchored on the surface of the framework structure of MCM-41
owing to the ease in the dissolution and incorporation of reactive monomeric
Al(OH)4- species [70]. Distortion of aluminium in the framework results in less
homogeneous and unsymmetrical environment as evidenced by the broadening and
shifting of the peak. Incorporation of aluminium into tetrahedral framework via
various means results in imperfect aluminium sites. Since the concentration of
aluminium in the sample is proportional to the intensities
Relative Intensity
48
Zeolite A
- 56 ppm
Dir-Al-MCM-41 (10)
Dir-Al-MCM-41 (20)
Dir-Al-MCM-41 (40)
Sec-Al-MCM-41 (0.25M)
Dir-Al-MCM-41 (80)
- 100
- 50
0
ppm
Al(H2O)63+
Figure 4.8 27Al MAS NMR spectra of Zeolite A and Al-MCM-41 samples with
various SiO2:Al2O3 ratios
49
Table 4.4: Quantitative peak intensities of 27Al MAS NMR of Al-MCM-41 samples
Sample
Relative Intensity of Peak
Dir-Al-MCM-41(10)
15.2
Dir-Al-MCM-41(20)
33.2
Dir-Al-MCM-41(40)
52.4
Dir-Al-MCM-41(80)
88.7
Sec-Al-MCM-41(0.25M)
62.6
of Al peak, quantitative study of peak intensity of spectra was carried out and shown
in Table 4.4. The relative intensities were calculated by comparing the tetrahedrally
coordinated aluminium peak intensities with that of (Si/Al=1) zeolite A. Dir-AlMCM-41(10) sample possesses the lowest Si/Al ratio of 15.2 and Dir-Al-MCM41(80) show the highest Si/Al ratio of 88.7. The data also revealed that Sec-AlMCM-41(0.25M) sample contains higher aluminium content (lower Si/Al ratio)
compared to Dir-Al-MCM-41(80). It is expected that more aluminium are
incorporated during the secondary synthesis of Al-MCM-41. However, the
incorporated aluminium atoms exist as distorted framework aluminium as discussed
earlier. The distorted framework aluminium also shows enhancement at higher Si/Al
ratio.
4.3.2
29
Si MAS NMR
The
29
Si MAS NMR results for Si-MCM-41 and Al-MCM-41 with various
SiO2:Al2O3 ratios are shown in Figure 4.9. For the Si-MCM-41 sample, well resolved
peaks at -108 ppm and -101 ppm are observed. The peak at ca. -108 ppm is attributed
to Q4 which corresponds to Si(4OSi) structural units, and the peak at ca. -101 ppm is
due to Q3 silicons in Si(3OSi)OH sites [71, 72]. However, the peaks in spectra are
broadened and shifted to lower field after incorporation of aluminium with
decreasing of peak Q4. The appearance of peaks at ca. -99 ppm and -95 ppm were
observed in Dir-Al-MCM-41 (10), Dir-Al-MCM-41 (20) and Dir-Al-MCM-41 (40)
50
Q4
- 108 ppm
Si
O
Si O Si O H
O
Si
Si
O
Si O Si O Si
O
Si
Q3
- 101 ppm
Si-MCM-41
Dir-Al-MCM-41 (80)
Sec-Al-MCM-41 (0.25M)
Dir-Al-MCM-41 (40)
Si
O
Si O Si O Al
O
Si
Q
Q3
2- 99 ppm
Dir-Al-MCM-41 (20)
- 95 ppm
Si
O
Al O Si O Al
O
Si
- 70
- 80
Dir-Al-MCM-41 (10)
- 90
-100
-110
-120
ppm TMS
Figure 4.9 29 Si MAS NMR spectra of Si-MCM-41 and Al-MCM-41 with various
SiO2:Al2O3 ratio
51
samples. The peaks have been assigned to Si(3OSi,1Al)
respectively. The results confirm the findings from
27
and Si(2OSi,2Al)
Al MAS NMR spectra that
aluminium has been incorporated into MCM-41 framework. It can be seen that SecAl-MCM41 (0.25M) sample contains a broader peak at - 99 ppm with decreasing of
peak Q4 (-107 ppm) compared to Dir-Al-MCM-41(80) sample indicating the
presence of higher quantity of tetrahedral aluminium incorporated into Sec-AlMCM41(0.25M) framework. The result indicates that Dir-Al-MCM-41(80) sample
contains lower aluminium content analogous to the high degree of amorphosity
compared to Sec-AlMCM-41(0.25M). The data are in agreement with calculated
XRD measurements. As a comparison, the Si/Al framework of Dir-Al-MCM-41(10),
Dir-Al-MCM-41(80) and Sec-Al-MCM-41(0.25M) samples were calculated by
Equation 4.1 and summarized in Table 4.5:
(Si / Al)NMR =
I4 + I3 + I2 + I1 + I0
(4.1)
I4 + 0.75 I3 + 0.50 I2 + 0.25I1
where I is the calculated phase area of the NMR signal attributable to Si(nAl) units,
(n = 1,2, 3, 4) indicates the coordinated Al atoms for a given peak.
Table 4.5: Calculated peak distribution and Si/Al ratio from 29Si MAS NMR
Si(4Al)
Si(3Al)
Si(2Al)
Si(1Al)
Si(0Al)
Si/Al
I4
I3
I2
I1
I0
(NMR)
Dir-AlMCM-41(10)
-
-
28.5
54.5
271.0
12.7
Dir-AlMCM-41(80)
-
-
-
16.0
348.5
91.1
Sec-AlMCM-41(0.25M)
-
-
-
25.5
322.0
54.5
Sample
The data in Table 4.5 show that the calculated Si/Al ratios from 29Si MAS NMR is
consistently higher than the corresponding ratio value of as-synthesized sample. The
data confirm that more aluminium atoms are incorporated into the framework of SecAl-MCM-41(0.25M) compared to Dir-AlMCM-41(80). The discrepancy of the result
is due to a lower proportion of total framework aluminium and distorted framework
aluminium as a result of loss in crystallinity.
3
Volume Absorbed cm /g STP
52
(a)
500
450
400
350
300
Desorption
250
Adsorption
200
150
100
50
0
0
0.2
0.4
0.6
0.8
1
Relative Pressure (P/P0)
(b)
Volume Absorbed cm 3/g STP
450
400
350
300
250
Desorption
200
Adsorption
150
100
50
0
0
0.2
0.4
0.6
0.8
1
Relative Pressure (P/P0)
Figure 4.10 Nitrogen adsorption isotherm of (a) Dir-Al-MCM-41 (10) (b) Sec-AlMCM-41 (0.25 M)
53
4.4
Nitrogen Adsorption and Desorption Analysis
Nitrogen adsorption isotherms of Al-MCM-41 by direct and secondary
synthesis are presented in Figure 4.10. At p/p0 = 0.5, the accessible pores are totally
filled with adsorbate and the isotherm reaches a plateau that remains fairly invariant
as p/p0 approaches unity. Both the nitrogen sorption isotherms show that the AlMCM-41 structure is of type IV; typically observed in mesoporous materials [33,
34]. The total mesopore volume was calculated from the amount of vapour adsorbed
at p/p0 = 0.50; assuming that Al-MCM-41 were then filled with condensed liquid
nitrogen in the normal liquid state. Adsorption at low pressure (p/p0 < 0.25) is
accounted for by monolayer-multilayer adsorption of N2 on the wall of mesopores.
The figures show a sharp step capillary condensation in mesopores region (p/p0) =
0.3-0.4, suggesting a narrow pore distribution. Hysteresis in both the lower (p/p0
=0.1-0.4) H2 hysteresis and higher (p/p0 =0.9-1.0) H3 hysteresis pressure region is
caused by particles porosity or by significant larger pores called macropores. The
absence of this hysteresis loops in the capillary condensation range is an indication
that the material possesses pores in a lower mesopore range.
The effect of aluminium insertion into the mesoporous silicate framework is
well illustrated. Table 4.6 shows the measurement of the BET surface area and pore
volume of Al-MCM-41 samples. As can be seen, Dir-Al-MCM-41(10) sample
contains the highest surface area and pore volume. Surface area of Al-MCM-41
sample after modification also remained higher with a percentage decrease of 3-16%,
indicating that incorporation of aluminium provides higher stability to the sample.
Meanwhile, surface area and pore volume of Dir-Al-MCM41(80) is significantly
lower than Sec-Al-MCM41(0.25M). The data are in agreement with those previously
calculated from XRD measurement. The result indicates that the existence of high
amorphous phase in Dir-Al-MCM-41(80) sample blocked the pore structure of
sample to give lower pore volume. The increment of surface area was expected due
to formation of defect sites on the surface of the sample [69, 70]. Therefore, AlMCM-41 samples with high crystallinity and large surface area (> 700m2g-1), are
potential efficient catalysts for Friedel-Crafts reaction.
54
Table 4.6: Surface properties of Al-MCM-41 with various SiO2:Al2O3 ratios
BET Surface
Surface Area (m2/g)
Pore Volume
Area (m2/g)
(H-form)
(cm3/g)
Dir-Al-MCM-41(10)
1040.2
1000.5
0.73
Dir-Al-MCM-41(20)
917.4
775.1
0.68
Dir-Al-MCM-41(40)
931.0
822.4
0.59
Dir-Al-MCM-41(80)
984.5
845.6
0.40
997.6
901.5
0.45
Sample
Sec-Al-MCM-41(0.25M)
4.5
Pyridine Adsorption – Fourier Transformed Infrared Spectroscopy
Measurement
In order to determine the type of acid sites on Al-MCM-41, samples were
measured using adsorption of pyridine as a probe molecule at room temperature and
desorption of pyridine molecule at different temperature under vacuum. Acidity of
samples with various SiO2:Al2O3 were characterized by Fourier Transformed
Infrared Spectroscopy (FTIR) in the 4000-3300 cm-1 region for hydroxyl groups
stretching and 1800-1300 cm-1 for pyridine molecule stretching. Before adsorption of
pyridine, protonic Al-MCM-41 samples were dehydrated under vacuum at 673 K
under 10-5 m bar pressure for 8 hours.
Figures 4.11 and 4.12 show highly intense bands at 3740 cm-1 for all H-AlMCM-41 samples. The band can be assigned to the non-acidic silanol (Si-OH)
groups. The silanol groups at 3740 cm-1 are located inside the channels of MCM-41
with and without strong hydrogen-bonding interactions. It is apparent that
incorporation of higher aluminium content into MCM-41 framework leads to an
increase in the intensity of band assigned to terminal hydroxyls. The increment is
probably due to the presence of lateral interaction of silanol groups.
55
3.0
2.0
(a)
(b)
(c)
1.0
(d)
0.0
4000.0
3900.0
3800.0
3700.0
3600.0
3500.0
3400.0
3300.0
Figure 4.11 FTIR spectra of hydroxyl region of Dir-Al-MCM-41 with
SiO2:Al2O3 (a) 10 (b) 20 (c) 40 (d) 80 dehydrated at 673K under 10-5 mbar
pressure
1.5
ABS
1.0
0.5
0.0
4000.0
3900.0
3800.0
3700.0
3600.0
3500.0
3400.0
3300.0
Figure 4.12 FTIR spectra of hydroxyl region of Sec-Al-MCM-41 (0.25M) with
dehydrated at 673K under 10-5 mbar pressure
56
3.0
H
ABS
2.0
B L
(a)
1.0
0.0
1800.0
L
B
B+L
(b)
(c)
(d)
1750.0
1700.0
1650.0
1600.0
1550.0
1500.0
1450.0
1400.0
1350.0
1300.0
1/cm
Figure 4.13 FTIR spectra of pyridine desorbed on Dir-Al-MCM-41(10) under
vacuum 10-5 mbar pressure at (a) 298K (b) 423K (c) 523K (d) 673K for every 1 hour
2.0
H
ABS
L
1.5
BL
1.0
(a)
B
B+L
(b)
0.5
0.0
1800.0
(c)
(d)
1750.0
1700.0
1650.0
1600.0
1550.0
1500.0
1450.0
1400.0
1350.0
1300.0
Figure 4.14 FTIR spectra of pyridine desorbed on Dir-Al-MCM-41(20) under
vacuum 10-5 mbar pressure at (a) 298K (b) 423K (c) 523K (d) 673K for every 1 hour
57
1.5
H
ABS
1.25
a)
1.0
b)
L
B L
B
B+L
c)
0.75
d)
0.5
1800.0
1750.0
1700.0
1650.0
1600.0
1550.0
1500.0
1450.0
1400.0
1350.0
1300.0
1/cm
Figure 4.15 FTIR spectra of pyridine desorbed on Dir-Al-MCM-41(40) under
vacuum 10-5 mbar pressure at (a) 298K (b) 423K (c) 523K (d) 673K for every 1 hour
3.0
ABS
2.0
1.0
B
(a)
(b)
L
H
L
B
B+L
1550.0
1500.0
(c)
(d)
0.0
1800.0
1750.0
1700.0
1650.0
1600.0
1450.0
1400.0
1350.0
1300.0
Figure 4.16 FTIR spectra of pyridine desorbed on Dir-Al-MCM-41(80) under
vacuum 10-5 mbar pressure at (a) 298K (b) 423K (c) 523K (d) 673K for every 1 hour
58
a)
B L
H
B
2.0
B+L
L
ABS
b)
1.0
c)
d)
0.0
1800.0
1750.0
1700.0
1650.0
1600.0
1550.0
1500.0
1450.0
1400.0
1350.0
1300.0
1/cm
Figure 4.17 FTIR spectra of pyridine desorbed on Sec-Al-MCM-41(0.25M)
under vacuum 10-5 mbar pressure at (a) 298K (b) 423K (c) 523K (d) 673K for every 1
hour
Figures 4.13 - 4.17 show the infrared spectra of pyridine in the 1300-1800 cm-1
region desorbed on the H-Al-MCM-41 at various temperatures. These thermal
treatments were carried out at 298 K, 323 K, 523 K and 673 K under 10-5 m bar
pressure for every one hour. All the samples possess six main peaks which can be
classified into 4 main categories which are:
i.
1450 and 1620 cm-1 (Lewis acid sites)
ii.
1550 and 1640 cm-1 (Brønsted acid sites)
iii.
1490 cm-1 (Brønsted and Lewis acid sites )
iv.
1590 cm-1 (Hydrogen bonded pyridine)
The generation of Lewis acid sites in the samples were contributed by extra
framework aluminium species (EFAL) or distorted framework aluminium present in
the form of Al3+, AlO+, Al(OH)2+ or charged AlxOyn+ clusters . Therefore, the band at
1450 and 1620 cm-1 occurred due to pyridine coordinated with extra framework
(EFAL) or distorted framework aluminium in the protonic Al-MCM-41. Meanwhile,
Brønsted acid sites in the samples are referred to aluminium in the framework in
which the pyridine was protonated to pyridinium ion form. It can be seen that with
59
increase of evacuation temperature, the intensity of main peak slightly decreases.
Tables 4.7 illustrate the relative concentration of Brønsted and Lewis acid sites in
(µmol/g) the samples with different SiO2:Al2O3 ratios. The results reveal that with
increasing SiO2:Al2O3 ratios, the amount of acid sites slightly decrease. According to
Hughes and White [65], the higher relative concentration (µmol/g sample) of these
acid sites will give stronger acid sites in the samples. Therefore, the results evidently
demonstrate that acidic Dir-Al-MCM-41(10) provides the highest acidity among the
samples. The data also shows that Sec-Al-MCM-41(0.25M) possesses a higher
number of acid sites compared with Dir-Al-MCM-41(80). This is probably related to
higher aluminium incorporation and crystallinity in the Sec-Al-MCM-41(0.25M)
framework. It is clear from Table 4.7 that as the desorption temperature increased
from 298-673 K, the number of acid sites decreased, correspondingly to the decrease
in crystallinity of the sample. Consequently, the results suggest that the presence of
amorphous phase in sample caused a decrease in the thermal stability of Al-MCM41, more significantly in sample with lower aluminium content [35].
In order to estimate quantitatively the ratio of the Brønsted to Lewis (B/L)
site for Al-MCM-41 at various temperatures, the approach by Chakraborty et al. [64]
was adopted. The approach assumed that the ratio of the molar extinction
coefficients, εB/εL remain the same in the temperature range of 323-573 K. Thus, the
values of integrated molar extinction coefficient for Brønsted and Lewis sites chosen
are 1.67 and 2.22 cm/µmol, respectively. From Table 4.8, the relative distribution of
acid sites (B/L ratio) for Dir-Al-MCM-41(10) at different temperature is in the 0.23 –
0.26 range. The result indicates that with high aluminium content and degree of
crystallinity, the ratio of both Lewis and Brønsted remained low even at high
temperature. Thus, samples with higher aluminium content provide higher thermal
stability and more Lewis acid sites. In contrast, in the 298–523 K temperature range,
the B/L acid site ratio of Sec-Al-MCM-41(0.25M) sample decrease with increasing
of desorption temperature. The decreasing of B/L ratio corresponds to the increment
of distorted tricoordinated aluminium formation which are anchored on the surface of
the framework structure of MCM-41. It is believed that the location and environment
of aluminium atom within the wall controls the acid strength. The closer the
aluminium atoms to the wall surface, the greater is their acid strength [74].
60
Meanwhile, Dir-Al-MCM-41(20) and Dir-Al-MCM-41(40) samples, the desorption
temperature generate an increase in B/L ratio, implying an increase in Brønsted
acidity. Loss in Lewis acidity have been attributed to removal of distorted
tricoordinated removal during calcination. Thus, in mesostructure Dir-Al-MCM41(20) and Dir-Al-MCM-41(40), Si-O(H)-Al sites are believed to be prefentially
located at or near the wall surface [36].
Table 4.7: Number of acid sites (µmol pyridine g-1) in H-Al-MCM-41 samples
µmol pyridine g-1
Sample
Brőnsted acid sites
Lewis acid sites
298K
423K
523K 673K
298K
423K 523K 673K
Dir-AlMCM-41(10)
48.9
37.9
21.1
14.7
108.6
93.1
52.2
36.8
Dir-AlMCM-41(20)
31.1
25.8
16.5
12.5
75.1
57.6
26.1
15.2
Dir-AlMCM-41(40)
25.4
23.3
13.4
9.6
56.2
43.7
21.0
8.5
Dir-AlMCM-41(80)
14.3
7.7
5.3
2.8
27.3
22.9
14.1
6.2
Sec-AlMCM-41(0.25M)
19.6
9.3
7.2
5.7
49.1
36.5
27.4
12.5
Table 4.8: Ratio of Brønsted (B) to Lewis (L) acidity in the H-Al-MCM-41 samples
at different desorption temperatures
Sample
B/L ratio
298K
423K
523K
673K
Dir-AlMCM-41(10)
0.26
0.24
0.24
0.23
Dir-AlMCM-41(20)
0.25
0.32
0.37
0.48
Dir-AlMCM-41(40)
0.27
0.31
0.38
0.69
Dir-AlMCM-41(80)
0.31
0.20
0.22
0.27
Sec-AlMCM-41(0.25M)
0.23
0.14
0.14
0.26
61
4.6
Temperature Programmed Desorption of Ammonia (TPD-NH3)
Temperature Programmed Desorption spectra of ammonia (TPD-NH3) on the
the H-Al-MCM-41 with various SiO2:Al2O3 ratios are depicted in Figure 4.18. The
amount of desorbed ammonia (µmol/g) which corresponds to the integral intensity of
the desorption curves recorded in the temperature range of 353-873 K is given in
Table 4.9. Dir-Al-MCM-41(10) has the largest number of strong adsorption sites for
ammonia. As shown in Figure 4.18, the desorption curve of this sample consists of a
low temperature peak with a maximum at 423K. The ammonia desorption at 423 K is
related to the low temperature peak value which corresponds to weak acid sites. The
medium temperature is overlapped by the large low temperature peak of desorption,
which is commonly assigned to Lewis and Brønsted sites of weak acidity strength
[73, 74]. Data in Table 4.9, shows that with increasing aluminium content in the
catalyst, the amount of gas absorbed is increased. The result indicates that Al-MCM41 has a moderate acidity and the acidity of catalyst increases with increasing
aluminium in the MCM-41 framework.
Table 4.9: Amount gas adsorbed of various H-Al-MCM-41
Dir-AlMCM-41(10)
Amount of desorbed ammonia (
µmol/g)
399.2
Dir-AlMCM-41(20)
237.3
Dir-AlMCM-41(40)
207.7
Dir-AlMCM-41(80)
167.6
Sec-Al-MCM-41(0.25M)
192.8
Sample
62
423 K
Relative Signal (mV)
(a)
(b)
b)
(c)
(d)
(e)
0
373
473
573
673
773
873
Temperature (K)
Figure 4.18 Temperature Programmed Desorption of Ammonia (TPD-NH3) spectra
of H-Al-MCM-41 of Dir-Al-MCM-41 of (a) 10 (b) 20 (c) 40 (e) 80 (d) Sec-AlMCM-41(0.25M)
63
4.7
Catalytic Activity of Mesoporous MCM-41 in Friedel Crafts Reaction
The catalytic activities of mesoporous MCM-41 as heterogeneous catalyst
were applied in hydroxyalkylation of benzene with propylene oxides. The
investigations are focused on optimization of the model reaction in order to get an
optimum parameter in the production of 2-phenyl-1-propanol. Figures 4.19 and 4.20
show the chromatograms of liquid sample of hydroxyalkylation of benzene with
propylene oxides without catalyst and after being catalyzed by mesoporous H-AlMCM-41 at 363 K for 3 hours. Analysis by gas chromatography indicates 5 main
peaks with different retention times, tR (min); identified and listed in Table 4.10.
Sampling at different parameters such as temperature, time, reactant / reactant mole
ratio give the same chromatogram. The results indicate that all the reactions give the
same product but different composition and yield. The 2 main peaks with retention
times, tR= 3.22 min and tR= 6.89 are assigned to reactant peak for propylene oxide
and benzene respectively. The peak with tR= 23.9 is assigned to 2-phenyl-1-propanol.
Table 4.10: Gas Chromatography data for hydroxyalkylation of propylene oxide with
benzene
Rt ( min)
Molecular Weight
Formula
Compound
3.22
59
C3H7O
Propylene Oxide
6.89
78
C6H6
Benzene
9.35
92
C6H5CH3
Toluene (IS)
22.0
123
C6H5NO2
Nitrobenzene
23.9
136
C6H5C3H6OH
2-phenyl-1-propanol
Toluene
Nitrobenzene
20.522
15.053
15.977
3.691
3.224
Propylene Oxide
22.043
Benzene
9.353
6.897
64
22.048
Toluene
6.826
Benzene
9.355
6.907
Figure 4.19 Chromatogram of liquid product hydroxyalkylation of
benzene with propylene oxide without catalyst at 363 K
Nitrobenzene
Propylene Oxide
23.937
22.620
20.617
8.980
3.701
19.331
3.228
2-phenyl-1-propanol
Figure 4.20 Chromatogram of liquid product hydroxyalkylation of benzene with
propylene oxide using Dir-Al-MCM-41(10) at 363 K after 3 hours
65
4.8
Determination of Amount of Desired Product ( 2-phenyl-1-propanol)
In order to investigate the amount of desired product which is
2 phenyl-1-propanol, calibration-curve method was employed. In this technique, a
plot of the amount of the standard injected (peak area)/ internal standard (peak area)
against the various concentration of the standard were constructed. In this research,
toluene (2.0 M) in nitrobenzene was chosen as the Internal Standard. The plot is
shown in Figure 4.21. Thus, the concentrations of desired product are determined
from the corresponding value of y / 0.9001 according to the respective calibration
curve.
Calibration Curve
5
y = 0.9001x
R2 = 0.9958
Standard / Internal Standard
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
0
0.5
1
1.5
2
2.5
3
3.5
Standard Concentration (M)
4
4.5
5
5.5
Figure 4.21 Calibration Curve of Standard with Internal Standard (2.0 M Toluene in
Nitrobenzene)
66
4.9
Effect of SiO2 : Al2O3 Ratios
The catalytic activity of aluminium containing MCM-41 has been
investigated for Friedel Crafts reaction for hydroxyalkylation of benzene with
propylene oxides as a model reaction. Figures 4.22 and 4.23 show that in the absence
of catalyst the performance of reaction was very low with 8.2% conversion of
benzene and no desired product (2-phenyl-1-propanol) was obtained. Hence, in
order to study the effect of aluminium content in mesoporous MCM-41 to catalytic
performance, Al-MCM-41 prepared with various SiO2 : Al2O3 ratios were tested in
the hydroxyalkylation of benzene with propylene oxide at 363 K. Figure 4.22 shows
the conversion of benzene and selectivity of the product at various SiO2:Al2O3 ratios.
The catalyst with SiO2:Al2O3 ratio of 10 (Dir-Al-MCM-41(10)) gives the highest
conversion and selectivity of benzene which are 74.5% and 63.9% respectively while
the catalyst with SiO2:Al2O3 ratio of 80 (Dir-Al-MCM-41(80)) gives the lowest
conversion and selectivity of benzene of 17.1% and 13.3% respectively. Both results
show that benzene conversion and selectivity of the product increase with increasing
framework aluminium content.
Meanwhile, the same trend also occurred to the amount of desired product
obtained as shown in Figure 4.23. 2-phenyl-1-propanol was obtained at the highest
amount of 2.49 mmol using SiO2:Al2O3 ratio of 10 (Dir-Al-MCM-41(10)) and lowest
amount of 0.14 mmol by SiO2:Al2O3 ratio of 80(Dir-Al-MCM-41(80)). The result is
in a similar order of acidity; measured by Temperature Programmed Desorption of
Ammonia (TPD-NH3). It indicates a relationship between acidity and Friedel Craft
reaction [59, 64]. Dir-Al-MCM-41(10) gave the highest total amount of acidity,
whereas that prepared with lowest aluminium content, Dir-Al-MCM-41(80), gave the
lowest acidity. Such poor performance is expected since the catalyst is almost
amorphous with smaller pore volume. It is known from literature that framework of
aluminium rich molecular sieves are generally less stable than those of the silicon
rich ones [42]. However, appropriate aluminium content create high acid sites
depend on location and environment of aluminium atom within the wall. In addition,
crystallinity of sample, surface area and B/L ratio also play a role in order to improve
catalytic activity of Al-MCM-41. It is evident from the data observed that maximum
67
catalytic activity is observed in samples not only with high aluminium content, but
high degree of crystallinity and large surface area. Sample Dir-Al-MCM-41(10) is
observed as the most active since it has high crystallinity and surface area which was
evidenced by XRD and Nitrogen Adsorption measurement.
In contrast, Sec-Al-MCM-41(0.25M) having the same order of magnitude of
selectivity as Dir-Al-MCM-41(40) gave much less product than the latter. The
increment of selectivity is due to the low B/L ratio in the Sec-Al-MCM-41(0.25M)
compared to Dir-Al-MCM-41(40). The B/L acid site ratio of Sec-Al-MCM41(0.25M) sample between 298 K to 423 K decreases dramatically with increasing of
desorption temperature. The decreasing of B/L ratio corresponds to the increment of
distorted tricoordinated aluminium formation which is anchored on the surface of the
framework structure of MCM-41. However, the amount of acid Lewis in the sample
is not good enough to convert all reactant. Thus, the conversion of benzene is still
lower compared to Dir-Al-MCM-41(40). In addition, Sec-Al-MCM-41(0.25M) is
less crystalline, rendering it less stable at high temperature.
80
Conversion
Selectivity
% (Percentage)
70
60
50
40
30
20
10
0
without
catalyst
10
20
40
Catalyst (SiO2/Al 2 O3 )
80
secAlMCM41
Figure 4.22 Conversion of benzene and selectivity of product (%) with various
SiO2:Al2O3 ratio at constant parameter (Temperature: 363 K; Reactant Mole Ratio:
0.5; Time: 24 hours; Solvent: Nitrobenzene)
Amount of desired product (mmol)
68
3
2.5
2
1.5
1
0.5
0
without
catalyst
10
20
40
Catalyst (SiO2:Al2O3)
80
secAlMCM41
Figure 4.23 Amount of 2-phenyl-1-propanol (desired product) (mmol)
with various SiO2:Al2O3 ratio at constant parameter (Temperature: 363 K;
Reactant Mole Ratio: 0.5; Time: 24 hours; Solvent: Nitrobenzene)
4.10
Effect of Temperature
The effect of reaction temperature on the conversion of benzene, selectivity
and the amount of desired product which is 2-phenyl-1-propanol is shown in Table
4.11 and Figure 4.24. In this reaction, Dir-Al-MCM-41(10) was selected due to good
conversion of benzene, selectivity and higher amount of 2-phenyl-propanol obtained.
In order to improve the benzene conversion with high yield of product, the reactions
were carried out at different temperatures which are in the range of 333-453 K.
During the first stage, the general trend for hydroxyalkylation of benzene with
propylene oxide reaction was that the conversion and selectivity of the product
increased with increasing temperature.
However, after 393 K, the data observed that the conversion of benzene was
significantly decreased. The result revealed that alkylation of benzene is also affected
by temperature. The lower conversion obtained can be attributed to the decrease in
crystallinity and number of acid sites. Meanwhile, selectivity of 2-phenyl-1-propanol
69
after 393 K decreased slightly with temperature, which maybe correlated to nonselective reactions taking place on the external acid surface on the catalyst or to the
weaker steric hindrance in pores of MCM-41 [70]. Another possible reason is due to
the stability of Brønsted acid sites at high temperature, enabling the catalyst to
further creates dehydroxylation of product to give 2-phenyl-propene. Thus, the result
proved that the reaction reached an optimum condition at the temperature of 393 K.
Table 4.11: The effect of temperature on the conversion, selectivity and yield
of desired product using Dir-Al-MCM-41(10) at constant parameter (Reactant
Mole Ratio: 0.5; Time: 24 hours; Solvent: Nitrobenzene)
Temperature
Conversion of
Selectivity
Product obtained
(K)
benzene (%)
(%)
(mmol)
333
67.4
56.9
1.46
363
74.5
63.9
2.49
393
92.3
87.5
3.91
423
22.9
81.0
0.58
453
11.3
82.2
0.39
100
% Percentage
90
80
70
Conversion of
benzene
Selectivity
60
50
40
30
20
10
0
333
363
393
423
453
Temperature (K)
Figure 4.24 Effect of temperature on the conversion and selectivity of
product using Dir-Al-MCM-41(10) at constant parameter (Reactant Mole
Ratio: 0.5; Time: 24 hours; Solvent: Nitrobenzene)
70
4.11
Effect of Propylene Oxide: Benzene Mole Ratio
The effect of varying the propylene oxide : benzene mol ratio on benzene
conversion, selectivity and amount of desired product at 393 K over Dir-Al-MCM41(10) after 24 hours is illustrated in Table 4.12. It is observed that the benzene
conversion, selectivity and the amount of desired product increased with decrease in
the propylene oxide: benzene mol ratio. At the lowest ratio of 0.5, propylene oxide
was the limiting reactant and exhibited the highest selectivity of 87.5% and amount
of desired product obtained (3.91 mmol). Whereas the mol ratio of 2.0 shows the
lowest selectivity of 27.7% and amount of desired product of 1.51 mmol.
Meanwhile, at the ratios of 1.0 and 2.0, propylene oxide became an excess reactant
so that the benzene exhibited a lower conversion and least amount of desired
product. The result revealed that non-stoichoimetric reaction occurred with lower
amount of propylene oxide was needed. The decrease in conversion and selectivity
were probably due to the formation of propylene oxide oligomers which blocked the
active sites of catalyst [12, 21].
Table 4.12: Effect of propylene oxide: benzene mole ratio on hydroxyalkylation of
benzene with propylene oxides over Dir-Al-MCM-41(10) at constant parameter
(Temperature: 393 K; Time: 24 hours; Solvent: Nitrobenzene)
(Propylene
Benzene
Selectivity of
Amount of desired
Oxides: Benzene)
Conversion
2-phenyl -1-propanol
product obtained
mole ratio
(%)
(%)
(mmol)
0.5
92.3
87.5
3.91
1.0
77.3
33.4
2.23
2.0
56.4
27.7
1.51
71
4.12
Effect of Reaction Time
The effect of reaction period on the hydroxyalkylation of benzene with
propylene oxides was studied using Dir-Al-MCM-41(10) at 393 K and reactant mole
ratio of 0.5. Figure 4.25 shows the conversion of benzene was increased from 26.9%
for a reaction time of one hour to a maximum of 92.3 % for a reaction period of 24
hours. Beyond this period the conversion started to decrease to about 16.8% after 28
hours. At the early stage, the data revealed that higher amount of Lewis acid sites in
Dir-Al-MCM-41(10) started to interact with both reactants. However, after 24 hours,
the benzene conversion decreased significantly, due to deactivation and saturation of
Lewis acid sites in catalyst. Basically, deactivations of catalyst are related to the
decrease in number and quality of acid sites. Furthermore, the formation of new
product located at the surrounding of active sites, prevent the accessibility of existing
reactant to react with the remaining Lewis acid sites in a Al-MCM-41 pore [43].
Meanwhile, the selectivity of 2-phenyl-1-propanol follows the same trend as
the conversion which is depicted in Figure 4.26. After 3 hours of reaction, there is a
significant increase in selectivity which reached a maximum of 87.5% at 24 hours.
Selectivity of 2-phenyl-1propanol is decreased due to the presence of side reaction.
Instead of Lewis acid sites, the appearance of Brønsted acid sites in catalyst at high
temperature probably enhances further dehydroxylation of 2-phenyl-1-propanol to 2phenyl-1propene. The result was evidenced through the appearance of additional
peak in gas chromatogram. The percentage of oligomerisation of propylene oxides
shows a gradual decrease from 25.4% at one hour of reaction to 4.6% after 28 hours.
Basically, the oligomeric products were formed from the oligomerisation of
propylene oxide on the basis of gas chromatography study [21]. The reduction of
oligomers product provides lower steric hindrance and both of reactant can diffuse
and react with acid sites without great difficulty.
Conversion of benzene (%)
72
100.0
90.0
80.0
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
1
2
3
4
20
24
28
Time (hour)
Percentage (%)
Figure 4.25 The effect of reaction time on benzene conversion over
Dir-Al-MCM-41(10) at constant parameter (Temperature: 393 K;
Reactant Mole Ratio: 0.5; Solvent: Nitrobenzene)
100
90
80
70
60
50
40
30
Selectivity
Oligomerisation
20
10
0
1
2
3
4
20
24
28
Time (hour)
Figure 4.26 The effect of reaction time on selectivity and oligomerisation
of propylene oxides over Dir-Al-MCM-41(10) at constant parameter
(Temperature: 393 K; Reactant Mole Ratio: 0.5; Solvent: Nitrobenzene)
73
4.13
Effect of Solvent
To understand the role of solvent in hydroxyalkylation of benzene with
propylene oxide, the reaction was carried out using 3 different solvents:
nitrobenzene, cyclooctene and dichloromethane. In order to investigate and compare
the performance of Friedel-Crafts reaction, non-solvent reaction also was carried out
as a blank. The reaction conditions and the results of hydroxyalkylation of benzene
with propylene oxides are presented in Table 4.13. The data show that in the absence
of solvent the catalytic activity of reaction was very low with 24.3 % conversion of
benzene and 0.21 mmol of 2-phenyl-1-propanol was obtained. It was observed that
the highest quantity of 2-phenyl-1-propanol was formed when nitrobenzene was used
as the solvent. The activity of catalyst increased considerably in the presence of
aprotic solvent due to interaction of negative charge or electron lone pair of solvent
with an intermediate of epoxy cations [25]. Through the interaction, an intermediate
of epoxy cations became stable and subsequently attacked by electrophiles on
aromatic cycle. The stable intermediate prevented oligomerisation from occurring in
Friedel Crafts reaction [26]. Thus, it is clearly observed that high degree of
oligomerisation of propylene oxide occurred in non-solvent reaction.
Meanwhile, dichloromethane possesses lower conversion and amount of 2phenyl-1-propanol compared to nitrobenzene. The observation was related to the
degree of polarity of solvent. According to Reichardt, nitrobenzene was classified as
Aprotic Highly Dipolar (AHD), meanwhile dichloromethane was classified as
Aprotic Dipolar (AD) [75]. The activity of the catalyst decreased considerably in the
presence of cyclooctene. The result was expected since cyclooctene provide nonpolar properties. Thus, the lower interaction of solvent with epoxy cations provides
higher formation of oligomerisation of propylene oxide. It was observed that the
catalytic activity of hydroxyalkylation of benzene with propylene oxide increases in
the following order: cyclooctene < dichloromethane < nitrobenzene. The results
indicate that in the presence of nitrobenzene as a solvent, the degree of
oligomerisation was minimized by the dilution effect and thus the reaction was
enhanced.
74
Table 4.13: Effect of solvent on hydroxyalkylation of benzene with propylene
oxides over Dir-Al-MCM-41 (10) at constant parameter (Temperature: 393 K;
Reactant Mole Ratio: 0.5; Time: 24 hours)
Benzene
Oligomerisation
Amount of 2-phenyl-
Conversion (%)
(%)
1-propanol (mmol)
Non - solvent
24.3
70.24
0.21
Nitrobenzene
92.3
4.89
3.91
Dichloromethane
69.4
25.43
2.54
Cyclooctene
48.9
50.75
0.95
Solvent
4.14
Effect of Autogenous Pressure
Basically, reflux is a technique used in chemistry to apply energy to reactions
over an extended period of time. Through this technique, a liquid reaction mixture is
placed in a vessel open only at the top. The vessel is connected to a vertical
condenser, such that any vapors given off are cooled back to liquid, and fall back into
the reaction vessel. The vessel is then heated vigorously during the course of the
reaction. Meanwhile, the autogenous pressure method has recently been extended
from zeolite synthesis to the formation of condensed inorganic solids, which find
uses in diverse areas due to properties such as low thermal expansion in closed
system [1, 7].
Thus, in order to investigate the effect of autogenous pressure compared to
reflux technique in hydroxyalkylation of benzene with propylene oxide, the reaction
was carried out using an autoclave or pressurized reactor. Two types of autoclaves
were used:
i. Teflon
ii. Stainless steel
75
The effects of autogenous pressure on the reaction are given in Table 4.14.
The reaction was carried out at 393 K with propylene oxide: benzene mol ratio = 0.5
over Dir-Al-MCM-41(10). The result shows that Teflon autoclave gave a higher
benzene conversion compared to stainless steel. Meanwhile the selectivity of desired
product using stainless steel was 11.7 %, which is higher than the selectivity
produced using Teflon autoclave. There is no appreciable difference in conversion
and selectivity of product using Teflon or stainless steel reactors. However the
conversion and selectivity of product are still inferior to those obtained using the
reflux technique. In addition, another advantage of reflux technique is that it can be
left for a long period of time without the need to add more solvent or fear of the
reaction vessel boiling dry.
Table 4.14: Effect of autogenous pressure on hydroxyalkylation of benzene with
propylene oxide at 393 K over Dir-Al-MCM-41(10) at constant parameter
(Temperature: 393 K; Reactant Mole Ratio: 0.5; Time: 24 hours; Solvent:
Nitrobenzene)
Benzene
Autoclave Type
Conversion
Selectivity of
Amount of
2-phenyl-1-propanol 2-phenyl-1-propanol
(%)
(%)
(mmol)
Stainless steel
25.1
11.7
2.2
Teflon
28.4
10.2
2.3
Reflux technique
92.3
87.5
3.9
4.15
Reusability of Al-MCM-41
In order to investigate the reusability of aluminium containing MCM-41, the
application of Dir-Al-MCM-41(10) catalyst was recycled three times. After each run,
76
the catalyst was thoroughly washed with dichloromethane and dried in oven at 370 K
for 2 hours. The catalyst was calcined at 823 K for 6 hours. The data given in Table
4.15, indicate that selectivity of 2-phenyl-1-propanol was retained with a slightl
decrease even after the second repetitions. Meanwhile, the conversion of benzene
significantly decreased. The observation was correlated to the increase of
deactivation of catalyst factor. This factor occurred due to decrease in number of acid
sites.
However, the conversion of benzene and product selectivity decreased
significantly after the third recycle, due to loss of catalyst during filtration.
Considering that, therefore, there is no loss in activity on the unit weight basis. The
structure and crystallinity of catalyst remained intact throughout all reactions as
proven by the XRD patterns in Figure 4.27. The results clearly suggest that the
catalyst is stable and provides a moderate regenerability.
Table 4.15: Reusability of Dir-Al-MCM-41(10) at 393 K at constant
parameter (Temperature: 393 K; Reactant Mole Ratio: 0.5; Time: 24
hours; Solvent: Nitrobenzene)
Reaction
Catalyst
Conversion
(Time Run)
Weight (g)
(%)
1
0.250
92.3
87.5
2
0.237
69.4
80.3
3
0.229
45.1
62.9
Selectivity (%)
77
Relative Intensity
First run
Second run
Third run
1.5
2
3
4
5
6
7
8
9
10
2-Theta - Scale
Figure 4.27 X-ray diffractogram patterns of H-Dir-Al-MCM-41(10) during three
recycles
78
4.16
Proposed Mechanism of Hydroxyalkylation of Benzene with Propylene
Oxide Catalyzed Al-MCM-41
The main product of hydroxyalkylation of benzene with propylene oxide as
the alkylating agent was 2-phenyl-1-propanol. The proposed reaction mechanism for
the formation of the main product over Al-MCM-41 is depicted in Figure 4.28. The
conversion and selectivity of reaction were influenced by various reaction parameters
such as SiO2: Al2O3 ratios, temperature, reactant ratios and types of solvent. In this
reaction, propylene oxide being a more polar molecule than benzene was
preferentially adsorbed on the Lewis acid sites. Basically, competitive adsorption
between benzene and propylene oxide always occur in Friedel-Crafts reaction.
However, the adsorption of alkylating agent on protonated surface like Al-MCM-41
is much stronger than that of benzene [24-26].
Al-MCM-41 generally possesses weak and mild acidity. However, the acidity
which was measured by TPD-NH3 and Pyridine Adsorption showed an increase in
acidity with increasing aluminium content. In this reaction, distorted tricoordinated
aluminium plays a roles Lewis acid sites interact with propylene oxide to produce
propoxy cations. However, the strength of Lewis acid sites still depends on
appropriate aluminium content, location and environment of aluminium atom with in
the wall. In addition, crystallinity of sample and surface area also play a role in order
to improve catalytic activity of Al-MCM-41. The results were evidenced by XRD
and Nitrogen Adsorption measurement in previous discussion. Thus, higher surface
area and larger pore size increase accessibility of alkylating agent (propylene oxide)
to interact further Lewis acid sites to produce propoxy cations. These intermediate
were stabilized through the presence of aprotic dipolar solvent. Otherwise the
unstable intermediate of propoxy cations will react further to produce propylene
oxides oligomer. The reduction of oligomers product provide lower steric hindrance
and both of reactant can diffuse and react with acid sites with out great difficulty.
Therefore, The results indicate that instead of aluminium content, solvent and
reactant mole ratio also play a role to give high conversion and selectivity of
79
H3C
+
O
O
O
Si
+
Al
Si
OO
OO
O
O
O
H3C
δ+ O
O
Si
+
O
δ- Al
Si
OO
OO
O
O
O
H3C
+
O
O
Si
δ
+
O
-
Al
Si
OO
OO
O
O
O
H3C
CH3
OH
H
+
+
O
O
O
Si
O
+
δ- Al
OO
O
Si
OO
O
O
Si
O
O
+
Al
OO
O
Si
OO
O
Figure 4.28 Proposed mechanism for Al-MCM-41 catalyzed hydroxyalkylation of aromatics
with propylene oxides
80
2-phenyl-1-propanol. Then, the propoxy cations are subsequently attacked by
electrophiles on the aromatic cycle by interacting with a positively charged site. The
result also reveal that alkylation of benzene also is affected by temperature and B/L
ratio. Dir-Al-MCM-41(10) catalyst contains the highest acidity with low Brønsted :
Lewis acid sites. The decreasing of B/L ratio at high temperature corresponds to the
increment of distorted tricoordinated aluminium formation which are anchored on
the surface of the framework structure of MCM-41. Thus, the quantity of Lewis acid
sites evidently affect the propoxy cations formation and consequently the conversion
of benzene and selectivity of main product. It’s observed that at high temperature,
selectivity of 2-phenyl-1-propanol decreased slightly with temperature, correlated to
non-selective reactions due to existence of Brønsted acid site in catalyst which
creates further dehydroxylation of desired product to give 2-phenyl-propene.
Previous works have demonstrated that highly acidic zeolites such as H-ZSM-5,
modernite, H-Beta and ZnNaY show low conversions and selectivity of desired
product [20,21]. Low Lewis acidity and restricted pore dimension are the possible
reasons for their low activities. On the other hand, Al-MCM-41 with larger surface
and pore size and higher Lewis acid sites, allows the reactant to pass through the
channels and enhance the formation of propoxy cations desired for the reaction to
take place. It is proposed that Lewis acids are desired if the reaction was to proceed.
This study has proven that Al-MCM-41 is a suitable catalyst for hydroxyalkylation
of benzene with propylene oxide
CHAPTER 5
CONCLUSIONS AND RECOMMENDATIONS
5.1
Conclusions
The thesis describes the potential of aluminium containing MCM-41 as a
heterogeneous catalyst on a Friedel-Crafts reaction. Hydroxyalkylation of benzene
with propylene oxide was chosen as a model reaction. Al-MCM-41 with various
SiO2:Al2O3 mole ratios were successfully prepared by direct and secondary
syntheses. Sodium aluminate was used as the aluminium source in the reactant
mixture of the following composition:
5.9 SiO2: (0.07-0.59) Al2O3 :1 CTAB : 1.5 Na2O : 0.15 (NH4)2O : 250 H2O
The results of XRD, FTIR and Nitrogen Adsorption-Desorption analysis
indicate that Al-MCM-41 sample prepared by both methods consist of a uniform
hexagonal pore volume of (0.40 – 0.73 cm3g-1) range with highly mesoporous
surface area (917-1040 m2g-1). The 27Al MAS NMR spectra support the insertion of
aluminium into the framework. The incorporation of aluminium into MCM-41 was
evidenced by the highly intense signal at 56 pm which is characteristic of
tetrahedrally coordinated aluminum sites. Directly synthesized sample with
SiO2:Al2O3 ratio of 10 was found to contain the highest framework Al with
82
calculated Si/Al ratio of 15.2.
27
Al MAS NMR and
29
Si MAS NMR data strongly
support the presence of distorted tetrahedral aluminium which are possible source of
Lewis acidity.
The acidity of Al-MCM-41 were analyzed using Pyridine Adsorption –
Fourier Transformed Infrared Spectroscopy Measurement and Temperature
Programmed Desorption of ammonia (TPD-NH3). The acidity measurement by
Pyridine Adsorption – Fourier Transformed Infrared Spectroscopy Measurement
evidenced that Al-MCM-41 generates both Brønsted and Lewis acid sites. The
generation of Lewis acid sites in the samples were attributed to framework distorted
aluminium rather than extra framework Al (EFAL). Meanwhile, Brønsted acid sites
in the samples are referred to aluminium in the framework in which pyridine was
protonated to pyridinium ion form. The result indicates that Lewis acid sites
dominate the acidity instead of Brønsted acid sites and the acidity of catalyst
increased with increasing aluminium in MCM-41 framework. The results also reveal
that at high aluminium content, the B/L ratio remained with lower decrease although
at high temperature. Thus, samples with higher aluminium content provide higher
thermal stability and more Lewis acid sites. The decreasing of B/L ratio corresponds
to the increment of distorted tricoordinated aluminium formation which are anchored
on the surface of the framework structure of MCM-41. It is believed that the location
and environment of aluminium atom with in the wall controls acids strength. The
closer the aluminium atoms to the wall surface, the greater their acid site strength.
The catalytic study of hydroxyalkylation of benzene with propylene indicates
that Dir-Al-MCM-41(10) produces the highest catalytic activity with a conversion of
benzene of 92.3% and selectivity of 87.5% respectively. The formation of 2-phenyl1-propanol was occurred at favorable condition with a temperature of 393 K and 24
hours of run with propylene oxide: benzene mole ratio of 0.5 using nitrobenzene as
the solvent. Through the reaction, tricoordinated aluminium plays a roles Lewis acid
sites interact with propylene oxide to produce propoxy cations. However, the main
factor to determine the strength of Lewis acid sites including appropriate aluminium
content, location and environment of aluminium atom with in the wall. In addition,
83
crystallinity of sample and surface area also play a role in order to improve catalytic
activity of Al-MCM-41. The higher surface area and larger pore size of catalyst
increase accessibility of alkylating agent (propylene oxide) to interact further Lewis
acid sites to produce propoxy cations. These intermediate were stabilized through the
presence of aprotic dipolar solvent. Otherwise the unstable intermediate of propoxy
cations will react further to produce propylene oxides oligomer. The reduction of
oligomers product became important due to provide lower steric hindrance. Thus,
both of reactant can diffuse and react with acid sites with out great difficulty. The
results indicate that instead of aluminium content, solvent and reactant mole ratio
also play a role to give high conversion and selectivity of 2-phenyl-1-propanol.
5.2
Recommendations
It is recommended that the hydroxyalkylation of aromatic with epoxides are
carried out by higher Lewis acid strength and crystallinity such as ZrO and SBA-15.
This catalytic study can be studied in future with further improvement in pressure
and flowrate reactor. Meanwhile, state of framework aluminium can be further
studied by quadrupole nutation 27Al MAS NMR.
84
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91
APPENDIX A
Mass spectra of 2-phenyl-1-propanol
H3C
OH
92
APPENDIX B
22.076
6.893
Chromatograms of reactant (a) benzene (b) propylene oxide (c) nitrobenzene as
a solvent
(a)
(b)
propylene oxide
22.056
3.224
benzene
22.093
93
(c)
nitrobenzene
94
APPENDIX C
NITROGEN ADSORPTION ISOTHERM
1. Nitrogen Adsorption Isotherm Data of Dir-Al-MCM41 (10)
Desorption
Adsorption
P/P0
4.853x10-5
3.269x10-4
0.0012
0.0043
0.0100
0.0274
0.0303
0.0625
0.0846
0.0996
0.1192
0.1391
0.1587
0.1780
0.1967
0.2322
0.2465
0.2731
0.2937
0.3387
0.3503
0.4106
0.4638
0.4985
0.5499
0.5999
0.6498
0.6998
0.7497
0.7997
0.8199
0.8499
0.8762
0.8996
0.9243
0.9488
0.9722
0.9808
0.9882
0.9941
Volume Adsorbed
(cm3g-1)
41.90
67.13
90.89
116.39
135.67
162.57
165.71
190.88
203.79
211.55
221.12
230.30
239.42
248.82
259.19
287.76
304.49
340.50
366.06
396.99
399.54
406.10
410.20
412.48
415.38
417.82
420.01
422.05
424.07
426.23
427.22
428.79
430.35
432.28
434.85
438.87
446.48
451.58
459.94
471.19
P/P0
Volume Adsorbed
(cm3g-1)
0.9820
0.9710
0.9441
0.9124
0.8841
0.8581
0.8501
0.8236
0.8000
0.7496
0.6997
0.6497
0.5997
0.5498
0.5002
0.4555
0.3942
0.3544
0.3048
0.2831
0.2647
0.2483
0.2285
0.1988
0.1438
462.69
454.88
444.66
439.56
436.94
435.10
434.50
433.08
431.92
429.73
427.70
425.70
423.65
421.47
418.78
411.26
404.48
400.04
378.12
354.81
330.51
307.76
284.32
260.20
231.84
95
2. Nitrogen Adsorption Isotherm Data of Sec-Al-MCM41 (0.25 M)
Adsorption
P/P0
5.770x10-5
1.415x10-4
0.0008
0.0038
0.0097
0.0257
0.0300
0.0559
0.0589
0.0794
0.0988
0.1182
0.1374
0.1566
0.1759
0.1951
0.2410
0.2615
0.3118
0.3601
0.4167
0.4494
0.5003
0.5500
0.6001
0.6501
0.7001
0.7500
0.8001
0.8200
0.8504
0.8749
0.9001
0.9249
0.9498
0.9737
0.9808
0.9898
0.9946
Volume Adsorbed
(cm3g-1)
36.16
63.04
95.45
126.97
150.13
179.28
184.70
209.72
212.20
227.82
241.93
255.79
269.88
284.32
299.38
314.97
349.66
359.86
369.84
373.66
377.03
378.78
381.08
383.11
384.97
386.70
388.38
390.07
391.74
392.54
393.61
394.57
395.72
397.23
399.40
403.94
406.74
412.65
418.89
Desorption
P/P0
Volume Adsorbed
(cm3g-1)
0.9765
0.9612
0.9357
0.9087
0.8827
0.8573
0.8312
0.8254
0.7996
0.7511
0.7002
0.6501
0.6002
0.5503
0.5002
0.4519
0.3981
0.3510
0.3011
0.2549
0.2109
0.1975
0.1679
0.1390
412.70
406.80
401.45
398.59
396.89
395.67
394.71
394.35
393.56
392.12
390.57
389.00
387.37
385.58
383.67
379.63
376.10
373.03
368.74
357.81
329.04
317.92
293.43
271.05
96
APPENDIX D
Calculation Method of Conversion, Selectivity, Yield and Percentage of
Oligomerisation
1. Conversion of Benzene (%) =
C (before reaction) – C’ (after reaction)
X 100%
C (before reaction)
C = Peak area of benzene / Peak area of internal standard
2. Selectivity of 2-phenyl-1-propanol (%) =
S’
X 100%
S (Total)
S = Peak area of 2-phenyl-1-propanol / Peak area of internal standard
STotal = Total of (Peak area of product/ peak area of internal standard)
3. Yield of product (mmol)
Concentration of 2-phenyl-propanol, Y = S / (0.9001) (Refer Figure 4.21)
= (X) mM x Amount of Solvent (mL)
1000
= (Z) mmol
4. Percentage of Oligomerisation (%)
Percentage of Oligomerisation, = P’ after reaction – P before reaction
X 100%
P before reaction
P – Peak area of propylene oxide oligomer / Peak area of Internal Standard