SYNTHESIS, CHARACTERIZATION AND CATALYTIC
PROPERTIES OF TITANIUM CONTAINING SILICA AEROGEL
LEE SOON CHAI
UNIVERSITI TEKNOLOGI MALAYSIA
PSZ 19:16 (Pind. 1/97)
UNIVERSITI TEKNOLOGI MALAYSIA
BORANG PENGESAHAN STATUS TESIS♦
JUDUL : SYNTHESIS, CHARACTERIZATION AND CATALYTIC
PROPERTIES OF TITANIUM CONTAINING SILICA AEROGEL
SESI PENGAJIAN: 2005/2006
Saya :
LEE SOON CHAI
(HURUF BESAR)
mengaku membenarkan tesis ( PSM / Sarjana / Doktor Falsafah )* ini disimpan di
Perpustakaan Universiti Teknologi Malaysia dengan syarat-syarat kegunaan seperti
berikut:
1. Tesis adalah hakmilik Universiti Teknologi Malaysia.
2. Perpustakaan Universiti Teknologi Malaysia dibenarkan membuat salinan untuk
tujuan pengajian sahaja.
3. Perpustakaan dibenarkan membuat salinan tesis ini sebagai bahan pertukaran antara
institusi pengajian tinggi.
4. **Sila tandakan ( √ )
√
SULIT
(Mengandungi maklumat yang berdarjah keselamatan atau
kepentingan Malaysia seperti yang termaktub 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:
3, Jalan Gemilang 10,
Taman UPC,
86100 Ayer Hitam,
Johor Darul Tak’zim.
Tarikh: 5.12.2005
________________________________
(TANDATANGAN PENYELIA)
PROF. DR. HALIMATON HAMDAN
Nama Penyelia
Tarikh: 5.12.2005
CATATAN: * Potong yang tidak berkenaan.
** Jika tesis ini SULIT atau TERHAD, sila lampirkan surat daripada pihak
berkuasa/organisasi berkenaan dengan menyatakan sekali sebab dan tempoh
tesis ini perlu dikelaskan sebagai SULIT atau TERHAD.
♦ Tesis dimaksudkan sebagai tesis bagi Ijazah Doktor Falsafah dan Sarjana secara
penyelidikan, atau disertasi bagi pengajian secara kerja kursus dan penyelidikan,
atau Laporan Projek Sarjana Muda (PSM).
“I hereby declare that I have read this thesis and in my opinion this thesis
is sufficient in terms of scope and quality for the award of the degree
of Master of Science (Chemistry)”.
Signature
:
………………………….
Name of Supervisor :
Prof. Dr. Halimaton Hamdan
Date
5.12.2005
:
BAHAGIAN A ⎯ Pengesahan Kerjasama*
Adalah disahkan bahawa projek penyelidikan tesis ini telah dilaksanalan melalui
kerjasama antara _______________________ dengan ________________________
Disahkan oleh:
Tandatangan : _________________________________ Tarikh: ______________
Nama
: _________________________________
Jawatan
: _________________________________
(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. Wan Ahmad Kamil Bin Mahmood
School of Chemical Sciences,
Universiti Sains Malaysia,
11800 Minden,
Pulau Pinang.
Nama dan Alamat
Pemeriksa Dalam I : Prof. Madya Dr. Hanapi Bin Mat
Fakulti Kejuruteraan Kimia & Kejuruteraan Sumber Asli,
Universiti Teknologi Malaysia,
81300 Skudai,
Johor Darul Takzim.
Nama Penyelia Lain
(jika ada)
:__________________________________________________
Disahkan oleh Penolong Pendaftar di Sekolah Pengajian Siswazah:
Tandatangan : _________________________________ Tarikh: ______________
Nama
: GANESAN A/L ANDIMUTHU
SYNTHESIS, CHARACTERIZATION AND CATALYTIC PROPERTIES OF
TITANIUM CONTAINING SILICA AEROGEL
LEE SOON CHAI
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
DECEMBER 2005
ii
I declare that this thesis entitled “Synthesis, Characterization and Catalytic
Properties of Titanium Containing Silica Aerogel” is the result of my own research
except as cited in the references. The thesis has not been accepted for any degree
and is not concurrently submitted in candidature of any other degree.
Signature
: ____________________
Name
: LEE SOON CHAI
Date
: 5.12.2005
iii
Dedicated to
My Parents
iv
ACKNOWLEDGEMENTS
I would like to express my sincere gratitude and appreciation to my research
supervisor, Prof. Dr. Halimaton Hamdan, for her guidance, support and patience
towards the completion of this work. Synthesis and modification of porous materials
have been an attractive topic for me as investigated intensively by the Zeolite and
Porous Materials Group (ZPMG) of Universiti Teknologi Malaysia.
Grateful acknowledgements are to Dr. Hadi Nur, Assoc. Prof. Dr. Zainab
Ramli, Assoc. Prof. Dr. Salasiah Endud, and Dr. Bogdan Sulikowski for their advice
and valuable suggestion particularly in the method of conducting a research.
To my all lab mates, especially Didik Prasetyoko, Lim, Yong and Ng, thank
you for their valuable discussion and friendship.
My sincere appreciations also extend to lab assistants and others who have
provided assistance at various occasions.
I wish to thank the Ministry of Science, Technology and Innovation (MOSTI)
for funding the research and my studies (UTM Fellowship Award; Project Vote:
74506).
Lastly, I would like to acknowledge my family, for their love and care that
convince me to always do my best.
v
ABSTRACT
Silica aerogel and titania silica aerogel were synthesized by chemical means.
The effect of titanium source, sulphuric acid and titanium loading were studied. The
structure and properties of the aerogels were examined by X-ray diffraction (XRD),
scanning electron microscopy (SEM), nitrogen adsorption (BET), energy dispersive
X-ray analysis (EDX), Fourier transform infrared (FTIR), and ultra violet-visible
diffuse reflectance spectroscopy (UV-Vis DRS). Both silica aerogel and titania silica
aerogel are amorphous. The surface area of the resulting titania silica aerogel was
significantly affected by the quantity of the acid used during synthesis. The
physicochemical properties were found could be engineered by the change of acid
loading and titanium loading. Isolated titanium in tetrahedral framework position,
well dispersed titania particle or crystalline titania (anatase) were formed in-situ
during the aerogel synthesis process. Catalytic reaction of cyclohexene and hydrogen
peroxide was carried out at 70 ˚C in a fixed batch reactor. The effects of
physicochemical properties of the catalyst, solvent, reaction temperature, oxidant
content and alkene to the reaction have been investigated. Both allylic and nonallylic oxidation process have occurred in the reaction. 1,2-cyclohexanediol was
formed as major compound in the reaction.
vi
ABSTRAK
Aerogel silika dan aerogel titania-silika telah disintesis melalui pendekatan
kimia. Pengaruh daripada sumber titanium, asid sufurik dan kepekatan titanium telah
dikaji. Struktur dan sifat aerogel telah dikaji menggunakan pembelauan sinar-X
(XRD), mikroskop imbasan elektron (SEM), penjerapan nitrogen, analisis
penyerakan tenaga sinar-X (EDX), Fourier transform infra merah (FTIR), and
spektroskopi pemantulan bauran ultra lembayung-nampak (UV-Vis DRS). Keduadua aerogel silika dan aerogel titania-silika bersifat amorfus. Luas permukaan
aerogel titania silika didapati amat dipengaruhi oleh kuantiti asid yang digunakan
semasa sintesis. Sifat fizikokimia didapati dapat dikawal dengan mengubah
penggunaan asid dan penggunaan titanium. Titanium terpencil dalam keadaan rangka
tetrahedral, partikel titania dalam penaburan sempurna and hablur titania (anatase)
didapati terbentuk in-situ dalam proses sintesis aerogel. Tindakbalas pemangkinan
bagi sikloheksena dengan hidrogen peroksida telah dijalankan dalam reaktor pukal.
Pengaruh daripada sifat fizikokimia mangkin, pelarut, suhu tindakbalas, kuantiti
pengoksida dan alkena terhadap keaktifan mangkin telah dikaji. Kedua-dua proses
pengoksidaan allilik and bukan-allilik didapati telah berlangsung dalam tindakbalas.
1,2-sikloheksanadiol didapati terbentuk sebagai hasil utama dalam tindakbalas.
vii
TABLE OF CONTENTS
CHAPTER
TITLE
PAGE
DECLARATION OF THE STATUS OF
THESIS
SUPERVISOR’S DECLARATION
CERTIFICATION OF EXAMINATION
TITLE PAGE
i
DECLARATION OF ORIGINALITY AND
ii
EXCLUSIVENESS
1
DEDICATION
iii
ACKNOWLEDGEMENTS
iv
ABSTRACT
v
ABSTRAK
vi
TABLE OF CONTENTS
vii
LIST OF TABLES
xi
LIST OF SCHEMES
xiii
LIST OF FIGURES
xiv
LIST OF ABBREVIATIONS AND SYMBOLS
xvi
LIST OF APPENDICES
xvii
INTRODUCTION
1.1 General Introduction
1
1.2 Research Background and Problem Statement
2
1.3 Research Objectives and Scope
4
1.4 Hypothesis
4
viii
2
LITERATURE REVIEW
2.1 Sol-Gel Science
6
2.2 Silica
8
2.2.1
The Chemistry of Aqueous Silicates
10
2.3 Titania and the Chemistry of Aqueous Titania
13
2.4 The Chemistry between Silica and Titania
15
2.4.1
Titania-Silica in Catalysis
2.5 Aerogel
2.5.1
16
18
History and Development of
18
Aerogel
2.5.2
Aerogel Synthesis
19
(i)
Drying Process
19
(ii)
Elimination of Surface
19
Tension
(iii)
Freeze Drying
20
(iv)
Supercritical Fluid
21
Extraction
2.5.3
Properties and Applications of
23
Aerogel
3
EXPERIMENTAL
3.1 Synthesis of Silica Aerogel
25
3.2 Synthesis of Titanium Containing Silica Aerogel
28
3.2.1
Post Synthesis: Synthesis of
29
Titania-Silica System
(i)
Grafting with Titinium
29
(IV) Tetrachloride
(ii)
Grafting with Titanium
29
(IV) Isopropoxide
(iii)
Precipitation of Titania
on Amorphous Silica
29
ix
3.2.2
Direct Synthesis: Synthesis of
30
Titania-Silica Aerogel System
3.3 Parameter Study for Synthesis (Direct Synthesis)
30
of Titanium Containing Silica Aerogel
3.3.1
Sources of Titanium
30
3.3.2
Si:Ti Molar Ratio
31
3.3.3
Sulphuric Acid Loadings
32
3.4 Characterization
3.4.1
Nitrogen Adsorption: Brunauer,
33
33
Emmett, Teller (BET) method
3.4.2
XRD Measurement
35
3.4.3
UV-Vis Diffuse Reflectance
36
Spectroscopy
3.4.4
Fourier Transform Infrared
37
Spectroscopy
3.4.5
Scanning Electron Microscopy
3.5 Catalytic Properties: Oxidation of Alkene
4
39
39
RESULTS AND DISCUSSION
4.1 Synthesis of Silica Aerogel
42
4.2 Synthesis of Titanium Containing Silica Aerogel
45
4.2.1
Post Synthesis
45
4.2.2
Direct Synthesis
51
4.3 Parameter Study for the Synthesis (Direct
53
Synthesis) of Titanium Containing Silica Aerogel
4.3.1
The Effect of Titanium Source
53
4.3.2
The Effect of Si:Ti Molar Ratio
58
4.3.3
The Effect of Loading of Sulphuric
62
Acid
4.4 Catalytic Properties: Oxidation of Alkene
4.4.1
The Influence of the Type of
66
66
Titanium
4.4.2
The Influence of Solvent
70
x
4.4.3
The Influence of Hydrogen
72
Peroxide Loading
4.4.4
The Influence of Reaction
74
Temperature
4.4.5
The Influence of Alkene
4.5 The Mechanism of the Reaction
5
76
77
CONCLUSIONS AND SUGGESTIONS
5.1 Conclusions
82
5.2 Suggestions
84
REFERENCES
85
APPENDICES
95
xi
LIST OF TABLES
TABLE NO.
TITLE
PAGE
2.1
The solubility of silica in different solvent [54]
22
2.2
The critical point of different solvents [55, 56]
23
2.3
Some properties of aerogel [59, 60]
24
3.1
Temperature programme implemented in the
27
supercritical drying process [64]
3.2
3.3
Titanium sources that have been used in the
synthesis of titanium containing silica aerogel
IUPAC classification of pores [67, 68]
31
34
3.4
Some assignments of infrared frequencies [78]
38
3.5
GC-FID instrument setting
40
4.1
The surface area of the silica aerogel
44
4.2
The BET surface area of titanium containing
50
silica
4.3
Effect of titanium source on the surface
54
characteristics of the Ti-Si aerogels
4.4
Effect of concentration of titanium on the
58
surface characteristics of the Ti-Si aerogels.
Titanium isopropoxide as titanium source, H+:
NaOH molar ratio = 1.25.
4.5
Effect of concentration of acid on the surface
63
characteristics of the Ti-Si aerogels. Titanium
isopropoxide as titanium source, Si:Ti molar
ratio = 33
4.6
Sample used in the catalytic testing and their
67
xii
characteristics
4.7
Catalytic activity of the titanium containing
69
silica aerogel, TS-1 and anatase. Reaction
condition: 10 ml cyclohexene, 10 ml acetone,
8.35 ml H2O2 35%, 156.3 mg catalyst, and 1 ml
toluene (internal standard) at 70 ˚C
4.8
Catalytic activity of the aerogel A250 as a
71
function of solvent. Reaction condition: 10 ml
cyclohexene, 10 ml solvent, 8.35 ml H2O2 35%,
156.3 mg catalyst, and 1 ml toluene (internal
standard) at 70 ˚C
4.9
Catalytic activity of the aerogel A250 as a
73
function of alkene: H2O2 molar ratio. Reaction
condition: 10 ml cyclohexene, 10 ml acetone,
respective amount of H2O2 35%, 156.3 mg
catalyst, and 1 ml toluene (internal standard) at
70 ˚C
4.10
Catalytic activity of the aerogel A250 as a
75
function of reaction temperature. Reaction
Condition: 10 ml cyclohexene, 10 ml acetone,
8.35 ml H2O2 35%, 156.3 mg catalyst, and 1 ml
toluene (internal standard)
4.11
Catalytic activity of the aerogel A250 as a
function of amount of hydrogen peroxide.
Reaction condition: 10 ml alkene, 10 ml
acetone, 8.35 ml H2O2 35%, 156.3 mg catalyst,
and 1 ml toluene (internal standard) at 80 ˚C
76
xiii
LIST OF SCHEMES
SCHEME
NO.
1
TITLE
The reactions in the oxidation of cyclohexene
PAGE
78
[95, 96, 97]
2
Reaction mechanism of the oxidation of
cyclohexene using hydrogen peroxide as
oxidant [102, 103, 109]
81
xiv
LIST OF FIGURES
FIGURES
NO.
2.1
TITLE
PAGE
Polymerisation behaviour of aqueous silica and
12
followed by the formation of gels and powders
2.2
The freeze drying process path (bolded arrows)
20
in pressure-temperature (P-T) phase diagram of
a pure substance
2.3
The supercritical drying process path (bolded
22
arrows) in pressure-temperature (P-T) phase
diagram of a pure substance
3.1
Synthesis of sodium silicate from rice husk ash
26
3.2
Synthesis of aerogel from sodium silicate
28
4.1
XRD diffractogram of silica aerogel
42
4.2
SEM micrograph showing the surface
43
morphology of silica aerogel
4.3
FTIR spectrum of silica aerogel
44
4.4
The X-ray diffractograms of titanium modified
silica aerogels
The X-ray diffractograms of titanium modified
45
4.5
46
amorphous silica (RHA)
4.6
The FTIR spectra of titanium modified RHA
47
4.7
The FTIR spectra of titanium modified aerogel
48
4.8
The UV-Vis spectra of titanium modified RHA
49
4.9
The UV-Vis spectra of titanium modified silica
49
aerogels
4.10
XRD diffractogram of titanium modified silica
52
xv
aerogels (Aph6)
4.11
FTIR spectrum of titanium modified silica
52
aerogels (Aph6)
4.12
UV-Vis spectra of titanium modified silica
53
aerogels (Aph6) and silica aerogel
4.13
The effect of titanium source on the
56
physicochemical characteristics of the Ti-Si
aerogels by UV-Vis DRS. (a) Titanium(III)
sulphate, (b) Titanium(IV) chloride, (c)
Titanium(IV) alkoxide, (d) Titanium(IV) oxide
in anatase form
4.14
X-ray diffractograms of aerogel samples with
60
various Si:Ti molar ratios compared with
anatase TiO2
4.15
UV-Vis spectra of samples synthesized with
61
various Si:Ti molar ratios
4.16
UV-Vis spectra of samples synthesized with
65
various H+: NaOH molar ratio
4.17
Time course study for the reaction mixture 10
ml cyclohexene, 10 ml acetone, 8.35 ml H2O2
35%, 156.3 mg TS-1, and 1 ml toluene (internal
standard) at 80 ˚C
79
xvi
LIST OF ABBREVIATIONS
λ
Wavelength
2θ
Bragg angle
BET
Brunauer, Emmet, Teller
Cu Kα
X-ray diffraction from copper K energy level
EDX
Energy dispersive X-ray analysis
etc
Etcetera
FTIR
Fourier Transform Infrared
GC-FID
Gas Chromatography – Flame Ionisation Detector
iep
Isoelectric point
IUPAC
International Union of Pure and Applied Chemistry
KBr
Potassium bromide
MCM
Mobil Crystalline Material
MS
Mass Spectroscopy
m/z
Mass-to-charge ratio
NMR
Nuclear Magnetic resonance
ppm
Part per million
RHA
Rice husk ash
Si:Ti
Silicon to titanium molar ratio of starting material
TMOS
Tetramethylortosilicate
TOF
Turnover frequency
UV-Vis DRS
Ultra Violet-Visible diffuse reflectance spectroscopy
XRD
X-ray diffraction
xvii
LIST OF APPENDICES
APPENDIX
1
TITLE
Component Table for GC-FID peaks
PAGE
95
identification
2
Chromatogram of the reaction mixture analysed
96
using gas chromatography
3
Calibration curve for quantify the concentration
97
of cyclohexene
4
Calibration curve for quantify the concentration
98
of cyclohexene oxide
5
Calibration curve for quantify the concentration
99
of 2-cyclohexen-1-ol
6
Calibration curve for quantify the concentration
100
of 2-cyclohexen-1-one
7
Calibration curve for quantify the concentration
101
of 1,2-cyclohexenediol
8
FTIR spectrum of 1,2-cyclohexanediol that has
102
been synthesized as standard
9
Mass spectrum of 1,2-cyclohexanediol that has
103
been synthesized as standard
10
Reaction mechanisms involving hydroxy radical
and cyclohexene [95, 96]
104
CHAPTER 1
INTRODUCTION
1.1
General Introduction
Aerogel is a gel in which the liquid phase has been replaced by air without
damaging the solid phase. Aerogel is a novel space-age super material. It is inert,
non-toxic, and environmental friendly new material. It has been used as a catcher’s
mitt in spacecraft to capture dust from a comet [1].
Silica aerogel is a very interesting material. It is extremely light (specific
gravity as low as 0.025 g/cm3), with the lowest thermal conductivity known to solid
material, high surface area and high porosity. This makes it suitable for many
applications. It has been applied as heat storage systems, catalysts and catalyst
supports. Silica aerogel is dielectric with air filled pores (can be as small as 10
nanometers in diameter) offers a better way to keep the interconnecting wires from
shorting across the narrow dividing space between transistors [2].
Many physical and chemical properties of a metal oxide can be modified by
interaction with a second oxide. Silica–alumina, for example, has stronger acidity
than both silica and alumina [3]. A screening study of silica-supported catalysts was
conducted by Hisao Yoshida et al. and they found that silica supported Ti system was
the most effective catalyst for epoxidation of propene [4]. It strongly suggests that
silica-titania mixed oxide might be the best combination to become the best catalyst
for the oxidation reaction compared to other oxides.
2
1.2
Research Background and Problem Statement
Titania (TiO2) is a technologically important material as catalyst and as
support. With its special properties, TiO2 attracts more attention recently, especially
for hydrodesulphurisation (HDS) or hydrodenitrogenation (HDN) in the petroleum
refining process [5, 6, 7]. The character of the catalyst with TiO2 carrier is superior to
that with γ-Al2O3 carrier. However, TiO2 is seldom used as a catalyst carrier in
commercial process due to two disadvantages. TiO2 has a small specific surface area
(usually 10 m2/g) and the mechanical strength is five times less than γ-Al2O3. In
addition, TiO2 in high surface area form has low mechanical strength, limited
extrudability and low thermal stability. Therefore, effort has been devoted in recent
years to coat titania onto high surface area supports such as silica and alumina to
improve the thermal stability and the surface area of TiO2 [8].
Despite the disadvantages, titania has the ability to modify catalytic
properties of the metal, thus attracts the studies of the interaction between titaniametal interfaces [9]. Since, it is very difficult to obtain high surface area titania (>100
m2/g); its use has been limited.
It is now established that nanoscale engineering of sol–gel TiO2–SiO2 mixed
oxides provides excellent epoxidation catalysts. The area of titanosilicate-catalyzed
epoxidation of olefins with hydrogen peroxides is largely because of the discovery of
TS-1 where Ti has been substituted for Si in the MFI framework by Shell in 1971
[10]. This molecular sieve was reported to be active in the following oxidation
reactions [11]: (i) oxidation of primary and secondary alcohols to the corresponding
aldehydes and ketones, (ii) hydroxylation of aromatics to phenol derivatives, (iii)
epoxidation of alkenes to epoxides, (iv) oxyfunctionalization of alkanes to alcohol
and ketones, (v) ammoximation of carbonyl compounds aldoxymes or ketoximes,
(vi) oxidation of thioethers to sulfoxides and sulfones, and (vii) oxidation of primary
and secondary amines to oximes or azoxy compounds and hydroxylamines. TS-1 is
the most prominent representative of epoxidation catalyst [12]. However, the use of
TS-1 is limited by inherently small pore size and only relatively few substrates can
be oxidized. Moreover, an obstacle in the commercialisation of TS-1 is that it is not
possible to be moulded.
3
The search for large pore analogues of TS-1 has led to the study of Ti
substituted into the framework or grafted onto the channels of zeolite beta or MCM
type silicalites. A series of new preparation methods of materials containing highly
dispersed titanium centres in a silica matrix were developed [13, 14, 15].
Smaller particles of metal oxide can be obtained when two oxide gel are
mixed at the same time. However, phase separation may occur due to different rates
of hydrolysis (sol-gel process) of silicon and titanium alkoxide, which results in
formation of larger TiO2 particles and prevents the homolytic substitution of titanium
in silica framework. Thus, Ti-MCM, Ti-aerogel or Ti-zeolite in several researches
are fail to be engineered the Ti-O-Si bonding as in TS-1 [3, 13, 15]. However, high
catalytic activity has been achieved by the use of organic based peroxide as oxidant
if the TiO2 particle was small enough. Thus, most studies avoid the use of hydrogen
peroxide in their catalytic oxidation. In addition, Dusi [16] has synthesized 20%
TiO2–80 wt% SiO2 aerogel from alkoxide sources and found that highly dispersed
titania in the silica matrix was obtained, showed outstanding performance in the
epoxidation of cyclic olefins with alkylhydroperoxides but inactive with hydrogen
peroxide. This was due to the formation of TiO2 particles inside the silica matrixes.
Therefore, it is a challenge to synthesize titania-silica aerogel to produce
homogeneous or well-dispersed mixed oxide by using aqueous solution. In recent
publications, there were several synthesis routes for the production of titania-silica
mixed oxide but alkoxide precursors are used. As the alkoxide is commonly more
expensive starting material, it will directly increase the cost of the final material and
limit its commercial value. Recently, Chan [17] have successfully synthesized silica
aerogel using organic waste precursor. Their innovation has resulted in a more
economical production of silica aerogel. Therefore it is feasible to find a better path
to synthesize well-dispersed titania-silica mixed oxide prepared from an organic
waste.
In addition, crystalline titanium oxide has great potentials in other various
applications, such as in photocatalysis [18], making the study of the titania-silica
aerogel more desirable.
4
Titania oxide is of interest as catalyst or support. A disadvantage of titania as
support is its low surface area. Therefore, inert oxide like silica aerogel is selected as
a support in order to obtain higher surface area dispersed titania.
1.3
Research Objectives and Scope
The objectives of this research are:
1) To synthesize titanium containing silica aerogel.
2) To investigate and characterize the physical and chemical properties of
titanium containing aerogel.
3) To identify the catalytic properties of the titanium containing silica aerogel in
the oxidation of cyclohexene by using hydrogen peroxide as oxidant.
4) To identify the influence of reaction conditions in the oxidation of
cyclohexene by titanium containing silica aerogel.
1.4
Hypothesis
To overcome these problems, inert oxides like silica have been used as
support to obtain high surface area dispersed titania. In this research work, direct
synthesis, precipitation and grafting of titania were implemented on the silica aerogel
as support. This approach not only increases the surface area of the titanium oxide
but also strengthens the silica aerogel.
Deposition or anchoring of Ti sites on silica circumvents the steric problem
by avoiding narrow channels. Sol–gel process provides an attractive route to the
preparation of multi-component oxide materials that show homogeneity in the
distribution of heterometal oxide bonds [19]. Catalysts prepared by sol-gel contain
accessible immobilized Ti within the silica framework. Since high specific surface
area is obtained and the resulting porous structure is very open, larger substrates can
5
access the active sites. Better accessibility may be obtained by having the active
component on the surface.
CHAPTER 2
LITERATURE REVIEW
2.1
Sol-Gel Science
Sol-gel process gives several advantages to material development as stated
below [19]:
•
Increase chemical homogeneity in multi-component system;
•
Produce high surface area gel or powder, which lead to relatively low
sintering temperatures;
•
Preparation of high chemical purity material due to the absence of
grinding and pressing process;
•
Prepared with relative ease from simple solution.
Sol-gel process involves sol, gel and colloidal chemistry. Colloid state
comprises of particles with a size range of 1 nm to 1000 nm and not to be affected by
gravitational forces. The interactions are dominated by short-range forces, such as
Van der Waals and surface forces. The International Union of Pure & Applied
Chemistry (IUPAC) defines colloid dispersion as a system in which the particles of
the colloidal size (1-1000 nm) of any nature (solid, liquid or gas) are dispersed in a
continuous phase of a different composition or state. In order to be treated as
colloidal, not all three dimensions need to be in colloidal range or even only one
dimension is in this range (e.g. fibre or thin film) may also be treated as colloidal.
The region of suspension may begin at about 1000 nm [20, 21].
7
Sol is a stable (does not settle or agglomerate at a significant rate) dispersion
of solid colloidal particles in a liquid phase. The dispersion of solid in water is
known as aquasol or hydrosol. Silica organosol can be obtained by transferring the
aquasol to an organic solvent. An aerosol is a colloidal dispersion of particles in gas.
Pyrogenic or fume oxides are powders made by condensing of precursor from a
vapour phase at elevated temperature. Silica made using this method is called an
Aerosil. Cryogel is a powder obtained by freeze-drying a sol. Sol is not stable against
mechanical force, such as centrifugation. It may consist of weakly cross-linked and
flexible polymer [22].
Colloidal particles can be linked together or be aggregated by gelation,
coagulation or flocculation or coacervation. Gelation is a link of colloidal particles to
form a continuous solid skeleton enclosing the liquid phase. Coagulation involves the
formation of close pack clumps of sol and followed by precipitation. Flocculation
occurs in the presence of flocculating agent that functions as a bridge to link the
particles in groups while remain open structure. When an adsorbed layer of material
that makes the colloidal particles becomes less hydrophilic, no bridge is formed
between particles, hence forming a concentrated liquid phase that is immiscible with
the aqueous phase. The process is termed as coacervation.
Gel point is the time for the last bond to form, which completes the giant
molecule. Gelation can occur after a sol is cast into mold, turning an object into a
desired shape. If the gel is greater than a few millimetres, it is generally called a
monolith. Aging is the process of change in structure and properties after gelation.
Further, dissolution, condensation and re-precipitation may occur during the aging
process. Shrinkage during aging process may due to syneresis, which attraction or
bond formation between particles induces the expulsion of the liquid from the pores.
Xerogel (xero means dry) is obtained by drying a gel under normal condition
and capillary pressure causes shrinkage of the gel network (often reduced in volume
by a factor of 5 to 10 compared to the original wet gel). Wet gel dried under
8
supercritical condition of the solvent may prevent the collapse of the wet gel
structure to produce a matter with low volume fraction of solid but high volume
fraction of air, known as aerogel. Since supercritical liquid has no interfacial between
liquid and vapour, there is relatively little shrinkage of the gel due to the absence of
capillary pressure. Aerogel has a very low particle coordination number, which is
usually macroporous and has high surface area. Nevertheless, they are usually
mechanically weak and unstable when exposed to water vapour. Both xerogel and
aerogel have high porosity and surface area that make them useful as filter, catalytic
substrates, or catalyst support. They are also useful in the preparation of dense
ceramics.
2.2
Silica
Silicon (Si) constitutes about 28% of the earth's crust. While, silica (SiO2) is
the most abundant elements in the earth's crust, viz. 59% mass of the earth's crust is
silica. The combination of silica with other oxide forms the silicate minerals in our
rock and soil. Silica can be in a form of crystalline (i.e. quartz, cristobalite, tridymite,
coesite) or amorphous. The tetrahedral silicate, [SiO4]4- is the building block of
silica. Four oxygen ions are in mutual contact and the silicon ion is located in the
tetrahedral hole [23]. The Si-O bond length is about 0.162 nm. The bond length is
shorter than the sum of the covalent radii of silicon and oxygen atoms (0.191 nm)
due to partial ionic and relatively high stability of the siloxane bond.
The polymorphism of silica is based on different linkages of the [SiO4]4units. Amorphous silica is formed by random packing of [SiO4]4- unit and it has
lower density compared to that of crystalline silica. Opal is one type of the natural
amorphous silica. Tridymite and cristobalite have much open structure. Meanwhile,
quartz has the densest structure and it is present in sand as a major component with
some metal oxides as impurities.
9
Silanol groups are formed on the silica surface during synthesis or due to the
rehydroxylation of thermally dehydroxylated silica. There are several types of
silanol. They are isolated (single or free silanol), vicinal (hydrogen bonded silanolsilanol), geminal (silanediol), and silanetriol. These silanols can be identified by the
29
Si NMR and Infrared spectroscopy. Internal silanols are present within the colloidal
particle during synthesis. H2O is physically adsorbed to all types of silanol groups
through hydrogen bonds. Adsorbed water molecules have direct effect on the
neighbouring weakened siloxane group, result in splitting of the group and formation
of new OH group on the surface.
Removal of physically adsorbed water may be completed by heating to 190 ±
10 °C. By about 450-500 °C, all the vicinal groups condense, yielding water vapour
and strained siloxane (Si-O-Si) bond. Strained siloxane bond may transform to stable
siloxane bond due to the calcinations at above 500 °C. Sintering at temperature
higher than 600 ± 10 °C may result in the loss of surface area of the silica. Internal
silanols start to condense at 600-800 °C. Above 800 °C [24], geminal silanols are
condensed. At temperature 1000-1100 °C, only isolated silanol groups remain on the
silica surface [25].
The protons on the silanol group may be exchanged with alkaline ions such as
Na+, K+, and NH4+ during synthesis in an alkaline medium. Silanol group can also be
esterified as a basis of silica in analytical and chromatography process.
Since the seventeenth century, it is known that sand and sodium or potassium
carbonate reacts at red heat to form a water soluble glass called water glass. Water
glass has been commercially manufactured in 1855 in Europe and America after
systematic investigation by Johann Nepomuk von Fuchs in 1850 [26]. Manufacture
has generally been carried out in large open-hearth furnaces above 1300 ºC by the
following reactions:
10
SiO2 + Na2SO4 + 1/2 C
1/2CO2 + SO2 + SiO2.Na2O
(2.1)
SiO2 + Na2CO3
CO2 + SiO2.Na2O
(2.2)
Recently, low temperature method has been employed to synthesize sodium
silicate from amorphous silica. In this thesis, amorphous RHA has been used as silica
source. Rice husk ash is widely available in Asia from the rice industry as a waste
product. This method is made possible by the high solubility of amorphous silica
under high pH [17].
Soluble silicates produced from silica are widely used in the glass, ceramics,
and cement as a major component. It also been used as bonding and adhesive agents
in pharmaceuticals, cosmetics, and detergents industries [27]. Silica has been used as
a major precursor for a variety of inorganic and organometallic materials, which have
applications in synthetic chemistry as catalysts, thin films or coatings for electronic
and optical materials [19].
2.2.1
The Chemistry of Aqueous Silicates
The oxidation state and the coordination number of silicon are +4 and four
respectively. Silicon (ionic radius = 0.42 Å) is less susceptible to nucleophilic attack
and coordination expansion does not spontaneously occur with nucleophilic reagents
[28]. Hence, the kinetic of hydrolysis and condensation of silicon system are slower
than in transition metal systems and in Group III systems.
The active silica is defined as one that will depolymerise completely to
soluble silicate in 100 min at 30 ºC in an excess of 10-2 M NaOH solution (pH 12).
Such solution contains monomeric silica and particles up to 10-20 Å.
11
Silicic acid, Si(OH)4 is predominant mononuclear species below pH 7. Silicic
acid can be formed by acidifying a soluble silicate or hydrolysing ester (e.g.
Si(OEt)4) in excess of water [28]. The earlier approach has been applied in making
colloidal silica, which involves making an acidic sol and precipitation of sodium salt
(by adding alcohol or acetone) in a strongly acidic medium (about pH 2). The
polysilicic acid can be made alkaline that colloidal particles are grown to desired size
and stabilize the product. Anionic species (e.g. Si4O8(OH)62-, Si4O8(OH)44-,
SiO8(OH)3-, SiO(OH)22-) are the predominant species above pH 7. The negative
charges are due to the adsorption of the hydroxyl ions above pH 7, but silica loses the
charge in acid solution. When a dilute solution of sodium silicate is partially
neutralized with acid to a pH of 8-9, a silica sol rather than a gel is obtained if the
concentration of the sodium salt is less than 0.3 M.
Na2SiO3 + 2HCl + H2O
Si(OH)4 + 2NaCl
(2.3)
Refer to Figure 2.1, polymerisation (gelation) of silicic acid occur in three
states as below [29]:
•
Polymerisation of monomer, Si(OH)4 to form particles.
•
Growth of particles.
•
Particles are linked together in branched chains; the overall medium becomes
viscous (thickening), and then solidifies to a coherent network of particles
that retains the liquid by capillary action.
After gelation, condensation and particle growth (Figure 2.1) may proceed in
the aging process. Condensation takes place to maximize the number of Si-O-Si
bonds and minimize the number of terminal hydroxyl groups through internal
condensation. Ostwald ripening mechanism further the growth of particle size due to
the solubility difference between particles especially at higher temperature and above
pH 7. At pH> 7, the dissolution of silica is more favoured, nucleation and growth is
the predominant mechanism [30, 31]. Particles grow in size and decrease in number
as highly soluble small particle dissolve and precipitate on larger nuclei (less
12
soluble). Growth stop when the difference in the solubility between the largest and
the smallest particles becomes only a few ppm. Due to the above reasons, Ostwald
ripening may result in larger particles formed and reduction in surface area.
Figure 2.1
Polymerisation behaviour of aqueous silica and followed by the
formation of gels and powders [32].
The classic silica aquasols with particle size 5-100 nm in diameter may be
prepared in aqueous medium. By autoclaving the solution, the particle size can
exceed 300 nm. Large particle silica sol (2000-3000 nm) can be prepared in organic
medium through Stöber Process in an alcohol-ammonia system with enough water
[33]. On the other hand, high porosity or high surface area can be prepared on the
basis of small dimension of building units. Small particles, “reverse” system (small
pores between units) or porosity generated by aggregation of small particles are
required in the model of high-surface-area materials. The upper limiting surface area
of silica composed of discrete primary particles would be about 2000 m2/g [32].
13
2.3
Titania and the Chemistry of Aqueous Titania
Titanium was first discovered in 1791 by William Gregor [34]. It is the fourth
most abundant metal in the earth’s crust, after iron, aluminium and magnesium. It is
the first member of the 3d transition series and has four valence electrons, 3d24s2.
The most stable oxidation state is +4, which involves the loss of all these electrons.
However, titanium may also exist in lower oxidation states, i.e. +3, +2, +1, 0, -1, and
-2.
Titanium dioxide is important in paint industry as white pigment due to its
high opacity, relative chemical inertness and the comparative abundance (and hence
cheapness) of titanium ores. The titanium dioxide also possesses a wide range of
semi-conductor and dielectric properties, which are highly depending on the density
of the point defects. Titanium dioxide exists in three crystalline structure, anatase,
brookite and rutile. They have been prepared synthetically. Titanium dioxide
precipitated from sulphate or chloride solution at room temperature is essentially
amorphous even after drying at 110 °C. Precipitated from the boiling sulphate
solutions is in a form of anatase. Rutile may be separated from boiling chloride
solution. Meanwhile, brookite crystals may be grown from amorphous TiO2 under
hydrothermal conditions with the presence of sodium hydroxide. Both anatase and
rutile are tetragonal, whereas brookite is orthorhombic. In all three forms, each
titanium atom is coordinated to six almost equidistant oxygen atoms, and each
oxygen atom to three titanium atoms [34].
Titanates do not contain discrete TiO44- ions (except barium salt) but are more
correctly regarded as mixed metal oxides. There are two types of titanate. They are
metatitanate MI2TiO3 or M IITiO3 and orthotitanate MI4TiO4 or M II2TiO4. Titanates
are usually water insoluble and are crystalline. The metatitanate of the type MI2TiO3
are prepared by fusion of titanium dioxide with the alkali metal carbonate.
Meanwhile, metatitanate MIITiO3 (MII =Mg, Co, Mn, etc.) are prepared by heating
the metal oxide with the stoichiometric quantity of TiO2 in a seal tube at temperature
of 1000-1300 °C for several hours.
14
Metal oxo species involves metal-oxygen multiple bonds. The TiO2+ exists
discretely in the titanyl sulphate, TiOSO4.H2O. The species present in aqueous
solutions of titanium (IV) are TiO2+, TiOH3+, Ti(OH)22+, and TiO(OH)+depends on
the pH of the solution [34].
Peroxo complexes may be formed in the presence of peroxide solution. The
addition of hydrogen peroxide to an acid solution of titanium (IV) will cause the
formation of an intense yellow-orange colour. They are being orange in acid
solutions, yellow in solutions of pH ~ 8 and colourless in strong alkaline media. The
red solid formed when H2O2 is added to a solution of oxotitanium (IV) sulphate in
concentrated sulphuric acid is Ti(O2)SO4.3H2O as monomer. The addition of alcohol
to a solution of oxotitanium sulphate containing H2O2 and adjusted to pH 8.6 with
potassium carbonate may give a yellow solid of a probable formula TiO3.2H2O.
There is one peroxo group per titanium atom in the molecule. Solid peroxo titanates
of the type MI4Ti(O2)4.H2O have been prepared by adding ice-cold solutions of the
alkali-metal hydroxide and H2O2 to the TiO3.2H2O. They are decomposed by
aqueous acid and are tetraperoxo species in the solid state, although no more than
two peroxo groups would bound to the metal in alkaline solution.
The alkoxides of titanium are prepared from titanium (IV) tetrachloride and
react with sodium alkoxides (NaOR) in alcohol or with alcohol (ROH) in excess of
anhydrous ammonia. The metal alcoxides are rapidly hydrolysed to metal hydroxide
in water. The alkoxides of titanium are most widely studied group of organic
compound of this element. This is because of their possible application to the
development of new polymeric materials with useful properties. For instance, tetra-nbutoxide has been used in the production of heat resistant paints due to the capability
of formation of highly dispersed titanium oxide [34].
The solubility of titania in water between pH 3 and 12 is only approximately
-6
10 mol/dm3 [35]. The growing particles are very sensitive to shear-induced
aggregation. Agitation at low level may produce uniform particle [22]. In an acidic
15
condition (0.001 molar H+), the particle surface charge increases, the final titania
particle size decreases, and larger shear rate is required to induce aggregation.
2.4 The Chemistry between Silica and Titania
There are three main approaches that can be applied to synthesize titaniasilica material. They are ionic interaction due to opposite surface charge of the
particles, reaction of the silicic acid with target oxide, and reaction with the silanol
group on the silica surface (grafting). However, the second method is only applied to
iron, uranium, chromium and aluminium but not yet explored to titania. The other
two methods involve surface reaction but not monomeric silica species.
The isoelectric point (iep, electrical mobility of silica particles is zero) and
point of zero charge (surface charge is zero) of silica is at pH 2 ± 0.5. Therefore,
silica is negatively charged above pH 2 [36]. It has been assumed that the catalyst
below pH 2 is the H+ ion, which forms an active cationic complex. Also, above pH 2
the OH- ion is the catalyst in that active anionic silica is generated. Vysotskii and
Strazhesko [37] have pointed out that in the presence of acid such as sulphuric acid,
the iep is not only the point of minimum rate of gelling but also the point which gels
of maximum strength and maximum specific surface area are obtained. This is
because the rate of aggregation is minimum at the iep and the rate of growth of the
ultimate particles from monomer at minimum, so that the ultimate particles are the
smallest as they form the gel. In contrast, titania has an iep around pH 5-6 [38].
To coat sol carrying positive charge, like Fe2O2 or Al2O3, it is necessary to
reverse the charge by adding the dilute sol into dilute (10%) sodium silicate under
intense agitation. Alternatively, chelating agent such as citrate could be applied
before adding silicate. The surface thus covered with a negatively charged molecular
layer of adsorbed silicate on which a layer of SiO2 can be applied [39]. For example,
16
cation-coated silica sol can be synthesized by adding basic salts of Al, Zr or Ti (e.g.
AlCl3) to an acidic or basic silica sol while stirring. Followed by addition of NaOH
until pH 4-6 and finally aged under 80-100 °C. This cationic-coated silica sol can
mix easily with water-soluble organic solvent and may be used in acidic condition
[40].
The negative charge on silica can be reversed by adsorbing an excess of
positively charged material on the surface. The coating included oxides of tri- and
tetravalent metals such as aluminium, chromium, gallium, titanium, and zirconium.
In making these products, some researchers mixed acidified silica sol with a basic
metal salts, which contained extremely small colloidal particle of metal oxide and
adsorbed on the silica surface. For instance, a titania-coated sol was made by
hydrolysing an organic titanium compound in an acid-stabilized silica sol at the pH
of less than 2 and heating the mixture to cause the titania to be deposited on the
surface of the particles [41, 42].
2.4.1
Titania-Silica in Catalysis
Since the development of the first heterogeneous titania/silica catalyst for the
epoxidation of olefins by Shell in 1971, a series of new preparation methods were
developed to synthesize materials containing highly dispersed titanium centres in a
silica matrix. Titanosilicate-catalyzed epoxidation of olefins with hydrogen peroxides
was made possible by the discovery of TS-1 in which Ti has been substituted for Si
in the MFI framework. The search for large pore analogues of TS-1 has led to the
study of Ti substituted into the framework or grafted onto the channels of zeolite beta
or MCM type silicalites.
Several researches have been directed at homogeneous analogues of TS-1,
such as CpTi-silsesquioxane immobilized in the mesopores of MCM-41. Deposition
or anchoring of Ti sites on silica circumvents the steric problem by avoiding narrow
channels. Catalysts prepared by sol-gel [43] contain accessible immobilized Ti
17
within the silica framework. Since high specific surface area is obtained and the
resulting porous structure is very open, larger substrates can access the active sites.
Better accessibility may be obtained by having the active component on the surface.
In this arrangement, the pore diffusion is better and no size selection takes place. In
search of a preparation method that would produce this geometric arrangement and a
high surface concentration of the active site, i.e. isolated Ti within a silica
environment, we investigated the preparation of titania/aerosilica catalysts by sol-gel
synthesis.
It is now established that nanoscale engineering of sol–gel TiO2–SiO2 mixed
oxides provides excellent microporous or mesoporous epoxidation catalysts. A
comparison of the catalytic performances of these amorphous oxides with molecular
sieve materials has been reported. Highly dispersed titania in the silica matrix,
mesoporous structure, and high surface area are the key characteristics of the 20 wt%
TiO2–80 wt% SiO2 aerogel obtained by this method. This catalyst showed
outstanding performance in the epoxidation of cyclic olefins with
alkylhydroperoxides [16].
Yoshida et al. [4] has conducted a screening study of silica-supported
catalysts. Silica supported Ti system was found to be the most effective catalyst for
the epoxidation of propene. All of these findings strongly suggest that silica-titania
mix oxides might be the best combination to become the best catalyst for the
oxidation reaction compared to others.
Besides as catalyst, TiO2–SiO2 mixed oxide materials are widely used in
optical films because of their chemical stability and large refractive index difference.
Optical coatings of TiO2–SiO2 can be produced as anti reflective thin films with
tailored refractive indices. Sol–gel process provides an attractive route to the
preparation of multi-component oxide materials that show homogeneity in the
distribution of heterometal oxide bonds [44].
18
2.5
Aerogel
2.5.1
History and Development of Aerogel
Aerogel was first discovered in 1931 by physicist Steven S. Kistler of the
College of the Pacific, Stockton, California, who wanted to prove that a ‘gel’, once
dried, contained a continuous solid network the same size and shape as the wet gel
[45]. His attempts with aqueous silica or alumina gels (hydrogels) from which water
was removed at above critical conditions (374.0 ˚C, 22.1 MPa) did not produce
satisfactory results. Eventually, he obtained silica aerogels by:
1. Preparation of a hydrogel in reaction of sodium silicate with hydrochloric
acid,
2. Careful removal of sodium and chlorine ions,
3. Converting the hydrogel into alcogel by replacing water with ethyl alcohol in
a lengthy process of multifold solvent replacement, and
4. Drying at above critical conditions for ethyl alcohol.
Later, Fricke obtained aerogels from alumina, tungsten, tin, and iron oxides,
as well as from organic gels such as gelatine, proteins, and cellulose. Rediscovery of
aerogels took place in the 1960s. In the late 1970s, the French government
approached Stanislaus Teichner at Universite Claud Bernard in Lyon seeking a
method for storing oxygen and rocket fuels in porous materials. Teichner et al.
substantially simplified the procedure by carrying out the sol-gel transition in the
vary solvent which was then removed at supercritical conditions. Because water and
alkoxysilanes are immiscible, a mutual solvent such as alcohol is normally used as a
homogenizing agent [45].
19
2.5.2
Aerogel Synthesis
(i)
Drying Process
At a normal drying process at ambient condition, gel will form a xerogel once
dried. This is due to the surface tension of the pore liquid causing the shrinkage and
fracture of the gel structure. Three stages are occurring in the drying process. First
stage is decreasing in gel volume (shrinkage) due to large capillary forces exerted by
the pore liquid during evaporation. Once the gel network may not be compressed
further, pore liquid starts to evaporate and causes the capillary force reduces in
second stage. Final stage starts with the evaporation of the liquid within the pores
and the vapour diffuse to the surface [46].
The increase of solvent tension will cause a linear decrease in the xerogel
surface area, pore volume and pore size [47, 48]. Hence, approaches to obtain lower
surface tension become significant in order to produce a dried gel with higher surface
area.
(ii)
Elimination of Surface Tension.
Capillary pressure, Pc is the factor that drives shrinkage of a gel during
drying process. By assuming that the contact angle is zero, Pc is given by equation as
shown below [49].
Pc =
2γ LV
(rp − t )
t
= Thickness of a surface adsorbed layer
γ LV = Surface tension of the pore liquid
rp
= Pore radius
(2.4)
20
Thus, capillary pressure could be minimized through reduce the surface
tension of the pore liquid by chemical additive or prepare gel with larger pore size.
Other alternative is dry the sample when the liquid has no surface tension, as
happened in freeze drying and supercritical drying methods where surface tension
ceases and meniscus no longer form. Aging is another approach to reduce fracture. It
strengthens the network and thereby the gel skeleton becomes stiffer and stronger
[50].
(iii)
Freeze Drying
Freeze drying is a way to avoid the presence of the liquid vapour interface. In
this method, the pore liquid is first frozen and thereafter dried by sublimation under
vacuum as shown in Figure 2.2. The materials obtained are termed cryogels. The gel
network may be destroyed by the nucleation and growth of the solvent crystals. Thus,
surface area and mesopore volume tend to be smaller than that of aerogel as well as
larger pores are formed in cryogels. Flash freezing has been developed to overcome
these problems. Solvent that has low expansion co-efficient and high pressure of
sublimation is recommended in order to reduce the time of the process and to obtain
better properties in cryogels [51].
Pressure
Liquid
Solid
Gas
Temperature
Figure 2.2
The freeze drying process path (bolded arrows) in pressuretemperature (P-T) phase diagram of a pure substance [52].
21
(iv)
Supercritical Fluid Extraction
Figure 2.3 shows the pressure-temperature (P-T) phase diagram of a pure
substance. The vapour pressure curve starts at the triple point and ends at the critical
point. The melting pressure curve starts at triple point and rises with increasing
temperatures and pressures. Meanwhile, the sublimation curve starts at triple point
and goes down with temperatures and pressures.
In the region below the critical point, phase transition take place by changing
temperature and pressure of the substance. Each phase can co-exist in the same
system with clearly separated phases.
In the region where the temperature above the critical point, the substance
cannot be liquefied by pressure increased. Critical temperatures and pressures (both
minima) show new “phase”: supercritical gas/fluid has a density like that of the
liquid but its flow properties and ability of molecules to be separated from one
another like a gas. There is no phase transition between gas and liquid. The substance
in this region is termed supercritical fluid. It is the gas-like, high diffusion coefficients and low viscosities. In addition to the liquid-like high solvating properties,
that makes supercritical fluids good solvents.
In the view of solvent properties, the surface tension of the solvent will be
reduced with the increase of temperature. Once critical point achieved, there is no
more surface tension exists. Therefore, ethanol can be extracted at this stage without
the collapse of the gel structure due to the surface tension.
The bolded arrows show the path how a critical point can be reached in the
aerogel synthesis process. Supercritical extraction is an extraction using a
supercritical fluid (must be above critical point). The solvent is heated in a pressure
reactor until exceeds its critical point, viz. formation of the supercritical fluid. The
fluid is released slowly from the system and then causes the decrease of pressure.
When the pressure is lower than the critical point, all the ethanol will be converted
into gas form. When the pressure reaches ambient pressure, the reactor will be
22
flushed with inert gas, e.g. nitrogen gas to remove the ethanol that remains as vapour
in the system. Clearly, the whole process to remove ethanol does not involve surface
tension. Hence, aerogel can be produced.
Pressure
Solid
Supercritical
Fluid
Liquid
Pc
Critical Point
Ptp
Gas
Triple
Point
Tc
Ttp
Figure 2.3
Temperature
The supercritical drying process path (bolded arrows) in pressuretemperature (P-T) phase diagram of a pure substance [52].
Ethanol, an alcohol, is used as a medium in the supercritical drying process.
Ethanol has lower surface tension compared to the other alcohols and other solvents.
In addition, ethanol is cheap and abundant. It is a good solvent that can dissolve in
both polar and nonpolar solvents [53]. At the same time provide low silica solubility
(Table 2.1), thus protect from damage of the silica skeleton along the synthesis
process.
Table 2.1: The solubility of silica in different solvent [54].
Solvent
Solubility of silica (mg/L)
Methanol
1890
Ethanol
164
Propanol
8
23
Nevertheless, once the critical point is approached, aerogel may be formed.
For example, supercritical carbon dioxide, CO2 (Tc = 31 ˚C, Pc = 73 atm) has been
used in many aerogel syntheses. Besides used in supercritical extraction, critical
point also can be applied in liquefaction of gases, e.g. fuels and air conditioning
(must be below critical point). Table 2.2 shows the critical point of some solvents.
Table 2.2: The critical point of different solvents [55, 56].
Substance
Formula
Tc (˚C)
Pc (MPa)
Pc (g/cm3)
Vc (cm3/mol)
Carbon Dioxide
CO2
31.0
7.38
0.47
94
Methanol
CH3OH
239.4
8.08
0.27
117
Ethanol
C2H5OH
240.9
6.14
0.28
168
Water
H2O
374.0
22.1
0.32
56
Note: 1 MPa = 106 Pa = 9.87 atm.
Tc = critical temperature, Pc = critical pressure, Vc = critical volume.
2.5.3
Properties and Applications of Aerogel
Aerogels composed of 65 to 90% of air, are the lightest solids ever produced.
Silica aerogel is a porous material with optical and thermal properties that makes the
material very interesting as an insulation material [3, 57]. Table 2.3 shows the
properties of silica aerogel.
Besides being the best thermal, electrical, and acoustic insulators known,
aerogels are finding its application as filters for seawater desalination,
micrometeoroid collectors, and subatomic particle detectors. In the future, aerogels
could be used in windows, building insulation, automobile catalytic converters, and
high-efficiency battery electrodes [58].
24
Aerogel catalysts are prepared by the sol-gel method associated with the
supercritical drying procedure. The resulting catalysts, in the form of simple or
mixed oxides and supported metals exhibit interesting high surface areas and large
pore volumes. Their very good resistance to heat treatments allow them to be used
for all types of catalysed reactions up to 450¯500 °C. Aerogel catalysts show in
general greater activity and selectivity than the corresponding xerogels. Their
stability with time on stream is also remarkable [43].
Table 2.3: Some properties of aerogel [59, 60].
Property
Value
Thermal conductivity
0.018- 0.3 Wm-1K-1
Bulk density
80- 140 kgm-3
Particle size
5- 500 µm
Specific surface area
400- 1000 m2/g
Temperature stability
Up to 600 ˚C
Pore size
< 50 nm
CHAPTER 3
EXPERIMENTAL
The experiment is generally divided into the following stages:
3.1
•
Synthesis of silica aerogel
•
Synthesis of titanium containing silica aerogel
•
Characterization of titanium containing silica aerogel
•
Catalytic testing of the titanium containing silica aerogel
Synthesis of Silica Aerogel
The white RHA taken from Sabak Bernam, Selangor was used in the
synthesis of sodium silicate. The synthesis is depicted in Figure 3.1. As the type of
silica in the RHA is present in amorphous form, this gives an advantage to synthesize
sodium silicate through low temperature alkali extraction method i.e. below 100 °C.
The solubility of amorphous silica is very low at pH < 10 and increases sharply
above pH 10. This unique solubility behaviour makes silica extractable from RHA by
dissolution under alkaline conditions and subsequently precipitating it at a lower pH.
This process of obtaining silica is an alternative method to the current high-energy
method that manufactured by smelting quartz sand with sodium carbonate at 1300 °C
[61].
26
Sodium
hydroxide
14.55 g
RHA
39.13 g
Filtrate
95 °C
Distilled
water
450 ml
1 day
Screw cap
Teflon bottle
Figure 3.1
Supernatant
(Sodium
silicate)
Synthesis of sodium silicate from rice husk ash.
200 g of sodium silicate was reacted with 167.32 g H2SO4 96% to form a gel.
The gel was crushed and kept in a Teflon bottle for 1 day (Figure 3.2). Later the gel
was washed with distilled water to remove soluble salt. Soxhlet process was carried
out to replace the water in the aquagel with ethanol forming an alcohol filled gel (is
termed alcogel) [62].
Supercritical extraction (Figure 3.2) was carried out using Parr instrument
autoclave fitted with a thermocouple, a pressure gauge and a temperature controller.
Ethanol was used as the supercritical drying solvent in this unit. Ethanol has a critical
temperature and pressure of 239 ˚C and 1200 psi, respectively. The volume of
ethanol that needs to be loaded is calculated using the equation 3.1 below.
y = (2000-Vgel)/3.72
where,
Vgel = volume gel loaded (ml)
If y < 500, 500 ml ethanol is used
If y > 500, y ml ethanol is used
(3.1)
27
The reactor was heated according to the heating programmed as shown in
Table 3.1. When the reactor reached 275 °C and kept for an hour, the pressure was
isothermally released at a rate of 20 psi/min. The reactor condition was maintained
slightly above the critical condition of ethanol to ensure that the whole mixture was
supercritical. The exiting solvent was collected in the condenser. The nitrogen gas
was flushed slowly through the autoclave for 15 minutes after the pressure reached
ambient pressure. Then, the reactor was left to cool overnight [63].
Table 3.1: Temperature programme implemented in the supercritical drying process
[64].
Time (hour) Temperature (°C)
1st
50
2nd
100
3rd
150
4th
200
5th
225
6th
250
7th
275
28
Magnetic stirrer
Sodium
silicate
solution
Washed
and
filtered
Aging
+ H2SO4
Aquagel
Gel
AEROGEL
Supercritical
extraction
Figure 3.2
3.2
Alcogel
Soxhlet
with ethanol
Synthesis of silica aerogel from sodium silicate.
Synthesis of Titanium Containing Silica Aerogel
In this work, three methods of preparation of titanium containing silica
aerogel were implemented. They were direct synthesis, grafting and precipitation.
Grafting and precipitation involved heterogeneous synthesis condition, where
between the silica (silica aerogel made) and the titanium source. In contrast, direct
synthesis involved homogeneous mixture of the sodium silicate with the titanium
precursor.
29
3.2.1
Post Synthesis: Synthesis of Titania-Silica System
(i)
Grafting with Titanium(IV) Tetrachloride
30 g of the silica support (RHA, or aerogel) was added to a solution
containing 12.56 g of TiCl4 in 200 ml n-hexane. After stirring for 4 hours, the solvent
was removed by evaporation in a rotary evaporator at 50 °C and then at 80 °C for 1
hour. The resulting solid was dried at 120 °C for 12 hours and calcined in air at 500
°C for 20 hours [64].
(ii)
Grafting with Titanium(IV) Isopropoxide
40 g of the silica support was added to a solution containing 25.11 g of
titanium(IV) isopropoxide in 350 ml of hexene. After stirring, the alcohol was
removed by evaporation in a rotary evaporator at 70 °C. The resulting solid was dried
at 120 °C for 12 hours and calcined in air at 500 °C for 20 hours [65].
(iii)
Precipitation of Titania on Amorphous Silica
The supports used were RHA (BET surface area of 30.06 m2/g), and silica
aerogel (BET surface area of 374.44 m2/g). The silica was first calcined at 600 °C for
12 hours before deposition of titania. 16.76 g pure TiCl4 was added to a diluted
solution of HCl (pH = 0.5-1.0, HCl 30% 8 g + H2O 432 g). 40 g of silica support
were then added and ammonium hydroxide was added under continuous agitation
until a final pH of 7.5 was reached. The resulting solid was dried at 120 °C for 12
hours and calcined in air at 500 °C for 20 hours [65].
30
3.2.2
Direct Synthesis: Synthesis of Titania-Silica Aerogel System
Titanium(IV) ethoxide (4 mmol Ti) and sulphuric acid were mixed with
sodium silicate (400 mmol Si) solution to get a transparent solution. The solution
was adjusted to pH 6 by adding NaOH solution or sulphuric acid. The gel was aged
for 3 days. This synthesis method was developed during this research work through
several trial and errors process.
After that, the silica gels were dispersed in distilled water and filtered to
remove the soluble salts. The washing step was repeated two more times. The
aquagel was then solvent exchanged with ethanol using soxhlet technique to remove
water from the gel matrix. Supercritical drying process was conducted in order to
synthesis an aerogel.
3.3
Parameter Study for Synthesis (Direct Synthesis) of Titanium
Containing Silica Aerogel
3.3.1
Sources of Titanium
Various titanium sources were used to prepare the titanium containing silica
aerogel as listed in Table 3.2.
In a typical preparation, titanium(IV) isopropoxide (10 mmol Ti) and
sulphuric acid solution (H+:NaOH molar ratio = 1) were mixed with sodium silicate
solution (330 mmol Si) to obtain a homogeneous mixture solution. The solution is
left to gel and aged for 3 days.
After that the silica gels were dispersed in distilled water and filtered to
remove the soluble salts. The washing step was repeated until the filtered cake near
to neutral. The aquagel was then solvent exchanged with ethanol using soxhlet
31
technique to remove water from the gel matrix. Supercritical drying process was
performed to remove ethanol in order to synthesis aerogel (Figure 3.2).
The same preparation procedure was implemented for other titanium source.
The loading of titanium was fixed to Si:Ti = 33 in the gel mixture. The gel matrixes
were obtained by adding stoichiometric volume of sulphuric acid for neutralizes the
sodium silicate. After supercritical dried, the sample was dried in vacuum oven. The
samples were characterized using nitrogen adsorption, X-ray diffraction (XRD),
Fourier transform infrared (FTIR), and UV-Vis diffuse reflectance spectroscopy
(UV-Vis DRS).
Table 3.2: Titanium sources that have been used in the synthesis of titanium
containing silica aerogel
3.3.2
Titanium Source
Formula
Titanium(IV) ethoxide
Ti(OCH2CH3)4
Titanium(IV) isopropoxide
Ti[OCH(CH3)2]4
Titanium(IV) propoxide
Ti(OCH2CH2CH3)4
Titanium(III) sulphate
Ti2SO3
Titanium(IV) chloride
TiCl4
Titanium(IV) oxide
TiO2
Si:Ti Molar Ratio
Various silica per titania ratios have been used to prepare the titanium
containing silica aerogel. The titanium(IV) isopropoxide was used in this part of the
experiment. Si:Ti molar ratios studied are 1, 6, 33, and 49.
In a typical preparation for molar ratio Si:Ti = 1, titanium(IV) isopropoxide
(33 mmol Ti) and sulphuric acid solution (H+: NaOH molar ratio = 1.25) were mixed
32
with sodium silicate solution (330 mmol Si) to obtain a homogeneous mixture. The
solution was left to gel and aged for 3 days.
After that, the silica gels were dispersed in distilled water (500 ml) and
filtered to remove the soluble salts. The washing step was repeated until the filtered
cake was almost neutral. Subsequently, the aquagel was soxhlet with ethanol to
replace water with ethanol. Supercritical drying process was carried out to remove
ethanol in order to synthesis an aerogel.
The same preparation procedure was implemented for other Si:Ti molar
ratios. The gel was obtained by adding 2.5 times the volume of acid that was needed
to neutralize the sodium silicate. After supercritical drying, the sample was dried in
vacuum oven. The samples were characterized using nitrogen adsorption, X-ray
diffraction (XRD), Fourier transform infrared (FTIR), and UV-Vis diffuse
reflectance spectroscopy (UV-Vis DRS).
3.3.3
Sulphuric Acid Loadings
Various loading of the sulphuric acid were used to prepare the titanium
containing silica aerogel. The acid was not only used to neutralize the sodium
silicate, but also to induce the formation of silicic acid and to change in aging pH.
The titanium(IV) isopropoxide was used for this purpose. The molar ratio of Si:Ti =
33 was selected. The acid loadings (mol H+: mol NaOH) were varied from 0.75 to
2.5, i.e. 0.75, 1.00, 1.25, 1.50, 1.75, and 2.50 times the volume to neutralize sodium
silicate.
In a typical preparation, titanium(IV) isopropoxide (10 mmol Ti) and
sulphuric acid solution (H+: NaOH molar ratio = 2.50) were mixed with sodium
silicate solution (330 mmol Si) to obtain a homogeneous mixture. The solution was
left to gel and aged for 3 days.
33
After that the silica gels were dispersed in 500 ml of distilled water and
filtered to remove the soluble salts. The washing step was repeated until the filtered
cake was near to neutral. The aquagel was then solvent exchanged with ethanol
using soxhlet technique to remove water from the gel matrix. Supercritical drying
process was performed to remove ethanol in order to synthesis an aerogel.
After supercritically dried, the sample was dried in vacuum oven. The
samples were characterized using nitrogen adsorption, X-ray diffraction (XRD),
Fourier transform infrared (FTIR), and UV-Vis diffuse reflectance spectroscopy
(UV-Vis DRS).
3.4
Characterization
The structure of the solid samples were characterized using nitrogen
adsorption, X-ray powder diffraction (XRD), Ultra Violet-Visible diffuse reflectance
spectroscopy (UV-Vis DRS), Fourier Transform Infrared Spectroscopy (FTIR), and
scanning electron microscopy (SEM).
3.4.1
Nitrogen Adsorption: Brunauer, Emmett, Teller (BET) method
Specific surface area can be determined through BET equation [66]:
p
1
c −1 p
=
+
,
Vads (p ° − p) Vm c Vm c p °
Where,
po = atm pressure
p
= partial pressure
c
= BET constant.
Vads = volume of gas adsorbed.
Vm = monolayer capacity.
(3.2)
34
The effective molecular area of nitrogen (am) is taken as 16.2 Å2 (16.2 x 10-20
m2/molecule), and NA is Avogadro number (6.0223 x 10-23 molecule/mole), which is
used to calculate the specific surface area of the sample (ABET).
ABET = Vm.NA.am
or
ABET = Vm(4.53) m2/g
(3.3)
Total pore volume can be calculated using formula: Vp = V0.95 (0.00156),
where V0.95 is volume adsorbed at pressure relative, p/p o = 0.95. By using these data,
the pore diameter, d can be calculated using formula: d = 4 (Vp/ABET). The
classification of the pores is shown as in Table 3.3.
Table 3.3: IUPAC classification of pores [67, 68].
Type of Pore
Pore Diameter (nm)
Micropores
0-2
Mesopores (also intermediate pore or transitional pores)
2-50
Macropores
50-7500
Megapores
> 7500
Note: 1 nm = 10 Å = 10-9 m
The BET method is not valid for the calculation of surface area for isotherm
Type I and Type III. On the other hand, both Type II and Type IV isotherms are
amenable to the BET analysis, provided that the value of c is not too high and the
BET plot is linear for the region of the isotherm containing “Point B”. “Point B” was
taken by Emmett and Brunauer to indicate the completion of the monolayer, and was
the point that often displayed a rather long straight portion started from this point in
the isotherm.
By assuming that the solid composed of similar size spheres [67], the mean
diameter (l) may be calculated from the surface area (A) using formula l =
6
,
ρA
where ρ is the density of the spheres. According to the Globular theory [68], the
35
particle size of silica can be estimated through the same equation with following
concerns:
l=
6
ρA BET
(3.4)
With l is the particle size (nm), ABET is the specific surface area (m2/g), and ρ
is the density (density of amorphous silica 2.2 x 106 g/m3).
Experimental:
The BET specific surface area and total pore volume (measured at p/po =
0.95) were obtained from the isotherms of nitrogen adsorption at 77 K, using
ThermoFinnigan S.p.A. Qsurf Surface Area Analyser M1. The flow rate for the gases
was set at 20 ml per minute or 40 psi (i.e. 3.5 unit on the flow meter scale) and room
temperature was set to the instrument. The sample (ca. 0.05 g) was previously
degassed at 200 ºC under flowing nitrogen for 30 minutes. Then, it was transferred to
test channel and analysis was started after nitrogen gas flushed for 25 minutes.
Standard calibration reading was kept in the range of 35 to 40 seconds. After
analysis, the total weight of the sample holder and sample (after degassed) was keyin to get the surface area or total pore volume. 1 cm3 nitrogen gas adsorbed is
equivalent to 2.84 m2 of surface area is considered for the instrument measurement.
3.4.2
XRD Measurement
XRD is a technique used to characterize solid materials. It is powerful in
determining the phase of the materials. The crystalline materials will have their own
diffraction pattern, which can be considered as their “fingerprint”. In contrast, small
particles or amorphous phases give either broad or weak diffractogram or no
diffraction at all. In particular, the surface region, where catalytic activity resides is
virtually invisible for XRD [69].
36
Powder diffraction pattern is a plot of the intensity of the diffracted beams,
which represents a map of reciprocal lattice parameter or Miller indices (hkl) as a
function of 2θ, which satisfies the Bragg condition:
nλ = 2d sin θ
Where,
(3.4)
n = order of the reflection (n = 1, 2, 3, …)
d = distance between two lattice planes
λ = wavelength of the X-rays
θ = diffraction angle
Commonly, first order diffraction (n = 1) is implemented. The Bragg relation gives
the corresponding lattice spacing, which are characteristic for a certain compound
[70].
Experimental:
All the samples were characterized by X-ray powder diffraction using Bruker
Advance D8 with Siemens 5000 diffractometer and the Cu Kα (λ = 1.5405 Å)
radiation as the diffracted monochromatic beam at 40 kV and 40 mA. Silicon powder
was used as an internal standard. Typically, powder samples were grounded and
spread on a sample holder. The diffraction pattern was scanned in the range between
2º to 60º at a step of 0.020º and step time 1.0 s (scanning speed of 1.2º/min).
3.4.3
UV-Vis Diffuse Reflectance Spectrometry
Electronic transitions of substrate can be studied by the UV-Visible light. The
UV-Vis spectra are very broad [71]. In the diffuse reflectance mode, samples can be
measured as loose powders. Diffuse reflectance is also the indicated technique for
strongly scattering or absorbing particles. The reflectance spectrum is described the
Kubelka-Munk function [72]:
37
K (1 − R∞ )
=
S
2 R∞
Where,
2
(3.5)
K
S
= absorption coefficient, a function of the frequency v
= scattering coefficient
R∞
= reflectivity of a sample of infinite thickness, measured as a
function of v.
Experimental:
UV-Vis DRS measurements under ambient conditions were performed using
Perkin-Elmer Lambda 900 UV/VIS/NIR Spectrometer equipped with a diffuse
reflectance attachment with a 76 mm integrating sphere using BaSO4 as a reference.
The samples were previously outgased at 120 °C for 12 hours before analysis in
order to eliminate the adsorbed water. The reflection in the percentage was measured
and presented by Kubelka-Munk function.
3.4.4
Fourier Transform Infrared Spectroscopy
Infrared has a wavelength in the range of 1-1000 µm. Infrared is classified to
far (10-200 cm-1), mid (200-4000 cm-1), and near (4000-10000 cm-1) infrared that
used for the detection of lattice vibrations, molecular vibrations, and overtones
accordingly. Mid infrared region is that of interest to us.
The infrared region between 4000 and 200 cm-1 can roughly be divided
into four regions:
1. The X-H stretch region (4000 - 2500 cm-1), where strong contributions
from OH, NH, CH and SH stretch vibrations are observed,
2. The triple bond region (2500 - 2000 cm-1), where contributions from
gas phase CO (2143 cm-1) and linearly adsorbed CO (2000 - 2200
cm-1) are seen,
38
3. The double bond region (2000 -1500 cm-1), where in catalytic studies
bridge bonded CO, as well as carbonyl groups in adsorbed molecules
(around 1700 cm-1). The fingerprint region (1500 - 500 cm-1), where
all single bonds between carbon and elements such as nitrogen,
oxygen, sulphur and halogens absorb,
4. The M-X or metal-adsorbate region (around 200 - 450 cm-1), where the
metal-carbon, metal-oxygen and metal-nitrogen stretch frequencies in
the spectra of adsorbed species are observed.
Correlation charts should be consulted for more precise assignments [73, 74, 75, 76].
The hydroxyl range between 3000 and 3800 cm-1 contains contributions
from adsorbed water and several hydroxyl groups on the SiO2 surface. The broad
absorption band around 3550 cm-1 is due to hydrogen-bonded OH groups. The
sharp peak at 3740 cm-1 corresponds to single OH group, which has no interaction
with other hydroxyls. The peak around 3660 cm-1 could belong to OH groups
inside the silica. Similar correlations exist for the O-H stretch frequencies of OH
groups on alumina and titania supports [77]. The assignments of some infrared
frequencies in silicon compounds are showed in Table 3.4.
Table 3.4: Some assignments of infrared frequencies [78].
Group
-SiOH
Range (cm-1) and Intensity
Assignments and Remarks
3700-3200 (s)
OH stretch, similar to alcohols
900-820 (s)
Si-O stretch
ca. 1430(m-s)
Ring mode
1100 (vs)
Ring mode
1100-1050 (vvs)
Si-O-C antisymmetric stretch
Si-O-Ar
970-920 (vs)
Si-O stretch
Si-O-Si
1100-1000 (s)
Si-O-Si antisymmetric stretch
Si-Ar
Si-O-C
(aliphatic)
Note: w = weak, m = medium, s = strong, vs = very strong, vvs = very very strong.
39
Experimental:
Infrared spectra of the samples were collected using a Perkin-Elmer Fourier
transform infrared (FTIR), with a spectral resolution of 2 cm-1, scanned for 10 s at 20
ºC by KBr pellet method. The framework spectra were recorded in the region of
1500 – 400 cm-1.
3.4.5
Scanning Electron Microscopy
Electron microscopy gives straightforward determination of morphology of
the surface. Scanning electron microscopy (SEM) is carried out by rastering a
narrow electron beam over the surface. The yield of either secondary or
backscattered electrons is detected as a function of the position of the primary
beam. An electron microscope offers additional possibilities for analysing the
sample. Emitted X-rays are characteristic for an element and allow for a
determination of the chemical composition of a selected part of the sample, i.e.
energy dispersive X-ray analysis (EDX).
Experimental:
The SEM stub was cleaned, and then placed double-sided tape on the top. The
sample is dispersed on the surface. A tin layer of gold coating is deposited on the
sample by gold spattering. Then, it is analysed using XL 40 Phillips type instrument.
3.5
Catalytic Properties: Oxidation of Alkene
Oxidation was carried out batch wise in a mechanically stirred 250 ml
thermostated glass reactor equipped with reflux condenser. In a typical run, 10 ml of
fresh distilled cyclohexene, 10 ml of acetone and 156 mg of the catalyst were mixed
in the reactor and the suspension was heated at 70 ºC. Then, 8.35 ml of 35%
40
hydrogen peroxide was added to the reaction mixture while maintaining vigorous
stirring. After the reaction, the mixture was cooled to room temperature.
The organic layer was separated by centrifugation and/or by extraction with
diethyl ether and then analysed by a Thermo Finnigan, Trace Gas Chromatography
(GC) using a capillary column (Equity-1, 30 m x 0.25 mm x 0.25 µm). The oventemperature programme was tabulated as in Table 3.5. A flame ionisation detector
(FID) was applied.
Table 3.5: GC-FID instrument setting
Oven Parameters
Setting
Initial temperature
40 ˚C
Initial time
2.00 min
Rate
10.0 ˚C/min
Final temperature
200 ˚C
Carrier mode
Constant pressure
Detector Parameters
Setting
FID base temperature
250 ˚C
H2 flow
35 ml/min
Air flow
350 ml/min
Makeup gas flow
30 ml/min
Injection Port Parameters
Setting
Base temperature
250 ˚C
Split flow
100 ml/min
Injection volume
1.0 µl
Products were identified by comparing with authentic samples (internal
standard) that have been checked using gas chromatography- mass spectroscopy
(GC-MS). Authentic samples that implemented in this study were cyclohexene
oxide, 2-cyclohexen-1-ol and 2-cyclohexen-1-one supplied by Fluka. While, standard
41
1, 2-cyclohexanediol has been synthesized through homogeneous synthesis as
described below [79].
140 ml of 30% hydrogen peroxide (1.4 moles) was added to 600 ml of 88%
formic acid (13.7 moles) in a three-necked flask equipped with a thermometer and a
motor-driven stirrer. Freshly distilled cyclohexene (82 g, 1.0 mole) was added slowly
from a dropping funnel over a period of 20–30 minutes while the temperature of the
reaction mixture was maintained between 40 °C and 45 °C by cooling with an ice
bath and by controlling the rate of addition. The reaction mixture was kept at 40 °C
for 1 hour after all the cyclohexene has been added. Then it was left overnight at
room temperature.
The formic acid and water were removed by distillation from a steam bath
under a reduced pressure. An ice-cold solution of 80 g of sodium hydroxide in 150
ml of water was added in small portions to the residual viscous mixture of the diol
and its formats. The temperature of the mixture was controlled so that it did not
exceed 45 °C. The alkaline solution was warmed to 45 °C, and an equal volume (350
ml) or more of ethyl acetate was added. After thorough extraction, the lower layer
was separated and extracted at 45 °C six times with equal volumes of ethyl acetate.
The seven ethyl acetate solutions were combined (total volume about 2.1
liter), and the solvent was distilled from a steam bath until the residual volume was
300–350 ml and the solid product begins to crystallize. The mixture was cooled to 0
°C, and the product was separated by filtration (77–90 g, melted in the range of 90–
98 °C). The mother liquor was concentrated on a steam bath to a volume of 65–75
ml, and more solid crystallized. The mixture was cooled and filtered as before and
yielded an additional 4–15 g of crude product melted in the range of 80–89 °C.
Trans-1, 2-cyclohexanediol, boiling point 120–125 °C/ 4 mm, was obtained
by distillation of the combined crude products, using an oil bath, a flask having a side
arm, with an air condenser sufficiently wide that they will not become plugged as the
product solidifies. The yield of product of melting point 101.5–103 °C was 75–85 g
(65–73%).
CHAPTER 4
RESULTS AND DISCUSSION
4.1
Synthesis of Silica Aerogel
The silica aerogel is a semi-transparent and very light fluffy powder. The
XRD pattern of silica aerogel in Figure 4.1 shows it is completely amorphous. This
result is well agrees with other reported study [45].
800
700
600
Lin (Counts)
500
400
300
200
100
2
Figure 4.1
10
20
30
40
50
2θ (degrees)
XRD diffractogram of silica aerogel.
60
70
80
90
43
The SEM micrograph in Figure 4.2 shows the surface morphology of the
silica aerogel. The sample was found having a rough and homogeneous surface. It
was similar with the morphology of the aerogel that prepared using TMOS as silica
source [80].
Figure 4.2
SEM micrograph showing the surface morphology of silica aerogel.
The EDX analysis of silica aerogel shows the presence of silicon and oxygen
as major elements. Trace amount of aluminium, which comes from the RHA source
was also detected. RHA sample taken from Bagan Sekincan, Perak was also found to
contain trace amount of alumina content [81]. This is resulted from the dissolution of
the alumina by the sodium hydroxide solution; similar to the Bayer process in the
extraction of alumina from bauxite mineral [82].
Al2O3 + 2OH- + 3H2O
2Al(OH)4-
(4.1)
FTIR spectrum of silica aerogel (Figure 4.3) shows a band at 969 cm-1, which
was attributed to the stretching of terminal silanol group, namely Si-OH or SiO-H
44
group. The band at 799 cm-1 was associated with symmetric Si-O-Si stretching. The
most intense band at 1090 cm-1 was assigned as the asymmetric Si-O-Si stretching
[83].
100
80
60
%T
40
799
969
20
1090
Figure 4.3
1100
1300
1500
cm -1
900
700
400
FTIR spectrum of silica aerogel.
The highest and the lowest surface area of the silica aerogel achieved in this
study are 882 m2/g and 387 m2/g respectively (Table 4.1). The particle sizes of the
silica in samples A1.0 and A2.5 are 7.03 nm and 3.09 nm respectively according to
the Globular theory [68].
Table 4.1: The surface area of the silica aerogel.
Sample Aerogel
BET Surface Area (m2/g)
Particle Size (nm)
A1.0
387
7.03
A2.0
784
3.48
A2.5
882
3.09
A3.0
867
3.14
A14.8
607
4.49
45
4.2
Synthesis of Titanium Containing Silica Aerogel
4.2.1
Post Synthesis
X-ray diffractograms in figures 4.4 and 4.5 indicate that both titanium-
modified aerogel and titanium-modified RHA are amorphous even after grafting with
titanium(IV) chloride and titanium(IV) isopropoxide. This properties also been
observed in the diffractograms for titania that precipitated in the silica matrixes
(aerogel and RHA). Crystalline form of titania was neither present in anatase, rutile
nor brookite. These results indicate that the titanium may either be in amorphous
form or dispersed as small crystalline titania with short-range order, which could not
be detected by XRD.
Intensity
Si aerogel
Si aerogel precipitated titanium(IV) chloride
Si aerogel grafted titanium(IV) chloride
Si aerogel grafted titanium(IV) isopropoxide
2
10
20
30
40
50
60
70
80
2θ (degrees)
Figure 4.4
The X-ray diffractograms of titanium-modified silica aerogels.
90
46
Intensity
RHA
RHA precipitated titanium(IV) chloride
RHA grafted titanium(IV) chloride
RHA grafted titanium(IV) isopropoxide
2
10
20
30
40
50
60
70
80
90
2θ (degrees)
Figure 4.5
The X-ray diffractograms of titanium-modified amorphous silica
(RHA).
The infrared spectra in Figure 4.6 show the vibration characteristic of RHA
and titanium-modified amorphous RHA. The FTIR spectra of RHA and titaniummodified RHA by grafting with titanium(IV) chloride and titanium isopropoxide
show the presence of Si-O-Si asymmetric stretching at 1099 cm-1 (internal
asymmetric Si-O-Si range: 1250-900 cm-1). The FTIR spectra of RHA and titaniummodified RHA precipitation of titanium(IV) chloride (Figure 4.6) also shows the
presence of Si-O-Si asymmetric stretching at wave number same as in grafting
method. The broad absorption band around 3450 cm-1 is due to hydrogen-bonded OH
groups due to the adsorption of the moisture from air [77].
47
RHA
3445
467
RHA grafted titanium(IV) isopropoxide
1099
3451
1097
462
RHA grafted titanium(IV) chloride
%T
3431
467
RHA precipitated titanium(IV) chloride
1097
466
1100
4000
Figure 4.6
3000
2000
cm -1
1500
1000 800
The FTIR spectra of titanium-modified RHA.
Additional weak band at 968 cm-1 was due to the presence of terminal silanol
in silica aerogel and titanium-modified aerogels by grafting with titanium(IV)
chloride and titanium isopropoxide as shown in Figure 4.7. Silica aerogel and
titanium-modified aerogel by precipitation (Figure 4.7) also show absorption at 968
cm-1 due to the presence of large amount of silanol groups. This band is absent in
RHA and titanium-modified RHA, as a result of high temperature burning process
during the combustion of rice husk.
Adsorbed water and silanol groups on the aerogels surface contribute the
hydroxyl absorption band in the range between 3000 and 3700 cm-1 [77]. Si-O-Si
asymmetric and Si-O-Si symmetric stretching is also present in all aerogel samples.
400
48
Si aerogel
968
3463
Si aerogel grafted titanium(IV) isopropoxide
1100
3423
Si aerogel grafted titanium(IV) chloride
%T
1100
968
3449
Si aerogel precipitated titanium(IV) chloride
1099
3451
1100
4000
Figure 4.7
3000
2000
cm-1
1500
1000
800
The FTIR spectra of titanium-modified aerogels.
UV-Vis spectroscopy has been extensively used to characterize the
coordination and the nature of titanium substituted in molecular sieves. The
ultraviolet peak position of the Ti4+ ion depends on its coordination and the size of
the extra framework TiO2 particles.
UV-Vis spectra (Figure 4.8 and Figure 4.9) of RHA and aerogel showed a
peak at 242 nm due to minor impurities present in the silica matrix. Post synthesis
samples showed the presence of extra framework titanium oxide (280-330 nm) [78].
It is in the range of the charge transfer of titanium in octahedral coordination for the
anatase titania. While, no diffraction peak was detected in their X-ray diffractograms.
These results support the hypothesis that the titania is present as small crystalline
anatase particles.
49
RHA
RHA grafted titanium(IV) isopropoxide
K-M
RHA grafted titanium(IV) chloride
RHA precipitated titanium(IV) chloride
190
0
Figure 4.8
250
300
350
400
nm
450
500
550
600
0
The UV-Vis spectra of titanium-modified RHA.
Si aerogel
Si aerogel grafted titanium(IV) isopropoxide
K-M
Si aerogel grafted titanium(IV) chloride
Si aerogel precipitated titanium chloride
190
0
Figure 4.9
250
300
350
400
450
500
nm
The UV-Vis spectra of titanium-modified silica aerogels.
550
600
0
50
The BET surface areas of the sample were determined from nitrogen
adsorption analysis as listed in Table 4.2. The surface area of the RHA was found to
increase after the titania was deposited on the silica. This mean that the titania has
higher surface area than that of RHA and the titania is located in the external surface
of the RHA. The use of titanium(IV) isopropoxide as starting material (in modifying
RHA) had result in the highest surface area, showing that it produce smaller titania
particle. Grafting with titanium(IV) isopropoxide also gave the best external
dispersion of titania and allowed the formation of small crystallites at its external
surface as observed by other study [78].
Table 4.2: The BET surface area of titanium containing silica.
Sample
BET Surface Area
(m2/g)
RHA
30
RHA grafted titanium(IV) isopropoxide
105
RHA grafted titanium(IV) chloride
39
RHA precipitated titanium(IV) chloride
60
Silica aerogel
391
Silica aerogel precipitated titanium(IV) chloride
381
Silica aerogel grafted titanium(IV) isopropoxide
383
Silica aerogel grafted titanium(IV) chloride
397
In the case of aerogel, precipitation and grafting with titanium showed little
change to the surface area of the original aerogel. The titania is well dispersed on the
silica aerogel. If compared to the titanium-modified RHA, this results shows that the
dispersion of the titania in aerogel is better than in RHA. It is due to aerogel has
thirteen times larger surface area than RHA, which provided much more space for
the deposition of titania and contributed to less agglomeration of the titania in the
silica matrix. Moreover, titania was dispersed in the inside of the pores rather than
distributed on the outer surface as titania has been trapped in the silica matrix during
51
the gel formation step. Aerogel shows it is so good as a support that it can still
maintain its high surface area (391 m2/g) even after treatment in the post synthesis
process (381-397 m2/g).
4.2.2
Direct Synthesis
The XRD pattern in Figure 4.10 shows titanium-modified silica aerogel is
similar to that of silica aerogel (Figure 4.4). No crystalline phase was observed for
the titanium compound.
FTIR spectrum of titanium-modified silica aerogel (Figure 4.11) shows a
band at 972 cm-1, which attributed to the Si-O-Ti bonds stretching. The stretching
band for Si-OH group was superimposed onto that 972 cm-1 peak. The most intense
band at 1090 cm-1 was assigned as the asymmetric Si-O-Si stretching. The Si-O-Si
symmetric stretching was indicated by the absorption at 796 cm-1 [83].
An UV-Vis spectrum of titanium-modified silica aerogel is shown in Figure
4.12. A band presents in the region of 210 to 225 nm (λ ≤ 230 nm) which was
corresponded to oxygen to tetrahedral titanium Ti(IV) ligand-to-metal charge
transfer, assigned to isolated Ti in tetrahedral framework position [84]. The reason is
that siloxane is a strong electron withdrawing ligand which results in charge transfer
from oxygen to titanium centre and causes blue-shifted to lower wavenumber. The
absence of absorption at about 340 nm indicates the absence of large TiO2 crystalline
particles [12].
The BET surface area and total pore volume of the titanium-modified silica
aerogel were 336 m2/g and 0.88 ml/g respectively. The particle size was less than
8.11 nm. This titanium-modified silica aerogel sample was a mesoporous material
because the pore size was about 10.43 nm.
52
400
Lin (Counts)
300
200
100
0
2
10
20
30
40
50
60
70
80
90
2θ (degrees)
Figure 4.10
XRD diffractogram of titanium-modified silica aerogel (Aph6).
100
80
60
%T
799
972
40
20
1090
1500
Figure 4.11
1300
1100
cm -1
900
700
FTIR spectrum of titanium-modified silica aerogel (Aph6).
400
53
222
Aph6
K-M
Silica aerogel
190
300
400
500
600
700
800
900
1000
1100
1200
nm
Figure 4.12
UV-Vis spectra of titanium-modified silica aerogel (Aph6) and silica
aerogel.
4.3 Parameter Study for Synthesis (Direct Synthesis) of Titanium Containing
Silica Aerogel
4.3.1
The Effect of Titanium Source
The BET surface area of the titanium containing silica aerogel prepared using
various titanium sources is listed in Table 4.3. The parameters that have been fixed in
this experiment are Si:Ti molar ratio = 33 (2.94 %mol Ti) and H+: NaOH molar ratio
= 1.
The surface area of the titanium-modified silica aerogels were not
significantly altered compared to that of unmodified silica aerogel. In general, the
54
incorporation of the titanium in silica reduced the bulk specific surface area by 16%.
However, the surface area of anatase-modified sample was reduced by 28%.
From this data, a trend can be observed, i.e., higher surface area would
provide larger total pore volume of the titanium–modified aerogel. Consequently, it
gives a similar trend in the pore diameter of the titania silica aerogel matrix.
Therefore, the source of the titanium had shown significant influence to the titania
form especially in particle size and homogeneity. If the titania was not bonded to the
silica, the silica could be a separator to avoid agglomeration of the titania particle.
Hence, produce fine and well-dispersed titania particles were produced.
Table 4.3: Effect of titanium source on the surface characteristics of the Ti-Si
aerogels.
Titanium Source
BET Surface Total Pore
2
Area (m /g)
Titanium(IV) ethoxide
Pore Diameter
Volume (ml/g) (nm)
394
0.89
9.01
Titanium(IV) chloride
352
0.77
8.80
Titanium(III) sulphate
347
0.73
8.46
Titanium(IV)
343
0.69
8.02
Titanium(IV) propoxide
332
0.69
8.26
Titanium(IV) oxide
285
0.55
7.71
395
1.98
20.00
(A250)
isopropoxide
(anatase)
Blank silica aerogel
(unmodified)
The highest surface area was obtained in titanium(IV) ethoxide prepared
aerogel. It was followed by titanium(IV) chloride, titanium(III) sulphate,
titanium(IV) isopropoxide, and titanium(IV) oxide. Since the other parameters were
same in the preparation, the silica structure may remain the same for all these
55
samples. Thus, it suggests that titanium(IV) ethoxide is the best titanium source to
produce high surface area and well dispersed titania in the silica matrixes. This
phenomenon is supported by the increase of the pore volume where smaller titania
particle may be formed in the silica matrix compared to other titania sources. As the
density of the titania is higher than silica, thus the mean particle size (l = 6/(ρ.ABET))
is less than 8.5 nm except for anatase (less than 10 nm).
The reduction of the surface area supported the presence of Ti in the aerogel.
There was a significant reduction of pore diameter and pore volume upon
introduction of titanium in the pore that reduced the total pore volume. This suggests
that titania presents as a separated phase from the silica and well dispersed in the
silica matrixes.
Not only the surface properties of the sample were altered, the
physicochemical properties of the sample could be varied by the changes in the
synthesis processes. Their physicochemical properties due to the changes in titanium
sources were figured out using UV-Vis DRS technique as shown in Figure 4.13.
The spectra in Figure 4.13(a) show that titanium(III) gives two types of
titanium. Homogeneity of the gel was poor as indicated by the formation of two
layers. A very strong absorption centred at 245 nm was observed for the precipitate
matter. The upper layer represents the tetrahedral form of titanium, indicated by
absorption of lambda max at 219 nm. This phenomenon resulted from phase
separation due to unsuitable pH condition. In general, trivalent titanium compound is
a preferred titanium oxide source in the synthesis of large-pored crystalline titanium
molecular sieve zeolites [85] and small pore titano-silicate [86], where initial pH of
10.5 or higher were employed in their synthesis.
56
31.
0
Upper
layer
27.5
Lower
layer
25
TiCl4
246nm
25
20
245nm
20
15
K-M
15
219nm
K-M
10
10
5
5
0.
2
0.1
190.
0
250
300
350
400
450
500
550
nm
190.0
600.
0
250
300
350
250nm
247nm
400
450
500
550
600.0
(b)
(a)
37.0
35
nm
Ti-ethoxide
Ti propoxide
30
245nm
Ti isopropoxide
25
20
K-M
15
10
5
0.1
190.0
300
400
500
600.0
nm
(c)
Anatase
onlyadded in aerogel synthesis)
Anatase
(before
6 .0
5 .0
Anatase
4 .0
K -M
3 .0
334nm
2 .0
1 .0
0 .0
1 9 0 .0
300
400
500
6 0 0 .0
nm
(d)
Figure 4.13
The effect of titanium source on the physicochemical characteristics
of the Ti-Si aerogels by UV-Vis DRS. (a) Titanium(III) sulphate, (b)
Titanium(IV) chloride, (c) Titanium(IV) alkoxide, (d) Titanium(IV)
oxide in anatase form.
57
When titanium(IV) chloride was used as starting material, UV-Vis spectra
show a band at 246 nm (Figure 4.13 (b)). It is due to the presence of hydrated
titanium in the silica matrix as a result of the hydrolysis of Ti-Cl by the water to form
Ti(SiO)3OH [87].
In the case of titanium alkoxides used as starting material, the spectrum
shows a predominant band centred at 245 nm, 247 nm, and 250 nm assigned to
titanium(IV) isopropoxide, titanium(IV) propoxide and titanium(IV) ethoxide
respectively (Figure 4.13 (c)). The absorption centred at 240-260 nm is likely due to
the presence of [Ti(SiO)3O]- species attributed to the higher electron density of the
negatively charged oxygen than the siloxane bond [87]. This implies that more OH
ligands were bonded to the titanium centre and caused blue shifting to higher
wavenumber. The absorption edges (at about 330 nm) are lower compared to other
titanium source.
Anatase is a crystalline titanium oxide that presents in octahedral
coordination, which each titanium atom is coordinated to six almost equidistant
oxygen atoms. Anatase powder has been recorded using UV-Vis spectroscopy. The
spectrum of anatase its alone (Figure 4.13(d)) shows a λmax at 335 nm and a shoulder
at 238 nm. Significant absorption above 300 nm is a good indication of the presence
of large TiO2 particles. It has been reported that bulk anatase would show an
absorption edge above 360 nm. Meanwhile, the absorption edge below 360 nm is
assigned to nanoparticles [13].
When the titanium oxide (anatase form) was dispersed in the gelling solution,
fast gelling process (less than 15 seconds) at pH 7 cause these particles to be trapped
and well-dispersed in the silica matrixes. The spectrum for this sample shows a band
centred at 334 nm with the absorption edge at 400 nm similar to that of starting
material, i.e. anatase powder. This confirms the presence of anatase form of titanium
oxide. While, the shoulder at 238 nm has disappeared in the spectrum after the
anatase powder was used in the aerogel synthesis. The reason is that the amorphous
titanium oxide that present in the starting material have been dissolved and
transformed to new anatase TiO2 that was induced by anatase crystal seed. It was
58
made possible by the ability of amorphous TiO2 to dissolve in sulphuric acid, similar
in the extraction of the titania from the titanium ore. In this case, silica aerogel
performed as a support for the anatase powder, improved the quality of the anatase
powder and immobilized foreign particle.
4.3.2
The Effect of Si:Ti Molar Ratio
The influence of the titania content on the dispersion of the species in silica
matrixes was examined. A series of samples has been prepared with H+: NaOH
molar ratio is 1.25 and titanium isopropoxide as titanium source. Nitrogen adsorption
has been carried out and the data are listed in Table 4.4.
Table 4.4: Effect of concentration of titanium on the surface characteristics of the
Ti-Si aerogels. Titanium isopropoxide as titanium source, H+: NaOH
molar ratio = 1.25.
Si:Ti
BET Surface Area
Total Pore Volume
Pore Diameter
Molar Ratio
(m2/g)
(ml/g)
(nm)
1 (A350)
469
0.90
7.29
6
743
1.66
8.94
33
947
1.75
7.39
49
917
1.72
7.50
Data in Table 4.4 shows that surface area of the bulk aerogel is significantly
reduced due to higher titanium loading. For the sample with Si:Ti = 1, the specific
surface area is the lowest (469 m2/g) and total pore volume (0.90 ml/g). This is due
to the formation of the extra framework titanium(IV) oxide in the silica matrix. The
titania is possibly located inside the pore of the aerogel as proven by the decrease in
the pore volume. The surface area and total pore volume are all increase or decrease
with the amount of titanium loading. Pore diameter for the Si:Ti = 6 shows such an
anomaly result that differ from the trend. It is not only give larger pore diameter than
59
those with lower titanium loadings but also sample with higher titanium loading.
Further study is necessary to clarify its reason, whether titania is combined together
with the silica wall building unit or is it caused by any other reasons.
X-ray diffractograms in Figure 4.14 of samples Si:Ti = 1, Si:Ti = 6 and
anatase modified-silica aerogel show diffraction peaks at the same position. All
peaks correspond to crystalline titanium(IV) oxide, synthetic anatase (PDF pattern
number: 04-0477), which matches the sample anatase; originally used as the starting
material in this study. This confirms that the titania in the silica matrix are in
crystalline form for samples with Si:Ti less than 6. This further supports the fact that
titania is located in the pore of the silica aerogel matrix with a reduction in pore
volume. The intensities of the peaks in Si:Ti = 6 diffractogram are much lower than
those in sample with Si:Ti = 1, implies less crystalline and lower amount of anatase
in the sample. No peak was observed for lower loading of titanium. Amorphous
titania or possibly very small size of crystalline titania may be formed in those with
low Ti-loading samples.
60
Intensity
Anatase
Si:Ti = 1
Si:Ti = 6
Si:Ti = 33
2
10
20
30
40
50
60
70
80
2θ (degrees)
Figure 4.14
X-ray diffractograms of aerogel samples with various Si:Ti molar
ratios compared with anatase TiO2.
90
61
UV-Vis DRS spectra in Figure 4.15 shows the aerogel samples that were
synthesized with various Si:Ti molar ratios. The UV-Vis DRS of Si:Ti = 1 has
further confirmed the formation of anatase with high titania loading. However, λmax
of Si:Ti = 1 is shown at 246 nm, suggesting the presence of [Ti(SiO)3O]- species. All
samples in this series have the same absorption edge at ca. 370 nm. This is an
indication of the presence of anatase in the system. The spectra for Si:Ti = 33 and
Si:Ti = 49 are similar to that of Si:Ti = 6, suggesting that similar phase is present.
These samples show a band centred at 251 nm and suggest the presence of
[Ti(SiO)3O]- species. Some literature suggested that the absorption bands centred in
the range of 240 to 280 nm were due to the charge transfer between framework
oxygen to octahedral coordinated Ti(IV) centre, and highly dispersed TiO2 with
particle size less than 5 nm (octahedral titanium species) [88].
251nm
Si:Ti = 33
32.4
30
Si:Ti = 6
25
20
K-M
Si:Ti = 45
15
246nm
10
5
Si:Ti = 1
305nm
Anatase
0.0
190.0
250
300
350
400
450
500
550
600.0
nm
Figure 4.15
UV-Vis spectra of samples synthesized with various Si:Ti molar
ratios.
Virtually, assignment for the bands that centred below 280 nm is due to
highly dispersed titania (either in nanosize segregated TiO2 particle or titania in silica
framework). The smaller the wavelength of the absorption may indicate smaller
particle size, lower Ti coordination number (λmax < 240 nm for tetrahedral Ti(IV))
and better dispersion of titania has formed in the silica matrix. The siloxane ligand
62
has lower electron density than OH ligand (or other ligands that have high electron
density), thus Ti(IV) bonded to siloxane bond will adsorb at lower wavenumber.
Titanium(IV) is possibly bonded to silica with coordination number more
than 4 (240-260 nm). This may resulted from the interaction between silica and
titania to form binary particle. In addition, there is also possible attribute to the
presence of very small titania particles that cannot be detected by the XRD,
specifically those with low titanium loading. This provides an alternative path for the
synthesis of nanosize particle anatase crystals in the silica matrix.
4.3.3
The Effect of Loading of Sulphuric Acid
From the nitrogen adsorption studies (Table 4.5), the loading of the acid has
significantly modified the surface physical properties of the titanium containing
silica aerogel. It is a combination of three effects: the effect of pH to silica, the effect
of pH to titania and the effect of interaction between titania and silica. Soluble silica
and titania were present in different form at different pH (or different concentration
of acid) as stated in Chapter 2.
At high pH values, the washing process will flush out the silica because
amorphous silica can easily dissolve at pH above 12. Practically, gel may be able to
form at pH below pH 12. However, washing process (for flushing out the salt
formed) dissolves the silica because of the increase in solubility of silica above
neutral condition. Thus, synthesis of the titanium containing silica aerogel above pH
7 is not favourable.
The aerogel that was synthesized under the neutral condition gives the lowest
surface area. This is due to the effect of gelling and aging at high pH, which was
affected by the solubility of the silica. At this pH, most of the silicates are present as
polynuclear and anionic species. At high pH values, particles are charged by
ionisation, therefore aggregation was reduced. In addition, particle growth by
monomer deposition and faster Ostwald ripening process. This factor causes bigger
63
primary particle to form and convex surface dissolves quickly during aging process.
Hence, aerogel with the lowest surface area (263 m2/g) in this series is produced.
Table 4.5: Effect of concentration of acid on the surface characteristics of the Ti-Si
aerogels. Titanium isopropoxide as titanium source, Si:Ti molar ratio =
33.
H+: NaOH
BET surface area
Total pore
Pore diameter
molar Ratio
(m2/g)
Volume (g/cm3)
(nm)
1.00 (A215)
263
0.51
7.70
1.25
791
1.44
7.30
1.50
1003
2.80
11.16
1.75
786
2.09
10.65
2.50
864
1.76
10.34
At lower pH value i.e. with excess sulphuric acid loading in the reaction, the
system is in the pH range that the particle growth is minimum. Most of the sodium
silicate exists in form of mononuclear silicic acid at pH lower than 7. It is
recommended that the size of the particle is limited to about 2 nm in pH below 7 due
to the solubility of silica [75]. Subsequently, the aggregation of small particle during
gelation produces high surface area gel. The highest surface area of the pure silica
aerogel that can be synthesized in this research was 882 m2/g as in Section 4.1.
However, the highest surface area in titanium containing silica aerogel is 1003 m2/g,
which greater than silica aerogel. This may be caused by the effect of titania, which
acts as a space separator between silica particles in the gel matrixes. This
phenomenon has been observed in other materials such as Ti-MCM-41 [89].
Therefore, the addition of titania in silica aerogel created opportunity to synthesize
higher surface area aerogels.
The isoelectric point (iep, electrical mobility of silica particles is zero) and
point of zero charge (surface charge is zero) of silica and titania may play a
significant rule to the gel obtained. The iep of the silica and titania is at pH around
64
2.0 ± 0.5 and at pH 5-6 respectively [36, 90, 91]. Therefore, silica is negatively
charged above pH 2 [92]. It is assumed that the catalyst below pH 2 is the H+ ion,
which forms an active cationic complex. Also, above pH 2 the OH- ion is the catalyst
in that active anionic silica is generated. Vysotskii and Strazhesko [37] have pointed
out that in the presence of acid such as sulphuric, the iep is not only the point of
minimum rate of gelling but also the point where gels of maximum strength and
maximum specific surface area were obtained. This is because the rate of aggregation
is minimum at the iep and the rate of growth of the ultimate particles from monomer
at minimum, so that the ultimate particles are smallest as they form the gel. Thus,
excess acid in the gel synthesis normally results in higher surface area aerogels. They
are above 700 m2/g in this experiment.
However, high loading of sulphuric acid produced lower surface area. This
could be due to excessive heat formed during the reaction that induced higher
solubility of silica, resulting in faster rate of coarsening process in the primary aging
stage. Moreover, the heat could cause evaporation of some portion of water from the
gel and fracture of the gel skeleton.
According to the UV-Vis spectra for different acid loading (Figure 4.16), the
absorption edge are above 360 nm, indicating the presence of anatase in the sample
except for the aerogel synthesized with H+: NaOH molar ratio is 1.
Interestingly, acid loading had modified the chemical properties of the titania.
Increase in acid loading resulted in blue shifting to higher wavenumber. The peak
centred at 227 nm indicates the presence of tetrahedral titanium in the silica
framework. The gel that formed at H+: NaOH molar ratio 1 had shown pH 6 in the
experiment. Since slightly excess acid has been used. In this case, it is believed that
ion-ion interaction between positively charged titania with negatively charged silica
in the reaction mixture has occurred due to the surface charge at pH 4. The gel was
later aged at pH 6 for 48 hours before washing process that provided low aggregation
environment for the titania. Thus, there was no absorption at 250 nm and no
absorption edge at above 360 nm.
65
25.8
227nm
27.4
257nm
25
20
20
15
15
K-M
K-M
10
10
340nm
375nm
5
5
0.2
190.0
0.3
300
400
nm
500
600.0
190.0
300
(a)
26.4
25
400
nm
500
600.0
500
600.0
(b)
18.8
258nm
270nm
16
20
14
12
15
10
K-M
K-M
8
10
6
380nm
280nm
4
5
2
0.2
190.0
0.1
300
400
nm
(c)
Figure 4.16
500
600.0
190.0
300
400
nm
(d)
UV-Vis spectra of samples synthesized with various H+: NaOH molar
ratios. (a) H+: NaOH molar ratio = 1.0, (b) H+: NaOH molar ratio =
1.25, (c) H+: NaOH molar ratio = 1.5, (d) H+: NaOH molar ratio =
1.75.
The samples which were synthesized at H+: NaOH molar ratio =1.25 and 1.50
showed same absorption at 258 nm. They have the same type of titania, as in the
samples that studied in Section 4.4.2. Higher acid loading of acid showed absorption
66
centred at 270 nm, indicates size of the titania particle is larger particle than those
showed absorption at 250 nm.
Therefore, this study suggests the most important parameter for engineering
of the surface area of the aerogel and chemical properties of the titania is the acid
loading in the gelling process.
4.4
Catalytic Properties: Oxidation of Alkene
The synthesized TiO2-SiO2 aerogels were tested in the oxidation reaction of
cyclohexene using hydrogen peroxide as oxidant. Various parameters have been
studied; including catalyst, solvent, loading of hydrogen peroxide, reaction
temperature and different alkene.
4.4.1
The Influence of the Type of Titanium.
Various materials have been tested for their catalytic properties in this study.
It is known that materials with the same physicochemical properties should have
similar chemical properties. Experimental results in previous section have shown a
trend that preparation at different concerntration of titanium is able to produce
sample with similar physicochemical properties. Thus, the samples used in this
testing are classified into four groups based on the type of titania due to the UV-Vis
absorption wavelength as stated in Table 4.6.
The catalytic test data is shown in Table 4.7. Silica aerogel without titanium
shows very low conversion (7%) and low 1,2-cyclohexanediol selectivity (31%).
Blank sample also have very low reaction activity with it low conversion of
cyclohexene [93].
67
A350, is a sample synthesized with Si:Ti = 1, shows the presence of anatase
in its X-ray diffractogram. TiO2 anatase powder was used as a standard. Sample
A350 with the highest titanium loading in the study shows the highest activity among
the aerogel samples with 26% conversion. However, A350 has the lowest selectivity.
The high conversion is enhanced by the presence of anatase in the sample. Anatase
powder above shows the highest activity in this series of samples but relatively has
lower selectivity in the reaction. It shows the catalytic properties of A350 is due to
the presence of crystalline TiO2 (anatase) in the aerogel sample [94].
Table 4.6: Sample used in the catalytic testing and their characteristics.
Characteristics
Sample
UV-Vis Absorption
TS-1
215 nm
(2 mol% Ti as supplied)
A215
(as in Table 4.5)
(as in Table 4.3)
(as in Table 4.4)
(as in Table 4.3)
Highly dispersed TiO2 particles
in silica matrix
350 nm
silica aerogel
Isolated tetrahedral titanium in
silica framework
250 nm
A350
Isolated tetrahedral titanium in
silica framework
227 nm
A250
Type of Titania
Crytalline TiO2 (anatase) in
silica matrix
no absorption
Silica matrix
(Blank silica aerogel)
Sample A250 shows the highest selectivity towards the formation of 2cyclohexen-1-one and shows the lowest epoxide content in the reaction mixture. The
formation of 2-cyclohexen-1-one is a result of allylic oxidation [95, 96, 97]. Lei
revealed that base and acid additives are able to afford the formation of
2-cyclohexen-1-one and 2-cyclohexen-1-ol respectively [98]. Thus, the higher
selectivity of 2-cyclohexen-1-one in sample A250 was believed to be caused by trace
amount of NaOH that remained in the aerogel which was synthesized with H+:
NaOH molar ratio =1. A250 used in other studies also indicated similar trend, where
68
the yield of 2-cyclohexen-1-one was always higher than 2-cyclohexen-1-ol. In fact, it
was the oxidation of the 2-cyclohexen-1-ol by hydrogen peroxide that produced
additional 2-cyclohexen-1-one [99]. The results suggest that A250 can potentially
undergo allylic oxidation and can be further studied by modifying its basicity.
TS-1 is known as the most prominent epoxidation catalyst [10]. It has an
absorption at 215 nm (210-230 nm), which indicates the presence of tetrahedral Ti
sites; isolated by SiO “ligands” and acts as Lewis acidic centres to activate the
peroxide [100, 101]. Results in Table 4.7 shows TS-1 has the highest epoxide
selectivity among the other catalysts. Although sample A215 that has the same UVVis absorption and shows the highest epoxide selectivity among the aerogel samples,
it is lower than TS-1. It proves that epoxidation is catalysed by the Ti-O-Si bonding
(adsorb at 210-230 nm). At the same time, A215 shows the most prominent glycol
selectivity. This may be due to the presence of alumina in aerogel sample, which
favours the formation of glycol. The conversion of the TS-1 is lower compared to
A215. In this case, the small pore size in TS-1 with the dimensional channel system
of 5.3 x 6.1 Å has caused diffusion limitation to the substrate and result in lower
conversion [102]. In contrast, aerogel is mesoporous and has relatively larger pore
volume. Both TS-1 and A215 have selectively converted cyclohexene to epoxide and
diol as the main mixture (total selectivity is about 80%) with H2O2, thus confirming
the importance of the titanium structure in the silica framework.
Table 4.7: Catalytic activity of the titanium containing silica aerogel, TS-1 and anatase. Reaction conditions: 10 ml cyclohexene, 10 ml acetone,
8.35 ml H2O2 35%, 156.3 mg catalyst, and 1 ml toluene (internal standard) at 70 ˚C.
TOF
Catalyst
(mM cyclohexene
/g catalyst/ hour)
Blank
Selectivity (%)
Conversion
(%)
Cyclohexene
2-cyclohexen-1-ol 2-cyclohexen-1-one
oxide
1,2-
others
cyclohexanediol
-
7
9
9
5
28
50
117
7
9
10
5
31
44
200
15
18
7
4
61
10
A215
260
20
12
6
6
70
6
Anatase
502
38
8
7
3
56
26
A350
342
26
9
9
7
46
29
A250
261
20
4
8
14
58
17
Silica aerogel
TS-1
(2 mol% Ti)
69
70
4.4.2
The Influence of Solvent
It is known that solvent have great effect on the activity and selectivity in the
liquid phase oxidations on the titanium silicates [103, 104, 105, 106]. The oxidation
of cyclohexene is conducted on the sample A250 using several solvents having
different polarity. The polarity of the solvent used in this study is in the order of
toluene< ethyl acetate < acetone [107]. The influence of solvent is presented in Table
4.8. It should be noted that the glycol (1,2-cychohexanediol) selectivity is the highest
when ethyl acetate is present in the reaction. It is observed that reaction without the
use of solvent has a two-fold turn over frequency higher than with toluene. It is due
to dilution of cyclohexene by the organic phase of toluene, reducing the contact with
catalyst that is present mostly in the aqueous phase. In contrast to high polarity
solvent, the mass transfer problems associated with the presence of different liquid
phase are minimized and are able to form a single phase with the organic substrate
and hydrogen peroxide. Therefore, it is as espected that acetone will give the highest
conversion among other solvents.
The activities of the sample A250 are suited with the polarity of the solvent
used in the reaction as proposed by other workers [104, 108]. Whereby the activity of
TS-1 in the oxidation of alkenes is enhanced by the use of polar solvents. The same
phenomena was also reported in the oxidation study of the Ti-Beta zeolite
synthesized by dry gel conversion [103]. However, it is contradict to those observed
by Corma that the activity of oxidation of alkene over hydrothermal synthesized Tibeta was higher in aprotic solvent acetonitrile than in protic solvent in the order of
MeCN> MeCOMe > MeCOEt [106]. They ascribed it is related to the hydrophilicity
of the hydrothermally synthesized Ti-beta which differs from TS-1. In this case, TiSi aerogel synthesized under high temperature supercritical extraction is relatively
hydrophobic as the case of TS-1.
Table 4.8: Catalytic activity of the aerogel A250 as a function of solvent. Reaction conditions: 10 ml cyclohexene, 10 ml solvent, 8.35 ml H2O2
35%, 156.3 mg catalyst, and 1 ml toluene (internal standard) at 70 ˚C.
TOF
Solvent
(mM cyclohexene
/g catalyst/ hour)
Without
Selectivity (%)
Conversion
(%)
Cyclohexene 2-cyclohexen-1-ol
2-cyclohexen-1-one
oxide
1,2-
others
cyclohexanediol
184
14
0
8
16
76
0
Acetone
261
20
4
8
14
58
17
Toluene
99
7
0
0
23
77
0
Ethyl acetate
250
19
0
7
9
84
0
solvent
71
72
4.4.3
The Influence of Hydrogen Peroxide Loading
The alkene to oxidant dependance of cyclohexene oxidation was investigated
and the reaction performance are given in the Table 4.9. There was no obvious
influence in the selectivity of allylic oxidation by changing this parameter. In
equilibrium amount of oxidant, the conversion was 20%. When the amount of
oxidant was doubled, the catalytic activity was increased more than 150%. At the
same time, the side products increased 2-fold to 30% selectivity. These significant
changes resulted from the excess oxidant that favoured additional oxidation to the
glycol and allylic oxidation products. The main side products would be accounted for
the formation of 2-hydroxycyclohexanone and adipic acid. This phenomenon has
also been observed in the oxidation of cyclooctene, where conversion and side
products increased as the molar ratio of alkene to oxidant decreased [93].
In contrast, the conversion of the alkene was reduced when the amount of
oxidant was decreased. However, the selectivity to glycol was apparently increased
up to 77%. The selectivity and activity of catalyst A250 did not differ much when the
molar ratio of alkene to oxidant was more than 5. At the same time, no quantitative
amount of side product was formed when hydrogen is used as the limiting reagent.
This implies that the molar ratio of alkene to hydrogen peroxide 10:1 is a suitable
condition to produce glycol with less side product in the reaction mixture. Aerogel
sample show it is so good as catalyst in catalyzing cyclohexene to 1,2cyclohexenediol.
Table 4.9: Catalytic activity of the aerogel A250 as a function of alkene: H2O2 molar ratio. Reaction conditions: 10 ml cyclohexene, 10 ml
acetone, respective amount of H2O2 35%, 156.3 mg catalyst, and 1 ml toluene (internal standard) at 70 ˚C.
TOF
Selectivity (%)
Alkene/H2O2
(mM
Conversion
molar ratio
cyclohexene /g
(%)
Cyclohexene
2-cyclohexen-1-ol
2-cyclohexen-1-one
oxide
1,2-
others
cyclohexanediol
catalyst/ hour)
0.5
658
49
4
6
7
54
30
1.0
261
20
4
8
14
58
17
5.0
156
12
5
9
10
76
0
10.0
165
12
5
9
9
77
0
73
74
4.4.4
The Influence of Reaction Temperature
By using the same catalyst (A250), the influence of the reaction temperature
was tested at 30 ˚C, 50 ˚C, 70 ˚C, and 80 ˚C. Table 4.10 shows that the catalytic
activity decreased with an increase of reaction temperature. This could resulted from
the higher temperature favour the decomposition of hydrogen peroxide to form water
and oxygen gas. At 80 ˚C, the reaction activity was the lowest. Even though, higher
temperatures (70 ˚C, and 80 ˚C) gave higher yields of expoxide and glycol; products
of nonradical reaction.
In contrast, radical based oxidation reaction (allylic oxidaton) was favoured at
lower reaction temperature, which produced larger amount of 2-cyclohexen-1-ol and
2-cyclohexen-1-one. This implies that a large number of peroxy radicals was formed
at lower temperatures (30 ˚C, and 50 ˚C). Therefore, there is a strong indication that
the formation of large amount of other side products (up to 30%) was induced by
these reactive radicals.
On the other hand, reaction carried out at 70 ˚C was prefered for higher
amount of glycol yield, with maintained activity.
Table 4.10: Catalytic activity of the aerogel A250 as a function of reaction temperature. Reaction conditions: 10 ml cyclohexene, 10 ml acetone,
8.35 ml H2O2 35%, 156.3 mg catalyst, and 1 ml toluene (internal standard).
Temperature
(˚C)
TOF
(mM cyclohexene
/g catalyst/ hour)
Selectivity (%)
Conversion
(%)
Cyclohexene 2-cyclohexen-1-ol
2-cyclohexen-1-one
oxide
1,2-
others
cyclohexanediol
30
276
21
5
15
17
33
30
50
224
17
7
16
19
42
16
70
261
20
4
8
14
58
17
80
41
3
13
9
11
67
0
75
76
4.4.5
The Influence of Alkene
The catalytic activity and selectivity of the A250 in the oxidation of linear
alkene i.e. 1-octene, was tested and the results are summarized in Table 4.11. The
activity of the linear olefin showed a large drop in conversion compared to cyclic
olefin. This complies to the previous study that the conversions are increased in the
order of 1-decane < 1-octene < 1-hexene < cyclohexene [93]. This result clearly
indicates that, besides the intrinsic reactivity of the double bond, olefin size and the
accessibility to the active sites of the catalyst are the major limiting factors of the
catalytic reaction. With this sample, the selectivity of the epoxide is as high as 64%
in the oxidation of 1-octene. Glycol was the only side product present in the 1-octene
oxidation reaction which indicates 1-octene oxide is less reactive to the hydrolysis
compared to cyclohexene oxide. The results suggest that catalytic activity and
selectivity of the catalyst (aerogel) are substrate dependent.
Table 4.11: Catalytic activity of the aerogel A250 as a function of amount of
hydrogen peroxide. Reaction condition: 10 ml alkene, 10 ml acetone,
8.35 ml H2O2 35%, 156.3 mg catalyst, and 1 ml toluene (internal
standard) at 80 ˚C.
TOF
Alkene
(mM cyclohexene
/g catalyst/ hour)
Selectivity (%)
Conversion
(%)
Epoxide
Diol
Others
Cyclohexene
41
3
13
67
20
1-octene
3
0.35
64
36
0
77
4.5
The Mechanism of the Reaction
There are a few reactions that can occur in the oxidation of cyclohexene.
Three reaction paths have been descried namely hydrolysis, epoxidation and allylic
oxidation [91]. Epoxidation reaction is resulted from the interaction between
oxometallic species and alkene. Other workers found cyclohexene converted into
epoxide and diol as predominant mixture over titanium containing zeolites with
aqueous H2O2 [102, 103].
The mechanism of the allylic oxidation involves radicals. These radicals
come from decomposition of peroxo titanium species formed by the reaction of
hydrogen peroxide with the titanium sites of catalyst as follows:
Ti
O H
Ti
O O H
Ti
O
+
+
H2O2
H2O
+
H2O
(4.2)
Ti O
+
OH
(4.3)
Ti OH
+
OH
(4.4)
Ti O O H
These peroxy radical may react with the alkene in several ways [95, 96, 97].
Allylic oxidation usually produces 2-cyclohexen-1-ol and 2-cyclohexen-1-one.
In this study, it is noted that the products of the reaction mixture are
combination of the yield from path 2 and path 3 as shown in Scheme 1. The
experimental data as shown in the line chart below in Figure 4.17 shows that only
epoxide is formed at the beginning of the reaction. The production and consumption
of the epoxide has reached equilibrium at about 45 mM in the reaction mixture. It is
followed by the formation of glycol which resulted from the epoxiran ring opening
reaction to the epoxide according to Scheme 1.
78
OH
O
1
OH
2
O
OH
3
O
OH
The reactions in the oxidation of cyclohexene [95, 96, 97].
Scheme 1
This reaction may result from acid catalysed hydrolysis, basic hydrolysis, or
direct hydrolysis process. The mechanism of acid catalysed hydrolysis is a
hydronium (H3O+) ion catalysed process. There is the first addition of a hydrogen
ion, and then an oxinium complex is formed. It is followed by the formation of
carbocation. The attack from the water to the carbocation results in the formation of
glycol and reproduces the hydroxonium ion.
C
O
+
C
H+
O
C C
H+
OH
C C
+
OH
C C
OH
+
H+ (4.5)
H OH
In comparison to the basic hydrolysis, the mechanism would be differ as
follows:
H OH
R1
O
C CH2
R2
-
-
+ OH
O
C CH2
R2 OH
R1
OH
C CH2 + OHR2 OH
R1
(4.6)
79
Meanwhile, the direct hydrolysis process is noted as below:
C
O
+
C
OH
C C
OH
H2O
(4.7)
The quantity of the glycol increased continuously during the first 40 hours.
While, the glycol formation rate was slow down after the first 4 hours. It is resulted
from the consumption of cyclohexene in allylic oxidation that forming 2-cyclohexen1-ol and later followed by 2-cyclohexen-1-one. These two allylic oxidation
compounds were also detected in the same reaction as byproducts by other
researchers [94].
Time Course Study for Oxidation of Cyclohexene
250
225
Concentration (mM)
200
175
Cyclohexene oxide
2-cyclohexene-1-ol
2-cyclohexen-1-one
1,2-cyclohanediol
Others
150
125
100
75
50
25
0
-25 0
10
20
30
40
50
Time (hour)
Figure 4.17
Time course study for the reaction mixture 10 ml cyclohexene, 10 ml
acetone, 8.35 ml H2O2 (35%), 156.3 mg TS-1, and 1 ml toluene
(internal standard) at 80 ˚C.
Subsequently, when the reaction was left for a longer period, other side
products with higher molecular weight were detected. At the same time, a significant
80
decrease of the amount of 1,2-cyclohexanediol was observed in the product mixture
at the end of the reaction. This suggests that side products were formed only from
further reaction of the glycol. It was found that glycol could be further oxidized and
the six membered ring was able to be cleaved in the way to the formation of adipic
acid, which can be explained by over-oxidation under the reaction condition [109].
Futhermore, minor portion of the side product may result from further
oxidation of the allylic products, where some researchers have used 2-cyclohexen-1ol and 2-cyclohexen-1-one as reactants in the catalytic oxidation reaction for the
formation of its epoxide and other derivatives, which directly proved the ability of
these compounds for further reaction [94].
The mechanism of the reaction is proposed in Scheme 2. Path 1 is the
formation of epoxide. In contrast, Path 2 indicates an allylic oxidation process. Due
to the amount of the product formed, Path 1 is the predominant mechanism in this
study.
81
Path 2
H2O
1/2 H2O2
[Ti]
+
OH
Cyclohexene
[Ti] H2O2
O2
Path 1
OO
O
Cyclohexene Oxide
OO
[Ti] H2O
O2
OH
OH
1,2-Cyclohexanediol
[Ti]
O
OH
H2O2
+
O
2-Cyclohexen-1-ol
2-Cyclohexen-1-one
OH
2-hydroxycyclohexanone
[Ti] H2O2
O
OH
O
Adipic Acid
Scheme 2
Reaction mechanism of the oxidation of cyclohexene using hydrogen
peroxide as oxidant [102, 103, 109, 110].
CHAPTER 5
CONCLUSIONS AND SUGGESTIONS
5.1
Conclusions
Titanium containing silica aerogel has been synthesized through high
temperature supercritical process. Sol-gel direct synthesis was demonstrated as the
potential technique of preparing high surface area titanium containing silica aerogel.
The surface area of the bulk aerogel system was increased when titanium was
introduced to the silica aerogel matrixes.
The study indicated that acid loading in the synthesis has the most important
influence in directing the type of titanium formed in the silica aerogel matrixes.
Homolytic substitution of the titanium in the silica framework was successfully
carried out proven by the UV absorption at 215 nm. Well-dispersed fine particles of
titanium oxide in the silica matrixes has also been obtained, which showed UV
absorption in the range of 240-290 nm. Crystalline TiO2 (anatase) was synthesized
in-situ during the sol-gel process when a high loading of titanium (Si:Ti = 6, 1) was
implemented.
83
The catalytic tests showed that titanium containing silica aerogel was active
to the oxidation of cyclohexene using hydrogen peroxide as oxidant. The products
from both radical (allylic oxidation) and non-radical oxidation (epoxidation) were
present. Meanwhile, the product mostly came from the non-allylic oxidation that
produces epoxide and 1,2-cyclohexanediol. The selectivity of 1,2-cyclohexanediol
was relatively high compared to other products and it always presented as the major
product. The aerogel containing highest amount of titanium(IV) oxide (Sample
A350) gave the highest conversion of 26%. However, the selectivity of the catalysts
was highest when sample A215 was applied. The mechanism of reaction is proposed
as in Scheme 2 of Section 4.5.
The catalytic oxidation reaction was greatly influenced by the solvent,
temperature, amount of oxidant, and type of alkene. The parameters were optimised
for selectivity to the glycol. The study established that favoured conditions for the
reaction were using oxidant as a limiting reagent, ethyl acetate as the solvent, and the
reaction conducted at 70 ˚C.
Combination of high surface area, greater strength of the mixed oxide as well
as tuneable physical and physicochemical properties of titania in the silica aerogel
matrix, results in titania-silica aerogel potential as heterogeneous catalysts. Another
advantage of aerogel is able to be molded to desired shape and size, which in most
cases are not possible in other systems.
84
5.2
Suggestions
Some suggestions for future work:
1. Incorporation of third oxide to the titania silica aerogel either during
the sol-gel synthesis or via post synthesis. The properties of the
physical and chemical properties of the ternary oxide system formed
may be varied.
2. Application of titania silica aerogel in photocatalytic reaction. The
advantage of in-situ formation of the crystalline anatase during
aerogel synthesis, in addtion to immobilization of anatase in the silica
matrix may overcome the anatase powder lost during the application.
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APPENDICES
Retention Time (min)
Component Name
2.68
Acetone
3.45
Ethyl Acetate
4.20
Cyclohexane
4.43
Cyclohexene
5.75
Toluene
7.35
Cyclohexane Oxide
7.90
2-Cyclohexen-1-ol
8.50
2-Cyclohexen-1-one
Appendix 1: Component table for GC-FID peaks identification.
Appendix 2: Chromatogram of the reaction mixture analysed using gas chromatography.
96
Cyclohexene Calibration Curve
1.8
1.6
mM Cyclohexene
1.4
1.2
1
0.8
y = 0.0002x
2
R = 0.9999
0.6
0.4
0.2
0
0
2000
4000
6000
8000
10000
12000
A(cyclohexene)/A(toluene)
Appendix 3: Calibration curve for quantify the concentration of cyclohexene.
97
Cyclohexene Oxide Calibration Curve
0.16
0.14
mM Cyclohexene Oxide
0.12
0.1
0.08
0.06
y = 0.0001x
R2 = 1
0.04
0.02
0
0
200
400
600
800
1000
1200
A(cyclohexene oxide)/A(toluene)
Appendix 4: Calibration curve for quantify the concentration of cyclohexene oxide.
98
2-Cyclohexen-1-ol Calibration Curve
0.35
mM 2-Cyclohexen-1-ol
0.3
0.25
0.2
0.15
y = 0.0001x
R2 = 0.9994
0.1
0.05
0
0
500
1000
1500
2000
2500
A(2-cyclohexen-1-ol)/A(toluene)
Appendix 5: Calibration curve for quantify the concentration of 2-cyclohexen-1-ol.
99
2-Cyclohexen-1-one Calibration Curve
0.16
0.14
mM 2-Cyclohexen-1-one
0.12
0.1
0.08
0.06
y = 0.0001x
R2 = 0.9995
0.04
0.02
0
0
200
400
600
800
1000
1200
A(2-cyclohexen-1-one)/A(toluene)
Appendix 6: Calibration curve for quantify the concentration of 2-cyclohexen-1-one.
100
1,2-Cyclohexanediol Calibration Curve
0.03
mM 1,2-Cyclohexanediol
0.025
0.02
0.015
0.01
y = 0.0001x
2
R = 0.9992
0.005
0
0
50
100
150
200
250
A(1,2-cyclohexanediol)/A(toluene)
Appendix 7: Calibration curve for quantify the concentration of 1,2-cyclohexenediol.
101
68.0
67
66
65
930
64
1602
63
856
%T
61
1281
2869
62
1453
3319 2931
669
1364
60
1067
59
58.0
4000
3000
2000
cm-1
1500
1000
102
Appendix 8: FTIR spectrum of 1,2-cyclohexanediol that has been synthesized as standard.
450.0
70
Abundance
75000
70000
65000
60000
55000
57
50000
45000
40000
35000
83
30000
98
25000
20000
15000
116
10000
220
5000
m/z-->
0
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
220
252
230
240
250
Appendix 9: Mass spectrum of 1,2-cyclohexanediol that has been synthesized as standard.
103
104
O
H2O
+
OH
H2O
O2
OH
OO
OOH
OH
O2
OO
O
O
O
OH
OH
Appendix 10: Reaction mechanisms involving hydroxy radical and cyclohexene
[95, 96].