CATALYTIC ESTERIFICATION OF BENZYL ALCOHOL WITH ACETIC
ACID BY ZIRCONIA –LOADED ON MESOPOROUS MATERIAL
MEHDI ERFANI JAZI
UNIVERSITI TEKNOLOGI MALAYSIA
CATALYTIC ESTERIFICATION OF BENZYL ALCOHOL WITH ACETIC
ACID BY ZIRCONIA-LOADED ON MESOPOROUS MATERIAL
MEHDI ERFANI JAZI
A Dissertation Submitted To The Faculty Of Science In Partial Fulfillment Of The
Requirement For The Award Of The Degree In Masters of Science (Chemistry)
Faculty of Science
Universiti Teknologi Malaysia
MARCH 2010
iii
To my Beloved Mother and Father
iv
ACKNOWLEDGEMENT
I would like to express my deep and sincere gratitude to my supervisor
Prof. Dr. Salasiah Endud. Her wide knowledge and patience have been of great value
for me. Her understanding, encouraging and personal guidance have provided a good
basis for the present thesis.
I also want to acknowledge all my friends from Zeolite Synthesis Laboratory
specially: Chin Tian Kae and Rozaina Saleh for their guidance, advice and
encouragements. They have contributed toward my understanding which without
that, this thesis has not been the same as it is presented here.
My sincere appreciation also extends to all my friends and others who have
provided assistance at various occasions. Unfortunately it is not possible to list down
all of them in this limited space.
Lastly, I would like to thank my family for their support and encourage all
along this project.
v
ABSTRACT
This research focuses on the synthesis and characterization of metalcontaining mesoporous silica for catalytic esterification of benzyl alcohol with acetic
acid. In this study Zr-containing MCM-41 (Zr-MCM-41) with different molar ratios
were synthesized successfully, and the influence of the Si/Zr molar ratio on the
crystalline structure, textural properties, morphological features and surface acidity
of Zr-MCM-41 mesoporous molecular sieves was investigated by X-ray diffraction
(XRD), N2 adsorption-desorption measurement, SEM and FTIR (Fourier transform
infrared) Spectroscopy, UV-Vis diffuse reflectance (UV-Vis DR), spectroscopy and
single point BET. It is observed that the structural ordering of Zr-MCM-41 varies
with the Si/Zr ratio, and highly ordered mesoporous molecular sieves could be
earned for a Si/Zr molar ratio larger than 5. Calcination may significantly improve
the structural regularity. After impregnation with 15 wt % of H3PW12O40 (denoted as
HWP hereafter),in esterification reaction of benzyl alcohol with acetic acid, the
benzyl alcohol conversion over all the HPW/Zr-MCM-41catalysts linearly increases
with increasing the reaction temperature, and selectivity to benzyl acetate was 100
%. The molar ratios of reactants also were investigated for final product yield; the
molar ratio of acetic acid to benzyl alcohol can be 2:1 for high yield. The presence of
zirconium in tetrahedral coordination was indicated by UV-Vis DR spectra, which
shows an absorption band around 220 nm in Zr-MCM41. The catalyst had more
active sites than pure Si-MCM-41 due to enhanced hydrophobicity properties and the
presence of framework zirconium species as Lewis active sites. Kinetics studies have
shown that the esterification reaction follows the Eley-Ridel mechanism. The energy
of activation for the reaction follows the order: HPW/Zr-MCM-41(Si/Zr=5) > ZrMCM-41(Si/Zr=10) > Zr-MCM-41(Si/Zr=20).
vi
ABSTRAK
Penyelidikan ini adalah terfokus pada sintesis dan pencirian silika mesoliang
yang mengandungi logam bagi pemangkinan pengesteran benzil alkohol dengan asid
asetik. Dalam kajian ini MCM-41 yang mengandungi Zr dengan nisbah molar yang
berbeza-beza telah berjaya disintesis, dan pengaruh nisbah molar Si/Zr terhadap
struktur hablur, ciri-ciri tekstur, morfologi dan keasidan permukaan penapis molekul
mesoliang Zr-MCM-41 mesoporous telah dikaji menggunakan pembelauan sinar-X
(XRD), penjerapan-penyahjerapan N2, SEM, spektroskopi FTIR (inframerah Fouriertransform), spektroskopi ultra-lembayung nampak pemantulan difusi (UV-Vis DR),
dan analisis BET titik tunggal. Didapati bahawa keteraturan struktur Zr-MCM-41
berubah mengikut nisbah Si/Zr, dan penapis molekul mesoliang bertertib julat jauh
dapat dihasilkan bagi sampel yang bernisbah molar Si/Zr lebih besar daripada 5.
Proses pengkalsinan secara jelas boleh meningkatkan keteraturan struktur. Setelah
pengisitepuan dengan H3PW12O40 15 wt% (diwakili sebagai HWP), dalam tindak
balas pengesteran benzil alkohol dengan asid asetik, penukaran benzil alkohol
bermangkinkan kesemua HPW/Zr-MCM-41 meningkat secara linear dengan
peningkatan suhu tindak balas, dan peratus pemilihan terhadap benzil asetat adalah
100%. Nisbah molar reaktan juga dikaji terhadap penghasilan produk tindak balas, di
mana nisbah molar asid asetik kepada benzil alkohol 2:1 telah menunjukkan
peratusan hasil paling tinggi. Kehadiran zirkonium dalam koordinatan tetrahedral
telah ditunjukkan oleh jalur serapan pada sekitar 220 nm dalam spektrum UV-Vis
DR bagi Zr-MCM-41. Mangkin tersebut adalah lebih aktif berbanding Si-MCM-41
tulen kerana peningkatan sifat hidrofobik dan kehadiran spesies zirkonium bingkaian
sebagai tapak aktif Lewis. Kajian kinetik telah menunjukkan bahawa tindak balas
pengesteran benzil alkohol dengan asid asetik berlaku menurut mekanisme EleyRideal. Tenaga pengaktifan bagi tindak balas tersebut adalah mengikut tertib:
HPW/Zr-MCM-41 (Si/Zr = 5) > Zr-MCM-41 (Si/Zr = 10) > Zr-MCM-41 (Si/Zr =
20).
vii
TABLE OF CONTENTS
CHAPTER TITLE
1
2
PAGE
DECLARATION
ii
DEDICATION
iii
ACKNOWLEDGEMENT
iv
ABSTRACT
v
ABSTRAK
vi
TABLE OF CONTENTS
vii
LIST OF TABLES
viii
LIST OF FIGURES
xi
LIST OF ABBREVIATIONS
xii
LIST OF APPENDICES
xv
INTRODUCTION
1.1 Research Background
1
1.2 Objectives of the study
2
1.3 Scope of research
3
1.4 Outline of research
4
LITERATURE REVIEW
2.1 Porous Materials
5
2.2 Mesoporous MCM-41
8
2.3 Synthesis of Mesoporous MCM-41
9
viii
2.4 Characterization of MCM-41
11
2.5 Mechanism of Formation of Mesoporous MCM-41
12
2.5.1
Liquid Crystal Templating Mechanism
13
2.5.2
Silicate Rod Assembly
13
2.5.3
Folded Sheet Mechanism
15
2.5.4
Mechanism of Transformation from Lamellar to Hexagonal
Phase
3
15
2.6 Incorporation of Zirconia into MCM-41
15
2.7 Zirconia as acid catalyst
17
2.8 Heteropoly acids as impregnated to the mesoporous materials
18
2.9 Esterification of benzyl alcohol with acetic acid
18
METHODOLOGY
3.1 Introduction
22
3.2 Chemical
22
3.3 Catalyst Synthesis
23
3.3.1
Synthesis of Zr-MCM-41 Supports
3.3.2
Preparation of H3PW12O4 supported Zr-MCM-41(HPW/ZrMCM-41
3.4 Characterization of HPW/Zr-MCM-41
23
24
25
3.4.1
Powder X-Ray Diffraction (XRD)
26
3.4.2
Fourier Transform Infrared Spectroscopy
26
3.4.3
Ultraviolet-Visible Diffuse Reflectance Spectroscopy (UVVis DR)
27
3.4.4
Scanning Electron Microscopy (SEM)
28
3.4.5
N2 adsorption Analysis
28
3.5 Catalytic testing
3.5.1
3.5.2
28
Esterification of benzyl alcohol with acetic acid in
presence of HPW/Zr-MCM-41
29
Analysis of the Reaction Products
30
ix
4
RESULT AND DISCUSSION
4.1 Synthesis of Zirconia containing MCM-41 (Zr-MCM-41)
32
4.2 Characterization of Zr-MCM-41 support
33
4.2.1
XRD Analysis
33
4.2.2
Textural properties
36
4.2.3
Morphology features
38
4.2.4
UV-Vis DR analysis
40
4.3 H3PW12O40/Zr-MCM-41 catalyst
4.3.1
FTIR Studies
4.4 Catalytic Test
5
42
42
45
4.4.1
Influence of molar ratio of the reactants
46
4.4.2
Influence of the catalyst concentration
48
4.4.3
Influence of the temperature
50
4.4.4
Influence of the reaction time
52
4.4.5
Kinetics of esterification of benzyl alcohol with acetic acid
54
4.4.6
Mechanism
58
CONCLUSION AND RECOMMENDATION
5.1 Conclusion
63
5.2 Recommendation
64
REFERENCES
65
APPENDICES
71
x
LIST OF TABLES
TABLE NO.
TITLE
PAGE
2.1
Classification of porous materials
5
2.2
Examples of zeolites and molecular sieves
7
2.3
Different molar ratios of surfactant /silica for
mesoporous synthesis and the typical phases formed
10
2.4
Routes for synthesis mesoporous materials
11
3.1
List of chemical used in synthesis of catalyst
23
3.2
Sample codes for different Si/Zr ratio of the
materials
24
GC-FID oven-programmed set up for identifying
benzyl acetate
30
GC-MSD oven-programmed set up for verifying
benzyl acetate
31
Reaction rate constants (10-3) and energy of
activation (kJ mol-1)
55
3.3
3.4
4.1
xi
LIST OF FIGURES
FIGURE NO.
TITLE
PAGE
1.1
Esterification of benzyl alcohol with acetic acid
2
1.2
Outline of research
4
2.1
The mesoporous M41S family
9
2.2
The structure of mesoporous MCM-41 material
9
2.3
(1) Liquid crystal phase initiated and (2) silicate
anion initiated
14
Scheme for generation of Brǿnsted and Lewis acid
sites
25
XRD patterns of the as-made and calcined ZrMCM-41(Si/Zr=20)
34
XRD patterns of the calcined Zr-MCM41(Si/Zr=10)
35
4.3
XRD patterns of the calcined Zr-MCM-41(Si/Zr=5)
35
4.4
(a) Pore diameter distribution of the sample calcined
at 600ºC. (b) The N2 adsorption-desorption isotherm
of the sample (Si/Zr=20)
37
(a) Pore diameter distribution of the sample calcined
at 600ºC. (b) N2 adsorption-desorption isotherm of
the sample (Si/Zr=10)
37
(a) Pore diameter distribution of the sample calcined
at 600ºC. (b) N2 adsorption-desorption isotherm of
the sample (Si/Zr=5)
38
4.7
SEM images of MCM-41
39
4.8
SEM images of Zr-MCM-41(Si/Zr=20)
39
3.1
4.1
4.2
4.5
4.6
xii
4.9
SEM images of Zr-MCM-41(Si/Zr=10)
40
4.10
UV-Visible Spectra for MCM – 41 and Zr-MCM41(Si/Zr=20 and 5)
41
4.11
FTIR Spectrum of HPW/Zr-MCM-41
43
4.12
FTIR Spectrum of Zr-MCM-41 and MCM-41
44
4.13
Esterification of BA with AA
45
4.14
Esterification of benzyl alcohol with acetic acid:
effect of catalyst type. Acetic acid(AA):benzyl
Alcohol(BA), 2:1(mol/mol); reaction time 1 h;
catalyst weight 0.5 g ; reaction temperature, 383 K.
Conversion (purple); Selectivity (light yellow), ester
46
Esterification of benzyl alcohol with acetic acid:
effect of AA:BA molar ratio (mol/mol); reaction
time 1 h; catalyst weight, 0.5 g; reaction
temperature, 383 K. Conversion (purple); Selectivity
(light yellow), ester (Si/Zr=10)
47
Esterification of benzyl alcohol with acetic acid:
effect of AA:BA molar ratio (mol/mol); reaction
time 1 h; catalyst weight, 0.5 g ; reaction
temperature, 383 K. Conversion (purple); Selectivity
(light yellow), ester (Si/Zr=20)
48
Esterification of benzyl alcohol with acetic acid:
effect of catalyst weight. AA:BA 2:1(mol/mol);
reaction time 1 h; catalyst weight, 0.5 g; reaction
temperature= 383 K. Conversion (purple);
Selectivity (light yellow), ester
49
Esterification of benzyl alcohol with acetic acid:
effect of catalyst weight. AA:BA 2:1(mol/mol);
reaction time 1 h; catalyst weight, 0.5 g; reaction
temperature= 383 K. Conversion (purple);
Selectivity (light yellow), ester
49
Esterification of benzyl alcohol with acetic acid:
effect of reaction temperature AA:BA 2:1(mol/mol);
reaction time 1 h; catalyst weight, 0.5 g; reaction
temperature, 383 K. Conversion (
); Selectivity (
), (ester)
51
4.15
4.16
4.17
4.18
4.19
4.20
Esterification of benzyl alcohol with acetic acid:
effect of reaction time AA: BA 2:1(mol/mol);
catalyst weight, 0.5 g; reaction temperature, 383 K.
xiii
Conversion (
4.21
); Selectivity (
), (ester)
53
Reaction pathway for the esterification of BA with
AA
54
4.22
Effect of catalyst weight on reaction rate
56
4.23
Plot of first-order rate equation for esterification of
BA with AA over Si/Zr=20 at 403K, 393K and
383K respectively from above
56
Plot of first-order rate equation for esterification of
BA with AA over Si/Zr=10 at 403K, 393K and
383K respectively from above
57
Plot of first-order rate equation for esterification of
BA with AA over Si/Zr=5 at 403K, 393K and 383K
respectively from above
57
Plot of first-order rate equation for esterification
of benzyl alcohol with acetic acid in absence of
any catalyst at 403 K, 393 K and 383K from above
58
Esterification of BA with AA: effect of acetic
acid concentration on the initial reaction rate.
Concentration of benzyl alcohol, 8.1 mol;
reaction temperature, 383 K; catalyst weight, 0.5g
59
Esterification of BA with AA: effect of acetic acid
concentration on the initial reaction rate.
Concentration of benzyl alcohol, 8.1 mol;
reaction temperature, 383 K; catalyst weight, 0.5g
60
Possible reaction mechanism for the esterification of
BA with AA over mesoporous materials
61
Plot of CB/rE vs CB/CA for esterification reaction of
BA with AA. Reaction temperature, 383 K, catalyst
weight 0.5g
62
4.24
4.25
4.26
4.27
4.28
4.29
4.30
xiv
LIST OF ABBREVATIONS
AAS
-
Atomic absorption spectroscopy
AA
-
Acetic acid
BA
-
Benzyl alcohol
CTABr
-
Cetyltrimethylammonium bromide
ER
-
Eley-Ridel
FTIR
-
Fourier transformer infrared spectroscopy
HPW
-
Tungsten phosphoric acid
KBr
-
Potassium bromide
LH
-
Langmuir-hinshelwood
MCM
-
Mobil composition of matter
RHA
-
Rice husk ash
SI – MCM – 41
-
Purely siliceous MCM-41
TEOS
-
Tetraethylorthosilicate
DR UV – Vis
-
Diffuse reflectance ultraviolet-visible Spectroscopy
XRD
-
X-ray diffraction
Zr – MCM - 41
-
Zirconia containing MCM-41
xv
LIST OF APPENDICES
APPENDIX
TITLE
PAGE
A
Quantitative analysis of gas chromatography
73
B
Reaction rate versus acetic acid concentration
74
C
Reaction rate versus type of catalyst
75
D
First order equation reaction versus time
75
E
Rate versus acetic acid concentration
76
CHAPTER 1
INTRODUCTION
1.1
Research Background
Esterification is an industrially important reaction, which is one of the
methods used to produce ester compounds. These organic esters are intermediates in
the
synthesis
of
fine
chemicals,
drugs,
plasticizers,
food
preservatives,
pharmaceuticals, cosmetics and auxiliaries [1].
Esters are normally produced by a batch process in industries using mineral
acid catalysts such as hydrofluoric acid, sulphuric acid or Lewis acid catalysts like
AlCl3 or BF3 [2]. Mineral acids are known as corrosive and virulent, so they need to
be neutralized after the completion of the reaction, while in the process catalyzed by
metal containing Lewis acid catalysts, the excess water has to be removed carefully
after the reaction [3].
These works up process however, leads to the formation of large amounts of
waste [4,5]. More ever, all these catalysts are typically categorized as hazardous
substances and hence undesirable from the environmental point of view. Therefore,
there is a global effort to replace hazardous and environmentally harmful catalysts
with ecofriendly alternatives [1].
2
Solid acid catalysts as zeolites are convenient alternatives to such
conventional acids which have been used as catalysts since 1960s in petrochemicals
manufacture, further expanding into areas of speciality and fine chemical synthesis
[6]. But zeolites are microporous materials and meet with diffusional resistance both
for reactants and products as well as applicable only for smaller molecular organic
compound.
Mesoporous silica possesses high specific surface areas, tunable pore
channels from 16 to 100Ǻ and high specific pore volumes, which show that
mesoporous silica is considerable to overcome the limitation of zeolites. Since,
mesoporous materials do not have efficient catalytic properties due to absence of
catalytically active sites, so MCM-41 is often modified by incorporating certain
active materials such as metal oxides, metal complexes and others. therefore, the
research is conducted in order to synthesize the zirconia loaded MCM-41 and the
resulting material tested in the esterification of benzyl alcohol with acetic acid.
Figure 1.1 gives the reaction scheme for esterification of benzyl alcohol with acetic
acid.
OH
Zr-MCM-41
O C CH3
O
CH3COOH
Figure 1.1 Esterification of benzyl alcohol with acetic acid
1.2
Objectives of Study
The research objectives are listed as below:
(a)
To synthesize high quality zirconia loaded on MCM-41
3
(b)
To characterize the physicochemical properties of the catalyst by, Fourier-
Transform Infrared (FTIR) spectroscopy, Diffuse reflectance UV-Visible (DRUVVis) spectroscopy, X-ray diffraction (XRD), and nitrogen adsorption desorption
measurement.
(c)
To investigate the catalytic properties of Zr-MCM-41 in the esterification of
benzyl alcohol with acetic acid
(d)
To study the chemical kinetics of the esterification of benzyl alcohol with
acetic acid.
1.3
Scopes of Research
The scopes of the research are listed as below:
(a)
Direct synthesis of zirconia loaded on MCM-41(Zr-MCM-41) with various
content of zirconium.
(b)
Characterization of physicochemical properties of Zr-MCM-41 using XRD,
nitrogen on adsorption desorption isotherm, DR UV-Vis and FTIR spectroscopies.
(c)
Optimization of the reaction parameters such as temperature, reaction time
and molar ratio of reactants.
(d)
acid.
Investigation on the chemical kinetic of reaction of benzyl alcohol with acetic
4
1.4
Outline of Research
The outline of research is shown in Figure 1.2.
Direct synthesis of Zr-MCM-41
Characterization using FTIR, XRD, DR UVVis, AAS and nitrogen adsorption desorption
isotherm
Catalytic testing of the prepared Zr-MCM-41 towards the
Esterifcation of benzyl alcohol with acetic acid
Product identification using GC
and GC-MS
Optimization studies: Temperature, Different
loading of Zr-MCM-41, Reaction time
Chemical kinetics studies
Figure 1.2 Outline of the research
Chapter 2
5
CHAPTER 2
LITERATURE REVIEW
2.1
Porous Materials
Crystalline porous solids are very important materials because of their wide
applications in various adsorption/separation, purification and catalytic processes.
According to IUPAC definition [7], based on pore width, three types of pore can be
present in a porous solid: micropore (<2 nm), mesopore (2 - 50 nm) and macropore
(>50 nm). The classification of porous materials is shown in Table 2.1.
Table 2.1 : Classification of porous materials [8]
Pore diameter
Microscope
2nm
mesopore
50nm macropore
(log scale)
Crystal
Zeolites and
Mesoporous
material
related material
Amorphous
Pillared clays
porous glasses
Material
Silica gels
Active carbons
6
Well known microporous materials are zeolites and zeolite-likes, such as
aluminophosphate molecular sieves which are inorganic composites, having a
crystalline three-dimensional framework with tetrahedral atoms (T atoms) like Al, Si,
P etc. bridged by oxygen atoms. These materials possess uniform channels or
cavities circumscribed by rings of a definite number of T atoms. The architectural
features of zeolites resulting with different acid sites and acid strengths,
exchangeable cations, shape and size selective channels and pores has been well
established by now. Most of the shape-selective reactions used in the chemical
industries today involve catalysts containing zeolites having pore diameters between
0.5 and 0.6 nm. This size is sufficient to accommodate a broad spectrum of small
molecules of technological interest [9].
However, the usefulness of present day heterogeneous catalysts in processing
high-molecular-weight hydrocarbons, which are of increasing importance, is limited
by the pore size of the zeolite used and/or by the pore geometry of the metal support.
The largest-pore zeolites commercially used (i.e., faujasites) have micropores with a
free diameter of only 0.72 nm. Hence there has been an ever growing interest in
expanding the pore sizes of the zeotype materials from the micropore region to
mesopore region. The requirement of larger pore materials having adsorption
capacity of larger molecules at their catalytic sites has triggered major synthetic
efforts in academic and industrial laboratories [10, 11]. In order to preserve the
remarkable adsorptive and catalytic properties of the zeolites, while expanding their
use to process bulkier molecules, new synthesis routes have been undertaken to
increase their pore diameters. This approach has led to synthesis of ultra large pore
molecular sieves, such as AlPO4-8, VPI-5 and UTD-1, as shown in Table 2.2. [12]
7
Table 2.2: Examples of zeolites and molecular sieves [12]
Definition
Small pore
Materials
Ring Size
Pore Diameter (Ǻ)
CaA
8
4.2
SAPO-34
8
4.3
ZSM-5
8
ZSM-48
10
5.3×5.6
ZSM-12
12
5.5×5.9
AlPO4-5
12
7.3
Faujasite
12
7.4
Cloverite
20
6.0×13.2
JDF-20
20
6.2×14.5
VPI-5
18
12.1
AlPO4-8
14
7.9 ×8.7
UTD-1
14
7.5×10
Medium pore
Large pore
5.3×5.6
5.1×5.5
Ultra large
pore
Researchers had taken great efforts to synthesize mesoporous materials such
as silicas, transitional aluminas and pillared clays [13]. In fact, there were reports of
the preparation of mesoporous carbons materials with uniform pore size [14, 15].
However, the pores in mesoporous materials are generally irregularly spaced and
broadly distributed in size. Thus, a gap has been bridged by the discovery of M41S
family, which have opened up new possibilities for preparing catalysts with uniform
pores in the mesoporous region that can be easily accessed by bulky molecules that
are present in crude oils and fine chemical productions.
8
2.2
Mesoporous MCM-41
In
1992,
researchers
silicate/aluminosilicate
reported
the
synthesis
of
a
family
of
mesoporous materials, M41S, which possesses a regular
hexagonal array of uniform pore openings with a broad spectrum of pore diameters
between 1.5 and 10 nm. M41S can be divided into three main sub-groups: MCM-41
with
hexagonal pore, MCM-48 with a cubic pore and MCM-50, which has a
lamellar structure [16]. The main characteristics of MCM-41 materials are their high
thermal stability, large surface area and narrow pore size distribution. The purely
siliceous Si-MCM-41 is structurally stable towards thermal treatment, hydrothermal
treatment with steam at mild conditions, mechanical grinding and also towards acid
treatment at mild condition. However, the structural Al in Al-Si-MCM-41 is unstable
even to mild thermo chemical treatment [17].
Si-MCM-41 has high potential for practical use as an adsorbent or a
mesoporous support for depositing active catalyst components, particularly useful in
the synthesis of fine chemicals involving bulky molecules. The wide spectrum of
pore diameter makes these materials readily accessible to large molecules, and has
major significance in the processing of those. The understanding about the synthesis
of these materials and the corresponding mechanism has opened up a new era of
molecular engineering. The most outstanding feature of the preparation of these
materials is the role of the templating agents. The surfactants (act as templates) are
large organic molecules having a long hydrophobic tail of variable length (e.g.
alkyltrimethylammonium cations with formula CnH2n+1(CH3)3N+, where n > 8) and a
hydrophilic head. The formation of mesoporous materials with a variety of
crystallographically well-defined frameworks has been made possible via a
generalized “liquid-crystal templating” (LCT) mechanism [16, 18]. The different
pore systems of the mesoporous M41S family are illustrated in Figure 2.1 and Figure
2.2.
9
Figure 2.1 The mesoporous M41S family
Figure 2.2 The structure of mesoporous MCM-41 material
2.3
Synthesis of Mesoporous MCM-41
Different synthesis strategies have been proposed and successfully used to
prepare nanostructures with a unique pore size distribution. Similar to zeolite and
molecular sieve synthesis, mesoporous molecular sieves can be synthesized
hydrothermally by mixing surfactants, silica, and/or silica-alumina source to form a
gel while maintaining the mixture at a temperature between 70 and 150º C for a fixed
period of time. It is interesting to note that mesoporous siliceous, MCM-41 as well as
metal containing, MCM-41 can also be synthesized at room temperature. Tatiana et
al. [19] synthesized Al containing MCM-41 mesoporous materials in a very short
time (a minute) at room temperature. The compound showed similar characteristics
to hydrothermally synthesized materials. Kazu et al [20]. have also reported the
characterization of V-MCM-41 and Ga-MCM-41 synthesized at room temperature.
The product obtained after crystallization is filtered, washed with distilled water,
10
dried at ambient temperature. It has been found that as the surfactant/silica molar
ratio increased, the siliceous products obtained could be grouped into four categories
as shown in Table 2.3.
Table 2.3: Different molar ratios of surfactant /silica for mesoporous
synthesis and the typical phases formed [21].
Surfactant/Silica
< 1.0
1.0 - 1.5
1.2 - 2.0
2.0
Typical phase
Hexagonal phase (MCM-41)
Cubic phases (MCM-48)
Thermally unstable materials
Cubic octamer [(CTMA)SiO2.5]8
The pore diameter of MCM-41 also depends on other factors such as
temperature, pH and crystallization time. The mechanism proposed a neutral
templating synthesis mechanism based on hydrogen bonding between primary
amines and neutral inorganic species [22]. The routes for synthesis of mesoporous
materials, according to pH changes are shown in Table 2.4.
Pure siliceous MCM-41 (Si-MCM-41) mesoporous materials are electrically
neutral, which limits their catalytic applications. In order to provide a specific
catalytic activity to the chemically inert silicate framework, researchers have
incorporated, in addition to Al, a variety of metals into the walls of nanostructures by
direct synthesis, ion exchange, impregnation or grafting.
11
Table 2.4: Routes for synthesis of mesoporous materials
Surface
pH
Example
Phase
S+I-
10-13
Cetyltrimethyl ammonium
ions + silicate species
Hexagonal,
cubic and
lamellar
S0I0
<7
C12H25NH2 + (C2H5O)4Si
Hexagonal
S+XI+
<2
Cetyltrimethyl ammonium
ions + silicate species
Hexagonal
.
The surfactants have amphiphilic nature which allows the silica source to
associate into supramolecular structures. This arrangement minimizes the
unfavorable interaction of the hydrocarbon tails with water, but it introduces a
competing unfavorable interaction, the repulsion of the charged head groups. The
balance between these competing factors determines the relative stability of the
micelles [21].
2.4
Characterization of MCM-41
Different techniques are used to characterize mesoporous materials. X-ray
powder diffraction, N2 adsorption/desorption isotherms and transmission electron
microscopy (TEM) are the essential characterization techniques to identify the
mesostructure of MCM-41 materials. Other techniques such as infrared (IR)
spectroscopy, magic angle spinning nuclear magnetic resonance (MAS NMR), X-ray
photoelectron spectroscopy (XPS), etc. have also been applied to obtain additional
structural information on MCM-41 mesoporous materials.
Detailed characterization (by XRD, N2 adsorption, IR, Raman, thermal
analysis, TEM, 29Si, and 27Al MAS NMR, and NH3-TPD for acidity measurement) of
MCM-41 has been reported by Lefvere and co-workers [23].
12
X-ray diffraction pattern of MCM-41 structures show a typical four-peak
pattern with a very strong peak at a low angle (d100 reflection line) and three weaker
peaks at a higher angle (110, 200, and 210 reflection lines). Powder X-ray diffraction
technique is used to identify the structure, phase purity, degree of 30 crystallinity,
unit cell parameters and crystallite size. In the case of MCM-41 the wall thickness of
hexagonal channels is usually calculated by subtraction of the inside pore diameters
obtained by gas adsorption from the unit cell dimensions determined by XRD.
Sorption capacities for probe molecules such as n-hexane, water, benzene, nitrogen,
argon, etc. yield information about the hydrophilicity/hydrophobicity, pore volume
and pore size distribution of the molecular sieves. The BET volumetric gas
adsorption technique using nitrogen, argon, etc. is a standard method for the
determination of the surface areas and pore size distribution of finely divided porous
samples [24].
The IR spectrum in the range 200-1300 cm-1 is used to characterize and to
differentiate framework structures of different molecular sieves. The use of
29
Si
MAS NMR spectra in determining the nature and chemical environment of the
atoms.
29
Si and
27
Al MAS NMR spectra provide information on Si/Al ordering,
crystallographically equivalent or non-equivalent Si and Al ions in various sites,
framework silica to alumina ratio, coordination of Si and Al, spectral correlation with
Si-O-T bond angles and Si-O bond lengths. Solid state MAS NMR spectroscopy of
27
Al can prove the presence of tetrahedrally and octahedrally co-ordinated Al in the
MCM lattice. The broad
29
Si NMR spectra of mesoporous materials show a close
resemblance to that of amorphous silica [25].
2.5
Mechanism of Formation of Mesoporous MCM-41
Various synthesis mechanisms have been proposed in the literature to explain
the formation of mesoporus materials. A few review articles are available on the
mechanism of mesoporous MCM-41 formation. A few of the proposed mechanism
are described below [26].
13
2.5.1
Liquid Crystal Templating Mechanism
A “liquid crystal templating” (LCT) mechanism was proposed by the Mobil
researchers [16]. It is based on the similarity between liquid crystalline surfactant
assemblies (i.e., lyotropic phase) and M41S. Two mechanistic pathways were
postulated for the synthesis of MCM-41 as the representative M41S material:
1) The aluminosilicate precursor species occupied the space between a preexisting hexagonal lyotropic liquid crystal (LC) phases and deposited on the micellar
rods of the LC phase.
2) The inorganic mediated, in some manner, the ordering of the surfactants
into the hexagonal arrangement. Initially three different mesophases in M41S family
were reported, viz.; lamellar, hexagonal, and cubic, in which the hexagonal
mesophase MCM-41 possessed highly regular arrays of uniform-sized channels.
Later additional phases such as SBA-1 (cubic phase with the space group, Pm3n),
SBA-2 (three dimensional hexagonal symmetry, P63/mmc) with super cages instead
of unidimensional channels and MSU-n having highly disordered hexagonal like
array of channels with diameters in the nanometre range were reported [26].
2.5.2
Silicate Rod Assembly
The silicate encapsulated rods are randomly ordered, eventually packing into
a hexagonal mesostructure. Heating and aging then completed the condensation of
the silicates into the as-synthesized MCM-41 mesostructure. The synthesis of MCM41 consists of four complementary routes:
i.
S+I-: direct co-condensation of anionic inorganic silicate species (I-) with
a cationic surfactant (S+)
ii.
S-I+: direct co-condensation of cationic inorganic silicate species (I+) with
an anionic surfactant (S-)
14
iii.
S+X-I+: counter-ion mediated assembly where X-=Cl- or Br-
iv.
S-M+I-: counter-ion mediated assembly where M+= Na +or K+
The routes are based on ion pairing between ionic silicon species and
surfactants. There is also a neutral route, which is based on hydrogen bonding
between neutral silicates species and neutral surfactant (S0I0). Basically the synthesis
of MCM-41 always involves a liquid template mechanism which contains two-steps.
The mechanism is summarised in Figure 2.3.
Figure 2.3 (1) Liquid crystal phase initiated and (2) silicate anion initiated
The first step is the co-condensation of inorganic silicon species with organic
surfactant. In this early step, there are three possible mechanisms. In the first
mechanism, hexagonal arrangements of micellar rods exist prior to the
polymerisation of the silicate species at the surface of the rods. Then, micellar rods
are encapsulated into 2-3 monolayers of silica. Subsequently, these rods interact to
form hexagonal arrangements. In the third mechanism, the hexagonal arrangement is
formed through the interaction of the surfactants with the silicate species. The silicate
species screen the charge of the surfactants, which renders possible the
agglomeration of micellar rods. Nevertheless, the real mechanism depends on the
reaction conditions. Finally, mesoporous MCM-41 is obtained through the removal
of surfactant from the structure. This may proceed via calcinations or via solvent
extraction.
15
2.5.3
Folded Sheet Mechanism
A folded sheet mechanism for the synthesis of mesostructures derived from
kanemite (layered silicate). The synthesized mesoporous silicate and aluminosilicate
materials designated as FSM-16 (Folded Sheet Mesoporous Materials). The
surfactant cations intercalate into the bilayers of kanemite by ion-exchange process.
MCM-41 and FSM-16 are similar but show slightly different properties in adsorption
and surface chemistry [27].
2.5.4
Mechanism of Transformation from Lamellar to Hexagonal Phase
The transformation from lamellar to hexagonal phase has been proposed by
Ryosuke Sueyoshi and co-workers [28]. They have proposed that in a
surfactant/silicate aqueous mixture with relatively low pH, low degree of
polymerization of silica, and low temperatures, small silica oligomers (3 - 8 silicon
atoms) interact with surfactant cations by coulombic interactions at the interfaces
forming multidentate binding between them. These subsequently polymerize to form
larger ligands, enhancing the binding between the surfactant and silicate species.
These surfactant silicate multidentate ligands lead to a lamellar biphase governed by
the optimal surfactant average head group area. During polymerization of silicate
species, the average head group area of surfactant assembly increases due to the
decrease in charge density of larger silicate layers and ultimately results in the
hexagonal mesophase precipitation.
2.6
Incorporation of Zirconia into MCM-41
Purely siliceous Si-MCM-41 does not possess acidity. Thus, it is difficult to
introduce and apply it as a solid acid. Incorporation of metal such as aluminum [29],
titanium [30] and zirconium [31] into the mesoporous structure have been
investigated and it was found to possess acidity. Basically, the incorporation of
16
zirconium into mesoporous materials is particularly important since it forms solid
acid catalyst possessing acid sites. The acidity generated is associated with the
presence of zirconium in the framework. The zirconium containing MCM-41 can be
synthesized by both direct and secondary synthesis using a wide range of Si: Zr
ratios, depending on the surfactant and synthetic conditions [32, 33].
The typical characteristic of Zr-MCM-41 with highly ordered mesoporosity,
large surface area, high thermal stability and some acidity, allude to the possibility of
applying these materials as catalyst in the synthesis and conversion of large
molecules.
Basically, the catalytic activity of protonic Zirconium containing MCM-48 is
attributed to the presence of acidic sites arising from the ZrO4 tetrahedral units in the
framework [31]. These acid sites may be Brønsted or Lewis in character. A purely
siliceous framework is electronically neutral due to +4 charge of Si and four -1
charges from oxygen atoms. However, the substitution of another element such as
zirconium atom does not affect the charge density of the framework. As a result,
purely siliceous MCM-41 retain neutrality when lattice Si4+ cations are replaced by
Zr+4 cations. This requires the Zr atoms to be tetra coordinated. Zirconium containing
mesoporous silica, Zr-MS, was found to have a ligand-to-metal charge transfer from
an O2- to an isolated Zr4+ ion in a tetrahedral configuration. In ZrO2 where there is
full connectivity of Zr-O-Zr linkages, the LMCT shifts to lower energy in Uv-Vis .In
direct synthesis of Zr-MCM-48 indicating isolated Zr4+ ions in the amorphous silica
walls and in post-synthesis of Zr implying a possible formation of Zr-O-Zr bonds on
the surface.
17
2.7
Zirconia as acid catalyst
ZrO2 is an important material due to its interesting thermal and mechanical
properties. The bulk properties of zirconia have been extensively studied. However,
there is little published information about the surface chemical properties of zirconia,
and it is these properties which are important in processing, lubrication, catalysis, etc
[34].
Zirconia, which is prepared by precipitation from solution, can exist in any of
three metastable morphologies depending on the thermal treatment. Silica and
aluminium substrates have been used for decades to support transition metal oxides
for catalytic applications. These supports are attractive for their high specific surface
area, mechanical stability, and promotion of well-dispersed active metal sites.
Photoactive species (such as Zr and Ti) can be easily incorporated into the
amorphous wall. In a preliminary study, this is found that a high dispersion of Ti-O
moieties in a framework of MCM-41 could enhance the photocatalytic
decomposition of H2O if compared with the bulk TiO2. Zirconium has been used in
catalysis due to both its moderate acidity and oxidizing capabilities. Recently,
zirconium was doped into MCM-41 and found to be active toward the photocatalytic
generation of hydrogen [32].
Another method of zirconium incorporation involves covalently bonding the
metal directly to the surface by reaction a metal alkoxide with template [33]. This
technique generates isolated and accessible Zr metal centres on the silica surface and
maintains the high surface area with minor constriction of the pores. X-ray powder
diffraction and nitrogen adsorption were used to determine the extent of structural
retention following incorporation of Zr into the silica matrix when compared to the
pure silica isomorphs. The nature of the structure and bonding of the local Zr
environment was determined by UV/Visible, photoacoustic (PAS)-FTIR, and
EXAFS techniques.
18
2.8
Heteropoly acids impregnated mesoporous materials
It has been proven that the HPW/Zr-MCM-41 catalysts exhibit superior
isomerisation catalytic properties in the n-heptane hydroisomerization reaction at
atmospheric condition.
The isomerisation selectivity reaches 100% until 260 ◦C. In the products, 2methylhexane is dominant in the monobranched isomers and 2,3-dimethylpentane is
the main compound accounting for more than 50% of the multibranched
products.The formation of multibranched isoheptanes has a close relation with the
pore diameter of the mesoporous catalysts. The ratio of multibranched to
monobranched isoheptanes varies within a narrow range between 0.8 and 1.2, which
is independent of the reaction temperature or conversion and is much higher than
zeolite-containing catalysts, showing the superiority of our mesostructured catalysts.
This important result reveals the possibility to obtain high-octane- number
gasolineby n-heptanes hydroisomerization by using mesoporous catalysts [35].
Acetylation of veratrole with Ac2O was carried out over HPW/ZrO2/MCM-41
catalyst calcined at 1123K in liquid phase conditions under N2 atmosphere. The
catalyst was fully characterized and the stability of HPW on the support has been
proved satisfactorily. The HPW/ZrO2/MCM-41 catalyst gave highest catalytic
activity at 353K with veratrole:Ac2O molar ratio 5 and 3 wt.% catalyst concentration
(of the total reaction mixture) with a maximum conversion of acetic anhydride
(43.9%) and 100% selectivity for acetoveratrone (3,4-dimethoxyacetophenone) [36].
2.9
Esterification of benzyl alcohol with acetic acid
Esters, which include a wide category of organic compounds ranging from
aliphatic to aromatic, are generally used in the chemical industry such as drugs,
plastizers, food preservations, pharmaceuticals, solvents, perfumes, cosmetics, and
chiral auxiliaries.
In manufacturing processes, esters are produced by a batch
19
process catalyzed using mineral acid catalysts such as hydrofluoric acid, sulphuric
acid or Lewis acid catalysts like AlCl3 and BF3. Benzyl acetate finds extensive uses
in perfumery, food, and chemical industries [2].
The chemical synthesis of benzyl acetate is carried out by acetoxylation of
toluene by using inorganic catalysts and the corresponding chemical synthesis
produces unwanted side products and it also has an associated probe mod catalyst
deactivation. The formation of benzyl ester also can be synthesized using enzymes.
SbCl3 efficiently catalyzes the acetylation of alcohols with acetic acid in high yields
[37]. Esterification reactions can be carried out without catalyst, although the
reaction is extremely slow, since the rate is dependent on the autoprotolysis of the
acetic acid. Consequently, esterification is enhanced by an acid catalyst, which acts
as donor to the acid. Both homogenous and heterogeneous catalysts are used in the
esterification reaction. Typical homogenous catalysts are mineral acids, such as
H2SO4, HCl, etc [9].
All these catalysts are hazardous and hence undesirable from the
environmental point of view. Therefore, there is global effort to replace hazardous
and environmentally harmful catalysts with ecofriendly alternatives. Solid acid
catalysts such as microporous crystalline aluminosilicate namely zeolites are
convenient alternatives to such conventional acids. They have been used as catalysts
since 1960s and although they are widely exploited in petrochemicals manufacture,
their applications as catalysts are also expanding into areas of speciality and fine
chemical synthesis. But zeolites are microporous materials with much diffusional
resistance both for reactants and products. It leads to unnecessary increase in the time
requirement for establishment of equilibration in the liquid phase reactions [12].
The mesoporous Zr-MCM-41 materials may be convenient candidates as they
have high surface area and large pore diameters with nearly diffusion constraint for
both the reactants to enter and the products to leave their mesopores. Their activity
for esterification in liquid phase has already been documented in the literature [3].
Generally in liquid phase esterification the equilibrium for the stoichiometric mixture
is reached at about 66–68% conversion for straight chain saturated alcohol; complete
20
conversion can only be achieved by elimination of the water formed. But it is known
that the same reaction may be thermodynamically favoured when performed in the
vapour phase due to the higher values of equilibrium constants in comparison with
those of the liquid phase [4].
The esterification of carboxylic acids and the acylation of alcohols are
fundamental reactions in organic chemistry. Conversions in esterification reactions
are limited by slow reaction rates and reversible reactions. A direct reaction of
carboxylic acid with alcohol is generally avoided because of the equilibrium that is
established between the reactants and the products. This requires the use of an excess
amount of one of the reactants or the elimination of water from the reaction mixture
to help the completion of the process [5].
Catalysts are always employed in liquid-phase esterification to accelerate the
reaction rate. The use of solid acids such as zeolite is convenient and also effective
for acid-catalyzed reactions. It has the following inherent advantages over catalysis
initiated by homogeneous catalysts: (a) it is non-corrosive, (b) the catalyst can be
easily removed from the reaction mixture by decantation or filtration, and (c) the
product selectivity can be achieved to a certain extent due to the shape-selective
nature of the micropore structure. Zeolites have been found to be efficient catalysts
in esterification reactions. In previous studies we understood that the esterification of
benzoic acid and substituted benzoic acids over zeolite Hß and HZSM5 using
dimethyl carbonate as the methylating agent. It was found that the pore architecture
of the zeolites comes into play when the molecular diameter of the reactant
molecules is greater than the pore size of the zeolites [38].
In the present investigation we report the efficiency of acid sites and the
effect of pore size of the mesoporous materials on the product selectivity during the
esterification of benzyl alcohol with acetic acid. We have also tried to correlate the
physico-chemical properties with the catalytic activities of the mesoporous materials
Kinetic data on the esterification over MCM-41 is not widely available. For the
design of a reactor configuration and for simulation purposes, it is essential to
describe the reaction rate precisely. We have in this study tried to obtain various
21
kinetic parameters during the esterification of benzyl alcohol with acetic acid. The
Langmuir–Hinshelwood (LH) and Eley–Ridel (ER) models are commonly used to
correlate kinetic data for esterification reactions catalyzed by solid catalyst [39, 40].
These two models are derived based on the assumption that the rate-limiting
step is the surface reaction between two adsorbed molecules (LH) or between an
adsorbed molecule and a molecule in the bulk (ER). We have tried to fit the kinetic
data into LH and ER models and we describe the reaction mechanism based on the
best fit [41].
The aim of the present work is to develop a new kind of catalyst containing
zirconia with ordered mesoporous structure and strong Bronsted acidity for the
esterification of benzylalcohol. The surface acidity of the silicate mesoporous
molecular sieves was greatly enhanced by simultaneous modification of the surface
and
the
framework
by
means
of
the
deposition
of
a
strong
acid
compound,H3PW12O40, on the surface and by the incorporation of Zr+4 ions into the
Si-MCM-41 framework.
22
CHAPTER 3
METHODOLOGY
3.1
Introduction
The entire research works were performed in the three different modes. In the
first mode the zirconium-containing mesoporous silica (Zr-MCM-41) with different
zirconium loadings were synthesized. In the second mode, characterization
techniques using powder X-ray diffraction (XRD), Fourier transform infrared
spectroscopy (FTIR), UV-Vis diffuse reflectance spectroscopy (UV-Vis DR),
scanning electron microscopy (SEM), single point BET were done on the prepared
catalyst to test its physicochemical properties. To examine activity of catalyst, the
esterification of benzyl alcohol by using aqueous acetic acid was elected as a kind of
reaction model.
3.2
Chemicals
The chemicals used for the synthesis of isomorphic substitution of ZrMCM41 are summarized in the Table 3.1. All the chemicals were used as received
without purification.
23
Table 3.1: List of chemicals used in the synthesis of catalyst
Chemicals
Chemical
Cetyltrimethylammonium
bromide (CTABr)
C19 H42 NBr
Fluka
99
Tetraethyl orthosilicate
(TEOS)
C8H20O4Si
Merck
98
Zirconium propoxide
(70% in propanol)
C12H28O4Zr
Aldrich
98
Ammonium hydroxide
(27 wt.%)
NH3.H2O
-
-
Aldrich
99
Tungsten Phosphoric Acid
3.3
H3PW12O40
Manufacturer
Purity (%)
Catalyst Synthesis
Direct synthesis or isomorphic substitution of Zr-MCM-41 has been prepared
by using the method proposed by Chen, L. F. et al. (2006) with some modifications
[42]. The direct synthesis helps to modify the surface of mesoporous materials in a
single step with controllable as well as homogenous distribution of active species.
3.3.1 Synthesis of Zr-MCM-41 supports
The Zr-MCM-41 solids were prepared using tetraethyl orthosilicate (TEOS)
as Si precursor and zirconium propoxide (70 % in propanol) as Zr source, along with
cetyltrimethylammonium bromide (CTABr) as surfactant template. The typical
preparation procedure of a Zr-MCM-41 sample with a molar ratio of Si/Zr =5, 10,
and 20 is as follow: first of all, two solutions were prepared, the first solution was
made by adding given amount of zirconium tetra-propoxide into given amount of
TEOS with stirring; the second solution was made by adding given amount of
24
CTABr into 110mL hot water (around 50º C) with stirring, followed by addition of
110 mL NH3.H2O (28wt %).
Then the first solution was added, drop by drop, into the second solution.
During the addition, the mixture was vigorously stirred for about 2 h, until a gel was
formed. The resultant gel was loaded into stoppered Teflon bottle without stirring
and kept at 100 ºC for 48 h. after cooling to room temperature, the resulting solid
product was recovered by filtration and was washed for 4 times with 500 mL of
deionized water. The white solid obtained was dried in air at 80 ºC for 24 h. Finally,
the sample was calcined at 600 ºC for 6 h in the air. The heating rate was 1ºC/min.
The
molar
composition
of
the
gel
mixture
is
the
following:
0.95TEOS:0.35CTABr:0.65NH3.H2O: X-zirconium tetrapropoxide:0.55H2O In this
study, the amount of X was varied according to desired Si/Zr molar ratios of 5, 10
and 20.
Table 3.2: Sample codes for different Si/Zr ratio of the materials
Sample
Si/Zr ratio
TEOS molar composition
Zr content(mmole)
SiZr20
20
0.95
2.3
SiZr10
10
0.95
0.65
SiZr5
5
0.95
0.55
3.3.2
Preparation of H3PW12O4 supported Zr-MCM-41(HPW/Zr-MCM-41)
In the first step, the 15 wt % H3PW12O40 /Zr-MCM-41 catalyst were prepared
by impregnating the Zr-MCM-41 solids with 10 mL of a ethanol solution containing
a given amount of H3PW12O40. The solvent was removed at 50º C in a vacuum
evaporator until dryness. The amounts of H3PW12O40 used depend upon the amount
of support. The dried catalyst were calcined at 300º C in air for 2 h. Quantitation of
25
Brönsted acid sites in the HPW/Zr-MCM-41 is according to the following scheme
shown in Figure.3.1.
Na+
Na+
O
3H+ PW12O403-
Impregnation with
heteropoly acid
_
_
O
Calcination
at 300º C
fohours
Lewis acid
sites
Brönsted
acid sites
O
Figure 3.1 Scheme for generation of Brönsted and Lewis acid sites
3.4
Characterization of HPW/Zr-MCM-41
Comprehensive characterization techniques were utilized in order to elucidate
and provide unambiguous structural information and physicochemical properties of
Zr-MCM-41 with tungsten phosphoric acid(HPW/Zr-MCM-41). These structure and
properties elucidation method include powder X-ray diffraction (XRD) analysis,
Fourier
transform
infrared
(FTIR)
spectroscopy,
ultraviolet-visible
diffuse
reflectance spectroscopy( UV-Vis DR), scanning electron microscopy (SEM), TEM,
26
single point BET and elemental analysis. All the samples used in the characterization
techniques had been calcined, unless stated otherwise.
3.4.1
Powder X-Ray Diffraction (XRD)
Powder X-ray Diffraction (XRD) analysis is a powerful technique for the
qualitative and quantitative characterization of zeolite and zeolite-like materials.
XRD measurement can signify whether the catalyst is amorphous, crystalline, or
quasi-crystalline, yield an estimate of average crystalline size, yield an estimate of
average crystalline sizes, and yield d-spacing and lattice parameters, allowing
identification of the phases present.
The sampling for this analysis is made by grinding sample manually into fine
powder to fit sample. The sample holder is placed in Bruker D8 Advance powder
Diffractometer with Cu-Kα as the radiation sources with λ= 1.5418 Ǻ at 40 KV and
40 mA. Samples are measured in the range of 2θ=1.5º-10º with 0.02º step size and 1
second step time.
3.4.2 Fourier Transform Infrared Spectroscopy
Fourier Transform infrared (FTIR) spectroscopy is a method for structure
characterization that gives information on short range and long range bond order
caused by vibrational coupling, lattice coupling, electrostatic and other effect. FTIR
can provide meaningful information due to the framework vibrations of zeolite
materials in the mid-infrared region (400 - 1400 cm-1). In addition, the method
provides fast yet easy identification of the presence or absence of important
functional groups.
27
The technique use for FTIR analysis is potassium bromide (KBr) pellet
technique. The analysis is done using a Perkin Elmer One FTIR Spectrometer. 1-3
mg of finely ground sample is well-mixed with 300 mg of KBr powder and the
mixture is then placed between two 13 mm evacuable die under 10 tons of pressure
for 2 minutes to form transparent pellet. The FTIR spectrums are recorded in a
spectral range of 4000-400 cm-1.
3.4.3
Ultraviolet-Visible Diffuse Reflectance Spectroscopy (UV-Vis DR)
UV-Vis diffuse reflectance spectroscopy (UV-Vis DR) is a powerful
technique for the qualitative and quantitative determination of the absorption spectra
of solids samples or molecules embedded on the solid surface. The UV-Vis DR can
reveal the chemical valence of incorporated transition metal ion. It measures the
amount of light reflected from the samples surface with and integrating sphere. The
data are reported as a percent of reflectance (% R) read on the transmittance scale of
the instrument and correspond to R=I/Io, where Io is intensity of the incident light and
I is the intensity of light reflected from samples.
In this study, the UV-Vis DR spectra of solid samples were recorded on a
Perkin-Elmer Lambda 900 spectrometer. About 50 mg of samples is replaced in the
sample holder and the spectrum was measured in the wavelength scale of 190-800
nm.
28
3.4.4
Scanning Electron Microscopy (SEM)
Scanning microscopy (SEM) is a type of electron microscope capable of
producing high resolution images of sample surface. It provides information on the
surface topography, morphology, and structure of the sample.
Samples in a powder form were mounted over carbon stubs using double
sided tape. Prior to sample scanning, samples were attached to the sample holder and
coated with platinum using BIO-RAD Polaron Division SEM Coating System
machine to prevent charge build-up on the sample surface. Then, the samples were
scanned using Philip XL40 field emission scanning electron microscope operating at
15kV.
3.4.5
N2 adsorption Analysis
A sorption isotherm (also adsorption isotherm) describes the equilibrium of
the sorption of a material at a surface (more general at a surface boundary) at
constant temperature. It represents the amount of material bound at the surface (the
sorbate) as a function of the material present in the gas phase and/or in the solution.
Sorption isotherms are often used as empirical models, which do not make
statements about the underlying mechanisms and measured variables. They are
obtained from measured data by means of regression analysis. The most frequently
used isotherms are the linear isotherm, Freundlich isotherm, the Langmuir isotherm,
and the BET model. The N2 physisorption measurements were carried out at 77 K on
ASAP 2010 volumetric adsorption analyzer.
3.5
Catalytic testing
The catalyst activity was tested in the liquid phase esterification of benzyl
alcohol with acetic acid as reaction model. The catalytic activity was examined in the
29
transformation of benzyl alcohol to benzyl acetate using HPW/Zr-MCM-41 as the
heterogeneous acid catalyst. The performance of HPW/Zr-MCM-41 was measured
in terms of conversion of benzyl alcohol and selectivity to benzyl acetate in
competition with etherification reaction.
3.5.1
Esterification of benzyl alcohol with acetic acid in the presence of
HPW/Zr-MCM-41
Protonated forms of Zr-MCM-41 means HPW/Zr-MCM-41 used in the
esterification of benzyl alcohol with acetic acid. The Zr-MCM-41was converted to
its protonated form following the impregnation of Zr-MCM-41 with tungsten
phosphoric acid. The esterification reaction was carried out in a round bottomed
(RB) glass flask (50 mL) fitted with a water cooled condenser in the temperature
region 383–403º K. The temperature was maintained using an oil bath connected to a
thermostat. The reactants benzyl alcohol and acetic acid were taken directly into the
RB flask along with the catalyst and also the reaction was done without catalyst for
comparison. The total volume of the reactants was kept at 12 mL.
The reaction mixture was continuously stirred during the reaction using a
magnetic stirrer. The reaction was carried out for a definite period of time after
which the catalyst was separated from the reaction mixture by filtration and washed
with acetone. The reaction products were analyzed using GC-FID and GC-MSD. Gas
chromatography (GC) model Agilent Technologies (6890N) using flame ionization
detector (FID) with HP-5% column (methyl siloxane, 30.0 m × 320 µm × 0.25 µm
nominal). The identification of the product was characterized by using GC-MSD
using Agilent 6890N-5973 Network Mass Selective Detector model using an Ultra-1
(methyl siloxane) column with the length and internal diameter 25 m × 0.2 mm.
Blank reactions were also carried out in the absence of a catalyst. The main product
was found to be benzyl acetate. Experiments were designed by varying the amount of
the catalyst, the molar ratios of the reactants, the reaction temperature and the
reaction period to obtain various kinetic parameters. The conversion and selectivity
30
(in percentages) were calculated based on the GC analysis using the following
expressions:
Conversion of benzyl alcohol =
100 – 100 × [benzyl alcohol]
[benzyl alcohol] + [benzyl acetate]+2[dibenzyl ether]
Selectivity (ester) (benzyl acetate) =
Selectivity (ether) (dibenzyl ether) =
3.5.2
100 × [benzyl acetate]
[benzyl acetate] +[dibenzylether]
100 × [dibenzyl ether]
[benzyl acetate] + [dibenzylether]
Analysis of Reaction Products
The withdrawn liquid samples were analyzed by Hewlett-Packard 6890 N gas
chromatography using an Ultra-1 (cross linked methylsilicone, 25x0.20 mm I.D)
column and flame ionization detector (FID). The setup of oven temperature was
shown in Table 3.3.
Table 3.3: GC-FID oven-programmed set up for identifying products
GC parameter
Temperature /Time
Oven temperature
80 ºC
Initial time
1 min
Rate
10 ºC
Final temperature
Hold time
270 ºC
2 min
Product identifications and its verifications were carried out using gas
chromatography-mass spectrometry (GC-MS) measurement and compared with
available standard compounds. Hewlett-Packard GC-MSD instrument is equipped
31
with HP-5 MS column (30 m x 0.25). The setup of oven temperature was shown in
Table 3.4.
Table 3.4: GC-MSD oven-programmed set up for verifying benzylacetate
GC parameter
Temperature / Time
Oven temperature
80 ºC
Initial time
1 min
Rate
10 ºC
Final temperature
270 ºC
Hold time
2 min
32
CHAPTER 4
RESULTS AND DISCUSSION
4.1
Synthesis of Zirconia Containing MCM-41(Zr-MCM-41)
Direct hydrothermal synthesis method was used to prepare Zr-MCM-41 with
different Si/Zr ratio (5, 10 and 20). In the preparation, a mixture solution of CTABr,
NH4OH (27 wt %) and H2O was prepared under room conditions with constant
stirring. The CTABr was used as template whereby NH4OH (28 wt %) was used to
provide alkanility of the reaction medium. To the mixture, a solution of TEOS as the
silica source and zirconium tetra-propoxide as zirconium source was added drop by
drop with vigorous stirring to ensure homogeneity between the organic and aqueous
phases during the gel preparation. The gel preparation procedure involves only room
temperature condition as high temperature synthesis is usually not preferred because
of a dissolution problem with the micelles.
In this study, the surface acidity of the silicate mesoporous molecular sieves
was greatly enhanced by simultaneous modification of the surface and the framework
by means the deposition of a strong acid compound, H3PW12O40, on the surface and
by the incorporation of Zr+4 ions into the Si-MCM-41 framework.
After following the synthesis procedure, white fine powders were obtained
and the samples were characterized by XRD, FTIR, UV-Vis DR, N2 adsorption
analysis and SEM to determine their physicochemical properties. Finally, the
33
catalytic activity of the samples prepared was tested in the liquid phase esterification
of benzyl alcohol with acetic acid.
4.2
Characterization of Zr-MCM-41 support
4.2.1
XRD analysis
XRD patterns of the as-made and calcined Zr-MCM-41n (n = 20) is displayed
in Figure 4.1.The Samples have three peaks that are indexed to (100), (110) and
(200) reflections, which correspond to well ordered hexagonal pore systems,
characteristic of MCM-41-type mesoporous materials. In comparison with the asmade solids, after calcination at 600ºC, several variations were observed:
1) The positions of the (110) peak shifted towards larger 2θ values. As a result, this
is related to the further condensation of the Si-OH and/or Zr-OH groups during
the calcinations, increasing to a contraction of the unit cell dimension relative to
the as-made solids.
2) The intensities of the diffraction peaks increase significantly after calcination,
which must be corresponded to the removal of the surfactant molecules in the
calcination.
Particularly, after calcination, the intensities of the XRD peaks increase
significantly in the Zr-MCM-41(Si/Zr=20), indicating that the structural ordering is
increased and ordered hexagonal pore system retained without collapsing.
It is remarkable that in the calcined MCM-41 solids, the intensities of
diffraction the peaks vary with zirconium content. The intensities of all the peaks
decrease as the Si/Zr molar ratio decreases from 20 to 10 and then decrease with
further increase of the zirconium content (Figures 4.2 and 4.3).
34
The calcined sample with a Si/Zr=20 shows the highest peak intensity, and
the sample with Si/Zr=5 shows the lowest peak intensity, indicating that a too high
zirconium content may lower the structural ordering in the resultant material. It is
possible that too many zirconium ions incorporated into the framework of Si-MCM41, might result in a partial collapse of the mesostructure (Figure 4.3). The wall
thickness largely increases by increasing the zirconium content, until the Si/Zr molar
ratio decreases to 5, indicating the incorporation of zirconium into the framework.
Relative Intensity (a.u.)
100
Calcined
As made
110
200
2-Theta-Scale ( CuKα)
Ǻ
Figure 4.1 XRD patterns of the as-made and calcined Zr-MCM-41(Si/Zr=20).
35
Relative Intensity (a.u.)
100
110
200
2-Theta-Scale ( CuKα)
Figure 4.2 XRD patterns of the calcined Zr-MCM-41(Si/Zr=10)
Relative Intensity (a.u.)
100
2-Theta-Scale ( CuKα)
Figure 4.3 XRD patterns of the calcined Zr-MCM-41( Si/Zr=5)
36
4.2.2
Textural properties
Figures 4.4, 4.5 and 4.6, show the loops of the N2 adsorption-desorption
isotherms of the calcined Zr-MCM-41 samples and the pore size distributions. Four
regions are observed:
1)
The first stage, at P/P0 < 0.2, is due to a monolayer adsorption of nitrogen
molecules on the walls of the mesopores.
2)
The second stage, at 0.3 < P/P0 < 0.4, determined by a high increase in
adsorption, is due to capillary condensation inside the mesopres.
3)
The third stage, the adsorption isotherm is the horizontal section beyond the
P/P0 of 0.4, which is corresponds to multilayer adsorption on outer surface of the
particles.
4)
The last stage at P/P0 > 0.9 can be related to capillary condensation in the
solid with high zirconium content.
The pore diameter distribution of the samples with low zirconium content,
Si/Zr=20 and 10, shows only a single peak around 21.90 and 22.5 Ǻ. However, in the
sample with high zirconium content, Si/Zr=5, some larger pores appear, which is due
to the formation of mesopores between particles.
37
(a)
Volume Adsorbed, cc/g
Pore Volume (cc/g)
21.90 Ǻ
Si/Zr=20
(b)
Pore diameter, (Ǻ)
P/P0
Figure 4.4 (a) Pore diameter distribution of the sample calcined at 600ºC. (b) The
N2 adsorption-desorption isotherm of the sample (Si/Zr=20).
22.5 Ǻ
(a)
Volume Adsorbed, cc/g
Pore Volume (cc/g)
Si/Zr=10
Pore diameter, (Ǻ)
(b)
P/P0
Figure 4.5 (a) Pore diameter distribution of the sample calcined at 600ºC. (b)
N2 adsorption-desorption isotherm of the sample (Si/Zr=10).
38
(a)
Volume Adsorbed, cc/g
Pore Volume (cc/g)
23.0 Ǻ
Si/Zr=5
(b)
P/P0
Pore diameter, (Ǻ)
Figure 4.6 (a) Pore diameter distribution of the sample calcined at 600ºC. (b)
N2 adsorption-desorption isotherm of the sample (Si/Zr=5).
4.2.3 Morphology features
The morphology of MCM-41 and Zr-MCM-41 materials was studied by
scanning electron microscopy. Typical SEM images are given in Figures 4.7, 4.8 and
4.9. It can be seen that, whatever the zirconium loading of MCM-41 and Zr-MCM41 (Si/Zr= 10 and 20), the parent MCM-41 and Zr-modified samples have similar
morphology consisting of particles of less than 1µm with irregular shapes, probably
due to agglomeration of particles.
39
Figure 4.7 SEM image of MCM-41
Figure 4.8 SEM image of Zr-MCM-41(Si/Zr=20)
40
Figure 4.9 SEM image of Zr-MCM-41(Si/Zr=10)
4.2.4
UV-ViS DR analysis
The calcined Zr-MCM-41 solids were also characterized with UV-vis DR
spectroscopy. For the purpose of comparison, the UV-vis DR analysis of the pure SiMCM-41 sample was also included as reference. All the Zr-MCM-41 solids show a
band around 200 nm which is due to the charge-transfer transition from an oxygen
ion to a Zr(IV) ion Figure 4.10, relating to the excitation of electrons from the
valence band (2p character in O) to the conduction band ( 4d character in Zr).
The Si-MCM-41 sample did not display any peak in the same wavelength
range, between 190 and 800 nm. Usually, in the UV-Vis spectrum, an absorption
band around 250 nm corresponding to Zr+4 in monoclinic ZrO2 phase and an
absorption band at around 300 nm of octahedral Zr+4 in the perovskite-type SiZrO3
can be observed [31].
41
Our results suggest that the zirconium ions in the Zr-MCM-41 samples are in
a different state than in pure ZrO2 or SiZrO3 (or pure Si-MCM-41 solid) and no
separated ZrO2 phase was formed in our samples. This is another strong evidence of
zirconium incorporation into the framework of the mesoporous materials. The
intensity of the band increases as the zirconium concentration increases, once again,
indicating that more zirconium ions are incorporated into the Si framework at higher
zirconium contents.
K-M
Si/Zr=5
Si/Zr=20
MCM-41
Wavelength ( nm)
Figure 4.10 Uv-Visible Spectra for MCM-41 and Zr-MCM-41(Si/Zr=20 and 5)
42
4.3 H3PW12O40/Zr-MCM-41 catalyst
4.3.1 FTIR studies
The FTIR technique was used for the surface characterization of the HPW/ZrMCM-41 catalyst. As shown in Figures, 4.11 and 4.12, the four fingerprint
absorption bands of the heteropolyanion vibrations at approximately 1088, 962, 893
and 816 cm-1 present in the H3PW12O40, were retained in our catalysts. The P-O
symmetric stretching is indicated by the vibrational transition at 1080 cm-1.
The normal mode associated with the band at 962 cm-1 is related to the W= O
stretch mode. The bands at 893 and 816 cm-1 are associated with the stretching
motion of W-O-W bridges. The band at 893 cm-1 is described as a W-Oc-W stretch
mode and the band at 822 cm-1 is related to W-Oe-W stretch mode.
The appearance of the four absorption bands related to the heteropolyanion
vibrations indicates that the primary structure of the H3PW12O40 is remained.
However, a blue-shift of all the band positions takes place in the HPW/Zr-MCM-41
catalyst when compared to those of the pure H3PW12O40 material. The band at 3500
cm-1 is characteristic band of the adsorbed water molecules;
The bands at 1621–1641 cm-1 are aroused by the vibration of the adsorbed
water molecules; the band around 811 cm-1 is the bending vibration of Si–O; the
band about 460 cm-1 is from the bendig vibration of Si–O. As can be observed in
Figure 4.12, for the pure silica MCM-41 sample, the band at 1081 cm-1 is from the
antisymmetric extension vibration of Si–O–Si. The Si–O–Si bands of the Zr-MCM41 shifted to 1079 cm-1, at the same time, we can also observe that the intensity of
the band 1081 cm-1 is gradually reduced as the zirconium content increases.
43
The red shift in the Si–O–Si band of Zr-MCM-41 sample with the increase in
the zirconium content is probably due to the replacement of Si ions in the framework
by Zr+4 ions.
%T
962
470
816
1088
1400
1200
1000
800
600
Wavenumber (cm-1)
Figure 4.11 FTIR Spectrum of HPW/Zr-MCM-41
400
44
1620
Zr-MCM-41
%T
MCM-41
1079
3500
460
0
1081
1000
2000
3000
4000
Wavenumber (cm-1 )
Figure 4.12 FTIR Spectrums of Zr-MCM-41 and MCM-41
45
4.4
Catalytic Test
The reaction takes place on mesoporous HPW/Zr-MCM-41 according to the
scheme in Figure 4.13:
O C CH3
O
OH
CH3COOH
Figure 4.13 Esterification of BA with AA
The expected products are benzyl acetate as the main product and
dibenzylether as the side product which relative amount of each product depends on
the catalyst type and reactants molar ratios.
Initial investigations of the effect of the catalyst type on the reaction yielded
some interesting results (Figure 4.14). Conversion of benzyl alcohol is between 18
and 48% for all different loadings of zirconium, selectivity for benzyl ester product is
100%. It seems that because of the being active acidic sites less than zeolite which
inhibit the ether formation, since ether formation required more active acidic sites
than ester formation, also the pore size of all HPW/Zr-MCM-41 samples, are
probably medium, therefore they cannot accommodate the large ether molecule, and
thus hinders its formation.
The fact that dibenzyl ether is not formed when the reaction is carried out in
the absence of any catalyst indicates the involvement of acid sites for the ether
formation and the presence of these active sites within the pores of the mesoporous.
Conversion and Selectivity (%)
46
100
80
60
40
20
0
Blank
Si/Zr=20
Si/Zr=10
Si/Zr=5
Figure 4.14 Esterification of benzyl alcohol with acetic acid: effect of catalyst type.
Acetic acid(AA):benzyl Alcohol(BA), 2:1(mol/mol); reaction time 1 h; catalyst
weight 0.5 g ; reaction temperature, 383 K. Conversion (purple); Selectivity (light
yellow), ester.
4.4.1 Influence of molar ratio of the reactants
The reaction was carried out over HPW/Zr-MCM-41(Si/Zr =10 and 20) using
different molar ratios of acetic acid to benzyl alcohol (Figures 4.15 and 4.16
respectively). In all case, only benzyl acetate was formed. The conversion of benzyl
alcohol was found to decrease with increase in concentration of benzyl alcohol in the
reaction mixture. In the all cases, the conversion decreased as the molar ratio of acid:
alcohol was varied from 2:1 to 1:3. The selectivity towards the ester was constant.
Also, the selectivity towards dibenzyl ether was unchanged.
Selectivity towards ester and dibenzylether was always 100% and 0%
respectively. For example in case of Si/Zr=20, the conversion of benzyl alcohol was
found to decrease with increase in concentration of benzyl alcohol in the reaction
mixture.
47
In the case of Si/Zr=20, the conversion decreased from 50 to 37% as the
molar ratio of acid: alcohol was varied from 2:1 to 1:3. In the case of Si/Zr=10, the
conversion decreased from 39 to 10% on varying the acid: alcohol molar ratio from
2:1 to 1:3. Selectivity towards ester was always 100%.
Si/Zr=10
Conversion and Selectivity
(%)
100
80
60
40
20
0
2:1
Figure 4.15
1:1
1:2
1:3
Esterification of benzyl alcohol with acetic acid: effect of AA:BA
molar ratio (mol/mol); reaction time 1 h; catalyst weight, 0.5 g; reaction
temperature, 383 K. Conversion (purple); Selectivity (light yellow), ester (Si/Zr=10).
48
Conversion and Selectivity (%)
Si/Zr=20
100
80
60
40
20
0
2:1
Figure 4.16
1:1
1:2
1:3
Esterification of benzyl alcohol with acetic acid: effect of AA:BA
molar ratio (mol/mol); reaction time 1 h; catalyst weight, 0.5 g ; reaction
temperature, 383 K. Conversion (purple); Selectivity (light yellow), ester (Si/Zr=20).
4.4.2 Influence of catalyst concentration
In the case of HPW/Zr-MCM-41(n=20), the amount of the catalyst was
varied from 0.5 to 1.5 g while keeping the molar ratio of acid: alcohol at 2:1. The
reaction was carried out at 383K for 1 h. The conversion of benzyl alcohol increased
from around 50 to 84% on increasing the weight of from 0.5 to 1.5 g (Figure 4.17).
The selectivity towards dibenzylether was zero percentage. In the case of HPW/ZrMCM-41(n=10), the conversion increased from around 40 to 68% in the same
catalyst range of 0.5–1.5 g (Figure 4.18). The selectivity was always 100%, as the
only product formed was the ester.
Conversion and Selectivity(%)
49
100
80
60
40
20
0
0.5
1.0
1.5
Fig 4.17 Esterification of benzyl alcohol with acetic acid: effect of catalyst weight.
AA:BA 2:1(mol/mol); reaction time 1 h; catalyst weight, 0.5 g; reaction
Conversion and Selectivity (%)
temperature= 383 K. Conversion (purple); Selectivity (light yellow), ester.
100
80
60
40
20
0
0.5
1.0
1.5
Figure 4.18 Esterification of benzyl alcohol with acetic acid: effect of catalyst
weight. AA: BA 2:1(mol/mol); reaction time 1 h; catalyst weight, 0.5 g; reaction
temperature, 383 K. Conversion (purple); Selectivity (light yellow), ester.
50
4.4.3.
Influence of temperature
The esterification reaction was carried out in the temperature region 383–403
K while keeping the acid: alcohol molar ratio at 2:1 and the catalyst weight at 0.5 g
(Figure, 4.19). In general, the conversion of benzyl alcohol increases with increase in
reaction temperature. The selectivity for the ester was found constant over HPW/ZrMCM-41(Si/Zr=5, 10 and 20).
Over HPW/Zr-MCM-41(Si/Zr=5, 10, 20), the selectivity for the ester was
found to be 100% regardless of the reaction temperature. This suggests that over all
samples of HPW/Zr-MCM-41, high temperatures favour the ester formation, whereas
such formation is less facile at lower temperatures. Also the temperature effect on
HPW/Zr-MCM-41(Si/Zr=20, 10) was higher than Si/Zr molar ratios of 5, because the
pores diameter of HPW/Zr-MCM-41(Si/Zr=20 and 10) was more regular and smaller
than Si/Zr molar ratios of 5 that increased formation of ester in higher temperatures.
Selectivity and conversion (%)
Selectivity and conversion (%)
51
Si/Zr=20
120
100
80
60
40
20
0
380
385
390
395
400
405
400
405
Reaction
Temperature
Series1
Series2(K)
Si/Zr=10
120
100
80
60
40
20
0
380
385
390
395
Selectivity and conversion (%)
Reaction
Temperature
Series1
Series2(K)
Figure 4.19
Si/Zr=5
Reaction Temperature (K)
Esterification of benzyl alcohol with acetic acid: effect of reaction
temperature AA:BA 2:1(mol/mol); reaction time 1 h; catalyst weight, 0.5 g; reaction
temperature, 383 K. Conversion (
); Selectivity (
), (ester).
52
4.4.4
Influence of reaction time
The conversion of benzyl alcohol increases rapidly in the beginning and
gradually levels off after 2 h (Figure.4.20). For example, over HPW/Zr-MCM41(Si/Zr=20) the conversion increased from around 21% in the first 30 min to around
57% in 3 h; on increasing the reaction time to 4 h, the conversion increased only to
60%. There is no significant difference in the formation of ether. The ether formation
is independent of the esterification reaction. The condensation of two molecules of
benzyl alcohol gives dibenzyl ether.
Conversion and selectivity (%)
53
120
Si/Zr=10
100
80
60
40
20
0
0
1
2
3
4
5
Conversion and selectivity (%)
Reaction
Series1time (h)
Series2
120
100
80
60
40
20
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Series1 time (h)
Series2
Reaction
Si/Zr=20
Figure 4.20 Esterification of benzyl alcohol with acetic acid: effect of reaction time
AA: BA 2:1(mol/mol); catalyst weight, 0.5 g; reaction temperature, 383 K.
Conversion (
); Selectivity (
), (ester).
This etherification does not take place in the absence of a catalyst.
Etherification of benzyl alcohol must be occurring simultaneously with the
esterification reaction (Figure 4.21).
54
Benzyl acetate
Benzyl alcohol
dibenzylether
Figure 4.21 Reaction pathways for the esterification of benzyl alcohol with acetic
acid.
4.4.5
Kinetics of esterification of benzyl alcohol with acetic acid
The rate of a reaction is proportional to the number of active sites, which in
turn is proportional to the weight of the mesoporous materials. A plot of rate as a
function of the weight of catalyst is linear.
The rate increased proportionally with the weight of mesoporous materials
(Figure 4.22), indicating the absence of any mass transfer limitations during the
reaction. Since the esterification and etherification are simultaneous reactions, they
could be treated separately to get the kinetic data.
Plots of –ln(1-yield(ester)) versus reaction time for the esterification reactions
carried out at different reactions Temperatures over HPW/Zr-MCM-41(Si/Zr=20 and
10) and HPW/Zr-MCM-41(Si/Zr=5), also in absence of any catalyst are given in
(Figures, 4.23, 4.24 4.25 and 4.26), respectively. The plots are nearly linear in all
cases indicating the esterification reaction to be a first-order reaction. The rate
55
constants obtained from the slopes of this linear regression of these plots and the
energies of activation calculated from the Arrhenius equation are given in Table
(4.1).
In this table, the activation energy was calculated from Arrhenius equation
K=Ae
-Ea/RT
, then we can earn activation energy from LnK2/K1= -Ea/R(1/T2-1/T1), A
was calculated from A=(rate constant in T+ rate constant in T+10)/( rate constant in
T),since the reaction is first-order , the k(rate constant) can be earned with this
equation, Rate= k [A]. The energy of activation is the lowest for, HPW/Zr-MCM41(Si/Zr=20) followed by HPW/Zr-MCM-41(Si/Zr=10 and 5) respectively.
However, HPW/Zr-MCM-41(Si/Zr=20) may be referred catalyst for the benzyl
acetate preparation due to its 100٪ selectivity towards the ester and the absence of
the ether formation.
Table 4.1: Reaction rate constants (10-3) and energy of activation (kJ mol-1) for
formation of the ester
Catalyst
Rate constant (ester)
Energy of activation
383K
393K
HPW/Zr-MCM-41(20)
7.8
10.5
30.56
HPW/Zr-MCM-41(10)
5.5
8.1
37.36
HPW/Zr-MCM-41(5)
4.5
5.3
42.48
Blank
2.6
3.9
58
56
100
Reaction Rate x 10-5
90
80
70
60
Si/Zr=5
si/zr=5
Si/Zr=10
si/zr=10
50
40
si/zr=20
Si/Zr=20
30
20
10
0
-10 0
0.5
1
1.5
2
2.5
Catalyst weight (g)
Fig 4.22 Effect of catalyst weight on reaction rate
Si/Zr=20Title
Chart
0.9
403 K
-Ln (1-Yield of ester)
0.8
0.7
393 K
0.6
0.5
383 K
0.4
0.3
0.2
0.1
0
-0.1
0
10
20
30
40
50
60
70
Time (min)
Figure 4.23 Plot of first-order rate equation for esterification of benzyl alcohol with
acetic acid over Si/Zr=20 at 403K, 393K and 383K respectively from above.
57
0.8
Si/Zr=10
-Ln (1-Yield of ester)
0.7
403 K
0.6
0.5
393 K
0.4
383 K
0.3
0.2
0.1
0
-0.1
0
10
20
30
40
50
60
70
Time (min)
Figure 4.24 Plot of first-order rate equation for esterification of benzyl alcohol with
acetic acid over Si/Zr=10 at 403K, 393K and 383K respectively from above.
2
Si/Zr=5
-Ln (1-Yield of ester)
1.8
403 K
1.6
1.4
1.2
393 K
1
0.8
383 K
0.6
0.4
0.2
0
0
20
40
60
80
100
120
140
160
180
Time (min)
Figure 4.25 Plot of first-order rate equation for esterification of benzyl alcohol with
acetic acid over Si/Zr=5 at 403 K, 393 K and 383K from above.
58
-Ln (1-Yield of ester)
1
Blank
403 K
0.8
393 K
0.6
383 K
0.4
0.2
0
0
20
40
60
80
100
120
140
160
180
-0.2
Time (min)
Figure 4.26 Plot of first-order rate equation for esterification of benzyl alcohol with
acetic acid in absence of any catalyst at 403 K, 393 K and 383K from above.
4.4.6
Mechanism
Generally, reactions over mesoporous surface follow the Eley-Ridel pathway.
The rate of reaction for Langmuir-Hinshelwood mechanism is given by equation (1)
rA= k KBKACA/(1+KACA+KBCB)2. The initial rate equation for ER is given by
equations (2) and (3).
(a) Without competitive adsorption of benzyl alcohol: rE=k KACA/1+ KACA
(2)
(b) With competitive adsorption of benzyl alcohol rE= k KACA/1+KACA+KBCB (3)
Where k s is the rate constant ks if adsorption of benzyl alcohol is the
controlling step, and k s= ksCB if the chemical reaction is the rate limiting step. KA
and KB are equilibrium adsorption constants for acetic acid and benzyl alcohol,
respectively; and CA and CB are the initial concentration of acetic acid and benzyl
alcohol, respectively. As already mentioned, there is no mass transfer limitation on
the MCM-41 surface, i.e. the chemical step is the rate determining step [41].
59
If the esterification reaction proceeds through a LH mechanism then a plot of
rate versus concentration must pass through a maximum, according to equation (1),
while if it follows an ER mechanism, then no such maximum is encountered. Figure
4.27 and Figure 4.28 indicates that the initial reaction rate increases linearly with
acid concentration.
This would suggest that the esterification of benzyl alcohol with acetic acid
follows an ER mechanism. A decrease in the rate was observed with an increase in
the alcohol concentration; this can be explained by the saturation of the catalyst
surface with alcohol, blocking the acid adsorption. Thus we can say that acid
adsorption is a must for the esterification to proceed and that there is a competitive
adsorption of the benzyl alcohol.
Si/Zr=20
Chart
Title
-3
Rate (mol/min)
Axis Title x10
7
6
5
4
3
2
1
0
0
2
4
6
8
10
-3
Concentration of Acetic
AxisAcid
Title (mol) x 10
Figure 4.27 Esterification of benzyl alcohol with acetic acid: effect of acetic acid
Concentration on the initial reaction rate. Concentration of benzyl alcohol, 8.1 mol;
reaction temperature, 383 K; catalyst weight, 0.5g.
60
Si/Zr=10
Si/zr=10
Rate (mol/min) x 10-3
6
5
4
3
2
1
0
0
1
2
3
4
5
6
7
8
-3
Concentration
of Acetic
Concentration
of Acetic
Acid acid(mol)10
(mol) x 10 ³
Figure 4.28 Esterification of benzyl alcohol with acetic acid: effect of acetic acid
concentration on the initial reaction rate. Concentration of benzyl alcohol, 8.1 mol;
reaction temperature, 383 K; catalyst weight, 0.5g.
The esterification reaction takes place between acetic acid adsorbed on the
mesoporous surface, forming an electrophile and benzyl alcohol in the liquid phase
(Figure 4.29).
61
O
H3C
Zr
CH3
C
C
OH
CH2OH
Zr
OH
CH2
CH2
CH2
H
O
O
O
H3CC
CHCH3
H3CC
OCH2
O
O
O
Zr
Zr
Zr
O
Zr
CH2
O
C
CH3
Zr+4 = Acid site on mesoporous Zr-MCM-41
Figure 4.29 Possible reaction mechanism for the esterification of benzyl alcohol
with acetic acid over mesoporous materials.
Then the initial rate equation can be given by equation (4):
rE=ksCBKACA/ 1+KACA+KBCB
(4)
Linearization equation (4), a plot of CB/rE against CB/CA (Figure 4.27) yields
a slope equal to KB/KAks and an intercept equal to 1/ks. From this the adsorption
coefficient KB/kA can be obtained. The fit was found to be suitable and the reaction
rate constant obtained from the intercept was found to be 7.67x10 -3min-1 for Si/Zr=20
and 4.45x10-3 min-1 for Si/Zr=10; benzyl alcohol is adsorbed more extremely over
Si/Zr molar ratio, Si/Zr=20 than over Si/Zr molar ratio, Si/Zr=10. Experiments were
carried out using silylated Zr-MCM-41. In this case it was observed that there was
only a decrease in the conversion of benzyl alcohol.
OH
62
This would offer that the active sites for the esterification are inside the pores
of the mesoporous and that the products are formed within the pores of the mesopore.
The continues observations may be summarized as follows: Assuming acidity is:
400
Y=71.11 x + 14.40 Si/Zr=20
350
300
CB/rE
250
200
150
Y= 45.30x + 92.36 Si/Zr= 10
100
50
0
0
0.5
1
1.5
CB/C
A
y=1
2
2.5
3
y=2
Figure 4.30 Plot of CB/rE vs CB/CA for esterification reaction of benzyl alcohol with
acetic acid. Reaction temperature, 383k, catalyst weight 0.5g.
An important parameter for the esterification, one could expect the required
acidic sites to be available in all mesoporous materials. If the reaction is to take place
on the outer surface or outside the pores, one could not have found any difference in
the type of product formed. In comparison, with previous researches, the pores size
in mesoporous MCM-41 is larger than zeolite, but the number of acidic sites is lower
than zeolite within pores, that is a reason for ester formation inside pores and no
ether formation was occurred.
63
CHAPTER 5
CONCLUSION AND RECOMMENDATION
5.1
Conclusion
In this study, effort has been devoted to direct synthesis of highly ordered Zrbased mesoporous molecular sieves MCM-41-type through a surfactant-template
approach. The molar Si/Zr ratio greatly influences the structural regularity and
textural properties. XRD data show that the range ordering of the mesoporous
structure of the catalyst decrease extremely as a result of the incorporation of higher
amount of zirconium species. The presences of zirconium in tetrahedral coordination
was indicated by UV-Vis DR spectra, which shows an absorption band around 200
nm in the Zr-MCM-41. Calcination may significantly improve the structural ordering
of the resultant materials. The Brönsted acidity of the Zr–MCM-41 solids could be
remarkably enhanced after promoted 12- tungstophosphoric acid.
Catalytic testing was focused on esterification of benzyl alcohol with acetic
acid. Results show that the Zr-MCM-41 are active in the esterification of benzyl
alcohol to produce benzyl acetate. The sample with Si/Zr=20 shows the highest
percentage of conversion and selectivity of 100%. The catalyst was shown to be
highly active in esterification of benzyl alcohol due to presence of bronsted acid sites
in the framework of zirconium species. Kinetic studies have shown that the
esterification follows the Eley–Ridel mechanism. The energy of activation for the
reaction follows the order: Blank> HPW/Zr-MCM-41(Si/Zr=5)> Zr-MCM41(Si/Zr=10) > Zr-MCM-41(Si/Zr=20).
64
5.2
Recommendation
In this research, zirconium species have been incorporated into mesoporous
silica by direct synthesis using zirconium (IV) propoxide as zirconium source. For
direct synthesis method, the pH adjustment and moisture play a crucial role in
suitable quality synthesis. Therefore further study should be carried out in vacuum
conditions such as in glove box.
It is also recommended that further characterization techniques can be applied
to determine the zirconium environments including
and AAS.
29
Si MAS NMR spectroscopy
65
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71
APPENDICES
APPENDIX A
Quantitative analysis of gas chromatography calibration plot of acetic acid (reactant)
Peak area of benzyl alcohol
Peak area of DMSO
and dimethyl sulfoxide (internal standard)
2.5
y = 0.212x - 0.175
R² = 0.989
2
1.5
1
0.5
0
0
2
4
6
8
10
Acetic acid (mmol)
For catalyst reaction:
Reaction rate for catalyzed reaction:
C
K1
A+B
P
K2
R= K [ AC][B] and R= K2K1[A][C][B]/ K1+K2[B]
K= rate constant, [A]= reactant concentration, [C]= catalyst concentration,
[AC]= reactant catalyst concentration], [B]= second reactant concentration
12
72
0=d[AC]/dt = K1 [A][C]-K1 [AC]-K2 [AC][B]
[AC]= K1 [A][C]/K1+K2 [B] then R= K2K1[A][C][B]/ k1+K2[B]
In the esterication reaction, we can suppose that ,K2
0
And also the catalyst concentration is constant that can be supposed to be [C]=1
Then, R= K1 [A].
An example of Si/Zr=20 for reaction rate calculation: eaquation comes from
[ ]
]
∫[
[ ]/
= -Kα ∫ dt, then we can get
Ln [A]t – Ln [A]0= -Kαt so, –Ln [A]t = Kαt –Ln [A0]. The diagram of
–Ln[A]t versus t (time) is linear, and the slpoe is Kα.
The [A]t is equal to (1- yield of ester).
The yield of ester can be obtained from GC, or conversion of benzyl alcohol gives
ester, because the amount of conversion of benzyl alcohol is equal to amount of
benzyl acetate produced that can be obtained from the calibration peak area of
benzylalcohol/peak area of DMSO versus acetic acid concentration), we can
calculate the reaction rate according to this: R=K [A] or
R= [A]-[A] 0/t.
As an example, we can see the below data.
73
APPENDIX B. Reaction rate versus acetic acid concentration
Rate x 10-3
3
4
5
8
[AA] x10-3
1
2
3
6
Rate x 10-3
1.5
2
3
6
[AA] x10-3
2
3
4
6
%Conversion = (peak area of benzyl alcohol / peak area of DMSO)- (peak area of
Acetic acid/product)
APPENDIX C. Reaction rate versus type of catalyst
Catalyst
Rate constant (ester)
Energy of activation
383K
393K
HPW/Zr-MCM-4(20)
7.8
10.5
30.56
HPW/Zr-MCM4(10)
5.5
8.1
37.36
HPW/Zr-MCM-41(5)
4.5
5.3
42.48
Blank
2.6
3.9
58
74
APPENDIX D. First order equation reaction versus time
-ln(1-yield of ester)
0
0.1
0.18
0.25
0.4
Time
0
10
20
30
60
-ln(1-yield of ester)
0
0.1
0.2
0.3
0.62
-ln (1-yield of ester)
0
time
0
10
20
30
60
time
0
0.1
10
0.28
0.4
0.85
20
30
60
-ln(l-yield of ester) versus time, that slope is rate constant K. We can calculate rate
according to R=K [A] or R=[A]/T.
Yield of ester = [benzyl acetate]/ [benzyl acetate]+[benzyl alcohol]+[acetic acid]
75
APPENDIX E. Rate versus acetic acid concentration
[AA] concentration x 10-5
2.1
3.7
5
7
Rate mol/min x 10-3
0.5
1.8
2.9
5