1. EFFECT OF PROCESS PARAMETERS ON PHYSICOCHEMICAL
CHARACTERISTICS OF ORDERED MESOPOROUS SILICA
SBA-15
By
KOH MING HOOI
Report submitted in partial fulfillment of the requirements for
the degree of Bachelor of Chemical Engineering
JUNE 2016

2. i
ACKNOWLEDGEMENTS
I would like to express my sincerest gratitude to my final year project supervisor, Dr
Azam Taufik Mohd. Din, for his generous and untiring guidance throughout my research.
Without his persistent help, this thesis would not be possible. I have learned most on
organizational and communication skills needed to meet deadlines along this research, thanks
to strings of useful advice and motivation given by Dr Azam.
Next, I would like to thank En. Ismail, Pn. Natasya, En. Masrul and En. Yushamdan for
their kind assistance in handling SEM, TEM, FTIR, XRD, BET and EDX physicochemical
analytical equipment with patience while willingly explain the working principle of these
analysis techniques in details. Much of my research effort has been eased thanks to their
willingness to share the knowledge with me.
Lastly, but most of all, I wish to thank my family and friends who have journeyed
together with me throughout this research with much love and support. It is their pillars of
encouragement that drove and propelled me further to the completion of this thesis.
Koh Ming Hooi,
JUNE 2016.

3. ii
TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS i
TABLE OF CONTENTS ii
LIST OF TABLES vi
LIST OF FIGURES viii
LIST OF SYMBOLS xii
LIST OF ABBREVIATIONS xiv
ABSTRAK xvi
ABSTRACT xvii
CHAPTER ONE : INTRODUCTION
1.0 Porous Materials 1
1.1 Mesoporous Materials 2
1.1.1 Ordered and Disordered Mesoporous Materials 2
1.2 Ordered Mesoporous Silica (OMS) 4
1.3 Santa Barbara Amorphous (SBA-15) Materials 6
1.3.1 Analogues to SBA-15 6
1.4 Pore Size and Morphological Control of SBA-15 8
1.5 Problem Statements 10
1.6 Research Objectives 11
1.7 Scope of the Research 12

4. iii
CHAPTER TWO: LITERATURE REVIEW
2.0 Synthesis of Ordered Mesoporous Silica (OMS) 13
2.0.1 Sol-Gel Chemistry in Synthesis of Ordered Mesoporous Silica 16
2.0.2 Hydrolysis/Condensation of Inorganic Precursor in Sol-Gel Route 17
2.1 Synthesis of SBA-15 by Precipitation Method 18
2.1.1 Self-Assembly of Surfactant 19
2.1.2 Addition of Silica Precursor and Ageing 20
2.1.3 Surfactant Removal by Washing and Calcination 21
2.2 Development of Morphology in Ordered Mesoporous Materials (OMM) 22
2.3 Influence of Synthesis Parameters on Characteristics of SBA-15 23
2.3.1 Types of Precursor 23
2.3.2 pH / Acid Concentration of Reaction Media 24
2.3.3 Ageing temperature and ageing time 27
2.3.4 Water to TEOS molar ratio, R 29
2.4 Physicochemical Characterization of SBA-15 31
2.4.1 Transmission electron microscopy (TEM) 31
2.4.2 Scanning electron microscopy (SEM) with energy dispersive x-ray
spectroscopy (EDX)
32
2.4.3 Surface area analysis by Brunauer-Emmet-Teller (BET) method 36
2.4.4 X-ray Powder Diffraction (XRD) 42
2.4.5 Fourier Transform Infrared (FTIR) Spectrophotometer 45
2.5 Industrial/ Commercial Application 47

5. iv
CHAPTER THREE: MATERIALS AND METHODS
3.0 List of Chemicals Required 48
3.1 List of Equipment Required 48
3.2 SBA-15 Synthesis By Conventional Precipitation
a) Effect of hydrochloric acid (HCl) concentration
b) Effect of ageing temperature
c) Effect of ageing time
49
49
50
50
3.3 Methylene blue Adsorption Test 50
3.4 Physicochemical Characterization
a) Surface area analysis
b) Scanning electron microscopy (SEM) with energy dispersive x-ray
spectroscopy (EDX)
c) Transmission electron microscopy (TEM)
d) Fourier Transform Infrared (FTIR) spectrophotometer
e) X-ray Powder Diffraction (XRD)
51
51
52
52
52
53
3.5 Safety Precaution 53
CHAPTER FOUR : RESULTS AND DISCUSSION
4.1 Physicochemical characteristics of SBA-15 54
4.1.1 Scanning Electron Microscopy (SEM)
(a) Effect of HCl Concentration
(b) Effect of Ageing Temperature
(c) Effect of Ageing Time
54
54
56
57
4.1.2 Energy Dispersive X-ray Spectroscopy (EDX) 59
4.1.3 Transmission Electron Microscopy (TEM) 60

6. v
4.1.4 X-ray Powder Diffraction (XRD) 60
4.1.5 Fourier Transform Infrared (FTIR) spectrophotometer 62
4.1.6 Surface Area Analysis by Brunauer-Emmet-Teller (BET) model 63
4.2 Methylene blue (MB) batch adsorption performance of SBA-15 Samples 65
CHAPTER FIVE: CONCLUSIONS AND RECOMMENDATIONS
5.1 Conclusion 69
5.2 Recommendations 70
REFERENCES 71

7. vi
LIST OF TABLES
Page
Table 2.1 Literature comparison on reactant amount ratio for SBA-
15 synthesis.
30
Table 2.2 Literature comparison on BET surface area of SBA-15
samples.
40
Table 3.1 List of Chemicals Required. 48
Table 3.2 List of Equipment Required. 48
Table 4.1 Surface area analysis and pore size of SBA-15 sample
(Run 2).
64
Table 4.2 Methylene blue batch adsorption performance of SBA-15
samples.
65
Table 4.3 Comparison on methylene blue batch adsorption
experimental condition.
67

8. vii
LIST OF FIGURES
Page
Figure 1.1 TEM images of aluminium oxide and Vycor glass. 3
Figure 1.2 TEM images of ordered mesoporous carbon, FDU-15 and
carbon nanotube.
4
Figure 1.3 TEM images of SBA-15. 6
Figure 1.4 Schematic illustration of (a) parallel channels of SBA-15 and
(b) nanocages of SBA-16.
7
Figure 1.5 TEM images of SBA-15 (A) and PHTS (B). 8
Figure 1.6 SEM images of examples of different SBA-15 morphologies
(spheres, fibers and rods from left to right).
9
Figure 2.1 Synthetic pathways to ordered mesoporous silica materials —
direct precipitation, true liquid-crystal template (TLCT),
evaporation-induced self-assembly (EISA) and exotemplate
(hard template).
14
Figure 2.2 Formation of mesoporous structures a) via cooperative self-
assembly, b) via true liquid-crystal templating route.
14
Figure 2.3 Molecular structure of tetraethoxysilane (TEOS) and
tetramethoxysilane (TMOS).
17
Figure 2.4 Sol-gel general reaction scheme showing (a) hydrolysis of
alkoxysilane, (b, c) water and alcohol condensation
respectively.
18

9. viii
Figure 2.5 Stepwise formation of mesoporous material, SBA-15. 19
Figure 2.6 Pluronic P123 as surfactant and TEOS as silica source (a), with
micelle formation of Pluronic P123 and TEOS (b).
20
Figure 2.7 Colloidal phase separation mechanism. 22
Figure 2.8 SEM images of SBA-15 synthesised by Jin et al. at different
acidity using HNO3 at (a) pH 1.02, (b) pH 1.43, (c) pH 2.05 and
(d) pH 2.61.
25
Figure 2.9 SBA-15 synthesised without glycerol at HCl concentration of
(a) 2.5 M, (b) 2.0 M, (c) 1.0 M and (d) 0.5 M.
27
Figure 2.10 SEM images of SBA-15 synthesised with glycerol at HCl
concentration of (a) 2.5 M, (b) 2.0 M, (c) 1.0 M and (d) 0.5 M.
27
Figure 2.11 Phase diagram of Pluronic P123 in water. 28
Figure 2.12 TEM of SBA-15 sample prepared at reaction temperature 35
C followed by hydrothermal treatment at 80 C for 48h.
Electron beam parallel (a) and perpendicular (b) to main axis
pores.
32
Figure 2.13 EDX spectra of SBA-15. 33
Figure 2.14 SEM of SBA-15 synthesised at HCl concentration of (a)
1.37M, (b) 1.68M, (c) 1.75M, (d) 1.83M, (e) 1.90M and (f)
1.98M.
34
Figure 2.15 SBA-15 samples synthesised: (a) using TEOS as silica source,
(b) using DMF and (c) THF as co-solvents, (d) synthesised in
35

10. ix
Na2SO4 solution and (e) in MgSO4 solution and (f) synthesised
using CTAB as co-surfactant.
Figure 2.16 SEM and TEM images of SBA-15 samples synthesised by
varying reaction temperature, stirring rate and surfactant
species.
36
Figure 2.17 Typical adsorption isotherms: Type I (microporous materials),
Type II (non-porous materials), and Type IV (mesoporous
materials).
37
Figure 2.18 N2 adsorption-desorption isotherm of SBA-15 prepared at
reaction temperature 35C and hydrothermal treatment at 80 C
for 48 h. Pore size distribution is shown inset.
39
Figure 2.19 BET surface area YS
a
, micropore area Y
b
, and total pore
volume YV
c
.
40
Figure 2.20 BJH model pore size distribution curve of SBA-15 samples
synthesised at (a) 1.87 M HNO3, (b) 1.87 M HCl, (c) 1.73 M
HNO3 and (d) 2.61 M HNO3.
41
Figure 2.21 Pore size distribution for SBA-15 sample synthesised (a) using
TMOS as silica source and (b) using DMF as co-solvent.
42
Figure 2.22 Schematic illustration of diffraction according to Bragg's Law. 42
Figure 2.23 Schematic illustration of hexagonal phase with characteristic d-
spacing and unit cell parameter, a.
43

11. x
Figure 2.24 XRD patterns of (a) pure SBA-15 and (b) SBA-15
functionalized with a fluorescent chromophore, 5-methoxy-2-
thiazoles.
44
Figure 2.25 XRD pattern of SBA-15 sample prepared at 35 C. 44
Figure 2.26 FTIR spectra of (a) pure SBA-15 and (b) SBA-15
functionalized with 5-methoxy-2-thiazoles.
45
Figure 2.27 FTIR spectra of SBA-15 and SBA-15 functionalized with
rhodium, Rh/SBA-15.
46
Figure 2.28 FTIR spectra of as-synthesised and calcined SBA-15. 46
Figure 4.1 SEM images of SBA-15 synthesised at HCl concentration of:
a) 2.5 M, b) 2.0 M, c) 1.5 M, (d, e, f) 1.0 M.
54
Figure 4.2 SBA-15 prepared at ageing temperature: a) 40 C, b) 50 C, c)
60 C, d) 70 C.
56
Figure 4.3 SBA-15 synthesised at ageing time: a) 48 h, b) 36 h, c) 24 h, d)
12 h.
58
Figure 4.4 EDX spectra of Run 2 SBA-15 sample. 59
Figure 4.5 TEM images of SBA-15 Run 2 sample at: a) perpendicular to
pore channels and b) parallel to pore channels.
60
Figure 4.6 Small angle XRD of SBA-15 sample (Run 2). 60
Figure 4.7 Wide angle XRD of SBA-15 sample (Run 2). 61

12. xi
Figure 4.8 FTIR spectra of SBA-15 sample (Run 2). 62
Figure 4.9 N2 BET adsorption isotherm (Top) and BJH adsorption model
dV/dlog(D) pore size distribution plot (bottom) of SBA-15
sample (Run 2).
63

13. xii
LIST OF SYMBOLS
Symbol Description Unit
BET surface area analysis
P equilibrium pressure of adsorbate at temperature
of adsorption
atm, Pa
Po saturation pressure of adsorbate at temperature of
adsorption
atm, Pa
v adsorbed gas quantity m3
vm monolayer adsorbed gas quantity m3
c BET constant -
SBET specific surface area m2
/g
Stotal total surface area m2
N Avogadro’s number -
s molecular cross-sectional area occupied by the
adsorbate molecule in the complete monolayer
m2
a mass of the adsorbent or solid sample g
X-ray powder diffraction (XRD)
n order of diffraction -

14. xiii
 Wavelength nm
d distance between lattice planes nm
d100 d-spacing of (100) nm
 angle of the incoming light 
a Unit cell parameter nm
t Wall thickness nm
D Mesopore size nm
Methylene blue adsorption test
qt mg of methylene blue adsorbed per gram of
adsorbent
mg/g
CO Initial dye concentration at time = 0 mg/mL
Ct Final dye concentration at time = t (one day) mg/mL
V Volume of solution mL
W Weight of dry adsorbent used (gram) gram

15. xiv
LIST OF ABBREVIATIONS
OMM Ordered mesoporous materials
OMS Ordered mesoporous silica
TEOS Tetraethyl orthosilicate
TMOS Tetramethyl orthosilicate
HCl Hydrochloric acid
PEO Polyethylene oxide
PPO Polypropylene oxide
SBA Santa Barbara Amorphous
FSM Folded Sheet Materials
HMS Hexagonal Mesoporous Silica
MSU Michigan State University
MCM Mobil Composition of Matter
CTAB Cetyltrimethylammonium bromide
DMF N,N-dimethylformamide
TLCT True liquid-crystal template
CSA Cooperative self-assembly
EISA Evaporation-induced self-assembly
CPSM Colloidal phase separation mechanism

16. xv
PHTS Plugged hexagonal template silica
TEM Transmission electron microscopy
SEM Scanning electron microscopy
EDX Energy dispersive x-ray spectroscopy
BET Brunauer, Emmett and Teller
XRD X-ray powder diffraction
FTIR Fourier Transform Infrared spectrophotometer
HPLC High performance liquid chromatography
MB Methylene blue
UV-Vis Ultraviolet-visible spectrophotometer

17. xvi
KESAN PARAMETER PROSES KE ATAS SIFAT FIZIKOKIMIA
SBA-15 SILIKA MESOPORE TERSUSUN
ABSTRAK
Kesan parameter proses atas sifat fizikokimia SBA-15 silika mesopore tersusun telah
disiasat. SBA-15 telah disintesiskan melalui kaedah templat halus menggunakan surfaktan
tanpa-ionik Pluronic P123 dan tetraetilena orthosilikat TEOS sebagai sumber silika serta asid
hidroklorik (HCl) sebagai pemangkin. Parameter proses yang diubah termasuklah konsentrasi
HCl (2.5 – 1.0 M), suhu penuaan (40 – 70 C) dan masa penuaan (12 – 48 jam). Sampel SBA-
15 yang disintesis telah diuji dengan eksperimen penjerapan metilena biru (MB) dan kondisi
proses yang menghasilkan SBA-15 dengan prestasi terbaik adalah 2.0 M HCl, pada 40 C dan
48 jam penuaan. Kecekapan penyingkiran metilena biru yang terbaik adalah sebanyak 19.63%.
Pencirian fizikokimia dilakukan melalui teknik pengimbasan elektron mikroskopi (SEM)
dengan tenaga serakan x-ray spektroskopi (EDX), transmisi elektron mikroskopi (TEM), BET
analisis kawasan permukaan, perubahan inframerah spektrofotometer Fourier (FTIR), dan
pembelauan x-ray (XRD). SBA-15 dengan prestasi penjerapan MB terbaik menunjukkan
dinding mesopore setebal 6.98 nm, saiz liang sebesar 4.06 nm (cawangan penjerapan BJH),
kawasan permukaan sebanyak 364.71 m2
/g dengan kawasan permukaan liang mikro sekecil
42.29 m2
/g. Turut diperhatikan dalam kajian ini adalah morfologi SBA-15 yang bertukar dari
bentuk rod panjang ke rod pendek sehingga kemunculan campuran bentuk rod dan sfera apabila
kepekatan HCl dikurangkan daripada 2.5 M ke 1.0 M. Apabila suhu penuaan dinaikkan
daripada 40 ke 70 C, SBA-15 berubah daripada struktur rod pendek ke struktur serat nipis.
Masa penuaan yang diubah daripada 12 ke 48 jam tidak menunjukkan apa-apa kesan ke atas
perubahan morfologi SBA-15.

18. xvii
EFFECT OF PROCESS PARAMETERS ON PHYSICOCHEMICAL
CHARACTERISTICS OF ORDERED MESOPOROUS SILICA SBA-15
ABSTRACT
Effect of process parameters on physicochemical characteristics of ordered mesoporous
silica SBA-15 has been investigated. SBA-15 was synthesised by soft-templating method using
non-ionic surfactant Pluronic P123 and tetraethyl orthosilicate TEOS as silica source with
aqueous hydrochloric acid (HCl) as catalyst. Process parameters varied include HCl
concentration (2.5 – 1.0 M), ageing temperature (40 – 70 C) and ageing time (12 – 48 hours).
The as-synthesised SBA-15 samples were tested with methylene blue (MB) batch adsorption
experiments and the process conditions which produces SBA-15 with best adsorption
performance is 2.0 M HCl, with ageing carried out at 40 C for 48 hours, with methylene blue
removal efficiency reported at 19.63%. Physicochemical characterisation of SBA-15 was
carried out using scanning electron microscopy (SEM) with energy dispersive x-ray
spectroscopy (EDX), transmission electron microscopy (TEM), BET surface area analysis,
Fourier transform infrared spectrophotometer (FTIR) and x-ray diffraction (XRD) techniques.
SBA-15 with the best MB adsorption capacity prepared exhibits thick mesoporous walls of
6.98 nm, pore size of 4.06 nm (BJH-adsorption branch), BET surface area of 364.71 m2
/g with
small micropore surface area of 42.29 m2
/g. It was observed that the surface morphology of
SBA-15 transformed from longer rods to shorter rods until a mixture of rods and spheres are
present upon reducing HCl concentration from 2.5 M to 1.0 M. Upon increasing ageing
temperature from 40 to 70 C, SBA-15 changed from short rods to thin-fibre like structure.
Ageing time varied from 12 to 48 hours did not show any significant effect on morphological
changes in SBA-15.

19. 1
CHAPTER ONE
INTRODUCTION
1.0 Porous Materials
Porous materials are generally defined as a continuous and solid network material filled
with voids (eg: channels/interstices). A material is thus considered porous if its voids can be
filled with gases. As for nanoporous materials, pore diameters are of the range 1 – 100 nm (Pal
and Bhaumik, 2013). In the past few decades, demand on usage of advanced structural
materials has led to abundance of research carried out on porous solids such as porous carbon,
synthetic silicate zeolites, mesoporous silicates and ordered porous metal oxides (Mejia, 2013)
in the field of catalysis and pollutants removal.
These pores can be classified as closed and open pores, based on pore accessibility to
surroundings. Materials containing closed pores are mainly used for thermal and sonic
insulation since they are completely isolated from their surroundings. In contrast, materials
with open pores have connectivity in between the pores which makes these materials suitable
for adsorption, filters, catalysis, etc. Materials with high open porosity normally have a large
available surface area compared to materials with no or closed porosity (Mejia, 2013).
Pore sizes in inorganic materials may range from nano-scale to macro-scale. According
to the International Union of Pure and Applied Chemistry (IUPAC), porous materials can be
classified into three classes based on their pore diameter (d), microporous d < 2 nm,
mesoporous 2 ≤ d ≤ 50 nm and macroporous d > 50 nm.
Microporous materials such as zeolites and metal-inorganic frameworks possess good
stability, selectivity and activity due to their crystallinity and the presence of incorporated
heteroelements in the structure. However, size limitation is a problem when it comes to large-

20. 2
molecular application using microporous materials (Buckley and Benito, 2007). This was
evident back in 1972, when Mobil Corporation experimented on converting methanol to
gasoline using microporous Zeolite ZSM-5 (Zeolite Socony Mobil). The research was to obtain
cheaper gasoline from acid-base reactions taking place within the micropores of zeolites
(Vallet-Regí, 2012). However, this approach did not work, given that zeolite pore size was too
small to enable the entry of larger organic molecules for reactions to take place. Due to its size
limitation, porous solids industry moved on to explore the possibility for mesoporous materials
to substitute the microporous zeolites.
1.1 Mesoporous Materials
Generally, mesoporous materials possess high surface area of 400 – 1000 m2
/g, large
pore volume and excellent thermal stability at 500 – 600 C (Pal and Bhaumik, 2013).
Mesoporous solids can be prepared either by soft template or hard template method, in which
organic molecules act as surfactant in soft template route, while porous solids such as porous
carbon is used in place of surfactant in hard template route (Pal and Bhaumik, 2013) . Due to
its physical characteristics mentioned, mesoporous materials are favoured in large-molecular
applications, such as cracking of heavy oil (which application of microporous zeolite is
unsuitable), polymer separation, enzyme immobilization and controlled-release of drugs
(Zhang and Wei, 2014).
1.1.1 Ordered and Disordered Mesoporous Materials
However, mesoporous materials can be ordered or disordered. The call for synthesis of
ordered mesoporous materials stems from the fact that these materials have well-defined
structural features such as pore sizes, pore shapes, pore arrangement and connectivities, which
can be precisely-controlled by its synthetic conditions (Zhang and Wei, 2014). In fact, ordered

21. 3
mesoporous materials possess better hydrothermal stability, mechanical stability and catalytic
activity in comparison to disordered mesoporous materials (Bonneviot et al., 1998).
Ordered and disordered mesoporous materials differ in terms of the order-ness of their
mesoporous channels, which can be observed from 2D images captured from transmission
electron microscopy (TEM).
Some popular examples of disordered mesoporous materials are Vycor glass, porous
glasses, aluminium oxides such as -alumina and activated carbons. Figure 1.1 as follows
shows TEM images of aluminium oxide and Vycor glass mechanical structure. From the TEM
images, it can be seen that there is very little or no order-ness structure observable for
mesopores located within disordered mesoporous materials.
Figure 1.1: TEM images of aluminium oxide (top) (Source: Sigma-Aldrich) and Vycor glass
(bottom) (Dandapat et al., 2009)
Ordered mesoporous materials on the other hand, exhibits distinct, ordered-arrays of
mesoporous channels in TEM images. Examples of ordered mesoporous materials are ordered
metal oxides, ordered mesoporous silica, ordered mesoporous carbon, carbon nanotubes (Nhut
et al., 2003) and mesoporous anodic alumina (Bruschi et al., 2014). Figure 1.2 as follows shows
the ordered arrays of mesoporous channels typically found in ordered mesoporous materials.

22. 4
Figure 1.2: TEM images of ordered mesoporous carbon, FDU-15 (top) (Supeng et al., 2010)
and carbon nanotube (bottom) (Source: NanoLab Inc.).
Thus, ordered mesoporous materials have attracted considerable attention due to their
high surface area, uniform pore size distribution and large pore size, which finds a variety of
applications in catalysis, drug delivery and optical devices. Ever since 1990s, various forms of
ordered mesoporous materials have been synthesised and further developed through extensive
research (Katiyar et al., 2006).
1.2 Ordered Mesoporous Silica (OMS)
One of the widely researched ordered mesoporous materials is ordered mesoporous silica
(OMS). Silica is preferably employed as the building block of OMS because it is inexpensive,
thermally stable, chemically inert, harmless and available in abundance in the Earth’s crust.

23. 5
Unlike traditional microporous zeolite which is synthesized using silica as a single template,
ordered mesoporous silica is formed in the presence of self-associating molecules, such as
cationic surfactant for MCM-41 synthesis and block copolymers for synthesis of SBA-15
(Rahmat et al., 2010).
To begin with, Folded Sheet Materials (FSM-16) was the first ordered mesoporous silica
developed from layered polysilicate kanemite by Kuroda in 1990. Mobil scientists later
discovered that by reacting silica with amphiphilic surfactant as structural-directing agent,
larger pores of ordered mesoporous silica could be produced. M41S family material, often
referred to as MCM material was thus produced in this manner. Examples of M41S materials
are hexagonal-MCM-41, cubic-MCM-48, and lamellar-MCM-50. Although this family of
materials offers large uniform pore structure, high specific surface area and specific pore
volumes (Vinu et al., 2006), M41S materials are limited to pore diameter to approximately 80
Å. This has limited usage of M41S in separations of large biomolecules such as proteins and
enzymes (Katiyar et al., 2006).
Discovery of M41S family material motivated further research on using different types
of structural-directing agent or templates for mesoporous material synthesis. For example,
hexagonal mesoporous silica (HMS) material was prepared using neutral amine as template.
HMS exhibits slightly disordered hexagonal structure and thicker walls, with superior thermal
stability upon calcination in air. Michigan State University (MSU-1) materials was synthesized
using polyethylene oxide (PEO) as template. It exhibits large wall thickness with considerable
amount of textural mesoporosity (Vinu et al., 2006).

24. 6
1.3 Santa Barbara Amorphous (SBA-15) Materials
Figure 1.3: TEM images of SBA-15(Huirache-Acuña et al., 2013).
It was until 1998 that Zhao et al. successfully synthesised hexagonally ordered Santa
Barbara Amorphous (SBA-15) (Figure 1.3) using amphiphilic triblock-copolymer of
poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) in highly acidic media, at a low
temperature range (35 C to 80 C). SBA-15 possess thick microporous silica pore walls (3-6
nm) which is responsible for its high hydrothermal stability compared to other OMS (Vinu et
al., 2006, Buckley and Benito, 2007). Due to the fact that SBA-15 has tunable pore size by
modification on its synthesis parameters, it is one of the favourite mesoporous materials in
modern industrial application.
1.3.1 Analogues to SBA-15
A wide variety of SBA materials reported by literatures are SBA-1 (cubic) (Che et al.,
2002), SBA-2 (3D hexagonal) (Perez-Mendoza et al., 2004), SBA-11 (cubic) (Zhao et al.,
2012), SBA-12 (3D hexagonal network) (Sakamoto et al., 2000), SBA-14 (lamellar)
(Kanellopoulos, 2011), SBA-15 (2D hexagonal network) (Zhang and Wei, 2014,
Kanellopoulos, 2011) and SBA-16 (cubic caged structure) (dos Santos et al., 2013).

25. 7
SBA-16 (see Figure 1.4 as follows) has similar formation mechanism with SBA-15. The
more hydrophobic Pluronic F127 is used instead as a surfactant to synthesise SBA-16 in acidic
media. However, both of these materials are different in morphology. SBA-15 contains parallel
pores, in 2D hexagonal arrangement, while SBA-16 contains a 3D structure of spherical body-
centered nanocages in cubic arrangement, in which each sphere is connected to another eight
neighbouring spheres (dos Santos et al., 2013).
Figure 1.4: Schematic illustration of (a) parallel channels of SBA-15 and (b) nanocages of
SBA-16 (dos Santos et al., 2013).
By increasing the silica over surfactant ratio during SBA-15 synthesis, plugged
hexagonal template silica (PHTS) is formed (Kanellopoulos, 2011) (see Figure 1.5) . PHTS is
essentially introduced as an analogue to SBA-15 due to its similar hexagonally-ordered
mesoporous structure with pore diameters similar to that of SBA-15. PHTS also contains
additional extra-microporous amorphous nanoparticles (plugs) (Kanellopoulos, 2011) in its
mesoporous channels, which are created from the large excess of silica source (Van Der Voort
et al., 2002). These ‘plugs’ act as pillars within PHTS, contributing to a better mechanical
stability compared to that of SBA-15. Upon applying higher synthesis temperature and longer
synthesis time, hydrothermal stability of PHTS can be improved further (Kanellopoulos, 2011).

26. 8
1.4 Pore Size and Morphological Control of SBA-15
A wide variety of SBA-15 morphology has been reported by literature, including rods,
fibers, spheres, gyroids, discoid-like and doughnut-like (Figure 1.6).
Spherical hollow silica with porous shell finds great usage in controlled delivery of
biomedical materials. They allow higher loading capacity to encapsulate drugs, genes or
biological molecules that are stimuli sensitive in the shell (Prokopovich, 2015). Jaroniec and
Sayari (2002) reported that spherical SBA-15 they had synthesised shows high elastic property
and mechanical stability, which is advantageous in drug-release applications. In fact, high
mechanical strength exhibited by enlarged pore diameter spherical SBA-15 finds great
application as stationary phase in high performance liquid chromatography (HPLC). Liu et al.
Figure 1.5: TEM images of SBA-15 (a) and PHTS (b)(Fei-hu et al., 2012).

27. 9
(2009) demonstrated that by adding co-surfactant cetyltrimethylammonium bromide (CTAB)
during synthesis, well ordered SBA-15 with enlarged pore diameter up to 8.5 nm can be
obtained and the functionalized SBA-15 showed excellent performance in adsorption of
aromatic compounds using ultra-high-performance liquid chromatography. Fibrous nanosilica
materials have been investigated for biomaterials design too. However, research finds limited
usage for nonporous fibrous nanosilica as drug vectors (Prokopovich, 2015).
Figure 1.6: SEM images of examples of different SBA-15 morphologies (spheres, fibers and
rods from left to right) (Kanellopoulos, 2011).
In year 2000, Stucky et al. (2000) successfully prepared rod-shaped SBA-15 in acidic
media using Pluronic P123 as surfactant and tetramethyl orthosilicate as silica source. It was
claimed that rod-shaped SBA-15 improved the adsorption of enzymes compared to spherical
particles.
Pore structure and its size also influence the adsorption capacity and molecular diffusion
through nanosilica materials. The micropores in mesoporous wall of SBA-15 are contributed
from hydrophilic PEO blocks, found in triblock copolymer used as surfactant for SBA-15
synthesis. On the other hand, internal structure of mesopores is contributed from the more
hydrophobic PPO blocks. By altering the lengths of PEO blocks, different amount of
micropores and wall thickness can be obtained, while changing the lengths of PPO blocks

28. 10
results in changing mesopore size. More hydrophobic PPO blocks present in the surfactant
results in larger pore diameter in SBA-15. Besides controlling the types of surfactant used,
synthesis conditions such as temperature, pH, ageing time, addition of additives such as co-
surfactants, swelling agents and salts also allow morphological and pore size tuning of SBA-
15 synthesised, which can be suited for different applications as described previously
(Kanellopoulos, 2011).
1.5 Problem Statements
Synthesis of SBA-15 is usually carried out at 2.0 M HCl (Kruk et al., 2000, dos Santos
et al., 2013, Gandhi et al., 2013, Pérez-Verdejo et al., 2014, Galarneau et al., 2001) or at 1.5 M
with the presence of co-surfactant (Katiyar et al., 2006). Extensive usage of such highly acidic
media poses safety, health and operational risks in industrial production scale and thus this
research investigates the possibility of attaining satisfactory physicochemical properties and
also MB adsorption performance of SBA-15 prepared at lower HCl concentration. However,
to compare with MB adsorption performance of SBA-15 prepared at high HCl concentration,
effect of hydrochloric acid concentration will be studied and compared at 1.0 M, 1.5 M, 2.0 M
and 2.5 M.
A number of research reported SBA-15 prepared at temperature range 35 C - 80 C. At
room temperature, only amorphous silica is obtained, while at high temperature (> 80 C) silica
gel is obtained (Ramalingam et al., 2013). Even though high ageing temperature is favoured
for SBA-15 synthesis, heating costs escalates if reactor temperature reaches 80 C or higher. It
is thus desirable to investigate whether if SBA-15 can be prepared at moderate ageing
temperature without compromising its desirable physicochemical properties. In this research,
the possibility of attaining satisfactory physicochemical properties and also MB adsorption

29. 11
performance of SBA-15 prepared at moderate ageing temperature of 40 C, 50 C, 60 C and
70 C will be assessed, so that an offset can be achieved between heating costs and desirable
physicochemical characteristics of SBA-15.
Due to low synthesis temperature employed, SBA-15 production by precipitation
method involves longer ageing time for precursor hydrolysis and condensation to complete. In
this case, a range of ageing time at 12, 24, 36 and 48 hours will be employed, to assess the
possibility for ordered mesoporous SBA-15 to be attained at shorter ageing time. Generally, a
convenient ageing time is either 24 hours (Gandhi et al., 2013, Galarneau et al., 2001) or 48
hours (Mohd Din et al., 2015). Optimal ageing time will be determined in this research based
on the physicochemical properties and adsorption performance of SBA-15 samples prepared.
The research will be carried out using one-factor-at-a-time (OFAT) technique, by first varying
HCl concentration, then ageing temperature, followed by ageing time. Methylene blue
adsorption test will be used to assess the adsorption performance of SBA-15 prepared.
1.6 Research Objectives
 To synthesise SBA-15 and investigate the effects of varying process parameters such
as ageing temperature, ageing time and HCl concentration on physicochemical
properties of SBA-15.
 To investigate physicochemical characteristics of SBA-15 using Brunauer, Emmett and
Teller (BET) surface area analysis, Fourier transform infrared spectrophotomer (FTIR),
scanning electron microscopy (SEM) with energy dispersive x-ray spectrophotometer
(EDX), x-ray diffraction (XRD) and transmission electron microscopy (TEM)
characterizations.

30. 12
 To determine the adsorption performance of SBA-15 samples using methylene blue
batch adsorption as performance indicator.
1.7 Scope of the Research
The scope of this research only covers on varying process parameters including ageing
temperature (40, 50, 60, 70 C), ageing time (12, 24, 36, 48 hours) and HCl concentration (1.0,
1.5, 2.0, 2.5 M), using sol-gel technique to prepare SBA-15. Functionalization of as-
synthesised SBA-15 would not be carried out in this research. Variation on other parameters
such as surfactant and precursor concentration ratio, types of surfactant/precursor, calcination
temperature etc, will not be carried out and these parameters shall remain constant whenever
applicable. Conventional precipitation is employed in synthesis of SBA-15. No further gel
production of SBA-15 and hydrothermal treatment for SBA-15 is carried out in this research.
Preparation of other types of mesoporous materials, such as MCM, FSM, etc. will not be carried
out as well. Comparisons and discussions on their physicochemical properties with SBA-15
sample synthesised in this experiment, will be reported based on relevant literature data though.

31. 13
CHAPTER TWO:
LITERATURE REVIEW
2.0 Synthesis of Ordered Mesoporous Silica (OMS)
At the beginning of this work, two methods of surfactant-assisted synthesis of OMS
materials have been mentioned: soft templating and hard templating method. Soft templating
or endotemplate method simply refers to utilising organic molecules or surfactant which are
amphiphilic in nature, that self-assembles in liquid media, so that inorganic precursor can
arrange themselves in ordered array around the self-assembled surfactant to form an inorganic-
organic composite solid (Pal and Bhaumik, 2013). Template remains within the inorganic
precursor in this manner, hence the name endotemplate (‘endo’ means within) method.
Examples of mesoporous materials synthesised by soft templating include SBA-15 and MCM-
41.
Further study shows there are few mechanisms for inorganic-surfactant molecule to
occur by endotemplate route (Figure 2.1): direct precipitation which involves cooperative self-
assembly (CSA) of surfactant and inorganic precursor (Pitchumani et al., 2006), true liquid-
crystal template (TLCT) (Pal and Bhaumik, 2013), and evaporation-induced self-assembly
(EISA) mechanisms (Kim et al., 2015).

32. 14
Figure 2.1: Synthetic pathways to ordered mesoporous silica materials—direct precipitation,
true liquid-crystal template (TLCT), evaporation-induced self-assembly (EISA) and
exotemplate (hard template) (Pitchumani et al., 2006).
Figure 2.2: Formation of mesoporous structures a) via cooperative self-assembly, b) via true
liquid-crystal templating route (Pal and Bhaumik, 2013).

33. 15
In this context, organic template is referring to the surfactant. In direct precipitation
(Figure 2.1), cooperative self-assembly (Figure 2.2) takes place, in which simultaneous
aggregation of organic template and inorganic precursor occurs via hydrolysis and
condensation reaction of inorganic precursor. The simultaneous aggregation can only take
place if both organic template and inorganic precursor are present at the same time. Later,
liquid–crystal phase with hexagonal, cubic, or laminar arrangement containing both the organic
template and inorganic precursor will be developed. This is followed by template removal to
obtain the desired ordered mesoporous materials (Kim et al., 2015).
In true liquid-crystal template (TLCT) mechanism (Figure 2.2), the surfactant
concentration present in solvent is so high that a liquid-crystal phase of surfactant with well-
defined geometry can be formed at the start of the synthesis, without the presence of the
inorganic precursor (Seddon and Raimondi, 2000). This is followed by infiltration of inorganic
precursor into the liquid-crystal phase of surfactant and subsequently inorganic framework is
obtained around the surfactant (Kim et al., 2015). Excess surfactant removal is required to
obtain porosity in the materials. TLCT mechanism is considered “true” because well-defined
dimensions of liquid-crystal phase of surfactant is allowed to occur at beginning of the
synthesis before adding inorganic precursor (Seddon and Raimondi, 2000). In cooperative self-
assembly route alone, proper formation of “true” surfactant liquid-crystal phase does not take
place due to simultaneous presence of inorganic precursor. Only liquid-crystal phase of both
surfactant and inorganic precursor are formed afterwards.
When sol-gel chemistry is combined with dip or spin coating, using organic template (eg:
low molecular weight surfactant or amphiphilic block copolymer), evaporation-induced self-
assembly (EISA) mechanism is employed (Kim et al., 2015) (see Figure 2.1).

34. 16
Hard templating or exotemplate refers to using porous solids such as silica and carbon
as template instead of organic molecule surfactant. Inorganic precursor fills up the hollow
space in porous template and this template remains outside surrounding the precursor material,
thus the name exotemplate (‘exo’ means outside) method. The exotemplate is then removed by
NaOH solution or by high temperature treatment and incorporated material is obtained (Pal and
Bhaumik, 2013). This method is frequently applied in synthesis of ordered mesoporous carbon,
such as CMK-1 (Pal and Bhaumik, 2013, Mohd Din et al., 2015). In this research, soft
templating approach is adopted for SBA-15 synthesis, in which non-ionic surfactant Pluronic
P123 and tetraethyl orthosilicate (TEOS) as silica source are employed.
2.0.1 Sol-Gel Chemistry in Synthesis of Ordered Mesoporous Silica
In order to understand the hydrolysis and condensation reaction of inorganic precursor
involved in precipitation route, a basic introduction to sol-gel chemistry would be discussed as
follows.
“Sol” is defined as colloidal particles or molecules which are suspended or dispersed in
a liquid solution. When the sol is mixed with another liquid, which causes formation of a
continuous three dimensional network, “gel” is formed. A gel is also defined as a rigid non-
fluid mass, usually made up of a continuous network including a continuous liquid phase. Thus,
sol-gel reactions involve hydrolysis and condensation reactions of inorganic alkoxide
monomers to develop colloidal particles (sol) and consequently convert them into a continuous
network (gel) (Othman, 2012). The only similarity between precipitation and sol-gel route is
the need for hydrolysis and condensation of inorganic precursor taking place, so that ordered
arrangement of the silica source can occur. In precipitation method, precipitate is obtained as
end product instead of a fully formed gel network as stated in sol-gel route (Kim et al., 2015).

35. 17
Figure 2.3: Molecular structure of tetraethoxysilane (TEOS) (left) and tetramethoxysilane
(TMOS) (right).
Most commonly used inorganic alkoxide as the precursor is metal alkoxide. Metal
alkoxide is composed of a metal or metalloid element bound to various reactive ligands. Due
to their ease of hydrolysis in the presence of water, metal alkoxide is the most common reagent
used for this purpose. Alkoxysilanes, such as tetramethoxysilane (TMOS) and
tetraethoxysilane (TEOS) (Figure 2.3), are largely used for the production of silica gels.
Aluminates, titanates, and zirconates, however, are usually used for the synthesis of alumina,
titania, and zirconia gels respectively (Othman, 2012).
2.0.2 Hydrolysis/Condensation of Inorganic Precursor in Sol-Gel Route
Referring to Figure 2.4, the hydrolysis step in Equation a) generates a silanol group (Si–
OH). Catalyst is required for hydrolysis to be carried out and hydrolysis rate depends on
solution pH in which the reaction is carried out (Othman, 2012).
In condensation step, the silanol group condense with either an alkoxide (Equation c))
or another silanol group (Equation b)) to build a strong siloxane linkage (Si–O–Si) with the
loss of either an alcohol (ROH) or a water molecule. As the number of Si–O–Si linkage
increases, the siloxane particles aggregate into a sol, which disperses in the solution into small
silicate clusters. Condensation of these silicate clusters leads to the formation of a network (a
gel). It should be noted that hydrolysis and condensation reactions go on concurrently, so that

36. 18
the full hydrolysis of tetra-alkoxysilane to Si(OH)4 does not necessarily occur before
condensation reactions begin (Othman, 2012).
Figure 2.4: Sol-gel general reaction scheme showing (a) hydrolysis of alkoxysilane, (b, c)
water and alcohol condensation respectively (Othman, 2012).
2.1 Synthesis of SBA-15 by Precipitation Method
In this research, synthesis of SBA-15 by conventional precipitation is adopted. As
mentioned earlier, in precipitation method, precipitates of SBA-15 is obtained instead of gel,
as produced in sol-gel route.
A slight difference between conventional and direct precipitation is the sequence of
adding surfactant and inorganic precursor. In this research, surfactant is added first for liquid-
crystal phase of surfactant to be formed in hydrochloric acid (HCl) solution, before adding the
inorganic silica precursor. Thus, true liquid-crystal template mechanism may take place. On
the other hand, both surfactant and inorganic silica precursor are added together at the start of
a)
c)
b)

37. 19
the synthesis for direct precipitation route. Thus, only cooperative self-assembly of surfactant
and inorganic precursor is possible to occur. Nevertheless, both precipitation methods involve
hydrolysis and condensation of inorganic precursor onto the surfactant micelles. Detailed
explanation on synthesis of SBA-15 in this research is elucidated as follows:
2.1.1 Self-Assembly of Surfactant
Three important stages in synthesising the SBA-15 begins with self-assembly of
surfactant (Figure 2.5) occurring through TLCT mechanism (Pal and Bhaumik, 2013), in which
excess amount of surfactant is present to form liquid-crystal phase in solvent media, under the
right temperature and pH. When critical micelle concentration of surfactant, eg: 0.03 wt% at
25 C for Pluronic P123, P123 is soluble in dilute solution in water (Giaquinto, 2012). Micelle
formation of the surfactant begins, with hydrophobic (PPO) core surrounded by hydrophilic
(PEO) chains, forming a corona around the core (Mejia, 2013) (Figure 2.6).
Figure 2.5: Stepwise formation of mesoporous material, SBA-15 (Pal and Bhaumik, 2013).

38. 20
Figure 2.6: Pluronic P123 as surfactant and TEOS as silica source (a), with micelle
formation of Pluronic P123 and TEOS (b) (Mejia, 2013).
2.1.2 Addition of Silica Precursor and Ageing
In second stage, silica source such as TEOS, TMOS or inorganic sodium silicate is added
to the surfactant solution (Figure 2.5) where it hydrolyses under acidic condition and silicate
oligomer sol is formed. Further condensation of these oligomers on surfactant micelle takes
place via cooperative self-assembly (CSA) route. Aggregation of these inorganic-organic
hybrid results in formation of precipitate. The precipitate formed will be left for ageing process
and also optionally, hydrothermal treatment to allow further condensation, solidification and
reorganization of the material to ordered structure (Pal and Bhaumik, 2013).
Ageing process takes place at a lower temperature, usually at around 35-50 C, with or
without stirring (Mohd Din et al., 2015, dos Santos et al., 2013). However, ageing is a slow
process (Bogatu et al., 2011). As a result, usually hydrothermal treatment is employed, in
which reaction temperature is increased significantly (dos Santos et al., 2013). This is to allow
reactive silanols remaining in the system to undergo further condensation. Heating at relatively
high temperatures (100-500 °C) is carried out to accelerate this phase, removes the organic
species and leads to formation of covalent siloxane bonds, a product of silanol condensation

39. 21
(Bogatu et al., 2011) (refer back to Figure 2.4). This treatment increases the mesopore size and
reduces microporosity by reducing the shrinkage of silica walls upon calcination. Hence,
hydrothermal treatment can be used for synthesising mesoporous silica with larger pore
diameters, useful for applications such as functionalization and metal incorporation into formed
channels (Mejia, 2013).
2.1.3 Surfactant Removal by Washing and Calcination
Final step of SBA-15 synthesis is surfactant removal. Removal of surfactants can be
done by using calcination or chemical removal. In this research, both methods are employed.
Normally, the calcination is carried out in a muffle furnace, by increasing the temperature from
room temperature to 500 °C for 6 hours in absence or limited supply of air to decompose excess
surfactant and remove volatile fractions such as water and alcohol produced from silanol
condensation (Yamada et al., 2002). In this research, calcination at 550C for 4 hours was
carried out.
The removal of surfactants by calcination produces polymerized and cross-linked silica
(Figure 2.5) with narrow pore size distributions and highly ordered mesostructures (Yamada et
al., 2002). Chemicals such as ethanol and deionized water can also be used to wash away
surfactant remains. During washing, only the surfactant is removed and pore size distribution
of the mesoporous silica remains the same (Mejia, 2013, Thielemann et al., 2011). According
to Thielemann et al. they discovered that combined washing using ethanol and water increases
the surface area of SBA-15, however, when multiple washing with plenty of solvent, eg: SBA-
15 washed with water, washed with ethanol, then washed again with water, narrowing or
widening at certain pore sections in SBA-15 is observed from nitrogen desorption data.
Thielemann et al. (2011) attributed this to hydrolysis and re-condensation reactions of the silica

40. 22
in the pore wall during washing with plenty of solvent. Their research ultimately proved that
controlled washing with reduced quantities of solvent is the optimum condition for obtaining
increased surface area and a narrow pore size distribution in SBA-15.
2.2 Development of Morphology in Ordered Mesoporous Materials (OMM)
In 2004, Zhao et al. proposed a mechanism called colloidal phase separation mechanism
(CPSM) to explain stages of formation of ordered mesoporous materials, and how these stages
affect development of the final morphology of OMM such as SBA-15 (Figure 2.7). By overall,
CPSM works similarly to synthesis route of OMM as explained previously, in which
cooperative assembly of surfactant/inorganic silica precursor and formation of liquid-crystal
phase comes into play.
A slight difference is that, CPSM theory suggests that after liquid-crystal phase
completes, phase separation of this liquid-crystal phase from the solution begins and precipitate
can be observed from solution. After phase separation, morphologies of ordered mesoporous
materials are developed and influenced by the competition mainly between the free energy of
mesostructure self-assembly, ∆G and the colloidal surface free energy, F (Figure 2.7)
(Chengzhong et al., 2004).
Figure 2.7: Colloidal phase separation mechanism (Chengzhong et al., 2004).

41. 23
Chengzhong et al. (2004) further explained that when phase separation stage occurs early,
∆G is dominant and the macrostructure of mesoporous materials is developed together with the
formation of mesostructure; therefore, mesoporous materials with crystal-like morphologies
can be generated. But if the phase separation occurs slowly, colloidal surface energy, F, will
have dominant influence upon the macrostructure. The morphology is now developed by effect
of surface energy during transformation from liquid-crystal phase to solid phase. With the
increasing influence of F, a morphology with large curvature will be formed in order to
minimize the surface energy (eg: spherical SBA-15). Process conditions identified for leading
to slower phase separation include low acidity, low temperature and low ionic strength.
2.3 Influence of Synthesis Parameters on Characteristics of SBA-15
2.3.1 Types of Precursor
In synthesis of ordered mesoporous materials, there are two general requirements for
suitable precursor, 1) the precursors have to be soluble in reaction media, 2) The precursors
must be reactive enough to participate in condensation reaction (Bogatu et al., 2011).
Common precursors that can be used include salts, oxides, hydroxides, alkoxides,
acylates and amines. Among these, alkoxides are the most commonly used. Among the
alkoxides, alkoxysilanes (known as silicon alkoxides) is commonly used, due to its gentle
reaction with water, with good homogeneity (Bogatu et al., 2011). Some popular examples of
alkoxysilane precursors for sol-gel technique are tetraethyl orthosilicate (TEOS), tetramethyl
orthosilicate (TMOS), methyl triethoxisilane (MTES), methyl trimethoxysilane (MTMS),
vinyl trimethoxysilane (VTMS), 3-aminopropyl trimethoxysilane (APS) and γ-
metacryloxypropyl trimethoxysilane (γ-MAPTS).

42. 24
According to Bogatu et al. (2011), the concentration and the type of silicon alkoxides
affect both hydrolysis and condensation rate, resulting in reactive monomers produced at
different rates. Branching and increasing chain length of precursor substituent reduces the
hydrolysis rate. However, the initial alkoxide concentration has no impact on the density of the
final material and no significant effects on the mechanical properties were reported. It has been
reported that when using TEOS as silica source, increasing TEOS concentration increases the
solution viscosity, but did not result in morphological change of the final product, provided
that other reaction parameters were constant. Concentration of silica precursor thus affects
hydrolysis and condensation rate of silica precursor, but not the morphology of mesoporous
silica.
2.3.2 pH / Acid Concentration of Reaction Media
The hydrolysis and condensation reaction of silica precursor is largely dependent on
solution pH. Under acid-catalyzed conditions, the hydrolysis kinetic is favoured instead of the
condensation (Bogatu et al., 2011, Othman, 2012). This leads to the production of more linear-
like networks with less siloxane bonds and a high concentration of silanol groups, and hence,
minimally branched polymeric species. In alkali-catalyzed reactions, condensation is faster
than hydrolysis, resulting in a highly condensed species that may agglomerate into fine
particles (Bogatu et al., 2011, Othman, 2012). TEOS is known to be less sensitive to hydrolysis,
due to silicon atom which is less electropositive. Thus, hydrolysis and condensation reaction
of alkoxysilane can be enhanced using acid or base catalyst. Hexagonal mesoporous SBA-15
is usually synthesised at highly acidic media with pH equals to or less than 1. At pH above
isoelectric point of silica (pH  2), no precipitation of silica gel occurs. At neutral pH of 7, only
amorphous silica is obtained (Ramalingam et al., 2013).

43. 25
Figure 2.8: SEM images of SBA-15 synthesised at different acidity using HNO3 at (a) pH
1.02, (b) pH 1.43, (c) pH 2.05 and (d) pH 2.61 (Zhengwei et al., 2008).
Zhengwei et al. (2008) showed that by increasing the acid concentration or reducing the
pH of reaction media to pH around 1.02, irregularly faceted, short column-like SBA-15
particles were obtained. After adjusting the pH to around 2, spherical SBA-15 particles were
obtained (see Figure 2.8 (a)). At higher pH around 2.61, the macro-spheres aggregated together
to form amorphous agglomeration.
Colloidal phase separation mechanism was used by Zhengwei et al. (2008) to explain
this phenomenon. According to them, upon approaching the isoelectric point (pH = 2) of silica,
rate of silica condensation and thus phase separation rate is slow. SBA-15 morphology will be
controlled by surface free energy, resulting in high curvature of particles such as formation of
spherical particles (see Figure 2.8 (c)). Further increasing the pH beyond silica isoelectric point,
silica condensation rate speeds up as explained previously. Competition occurs between
minimization of surface free energy and total free energy, resulting in formation of irregular
amorphous silica particles (see Figure 2.8 (d)). However at high acidity, at which pH is adjusted

44. 26
to around 1, the condensation and colloidal phase separation rate is much faster, thus irregular
faceted particles were obtained.
When SBA-15 synthesis is carried out at acidic condition, suitable acid concentration is
affected by the type of silica source used. Xiao Ying et al. (2004) compared the effect of HCl
acid concentration using three types of silica source: 1,2-bis(trimethoxysilyl)ethane (BTMSE),
1,2-bis(triethoxysilyl)ethane (BTESE) and tetraethyl orthosilicate (TEOS). All of the synthesis
were carried out using the same surfactant, Pluronic P123. They discovered that organosilica
precursor such as BTMSE and BTESE interact poorly with surfactant and condense much
faster than silica precursor such as TEOS. As hydrochloric acid act as catalyst to improve
hydrolysis and condensation rate of the precursors, high acid concentration results in rapid
condensation of organosilica framework, which results in poorly defined pores. Significantly
lowering the acid concentration proved to reduce the condensation rate of BTMSE and BTESE
and only then well-defined hexagonal pore arrangement in SBA-15 could be obtained. This is
in contrast to synthesis using TEOS, because TEOS condenses very slowly in aqueous solution
and high acid concentration is generally required to increase the condensation rate of silicate
framework (Xiao Ying et al., 2004).
In another study carried out by Wang et al. (2009), when rod-shaped SBA-15 particles
are synthesised using TEOS at different HCl concentration (2.5 M, 2.0 M, 1.0 M and 0.5 M)
without any additives, there is no significant difference in the rod-shape and length of SBA-15
synthesised at these acidic condition (Figure 2.9). However when glycerol is added in the
synthesis, increasingly well-defined SBA-15 rod particles are obtained as the acid
concentration is further reduced to 0.5 M. Wang et al. (2009) attributed this effect to the
hydrogen bonding interaction between glycerol and TEOS that form glycerol-modified silane.
These polymeric precursors are then arranged via side-by-side anchoring to form elongated rod
particles (Figure 2.10).

45. 27
Figure 2.9: SBA-15 synthesised without glycerol at HCl concentration of (a) 2.5 M, (b) 2.0
M, (c) 1.0 M and (d) 0.5 M (Wang et al., 2009).
2.3.3 Ageing temperature and ageing time
In terms of effect of temperature, there are two different temperatures to discuss about
in SBA-15 synthesis, one is reaction or synthesis temperature (when precipitation/gelation
takes place) and ageing temperature (after precipitation/gelation completes). Based on
experimental data and statistical analysis model built from full 23
factorial design at two levels,
Figure 2.10: SEM images of SBA-15 synthesised with glycerol at HCl concentration of
(a) 2.5 M, (b) 2.0 M, (c) 1.0 M and (d) 0.5 M (Wang et al., 2009).

46. 28
Klimova et al. (2006) described influences both these temperatures have on SBA-15 synthesis.
It is noteworthy that temperature at synthesis stage affects the surface area of SBA-15 more
strongly than ageing temperature. Ageing temperature on the other hand, affects micropore
area more strongly than reaction temperature.
Figure 2.11: Phase diagram of Pluronic P123 in water (Johansson, 2010).
Klimova et al. (2006) suggested that structure formation of surfactant micelles during
synthesis is much more important than the mesophase structure further developed by surfactant
micelles during ageing. This is especially true for Pluronics-type surfactant in which its micelle
size and structure strongly depends on solution temperature (see Figure 2.11). A rise of
temperature brings about a partial dehydration of the PEO blocks and reduces the volume of
the hydrophilic corona, and so decreases the surface area of the hydrophilic part of the micelle.
The corresponding decrease of the surface/volume ratio of the micelle is the driving force for
an increase of the aggregation number and the volume of each micelle, leading to an increase
of pore size (Galarneau et al., 2001). Thus synthesis temperature particularly plays a part in
affecting surface area of SBA-15 compared to its ageing temperature.
On the other hand, ageing temperature affects micropore area better because during
ageing or especially during hydrothermal treatment, the structure of the previously formed
mesophase encounters polymerization and condensation of silica species in the walls, take

47. 29
place with the formation of Si–O–Si linkages solidifying inorganic network. Such restructuring
of the silica wall results in disappearance of micropores and some increase of mesopore size
and volume (Klimova et al., 2006).
On the effects of ageing time, it is reported that although the influence of ageing time is
not as strong as ageing temperature, they observed that micropore area decreased with an
increase in ageing time (Klimova et al., 2006).
2.3.4 Water to TEOS molar ratio, R
As shown in Equation (a) in Figure 2.4, the presence of sufficient amount of water is
important for hydrolysis of alkoxysilane or silica precursor, to allow production of silanol
groups for subsequent condensation. At a fixed TEOS concentration, an increase in water
content leads to increased hydrolysis and condensation rate. McDonagh et al. (1996) proved
that in the production of silica film, a water: TEOS molar ratio (denoted as R) of at least 4 is
required for complete hydrolysis of alkoxysilane. They suggested however, that this
observation might only be true for low pH (eg: pH =1) reaction media. At higher pH, the role
of R values in hydrolysis rate of alkoxysilane might not be significant. This is due to the fact
that hydrolysis rate of silica precursor is much faster compared to condensation rate at low pH
value. Hence, higher amount of water content at acidic condition speeds up the hydrolysis of
silica precursor. Subsequent leaching testing on silica film produced by McDonagh et al. (1996)
showed that at low R-value, a matrix with a more open structure is produced, and the leaching
phenomenon was reduced when R value was increased. This may be due to incomplete
hydrolysis and, consequently, less cross-linking in the sol– gel material (Butler et al., 1998,
McDonagh et al., 1996).

48. 30
In this research, comparisons on water: TEOS molar ratio based on literature data on
synthesis of SBA-15 are made as shown in Table 2.1. In this research, a general water:TEOS
ratio of around 5.38 is adopted as commonly listed in literature data to ensure near completion
of hydrolysis of silica precursor.
Table 2.1: Literature comparison on reactant amount ratio for SBA-15 synthesis.
HCl
Concentration
(M)
HCl
Amount
Pluronic P123
Amount
TEOS
Amount
Water
Amount
Ref.
2.0 20 mL 4 g 8.40 g -
(Katiyar et al.,
2006)
2.0 50.4 mL 1.7 g 3.75 mL -
(dos Santos et
al., 2013)
2.5 450 mL 12 g 23.00 g -
(Mohd Din et
al., 2015)
2.0 60 g 5 g 8.00 g 60 g
(Gandhi et al.,
2013)
2.0 120 g 4 g 8.50 g 30 g
(Kruk et al.,
2000)
2.0 474 mL 16 g 34.4 mL -
(Pérez-Verdejo
et al., 2014)
2.0 30 g 1 g 2.10 g 15 g
(Galarneau et
al., 2001)
2.0 120 mL
4 g in 30 mL
of water
8.50 g -
(Dong et al.,
2011)
1 TEOS: 0.03 P123: 1.0 MgSO4 or Na2SO4: 65 H2O: 0.005 HCl: 40 ethanol
(By mole ratio)
(Zhao et al.,
1998)
1 TEOS: 0.017 P123: 5.7 HCl: 193 H2O (By mole ratio)
(Yamada et al.,
2002)

49. 31
2.4 Physicochemical Characterization of SBA-15
In this research, various physicochemical characterization on synthesised SBA-15 were
carried out include using transmission electron microscopy (TEM), scanning electron
microscopy (SEM), surface area analysis (BET), x-ray powder diffraction (XRD) and Fourier
Transform Infrared (FTIR) spectrophotometer .
2.4.1 Transmission electron microscopy (TEM)
With transmission electron microscopy (TEM) it is possible to resolve features in the
range of 1 Å. In a TEM a beam of electrons is transmitted through a thin sample and the
electrons are scattered in the specimen. The transmitted electrons are focused on a fluorescent
screen or CCD camera by electromagnetic coils and the image is formed. The image contrast
is caused by mass-thickness differences where thicker regions of the specimen (in this case the
silica walls) absorb or scatter more of the electrons compared to thinner regions. Furthermore,
it is possible to increase the contrast by blocking out some of the diffracted beams which will
result in an image where areas that strongly diffract the electrons, which is the silica walls, will
appear darker in the micrograph (Johansson, 2010) .
Figure 2.12 shows typical TEM images for SBA-15. Based on literature data, this SBA-
15 was synthesised using sol-gel technique at the synthesis condition as labelled. Well-ordered
2D hexagonal array of mesopores are observed when the electron beam is parallel to the main
axis of the pores. When the electron beam is perpendicular to the pore main axis, ordered
striped mesoporous channels are observed for this sample (Klimova et al., 2006). It is observed
too that the characteristic size and morphology of SBA-15 shown by TEM always involve 2D
hexagonal arrays and 1D long cylindrical channels of the mesopores

50. 32
2.4.2 Scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy
(EDX)
Scanning electron microscopy (SEM) is used to study the topography of materials and
has a resolution of ~2 nm. An electron probe is scanning over the surface of the material and
these electrons interact with the material. Secondary electrons are emitted from the surface of
the specimen and recorded. The height differences in the sample give contrast in the image. In
this research, SEM has been used to study surface morphology of the particles and the pore
direction in SBA-15 (Johansson, 2010). Used in conjunction with SEM, EDX is a chemical
microanalysis technique which detects x-rays emitted from the sample during bombardment
by an electron beam to characterize the elemental composition of the analysed volume.
Features or phases as small as 1 µm or less can be analysed. When the sample is bombarded
Figure 2.12: TEM of SBA-15 sample prepared at reaction temperature 35 C followed by
hydrothermal treatment at 80C for 48h. Electron beam parallel (a) and perpendicular (b) to
main axis pores.

51. 33
by the SEM electron beam, electrons are ejected from the atoms comprising the sample's
surface. The resulting electron vacancies are filled by electrons from a higher energy state, and
an x-ray is emitted from these high-energy state electrons to balance the energy difference
between the two electrons states. The x-ray energy is characteristic of the element from which
it was emitted, while EDX detector measures the relative abundance of emitted x-rays versus
their energy in keV (Materials Evaluation and Engineering Inc., 2014). Figure 2.13 shows an
example of EDX spectra of pure SBA-15 without functionalization. It can be observed that
only Si and O atoms are detectable, indicating the formation of pure silica.
Figure 2.13: EDX spectra of SBA-15 (Tomer et al., 2015).
Depending on synthesis condition, SEM images of SBA-15 can show various type of
morphology such as spherical, rod or fibrous shaped particles. Figure 2.14 below shows
different morphology of SBA-15 when synthesised at different HCl concentration in the
presence of heptane and ammonium fluoride (NH4F). It is observed that at lowest acid
concentration, the sample synthesised is rather amorphous in structure. However, as the acid
concentration is increased up to 1.98 M, SBA-15 particles elongated into rod shapes (Johansson,
2010).

52. 34
Figure 2.14: SEM of SBA-15 synthesised at HCl concentration of (a) 1.37M, (b) 1.68M, (c)
1.75M, (d) 1.83M, (e) 1.90M and (f) 1.98M (Johansson et al., 2011).
It has been observed that rod-shaped particles of SBA-15 are commonly observed when
synthesis is carried out without presence of co-solvent or co-surfactant. In Figure 2.15 as
follows, when synthesis is carried out in the presence of co-solvent or co-surfactant, gyroid-
shaped SBA-15 is attained using Na2SO4 as co-surfactant; large spherical SBA-15 is attained
using cetyltrimethylammonium bromide (CTAB) as co-surfactant; doughnut-like SBA-15 is
synthesised using N,N-dimethylformamide (DMF) as co-solvent.

53. 35
Figure 2.15: SBA-15 samples synthesised: (a) using TEOS as silica source, (b) using DMF
and (c) THF as co-solvents, (d) synthesised in Na2SO4 solution and (e) in MgSO4 solution
and (f) synthesised using CTAB as co-surfactant (Dongyuan et al., 2000).
Hyung Ik et al. (2010) has shown that different surface morphology of SBA-15 (Figure
2.16) can be obtained when varying the reaction temperature, stirring rate and surfactant
species in the process. However, by further observation, TEM images of the samples
synthesised shows similar characteristic 1D ordered mesoporous channels in all of the samples
synthesised, regardless of the particle morphology obtained.

54. 36
Figure 2.16: SEM and TEM images of SBA-15 samples synthesised by varying reaction
temperature, stirring rate and surfactant species (Hyung Ik et al., 2010).
2.4.3 Surface area analysis by Brunauer-Emmet-Teller (BET) method
The most common procedure to determine the surface area of a porous material is by
Brunauer-Emmet-Teller (BET) method. This method is an extension to Langmuir theory,
which is monolayer adsorption to multilayer adsorption. It is assumed that:
1. gas molecules physically adsorb on a solid in layers infinitely;
2. there is no interaction between each adsorption layer; and
3. the Langmuir theory can be applied to each layer (Brunauer et al., 1938).

55. 37
Figure 2.17: Typical adsorption isotherms: Type I (microporous materials), Type II (non-
porous materials), and Type IV (mesoporous materials) (Naumov, 2009).
Referring to Figure 2.17, adsorption isotherms of microporous materials are usually
represented by Type I isotherm. The micropores are filled at comparatively low relative
pressure. Upon completion of the micropore adsorption, the slope of the isotherm levels off
and reaches a plateau. Non-porous materials often follows Type II isotherm. Adsorption at low
pressures is much less pronounced due to the lack of porosity; after monolayer adsorption, a
second part of the curve steeps up due to multilayer adsorption (Naumov, 2009).
Mesoporous materials typically show Type IV isotherm (Figure 2.17). After a steep
increase at low relative pressures due to the monolayer adsorption (possibly due to additional
microporosity in the samples), the slope levels off into the second part representing the
multilayer adsorption followed by a steep increase of adsorbed gas volume due to capillary
condensation within the mesopores at higher relative pressures. According to IUPAC definition,
capillary condensation is said to occur when, in porous solids, multilayer adsorption from a
vapour proceeds to the point at which pore spaces are filled with liquid separated from the gas
phase by menisci. Usually a hysteresis loop of the adsorption and desorption curves is observed
in that range. The steeper the curve is in the capillary condensation regime, the narrower is the
pore size distribution. After capillary condensation, a plateau is reached. The mesopore volume

56. 38
is calculated from that plateau. And BET surface area can be calculated from the shape of the
isotherm prior to capillary condensation (Naumov, 2009).
To calculate the BET surface area the monolayer adsorbed gas quantity, vm, of the
material is determined from the BET-plot. Linear fit to BET adsorption isotherm is derived by
the linear BET equation as follows (Johansson, 2010):
1
𝑣 (
𝑃𝑜
𝑃
) − 1
=
𝑐 − 1
𝑣 𝑚 𝑐
(
𝑃
𝑃𝑜
) +
1
𝑣 𝑚 𝑐
𝐸𝑞𝑛 (2.1)
which:
P = equilibrium pressure of adsorbate at temperature of adsorption
Po = saturation pressure of adsorbate at temperature of adsorption
v = adsorbed gas quantity (eg: in volume)
vm = monolayer adsorbed gas quantity
c = BET constant
The specific surface area of the adsorbent can be calculated using following equation
(Johansson, 2010):
𝑆 𝐵𝐸𝑇 =
𝑆𝑡𝑜𝑡𝑎𝑙
𝑎
=
𝑣 𝑚 𝑁𝑠
𝑎
𝐸𝑞𝑛 (2.2)
which:
SBET = specific surface area
Stotal = total surface area
vm = monolayer adsorbed gas quantity

57. 39
N = Avogadro’s number
s = molecular cross-sectional area occupied by the adsorbate molecule in the complete
monolayer
a = mass of the adsorbent or solid sample
It has been observed from literature data that well-formed SBA-15 always shows Type
IV adsorption isotherm with H1-type hysteresis loop (Figure 2.18) (Klimova et al., 2006,
Dongyuan et al., 2000). Due to capillary condensation in hysteresis loop region, larger pressure
drop is required during desorption to overcome the van der Waals interactions among adsorbed
liquid molecules in the confined pores or capillaries. This is a typical phenomenon observed
for adsorption-desorption isotherm of mesoporous materials (Naumov, 2009) .
Figure 2.18: N2 adsorption-desorption isotherm of SBA-15 prepared at reaction temperature
35C and hydrothermal treatment at 80 C for 48h. Pore size distribution is shown inset
(Klimova et al., 2006).

58. 40
Figure 2.19: BET surface area YS
a
, micropore area Y
b
, and total pore volume YV
c
(Klimova
et al., 2006).
Using BET surface area analysis, information on BET surface area, micropore surface
area, total pore volume, micropore volume and average pore diameter can be calculated (Figure
2.19). Similar to TEM and SEM images, variation in process parameters results in different
range of BET surface area reported (Table 1.2):
Table 1.2: Literature comparison on BET surface area of SBA-15 samples.
BET Surface
Area (m2/g)
Process parameter varied Ref.
572.0 – 809.0
Reaction temperature, stirring rate, surfactant
species
(Hyung Ik et al., 2010)
522.0 – 853.0 Hydrothermal treatment temperature (Ma et al., 2003)
410.0 – 776.0
HCl concentration, ageing time and ageing
temperature
(Johansson et al., 2011)
759.5 - 963.6 Reaction pH, Fe/Si molar ratio (Li et al., 2005)
488.0 – 679.0 Reaction pH, TEOS/Pluronic P123 ratio (Abdullah et al., 2010)
550.0 – 700.0 Synthesis temperature (Galarneau et al., 2001)
531.0 – 992.0
Acid species, acid concentration, reaction
temperature, concentration of inorganic salt
(Zhengwei et al., 2008)

59. 41
Consecutively, pore size distribution curve of the SBA-15 samples can be plotted using
Barret-Joyner-Haleda (BJH) model. Based on the highest pore volume obtained, average pore
diameter of the SBA-15 sample is determined. Narrow pore size distribution is a characteristic
feature of SBA-15 materials, which is usually shown by a narrow spike in the pore size
distribution curve. In Figure 2.20, range of average pore size for SBA-15 samples synthesised
are 5-10 nm, in the pore size range classified for mesoporous materials.
Figure 2.20: BJH model pore size distribution curve of SBA-15 samples synthesised at (a)
1.87M HNO3, (b) 1.87 M HCl, (c) 1.73 M HNO3 and (d) 2.61 M HNO3 (dos Santos et al.,
2013).
In another example, pore size distribution of SBA-15 samples are always in a narrow
range, which can be identified from the sharp spike in the pore distribution curve as shown by
Figure 2.21. Here, SBA-15 samples are synthesised by using TMOS as silica source and DMF
as co-solvent separately (Dongyuan et al., 2000).

60. 42
Figure 2.21: Pore size distribution for SBA-15 sample synthesised (a) using TMOS as silica
source and (b) using DMF as co-solvent (Dongyuan et al., 2000).
2.4.4 X-ray Powder Diffraction (XRD)
X-ray diffraction (XRD) is a technique used to study periodically ordered structures at
atomic scales. The wavelengths of X-rays are in the same order of magnitude as the distance
between lattice planes in crystalline materials. When the X-rays enter the material they will be
scattered by the electron clouds around the atoms. The periodicity of the lattice planes gives
rise to constructive interference of the X-rays (Figure 2.22) and the intensity of the scattered
X-rays is plotted against the angle 2 (Johansson, 2010).
Figure 2.22: Schematic illustration of diffraction according to Bragg's Law.

61. 43
From the plotted peaks the lattice distance can be calculated using Bragg’s law:
𝑛 = 2𝑑 ∗ 𝑠𝑖𝑛 𝐸𝑞𝑛 (2.3)
which:
n = order of diffraction
 = wavelength
d = distance between lattice planes
 = angle of the incoming light
Figure 2.23: Schematic illustration of hexagonal phase with characteristic d-spacing and
unit cell parameter, a (Linton, 2009).
The unit cell is the smallest repeating unit of a crystalline structure. Figure 2.23 display
the d-spacing and the cell parameter, a, for a hexagonal phase in SBA-15. For a d-spacing (100),
d100, the unit cell, a, can be calculated from the the first Bragg peak position as follows:
𝑎 =
2
√3
𝑑100 𝐸𝑞𝑛 (2.4)

62. 44
Figure 2.24: XRD patterns of (a) pure SBA-15 and (b) SBA-15 functionalized with a
fluorescent chromophore, 5-methoxy-2-thiazoles (Li et al., 2007) .
Figure 2.25: XRD pattern of SBA-15 sample prepared at 35 C (Klimova et al., 2006).
From Figure 2.24 and Figure 2.25, XRD pattern of SBA-15 shows three main diffraction
peaks, referred to as crystal phase corresponding to Miller indices (100), (110) and (200). These
three peaks are characteristics of 2D hexagonal pore arrangement, commonly found in SBA-
15. These XRD patterns indicate that well-defined mesostructure is present in sample analysed
(dos Santos et al., 2013).

63. 45
2.4.5 Fourier Transform Infrared (FTIR) Spectrophotometer
Fourier transformed infrared (FTIR) spectrophotometer is used to study functional
groups on the surface of materials using the discrete energy levels for vibrations of atoms in
these groups. In this research, FTIR analysis is carried out using KBr pellets. Basically when
light with a specific energy is transmitted through the sample it can be absorbed by groups of
atoms in the material. This occurs when the frequency of the incoming light corresponds to the
frequency of vibrations in bonds between atoms. The vibration energy depends on the masses
and chemical environment of the atoms, the type of vibration. By scanning over a range of
wavelengths (in this case 4000-750 cm-1
) and recording the amount of transmitted light for
each wavelength it is possible to determine which functional groups that are present on the
surface of the material (Johansson, 2010).
Figure 2.26: FTIR spectra of (a) pure SBA-15 and (b) SBA-15 functionalized with 5-methoxy-
2-thiazoles (Li et al., 2007).

64. 46
Figure 2.27: FTIR spectra of SBA-15 and SBA-15 functionalized with rhodium, Rh/SBA-15
(Giraldo et al., 2014).
Referring to Figure 2.26 (a) and Figure 2.27, bands at around 3445 cm–1
can be assigned
to the -OH stretching vibrations mode of the silanol groups involved in hydrogen interactions
with the adsorbed water molecules, while siloxane bond (-Si-O-Si-) is shown by the broad
strong peak located at around 1100 cm-1
for both figures (Giraldo et al., 2014, Li et al., 2007).
Figure 2.28: FTIR spectra of as-synthesised and calcined SBA-15 (Gandhi et al., 2013)..
Difference in FTIR adsorption bands for as-synthesised and calcined SBA-15 samples
can be detected as well. Referring to Figure 2.28, the absence of -CH2- stretching bands at 2975
cm-1
, 2926 cm-1
and -CH2- bending at 1456 cm-1
in the calcined SBA-15 confirms the complete
removal of the Pluronic P123 triblock copolymer on calcination. Similar to other FTIR spectra

65. 47
reported in other literature, the absorption bands at 1089 and 3427 cm-1
in both as synthesized
and calcined samples show the presence of siloxane bond (-Si-O-Si-) and -OH groups which
confirms the silica formation (Gandhi et al., 2013).
2.5 Industrial/ Commercial Application
If lower ageing temperature, shorter ageing time and less acidic media can be used to
prepare hexagonally ordered SBA-15, increased industrial production of SBA-15 with low
heating demand at a safer operating environment can be achieved. SBA-15 is now widely used
in biotechnology as drug delivery system. Different guest materials such as anti-inflammatory
agents, antibiotics, vaccines and hormones can be loaded into the mesoporous pores of SBA-
15. This is very useful for traditional oral drug that are difficult to distribute or when there’s a
need for slow release of water-soluble drugs. Besides, these purely siliceous mesoporous
materials are found to be biocompatible and bioactive, which attracts research interest in using
mesoporous silica as bioceramics to act as bone substitution materials. Using biocompatible
SBA-15 thus provides a safer platform for controlled drug release in humans. Amorphous silica
is also degradable in aqueous solution and so problems related to waste removal after usage
can be very much avoided (Andersson et al., 2008).
In terms of removing harmful contaminants from wastewater such as phenol, Ti-
containing SBA-15 has been found to exhibit good photocatalytic activity in degrading phenol
present in wastewater (Yang et al., 2014). Phenol is highly irritating to eyes, skin and mucous
membrane in humans after acute or short term exposure, thus efficient removal of this harmful
chemical can be made possible with mesoporous silica. With the ease of SBA-15 production,
various catalytic and biotechnology applications can be carried out to improve public health
and solve environmental pollution problems.

66. 48
CHAPTER THREE:
MATERIALS AND METHODS
3.0 List of Chemicals Required
Table 2.1 summarises the important chemicals needed with supplier details as follows:
Table 2.1: List of Chemicals Required.
Chemicals/ Materials Supplier Purpose
1. Pluronic P123, (PEO20PPO70PEO20) Sigma-Aldrich
Malaysia
Act as surfactant.
2. Tetraethyl orthosilicate, TEOS (98 %) Acros Organics
Malaysia
Act as silica inorganic
precursor.
3. Hydrochloric acid, HCl (Fuming, 37
wt%)
R & M Chemicals
Malaysia
Act as catalyst in
reaction media.
4. Methylene blue (C.I. 52015) Reag. Ph
Eur
Merck Malaysia As adsorbate for
SBA-15.
3.1 List of Equipment Required
Table 3.2 summarises the equipment and facilities used in this research as follows:
Table 3.2: List of Equipment Required.
Chemicals/ Materials Model Manufacturer Purpose
1. Scanning electron
microscopy, SEM
FEG 450 SEM Quanta ,
USA
To study surface topology
of SBA-15.
2. Transmission
electron microscopy,
TEM
CM12 Philips, The
Netherlands
To study internal
microstructure of SBA-15.

67. 49
3. X-ray powder
diffraction, XRD
X’Pert Pro
PW3040
PANalytical, The
Netherlands
To study periodically
ordered structures in atomic
scale in SBA-15.
4. Fourier Transform
Infra-red, FTIR
spectrophotometer
IRPrestige-21 Shimadzu, Japan To study functional groups
on SBA-15 surface.
5. Surface Area
Analyzer
ASAP 2010 Micrometrics,
USA
BET and BJH model
surface area
characterization of SBA-15.
6. UV
spectrophotometer
UV-1800 SHIMADZU,
Japan
To analyse dye
concentration.
7. Universal Oven UF55 Memmert,
Germany
Sample drying.
8. Water bath shaker 903 PROTECH, USA For ageing and adsorption
test.
9. Muffle furnace CWF 1300 CARBOLITE,
UK
For calcination of as-
synthesised SBA-15.
3.2 SBA-15 Synthesis By Conventional Precipitation
a) Effect of hydrochloric acid (HCl) concentration
150 mL of 2.5 M HCl is prepared and mixed with 5 g of Pluronic P123 in a 500 mL size
glass beaker. The mixture is then stirred at speed 1100 rpm with temperature 50C, until the
solution turn clear and colourless. At this point, 9.67 g of tetraethyl orthosilicate (TEOS) is
added dropwise into the solution. The mixture is then left stirring at speed 350 rpm at same
temperature for it to turn from cloudy into white solution in 2 hours. This mixture is then left
for ageing in water bath shaker for 48 hours at ageing temperature of 40 C. White precipitate
is filtered out and washed continuously with deionized water. The filtered white solid is then
left for natural drying before calcination is carried out at 823 K for 240 minutes with heating
rate of 5 C/ min. Subsequent trials are carried out at HCl concentration of 2.0, 1.5 and 1.0 M,

68. 50
while maintaining constant on other process parameters. These powdered samples are then sent
for methylene blue adsorption test. The sample that shows the highest adsorption capacity for
methylene blue will be sent for physicochemical characterization to determine the presence of
ordered structure in SBA-15 produced.
b) Effect of ageing temperature
Hydrochloric acid (HCl) concentration that produces SBA-15 sample with the best
methylene blue adsorption capacity is applied in Experiment B. In Experiment B, ageing
temperature is varied by 40, 50, 60 and 70 C, while maintaining constant on other process
parameters. Synthesised samples are sent for methylene blue adsorption test and the sample
with the best adsorption performance is then sent for physicochemical characterization.
c) Effect of ageing time
Ageing temperature that produces SBA-15 with the best methylene blue adsorption
capacity is applied in Experiment C, using the same HCl concentration as employed in
Experiment B. In Experiment C, ageing time is varied by 12, 24, 36 and 48 hours. Synthesised
samples are sent for methylene blue adsorption test and the sample with the best adsorption
performance is then sent for physicochemical characterization.
3.3 Methylene Blue Adsorption Test
The methylene blue adsorption test is carried out with adaptations to method reported by
Dong et al. (2011).

69. 51
The adsorption test is carried out by preparing a 0.1 g/L (100 ppm) of methylene blue
stock solution. The test is carried out at a 250 mL conical flask by placing 0.1 g of SBA-15
with 100 mL of methylene blue stock solution. The mixture is left in water bath at 30 C for
one day. Dye concentration of both methylene blue solution and resulting dye mixture is
determined using UV spectrophotometer at max = 665 nm, with a dilution factor 5, in which 1
mL of dye solution is pipetted and diluted with distilled water to 5 mL using a 5 mL volumetric
flask. SBA-15 sample is then filtered out under reduced pressure, dried and kept in sample cell
for further physiochemical testing. Data obtained is used to calculate the adsorption capacity
of SBA-15 using the following formula (Dong et al., 2011):
𝑞𝑡 = (𝐶 𝑜 − 𝐶𝑡) ∗
𝑉
𝑊
𝐸𝑞𝑛 (3.1)
In which:
qt = mg of methylene blue adsorbed per gram of adsorbent
CO = Initial dye concentration at time = 0
Ct = Final dye concentration at time = t (one day)
V = Volume of solution
W = Weight of dry adsorbent used (gram)
3.4 Physicochemical Characterization
a) Surface area analysis
The following experimental method is adopted from Mohd Din et al. (2015) with
modifications. Surface area properties of SBA-15 is determined using surface area analyser
(Micrometrics ASAP 2010, USA) at 77 K. SBA-15 samples are degassed at 373 K for 9 hours

70. 52
before measurement. The N2 adsorption-desorption isotherm can be determined by admitting
known volumes of nitrogen in and out of the sample and measuring the equilibrium pressure.
Pore size distributions were calculated by using Barret-Joyner-Haleda (BJH) model while
surface area properties is determined by using Brunauer-Emmet-Teller (BET) model.
b) Scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy
(EDX)
An appropriate amount of SBA-15 sample is coated with gold and placed on carbon tape
before inserted into the microscope (Mejia, 2013) for SEM and EDX analysis (SEM Quanta
FEG 450, USA) (Mohd Din et al., 2015). The micrograph shows the particle size and
morphology, in particularly the topography of the sample.
c) Transmission electron microscopy (TEM)
Textural images are captured using TEM (Philips TEM CM12, The Netherlands) (Mohd
Din et al., 2015). SBA-15 samples are dispersed in acetone and deposited onto carbon grids
and allowed to dry before measurement (Mejia, 2013).
d) Fourier Transform Infrared (FTIR) spectrophotometer
Surface chemistry of SBA-15 sample is determined using FTIR spectrophotometer
(Shimadzu IRPrestige-21, Japan) using KBr pellets within 4000-400 cm-1
wavelength range
(Mohd Din et al., 2015). To prepare the pellets, 0.8 mg of SBA-15 sample and 120mg of KBr
powder are ground and mixed to remove scattering effects (Mejia, 2013). The powder mixture
is finally pressed into pellet form.

71. 53
e) X-ray Powder Diffraction (XRD)
The crystal structure of SBA-15 is determined at 2 range of 0.5-50 by using Materials
Research Diffractometer (Mohd Din et al., 2015). The (100) d-spacing from diffraction pattern
is calculated to obtain the unit cell parameter, a, which is mesopore center-to-center distance
(Eun Young, 2007):
𝑎 =
2
√3
∗ 𝑑100 𝐸𝑞𝑛 (2.4)
Calculated a-parameter value combines with adsorption results to further calculate the
thickness of mesopore wall, t (Eun Young, 2007).
𝑡 = 𝑎 − 𝐷 𝐸𝑞𝑛 (3.2)
In which
t = wall thickness
D = mesopore size
3.5 Safety Precaution
All safety precaution in handling hazardous chemicals such as fuming 37 wt% HCl,
TEOS and Pluronic P123 has been noted and carried out as instructed in the Material Safety
Handling Sheet (MSDS) written for each hazardous chemical listed. Personal protective
equipment such as goggles, safety gloves, respirators and lab coat are worn throughout the
experiment. Proper and safe disposal of the chemicals used has been carried out as instructed
in the MSDS.

72. 54
CHAPTER FOUR
RESULTS AND DISCUSSION
4.1 Physicochemical characteristics of SBA-15
In scanning electron microscopy (SEM) characterization with energy dispersive x-ray
spectroscopy (EDX), all samples were sent for testing. However, for subsequent transmission
electron microscopy (TEM), surface area analysis by Brunauer-Emmet-Teller (BET) model,
Fourier transform infrared (FTIR) spectrophotometer and x-ray powder diffraction (XRD)
testings, only SBA-15 sample that showed the highest methylene blue batch adsorption
capacity is tested. Results obtained are as follows:
4.1.1 Scanning Electron Microscopy (SEM)
(a) Effect of HCl Concentration
In this experiment, the synthesis condition of SBA-15 was varied by changing its HCl
concentration from 2.5 M, 2.0 M, 1.5 M to 1.0 M, by maintaining ageing temperature at 40 C
for 48 hours. The SEM images for SBA-15 are shown as follows:
b
b
a

73. 55
Figure 4.1: SEM images of SBA-15 synthesised at HCl concentration of: a) 2.5 M, b) 2.0 M,
C) 1.5 M, (d, e, f) 1.0 M.
From Figure 4.1, it is observed that as HCl concentration is reduced from 2.5 M to 1.0
M, SBA-15 morphology evolve from longer rectangular rods (2.5 M) to shorter rods (2.0 M,
1.5 M) until mixture of rods and spheres are present when 1.0 M HCl is used. This phenomenon
can be explained by colloidal phase separation mechanism (CPSM) proposed by Zhao et al.
(2004). This theory suggests that when phase separation occurs, precipitates will be observable
in the solution and the rate of this phase separation affects the final morphology of ordered
mesoporous structures. In this case, when phase separation occurs slower at lower acidity (eg:
c d
e f

74. 56
1.0 M), morphologies of SBA-15 are influenced by colloidal surface free energy, F. With
increasing influence of F, materials with large curvature, such as spherical shapes, will develop
to minimize the surface energy, thus more spherical structures are observable in SBA-15 as
HCl concentration is reduced. However, concentrated HCl results in faster hydrolysis and
condensation reactions, which promotes faster rate of phase separation. When this occurs,
morphologies of SBA-15 are influenced by free energy of mesostructure self-assembly, G.
With increasing influence of G, the overall macrostructure of SBA-15 is formed together in
the presence of mesostructures, to minimize the free energy of mesostructure assembly. This
is why mesoporous, crystal-like morphologies of SBA-15 can be obtained (eg: fibers, rods,
flakes) when higher HCl concentration is applied in its synthesis (Chengzhong et al., 2004).
(b) Effect of Ageing Temperature
In this experiment, HCl concentration which produced SBA-15 with the highest
methylene blue adsorption capacity is employed, with ageing time maintaining at 48 hours as
well. However, ageing temperature is varied at 40, 50, 60 and 70 C.
a b

75. 57
Figure 4.2: SBA-15 prepared at ageing temperature: a) 40 C, b) 50 C, c) 60 C, d) 70 C.
From Figure 4.2, it is observed that as the ageing temperature is increased from 40 - 70
C, SBA-15 morphology changes from short rods (40, 50 C) to thinner fibre-like structure (60,
70 C). This phenomenon can be explained similarly with CPSM theory. At higher ageing
temperature, polymerization and condensation of silica species in the walls increases, resulting
in higher phase separation rate. Thus G dominantly affects the final morphology of SBA-15.
As mesostructure is continuously formed to minimize G, more crystal-like structures of SBA-
15 can be obtained, which in this case, the SBA-15 with fibre crystal-like structure is obtained
at 60 and 70 C.
(c) Effect of Ageing Time
In this experiment, ageing temperature and HCl concentration producing SBA-15 with
the highest adsorption capacity is employed, but ageing time is varied at 48, 36, 24 and 12
hours.
c d

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Figure 4.3: SBA-15 synthesised at ageing time: a) 48 h, b) 36 h, c) 24 h, d) 12 h.
From Figure 4.3, not much changes in SBA-15 topology is observable when ageing time
is reduced. By rough judgement, it is more difficult to pin-point a clear type of morphology for
SBA-15 samples synthesised at 24 and 12 hours of ageing. More well-defined rod-shape SBA-
15 particles are observable when ageing time is increased to 36 and 48 hours. This is
explainable since longer ageing time ultimately allows more polymerization and condensation
of silica species on the walls to form ordered mesostructure of SBA-15. Shortening the ageing
time prevents hexagonal structure of SBA-15 to be formed properly. This remains true if the
rest of operating conditions (eg: HCl concentration, ageing temperature) are constant. Judging
a b
c d

77. 59
from the SEM images, it is not apparent that ageing time has strong effect on the surface
morphology of SBA-15 compared to HCl concentration and ageing temperature factors.
However, methylene blue adsorption results is different when ageing time is reduced. This shall
be discussed further in section 4.2.
4.1.2 Energy Dispersive X-ray Spectroscopy (EDX)
In this discussion, only EDX spectra of Run 2 SBA-15 sample (with highest methylene
blue adsorption capacity) is shown as follows, as similar trends are observed for rest of the
SBA-15 prepared (available in Appendices section).
Figure 4.4: EDX spectra of Run 2 SBA-15 sample.
The EDX spectra shows the respective weight percentage contributed by each element
in SBA-15 sample. Si and O element are detected, confirming the formation of silica. However,
slight carbon residue is detected as well. This could be the impurities attached to the sample
during calcination in muffle furnace used for carbon samples previously. Nevertheless, the
intensity peaks detected for element Si and O are comparable with literature data (Tomer et al.,
2015).
8.33%
58.00%
33.67%

78. 60
4.1.3 Transmission Electron Microscopy (TEM)
Figure 4.5: TEM images of SBA-15 Run 2 sample at: a) perpendicular to pore channels and
b) parallel to pore channels.
For TEM testing, only SBA-15 sample with highest methylene blue adsorption capacity
(Run 2) is sent for characterization. Based on Figure 4.5, well-defined mesoporous channels
are obtained in this sample as characteristic 1D cylindrical channels and 2D hexagonal pore
arrangement can be observed, which are commonly found in SBA-15.
4.1.4 X-ray Powder Diffraction (XRD)
Figure 4.6: Small angle XRD of SBA-15 sample (Run 2).
0
100000
200000
300000
400000
500000
600000
700000
800000
900000
1000000
0 1 2 3 4 5 6
Intensity
2
Intensity vs. 2
(100)
a b

79. 61
From Figure 4.6, only one well-resolved diffraction peak is observable, which shows the
crystal phase corresponding to Miller index (100). This peak is characteristic of 2D hexagonal
pore arrangement, which is commonly found in SBA-15 materials. However, this SBA-15
sample is not as highly ordered as high quality SBA-15 usually shows distinct (110) and (200)
peaks in XRD analysis and much sharper (100) peak than that obtained in this research (Eun
Young, 2007). Nevertheless, it can be concluded that well-ordered mesostructure is still
available in sample analysed. Unit cell parameter, a, can be calculated based on first Bragg
peak position, d100 = 9.4217 nm as obtained in this analysis.
Figure 4.7: Wide angle XRD of SBA-15 sample (Run 2).
From Figure 4.7, wide angle XRD analysis is carried out for the same sample. A broad
peak is present between 2 range of 15  and 30  due to the presence of amorphous silica
(Gandhi et al., 2013, Yang et al., 2014).
0
50
100
150
200
250
0 10 20 30 40 50 60
Intensity
2
Intensity vs. 2

80. 62
4.1.5 Fourier Transform Infrared (FTIR) spectrophotometer
Figure 4.8: FTIR spectra of SBA-15 sample (Run 2).
Types of functional groups present on Run 2 SBA-15 sample is determined using FTIR
testing using KBr pellets within 4000-400 cm-1
wavelength range. From Figure 4.8, strong
broad peak at 1094 cm-1
can be assigned to siloxane bond (-Si-O-Si-) stretch while -OH
stretching vibrations mode of the silanol groups involved in hydrogen interactions with the
adsorbed water molecules is observed at 3454 cm-1
peak. Bending H2O band at 1637 cm-1
is
also ascribed to adsorbed water molecules on the material. The presence of these two functional
groups confirm the silica formation. Another obvious band at around 802 cm-1
can be ascribed
to symmetric stretching from –Si-O bonds, while another peak at around 466 cm-1
can be
attributed to bending –Si-O-Si- bonds (Yang et al., 2014, Uchoa et al., 2012). Absence of peaks
at 3000-2850 cm-1
range (-CH2- stretch) and 1470-1450 cm-1
(-CH2- bend) confirms the
complete removal of Pluronic P123 in this calcined SBA-15 sample (Gandhi et al., 2013).

81. 63
4.1.6 Surface Area Analysis by Brunauer-Emmet-Teller (BET) model
Figure 4.9: N2 BET adsorption isotherm (Top) and BJH adsorption model dV/dlog(D) pore
size distribution plot (bottom) of SBA-15 sample (Run 2).
From Figure 4.9, Type IV adsorption isotherm with H1-type hysteresis loop is obtained
from N2 adsorption-desorption isotherm. This is consistent with adsorption isotherm reported
for well-formed SBA-15 as capillary condensation occurs at hysteresis-loop region (Naumov,
2009). The steep increase of N2 adsorption at P/Po = 0.52 indicates mesopore uniformity in

82. 64
size, which is confirmed by the pore size distribution curve plotted using Barret-Joyner-Haleda
(BJH) adsorption model showing a sharp peak at pore diameter of 4.06 nm, located in the
mesopore diameter range of 2 - 50 nm (Xiao Ying et al., 2004).
Table 4.1: Surface area analysis and pore size of SBA-15 sample (Run 2).
SBET
(m²/g)
S
(m²/g)
Sext
(m²/g)
a
(nm)
t
ad/des
(nm)
dBET
ad/des
(nm)
dBJH
ad/des
(nm)
VTotal
(cm³/g)
V
(cm³/g)
364.71 42.29 322.42 10.88 6.98/
6.98
3.90/
3.89
4.06/
4.17
0.36 0.021
Note: BET surface area, SBET = Micropore area, S + External surface area, Sext , a = unit cell parameter calculated from:
2𝑑100/√3, t = wall thickness calculated from: 𝑎 − 𝑑 𝐵𝐽𝐻 , dBET ad/des = pore diameters calculated from BET model in
adsorption/ desorption branch , dBJH ad/des = pore diameters calculated from BJH model in adsorption/ desorption branch,
VTotal = total average pore volume, V = micropore volume.
From Table 4.1, it is observed that wall thickness, t, of SBA-15 sample calculated is
much larger than its pore size calculated from BJH and BET models. We can expect good
hydrothermal stability of SBA-15 analysed. Pore size reported for this sample using BET
adsorption branch is 3.90 nm, while for BJH adsorption branch is 4.06 nm. This data reported
is comparable to literature data (Mohd Din et al., 2015). It is also observed that BET surface
area (364.71 m²/g) obtained is much smaller than typically reported for SBA-15 (400-500 m²/g).
At 2.0 M HCl concentration, with ageing carried out at 40 C for 48 hours, the BET surface
area obtained in this SBA-15 sample is much less than expected. Upon comparison with
literature data which employs similar HCl concentration, SBA-15 with high surface area above
or around 600 m²/g is usually obtained by carrying out hydrothermal treatment by heating SBA-
15 at around 60/ 80/ 100 C for around 1 or 2 days (Gandhi et al., 2013, Sabri et al., 2015,
Klimova et al., 2006, Katiyar et al., 2006). It is inferred that by using the moderate temperature

83. 65
in this synthesis, BET surface area is affected negatively due to the absence of high temperature
treatment (hydrothermal treatment) on SBA-15. As mentioned earlier, hydrothermal treatment
is not applied in this experiment and therefore synthesis and ageing temperature for Run 2
sample is mostly maintained at around 40 C. Larger surface area could be obtained if
hydrothermal treatment is employed, since at increased temperature above 60 C, hydrolysis
of PEO chains in Pluronic P123 allows increment in aggregation volume of micelle, leading to
enlarged pore size and surface area, with reduction in wall thickness (Klimova et al., 2006).
Indeed, BET analysis for this sample shows exceptionally thick wall, smaller pore size and
small total surface area.
4.2 Methylene blue (MB) batch adsorption performance of SBA-15 Samples
Table 4.2: Methylene blue batch adsorption performance of SBA-15 samples.
Run [HCl] (M)
Ageing Temp
(°C)
Ageing Time
(hours)
MB Removal Efficiency (%)
1 2.5 40 48 13.27
2 2.0 40 48 19.63
3 1.5 40 48 9.96
4 1.0 40 48 11.36
5 2.0 40 48 19.63
6 2.0 50 48 5.89
7 2.0 60 48 3.61
8 2.0 70 48 0.26
9 2.0 40 48 19.63
10 2.0 40 36 3.57
11 2.0 40 24 9.51
12 2.0 40 12 9.90

84. 66
From Table 4.2, the methylene blue (MB) batch adsorption capacity for all samples are
generally poorer than reported in literature. Examining the effect of HCl concentration on
adsorption capacity of SBA-15 samples, SBA-15 synthesised at 2.0 M HCl has the best
performance, although samples synthesised at 2.5 M HCl has comparable performance as well.
Although by theory that SBA-15 surface area can be increased by increment in ageing
temperature, which in turn can potentially improve the adsorption capacity for MB, in actual
the results run in contradict to theory by dropping capacity tremendously from Run 5 until Run
8. Next on, methylene blue adsorption results for samples synthesised at increasing ageing time
also shows irregular patterns, by decreasing from 12 to 36 hours, then improved again
significantly for samples synthesised by 48 hours of ageing.
In overall, the poor MB adsorption performance could be attributed to various factors.
In the case for Run 2 sample, the poor performance could be due to the small surface area
(364.71 m²/g) obtained. As less surface area is available for methylene blue particles to be
adsorbed, the adsorption performance is generally expected to be lower. There were other
studies made on methylene blue adsorption using bare SBA-15 synthesised, which reported
good performance of dye removal at 80 % or above, some even achieved near 100 % of removal
using SBA-15 synthesised. Comparison is made further to determining the reason behind the
poor adsorption performance of Run 2 SBA-15.