1 GENERAL INTRODUCTION 
 1
 CHAPTER 1. GENERAL INTRODUCTION 
 
1.1. Extension from Microporous to Mesoporous Dimensions 
 
According to the IUPAC system [1], porous materials are classified according to their 
pore sizes into three classes: (i) Microporous, having pore sizes below 20 ?; (ii) 
Mesoporous, with intermediate pore sizes in the range 20 - 500 ?; and (iii) 
Macroporous, with pore sizes exceeding 500 ?. 
 
Zeolites and zeolite-like molecular sieves belong to the microporous class, and have 
attracted particular attention in the fields where molecular recognition is needed, e.g. 
catalysis, separations and chemical sensing [2, 3]. They have been extensively used in 
acid-catalyzed reactions for the production of petrochemicals and fine chemicals, 
where they serve both as catalysts and catalyst supports. Their extensive usage was 
prompted by attractive features like their high thermal and hydrothermal stability, as 
well as tunable acidity. However, the limitation in the applicability of zeolite catalysts 
and supports lies in the diffusional constraints (so-called shape selectivity) in 
reactions involving bulky reactants or those generating bulky products because of the 
restricted pore sizes. The need to extend zeolitic dimensions beyond this shape-
 selective limit, i.e., making zeolites with pores comprised of larger than 12 T-atoms 
(pore diameters 6-8 ?), has seen much research effort. Pore systems that contain rings 
circumscribed by more than 12 T-atoms are classified as extra-large pores [4]. 
 
One of the initial attempts to improve the diffusion of reactants involved the use of 
pillared clays (PILCs), prepared by intercalating voluminous metallic complexes 
between structural layers of swelling clays like smectites in order to separate the 
layers [5-10]. Although the resulting materials possess high surface area and large 
interlayer separations, they are still of limited practical utility [11].  
 
1 GENERAL INTRODUCTION 
 2
 Another attempt involved synthesizing zeolites and/or zeotypes comprising larger 
ring sizes than conventional zeolites. In this light, the first molecular sieve with extra-
 large pores was an aluminophosphate material with 18-membered ring pores and 
three-dimensional topology, designated VPI-5 [12]. Its silicon-containing analogue, 
MCM-9, was reported immediately thereafter [13]. Subsequently, the synthesis of 
two other phosphate-based extra-large pore materials with 20-membered rings were 
reported, cloverite (14) and JDF-20 [15, 16], which are grouped with the naturally 
occurring 20-membered ring phosphate, cacoxenite [17]. The main shortcoming of 
these phosphate-based molecular sieves is the lack of the desired properties of combi-
 ned high acidity and thermal/hydrothermal stability, which limits their practical pot-
 ential.  
 
 Recently, the first extra-large pore zeolite with 14-membered ring size, designated 
UTD-1, was reported and shown to possess good acidity and thermal/hydrothermal 
stability [18, 19]. This extra-large pore zeolite, with a DON-type structure and a ring 
size of 14 tetrahedral atoms, was synthesized using an organometallic structure-
 directing agent (SDA), the bis(pentamethylcyclopentadienyl)cobalticenium cation. 
Another 14-membered ring material with one-dimensional pores, high-silica CIT-5 
with a CFI-type structure, has been subsequently synthesized under hydrothermal 
conditions in the presence of N(16)-methylsparteinium and Li+ [20, 21]. 
 
To summarize this endeavour of preparing large pore materials with open-framework 
structures, it can be stated that while a ring size of only 14 tetrahedral atoms in known 
in zeolites, extra-large pore phosphates come with various ring sizes, including 14, 
16, 18, 20 and 24. Unfortunately phosphate-based molecular sieves have lower ther-
 mal stability. 
 
Recently, Corma and coworkers [22, 23] realized that, as in layered clay materials, 
zeolites can be delaminated to improve the accessibility of catalytic sites to large 
molecules. 
1 GENERAL INTRODUCTION 
 3
  
Another conventional approach to prepare molecular sieve materials with larger pore 
sizes is the so-called ?secondary synthesis method?. In this method, microporous 
zeolite molecular sieves such as A, X, and Y were chemically treated with a 
(NH4)2SiF6 solution or steamed. Such a treatment generated mesopores but the pore 
sizes of these treated materials did not have a narrow distribution [24].  
 
A remarkable breakthrough in the templated synthesis of large-pore zeolites was the 
discovery of the M41S family of mesoporous materials which were synthesized by 
researchers at the Mobil Research and Development Corporation (Ltd) [25, 26]. 
These materials show a very narrow pore size distribution. The members of this 
family are MCM-41, MCM-48 and MCM-50, which differ from each other in their 
unit cell types and overall crystallographic point groups. MCM-41 is hexagonal, 
MCM-48 is cubic (space group Ia3d) while MCM-50 is lamellar. The formation of 
each of these phases is dependent on the synthesis conditions, particularly the 
surfactant to silica molar ratio of the synthesis gel. This discovery of the M41S 
family of materials has, therefore, dramatically expanded the range of pore sizes from 
micropore (13 ?) to mesopore (20-100 ?), with MCM-41 being the most studied 
member of the M41S family. The Mobil researchers have named the formation 
mechanism of the MCM series of materials a ?liquid crystal templating? (LCT) 
mechanism. It involves electrostatic interaction between quaternary ammonium salts 
[CnH2n+1(CH3)3NBr] and the silicate polyanion. The phases that result from the LCT 
mechanism involving silicates and amphiphilic alkyltrimethylammonium surfactants 
are depicted in Figure 1.1: 
 
1 GENERAL INTRODUCTION 
 4
  
 
Fig. 1.1. Three structure types observed for silica-surfactant mesophases : (a) MCM-41 
(Hexagonal, 1-D), (b) MCM-48 (Cubic, bicontinuous, 3-D), (c) MCM-50 (Lamellar, 2-D) 
[25].  
 
Vartuli et al [27] have reported the formation of an additional molecular species, a 
cubic octamer [(CTMA)SiO2.5]8 (see Table 1.1). 
 
Table 1.1. Composition of the synthesis mixtures for the formation of various M41S 
phases [ref. 27] 
 
Product aSur/Si molar ratio of gel bTEOS/100 g of cCTMAOH 
MCM-41 0.6 30 g 
MCM-48 1.0 20 g 
Lamellar 1.3 15 g 
Cubic octamer 1.9 10 g 
aSurfactant to silicon mol ratio 
bTetraethyl orthosilicate 
cCetyltrimethylammonium hydroxide 
 
A highly ordered MCM-41 analogue, designated FSM-16, was discovered at about 
the same time as the M41S materials [28, 29]. Its synthesis involved ion exchanging 
the interlayer Na+ ions of kanemite with CTAB. It has also recently been prepared by 
cetylpyridinium bromide templating of water glass [30].   
1 GENERAL INTRODUCTION 
 5
 1.2. Synthesis and Fabrication of Mesoporous MCM-41 
 
The preparation methods of M41S materials are reminiscent of the synthesis of 
zeolites [31] in that they involve the hydrothermal transformation of alkaline silicate 
or aluminosilicate gels in the presence of a template. Unlike conventional zeolite 
synthesis, MCM-41 materials are prepared from a cationic surfactant template and 
silica polyanions through an electrostatic interaction (S+I-). The main synthesis 
methods used to prepare the Mobil mesoporous materials thus far are liquid crystal 
templating and sol-gel processing. A summary and the background to these two 
methods is given below. 
 
1.2.1 Liquid-Crystal Templating (LCT): Supramolecular Self-Assembly 
 
Through charge matching between the hydrophilic head groups of surfactant 
molecules and inorganic precursors, silicates and surfactants can be self-assembled 
into supramolecular aggregates with long-range packing order [32 - 34]. Upon 
removal of surfactants through calcination or organic washes, mesoporous silicates 
with ordered porous arrays can be achieved.  
 
A distinguishing feature of the M41S synthesis is that quaternary ammonium 
surfactants, CnH2n+1Me3N+, with different alkyl chain lengths (n = 8-18) are used as 
templates [25, 26]. Surfactants are large organic molecules [CnH2n+1(CH3)3N+] with a 
hydrophilic head group and a long hydrophobic tail of variable length, and are 
capable of forming micelles or liquid crystal phases in aqueous solutions at a critical 
micelle concentration (CMC). Researchers have termed the mechanism of formation 
of these mesoporous materials a LCT mechanism, (see Figure 1.2). 
 
1 GENERAL INTRODUCTION 
 6
  
 
Fig. 1.2. Liquid-crystal templating (LCT) mechanism for self-assembly proposed by Beck et 
al [26] showing two possible pathways for the formation of MCM-41: (1) liquid-crystal 
initiated and (2) silicate-initiated.  
 
Pathway 1 assumes that the primary structure-directing element is the water/surfact-
 ant liquid-crystal phase, whereas pathway 2 suggests that the addition of the silicate 
orders the subsequent silicate-encased surfactant micelles. The investigation of this 
system by Chen et al [35] revealed that the formation of the mesoporous structure 
occurs by the second mechanism, and does not follow a templating process by an 
initially ordered liquid-crystalline phase. A charge matching interaction between the 
hydrophilic head groups of surfactant molecules and inorganic precursors has also 
been proposed [32-34], in which silicates and surfactants can be self-assembled into 
supramolecular aggregates with long-range order of packing. In short, the highly 
ordered MCM-41 porous structure is not caused by single guest ions or molecules, 
but by a supramolecular self-assembly of surfactant molecules in aqueous solutions.  
 
Subsequent research has shown that not only milder hydrothermal conditions give 
good quality Si-MCM-41 materials, but that materials can also be prepared at room 
temperature [36-38], under ultrasound radiation [39], as well as under microwave 
radiation [40, 41]. High Al-containing MCM-41 (Si/Al = 1.5) has also been prepared 
at room temperature [42]. The syntheses by ultrasound and microwave radiation were 
1 GENERAL INTRODUCTION 
 7
 efficient in yielding the best quality Si-MCM-41 over a relatively short time (3.5 and 
4 h, respectively). Thus the usually longer crystallization times required for synthesi-
 zing advanced materials is significantly reduced. 
 
This versatility in the conditions under which the MCM-41 materials can be synthesi-
 zed, as opposed to the normal difficulties in synthesizing conventional microporous 
zeolites, makes them amenable to study in various fields.  
 
1.2.2. Sol-Gel Synthesis Route (Alkali-free Synthesis) 
 
Sol-gel chemistry originated in the mid-1800s with several studies on the creation of 
silicon dioxide (SiO2) glass from the tetraethoxysilane (TEOS) precursor [43, 44]. It 
has subsequently been defined as the preparation of a ceramic material (e.g. glass) by 
the preparation of a sol, gelation of the sol, and subsequent removal of the solvent [45 
- 47]. Depending on the nature of the precursors, the sol-gel process can be classified 
into two different routes: a) inorganic route where the precursor is an aqueous 
solution of an inorganic salt (e.g. sodium silicate solution), or b) organic route where 
the precursor is a metal-organic compound (e.g. TEOS) [48]. Metal alkoxides or 
tetraalkoxysilanes are most commonly preferred as precursors, because: they are 
soluble in organic solvents (providing high homogeneity), they can easily be 
converted to the corresponding oxides, and they are known for almost all elements 
[49-51]. Metal oxides can be created from tetraalkoxysilanes via a two-step process 
shown below for the formation of SiO2 from TEOS: 
 
Si(OR)4 + n H2O ? Si(OH)n(OR)4-n + n ROH, 
 
where R is, for example, a ?CH2CH3 group. This step represents the nucleophilic 
substitution of the alkoxide groups with hydroxyl groups. The second step is a 
condensation process that initiates the silicate glass structure: 
 
1 GENERAL INTRODUCTION 
 8
 ?Si(OH) + RO-Si? ? ?Si-O-Si? + ROH, or 
 
?Si(OH) + HO-Si? ? ?Si-O-Si? + H2O 
 
The condensation process continues to make an extended three-dimensional silicate 
glass matrix. 
 
In general, the sol-gel procedure consists of the steps outlined below [52]: 
 
1. Preparation of a homogeneous solution either by dissolution of a metal 
organic precursor (TMOS, TEOS, Zr(IV)-Propoxide, Ti(IV)-Butoxide, etc.) in 
an organic solvent that is miscible with water, or by dissolution of inorganic 
salts in water. 
2. Converting the homogeneous solution to a sol by treatment with a suitable 
reagent (pure water or water containing either HCl, NaOH or NH4OH). 
3. Aging: The sol changes into a gel by self-polymerization. 
4. Shaping the gel to the finally desired form (thin films, fibres, spheres, etc.). 
5. Converting (sintering) the shaped gel to the desired ceramic material generally 
at temperatures around 500 oC.  
 
Removal of the pore liquid under hypercritical conditions yields a network that does 
not collapse and results in the production of an aerogel [53]. When the pore liquid is 
removed by evaporation under normal conditions, shrinkage of the pores occurs and a 
xerogel is formed.  
 
Over the years, the sol-gel chemistry approach has been developed into a powerful 
tool for making films and coatings, optics, fibres, powders, and membranes with 
desired characteristics [45, 54]. The potential benefits of sol-gel processing over the 
conventional powder route [45, 54, 55], include the following: (i) good homogeneity, 
(ii) ease of chemical composition control, (iii) high purity, (iv) low-temperature 
1 GENERAL INTRODUCTION 
 9
 processing, and (v) large area and versatile shaping over vacuum deposition techniq-
 ues.  
 
However, unless structure-directing materials are employed, the materials obtained in 
a simple sol-gel process are amorphous and randomly ordered [45]. Thus, the 
synthesis approach to ordered mesoporous silicate frameworks of the MCM-41 
architecture, requires that a surfactant template be edded to the sol-gel mixture [56-
 59]. The sol-gel method has a great potential in making nanocomposite materials. 
 
1.2.3. Other Mesoporous Materials: Post-LCT Syntheses 
 
The novel structures and properties of MCM-type mesoporous molecular sieves, have 
prompted the extension of the ideas of liquid crystal templating used to synthesize the 
M41S silicates, to the synthesis of other silica and non-silica analogues [33, 60]. The 
organic/inorganic interface chemistry for the synthesis of these mesostructured 
surfactant-inorganic arrays involves four different interactive routes [33, 61]: (S+I-), 
(S-I+), (S+X-I+) and (S-M+I-), where S+ = cationic surfactant, S- = anionic surfactant, I+ 
= cationic inorganic species, I- = anionic inorganic species, X- = (Cl-, Br-) and M+ = 
(Na+, K+, etc.). Consequently, the synthesis involving these interactions has been 
referred to as the generalized or cooperative synthesis mechanism. 
 
1.2.3.1. Silica-Based Mesoporous Materials 
 
Ever since the disclosure of the M41S family of mesoporous materials in 1992, a 
veritable explosion of research effort has occurred in macromolecular and micelle 
templated syntheses of mesostructured materials. Consequently, many synthetic 
routes and strategies have been developed to yield a wide diversity of various 
frameworks with different chemical compositions and pore structures. The actual 
differences are due to the nature of the electrostatic interactions between the organic 
and inorganic phases. The initial Mobil silicate-based materials were prepared in a 
1 GENERAL INTRODUCTION 
 10
 process involving direct ionic bonding between the liquid-crystalline cationic 
surfactant assemblies and an anionic silicate species (S+I- interaction) [25, 26], giving 
rise to the so-called liquid-crystal templating (LCT) mechanism. Post-M41S advances 
have led to the development of synthesis models based on the interfacial interactions 
between the surfactant and the inorganic species, and also on the fact that the silicate 
species promote the formation of a liquid crystal phase below the critical micelle 
concentration (CMC). In addition to the previously mentioned (S+I-) pathway, the (S-
 I+) pathway has been developed [62] which involves the condensation of an anionic 
surfactant with cationic inorganic species. Two other routes were also developed in 
which the surfactant and the inorganic phase have similar charges, but are separated 
from each other by small ions with opposite charge: (S+X-I+) with (X- = Cl-, Br-) and 
(S-M+I-) with (M+ = Na+ or K+) [33, 62]. At the same time, an elegant route was 
proposed by Tanev et al [63-65] to prepare mesoporous silicas at room temperature 
by a neutral templating route (S0I0). In this case, the organic surfactant is not a 
quaternary ammonium cation but a primary amine (dodecylamine), and the self-
 assembly involves hydrogen-bonding interactions between the neutral primary 
amines and neutral inorganic precursors. The resulting materials, denoted HMS 
(hexagonal mesoporous silica), can be considered as special members of the M41S 
family, but differ significantly from the molecular sieves obtained by electrostatic 
assembly pathways. They usually possess thicker framework walls (2-3 nm compared 
to ca. 1 nm for MCM-41), smaller X-ray scattering domain sizes and a textural 
mesoporosity. The small X-ray scattering domain sizes, evidenced by the absence of 
narrow (110) and (200) reflections in the X-ray pattern, reflect a short range 
hexagonal order compared with MCM-41. One of the most important advantages of 
HMS compared with MCM-41 is that the organic phase (SDA) can be removed from 
the as-synthesized samples by solvent extraction, which is not possible in the case of 
other pathways where strong electrostatic interactions exist between the organic and 
inorganic phases [64]. The solvent extraction prevents the partial degradation of the 
mesoporous structure that could occur during calcination in air at relatively high 
temperatures. Hexagonal mesoporous silica (HMS), due to their thicker and more 
1 GENERAL INTRODUCTION 
 11
 condensed pore walls, are more hydrothermally stable than the materials prepared 
through cationic templating [66].  
 
Other models describing the micelle-templated synthesis of mesoporous silica have 
been proposed. These include silicate rod assembly [67], silicate layer puckering [68], 
charge-density matching with the possibility of deriving MCM-41 from a lamellar 
phase [32], true liquid crystal templating [69], and cooperative templating (a.k.a. 
generalized LCT model) [33, 61].  A wide spectrum of novel structures have been 
obtained through these templating mechanisms. 
 
The use of gemini surfactants of the form CnH2n+1N+(CH3)2(CH2)sN+(CH3)2CmH2m+1 
as templates at room temperature has produced mesoporous silica frameworks like 
SBA-2 (3-D hexagonal mesophase, space group P63/mmc) [61], SBA-3 (similar to 
MCM-41) and SBA-1 (cubic, (Pm3n)) [33, 62], as well as MCM-48 [61]. However, 
the use of neutral gemini surfactants of the type CnH2n+1NH(CH2)2NH2 gave rise to 
MSU-G mesoporous silica from TEOS [70].  
 
SBA-2 synthesized at room temperature in alkaline pH with the gemini surfactant 
[CH3(CH2)15N(CH3)2(CH2)3N(CH3)3]2+ has been found to adopt a number of 
morphologies [71], including hollow spheres (ca. 100 ?m diameter), flat plates (50 
?m across), and small spheres (1-2 ?m diameter). As seen in Figure 1.3 below, the 
plates appear to be formed directly by rupturing the hollow spheres: 
 
1 GENERAL INTRODUCTION 
 12
  
 
Figure 1.3. SEM micrograph of SBA-2 showing different morphologies [ref. 71] 
 
An ordered mesoporous silica with a novel cubic structure (space group Fd3m) has 
been synthesized by using tri-head group quaternary ammonium surfactants as the 
structure-directing agents under basic conditions at low temperatures [72]. Tri-head 
group surfactant amphiphiles are a kind of multicharge cationic surfactant with three 
head groups separated by different methylene chains and a variable length 
hydrophobic tail, and are closely related to gemini surfactants [34, 73, 74]. The higher 
charge in the former type of surfactants enables it to interact strongly with water 
molecules and to easily give new amphiphile self-aggregation [75]. The resulting 
mesoporous materials have uniform nanopore size (~3.0 nm) and large surface areas 
(~960 m2/g). 
 
A number of neutral templating methods used to synthesize mesoporous silicas, such 
as the use of polyethers, have been reported in the literature [64, 65, 76-78]. The 
templating mechanism in this case is presumed to involve a phase separation of the 
hydrophobic and the hydrophilic part of the long chain polyethylene molecules prior 
to the formation of the molecular sieves. A prototype of a mesoporous silica obtained 
via this route is SBA-15, with periodic pores of dimensions in the range 50-300 ?, 
and synthesized using an amphiphilic tri-block copolymer, [EO20-PO70-EO20] as 
1 GENERAL INTRODUCTION 
 13
 organic SDA in aqueous acidic medium [76, 77]. Further developments in the 
triblock copolymer-templated syntheses have also given rise to FDU-1, a cubic 
(Fm3m structure) mesoporous silica with a 3-D hexagonal channel system [79, 80]. 
The template used in this synthesis is a tri-block copolymer (EO39BO47EO39) and 
TEOS serves as a silica source. A related type of nonionic surfactant-templated 
synthesis of mesoporous silica uses Vitamin E TPGS as structure-directing template 
[81]. The resulting mesoporous silica referred to as DAM-1, has a highly ordered, 
hydrothermally-stable, 2-D hexagonal mesostructure with various morphologies 
being achieved, including spheres, gyroids, discoids, hexagonal plates and rods. 
 
A novel mesoporous silica molecular sieve (designated KSW-2) is also obtainable 
from mild acid treatment of a layered alkyltrimethylammonium (CnTMA)-kanemite 
complex [82, 83]. This material is different from the hexagonal mesoporous silica 
(FSM-16) derived from kanemite [29, 84] in that it has an orthorhombic structure 
with rectangular arrangements of semi-squared one-dimensional channels. A 3-D 
cubic mesoporous structure, SBA-12, has also been synthesized using the oligomeric 
alkyl-ethylene oxide surfactant, Brij 76 (C18EO10) [77]. 
 
Although the mesoporous silica material MSU-X was prepared by a neutral surfactant 
assembly pathway using TEOS and nonionic surfactants [65], a related mesoporous 
silica molecular sieve, with a wormhole-like framework typical of MSU, was 
obtained under acidic conditions using a low-cost and biodegradable amphoteric 
surfactant template, tetradecyl betaine [85].  
 
Inexpensive non-surfactant templates, e.g. triethanolamine (TEA), have been found to 
direct the synthesis of well-defined, hydrothermally-stable mesoporous siliceous 
oxides, denoted TUD-1 [86]. The mesopore size distribution of these mesoporous 
materials can be controlled by altering the reactant composition and the preparation 
variables [87]. Their pore sizes can be tuned in the range 25-250 ? and the surface 
areas varied between 400 and 1000 m2/g. 
1 GENERAL INTRODUCTION 
 14
  
1.2.3.2. Metal Oxide-Based  Mesoporous Materials  
 
The ?generalized liquid crystal templating? mechanism, also known as ?cooperative 
templating model?, [33, 61] can be extended to non-silica mesostructured materials 
synthesis. This method is based on the specific type of electrostatic interaction 
between a given inorganic precursor and surfactant head group. So far, this coopera-
 tive model has been regarded as the most plausible mechanism for the formation of 
mesoporous materials. Following this approach, a number of mesoporous metal 
oxides such as antimony oxide and tungsten oxide have been synthesized [33, 62]. 
Other research groups have also reported the templated syntheses of mesoporous 
metal oxides and representatives include: alumina [62, 88-90], titania [91, 92], vanad-
 ium oxide [93, 94], zirconia [95-98] and zirconium oxophosphate [99, 100], hafnium 
oxide [101], manganese oxide [102], iron oxide [103] and iron sulfides [104]. 
 
Ying and his group have developed an approach called ?ligand-assisted templating? 
(LAT) to ensure a proper precursor-template interaction and micellar self-assembly 
prior to precursor hydrolysis [105, 106]. In the ligand-assisted templating approach 
the inorganic components (alkoxides) are covalently bonded to the long-chain amine 
template before the hydrolysis and condensation of the precursor. This approach 
allows the selective hydrolysis and condensation of the oxide precursor on the 
micellar surface and prevents the uncontrolled precipitation of particulate oxide 
materials that is commonly observed in non-silicate materials. Meanwhile, Ying and 
coworkers have used the ?ligand-assisted templating? mechanism to form non-silica 
based metal oxides using amine surfactants. This synthesis approach resulted in the 
preparation of a new family of hexagonally packed mesoporous transition metal 
oxides termed TMS1 [91,105-109], based mainly on niobium oxide, for example, Nb-
 TMS1, a niobium oxide analogue of the hexagonally packed mesoporous silicate-
 based MCM-41 was prepared using this approach. Tantalum oxide molecular sieves 
were also obtained by this templating approach [107].   
1 GENERAL INTRODUCTION 
 15
  
Other approaches, including the use of amphoteric surfactants for preparing mesopor-
 ous zirconia [110], have also been reported. 
 
1.2.3.3. Carbon-Based Mesoporous Materials 
 
Most work in the field of carbon-based mesoporous materials has been carried out by 
Ryoo?s group at KAIST (Korea Advanced Institute of Science and Technology). The 
resulting materials are designated CMK-n, where CMK is an acronym for Carbon 
Mesostructures at KAIST: 
 
CMK-1 was the first member of the ordered mesoporous carbon molecular sieves to 
be synthesized [111]. It was prepared by using MCM-48 mesoporous silica molecular 
sieves as template and sucrose as the carbon source. Substitution of the MCM-48 
template with SBA-15 gave rise to CMK-3 [112], which shows retention of the 
template structural symmetry (See Figure 1.4): 
 
1 GENERAL INTRODUCTION 
 16
  
 
Fig. 1.4. (Left): Changes in powder XRD patterns during synthesis of the carbon molecular 
sieve CMK-1 with its silica template MCM-48: (a) The mesoporous silica molecular sieve 
MCM-48, (b) MCM-48 after completing carbonization within pores, and (c) CMK-1 [ref. 
111]. (Right): Powder XRD patterns of CMK-3 carbon and SBA-15 silica used as template 
for the CMK-3 synthesis [ref. 112]. 
 
After removal of the template (silica framework) using an aqueous solution of sodium 
hydroxide or HF, the mesoporous carbon structure was obtained. The XRD patterns 
of both CMK-1 and CMK-3 showed no peaks in the region 2? > 10o, indicating that 
the carbon framework was atomically disordered. These properties coupled with the 
high specific surface area (BET area 1380 m2/g and 1520 m2/g, respectively) suggest 
that the carbon molecular sieves can be useful as catalytic materials in hydrophobic 
applications and as adsorbents for the removal of organic pollutants, as well as in 
organic-sampling applications. Importantly, Kang et al [113] have recently reported 
1 GENERAL INTRODUCTION 
 17
 the regeneration of highly ordered mesoporous silica from CMK-3, which showed the 
reversible replication between carbon and inorganic materials. 
 
By analogy with the synthesis of CMK-1 above, a mesoprous carbon molecular sieve 
was formed using UTD-1 as structure directing template [114], which was found to 
have an ?inverse? TUD-1 structure, with surface area > 1000 m2/g and tunable mesop-
 ore size.  
 
1.2.4. Methods of Template Removal from the Mesostructure 
 
The surfactant structure-directing agent (SDA) used to organize the silicate oligomers 
during preparation is usually trapped inside the solid inorganic matrix, forming a cast 
of the ordered nanoscale structure. Thus, the method by which it is removed is critical 
in obtaining the final mesoporous molecular sieves, as it can alter the structure and 
the porosity of the material. By far the most common method of removing the SDA 
(by analogy with microporous zeolites synthesis) after material synthesis is calcina-
 tion. In this process the material is heated slowly in an oxidizing environment, that 
converts the organic species into gaseous oxides. In addition to this conventional 
method, used in the initial reports on the synthesis of MCM-41, various other relati-
 vely new methods for removing the occluded surfactant template from the pores of 
the resulting mesophase have been reported: 
 
(a) Sample controlled thermal analysis (SCTA) [115], has been successfully used 
[116] to remove the template from as-synthesized Si-MCM-41. This technique also 
allowed a study of the mechanism of thermal degradation of the cationic surfactant 
CTAB occluded in a pure silica MCM-41-type material, as well as the consequences 
on the porous properties of the final material. 
 
(b) Solvent extraction of the surfactant template using ethanol solutions of acids or 
salts [66,117-120]. Advances in the solvent extraction of the template after MCM-41 
1 GENERAL INTRODUCTION 
 18
 synthesis led to supercritical fluid extraction (SFE), reported by Kawi and Lai [121]. 
This method allows the surfactant template to be recovered and re-used.   
 
(c) Extraction by ozone treatment [122, 123] of the as-prepared MCM-41 meso-
 structure in its mother liquor results in mesoporous MCM-41 with larger pores than 
the similar material prepared by traditional calcination. 
 
(d) Silylation with ethanolic solutions of alkylsilanes and alkylsiloxanes [124], was 
found to achieve simultaneous template extraction and direct surface modification.  
 
(e) Treatment of uncalcined MCM-41 with trialkylchlorosilanes [125] has also been 
identified as a method for template removal after MCM-41 synthesis. 
  
Template removal is typically achieved by either calcination or solvent extraction or a 
combination of the two methods.    
 
1.3. Characteristics of Mesoporous Materials (MCM-41) 
 
1.3.1. Bulk and Textural features of MCM-41 
 
Mesoporous MCM-41 materials are characterized by exceptionally high specific 
surface areas (?1000 m2/g), high thermal stability and a narrow monodisperse pore 
size distribution. Other attractive properties of MCM-41 materials are high hydro-
 carbon sorption capacities (> 50 wt% cyclohexane at 40 torr, 49 wt% n-hexane at 40 
torr, and 67 wt% benzene at 50 torr) and pore volumes in the range 0.7-1.2 cm3/g. 
MCM-41 consists of a honeycomb structure that results from a regular hexagonal 
array of uniform unidimensional cylindrical pores. The initial results from the Mobil 
researchers [25, 26, 64] showed that it is possible to tailor the pore sizes of these 
materials to be in the range 20-100 ?, by both manipulating the alkyl chain length of 
the cationic surfactant template used in the synthesis and by adding auxiliary hydro-
1 GENERAL INTRODUCTION 
 19
 carbons, such as mesitylene to the synthesis mixture. Materials with pore sizes greater 
than 40 ?, and up to 100 ?, could be synthesized using reaction mixtures containing 
auxiliary organics (mesitylene). The pore structure of the resulting materials, as seen 
under transmission electron microscopy (TEM), is depicted in Figure 1.5:  
 
 
 
Fig. 1.5. TEM images of MCM-41 materials with Ar pore sizes of (a) 20, (b) 40, (c) 65, and 
(d) 100 ?. [ref. 26] 
 
From Figure 1.5 above, it is apparent that all samples show the hexagonal structure, 
although smaller pore size materials (< 40 ?) show the most regular hexagonal arran-
 gement of uniform pores, i.e. those prepared in the absence of any organics. The reg-
 ularity decreases as the pore size increases.  
 
In contrast to the addition of auxiliary organics to swell the pore diameters of MCM-
 41, Corma et al [126] were able to prepare swollen, highly ordered MCM-41-type 
materials by adjusting the CTMA/SiO2 ratio of the synthesis gel in the range 0.09 to 
0.15 and by carrying out the synthesis under hydrothermal conditions. 
1 GENERAL INTRODUCTION 
 20
  
A typical XRD pattern of MCM-41, together with a nitrogen adsorption isotherm are 
shown in Figure 1.6 below. The powder XRD pattern shows three to five Bragg peaks 
in the region 2o < 2? < 10o, indicating the long-range order present in this material. 
Since the reflections are due to the ordered hexagonal array of parallel silica tubes, 
they can be indexed assuming a hexagonal unit cell as (100), (110), (200), (210) and 
(300). A useful parameter is the lattice constant or repeat distance (ao) between two 
pore centres. This may be calculated by ao = 2d100/?3, where d100 is the interplanar 
spacing corresponding to the (100) Bragg reflection. For the material whose inter-
 planar spacing d100 is about 40 ?, the lattice parameter ao is about 45 ?.  
 
                                (a)                                                              (b) 
 
                          
 
 
 
 
 
 
 
 
 
 
 
Fig. 1.6. XRD pattern (a) of high-quality calcined MCM-41 reported by Huo et al [34], and 
N2 adsorption isotherm (b) reported by Branton et al [127].  
 
The adsorption isotherm (Figure 1.6b) gives information on the predominant type of 
pores in the MCM-41 material. It corresponds to a reversible type IV isotherm, 
indicating the presence of mesopores. A sharp increase in the adsorbed and desorbed 
volumes at a relative pressure P/Po ~ 0.42 reflects the uniformity of the pore-size 
1 GENERAL INTRODUCTION 
 21
 distribution, and corresponds to capillary condensation in the mesopores. The average 
pore diameter in this sample, calculated using the Barrett-Joyner-Halenda (BJH) 
method, was very narrow at 3.3 nm.  
 
1.3.2. Morphologies and Shapes in Mesoporous MCM-41 Materials 
 
Controlling the macroscopic morphology of mesoporous materials could generate 
potential practical applications for these systems. Therefore, there has been a great 
deal of work performed on the development of mesoporous MCM-41-type materials 
with controlled morphologies and shapes such as films [128-132], fibres [130-135], 
spheres [136-140], monoliths [69, 141, 142], ropes and gyroids [143], etc.  
 
Si-MCM-41 can also be prepared such that a bimodal distribution of morphologies is 
observed [144, 145]. Figure 1.7 below shows the bimodal morphologies of MCM-41 
reported by Schulz-Ekloff et al [144], consisting of both worm-shaped and spherical 
MCM-41 particles and called bimodal silica (BMS): 
 
 
 
 
 
 
 
 
 
 
Fig. 1.7. SEM images of (a) BMS-1, (b) BMS-2, and (c) BMS-3. The length of the bar for (a, 
b) corresponds to 5 ?m and for (c) corresponds to 10 ?m. Varying amounts of ethyl acetate 
were added to the synthesis mixture for the preparation of the BMS materials, with BMS-1, 
a b c 
1 GENERAL INTRODUCTION 
 22
 BMS-2 and BMS-3 prepared by adding 15, 25 and 35 ml, respectively, of ethyl acetate to the 
synthesis gel. [ref. 144]  
The above distribution has been achieved by homogeneous precipitation from an 
isotropic reaction mixture, wherein the hydrolysis of ethyl acetate led to a decrease in 
the pH of the reaction mixture and consequent formation of solid particles.  
 
A critical property of MCM-41 materials, which has a direct bearing on their practical 
utility, is the hydrothermal stability in reactions that produce water as a by-product. 
Section 1.3.3 is dedicated to the origin of this property and ways or methods through 
which it can be circumvented.  
 
1.3.3. Hydrothermal Stability of MCM-41 Materials 
 
While MCM-41 has been reported to have high thermal stability in air, O2 and water 
vapour [119, 146, 147] it has low hydrothermal stability in boiling water and aqueous 
solutions. This poor hydrothermal stability is the limiting factor in the potential 
application of these materials to several types of hydrocarbon processing procedures 
and adsorption processes requiring heavy-duty regeneration. The origins of this poor 
stability is related to their thin pore walls of ca. 5 ? [25, 148], which are also amorp-
 hous [100], and can be easily destroyed by hydrolysis of the Si-O-Al or Si-O-Si 
bonds. Dealumination can also occur during calcination. The laws of sintering state 
that the existence of a small hole between parallel pores creates a highly unstable 
situation in aluminosilicates and silicates [45], and since MCM-41 materials possess 
such pores, dealumination should lead to unstable materials. Other groups have 
proposed that the loss of structure in MCM-41-type materials under hydrothermal 
conditions involves silicate hydrolysis by water adsorbed onto the silanol groups 
[149, 150]. It has also been found that mesoporous silicates undergo a temperature-
 induced phase transformation during synthesis [151], resulting for example, in MCM-
 48 being easily and rapidly (less than 6 h) produced from MCM-41. Thus, control of 
the temperature is also an important factor in producing MCM-41 under hydro-
1 GENERAL INTRODUCTION 
 23
 thermal conditions. Enormous efforts have been made to improve the stability of 
these mesoporous materials either by changing the synthesis procedure or by 
modifying the materials after synthesis, and a review of the several methods advanced 
in this regard are given in the next section. 
 
1.3.3.1. Improving the thermal/hydrothermal stability of Si-MCM-41 
 
The structural stability of MCM-41 samples in boiling water can be improved by 
changing synthesis variables such as the hydrothermal synthesis temperature [152], 
alkyl chain length of the templates [152], type of silica source [153], pH during synt-
 hesis [146, 154-156], as well as inorganic and organic salt addition (sodium chloride, 
sodium acetate, sodium fluoride, tetraalkylammonium ions, EDTA) during the cry-
 stallization process of the pure silica material [146, 158-163]. A combination of 
repeated pH adjustments during synthesis and the salt effect [164, 155] has been 
found to significantly enhance the hydrothermal stability of the resulting pure silica 
MCM-41.  
 
Several approaches have also been proposed that include enhancement of the conden-
 sation of the silanol groups by replacing NaOH with small alkylamines as minerali-
 zation [165], preparation of materials with thicker pore walls by controlling the synth-
 esis gel composition (the OH-/SiO2 ratio) [76, 166] or by secondary synthesis using 
Si-MCM-41 as a silica source in a two-step synthetic process over long crystallization 
times [167, 168], inclusion of fluoride ions in the synthesis gel for MCM-41 or SBA-
 15 [162, 169-173], one-step organic modification [174] and post-synthesis hydro-
 thermal restructuring in water [175, 176]. Incorporation of elements such as titanium 
or aluminium into the walls [177 - 180], or even ion exchanging calcined AlMCM-41 
with Na+, K+, Ca2+ and Y3+ [146] has been found to improve the hydrothermal 
stability of the MCM-41. 
  
1 GENERAL INTRODUCTION 
 24
 Several other post-synthesis treatments have been shown to improve the hydrothe-
 rmal stability of siliceous MCM-41. These include grafting of Al onto Si-MCM-41 
materials [181] or incorporating Al into Si-MCM-41 [182, 183], treatment with acid 
[120, 184], coating of pore interiors with further layers of silica [185, 150] or alumina 
[186], dispersion of a ZrO2 monolayer on MCM-41 [187] and surface modification of 
MCM-41 with Grignard reagents [188, 189]. The latter approach gives stable Si-C 
bonds that render the material highly hydrophobic and consequently hydrothermally 
stable. Other effective modifications are the complete [150, 190, 191] or partial [192] 
hydrophobization of the surface by trimethylsilylation to preclude water attack. 
However, Pan et al [193] found that the BET surface area and pore size of the MCM-
 48 silylated by trimethyl chlorosilane (TMCS) decreased markedly, hence retarding 
the advantages of MCM mesoporous materials possessing high surface areas and big 
pore sizes. 
 
A unique method for addressing the hydrothermal instability of mesoporous materials 
was reported by Galarneau et al [194]. The method involves using intercalated 
surfactants in the interlayer spaces of smectite clays to template the polycondensation 
of TEOS, resulting in materials with thermally stable mesopores, referred to as 
porous clay heterostructures (PHCs).  
  
The importance of Al incorporation has been emphasized by the results of Kawi and 
Chen [195], who found that substitution of Al in the material framework or by 
introduction of non-structural Al species onto the pore surface of MCM-41 has the 
effect of stabilizing the mesoporous framework of the resulting materials in boiling 
water. This is indicated by the XRD patterns (Figure 1.8) and BET surface analysis 
data (Figure 1.9). 
 
1 GENERAL INTRODUCTION 
 25
  
Fig. 1.8. Powder XRD patterns of Si-MCM-41 and Si-Al-MCM-41 (a) before and (b) after 
treatment in boiling water for 1 week [ref. 195] 
 
 
Fig. 1.9. SBET of Si-MCM-41, Si-Al-MCM-41, and Al/Si-MCM-41 before and after treatment 
in boiling water for 1 week: ? fresh sample, ? in boiling water. The last material was 
prepared by Al impregnation of Si-MCM-41. [ref. 195].  
1 GENERAL INTRODUCTION 
 26
  
It can be seen from the data above that Si-MCM-41 loses most of its surface area 
(from 1301 to 107 m2/g) after it had been soaked in boiling water for 1 week, whereas 
Si-Al-MCM-41 still maintains its surface area around 1000 m2/g under treatment in 
boiling water for 1 week. In contrast to these observations, He et al [196, 197] 
reported that Al(III) decreased the thermal and hydrothermal stability of MCM-41 
materials, whereas La(III) and Fe(III) improved these properties.  
 
Recent advances in the synthesis of MCM-41 materials with thickened pore walls 
have led to the use of high-pressurized (12 MPa) synthesis conditions [198], decrea-
 sing the pore diameter of the template micelles [199], and increasing the hydrother-
 mal synthesis time and temperature [200].    
 
1.4. Catalysis and Technical Applications of MCM-41 Materials 
 
Si-MCM-41 mesoporous materials lack intrinsic catalytic applicability by themse-
 lves, but can be modified with organic functional groups or heterogenized with transi-
 tion metals in order to make them suitable for various applications. Work in these two 
fields is briefly reviewed in the next subsections. In the metal-containing variants of 
MCM-41, Fe and Co-derivatized materials will be discussed separately from the other 
metallated species, because such materials form part of thesis study. The catalytic 
applications of the rest of the metal-variants of MCM-41 will be described in section 
1.4.3. 
 
1.4.1. Organically-Modified MCM-41 Materials 
 
Many applications (such as adsorption, ion exchange, catalysis and sensing) require 
mesoporous materials to have certain attributes such as binding sites, stereochemical 
configuration, charge density and acidity [201-203]. Therefore, by analogy with the 
chemistry of organically-modified silicates or ORMOSILs [204-206], MCM-41-type 
1 GENERAL INTRODUCTION 
 27
 materials can also be modified by attachment of organic functionalities to their 
surfaces in order to increase their potential applicability, an aspect that forms an 
important research area in heterogeneous catalysis and green chemistry [207]. The 
interest in inorganic-organic hybrid materials arises from the advantages that the 
inorganic and organic moieties bring to the systems [208, 209], viz., that the 
inorganic components can provide mechanical, thermal or structural stability, while 
the organic features are often more readily modified for specific applications in 
catalysis, separation, or sensing [210]. The typical precursors in the preparation of 
inorganic-organic hybrid materials are organically functionalized metal alkoxides, 
whose Si-C bonds should be hydrolytically stable under acid or base conditions. The 
ordered mesoscopic inorganic-organic hybrid materials contain easily modifiable 
organic functional groups for applications as catalysts, catalyst supports, and statio-
 nary phases in chromatography which take advantage of the high surface areas and 
specific size selectivities of the mesoporous silicate structures [211].  
 
Since the discovery of MCM-41, its organically-modified derivatives have also been 
prepared by either post-synthesis grafting or co-condensation of tetra-alkoxysiloxane 
and one or more organo-alkoxysilanes through the sol-gel process [212, 213]. The 
organic functionalization of silicates has been found to play an important role of 
permitting precise control over the surface properties, modification of the hydrophilic 
or hydrophobic character of the surface, alteration of the surface reactivity, protection 
of the surface from attack, modification of the bulk properties of the materials and at 
the same time stabilizing the materials towards hydrolysis [209, 214-217].  
 
Representative catalytic reactions catalyzed by organically-modified mesoporous 
silica are described in this section, separate from those of the Fe- or Co-MCM-41 
materials and those of other derivatives of the M41S materials. Both direct and post-
 synthesis methods are used in their preparation as outlined below. 
 
 
1 GENERAL INTRODUCTION 
 28
 1.4.1.1. Co-condensation or Direct Organic Modification of MCM-41   
 
Co-condensation of the reactive species during the mesopore synthesis is a method to 
incorporate functionality into the walls of a channel system [218]. This approach is an 
extension of the organically-modified sol-gel derived materials. The co-condensation 
method is the most common and direct synthesis route to the introduction of organic 
groups into a silica network [219-221], and provides better control over the amount of 
organic groups incorporated into the matrix and ensures the uniform surface coverage 
with organic groups under mild conditions. The preparation method involves co-
 condensation of TEOS and organosiloxanes in the presence of surfactant templates at 
room temperature, giving rise to covalently linked, ordered, hybrid inorganic-organic 
networks. The role of TEOS is to act as a primary silica source and as reagent for the 
formation of fully saturated Q4 linkages which are required to maintain structural 
integrity in the organo-silica-surfactant mesophases [222]. 
 
Although many research groups have synthesized organically-modified mesoporous 
silicates through this method [211, 222-226], of particular interest is the work of 
Macquarrie [227, 228], who showed that functionalizing MCM-41 with aminopropyl 
functional groups by a one-pot method gives rise to an effective Knoevenagel 
condensation catalyst. Direct organic modification of Ti-MCM-41 materials has been 
found to produce highly efficient catalysts for epoxidation reactions [229-231]. 
 
1.4.1.2. Post-synthesis Organic Modification of MCM-41 
 
In the post-synthesis grafting method, the surface of the pre-fabricated inorganic 
mesoporous materials (MCM-41 has a high density of surface silanols) is reacted in a 
suitable solvent with organosilane coupling reagents through the surface silanol 
groups. This method has the advantage that the original structure of the mesoporous 
matrix is usually maintained intact. Post-synthesis grafting techniques have been used 
for many years to attach organic functionalities to mesoporous silicate surfaces [232], 
1 GENERAL INTRODUCTION 
 29
 making this a relatively older method when compared to the co-condensation in 
mesopores.  
 
Through the grafting technique, [233] micelle-templated silicas (MTS) were function-
 alized with primary or secondary amine functions to achieve effective catalysts for 
the Knoevenagel condensation reaction. Quaternary tetraalkylammonium hydroxide-
 grafted MCM-41 was also found to be a strong Bronsted base catalyst [234] and an 
excellent alternative to homogeneous catalysts for Knoevenagel condensations, 
Michael additions and aldol condensations because of their high catalytic activity and 
good selectivity under mild conditions. Post-synthesis double organic modification 
[235] of Si-MCM-41 and Ti-MCM-41 using 3-chloropropyl and methyl groups 
produced efficient catalysts for the epoxidation of cyclododecene. 
 
 1.4.1.3. Decontamination of Ground Water: Heavy Metals and Oxyanions 
 
Heavy metals, particularly mercury and lead, are important environmental pollutants, 
threatening the health of human populations and natural ecosystems alike. Removal 
of these species from the environment by selective binding on solid adsorbent 
surfaces is a major focus of waste treatment and cleanup efforts. The high surface 
areas combined with large and accessible pores of ordered mesoporous silicas (OMS) 
of the M41S family, make them good candidates for the creation of novel adsorbents 
of tailored surface properties for environmental applications [216, 236]. This tailoring 
can be achieved by post-synthesis coating of the inside surfaces of mesopores with an 
organic monolayer whose molecules are covalently bound to the silica. The free ends 
of the tethered molecules can be chemically tailored to sequester particular species 
from solution. 
 
The adsorption of toxic metal cations has been explored for functional groups fixed 
on mesoporous silica. The divalent cations (Cu, Zn, Cr and Ni) present in waste-water 
are adsorbed to a greater extent on amino-functionalized SBA-15 than on thiol-
1 GENERAL INTRODUCTION 
 30
 functionalized SBA-15, whereas Hg2+ is preferably adsorbed on thiol-functionalized 
SBA-15 [237]. The mercury adsorption on thiol-functionalized mesoporous silicas 
has been intensively studied [216, 236, 238-240]. Mercier and Pinnavaia?s thiol-
 functionalized mesoporous material, denoted MP-HMS [216], exhibited Hg(II) 
binding with sufficient efficiency to meet federal drinking water standards, along 
with an unprecedented loading capacity of 310 mg/g). Concurrent with this report, on 
thiol-functionalized HMS silica, Feng et al [236] reported an analogous heavy metal 
ion adsorbent based on the mercaptopropylsilyl functionalization of a calcined form 
of mesoporous MCM-41 silica. Although their grafting approach was similar to that 
reported previously for the functionalization of MCM-41 [241, 242], a much higher 
concentration of immobilized ligand was achieved through the use of a larger pore 
framework (5.5 nm) and a functionalization process that involved repeated surface 
hydrolysis and silylation cycles. The mercury loading (505 mg/g) was even higher 
than that observed for functionalized HMS. This study showed that the formation of 
almost a monolayer of Si-CH2-CH2-CH2-SH was followed by bridging by Hg-O-Hg 
units to sulphur atoms, which earned these materials the codename FMMS 
(functionalized monolayers on mesoporous supports) and eventually SAMMS (self-
 assembled monolayers on mesoporous supports). An advantage of this type of 
strategy is that the adsorption site has a molecular nature so that the adsorption 
capacity can be estimated on the basis of stoichiometry. However, it was demons-
 trated that the ratio of Hg to S depended on the kind of mesoporous silica used [238].  
 
Over the years, the FMMS or SAMMS technology has been developed at the Pacific 
Northwest National Laboratory (PNNL) in the USA) , since these materials are 
extremely efficient in removing mercury, heavy metals, radionuclides and anions 
from both aqueous and nonaqueous streams. They can adsorb metals 30 to 10,000 
times more effectively than current commercial filters, as the thiol groups of 
TMMPS-coated silica bind strongly to heavy metal ions (e.g. Hg2+ and Ag+). The 
adsorptive power of the material is measured in terms of the distribution coefficient, 
Kd. The functionalized monolayers have shown a Kd as high as 108 for mercury, 
1 GENERAL INTRODUCTION 
 31
 compared to 104 for other resin materials [243]. The stability and potential to 
regenerate and reuse these materials has also been demonstrated. A schematic 
summary of the synthesis of the silica sponge (SAMMS) and its final structure [236, 
244, 245] is depicted in the figure below: 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 1.10. Recipe for the formation of a silica sponge (A), and retention of the hexagonal 
structure B (ref. 245). 
 
B 
A 
1 GENERAL INTRODUCTION 
 32
 Studies on the nature of the interface chemical interaction between mesoporous silica 
and the substrate [246] have revealed that high-quality, close-packed monolayers can 
be formed on the nanoporous supports through the introduction of several layers of 
physically adsorbed waters on the nanoporous surface. The role of this physisorbed 
water is to physically confine all the hydrolysis and condensation reactions of the 
organosilanes to the interface. Figure 1.11 (a) and (b) below show direct chemical 
bonding between heavy metals and alkyl thiols for heavy-metal adsorption: 
                                    (a)                                                                           (b) 
                                  
 
 
 
 
Fig. 1.11. Heavy metal removal by SAMMS [ref 246] 
 
In summary, the SAMMS technology utilizes the unique properties of mesoporous 
materials (surface areas ~1000 m2/g and uniform pore size ranging from 1-50 nm) to 
functionalize up to 100 percent of the available surface area using self-assembly 
techniques. This results in a highly selective material with exceptional material 
loading and kinetic properties.  
 
Numerous reports on the syntheses of metal-coordinated functionalized mesoporous 
silicas [218, 247-252] indicate how the structure of the adsorption site can capture 
cations (or anions). 
  
The risk of exposure to toxic oxyanions in ground water is a rising environmental 
problem worldwide. These anions include arsenate and chromate that are particularly 
hazardous. The EPA maximum contaminant level for As in drinking water is 10 
?g/dm3, and naturally occurring excessive arsenic has been widely detected in wells 
in the Chikugo plain, Japan [253]. Under a range of conditions, arsenic and other 
1 GENERAL INTRODUCTION 
 33
 environmentally toxic metals such as chromium, exist in nature as tetrahedral oxy-
 anions. Because efficient adsorption of target ions is not expected on the neutral silica 
surfaces of MCM-41 and other mesoporous silicas, the strategy of functional-ization 
of the surface must be designed for oxyanion adsorption. The high density of silanol 
groups (from 0.5 to 3.0 OH per nm2) [118] helps to fix large amounts of silanes with 
an adsorption activity onto a silica surface.  
 
Unlike cation capture by amino- or thiol-functionalized groups, the studies of anion 
adsorption by functional groups fixed in the pores of mesoporous silicas are rare. 
Fryxell et al [254] have shown that diamino-functionalized MCM-41 exhibits a 
marked capacity for adsorption of Cu2+, giving rise to a positively charged 
mesoporous host, which achieves nearly complete removal of arsenate and chromate 
by selective binding of the anions. The leaching of Cu2+ during the adsorption was 
not significant because of the affinity of the ethylenediamine ligand. In a related 
study on the adsorption of chromate and arsenate by amino-functionalized MCM-41 
and SBA-1, Yoshitake et al [255] found that triaminosilane-grafted mesoporous silica 
adsorbed more oxyanions than mono- and di-functionalized mesoporous silicas. Fe 
centres in functionalized mesoporous silica have been found to be environmentally 
benign and can readily remove pollutant oxyanions from waste water [256]. 
 
Another technical application of MCM-41 is based on the fact that SiO2-based 
mesoporous materials are suitable as host materials for semiconductors, due to their 
high degree of order as well as the large band gap of the SiO2 (which serves as a 
barrier material). Several semiconductor compounds have been implanted into the 
MCM-41 silica to obtain regular arrays for quantum wires, e.g. CdS [257], CdSe 
[258], GaAs [259], InP [260], Ge [261] and SiGe [262]. In this light, Chen et al [263] 
reported the first successful incorporation of the semimagnetic semiconductor Cd1-
 xMnxS into MCM-41 SiO2, as an alternative way of creating highly ordered arrays of 
magnetic quantum wires for possible applications in magneto-electronics. In other 
developments, ?ship-in-a-bottle? synthesis has been used in the preparation of tailored 
1 GENERAL INTRODUCTION 
 34
 metal structures in mesoporous silica like nanowires, which are interesting because of 
their novel electronic, optical and catalytic properties [264, 213]. Examples of metal 
nanowires prepared in mesoporous silica matrices are Cu [265], Pt and Rh [266, 267], 
Au [268], Bi2Te3 [269], etc. Conducting filaments of polyaniline have also been 
prepared in a mesoporous channel host [270], and their a.c. conductivity has been 
demonstrated.  
 
1.4.2. Metal-containing Mesoporous Materials (Me-MCM-41) 
 
The incorporation of transition metals into microporous and mesoporous materials 
has drawn considerable interest because it results in materials with acid and/or redox 
properties [271-274]. However, the resulting redox molecular sieves need to be 
stabilized against leaching of the active metal component if they are to be used in 
liquid-phase reactions like the oxidation of alkenes, alkanes and alcohols with H2O2 
as oxidant [273]. Due to the special pore structure of M41S materials, they can be 
used as a matrix to immobilize catalytically active species onto or within the silica 
walls or as a host to accommodate certain guest compounds. Indeed, MCM-41 and 
MCM-48 are very widely studied members of this family, both as such and when 
they are substituted with redox metals such as Ti and V [275]. Metals across the 
periodic table have been incorporated into mesoporous M41S materials for various 
applications, and both their preparation methods and catalytic applications can be 
found in review articles such as [100, 276, 277]. Since this thesis will concentrate 
mainly on the chemistry of Fe- and Co-MCM-41 composite materials, with particular 
emphasis on their synthesis, characterization and applications, discussion will thus be 
restricted to these metals. 
 
Several approaches have been used to incorporate transition metals into mesoporous 
MCM-41-type materials. These include direct [278-280], ion exchange [281] and 
template ion exchange (TIE) [282, 283], impregnation [284-286], metallocene 
1 GENERAL INTRODUCTION 
 35
 grafting [287 - 290], immobilization (Si-MCM-41s) [291] and ship-in-a-bottle (SIB) 
synthesis [292-298]. 
 
TIE as a preparation technique of metal ion-planted MCM-41 [282] has the advantage 
that metal loadings in the range Si/Metal = 20-320 can be achieved without collapse 
of the mesoporous structure. The technique is beginning to gain popularity in hetero-
 geneous catalysis. 
 
1.4.2.1. Iron-containing Mesoporous MCM-41 (Fe-MCM-41) 
 
Ferrisilicates are an important branch of iron-containing materials that catalyze a 
wide variety of reactions, ranging from acid-catalyzed processes to dehydrogenation 
and oxidation, e.g. oxidation of benzene to phenol by H2O2 using a sol-gel-derived 
Fe/amorphous silica catalyst [299] and selective oxidation of benzene to phenol with 
N2O using Fe-ZSM-5 [300]. Metal-containing MCM-41, which possesses uniform 
nano-order mesopores and high concentrations of isolated active sites, has attracted 
much attention as a new type of oxidation catalyst, especially in liquid-phase oxida-
 tion reactions since facile diffusion of relatively large molecules can be expected.  
 
The first direct synthesis of the Fe-MCM-41 was reported by Yuan et al [301]. This 
was a hydrothermal type of synthesis in which ferric nitrate was part of the initial 
synthesis gel mixture, although He et al [302] reported the first synthesis of similar 
materials at room temperature. Since then a number of reports have appeared on the 
synthesis and characterization of Fe-MCM-41 using different methods, including  
encapsulation of Fe2O3 nanoparticles [303-306], wet impregnation [307], template 
ion exchange [308, 309], incipient wetness impregnation [310, 311], etc. However, all 
the iron-containing mesoporous silica materials prepared by direct synthesis with 
Fe(III) forming part of the synthesis gel, contained ? 1 wt% Fe. Remarkably, Pasqua 
et al [312] were able to control the incorporation of Fe to a loading of up to 5 wt%. 
This was achieved by incorporating the Fe precursor in the presence of the F- ions in 
1 GENERAL INTRODUCTION 
 36
 the synthesis gel at a neutral pH, giving materials with iron exclusively in the silicate 
lattice. At such a pH, stable mesoporous materials with thick silica walls with the iron 
loading up to 5 % were formed. 
 
Furthermore, FeMCM-41 prepared under hydrothermal conditions showed evidence 
for the encapsulation as well as isomorphous substitution of trivalent iron in the 
MCM-41 [313].  
 
Characterization of the Fe(III) environment in MCM-41 is important in accounting 
for the observed properties of the resulting materials. In zeolites, the Fe3+ can occupy 
tetrahedral framework positions or octahedral extraframework positions [314]. 
Iron(III) has five d electrons and in aqueous solutions it forms mainly octahedral 
complexes. With weakly coordinating anionic ligands (like H2O, OH-, Cl-, NO3-, 
etc.), as is the case in zeolite environments, it forms high-spin complexes with 
magnetic moments close to the spin-only value of 5.9 ?B. These complexes are 
usually pale in colour as a result of the 6A1g ground state and the occurrence of 
mainly spin-forbidden bands in the visible region of the spectrum. Thus, UV/Vis 
cannot provide reliable information about the fate of Fe(III) in zeolite matrices. 
 
Unlike Al-, B-, or Ga-modified mesoporous silicas, where the nature and the coordin-
 ation environment of the heteroatom can be probed with solid-state NMR spectros-
 copy [315-319], information about Fe-modified materials is scarce [196, 320]. From 
EPR studies of Fe3+ impurities in NH4-faujasite, Derouane et al [321] showed that 
iron can be simultaneously present in three forms: Fe3+ species in the aluminosilicate 
framework, Fe3+ ions acting as counterions and Fe3O4 or another Fe3+ compound with 
strong exchange spin-spin interactions, precipitated on the zeolite. The synthesis, 
structure and characterization of a ferrisilicate analogue of a ZSM-5 zeolite was first 
reported in the open literature in 1985 [322]. By analogy with ferrisilicate analogues 
of zeolites described above, the Fe(III) in mesoprous MCM-41 can exist either as 
framework (FW) or extraframework species [323], and thus the structural arrangeme-
1 GENERAL INTRODUCTION 
 37
 nt of the iron species is easily detectable by ESR techniques [324, 325]. Indeed, EPR 
spectroscopy has been successfully used to characterize the Fe species in Fe-MCM-
 48 [326,], Fe-MCM-41 [301, 308, 311], Fe-HMS [327] and Fe-Kenyaite [328]. In all 
cases, the ESR spectra of the as-synthesized (surfactant-containing) Fe-mesopore 
were dominated by two signals at g = 4.3 and 2.0, although additional peaks could be 
observed in certain instances. The assignment of these signals was based on previous 
literature on Fe-containing zeolites [321, 324, 329], according to which the signal at g 
= 4.3 corresponds to Fe(III) in a distorted tetrahedral environment whereas, that at g = 
2.0 is attributed to Fe(III) in a highly symmetrical octahedral environment.  
 
The ionic radii of Si4+, Al3+ and Fe3+ are 0.039, 0.057 and 0.067 nm, respectively 
[330]. From this difference in ionic size, it may be possible to deduce framework 
substitution in zeolites by XRD, where incorporation of the larger ion is expected to 
cause some amount of lattice expansion. 
 
Other supplementary information on the location of Fe in MCM-41 can be obtained 
from temperature programmed reduction (TPR) studies. TPR has already been used 
to compare the redox properties of Fe incorporated in Fe-MCM-41 [331]. The 
materials were prepared by direct hydrothermal treatment of an Al-free recipe at 370 
K for 170 h, and contained different framework iron contents. In samples with Si/Fe 
ratios of 20 and 12 all of the iron was found to be reducible with H2, but in materials 
with lower iron content only a part of the Fe(III) species could be reduced to Fe(II). 
In neither of the materials could Fe(III) be fully converted with CO to the divalent 
oxidation state. A significant part of the iron could be reversibly reduced and reoxidi-
 zed in successive redox cycles. The TPR profiles of the prepared Fe-MCM-41 
samples, pretreated in situ in a flow of O2/He prior to the TPR runs, exhibited 
pronounced H2 consumption in the same temperature range, between 520 K and 840 
K. 
 
1 GENERAL INTRODUCTION 
 38
 In the case of Fe-MCM-41(Si/Fe = 77) and Fe-MCM-41(Si/Fe = 139), i.e., at low 
iron content, the areas of this TPR peak were found to correspond to reduction of 
only 76 and 66 % of the incorporated iron from the tri- to the divalent oxidation state, 
respectively. In contrast, the amount of consumed hydrogen calculated from the peak 
area in the TPR curve of Fe-MCM-41(20) corresponded exactly to that needed for the 
reduction of the incorporated iron to the divalent oxidation state. Reduction to 
zerovalent iron started at temperatures higher than 920 K. This points to the stabili-
 zation of the divalent iron species in MCM-41, since hematite was found to be 
completely reduced to metallic iron already at 920 K. The TPR curve of Fe-MCM-
 41(Si/Fe = 12) exhibited a second peak, starting at about 840 K, which was 
tentatively attributed to the reduction of extra-framework iron (EF-Fe) species. 
Furthermore, the H2 consumption calculated from the main peak (between 500 and 
840 K) slightly exceeded the value expected for the Fe3+ ? Fe2+ conversion. 
Possibly, part of the EF-Fe present in the form of large clusters, i.e., as iron oxide 
phases, may be reduced to the zerovalent state (see Table 1.2). 
 
Table 1.2. Iron contents and H2 consumption of different Fe-MCM-41 samples 
 
Sample Fe(mmol/ga) H2 consumptionb 
(mmol/ga) 
H/Fe (atom ratio) 
Fe-MCM-41(139) 0.119 0.039 0.66 
Fe-MCM-41(77) 0.215 0.0817 0.76 
Fe-MCM-41(20) 0.788 0.394 1.00 
Fe-MCM-41(12) 1.254 0.677 1.08 
 
asamples fired at 1273 K; bcalculated from TPR peak between 500 and 840 K (ref. 331) 
 
In any case, as from the TPR data, most of the iron incorporated into the pore walls of 
MCM-41 during hydrothermal synthesis is reducible to Fe(II) below 840 K. Only in 
low-iron-containing materials is a significant part of the iron resistant to reduction. 
1 GENERAL INTRODUCTION 
 39
  
Mesoporous MCM-41 materials isomorphously substituted with iron were reported to 
be catalytically active in the conversion of N2O to NO [332], acetone to mesityl oxide 
and trimethyl-benzene [333], and SO2 to SO3 [334]. Other Fe-MCM-41-catalyzed 
oxidation reactions included the liquid-phase oxidation of cyclohexane with aqueous 
30 % H2O2 [335], hydroxylation of phenol using 30 % H2O2 [336, 337], the 
epoxidation of styrene with H2O2 [309, 338], and gas-phase CO oxidation [331]. Non-
 oxidative catalytic reactions include Friedel-Crafts benzylation of benzene with 
benzyl chloride [339], selective catalytic reduction (SCR) of NO with NH3 [340], and 
ethylbenzene dehydrogenation [341]. 
 
Highly dispersed Fe2O3 was incorporated almost exclusively into the mesoporous 
host (MCM-48) when prepared by impregnation [306], and showed a high catalytic 
activity in the methanol decomposition reaction, with high methane selectivity [342]. 
Direct hydrothermal synthesis of Fe-MCM-48 using a mixture of sodium laurate and 
CTAB as template, afforded an active catalyst for the hydroxylation of phenol [326]. 
The phenol conversion in this reaction was 43.6 % and the diphenol selectivity was 
97.7 %.    
 
1.4.2.2. Cobalt-containing Mesoporous MCM-41 (Co-MCM-41) 
 
Because of its neutral framework, pure silica MCM-41 has had limited applications in 
catalysis, molecular sieving, catalyst supports, adsorbents, etc., [343]. However, an 
active catalyst can be prepared when the silicate framework of MCM-41 is modified 
by partial substitution with elements (or their oxides) such as Al, Ga, Fe, Ti, Pd, Mn, 
and Co [344-348]. Furthermore, high-metal-loaded CoMo/MCM-41 has been 
prepared by co-impregnation of MCM-41 with the metal precursors [349]. Jentys et 
al [348] have synthesized highly dispersed Co/MCM-41 catalysts by direct hydro-
 thermal synthesis involving the addition of Co(II) ions directly to the synthesis gel. 
The degree of dispersion was confirmed by the XRD patterns of the resulting calcined 
1 GENERAL INTRODUCTION 
 40
 materials, which showed the absence of higher order metal oxide peaks (see Figure 1. 
12): 
 
 
 
Fig. 1.12. XRD patterns of Co/MCM-41 and MCM-41 [ref 348] 
 
Interestingly, the small clusters of cobalt did not sinter during reduction or sulphida-
 tion. Incorporation of Co into the MCM-41 lattice was not observed. The addition of 
Co to the synthesis gel did not alter the structural characteristics of the MCM-41 
samples. Co/MCM-41 was also prepared by solid-state ion exchange for comparison 
purposes. In addition, these authors have compared the structural properties of 
Co/MCM-41 prepared by direct synthesis and impregnation methods [350]. For both 
preparation methods the size of the metal particles decreased with the pore diameter 
(pore diameters between 2.9 and 3.6). Comparison of the Co/MCM-41 materials with 
the same pore diameter revealed that the direct synthesis method led to significantly 
smaller metal clusters than the impregnation method. MCM-41 materials functiona-
 lized by cobalt complexes have also been reported [251, 351]. The cobalt loading 
achieved in these studies, in which deposition is based on ion exchange, direct synth-
1 GENERAL INTRODUCTION 
 41
 esis, or impregnation methods, was relatively low. To increase the Co loading, 
Co2(CO)8 was deposited onto Si-MCM-41 surface [352] from the gas phase in a 
fluidized bed reactor, and loadings of up to 41 wt% Co were achieved using a pulse 
deposition method. A multi-stage grafting method has been used to immobilize 
optically-active Co(II) salen complexes in MCM-41 [353]. These materials showed a 
very high enatioselectivity in the asymmetric borohydride reduction of aromatic 
ketones, as compared to similar homogeneous salen catalysts.  
 
Highly dispersed Co-Ru nanoparticles in mesoporous silica xerogels and MCM-41 
were also prepared by impregnation from organic solutions of heterometallic 
carbonyl clusters [NEt4][Co3Ru(CO)12] [354], followed by thermal treatment under 
an inert atmosphere. Co/MCM-41 [355] and Ru-promoted Co/MCM-41 [356, 357] 
have been prepared by Goodwin Jr?s group, and showed activity towards CO 
hydrogenation reactions (i.e. the Fischer-Tropsch reaction). 5 % Co/SCMM (SCMM 
= silicate crystalline mesoporous material) prepared by impregnation with Co2(CO)8 
from its hexane solution under a deoxygenated atmosphere, also showed high CO 
conversion in FTS compared to that based on silica gel [358]. On the other hand, 
high-metal-loaded CoMo/MCM-41 materials prepared by coimpregnation have 
proved to be substantially more active than conventional CoMo/Al2O3 catalysts [349] 
in the hydrodesulphurization (HDS) of dibenzothiophene in distillate fuels. A mono-
 metallic cobalt-containing mesoporous smectite-like material [SM(Co)] prepared by a 
hydrothermal method was active as a catalyst for the hydrodesulfurizat-ion (HDS) of 
thiophene [359], and the addition of platinum to this system was found to enhance the 
HDS activity by 50 % with no significant change in the product distribution. Thus, 
the synthetic Co-containing smectite-like material is an active catalyst for the thio-
 phene HDS reaction. Cobalt was also loaded onto SiO2 by wet impregnation using a 
cobalt acetate solution and comparative studies showed that the activity of SM(Co) 
was much higher than that of Co/SiO2.  
 
1 GENERAL INTRODUCTION 
 42
 Ram?rez et al [360] prepared CoMo HDS catalysts supported on alumina with 
different amounts of aluminium-modified MCM-41 by pore volume impregnation 
from aqueous solutions of Co and Mo precursors. The materials were tested in the 
dibenzothiophene (DBT) hydrodesulfurization reaction. The results showed that the 
incorporation of MCM-41 into the catalyst formulation leads to higher catalytic 
activity. The TPR results indicated a diminished interaction of the Co and Mo phases 
with the support, compared to those existing in the alumina-supported catalyst. The 
decrease in surface area indicates that under the conditions of impregnation, the Mo 
and Co metal salts are not adequately dispersed. This lack of dispersion, which under 
calcination causes the partial blockage of the MCM-41 pores, may be due to the 
isoelectric point of the MCM-41 (pH = 2.3) being below the pH of the impregnating 
solution (pH = 5.5) (see Figure 1.13) 
 
 
 
Figure 1.13. Zeta potential of MCM-41 as a function of pH. The dotted line indicates the 
isoelectric point (ref. 360). 
 
Under this preparation condition, the surface of the MCM-41 is negatively charged, 
and therefore, the molybdate anions do not readily adsorb on the surface causing 
1 GENERAL INTRODUCTION 
 43
 some agglomeration of the Mo phases. This result is in line with the behaviour of 
pure silica supports, on which the molybdenum ions cannot be well dispersed. In 
order to envision the type and reducibility of the Co and Mo species present in the 
catalyst precursors, TPR experiments were performed and the corresponding reducto-
 grams observed are shown in the Figure 1.14 below: 
 
 
 
Figure 1.14. TPR patterns of CoMo/Al-MCM(x) catalysts, x is the weight % of MCM-41 in 
the support [ref. 360]. 
 
For a pure alumina-supported catalyst, two main reduction peaks are observed and are 
assigned to the reduction of octahedral Mo species (peak at Tmax = 706 K), and 
tetrahedral Mo species in strong interaction with the alumina support (peak at Tmax = 
1040 K). Also observed is the presence of shoulders at 773 K, due to the reduction of 
polymeric octahedral Mo species, and at 1004 K, due to the second reduction step of 
1 GENERAL INTRODUCTION 
 44
 octahedral Mo species. The reduction of Co species takes place at about 873 K. As 
MCM-41 is incorporated into the support, a decrease in the peak assigned to the 
reduction of tetrahedral Mo species in strong interaction with the support is observed. 
The maximum of this reduction peak is, therefore, shifted towards 973 K, where the 
reduction associated with the second reduction step of octahedral Mo(VI) takes place. 
In line with this, a significant increase in the size of the two low-temperature peaks 
corresponding to octahedral Mo species is observed. In fact, the magnitude of the 
reduction peak at about 773 K increases so much that a change in the relative size of 
the two low-temperature peaks is clearly evident. In line with the above, the total 
hydrogen consumption, and therefore, the reducibility of the Co and Mo species, incr-
 eases with MCM-41 loading. These results indicate a clear shift in the population of 
Mo species, from tetrahedral to octahedral ones, as the MCM-41 content is increased 
in the catalyst support. The TPR traces also show an increase in the hydrogen 
consumption in the region around 873 K, where the Co species are reduced. This 
indicates that the population of reducible Co (Co in octahedral coordination) also 
increases with MCM-41 content. This observation is well in line with the results 
obtained with Co/SiO2 catalysts, in which the Co species remain in octahedral coordi-
 nation and are easily reducible. In contrast, in alumina-supported catalysts part of the 
Co forms tetrahedrally coordinated CoAl2O4, that is hard to reduce. Clearly, the 
higher total hydrogen consumption, due to increased reduction of both Co and Mo, 
observed in MCM-41-containing catalysts, is due to a shift in the type of Co and Mo 
species from tetrahedral to octahedral ones. This result is clearly related to the milder 
interaction of the metal phases with the MCM-41 in the support compared to the 
alumina-supported catalysts. This milder interaction of the Mo and Co species with 
the support may induce interactions between Mo and Co. Because of this, the format-
 ion of species such as CoMoO4 cannot be discounted, especially when the form and 
position of the maximum of the TPR peak observed at 771-785 K and the shoulder at 
about 973 K are noted. These features coincide with the data reported by Arnoldy et 
al [361] for the reduction of CoMoO4, which reduced in two steps, leading to two 
reduction peaks at 790 and 980 K. The observed increase in hydrogen consumption 
1 GENERAL INTRODUCTION 
 45
 with MCM-41 content would also be in line with the presence of an increased amount 
of reducible Co, in the form of CoMoO4 or other reducible Co species, at the expense 
of Co in CoAl2O4, which is hardly reducible.    
 
1.4.3. General Catalytic Reactions of Mesoporous Materials 
 
It has already been mentioned that Si-MCM-41 materials possess a neutral 
framework, which limits their practical utility [343]. Since most of the applications of 
silicate type of materials are in the field of catalysis, the catalytic properties can be 
adjusted by the incorporation of different metals such as titanium and aluminium into 
the MCM-41 framework [63, 362, 363]. The acidity of aluminosilicate MCM-41 
materials is of medium strength when compared to USY zeolite [363].  
 
Some of the reactions catalyzed by metal-containing MCM-41 materials are: selective 
oxidation of hydrocarbons using Ti-MCM-41 [362], catalytic oxidation of aromatic 
compounds using Ti-MCM-41 [63], liquid-phase oxidation of hydrocarbons using Zr-
 MCM-41 [364], n-hexane aromatization using Ga-MCM-41 [365], peroxidative 
halogenation using Ti(IV)-grafted MCM-41 and MCM-48 [366], hydrocracking of 
vacuum gasoil using NiMo/MCM-41 [367], hydrogenation of aromatics in diesel 
fuels on Pt/MCM-41 [284], alkylation of naphthalene with alcohols over mesoporous 
MCM-41 [368], epoxidation of cyclohexene and pinene with tetrabutylperoxyhydrox-
 ide (TBHP) on Cp2TiCl2-grafted MCM-41 [298], Knoevenagel condensation with 
aminopropyl-functionalized MCM-41 [228], selective synthesis of jasminaldehyde at 
low benzaldehyde to heptanal ratios using aluminosilicate MCM-41 [369], Knoeven-
 agel and Michael condensations using alkali-MCM-41 [370], epoxidation of stilbene 
and its derivatives with TBHP on Mn-MCM-41 [347], etc.  
 
 
 
1 GENERAL INTRODUCTION 
 46
 1.5. Pollution Control Through Catalysis 
 
One of the most significant problems facing mankind relates to the quality of our 
environment. All environment-related issues are multidisciplinary, ranging from 
science and engineering to social, economic and regulatory issues [371]. Furthermore, 
these issues are global in nature, requiring multidisciplinary, multiorganizational and 
multinational educational efforts. Pollution is one such problem directly affecting the 
environment and the quality of life. The effects of pollution, leading to health and 
other problems in certain parts of the USA (notably in California), were recognized as 
long ago as the 1950s. The recognition of photochemical smog as a major contributor 
to these problems, and the part played by the motor vehicle, led to the formulation of 
the US Clean Air Act of 1970. This act laid down a programme to reduce emissions 
from motor vehicles by 90 % compared with an average 1960 model car. Initially, 
some improvements were achieved by engine modifications, but this did not achieve 
the target levels and alternative technologies needed to be considered. Catalysts had 
already been evaluated as possible solutions to the problem, but had been rejected 
because of their short lifetime. Both base metal and precious metal catalysts were 
very susceptible to the presence of lead additives in the fuel, and the sulphur oxides 
generated when the fuel-bound sulphur was combusted in the engine. It was 
eventually agreed that catalysts could be successful, provided fuel without lead 
additives was used. This required the setting up of a distribution chain for the so-
 called ?unleaded? fuel, and finding alternative solutions to problems of valve wear 
caused by the removal of the lead additive. 
 
In short, many of our current pollution problems are associated with the energy 
technologies that rely on the combustion of carbon-based fuels [371]. These involve 
the emission of greenhouse gases, acid rain precursors (NOx and SOx) and carbon 
monoxide, formation of photochemical oxidants and mining-related pollution. Thus, 
it is a general society aim to make environmental issues a major factor in process 
1 GENERAL INTRODUCTION 
 47
 design considerations, particularly in the design of highly active pollutant-abatement 
catalysts and to minimize the level of these toxic emissions.  
 
1.5.1. Carbon Monoxide Poisoning 
 
Carbon monoxide is an invisible, odourless, colourless, tasteless and poisonous gas 
that is widely known as the ?silent killer? because it cannot be detected by human 
senses. When inhaled, CO is rapidly absorbed by the lungs into the bloodstream 
where it diffuses rapidly across the alveolar capillary membrane and binds tightly to 
iron centres in hemoglobin and other hemoproteins. CO induces its toxicity effects by 
binding to hemoglobin with an affinity 240 times greater than oxygen, to form 
carboxyhemoglobin [372, 373], i.e. the oxygen-carrying oxyhemoglobin (O2Hb) is 
replaced by the more stable carboxyhemoglobin (COHb), which decreases the blood 
oxygen content and hinders the allosteric release of oxygen from hemoglobin to 
tissues. Carbon monoxide can be lethal at concentrations of less than 0.1 % and the 
kinetics and mechanism of its toxicology has been described elegantly by Coburn et 
al [374].  
 
Sources of carbon monoxide include incomplete combustion of burning fuels like 
natural gas, liquid propane gas, gasoline, kerosene, oil, coal, wood, charcoal and even 
tobacco. Automobiles, with their internal combustion engines, are the primary 
sources of CO pollution and the most dangerous when parked inside a garage. Carbon 
monoxide emissions from modern engines, though controlled by regulatory stand-
 ards, are still highly toxic in poorly ventilated spaces [375]. CO oxidation is an effici-
 ent way to remove traces of CO from the exhaust emissions at low temperatures, and 
will be discussed in the sections that follow. 
 
 
 
 
1 GENERAL INTRODUCTION 
 48
 1.5.2. CO Oxidation and Catalysis by Gold  
 
Typical emissions in the exhaust of gasoline-powered engines consist of noxious 
gases such as carbon monoxide (CO), unburned hydrocarbons (HC) and oxides of 
nitrogen (NOx). These are effectively controlled by adoption of a three-way catalytic 
converter (TWC) whereby the three gas types are oxidized or reduced to harmless 
carbon dioxide (CO2), nitrogen (N2) and water vapour (H2O). The name three-way 
catalytic converter derives from the fact that three different components of the 
automobile emissions are controlled simultaneously by a catalyst [376]. Pt and/or Pd 
and Rh are the most common active phases in three-way catalysts. The role of 
platinum in the TWC is to oxidize the CO and hydrocarbons (HC), especially during 
cold start-up of the engine. Rhodium is active to reduce nitrogen oxides (NOx) 
especially in the presence of a high concentration of CO [377].  
 
However, the catalytic converter cannot effectively remove HC and CO from the 
outlet of internal combustion engines in the cold-start phase of running the engine, 
and these emissions must be reduced to meet new regulations and improve the 
environment [378]. The cold-start period refers to the first few minutes after engine 
ignition before the catalyst reaches its light-off temperature (250-300 oC), during 
which 50-80 % of the total unburned hydrocarbon fuel simply passes out of the 
tailpipe into the atmosphere. The light-off temperature is defined as the temperature 
at which a catalyst becomes functional. 
 
Over the years, significant effort has been put in improving the cold-start behaviour 
[379] of catalytic converters around the world. Examples of new technologies that 
have been developed include close-coupled catalysts [380], electrically-heated 
catalysts (EHC) [381-383] and hydrocarbon traps [384-386]. The major advantage of 
close-coupled catalysts is that no additional power is needed. However, the volume of 
catalyst connected to the engine must be less than 200 ml and must be ?warmed-up? 
rapidly. A large quantity of noble metal must also be used to achieve high 
1 GENERAL INTRODUCTION 
 49
 purification. The cost of a catalyst is directly linked to that of noble metals, which are 
generally very expensive. The EHC technique has attracted wide attention since 
catalysts can be rapidly heated and the light-off time is shorter than 40 s. The major 
disadvantage of the EHC technique is that the EHC carrier material is made of metals 
which have low electrical resistivity (~10-5 ?.cm) and high thermal conductivity, so 
high power (> 1.5 kW) and therefore, high current (>150 A) are usually required 
[387] for EHC to take effect and achieve reasonable cold-start behaviour. Such high 
current and power are not available from present vehicle power systems, and conseq-
 uently the EHCs have not been deployed because these devices are cost- and energy-
 intensive. The HC trap technique involves a complex preparation of a hydrocarbon 
absorber, which makes them impractical in the near future.  
 
It is thus of interest to develop a highly active CO oxidation catalysts to remove trace 
amounts of CO at low temperatures according to the following reaction: 
 
CO + ? O2 ? CO2,  ?G298 = -257.1 kJ/mol 
 
 Gold has long been regarded as catalytically far less active than platinum group 
metals [388-390], and has consequently found very little application in heterogeneous 
catalysis. This inactivity or unreactivity has its origins in its electronic structure, 
([Xe] 4f145d106s1), since gold has a filled d-band, and filled bands are usually associa-
 ted with low activities [391]. The other factor normally cited as a cause for the unrea-
 ctivity of gold is its high electronegativity, which makes it unable to form stable 
compounds with other electronegative elements such as oxygen and sulphur [392]. 
However, the realization by Haruta and coworkers [393, 394] that the nobleness of 
gold breaks down at nanometric sizes when it is in contact with reducible oxidic 
supports, has triggered global interest in the use of gold as a catalyst. In this pioneer-
 ing work, Haruta and coworkers [393, 394] showed that gold is remarkably active for 
low-temperature CO oxidation when it is highly dispersed and deposited on supports 
1 GENERAL INTRODUCTION 
 50
 such as reducible semiconductor metal oxides, hydroxides of alkaline earth metals, or 
amorphous ZrO2.  
 
Since the initial results on CO oxidation suggested the importance of the nature of the 
support on the resulting catalytic activity, two different classes of supports have been 
identified: (i) active supports are materials that are relatively easy to reduce like 
Fe2O3 or TiO2, and (ii) inert or inactive supports like Mg(OH)2, Al2O3 and SiO2. 
Although active supports often result in very active gold catalysts, inert supports can, 
in some cases, lead to catalysts as active as those with reducible supports, but it is 
very important to ensure a high dispersion of very small particles of gold on the oxide 
[395, 396].  In general, silica supported gold samples were found to be less active 
than the alumina-supported ones [397], probably due to the presence of large gold 
particles.  
 
Since this breakthrough discovery, much attention has recently focused on the use of 
gold as a catalyst [398, 391], and below are some of the heterogeneous reactions that 
are catalyzed by gold [399]: 
 
?Hydrochlorination of ethyne 
?Oxidation of CO and hydrocarbons 
?Selective oxidation, eg epoxidation of olefins 
?Selective hydrogenation 
?Hydrogenation of CO and CO2 
?Water gas shift 
?Reduction of NO with propene, CO or H2 
?Reactions with halogenated compounds 
?Hydrogen plus oxygen reaction 
?Removal of CO from hydrogen streams  
 
1 GENERAL INTRODUCTION 
 51
 1.5.3. Commercial Applications of Gold Catalysts 
 
Gold catalysts have already been commercialized to remove domestic odour and CO 
in air at room temperature [400]. Catalysts for the oxidation of carbon monoxide at 
low temperature also find practical applications in several devices: active catalysts are 
used in closed-cycle CO2 lasers, in gas sensors for detection of trace amounts of CO 
in air, in CO safety masks, etc., [401-403]. Gold-palladium alloy catalysts are used in 
the commercial production of vinyl acetate monomer (VAM) by BP Chemicals at 
Hull (UK) using a new ?Leap Process?. This monomer is used in the manufacture of 
emulsion-based paints, wallpaper paste and wood glue [404]. Possibilities are being 
investigated for putting some gold in car engines or to have twin catalysts, gold and 
platinum with the former covering the cold period and platinum the hotter one [405]. 
A future is also envisaged for using gold in fuel cell systems [406], and this 
application is on the verge of a breakthrough in Europe [406].   
 
1.5.4. Supported Au Catalysts 
 
The high reactivity of gold nanoparticles has been a subject of many studies since its 
discovery by the group of Haruta in the eighties [393, 394]. Several studies have 
pointed out different factors as responsible for the enhancement of the reactivity 
observed mainly in oxidation processes. However, some questions related to the role 
of the support, the existence of charged atoms at the interface and the availability of 
low coordinated sites have yet to be answered. 
 
Gold can be highly dispersed on a variety of metal oxides by coprecipitation and 
deposition-precipitation [394] preparative techniques, followed by calcination in air. 
Such supported gold differs in catalytic nature from unsupported gold particles and 
has been found to exhibit high catalytic activities for low-temperature oxidation of 
CO. The TEM photograph of the highly dispersed Au/?-Fe2O3 prepared by co-
 precipitation is shown in Figure 1.15 below: 
1 GENERAL INTRODUCTION 
 52
  
 
 
Figure 1.15. TEM micrograph of Au-Fe(1:19) coprecipitate calcined at 400 oC: dispersion of 
gold particles over ?-Fe2O3 [ref. 394]. 
 
Figure 1.16 (see below) compares the catalytic activities for the oxidation of H2 and 
CO of unsupported gold particles with diameters around 20 nm, and gold supported 
on ?-Fe2O3 and ?-Fe2O3 alone. The corresponding Arrhenius plots and the calculated 
activation energies of the representative gold catalysts on different supports, for the 
CO oxidation results in Figure 1.16, are shown in Figure 1.17. 
 
 
 
1 GENERAL INTRODUCTION 
 53
  
 
Figure 1.16. Conversions of H2 and CO in catalytic oxidation reactions as a function of 
catalyst temperature. CO or H2 1 vol% in air, SV = 2 ? 104 h-1.ml/g-cat: oxidation of H2(?) 
and CO(?) on Au-Fe(1:19) coprecipitate calcined at 400 oC in air; oxidation of H2(?) and 
CO(?) on Au powder prepared from colloidal metal particles with diameters around 20 nm; 
oxidation of H2(?) and CO(?) on ?-Fe2O3 powder prepared by calcination of ferric 
hydroxide at 400 oC. [ref. 394] 
 
 
Figure 1.17. Arrhenius plots for the rate of CO oxidation over supported catalysts: (?) 
Au(1.2 wt%)/Co3O4, (?) Au(0.66 wt%)/?-Fe2O3, and (?) Au(3.3 wt%)/TiO2. [ref. 394] 
 
1 GENERAL INTRODUCTION 
 54
 It can be seen from the Arrhenius plots above that the activity of Au catalysts is influ-
 enced by the type of oxidic support used, with the cobalt-based catalyst exhibiting the 
lowest activation energy for the oxidation of CO.  
 
1.5.5. Preparation of Supported Au Catalysts 
 
The use of gold as a catalyst requires a careful and unconventional preparation of the 
gold metal, centred on achieving a very small gold particle size. Preparation methods 
like co-precipitation (CP) [393, 407, 408], deposition-precipitation (DP) [394, 409], 
chemical vapour deposition (CVD) [396, 410], adsorption of gold colloids [411], 
supporting gold phosphine complexes on as-precipitated metal hydroxides [412] and 
co-sputtering [413] can produce highly dispersed gold catalysts, which exhibit unique 
catalytic nature in many different reactions depending on the type of metal oxide 
supports used [414, 415]. The latter methods are more demanding and less suitable in 
the production of larger quantities of materials, and therefore uneconomic. Indeed the 
most popular and reliable synthesis methods to prepare supported gold catalysts are 
the initial methods reported by Haruta and coworkers [393, 414], namely co-
 precipitation and deposition-precipitation, which result in highly dispersed and highly 
active supported gold catalysts for low-temperature oxidation of CO. In the light of 
the coprecipitation method, Wagner et al [416] coined the name ?inverse coprecipitat-
 ion? for the preparation method involving a reversed order of addition of the copreci-
 pitate precursor solutions.  
 
Haruta [414] has shown that a strong interaction between the support and the Au 
particles is necessary to avoid the coalescence of particles to form bigger and inactive 
crystallites. This is normally achieved by using the coprecipitation method, although 
this method has the drawback of leaving part of the gold particles buried inside the 
support and inaccessible to the reactant gas stream. Therefore, a preparation method 
is still needed to produce almost all gold particles being accessible to the reactants.  
 
1 GENERAL INTRODUCTION 
 55
 Typically, coprecipitation or deposition-precipitation methods are more desirable than 
impregnation, especially when chloroauric acid (HAuCl4.xH2O) is used as the precur-
 sor. Various processing variables in the synthesis of supported gold catalysts have 
been studied, which include pH, temperature and concentration of the preparation 
solution, calcination temperature, reduction temperature, and reduction/oxidation 
cycling. Some of these studies have helped elucidate the underlying chemistry that 
causes variation in the properties of the final solid. 
 
One of the better understood variables in the deposition-precipitation procedure is 
pH. In such a preparation method, HAuCl4 is used as the metal precursor. The chloro-
 auric anion hydrolyzes in solution to produce Au(OH)xCl4-x-. The extent of hydrolysis 
depends on the pH, as well as Au and Cl concentrations [417]. It has been found that, 
depending on the oxide support, catalyst preparation at a pH ranging from 7 to 8 is 
preferable [418]. At this pH, the value of x is close to 3. At lower pH values, when 
there is less hydrolysis of the Au-Cl bond, large Au particles are formed after 
calcination [414]. At pH values above the isoelectric point of the oxide, adsorption of 
the negatively charged Au(OH)xCl4-x- complex decreases rapidly. Therefore, there is a 
narrow range of pH values where sufficient Au can be deposited onto the support 
with minimal chloride in the Au complex. It is interesting to note that in order to 
obtain a high dispersion of Au in the final catalyst, it is desirable that the precursor 
Au complex is adsorbed as an anion. If the AuCl(OH)3- precursor is added to a dilute 
suspension of the support while keeping the pH at 7, Au(OH)3 would be the predomi-
 nant species due to the low initial chloride concentration [417]. Its low solubility 
causes precipitation onto the support material, resulting in a poor catalyst. 
 
Recently, another effect of the pH of the solution was identified [419]. In addition to 
Au(OH)xCl4-x-, Cl- ions are also adsorbed onto the oxide support. The amount of 
chloride adsorbed decreases rapidly as the isoelectric point is approached (alumina 
support), when the oxide surface is no longer positively charged. The adsorbed Cl- 
has two effects: (1) it causes agglomeration of Au particles during calcination, such 
1 GENERAL INTRODUCTION 
 56
 that the average Au particle size is larger in samples with a higher residual chloride 
content, and (2) it inhibits the catalytic activity by poisoning the active site. The 
adsorbed chloride can be displaced by other anions. In the earlier preparations, 
magnesium citrate was added to the preparation solution to obtain an active catalyst 
[420]. It is believed that the primary function of the citrate is to displace the chloride 
from the support and possibly from the Au(OH)xCl4-x complex. Chloride can also be 
removed by reduction or calcination [421, 422]. However, high temperature 
calcination or reduction causes agglomeration of Au particles.  
 
More recently, attention has turned to the effect of calcination temperature as a factor 
influencing the activity of supported gold catalysts. Haruta et al [393, 394, 423] found 
that coprecipitated gold on iron oxide is active only after calcination at 400 oC. At 
this temperature the gold forms small metallic particles that are the active species. 
However, Tanielyan and Augustine [424] prepared coprecipitated Au on iron oxide 
catalysts that were already active after drying at 95 oC, whereas calcination at high 
temperature (380 oC) led to a significant decrease in activity. These contradictory 
observations suggest that the role of calcination in the preparation of the catalysts is 
still unknown. On the other hand, Kozlova et al [425] studied a series of samples 
prepared by impregnation of several iron oxide supports prepared from the same 
precursors but with different water or hydroxyl group content. The activity of these 
catalytic systems, even after calcination, has been found to decrease as the amounts of 
water and hydroxyl groups decrease before impregnation with the gold precursor. It 
was reported that for Au/TiO2 [426, 427], Au/iron oxide [427-429], and Au/MnOx 
[430] catalysts, calcination at a mild temperature (100-200 oC) resulted in more active 
catalysts than calcination at higher temperatures. There are also reports that 
uncalcined Au/Al2O3 [418] or Au/Y [431] can be very active.  
 
The activity of Au/TiO2 and Au/ZrO2 CO oxidation catalysts prepared by adsorption 
of gold colloids on a support was also found to be influenced by the calcination 
temperature [432]. Maximal activity was obtained for the catalysts calcined at 500 oC 
1 GENERAL INTRODUCTION 
 57
 (Au/TiO2) and 560 oC (Au/ZrO2), respectively. Catalysts calcined at higher tempera-
 tures exhibited lower activity due to sintering of the gold particles (T > 600 oC), as 
metals on supports are theoretically mobile at their Tammann temperature (for gold, 
TTammann = 395 oC). As a consequence of the low melting point and high surface 
mobility of gold, interest has focused on preparation methods which minimize gold 
agglomeration [412]. To prepare gold of high dispersion, Guillemot et al, Kang et al, 
and Ichikawa et al have used the zeolites Y [433-439] or MFI (ZSM-5) [440, 441]. 
The catalysts were probed for the decomposition and reduction of nitric oxide and the 
oxidation of carbon monoxide.  
 
For the preparation of highly dispersed gold catalysts on zeolitic supports, different 
synthetic methods have also been used, which also differ from the unconventional 
synthetic methods mentioned above. These include: cation exchange of the iron-
 impregnated zeolite-Y with chloroauric acid solution [442], autoreduction of Au3+ by 
ethylenediamine ligands inside NaHY [433], co-exchanging iron(II)ethylenediammo-
 nium and Au(III)ethylenediamine complex ions in HY [443] and sublimation of 
AuCl3 onto MFI [444].  
 
In spite of the success with some inert supports, the preparation of active Au/SiO2 
catalysts with solution-based methods is difficult, and the resulting materials have 
very low activity in CO oxidation [396, 418]. In the case of SiO2, the CP and DP 
techniques fail to poduce active CO oxidation catalysts, because the point of zero 
charge (PZC) of SiO2 is around pH 2, and at pH 7 this oxide exhibits a large negative 
surface charge, which prohibits the deposition of Au(OH)3. Impregnation of SiO2 
with HAuCl4 [396] or cationic Au-phosphine cluster solutions [445] produced very 
large supported Au particles (mean size ? 16 nm), which were inactive for CO 
oxidation at room temperature even after calcination. However, the use of amines as 
stabilizers of the gold metal phase during the sol-gel deposition of Au on SiO2 
resulted in the formation of catalytically active Au particles with an average size of 5-
 6 nm [446, 447]. Chemical vapour deposition (CVD) was also successfully applied to 
1 GENERAL INTRODUCTION 
 58
 prepare small (mean size ca. 6.6 nm) and active silica-supported gold particles, which 
showed CO oxidation activity even at subambient temperatures [396]. Also, function-
 nalizing the mesoporous SiO2 surface with cationic organosilanes to generate a 
monolayer of positively charged groups on the pore surface [448], as well as stabiliz-
 ing the Au particles by using self-assembled monolayers (SAMs) to surround the 
particles [449, 450] give small active silica-supported gold nanoparticles. This 
provides evidence that SiO2 can also be a useful support for catalytically active gold 
nanoparticles. Recently, the attractiveness of silica materials as supports for gold 
catalysts has increased following reports on Au particle growth within the channels of 
MCM-type mesoporous silicas [451]. 
 
1.5.6. Controversy over the active species in CO oxidation 
 
The outstanding activity of highly dispersed gold in the low temperature CO oxida-
 tion on supports like TiO2, Fe2O3, MnOx, ZrO2, etc. has been the subject of several 
review articles [391, 412, 414, 415, 452].  
 
Although active catalysts commonly contain small, 2-5 nm Au crystallites, size alone 
does not seem to be a sufficient factor for the high activity of supported Au catalysts 
in CO oxidation [418]. The actual cause of this high activity is still uncertain, and 
there is a wide variation in the activities reported among different laboratories for 
apparently similar catalysts. There have been a number of proposals as to the active 
catalytic site, which include: gold-support interface [394], small Au clusters that 
possess nonmetallic electronic properties due to quantum-size effects [453], and 
surface step and strain defects [454]. A chemical model that involves an ensemble of 
metallic Au atoms and Au cations with hydroxyl ligands has been suggested [455-
 457]. There appears to be increasing support that the active site involves the 
perimeter of the Au particles or the Au-support interface [426,458, 459]. Although 
there is still an ongoing debate on what causes the high activity of finely dispersed 
gold on oxide carriers [391, 411, 414, 453, 454, 460-462], it is well accepted that the 
1 GENERAL INTRODUCTION 
 59
 final activity of gold is dependent on the particle size, on the support properties (e.g. 
semiconducting or insulating), the preparation procedure, and the pretreatment [391, 
411, 414, 460, 462]. On the other hand, Finch et al [463] have attributed the room 
temperature activity of the Au/Fe2O3 catalysts to a synergistic interaction between 
AuOOH.xH2O and a ferrihydrite species.  
 
To summarize, supported gold catalysts [464] are very active for CO oxidation and 
other environmentally important reactions when: (i) there is strong contact between 
the support and the gold particle perimeter, (ii) there is an appropriate metal oxide 
support, and (iii) the particle size is < ca. 5 nm. Three broad areas for the new 
applications of gold catalysts have been identified [465]: (a) Automotive pollution 
control (cold-start, diesel engines), (b) Clean energy generation (fuel cells, hydrogen 
purification, electrocatalyst extender, etc.) and (c) Environmental pollution control 
(odour abatement and toxin removal; air conditioning in automotive, aircraft, subma-
 rines, office buildings, hospitals, factories and restaurants; waste incinerators; gas 
masks). 
 
The mesoporous gold-silica nanocomposite materials of the MCM-41 type are expec-
 ted to be potentially useful as sensors and catalysts.  
 
1.6. Hypotheses and Aims of the study 
 
Nanoscience developments are increasingly being applied in catalysis to prepare 
materials and develop structure-performance relationships. Precisely controlling size, 
shape, spatial distribution, composition, and thermal and chemical stability of the 
active catalyst nanocomponents can provide catalysts with selectivity approaching 
100% and very high activity plus low process energy consumption. The Mobil meso-
 porous material, MCM-41, possesses a myriad of catalytically sought-for properties, 
including high BET surface area, narrow pore size distribution, large pore sizes and 
high hydrocarbon sorption capacity. These properties make it a promising support for 
1 GENERAL INTRODUCTION 
 60
 active metal components in various fields of heterogeneous catalysis. Combining 
these properties of mesoporous MCM-41 with the unrivalled low temperature activity 
of gold can result in highly active, easily prepared nanocomposite catalysts. 
 
It is thus the aim of this thesis to study the preparation parameters for Au-based 
mesoporous MCM-41-supported catalysts for the total oxidation of CO at low tempe-
 ratures. Starting with MCM-41 in its pure silica form, as well as starting with metal 
precursors at the synthesis stage of MCM-41, we hope to prepare Fe- and Co-MCM-
 41 materials with maximum retention of the important properties of the mesoporous 
silicate. The presence of such base metals within the silicate framework is expected to 
induce reducibility, strengthen the material, etc. The base metal-modified MCM-41 
materials are to be used as supports for Au catalysts, which are known to catalyze the 
oxidation of carbon monoxide at low temperatures. The principal role of the Fe or the 
Co component is to aid in the redox reactions of Au, and to provide anchoring sites 
for the gold nanoclusters in order to reduce their chances of coalescing during 
calcination above their Tammann temperature (395 oC). The use of this high surface 
area support should be beneficial in keeping the loading of precious metals, because 
of their high prices, low but yet achieve reasonable catalytic activity. 
 
1.7. Rationale/Motivation for Research in this Thesis 
 
Carbon monoxide (CO) and unburned hydrocarbons (HCs) are ubiquitous pollutants 
in the air, arising from incomplete combustion in gasoline-powered car engines. CO 
is of immediate environmental and public health concern as even small exposures 
(ppm levels) can be lethal. This gas effects its toxicity by binding to hemoglobin with 
an affinity of 240 times that of oxygen, and thus reduces the oxygen-carrying 
capacity of the red blood cells by forming a more stable carboxyhemoglobin (COHb). 
It is thus desirable to achieve 100% oxidation of these pollutants to CO2 and H2O 
through catalytic oxidation. New stricter emission control regulations for exhaust 
gases and the high cost of the PGMs used in TWCs have led to investigations to 
1 GENERAL INTRODUCTION 
 61
 reduce the cost of these catalysts by improving their activity and durability. The 
present formulation of the TWCs, comprised of PGMs, is inefficient for these 
oxidation processes at ambient temperatures (during the engine?s cold-start phase), 
and only become effective when their temperature is above 250 to 300 oC (known as 
the light-off temperature). Therefore, new catalyst designs are necessary to take care 
of these noxious emissions at low temperatures. 
 
The realization that highly dispersed nanoparticulate Au on reducible oxide supports 
can accomplish the oxidation of CO oxidation at room temperature and in the 
presence of moisture (which is not the case with other metals), opens up a space for 
Au in automotive pollution control during the cold-start phase of running the engine. 
In South Africa, the project AuTEK has been launched as a joint research venture 
between AngloGold and Mintek, which focuses on the application of gold in catalysis 
for combating air pollution by removing noxious gases such as CO and NOx. Thus, a 
detailed study of the various preparation parameters for active Au catalysts in CO 
oxidation reactions is in line. However, due to the limitation with the price of Au 
precursors, it is desireable to use economic amounts of Au in the catalyst formulat-
 ions (~1-3 wt% Au). This can be achieved by employing a porous support of reason-
 ably high specific surface area. The Mobil mesoporous material, MCM-41, matches 
this requirement and will be explored as support for Au catalysts, which can be used 
to purify the air in office buildings, aircraft and for the general remediation of air 
pollution. 
 
The relatively greater availability of gold compared to the PGMs and the sky-
 rocketing prices of platinum and palladium since 1998 [465, 466], are also a good 
motivation for carrying out extensive research with Au-based systems in order to 
improve the price of this metal. In his paper entitled ?The Golden Age of Catalysis?, 
G. C. Bond [467] backs up this motivation and further revolutionizes gold-based 
research with the words, ?We may confidently expect further developments, and early 
1 GENERAL INTRODUCTION 
 62
 in the new millennium gold should take its place alongside its neighbours as a vital 
member of the armoury of transition metal catalysts?. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
1 GENERAL INTRODUCTION 
 63
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