Chemistry Reference
In-Depth Information
and this opened up a wide range of important trans-
formations for these new catalysts. For example, the
oxidation of a-terpineol proceeded smoothly in Ti-
MCM-41 but not in TS-1, where the substrate is too
bulky to enter the pores. A large-pore Ti-zeolite, Ti-
BETA, also was evaluated and shown to be less active
than Ti-MCM-41 in this reaction. Selectivity towards
epoxide was moderate in both cases but better with
the MCM material [89]. Pinnavaia's group [90]
showed that Ti-HMS materials (prepared via neutral
amine templating) were more active than Ti-MCM
materials, due to the presence of textural porosity,
which allows better access to the mesopores (Fig.
7.16). The Ti-HMS and Ti-MCM materials prepared
by Pinnavaia also displayed the ability to oxidise aro-
matics, and this will be discussed later.
One drawback that was rapidly evident from this
initial work was that the catalysts caused a sub-
stantial amount of non-productive decomposition of
hydrogen peroxide. Several subsequent papers have
addressed this problem, which is thought to be
related to the hydrophilicity of the catalysts [91,92],
being high enough to promote a substantial amount
of peroxide decomposition but low enough to allow
access and reaction of alkenes with the hydrogen
peroxide. This is in apparent contrast to the work
described above relating to fatty acid esterification,
where the hydrophobicity of the catalysts was cited
as a reason for their high activity. One possible expla-
nation may lie in the different requirements of the
different reactants. If we assume that the surface of
the catalyst is strongly H-bonding active (which is
likely to be the case due to the high numbers of polar
hydroxyl groups on the surface), this would allow a
strong interaction between the surface and the acid
group in the fatty acid (beneficial due to activation
of the reactant). This adsorption would render the
interior of the pores more hydrophobic due to the
long alkyl chain of the acid, and encourage adsorp-
tion/desorption of products and reactants (even
taking into account some pore size reduction due
to partial blockage). A similarly strong interaction
thus would be expected with hydrogen peroxide.
However, in this case the interaction would be detri-
mental due to the increased likelihood of decompo-
sition to water and oxygen. Thus, two modes of
adsorption of hydrogen peroxide on Ti-substituted
mesoporous silicas are possible: an interaction with
SiOH, leading to destruction of the reagent; and an
interaction with Ti=O, leading to activation and
epoxidation. An alternative explanation may be that
the active sites of the two catalysts lie in different
regions of the two materials (calcined silicas and
MTS materials consist of hydrophobic and
hydrophilic patches [66]).
Interestingly, TS-1, which is known to be
hydrophobic, causes very little hydrogen peroxide
decomposition, which is another instance where the
detailed chemistry of the zeolite systems and their
templated analogues differs significantly. Such dif-
ferences between the exact details of the surface
chemistry of various silica materials (zeolitic, amor-
phous silica and the various micelle-templated
materials) appear often, and it is important not to
assume that the surface chemistry of all these
systems is similar and will have little influence on
the catalytic properties of the materials.
Efforts to enhance the hydrogen peroxide utilisa-
tion of these new catalysts have been made, with
great success. Coverage of the surface of these ma-
terials with non-polar trialkylsilyl groups has been
shown to enhance the performance of the catalysts
substantially. In these cases, large increases in
epoxide yield [92] and improvements in hydrogen
CHO
Ti-cat
H
2
O
2
Fig. 7.16
The influence of textural
porosity on the efficiency of reaction
between styrene and hydrogen
peroxide. Similar trends were seen for
the oxidation of methyl methacrylate
to methyl pyruvate and the oxidation
of phenols to quinones.
Catalyst
pore volume
conversion selectivity
H
2
O
2
framework
textural
decomposition
Ti-MCM 0.68 cm
3
g
-
1
0.02 cm
3
g
-
1
10%
82%
3.8%
Ti-HMS 0.68 cm
3
g
-
1
0.72 cm
3
g
-
1
28%
77%
2.2%
