Geoscience Reference
In-Depth Information
Figure 6.46 Effective pore size
distribution
[131]
.
4
3
Na-X
2
VPI-5
1
0
4 8
Effective pore diameter (Å)
12
16
20
Though zeolites were not regarded as promising catalysts for oxidation reac-
tions, considerable changes have taken place in the last decade. It was during the
early 1980s, that titanosilicate zeolites, having a structure of the ZSM-type, were
identified and the discovery of their unique properties in oxidation reactions involv-
ing H
2
O
2
took place. Thus, titanosilicates become the effective catalysts of a wide
range of reactions occurring in the liquid phase, such as the oxidation of alcohols,
epoxidation of alkenes, hydroxylation of aromatic compounds, and the conversion
of cyclohexanone into an oxime
[151
157]
. These advances in liquid phase oxida-
tion of primary and secondary alcohols to the corresponding carbonyl compounds
have been assessed as one of the most important achievements in the field of
organic synthesis. The partial oxidation of hydrocarbons is achieved, as a rule, in
the presence of transitional metal oxides. Their activity is due to more or less facile
electron transfer and their capacity for the reversibility of their oxidation state in
interaction with oxygen in the substance. Oxidized zeolites themselves cannot serve
as oxidation catalysts, but the possibility of their use as carriers, nevertheless, is of
great interest. However, because of the serious environment problems associated
with Cr-containing effluent, attention has been focused on the use of catalytic
amounts of soluble chromium compounds in conjunction with, for example, tetra-
butyl hydroperoxide (TBHP) as the stoichiometric oxidant. The use of heteroge-
neous catalysts in the liquid phase, on the other hand, offers several advantages
compared with their homogenous counterparts, e.g., ease of recovery and recycling
and enhanced stability. Kharitonov et al.
[157]
have studied the reactions involving
gasoline oxidation with nitrous oxide, and the best results were obtained in the
oxidation of benzene to phenol in the presence of iron-containing ZSM-5 zeolites.
An approach to create solid catalysts with novel activities is to incorporate redox
metals by isomorphous substitution into the lattice framework of zeolites and
related molecular sieves. Since the discovery of aluminophosphate molecular sieves
in 1982, much attention has been focused on the incorporation of various elements
into the framework of these molecular sieves
[158]
. Modified compositions of these
silica or aluminophosphate-based materials are expected to expand the scope of
their catalytic applications as mesoporous materials and optimized advanced
