Chemistry Reference
In-Depth Information
position. What is particularly interesting is that Type
2 catalyst is very effective for alkylation chemistry
but the reverse is true in the case of AMS dimerisa-
tion and hydroperoxide decomposition. In the case
of olefin isomerisation there appears to be only a
small difference between the two materials investi-
gated. For each reaction the catalyst and conditions
have been optimised to give very high conversions
with good selectivity. In the case of AMS chemistry
the only three products observed were 2,4-diphenyl-
4-methyl-1-pentene (I), 2,4-diphenyl-4-methyl-
2-pentene (II) and the saturated dimer 1,1,3-
trimethyl-3-phenylindan (III).
The reactions that are faster for the Type 1 catalyst
are those that are the most likely to be rate-limited
by the initial proton transfer. We have demonstrated
that proton transfer
is
rate-limiting for the AMS
dimerisation. It seems logical that the reaction rates
are optimised for the catalysts that have the better
dispersed, more accessible acid site distribution when
proton transfer is rate-limiting. Palinko
et al
. [104]
have reported on the surface characterization of
both Nafion
®
and a Nafion
®
/silica composite catalyst
by infrared microscopy. The composite was made
using silicon alkoxide and would correspond to a
Type 1 catalyst. One conclusion reached was that of
interaction of the highly dispersed sulfonate groups
(SO
3
H) containing pockets of the Nafion
®
and the
hydroxyl groups of the silica within the composite.
It was suggested also that this may lead to some
decrease in acidity. The influence of the composition
within the Nafion
®
/silica composites on the isobu-
tane/2-butene alkylation also has been reported [93]
and a similar conclusion was observed. In the highly
dispersed Nafion
®
/silica systems, the sulfonic groups
of the polymer interact to a greater extent with the
silanol groups of the silica, resulting in a decrease in
the activity of the sulfonic acid groups. In the Type
2 materials, where the Nafion
®
is more aggregated,
one would expect the interaction of the sulfonate
groups (SO
3
H) and the hydroxyl groups (SiOH) of
the silica to be reduced considerably, leading to a
material with an acidity that is more 'Nafion
®
-like.'
Overall, then, we are inclined to think of the ratio
of rates for Type 1 and Type 2 catalysts as the result
of a trade-off between acid site accessibility (more
favourable for Type 1 catalysts) and acid site ion-pair
microsolvation effects (which may have a higher
inherent acidity that is more favourable for Type 2
catalysts) [42]. It appears that one can tune the cat-
Scheme 6.26
We have found also that the catalytic activity of
these new materials can be tailored by varying the
catalyst synthesis. This opens up the possibility of
fine tuning these catalysts. The extent of the
Nafion
®
dispersion can be varied within the silica.
This change of the Nafion/silica microstructure in
turn affects the catalytic activity. In the dimerisation
of AMS a highly dispersed form of the Nafion
®
gives
the optimum activity. However, in the case of acyla-
tion or alkylation chemistry, a slightly aggregated
form of the Nafion
®
is preferred. A comparison of the
two types of catalysts developed can be found in the
literature [90]. In the nanocomposite, acid sites are
localised within discrete Nafion
®
domains. Decreas-
ing the domain size can be thought of as increasing
the acid site dispersion by increasing the effective
surface area of the acidic regions. Using high-
resolution transmission electron microscopy and
scanning electron microscopy we have found that the
microstructures obtained from alkoxide precursors
contain a highly dispersed form of the Nafion
®
resin,
which we refer to as Type 1 catalysts. For the Type 1
catalyst, Nafion
®
domains are ca. 10-30 nm in size
and distributed uniformly throughout the porous
silica network. The Nafion
®
dispersion is ca. 5000
times greater than the pure unswollen polymer.
Table 6.7 shows a comparison of a number of
catalytic reactions investigated using the Nafion
®
resin/silica nanocomposites prepared from the two
alternative silica precursors. In general Type 2
(which is made from sodium silicate as the silica
source) leads to a Nafion
®
resin/silica microstructure
that is about 10 times more active in the industrially
important area of alkylations than reported previ-
ously using the alkoxide route (Type 1) [90], specifi-
cally the formation of cumene and linear alkyl
benzenes.
The catalytic activity was compared using the Type
1 and Type 2 catalysts for seven different reactions,
including alkylations, acylation, olefin isomerisation,
the dimerisation of AMS and hydroperoxide decom-
