Chemistry Reference
In-Depth Information
Scheme 6.24
the equilibrium distribution of the
n
-butenes was
formed at 50°C. The composite catalyst has higher
activity than Amberlyst
®
-15 resin for aluminosili-
cates and its catalytic lifetime is much longer than H-
ZSM-5 under the reaction conditions.
The composite is very effective for catalysing long-
chain olefin isomerisation. 1-Dodecene isomeriza-
tion to its linear isomers was carried out in the liquid
phase at 75°C over the 13 wt.% Nafion
®
resin/silica
composite, plain Nafion
®
resin, Amberlyst
®
-15 resin
and H-ZSM-5 catalysts. The first-order rates are 190,
<1, 13 and 4 (mM g
-1
calyst h
-1
), respectively. The
data clearly indicate the drastically enhanced acces-
sibility of the sulfonic acid sites inside the composite
versus the plain Nafion
®
polymer resin. The signifi-
cantly higher activity of the composite in compari-
son with that of the Amberlyst
®
-15 resin could be
due mostly to the higher inherent acid strength of
the Nafion
®
resin.
The composite is effective also in the solid-acid-
catalysed dimerisation of a-methylstyrene (AMS)
[100]. The unsaturated dimers of AMS, particularly
2,4-diphenyl-4-methyl-1-pentene, are industrially
important and are useful as chain-transfer agents or
molecular weight regulators in the production of
polymers. The first-order rate constants for 13 wt.%
composite, supported 2 wt.% Nafion
®
resin on shot
coke (carbon) and Amberlyst
®
-15 resin are 110.1,
0.1 and 0.6 mM meqH
+-1
h
-1
, respectively, in cumene
Almost three orders of rate enhancement are
observed, which reflects an increased access to the
acid sites. These types of catalysts are efficient also in
propylene oligomerisation.
The traditional catalysts used in acylation chem-
istry suffer from several drawbacks: from the use of
stoichiometric amounts of catalysts (AlCl
3
, BF
3
) to
extensive problems with disposal of the associated
waste stream. Ion-exchange resins have not played
a strong role in this area compared with some more
recent developments in both zeolite chemistry and
the careful design of aluminum-chloride-modified
supports. The acylation of anisole with carboxylic
acid chlorides (see Scheme 6.24) occurs very rapidly
with these catalysts, with short reaction times and
high selectivity. In some cases 100% conversion with
Scheme 6.25
97% selectivity was obtained. Using Nafion
®
-based
catalysts, from traces to ca. 98% conversion were
obtained using the following aromatics: toluene
(trace),
o
-xylene (1.3%),
m
-xylene (18%), 2,4,6-
trimethylbenzene (98%) and anisole (40%) [101].
Heidekum
et al
. [102] recently reported on the Fries
rearrangement of phenyl acetate (to hydroxyace-
tophenones) and found that both Nafion
®
-based
and zeolitic-type catalysts were very effective (see
Scheme 6.25). Hydroxyacetophenone is a valuable
precursor in the pharmaceutical industry for the
formation of
p
-hydroxyacetanilide (painkiller). The
reaction reached high selectivity when high phenol
levels were used. The excess phenol reacts with one
of the intermediates (a ketene) to prevent an irre-
versible consecutive reaction leading to poor selec-
tivity. Both an H-BEA zeolite and Nafion
®
resin/silica
nanocomposite gave >90% selectivity.
A recent study on the esterification of several
cyclic olefins with saturated and unsaturated car-
boxylic acids has been studied over Amberlyst
®
-15,
Nafion
®
and the highly dispersed form of Nafion
®
(Nafion
®
/silica nanocomposites) [103]. The Nafion
®
-
based catalyst materials are superior to the weaker
acidity Amberlyst
®
catalysts when less-active olefins
are used and especially at low temperatures. The
esterification of cyclic olefins with carboxylic acids
such as dicyclopentadiene or naturally occurring ter-
penes yields compounds of industrial value (for the
flavour and fragrance industry) (see Scheme 6.26).
A 91% yield of the esterified product is obtained
over the composite compared with <5% for
Amberlyst
®
-15. In the simple esterification of
benzoic acid with
n
-butanol, an Amberlyst
®
catalyst
and a pure Nafion
®
resin catalyst showed good activ-
ity. In summary, in many esterification reactions ion-
exchange resins offer high reactivity.
