Chemistry Reference
In-Depth Information
support is silica, but supports also include the meso-
porous pure silica molecular sieve MCM-41 and in
some cases carbon. In all of these approaches the
potential for leaching exists and careful considera-
tion has to be given to catalyst solubility in the
catalytic reaction of interest. Mizuno
et al
. [31] have
described the use of insoluble solid acid catalysts for
liquid-phase reactions using both supported and
non-supported HPAs. The Cs
2.5
H
0.5
PW
12
O
40
catalysed
the alkylation of isobutane with butenes, and the
catalytic activity was higher than that of H
3
PW
12
O
40
and aluminosilicates. The usual problem of catalyst
deactivation was observed for this reaction. The
(NH
4
)
2
HPW
12
O
40
HPA was an efficient catalyst for the
liquid-phase Friedel-Crafts alkylation (benzylation)
of benzene and the acylation of
p
-xylene with
benzoic anhydride or benzoyl chloride. In the case of
(NH
4
)
2
HPW
12
O
40
, however, the catalyst partially dis-
solved during the reaction and also began to lose
activity. In contrast, no deterioration was observed
for the benzoylation of
p
-xylene with benzoic anhy-
dride catalysed by the same HPA. It was noted
that these types of acids (HPAs) were more
active than Nafion
®
and some modified clays. The
Cs
2.5
H
0.5
PW
12
O
40
HPA was extremely active for the
hydrolysis of 2-methylphenyl acetate in excess water
compared with H-ZSM-5 and sulfated zirconia
(which was almost inactive). The HPAs were found
to be active catalysts in the liquid-phase esterifica-
tion of 2,6-pyridinedicarboxylic acid with
n
-butanol,
although again it was pointed out that some disso-
lution of the HPAs may be occurring. Some Pt-
promoted Cs
2.1
H
0.9
PW
12
O
40
catalysts also have been
shown recently to be very effective catalysts in the
demanding reaction of skeletal isomerisation of
n
-
butane [7].
Okuhara & Nakato [7] have described a number
of catalytic reactions based upon mesoporous
heteropoly compounds. The Cs
2.5
H
0.5
PW
12
O
40
HPA
catalysed the isomerisation of
n
-butane at 573 K with
smaller deactivation, and the rate of isobutane for-
mation and the selectivity were much higher than
those of sulfated zirconia. The Cs
2.5
H
0.5
PW
12
O
40
HPA
also was an efficient insoluble catalyst in the esteri-
fication of acetic acid with ethanol, although
Amberlyst
®
-15 was found to be a superior catalyst.
These kinds of esterification reactions do not, in
general, need very high acid strength. The
Friedel-Crafts acylation over Cs
2.5
H
0.5
PW
12
O
40
and
H
3
PW
12
O
40
was studied using a range of acylating
Scheme 6.9
agents and aromatic compounds, such as
p
-xylene.
It was reported that anisole and
p
-xylene are acy-
lated with benzoic anhydride and acetic anhydride
by the Cs salt without dissolution of the catalyst (see
Scheme 6.9).
Typical conversions of the partial salt and the acid
were 57% and 3%, respectively. The HPAs were not
very active for the more demanding reaction of
benzene with acetic acid. It is clear that HPAs, espe-
cially based upon insoluble Cs salts, may be attrac-
tive acylation catalysts for applications in fine
chemicals.
Izumi
et al
. [35] attempted to immobilise het-
eropoly compounds within a silica matrix and used
these to study some water-based hydrolysis reac-
tions. The Cs
2.5
H
0.5
PW
12
O
40
HPA was active for the
hydrolysis of ethyl acetate and retained its catalytic
activity after a hydrothermal treatment at 393 K.
These salts, however, form colloidal suspension after
the reaction in the presence of a large excess of
water, and recovery of the catalyst is difficult. In the
case of the silica-immobilised catalyst, the catalyst
exhibited activities for hydrolysis of ethyl acetate as
high as that of unsupported Cs
2.5
H
0.5
PW
12
O
40
, and
the catalyst used was recovered successfully from the
solution by filtration. Okuhara also points out the
potential use of meso- and microporous HPAs as
shape-selective catalysts (namely product shape
selectivity) for
n
-butane skeletal isomerisations. The
HPAs are very active also in the formation of MTBE,
with a selectivity of 95% [36,37], particularly a
dimeric form of the Keggin ion known as Dawson
type (H
6
P
2
W
18
O
62
). Other reactions include iso-
propanol dehydration, the dehydration of diols, the
oligomerisation of propene and the etherification of
phenethyl alcohol with alkanols [38]. In the case of
MTBE, the gas-phase synthesis of MTBE over HPAs
has been described using unsupported and supported
catalysts [39,40]. The superior activity of dodeca-
tungstophosphoric acid supported on K-10 clay in
