Chemistry Reference
In-Depth Information
The importance of Brønsted acidity has been
pointed out in a very interesting paper by Clark
et al
.
[46]. They have investigated the use of sulfated
zirconia as new solid acid-based routes to linear
alkyl benzenes. The alkylation of benzene with 1-
dodecene was reported in 1999. The 1-dodecene
conversion was quite low, with conversions of
around 12%. Supported HPAs, acid clays and
alumina pillared clays showed conversions close to
100%. In contrast to this, Clark showed that a very
high conversion could be achieved using a commer-
cial supply of sulfated zirconia [46].
As pointed out by Clark, sulfated zirconia has been
reported to possess high Lewis acid activity and
this has encouraged some studies on its use in
liquid-phase Friedel-Crafts benzoylation reactions.
However, these have met with only moderate
success. Clark points out that under normal handling
conditions these catalysts pick up sufficient water to
render these almost exclusively Brønsted-type acids.
Thus, these types of catalyst may not be expected to
show high activity in the area of acylations that
prefer Lewis-type acidity. Using these commercial
'Brønsted'-type catalysts did indeed show low ac-
tivity for the acylation of benzene with benzoyl
chloride. The formation of linear alkylbenzenes was
studied due to the commercial importance of this
reaction (for surfactants, etc.) and the fact that it is
known that this reaction requires Brønsted acid sites.
The alkylation of benzene with 1-dodecene over
sulfated zirconia resulted in complete conversion of
the alkene with the formation of 93% of the mon-
ododecylbenzene, 43% of which is the preferred
2-isomer. Interestingly, if the Brønsted sites are con-
verted to Lewis sites by calcination above 500°C,
then the activity drops dramatically. Activity and
selectivity are comparable to those of aluminium
trichloride. A microporous sample of sulfated zirco-
nia was found to deactivate after the initial reaction,
whereas mesoporous sulfated zirconia was reusable.
The authors found that deactivated catalysts can be
regenerated by both solvent extraction and thermal
treatment; the latter resulted in complete regenera-
tion of catalyst activity. Under the right preparation
and handling conditions, sulfated zirconia appears
to be a very attractive catalyst for the alkylation of
benzene. In view of this work, it may be interesting
to reinvestigate a number of earlier reported reac-
tions in a controlled way to access in more detail the
effect of the relative amounts of Lewis and Brønsted
sites upon reactivity.
3.4 Ion-exchange resins
Ion-exchange resins have been used for a range of
commercial applications. For example:
(1)
The etherification of olefins with alcohols, e.g.
the coupling of isobutene with methanol to form
methyl
tert
-butyl ether (MTBE). Owing to the
concern of water contamination by MTBE, it
seems likely that MTBE will be phased out in
the USA. Other reactions include dehydration
of alcohols to olefins or ethers, e.g.
t
-butanol
dehydration to form isobutene.
(2)
Alkylation of phenols to alkyl phenols.
(3)
The condensation reactions, e.g. manufacture of
bisphenol-A (more than 2 billion lb per annum)
from phenol and acetone.
(4)
Olefin hydration to form alcohols, e.g. propene
hydration to form 2-propanol.
(5)
Purification of the phenol stream after decom-
position of the cumene hydroperoxide to phenol
and acetone.
(6)
Ester hydrolysis and other reactions.
A wide variety of reactions have been described,
ranging from alkylation with olefins, alkyl halides,
alkyl esters, isomerisation, transalkylation, acyla-
tion, nitration, ether and ester synthesis, acetals,
thioacetals, hydration and rearrangement chemistry
[55,56]. Sharma
et al
. give a detailed account on both
the microstructure and applications of polystyrene-
based ion-exchange resins, with approximately 300
different reactions described [57,58].
It is also noteworthy how the use and application
of these types of resins grow dramatically as the
microstructure of these materials is improved. Prior
to about 1960, polystyrene-based resins were essen-
tially gel-type resins whose swelling characteristics
depended upon the solvent or reactants. In non-
swelling media the active sites were largely inacces-
sible for reactivity. This problem was overcome with
the development of 'macroporous' ion-exchange
resins in the early 1960s [59]. A good understand-
ing and optimisation of this material for a range of
catalytic reactions has pioneered the way for these
ion-exchange resins to become industry's catalyst
of choice in several key areas. These include MTBE
