Chemistry Reference
In-Depth Information
A kinetic study of benzene isopropylation with
isopropyl alcohol on alumino- and ferrisilicate has
shown that the rate of the overall reaction is
controlled by the desorption rate of cumene. The
activity does not follow the acidity of the catalysts.
This is a good illustration that acidity alone does
not always determine reactivity, so any correlations
need to take into account a wide range of factors.
It was observed that MFI ferrisilicates possessing
low-strength acid sites are the most convenient
catalysts for achieving
para
-selectivity in the
alkylation.
Corma
et al
. [22] are careful to point out that
factors other than acid strength may play a large role
in both the activity and selectivity of solid acid cata-
lysts. Zeolites exhibit better selectivities than the
commercially used Amberlyst
®
-15 resin for the gas-
phase methylation of isobutene with methanol. This
was rationalised in terms of the differences in
adsorption characteristics of the two reactants.
High amounts of methanol relative to isobutene
were adsorbed on the zeolites compared with the
ion-exchange catalysts. This preferred adsorption
of methanol in zeolites suppresses the formation
of by-products arising from isobutene adsorption on
acid sites, such as dimerisation and oligomerisation.
An interesting contrast for the zeolites H-
mordenite and H-ZSM-5 is observed in propene
hydration where the rate was measured as a func-
tion of concentration of acid sites. Mordenite shows
a linear dependence (higher rate with higher
number of acid sites), whereas H-ZSM-5 peaks at
about 0.4 (meq protons/gram) and then drops off
dramatically. In the case of H-ZSM-5, the rate of
hydration is influenced by intracrystalline diffusion
and by the strength of reactant adsorption. Careful
optimisation of acidity, pore structure and adsorp-
tion/desorption is always needed to tailor these
catalysts for specific reaction chemistries. Large-scale
production of isopropanol by catalytic propene
hydration over a modified beta zeolite has been
demonstrated. Zeolites also are effective catalysts
for both the dehydration of alcohols to olefins (the
reverse of the hydration reaction above) and the for-
mation of ethers due to intermolecular reaction.
It is clear from the above examples that zeolites
play a dominant role in the use and application of
solid acid catalysts. One potential limitation of zeo-
lites is the size limitation of the pore structure (typ-
ically 12 Å). Corma
et al
. [22] provided a very nice
example of the use of one of the so-called MCM-41
zeolitic-type materials. These materials have larger
uniform pores that lie in the range 10-100 Å. These
materials were developed originally by scientists at
Mobil. The general acid strength of these is specu-
lated to be lower than some of the zeolitic-type
materials. The advantage of larger pore size com-
pared with beta zeolites was demonstrated via a
series of acetalisation reactions. The acetalisation
of aldehydes using different molecular sized alco-
hols was studied: heptanol, phenylheptanol and
diphenylheptanol. Although the conversion of
the smaller molecular sized heptanol/aldehyde (ca.
95%) was comparable for the zeolite and MCM-41,
using diphenylacetaldehyde gave only 8% conver-
sion over beta zeolite compared with 80% conver-
sion over the MCM-41 material.
The second implication of the larger MCM-41
material is that these materials should deactivate at
a slower rate than the smaller pore zeolites (such as
HY zeolite), where plugging occurs in the smaller
pore structures. It should be noted, however, that
many of the larger pore MCM-41 materials will not
have the stereospecificity that a small-pore zeolite
imposes and thus one may expect poorer selectivity
than the more traditional zeolites. A second [25]
example for the catalytic application of mesoporous
materials is in the use of MCM-41 aluminosilicate for
alkylation of the bulky 2,4-di-
tert
-butylphenol with
cinnamyl alcohol. It was shown that although this
alkylation does not occur in the restricted environ-
ment of an HY zeolite (pore size 7.4 Å), the primary
alkylation product 6,8-di-
tert
-2-phenyl-2,3-
phenyl-2,3-dihydro[4H]benzopyran is formed with
MCM-41.
A nice example of the control of molecular selec-
tivity is shown in the use of the zeolite ferrierite (FER
is a one-dimensional pore structure with a 10-
membered pore ring opening) [26]. Isobutene is
used in the formation of MTBE (methyl-
tert
-butyl
ether) as an oxygenate octane booster in gasoline,
although, as pointed out later, the use of MTBE is
being restricted due to the potential effects of
groundwater contamination. These medium-pore
zeolites are highly selective for the olefin skeletal
isomerisation reaction. A mechanism has been pro-
posed in which the highly branched C8 olefins are
trapped within the zeolite pores as a result of geo-
metric constraints and are selectively cracked to yield
isobutene. A more detailed account of the isomeri-
