Chemistry Reference
In-Depth Information
isomer, sufficient time occurs to isomerise within the
framework to the
p
-isomer. This is a good example
of controlling and reducing bimolecular reactions
and optimizing selectivity based upon size constraint
effects.
The high selectivity imparted within a zeolite
structure is exemplified by the catalysed nitration of
mono-substituted benzenes using stoichiometric
amounts of acetic anhydride in the presence of the
H-beta zeolite. The conventional catalysts in nitra-
tions employ sulfuric acid as a catalyst. The
para
selectivity is greater than 99% with a phenyl sub-
stituent on the benzene ring, as compared with
about 82% for ethylbenzene.
Zeolites are well known as excellent catalytic
cracking catalysts in the oil refining industry. Gas-oil
cracking is the basis for the production of at least
30% of the gasoline, olefins and isoparaffins in the
petrochemical industry. The reaction mechanism
appears to go via carbenium- and carbonium-ion-
like intermediates (see Scheme 6.7), as described by
Corma [24].
It has been proposed that one way to stabilise the
carbenium ion pair (to the zeolite) is through a beta
scission process, which produces an olefin in the
gas phase and a new alkoxy group that remains
adsorbed. Olah
et al
. suggested that the transition
state appears as a two-point interaction of ethane
with the OH and a neighbouring oxygen [5].
The reaction coordinate of protolytic cracking repre-
sents concerted stretching of the O-H bond and
the bridging hydroxyl group, combined with
heterolytic splitting of the carbon-carbon bond in
the protonated ethane molecule. A free alkane is
formed, along with the formation of an alkoxy group
on the neighbouring basic oxygen of the former acid
site.
In the case of
n
-alkanes, protolytic cracking occurs.
When the shorter carbenium ion is on the surface,
cracking may occur via two routes: protolytic crack-
ing if the carbenium ion desorbs and regenerates the
original Brønsted site; and b-scission if hydride trans-
fer from a reactant molecule to the adsorbed car-
benium ion occurs. Reaction pathways and yields are
very dependent upon zeolite chemical composition
and geometry. A high Si/Al framework ratio, while
reducing coke, increases recracking. The ZSM-5
catalyst exhibits selective cracking of molecules in
the gasoline range, which produces an increase in
the gasoline research octane number (RON) and also
an increase in the production of propylene. Using
beta or MCM—22 increases RON and shifts the C4
and C5 products to the more desirable isobutylene
and isoamylene products. In summary, zeolites have
played a prolific role in the control and optimisation
of gasoline products. The relative rate of cracking of
linear and branched alkanes depends upon and can
be controlled by the geometrical constraints within
the zeolite feedstocks, which ultimately help to opti-
mise fuel use and energy consumption.
One very important reaction in which zeolites are
not very effective is in the alkylation of isobutane
and butene. The product, trimethylpentane, is a key
ingredient in high-octane fuels. The drive here is to
replace sulfuric acid and HF with solid acid catalysts
that are clearly safer to handle. Although zeolites
show very high initial activity, these catalysts deac-
tivate very rapidly and no suitable zeolite catalyst has
been identified at this point.
Zeolites are well known for the effective
oligomerisation of olefins. Olefin oligomerisation
and aromatisation of C2-C10 olefins are the basis of
Mobil's olefin to gasoline distillation process. Reac-
tion products range from iso-olefins, which can be
hydrogenated to form premium-quality low-point
jet fuel and distillate fuel, to aromatics using higher
reaction temperatures. Again, the desired products
can be controlled carefully by temperature, zeolite
composition (Si/Al ratio), texture (crystal size) and
catalyst conformation, as shown for ZSM-5.
Scheme 6.7
