Chemistry Reference
In-Depth Information
+H
+
-H
+
-H
2
+H
2
sec
-C
+
tert
-C
+
n
-paraffin
n
-olefin
iso-olefin
isoparaffin
Scheme 6.5
Scheme 6.6
sation of olefins using zeolites has been described by
Corma [24,27-29].
In the skeletal isomerisation of alkanes, the con-
version and selectivity are strongly increased when
Pt or Pd is incorporated into the acidic zeolite and
the reaction is carried out in the presence of hydro-
gen. The reaction sequence is as shown in Scheme
6.5, where the proposed mechanism involves the
formation of an intermediate olefin that is produced
by dehydrogenation of the alkane on the metallic
site. The olefin then is protonated to form a
carbenium-ion-like transition state, which can iso-
merise. It has been proposed also that the reaction
pathway may go via a penta-coordinated carbonium
ion rather than a true carbonium ion.
Very strong acid sites (such as mordenite) are
required to isomerise short-chain alkanes such as
C
4
-C
5
, because the reaction goes through primary
carbenium ions. With mordenite the maximum
strength is produced by modifying the framework
Si/Al ratios of ca. 10, with essentially isolated Al
atoms. With long-chain
n
-paraffins, milder acidities
and larger pores are desired in order to minimise
recracking of the branched products formed. Bifunc-
tional catalysts based upon faujasite, ZSM-5 and
phosphate-based zeolites (SAPO-5) show good selec-
tivities for branching isomerisation of long-chain
paraffins.
One of the best examples to illustrate the role
played by shape selectivity is shown by the
isomerisation-transalkylation of alkyl aromatics. The
p
/
o
selectivity for ZSM-5 is about 2.9 compared with
a value of about unity for mordenite. The preferable
isomerisation reaction of
m
-xylene to the dispropor-
tionation reaction for ZSM-5 is ca. 33 compared with
a ratio of ca. 1-2 for mordenite. The isomerisation of
alkylaromatics is always accompanied by competitive
transalkyation reactions (see Scheme 6.6).
In the case of
m
-xylene, the most important
product commercially is the
para
compound, a
precursor to terephthalic acid. The thermodynamic
equilibrium ratio at room temperature is 24, 60 and
16 for the
p
-,
m
- and
o
-isomers, respectively. When
using large-pore zeolites, transalkylation occurs via
a biomolecular mechanism of the intermediates
to form unwanted transalkylation products. The
transalkylation reaction can be reduced by using
zeolites with smaller pore size, which favours the
unimolecular internal isomerisation. Thus, with
mordenite (the size of the window in the intracrys-
talline cavity being ca. 6.8 Å, the transalkyation reac-
tion is much more prominent than ZSM-5 (ca. 5.7 Å
cavity size). The conversion to the preferred
p
-isomer
(
p
-xylene) is also enhanced relative to the
o
-isomer,
due to the higher diffusion coefficient of the
p
-
isomer compared with the
o
-isomer within the ZSM-
5 zeolite (two orders of magnitude for the
p
-isomer).
This difference allows the
p
-xylene to escape more
readily than the
o
-isomer outside the zeolite crys-
talline cage. With a slower diffusion time for the
o
-
