Chemistry Reference
In-Depth Information
the stability and to introduce additional acid sites
(Lewis and Brønsted). A number of mixed-oxide
pillared clays also have been developed, such as
Zr-Al pillared clays. A detailed review on the syn-
thesis and applications of these materials has been
described [85]. As with acid-treated clays, pillared
clays are active in applications such as cumene crack-
ing,
n
-heptane isomerisation, gas-oil cracking and
toluene alkylation. We do note, however, that
many of the earlier commercially used clays have
since been replaced by the more selective zeolite-
type catalysts.
M
n+
• xH
2
O
4 Some Recent Developments in Catalytic
Materials and Processes
4.1 The 'Kvaerner Process' and esterification
chemistry
Rohm & Haas have described the use of ion-
exchange resin catalysts as solid acid replacements
for homogeneous catalysts in a number of esterifica-
tion reactions [89]. The use of these polymeric cata-
lysts minimises waste streams compared with
homogeneous catalysts in the manufacture of butane
diol. The esterification of maleic anhydride with
ethanol to produce diethyl maleate (see Scheme
6.17) has been reported with high yield [89]. The
product then is hydrogenated to give the linear
alcohol butane diol.
This process has been commercialised by Rohm &
Haas with Kvaerner Process Technology and has
been used successfully by BASF in three large-scale
plants. The conventional esterification process
employs sulfuric acid or methane sulfonic acid and
results in sulfur contamination of the final ester
product. This also has the effect of poisoning a
hydrogenation catalyst that is used downstream. In
general the process efficiency is less than 96% and
the neutralisation step creates a waste disposal
problem.
One of the key breakthroughs in this area came
from an improved design of the catalyst. The origi-
nal types of catalysts used would allow high propor-
tions of the alcohol to penetrate deep inside the
catalyst. As a result, alcohol dehydration to the ether
occurs, which reduces the overall yield. This problem
was circumvented by designing a catalyst where the
majority of the acid sites were accessible on the
surface to both the acid and alcohol, yielding a very
Fig. 6.7
Structure showing a 2 : 1 layer lattice silicate, with
cations in-between the sheets.
water molecules in the interlayer. During activation,
Al
3+
and Mg
2+
are removed from the octahedral sites
in the clay layers. These cations are relocated in the
interlayer space, where they act as acid centres. Clays
are known to catalyse a wide range of reactions,
including alkylation-type chemistry (e.g. benzene
and 1-dodecene), acylation chemistry (the acylation
of diisobutylene with acetic acid using a commercial
acid-treated clay F-240), dimerisation chemistry (of
a-methylstyrene), oligomerisation chemistry (of
C
10-24
linear olefins for lubricant base oils), etherifi-
cation reactions (
t
-butanol with methanol), esterifi-
cation reactions (reaction of myristic acid with
propylene over Englehard F-24) and condensation
reactions (such as the condensation of barbituric acid
with arylcarboxyaldehydes). Acid-treated clays
effectively catalyse a variety of reactions that are
useful for the synthesis of a wide range of industri-
ally important products.
One of the limitations of clays is the lack of
thermal stability. This problem has been addressed
by developing a wide variety of pillared clays.
The layered clays are intercalated with inorganic
polyoxocations such as the Al
13
polycation. These
serve to increase the pore size of the clays, to increase
