Chemistry Reference
In-Depth Information
Scheme 14.1
Scheme 14.2
Scheme 14.3
of the world's production has shifted to this
H
2
O
2
/zeolite-based approach (Scheme 14.2) [22].
Zeolite TS-1 also has been reported as being useful
in a new manufacturing route for propylene oxide.
Over 3 ¥ 10
6
t of this material are produced annually,
primarily by the reaction of base with propylene
chlorohydrin, which again generates considerable
amounts of salt. In contrast, treatment of propylene
in aqueous methanol with H
2
O
2
under mild condi-
tions (<60°C and 4 bar) produces propylene oxide in
selectivities as high as 97% (Scheme 14.3) [22].
Traditionally, the commercial use of heteroge-
neous catalysis has been limited to the production
of large-scale commodity chemicals. More recently,
the technology is being used in the production of
fine chemicals and pharmaceuticals. Sheldon and
Downing describe several examples of heteroge-
neously catalysed pilot plant and demonstration
operations reported by industry [27]. The BASF
production of citral (Scheme 14.4) [28,29] and
Eastman's method for manufacturing 3,4-epoxy-1-
butene (Scheme 14.5) are illustrative [30].
Citral manufacture starts from the reaction of
isobutylene and formaldehyde to give 3-methyl-3-
butenol, which serves as the starting material for two
components: 3-methyl-2-butenol via Pd-catalysed
isomerisation of the double bond to the more highly
substituted position; and 3-methyl-3-butenal via
oxidation of the hydroxyl group over a silica-
supported silver catalyst. Addition of these two
materials gives an ether, which in two thermal steps
rearranges to form citral.
The Eastman process (Scheme 14.5) for the selec-
tive oxidation of only one of the double bonds in
butadiene is carried out over a silver catalyst with
oxygen. Epoxybutene is being positioned by
Eastman as a fine chemical platform that can be con-
verted to a number of different intermediates,
including cyclopropanes and dihydrofurans.
Direct amination of olefins can be catalysed by
heterogeneous systems. Workers at BASF have
found that treatment of isobutylene with ammonia
under supercritical conditions gives a 99% selectiv-
ity to
t
-butylamine at 12-15% conversion. This
direct formation of
t
-butylamine is a marked
improvement over the conventional three-step
Ritter route [30]. The conventional route requires
4.5 kg of raw material to generate 1 kg of product,
and results in the formation of about 3 kg of Na
2
SO
4
as a waste product (Scheme 14.6).
