Chemistry Reference
In-Depth Information
proper synthesis, leading to shape-selective catalysts
able to induce reaction selectivity on the basis of
molecular size. These materials have proven useful
for the promotion of several industrially important
oxidations in the liquid phase using aqueous H
2
O
2
as the oxidant. A new process using TS-1 catalysts
offers a route to caprolactam, a component of Nylon-
6. In 1995, the world capacity for caprolactam
was 3.7 ¥ 10
6
t, mostly produced by the Beckmann
rearrangement of cyclohexanone oxime [24]. Recent
developments have greatly improved this route
for the synthesis of both caprolactam and its pre-
cursors by using several sequential catalytic steps
(Fig. 14.1).
Partial hydrogenation of benzene to cyclohexene,
followed by hydration over an H-ZSM-5 zeolite,
gives a new process for cyclohexanol synthesis now
being carried out at a scale of 6 ¥ 10
4
t year
-1
by Asahi
[17]. Cyclohexanol is oxidised with O
2
to cyclo-
hexanone, which is treated with NH
3
and H
2
O
2
over
TS-1 to form cyclohexanone oxime. Enichem oper-
ates this conversion at a demonstration scale of 1.2
¥ 10
4
t year
-1
. Conventional manufacture of cyclo-
hexanone oxime requires the use of stoichiometric
amounts of H
2
SO
4
, resulting in the generation of a
large amount of waste salt. Vapour-phase Beckmann
rearrangement over a zeolite can be used to prepare
the caprolactam [25] but it is not yet commercialised,
so the final step in practice still generates (NH
4
)
2
SO
4
as a waste. A recent report indicates that caprolac-
tam could be produced from a completely catalytic
route, ultimately starting from butadiene (Scheme
14.1) [2].
Adiponitrile, produced by the Ni(0)-catalysed
hydrocyanation of butadiene, can be reduced selec-
tively to the monoamine. Caprolactam is formed in
the vapour phase by heating of the monoamine over
an alumina catalyst. Rhodia is planning a 1 ¥ 10
5
t
year
-1
plant based on this technology by 2003. The
zeolite-catalysed cyclohexanol process also provides
a new route to adipic acid, because commercial
production of adipic acid occurs by oxidation of
cyclohexanol with HNO
3
or O
2
in the presence
of a catalyst (NH
4
-metavanadate/Cu-nitrate or Cu-
Mn-acetate) [26].
Zeolite TS-1 has proven to be remarkably versatile
in its ability to promote clean oxidation technology.
Hydroquinone is produced at an annual level of
about 5 ¥ 10
4
t. The classic approach oxidises aniline
in the presence of acidic MnO
2
, followed by a reduc-
tion over Fe/HCl, generating large amounts of waste
salt. In contrast, reaction of phenol with H
2
O
2
in the
presence of TS-1 gives a very clean route to hydro-
quinone via direct hydroxylation of the aromatic
ring. The only by-product is catechol, which is also
an item of commercial interest. Currently, about half
Fig. 14.1
New routes to caprolactam and adipic acid.
