Chemistry Reference
In-Depth Information
chemistry and plant design, significant material
savings can be made.
being, for all practical purposes, completely recycled.
Although the purist could argue that this process
may not be particularly 'intrinsically safe' due to the
potential hazard associated with handling HF, it is
considerably greener in the context of reduced ma-
terials consumption.
Much academic and industrial research effort has
gone into the greening of Friedel-Crafts processes,
with the aim of developing benign, easily recyclable,
inexpensive, active solid catalysts that are highly
selective and avoiding wasted raw materials and by-
products. For the alkylation reaction zeolites gener-
ally have provided the commercial solution to this
problem, with major areas of research work centred
on the avoidance of olefin oligomerisation and the
development of catalysts stable to the operating con-
ditions. Returning to the production of phenol from
cumene (see above), the first step involves the alky-
lation of benzene with propene, which was carried
out originally with AlCl
3
; today, several commercial
zeolite-based processes have been developed. The
Mobil process, developed in 1993, employs a high-
silica catalyst ZMS-5 that gives almost stoichiometric
yields, whereas Dow Chemical have developed a
process based on de-aluminated mordøenite [21].
The development of solid acid catalysts to solve the
many problems associated with the acylation reac-
tion generally has proved more problematic, but for
activated substrates such as aryl ethers Rhone-
Poulenc have developed an H-beta-zeolite catalyst
[22].
4.1 Catalytic solutions
Organic chemists very rarely write balanced equa-
tions and this can hide a multitude of sins. Taking
Friedel-Crafts reactions as an example, alkylation
and acylation reactions often are referred to as being
catalysed by lewis acids such as aluminium chloride;
although this is partially true, it is frequently ignored
that the acylation reaction requires more than stoi-
chiometric amounts of AlCl
3
(Fig. 2.10) [19].
In the alkylation reaction AlCl
3
is required only in
small amounts, but in the acylation reaction it com-
plexes with the ketone product and is taken out of
the catalytic cycle. In both cases, reactions usually
are quenched with water, leading to copious
amounts of aluminous waste and releasing three
equivalents of HCl. In the case of the acylation of
1,3-dimethylbenzene and assuming a quantitative
yield, more than 0.9 kg of AlCl
3
are wasted per kilo-
gram of dimethyl acetophenone produced.
A recent, but classic, example of overcoming the
wasted materials issue in aromatic acylations is the
Hoechst Celanese route to the analgesic ibuprofen
[20]. The reaction involves acylation of isobutylben-
zene with acetic anhydride, a process that had been
carried out traditionally with AlCl
3
in an organic
solvent. The Hoechst Celanese process employs
liquid HF as both a true catalyst and solvent, the HF
Fig. 2.10
Typical Friedel-Crafts
reactions.
