Chemistry Reference
In-Depth Information
encouraging our young chemists and engineers to
adopt.
In the cyclohexane oxidation stage, small
improvements to rate and conversion have been
made by improved design of air/liquid mixers and
heat exchangers (process intensification) [51], but
the inherent hazards associated with stage 1 of the
process largely remain.
Perhaps the preferred answer to achieving a
greener, more inherently safe process is to find an
alternative route. Production of cyclohexanol by cat-
alytic hydrogenation of phenol proceeds in high
yield and high selectivity, and at first sight overcomes
the cyclohexane oxidation issues. However, the
starting material for both routes is benzene, and
phenol production involves a similar oxidation step
(see cumene route discussed above). Thus, it may
be argued that cyclohexanol production has been
'greened' but the issues have been put back a stage.
On the other hand, it could be argued that because
phenol is produced anyway the number of oxidation
plants required has been reduced and therefore the
overall risk has been reduced. Although the phenol
route is practised, unfavourable economics have pre-
vented widespread adoption of this process.
What could be—at least if the economics were
more favourable—the perfect green and sustainable
solution has been identified by Frost [56]. The
process involves synthesis of adipic acid from glucose
via catechol (Fig. 2.17) using a genetically modified
Escherichia coli
biocatalyst.
Several other processes, with varying degrees of
greenness, have been developed over the years,
including carboalkoxylation of butadiene [57] and
dicarbonylation of 1,4-dimethoxybut-2-ene [58],
but for various reasons these have not been com-
mercially successful.
The most successful move towards sustainability
has been made by Asahi Kasei Corp. [59], who have
developed a commercial route to cyclohexanol based
on the hydration of cyclohexene (Fig. 2.18). The
process relies on the use of a novel high-silica H-
• Is the product (adipic acid) required or is there an
alternative?
• Are there any alternative routes that can be
evaluated and compared?
• Can the ketone/alcohol mix be produced avoiding
the inherent hazards involved with oxidation?
• Can the ketone/alcohol mix be produced with
lower risk?
• Can the large hydrocarbon inventory be reduced?
• Is there an alternative to the nitric acid oxidation
step?
• Can nitrous oxide be recycled in the process to a
useful material?
• Can nitrous oxide emissions be avoided?
These types of questions need to be addressed from
both the technical and economic viewpoint, with the
aim of identifying a more environmentally friendly
product or route that is also commercially viable.
Of course many of these questions have been
addressed, sometimes in an attempt to improve
process and eco-efficiency, and at other times indus-
try has been forced to consider alternatives based on
public and legislative pressure following Flixborough
and increased concern over the greenhouse effect for
example.
Following pressure to reduce nitrous oxide emis-
sions, all major adipic acid manufacturers agreed
to adopt some form of nitrous oxide abatement
measure by 1998. Most of these procedures involve
end-of-pipe technology that overcomes the immedi-
ate problem, at some significant cost, but does not
address the real issue of avoiding nitrous oxide pro-
duction. Some of the abatement technologies in-
volved are: catalytic reduction in the presence of
methane to nitrogen and carbon dioxide; catalytic
dissociation into nitrogen and oxygen; and oxidation
to nitric oxide, which can be used to make nitric acid.
Fig. 2.17
Green route to adipic acid.
