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Nitrous Oxide
One reaction that is being developed on a commercial
scale is the AlphOx process developed by Solutia and the Boreskov
Institute of Catalysis in Moscow. This has been running as a pilot
plant since 1996 and may come on stream commercially in the next
few years.
This reaction uses nitrous oxide (N2O) as the
oxidising agent, which reacts with Benzene in the vapour phase
to give phenol and nitrogen. The process uses metal modified zeolite
catalysts, such as V2O5/ MoO5/ ZSM-5 and Fe2O3/ MoO3/ ZSM-5, which
transfer atomic oxygen, from the decomposition of the N2O on their
surface, to benzene. The active catalyst appears to be the metal
species occupying the pores in the zeolite structure.
This reaction is of particular value to Solutia,
as they are a major producer of hexanedioic acid (adipic acid),
used in
nylon production, and N2O is produced is
a by-product. This cannot simply be released into the atmosphere,
as it is a pollutant under strict control, so its use to make
phenol removes the need to treat it as waste, and generates a
valuable product.
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Biomass
Though pure benzene and phenol will continue
to be petrochemical products for the foreseeable future, one area
where the use of biomass feedstock is already making an important
contribution is in the production of phenol-based resins used
in wood panel products like plywood, medium density fibreboard
(MDF) and laminates.
Heating the biomass, mainly softwood waste,
produces a pyrolysis oil which, although containing a mixture
of many phenol-based materials, can be used to make phenol-methanal
resins without requiring further purification, as it can still
form the cross-links. It is generally used in a 50-50 mixture
with pure phenol.
The lack of a purification stage reduces the
energy needed, and the gases and charcoal also produced can be
used as fuel for heating the biomass. Cost savings can be as high
as 25%.
Research aimed at increasing the proportion of biomass feedstock
to 100% for this particular application is being undertaken.
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Microreactor Technology
In future, many chemicals including phenol may
be produced in relatively small reactors about the size of a large
desktop.
One potential micro-reactor to produce phenol
involves the use of a small diameter (2 mm), porous tube of alumina
coated with a layer of palladium metal. A mixture of benzene and
oxygen is fed through the tube, and hydrogen gas is passed over
the tube.
The tube is heated to 150 - 250°C. Hydrogen
permeates though the alumina tube, and is converted to atomic
hydrogen by the palladium catalyst. The hydrogen atoms react with
oxygen gas, releasing oxygen atoms, which in turn react with the
benzene forming benzene epoxide. This isomerises to phenol.
The boiling points of phenol (182°C) and benzene (80°C)
mean that phenol is easily separated from unreacted benzene, and
the final liquid phenol is in a highly pure form.
Researchers claim that this method saves on
capital cost, reduces energy use, reduces waste, and can easily
be scaled up by adding more tubes, effectively using a modular
approach. One single micro-reactor could produce up to 100,000
tonnes per year. The technology can also be applied to the manufacture
of other materials.
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Oxidation of Benzene
The direct oxidation of benzene can be achieved
using several different oxidising agents, but none of these have
yet proved economically viable on a large scale. Considerable
research is going on into finding catalysts, which might allow
the direct oxidation of benzene to phenol.
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