Amoco process In the Amoco process, which is widely adopted worldwide, terephthalic acid is produced by catalytic
oxidation of
p-xylene: : The process uses a
cobalt–
manganese–
bromide catalyst. The bromide source can be
sodium bromide,
hydrogen bromide or
tetrabromoethane. Bromine functions as a regenerative source of
free radicals.
Acetic acid is the solvent and
compressed air serves as the oxidant. The combination of bromine and acetic acid is highly
corrosive, requiring specialized reactors, such as those lined with
titanium. A mixture of
p-xylene,
acetic acid, the
catalyst system, and compressed air is fed to a reactor.
Mechanism The oxidation of
p-xylene proceeds by a free radical process. Bromine radicals decompose cobalt and manganese hydroperoxides. The resulting oxygen-based radicals abstract hydrogen from a methyl group, which have weaker C–H bonds than does the aromatic ring. Many intermediates have been isolated.
p-xylene is converted to
p-toluic acid, which is less reactive than the p-xylene owing to the influence of the
electron-withdrawing carboxylic acid group. Incomplete oxidation produces
4-carboxybenzaldehyde (4-CBA), which is often a problematic impurity. :
Challenges Approximately 5% of the acetic acid solvent is lost by decomposition or "burning". Product loss by
decarboxylation to
benzoic acid is common. The high temperature diminishes oxygen solubility in an already oxygen-starved system. Pure oxygen cannot be used in the traditional system due to hazards of flammable organic–O2 mixtures. Atmospheric air can be used in its place, but once reacted needs to be purified of
toxins and
ozone depleters such as
methylbromide before being released. Additionally, the corrosive nature of bromides at high temperatures requires the reaction be run in expensive titanium reactors.
Alternative reaction media The use of
carbon dioxide overcomes many of the problems with the original industrial process. Because CO2 is a better flame inhibitor than
N2, a CO2 environment allows for the use of pure oxygen directly, instead of air, with reduced flammability hazards. The solubility of molecular oxygen in solution is also enhanced in the CO2 environment. Because more oxygen is available to the system,
supercritical carbon dioxide (
Tc = 31 °C) has more complete oxidation with fewer byproducts, lower
carbon monoxide production, less decarboxylation and higher purity than the commercial process.
Promotors and additives As with any large-scale process, many additives have been investigated for potential beneficial effects. Promising results have been reported with the following. Terephthalic acid can be prepared in the laboratory by oxidizing many
para-disubstituted derivatives of
benzene, including
caraway oil or a mixture of
cymene and
cuminol with
chromic acid. Although not commercially significant, there is also the so-called "
Henkel process" or "Raecke process", named after the company and patent holder, respectively. This route involves the transfer of carboxylate groups. Either
potassium benzoate disproportionates to potassium terephthalate and
benzene or
potassium phthalate rearranges to the terephthalate.
Phthalic anhydride can be used as a raw material and then potassium can be recycled. ==Applications==