Preparation and Purification of Biphenyldicarboxylic Acids

A process for producing 3,4′- and/or 4,4′-biphenyldicarboxylic acid diester, the process comprising:

• (a2) contacting 3,4′- and/or 4,4′-dimethylbiphenyl with a source of oxygen in the presence of an oxidation catalyst to produce an oxidation product comprising 3,4′- and/or 4,4′-biphenyldicarboxylic acid and one or more isomers of formylbiphenylcarboxylic acid;
• (b2) contacting at least a portion of the oxidation product with an alcohol under conditions effective to esterify at least part of 3,4′- and/or 4,4′-biphenyldicarboxylic acid and at least part of the formylbiphenylcarboxylic acid and produce an esterification effluent containing 3,4′- and/or 4,4′-biphenyldicarboxylic diester and formylbiphenylcarboxylic acid ester;
• and
• (c2) separating at least part of the 3,4′- and/or 4,4′-biphenyldicarboxylic diester from the esterification effluent by crystallization.