Oxidative Ring-Opening of Aromatics: Decomposition of Biphenyl Carboxylic Acids and Zinc Biphenyl Carboxylates

Ring-opening conversion of multinuclear aromatics can be used to upgrade heavy aromatic oils to lighter products, and it is usually performed reductively with H₂. Oxidative ring-opening is an alternative strategy that involves three steps: (i) oxidation of multinuclear aromatics to quinonoids, (ii) further oxidation and ring-opening to produce aromatic carboxylic acids, and (iii) decarboxylation of aromatic carboxylic acids. In the last step, decomposition by ketonization is an undesirable side reaction that leads to a ring-closed product. Selectivity control during aromatic carboxylic acid decomposition was investigated using biphenyl-2-carboxylic acid, biphenyl-2,2′-dicarboxylic acid, zinc(II) biphenyl-2-carboxylate, and zinc(II) biphenyl-2,2′-dicarboxylate. The reaction networks of thermal decomposition of the aromatic carboxylic acids were determined. Decomposition of biphenyl-2-carboxylic acid took place mainly by decarboxylation to produce biphenyl, dehydration and ring-closure to produce fluorenone, and the formation of diphenic anhydride as intermediate product leading to fluorenone. Decomposition of biphenyl-2,2′-dicarboxylic acid proceeded through decarboxylation to biphenyl-2-carboxylic acid as intermediate, as well as two seemingly related pathways, leading to the formation of a hydroxy-fluorenone and a cyclic trione. Over the temperature range from 340 °C to 400 °C, thermal decomposition invariably resulted in a higher ketonization than decarboxylation selectivity. Decomposition of the analogous zinc carboxylates demonstrated that ketonization could be suppressed and the most abundant products were biphenyl > fluorenone > fluorene. It was possible to achieve a biphenyl (decarboxylation) to fluorenone (ketonization) selectivity ratio of 17:1 during batch reactor decomposition of zinc(II) biphenyl-2,2′-dicarboxylate at 380 °C. Reaction stoichiometry indicated that water should affect selectivity, which is consistent with observations in the literature, but this aspect was not investigated further.