Oxidative ring-opening of aromatics: Thermochemistry of sodium, potassium and magnesium biphenyl carboxylates

Oxidative ring-opening is a potential alternative to hydrodearomatization and cracking for the conversion of multinuclear aromatic compounds to lighter products. Oxidation of multinuclear aromatics produces quinonoids that can be ring-opened by forming carboxylic acids, which can be decarboxylated to produce lighter oxygen-free hydrocarbons. The efficiency of this reaction depends on the thermochemistry of the metal carboxylate intermediate. Undesirable side-reactions compete with decarboxylation. Using metal carboxylates as catalytic surrogates is a convenient strategy to investigate the catalytic decomposition of carboxylic acids. The thermal behavior of sodium, potassium and magnesium biphenyl-2,2′-dicarboxylates was investigated, over the temperature range 25 to 600 °C, to evaluate the suitability of these metals as catalytic materials for decarboxylation, as well as to gain an understanding of the steps involved in the decomposition. In all cases, thermal analysis revealed quite complex behavior. Thermal events including the loss of lattice water, phase transitions and decomposition were observed. The strong basic properties of alkali and alkaline earth metals resulted in the formation of thermally stable carboxylates with decomposition temperatures above 400 °C. Infrared spectroscopy indicated that the Na, K and Mg carboxylates all existed in ionic (or bridging) configuration, but a bidentate type of interaction was additionally observed for the magnesium carboxylate. Formic acid was also identified during the initial stages of sodium biphenyl-2,2′-dicarboxylate decomposition suggesting the formation of an ester intermediate (not confirmed). The thermal stability of the biphenyl-2,2′-dicarboxylates increased in the order Na < K < Mg, which differed from thermal stability sequence of the alkanoates.