Cross-linking Reactions between Phenols and Benzoic Acid: the Role of Aryl Esters

Introduction It has been proposed that oxygen functional groups (i.e., carboxylic acids and their salts, phenols, and ethers), prevalent in low rank coals, are responsible for cross-linking reactions that inhibit the efficient conversion of low-rank coals to liquid fuels and chemicals. In the pyrolysis and liquefaction of low-rank coals, cross-linking has been correlated with evolution of CO2 and H2O. To determine if decarboxylation leads to cross-linking, the thermal decomposition of aromatic carboxylic acids was investigated. It was determined that carboxylic acids predominately undergo decarboxylation by an acid promoted ionic pathway at 400 °C, but depending on the reaction conditions, small amounts (<5 mol%) of cross-linked products (i.e., arylated products) could be formed by formation and radical induced decomposition of anhydrides. Anhydride formation and cross-linking reactions were enhanced by incorporating the aromatic carboxylic acids in a polymer (poly(mxylylene)-co-(5-carboxy-m-xylylene) at concentrations similar to those found in low rank coal (ca. 2.4 carboxyls per 100 carbons). Since these condensation reactions readily occur at temperatures found in the thermal processing of coal (350-425 °C), additional types of cross-linking reactions, such as the condensation of carboxylic acids and phenols to make aryl esters, might be possible. Since phenols are 2-5 times more concentrated than carboxylic acids in low rank coal, aryl esters could form from condensation reactions. Aryl esters are predicted to be thermally stable up to temperatures of 650 °C based on the Arrhenius parameters reported for the C-O homolysis of phenyl acetate (log k (s) = 15.3 – 72.3 kcal mol / 2.303RT). Pyrolysis of phenyl benzoate at 700 °C produced 90% conversion, and produced phenol, biphenyl, benzene, and dibenzofuran as the major products, but the mass balance was poor (<60%). Thus, aryl esters could act as low temperature cross-links in low rank coal. However, there is little data on the formation and pyrolysis of aryl esters at temperatures relevant to those found in the thermal processing of coal (350-425 °C). In this study, we have investigated the pyrolysis benzoic acid in the presence of excess mphenylphenol to study the formation and decomposition of aryl esters, and we have investigated the pyrolysis of phenyl benzoate at 400 °C to determine if aryl esters act as a thermally stable lowtemperature cross-link.