Molybdenum carbide, supercritical ethanol and base: Keys for unlocking renewable BTEX from lignin

Supported molybdenum hemicarbide catalysts were prepared and employed for the depolymerization and deoxygenation of waste lignins to aromatics in supercritical ethanol. Titanium nitride appears be a particularly beneficial support material. Depolymerization at 330 °C led to a higher yield and selectivity for arenes than at 280 °C. Base is essential for efficient substrate conversion: the basic constituents inherent in waste lignins are suitable, as is added sodium hydroxide. Without base, depolymerization is poor, the yield of aromatics is very low, and substrate defunctionalisation does not occur. A significant proportion of aromatic products is formed from supercritical ethanol itself, including benzene, which is likely to be present in all catalytic runs due to ethanol aromatization. Hence the total arene content produced is actually higher than has been widely reported. Catalytic cracking of ethanol leads to C₁-units, which add to reaction intermediates to give aromatics containing odd numbers of carbons (e.g. toluene).