Gas–solid conversion of lignin to carboxylic acids

Abstract

Lignin represents 15% to 40% of the dry weight of lignocellulosic biomass but remains under exploited as a sustainable feedstock for chemical and fuels even though it is the only bio-polymer with aromatic units. Technology that selectively converts lignin to value added specialities would improve the economics of the burgeoning bio-refinery industry. Here, we introduce a two stage gas-phase catalytic process that produces carboxylic acids and aromatics from lignin while minimizing coke and char and maintaining catalyst activity. In the first step, a mixture of 50% water vapour in air crack and partially oxidize this complex macromolecule (<550 °C). The effluent gas contacts heterogeneous mixed-metal oxides or metal catalysts in the second step. The product profile from the second step includes aromatic compounds but mostly C₄ carboxylic acids such as maleic acid and butyric acid. Vanadium catalysts cleave lignin bonds, open aromatic rings and oxidize lignin to carboxylic acids, especially maleic acid. WO₃/TiO₂ mostly gave butyric acid. Basic catalysts produced more aromatic compounds. The maximum amount of coke was 5% of the total carbon in lignin.