(669c) Selective Conversion of Biorefinery Lignin to Dicarboxylic Acids

Lignin is a ubiquitous component in almost all plant biomass. Large quantities of industrial lignin are already produced annually as a waste product of the pulp and paper industry, where the vast majority is burned as a low cost fuel to provide energy for the papermaking process. The emerging biomass refinery industry will further introduce an enormous amount of lignin. Thus, there is an urgent need to develop technologies that can create new applications for biorefinery lignin. Lignin is the largest source of renewable material with an aromatic skeleton.

Majority of previous in area have been focused on depolymerizing lignin to low molecular weight phenolic compounds (LMWPC). However, one potential group of chemicals has been overlooked among oxidative conversion products: dicarboxylic acids (DCA). DCA such as muconic, maleic, and succinic acids are important and highly valuable industrial chemicals and intermediates used in many industries including the biopolymer, pharmaceutical, and food additives industries. Current commercial dicarboxylic acids are all produced from petroleum based feedstock. Developing a green route to producing DCA from renewable biomass lignin will be of a prime interest to both chemical and biomass conversion industry. Oxidative aromatic ring cleavage to yield muconic acid and its derivatives is a well-recognized reaction. Although DCA and their derivatives have been detected in the reaction mixture  from oxidative delignification of wood pulp, there has not been successful attempt to produce DCA from biomass lignin. Discovering a selective route to convert lignin to DCA will pave a new avenue towards lignin utilization.

This paper demonstrates a novel route to creating dicarboxylic acids from biorefinery lignin via ring cleavage by chalcopyrite catalyzed oxidation in a highly selective process. We reported a method of selective production of DCA via chalcopyrite (CuFeS₂) catalysed oxidation of biorefinery lignins in the presence of hydrogen peroxide (H₂O₂) under mild reaction conditions. A clear two stage reaction mechanisms is proposed, including depolymerization and oxidative aromatic ring cleavage, as explaination of the products profiles through lignin and model compounds study. Up to 95% selectivity of stable, saturated dicarboxylic acids was obtained from several types of biorefinery lignin and model compounds under mild, environmentally friendly reaction conditions.