Herbaceous crop lignin depolymerization using transition metal supported molecular sieves catalysts

Lignin has received great attention as a sustainable precursor for basic aromatic building blocks, such as phenols, which are currently obtained by synthetic chemistry using fossil-based feedstocks like crude oil. Several researchers are trying to upgrade lignin into value-added products with very little success. In 2004, the pulp and paper industry alone produced about 50 million tons of extracted lignin (using Kraft pulping process), yet the existing markets for lignin products remain limited and focus on low-value products such as dispersants or binding agents. Most of the produced lignin is still combusted to generate energy for the pulping industry due to market limitations.

Lignin depolymerization into aromatic products under mild conditions is an attractive approach for its valorization. Prominent linkages present in lignin include ether bonds that account for ~70% of the existing cross-links. Most of the reported methods for lignin depolymerization are based on hydrolysis, oxidation or reduction of the prominent ether bonds. In this work, lignin was extracted during Extractive Ammonia (EA) pretreatment and enriched through a fractionation process using common solvents like water and ethanol. This enriched lignin fraction was depolymerized into monomeric phenolic components in the presence of organic solvents through catalytic fragmentation–hydrogenolysis. Transition metal supported molecular sieve catalysts were used in this technique. Electro-catalysis supported on Raney Nickel was sequentially applied to further reduce depolymerized lignin molecules to fuel grade chemicals. Details about different catalysts used in the process, mass balances and mechanisms through which depolymerization reactions took place will be presented herein.