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HYDROCHAR-SUPPORTED CATALYSTS FOR LIGNIN OXIDATION

Antonina Stepacheva, Mariia Markova, Yury Lugovoy, Alexander Sidorov, Vladimir Molchanov

Abstract

Lignin is a heterogeneous phenolic polymer that gives rigidity to plant cell walls and accounts for 15-40% of the dry weight of biomass [1]. Natural lignocellulosic materials have low solubility and processability, which limits their effective practical application [2]. Modern research on lignin processing focuses on depolymerization processes followed by conversion of the resulting monomers either by hydroprocessing or oxidation. Oxidative conversion of lignin is carried out using biotechnological methods (enzymes, microorganisms, and fungi), as well as thermochemical methods involving oxygen, peroxides, and harsh oxidants. The target products of oxidation are aromatic aldehydes and acids (vanillin, lilac aldehyde, quinones, etc.). In this study, the hydrochar-based catalysts for lignin oxidation were synthesized. Ag, Cu, and Ni were used as active metals because of their high activity in the dissociation of hydrogen peroxide as well as in the oxidation of main biomass components. The catalysts were synthesized by the hydrothermal carbonization of sawdust in the presence of metal salts to form the active phase directly in the hydrochar pores. The morphology, structure and composition of catalysts was studied using the low-temperature nitrogen physisorption, XPS, XRD, and TEM methods. The activity of the catalysts was studied in the lignin oxidation by hydrogen peroxide in mild conditions. Cu-containing catalyst showed higher activity providing 80% of lignin conversion at 80 C for 3 h. The main products of lignin oxidation were found to be mandelic acid and its esters, and the selectivity towards quinones increases with an increase in temperature to 90°C.

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