Highly efficient metal-acid synergetic catalytic fractionation of lignocellulose under mild conditions over lignin-coordinated N-anchoring Co single-atom catalyst

Abstract

Lignin reductive depolymerization via a lignin-first strategy is extremely attractive but practically challenging due to the harsh reaction conditions, such as high temperature and high external H2 pressure. Herein, we demonstrate a highly efficient acid-assisted reductive fractionation of lignocellulose over a cobalt monatomic catalyst (Co SAs-N@LC) under mild temperature without H2. The Co SAs-N@LC with a Co-N4 conformation was fabricated by pyrolyzing phenolic-OH rich lignin-metal complex mixed with melamine. When subjected to poplar reductive fractionation at 160 °C with 0.1% H2SO4, the Co SAs-N@LC achieved a high monophenol yield of 41.7% with near complete delignification (99.7%), which was credited to the acid-metal synergy on effective lignin depolymerization, more importantly on condensation inhibition. Furthermore, thanks to the acid-catalyzed hydrolysis, xylan was simultaneously liberated, leaving a cellulose-rich fraction as the solid fraction. DFT calculations revealed that the Co-N4 conformation endowed the catalyst superior hydrogen evolution capacity and the monatomic catalyst showed satisfactory recycling stability due to the acid-resistant N-anchoring and the minimized catalyst fouling. Therefore, this work provides an effective approach for reductive fractionation of lignocellulose under mild conditions, more importantly a total biomass fractionation, through the combined use of atomically dispersed Co catalyst and acid co-catalyst.