Understanding the cleavage of inter- and intramolecular linkages in corncob residue for utilization of lignin to produce monophenols

Abstract

The cleavage of inter- and intramolecular linkages is a crucial but complicated issue for isolation and depolymerization of lignin in lignocellulosic biomass to produce biofuel. Herein, a Na₂CO₃–H₂O–tetrahydrofuran system was adopted for studying the cleavage of these linkages in corncob residue. Almost all ether, ester and hydrogen bonds between lignin and cellulose were broken by Na₂CO₃ at 140 °C, resulting in 94.6% dissolution of lignin, meanwhile the C–O bond in the β-O-4 linkage and the C_α–C_β bond in the aliphatic side-chain of lignin could be broken to obtain aryl aldehydes. At 300 °C, the C_Ar–C_α bond could be broken down, bringing about high selectivity to monophenols without substituted alkyl groups, and the total yield of monophenols reached up to 26.9 wt% without additional hydrogen. Na₂CO₃ could also inhibit the formation of a C_α-oxidized side-chain product and the elimination reaction of an –OH group on C_α forming p-coumaric acid. G and H units in the corncob residue were mostly connected by a β-O-4 linkage, whereas S units were possibly linked by an α-O-4 linkage. The hydrogen bonds at –OCH₃ groups in S and G units in lignin could be disrupted by Na₂CO₃ to facilitate the dissolution and degradation of lignin.