Cleavage of the lignin β-O-4 ether bond via a dehydroxylation–hydrogenation strategy over a NiMo sulfide catalyst

Abstract

The efficient cleavage of lignin β-O-4 ether bonds to produce aromatics is a challenging and attractive topic. Recently a growing number of studies have revealed that the initial oxidation of C_αHOH to C_αO can decrease the β-O-4 bond dissociation energy (BDE) from 274.0 kJ mol⁻¹ to 227.8 kJ mol⁻¹, and thus the β-O-4 bond is more readily cleaved in the subsequent transfer hydrogenation, or acidolysis. Here we show that the first reaction step, except in the above-mentioned pre-oxidation methods, can be a C_α–OH bond dehydroxylation to form a radical intermediate on the acid-redox site of a NiMo sulfide catalyst. The formation of a C_α radical greatly decreases the C_β–OPh BDE from 274.0 kJ mol⁻¹ to 66.9 kJ mol⁻¹ thereby facilitating its cleavage to styrene, phenols and ethers with H₂ and an alcohol solvent. This is supported by control experiments using several reaction intermediates as reactants, analysis of product generation and by radical trap with TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) as well as by density functional theory (DFT) calculations. The dehydroxylation–hydrogenation reaction is conducted under non-oxidative conditions, which are beneficial for stabilizing phenol products.