Enhancing β-aryl ether bond cleavage of lignin model dimer via benzylic alcohol dehydration

Abstract

Lignin has immense potential for the production of monoaromatics that can be further prepared as target chemicals or alkyl biofuels. In this work, experiments and density functional theory (DFT) calculations were employed to investigate the effect of benzylic alcohol (α-hydroxyl group, CαH─OH) dehydration to α,β-alkenyl group (Cα = Cβ) on the enhancement of the β-aryl ether (β-O-4) bond cleavage reactivity of lignin model dimer over Pd/C by formic acid. This study demonstrated that the β-O-4 bond cleavage reactivity was significantly enhanced via the benzylic alcohol dehydration to Cα = Cβ. The cleavage reactivity of the β-O-4 bond attached to Cα functional groups was in the order of α-hydrogen atom (CαH─H) > CαH─OH > α-carbonyl group (Cα = O). An easy formation of the CαH─H from the Cα = Cβ hydrogenation led to the enhancement of the β-O-4 bond cleavage reactivity being achieved. The β-O-4 bond cleavage rate was increased by 6.2 times in the presence of the CαH─H compared to the Cα = O. Due to few and lower required energy barriers after CαH─OH dehydration, the β-O-4 bond attached to the Cα = Cβ was more thermodynamically favorable to be cleaved. The work highlights the prospect of efficient depolymerization of lignin into monoaromatics.