Selective hydrogenolysis of lignin over hierarchical CoNC catalysts for the sustainable production of 4-propylsyringol

Abstract

Production of chemicals through the conversion of lignin can not only improve the utilization of renewable biomass resources but also reduce the dependence on traditional fossil resources. In this work, 4-propylsyringol (4-PS), a high value-added chemical for medicine and materials, was obtained via selective hydrogenolysis of lignin in the presence of a novel hierarchical Co, N co-doped carbon (CoNC) catalyst. When organosolv bagasse lignin was depolymerized at 230 °C for 4 h, the yield of monophenols reached 28.8 wt%, and 34.3% of the products were 4-PS (yield of 9.9 wt%). Extensive characterizations demonstrated that the high ratio of mesopores (92.9%) and the suitable pore-size (>1.9 nm) distribution of CoNC promoted the adsorption and mass transfer of lignin on the catalyst, while the synergistic catalytic effect between the Co single atom and acid site (pyrrolic–N) played a key role in the superior catalytic activity of CoNC. The structural evolution of lignin, control experiments with lignin models, and DFT calculations showed that the β-O-4 bond in the S unit of lignin was the most reactive amongst the lignin fragments, and it was responsible for the excellent selectivity of 4-PS. In addition, a plausible reaction mechanism was proposed, where the β-O-4 bond cleaved according to a carbon center radical pathway, as revealed by electron paramagnetic resonance (EPR) spectroscopy results. Therefore, this work provides a sustainable alternative strategy to petroleum routes for producing typical value-added fine chemicals.