Selective catalytic depolymerization of lignin to guaiacols over Mo-Mn/sepiolite in supercritical ethanol

Abstract

Lignin catalytic depolymerization (LCD) into liquid fuels and aromatic chemicals in supercritical ethanol systems is one of the most effective strategies for efficient utilization of biomass resources. In this study, a series of sepiolite (SEP) supported Mo-Mn catalysts with different Mn/Mo ratios were prepared and applied to LCD. Experimental results for lignin and 2-phenoxy-1-phenyl ethanol (PP) confirmed that Mo/Mn species promoted the breakage of β-O-4 bonds, Mn additive effectively inhibited the alkylation of intermediates, and Mn/Mo ratios distinctly affected the yields of lignin oil (LO) and guaiacols. All as-prepared Mn-Mo/SEP catalysts achieved approximately 100 % conversions of lignin and PP. Various characterization results demonstrated that Mn/Mo ratios could regulate the particle sizes of metal oxides as well as the concentration of oxygen vacancies (OV) and surface acidic sites. In 3Mn1Mo/SEP, the appropriate interaction among SEP, Mn and Mo species reduced the particle sizes of metal oxides and rendered the crystalline phase structure in a relatively complex state, which undoubtedly generated massive steps/corners on its surface and then enhanced the hydrogenation reaction. Furthermore, the 3Mn1Mo/SEP contained many OV and suitable distribution of Lewis and Brønsted acid sites, which facilitated the adsorption of oxygen-containing intermediates and further hydrogenation and deoxygenation. Therefore, 3Mn1Mo/SEP achieved 45.7 % of LO yield with an HHV of 32.31 MJ/Kg and the highest selectivity to phenols (85.61 %) and yield of guaiacols (308.9 mg/g lignin). In addition, the plausible reaction network for LCD was proposed based on the results of experiments and characterizations.