Lignin Catalytic Depolymerization for Guaiacols and Alkylphenols over Co–Zr/Sepiolite under Supercritical Ethanol

Lignin catalytic depolymerization (LCD) in supercritical ethanol for the preparation of liquid fuels and phenolic monomers is one of the promising strategies for the valorization of lignocellulosic biomass. This paper reported sepiolite (SEP)-supported Co–Zr catalysts (Co–Zr/SEP) with different Co/Zr ratios for LCD in supercritical ethanol. The characterization results revealed that the cobalt oxides interacted with the SEP framework to create abundant pores, which increased the accessibility of the active sites with lignin fragments. Furthermore, the Zr additive promoted the conversion of Co(OH)2 to Co3O4 to increase Lewis acid sites (LAS) and oxygen vacancies (VO), which could enhance the adsorption and stabilization of reactive intermediates. Among the samples, 3Co1Zr/SEP exhibited the superior surface area and the highest LAS and abundant VO. The experimental results showed that the Co/Zr ratio significantly affected the yields of liquid products and phenolic monomers, and 3Co1Zr/SEP achieved the outstanding liquid product yield (60.11%) and the optimal yields of guaiacols (445.2 mg/g lignin) and alkylphenols (109.8 mg/g lignin) under the optimal reaction conditions. Additionally, 3Co1Zr/SEP significantly reduced the apparent activation energy for breaking β-O-4 bonds to 71.46 from 89.94 kJ·mol–1 over SEP by model experiments; furthermore, the possible reaction pathways of LCD over 3Co1Zr/SEP were proposed.