Mechanistic insights into the co-pyrolysis of lignin and spent bleaching clay over a Mo-modified carbon catalyst

Abstract

Lignin pyrolysis is a sustainable pathway for synthesizing high-value alkylphenol chemicals and fuels. In this study, the co-pyrolysis process of lignin and spent bleaching clay was investigated for producing alkylphenols. The effects of different metal-loaded biochars, pyrolysis temperatures, and lignin-derived phenolic compounds on the distribution of pyrolysis products were examined. The results showed that the total acid content in Mo/C was 3.39 times higher than that in the pristine carbon. Additionally, Mo/C exhibited the highest number of weak acid sites (56.0%), abundant surface oxygen-containing functional groups, and a microporous structure. These characteristics facilitated both the cleavage of methoxy groups (C Ar –OCH 3 ) and the occurrence of aromatic C–C coupling reactions, thus increasing the selectivity of alkylphenols from 6.5% to 33.0%. Alkylphenol yield peaked at 500°C, whereas higher temperatures favored demethoxylation over coupling. Methoxyphenol exhibited an approximately 20–25% higher alkylation rate than phenol. Electrostatic potential calculations attributed this enhancement to the electron-donating effect of the methoxy group, which facilitated its attack by electrophilic alkyl fragments. In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) revealed the anchoring of phenolic intermediates on the Mo/C surface through Mo–O bonds. Charge difference calculations revealed electron transfer between the oxygen-containing functional groups of phenolic compounds and the Mo/C catalyst, thereby activating the C–O bond. Alkylphenols were generated through two distinct pathways: the deoxygenation of methoxyphenols and the coupling reaction between phenol and alkyl fragments. The deoxygenation pathway was more energetically favorable. This study provides insight into the low-cost and efficient production of green alkylphenol compounds.