Analytical Py-GC/MS study on conversion behaviors and reaction mechanism of biochar-mediated catalytic upgradation of primary pyrolytic products from lignocellulosic biomass

•Carbocatalytic conversion of biomass-derived primary pyrolytic products are presented.•Product distribution under different temperatures and mixing ratios are analyzed.•Selectivity towards side-chain elimination, ring opening and recombination are compared.•Low temperature and high biochar ratio favors producing phenols and furans.•Carbon deposition should be avoided when producing aromatics at high temperature. This study aims to demonstrate the general carbocatalytic conversion behaviors and reaction mechanisms of representative biomass-derived primary pyrolytic products towards product upgradation. Specifically, 4-Ethylguaiacol (4-EGC) and 5-hydroxymethylfurfural (5-HMF) were selected as lignin- and cellulose/hemicellulose-derived model compound, respectively, to elucidate the origin of products under different temperatures and biochar-reactant mixing ratios. Three reaction routes were distinguished, including side-chain elimination, ring opening and recombination, with each route exhibiting unique characteristics in terms of initiation temperature and selectivity. For 4-EGC, elimination of side-chains began at 500 °C, accompanied by alkyl transfer reactions to generate multi-alkyl-substituted phenols, while ring-opening started at 700 °C, and aromatics could be simultaneously produced. Biochar did not change the initiation temperature of each route but significantly influenced the product distribution. Comparing the biochar-reactant mixing ratio of 0 and 10:1, at 600 °C the selectivity towards phenols increased from 20.52 % to 54.98 %, and at 800 °C the selectivity towards aromatics increased from 14.28 % to 53.01 %. For 5-HMF, the elimination of side chains already began at 400 °C, and the ring-opening started at around 600 °C, but recombination products (phenols and aromatics) would not notably emerge until 700 °C. Biochar could both lower the initiation temperature of each reaction route and change the conversion efficiency for 5-HMF owing to the lower stability of furan ring. However, the impact of biochar on selectivity towards aromatics was less pronounced compared to 4-EGC. Even at 800 °C, the relative content of aromatics only reached 28.88 % with a biochar-reactant mixing ratio of 10:1 for 5-HMF. Comparative investigations on biochar structure before and after co-pyrolysis reactions further revealed that, at 600 °C, biochar maintained its structural integrity, but at 800 °C, it underwent significant changes, including amorphous carbon deposit