Towards understanding the chemical reactions between KOH and oxygen-containing groups during KOH-catalyzed pyrolysis of biomass

In this study, the effect of KOH/biomass ratios and pyrolysis temperatures on products distribution, the evolution of oxygen-containing groups in pyrolytic tri-state products and the possible chemical reactions between oxygen-containing groups and KOH were investigated. The results indicated that KOH can react with oxygen-containing groups in biomass to facilitate gas generation at lower ratios (≤1:2) or higher temperature (∼800 °C). When at higher ratios (>1:2) and 600 °C, KOH has an inhibitory effect on gas production. With a significant decreased in the oxygen-containing species and acids, the phenols and hydrocarbons became the main species in bio-oil. Moreover, KOH facilitates methoxyphenol conversion and has high selectivity for alkyl phenol generation. For biochar, at lower ratios (<1:2), K can attack the oxygen sites of phenolic hydroxyl groups, forming dangling bonds and promoting the formation of aromatic rings with anhydride groups. However, at higher ratios (>1:2), the oxygen-containing groups on the surface of biochar gradually changed from anhydride groups to quinone groups, and finally form a biochar with large aromatic ring and rich in quinone groups. As analyzed above, a possible chemical reaction between KOH and oxygen-containing groups during KOH-catalyzed pyrolysis of biomass was proposed.