Effect of intrinsic alkali (earth) metals on hydrothermal conversion of lignocellulosic biomass

This study investigates the effects of intrinsic alkali and alkaline earth metals (AAEMs) (K⁺ and Ca²⁺) on the reactivity of lignocellulosic biomass during hydrothermal conversion at 290 °C for 1 h, using the three primary lignocellulosic biomass components (cellulose, xylan, and lignin), as well as synthetic biomass prepared by mixing these three components. Results indicate that K⁺ and Ca²⁺ exhibited distinct effects on product distribution and reaction pathways. In the mixed cellulose-hemicellulose-lignin system, K⁺ and Ca²⁺ mainly regulated product distribution without markedly altering the overall conversion (66.42%) while reducing bio-oil yield from 16.96% to 13.02% (K⁺) and 10.29% (Ca²⁺). Specifically, K⁺ provided optimal bio-oil quality, as its aqueous mobility promoted bond cleavage and ketonization, thereby enhancing fuel properties. Ca²⁺ promoted dehydration-condensation reactions that led to the formation of thermally stable solid residue (SR). The observed differences between K⁺ and Ca²⁺ arose from their distinct charge density and Lewis acidity. K⁺, with weak coordination ability, favored bond cleavage and bio-oil upgrading, whereas Ca²⁺, with strong Lewis acidity, induced dehydration-condensation and cross-linking to form stable SRs.