Synergistic binary acidity in metal sulfates drives efficient alcoholysis of high-solid xylose residues: An experimental and DFT investigation

Xylose residue (XR), a waste product from the xylose industry, is currently primarily used for incineration, animal feed, and cultivation substrates, representing a significant waste of resources. Due to its rich cellulose content, this study demonstrates the valorization of XR into methyl/ethyl levulinate (ML/EL) under high-solid conditions catalyzed by sulfate, through an integrated approach combining experimental and DFT investigations. Under the same optimized conditions (catalyst loading: 6 g/200 mL, 180 °C, 2 h), an increase in the solid content of XR from 15 % to 30 % in methanol, catalyzed by CuSO 4 , enhanced the ML concentration by 58 % (46.73 → 74.06 g/L), though at the expense of a 20 % yield reduction (24.89 → 19.84 wt%). Analogous trends were observed in ethanol systems catalyzed by Al 2 (SO 4 ) 3 under identical conditions (EL: concentration +60 % (26.0 → 41.59 g/L), yield −19 % (22.96 → 18.64 wt%)). DFT simulations revealed that Brønsted-Lewis acid synergy (e.g., Cu(CH 3 OH) 2 + , Al(CH 3 OH) 4 3+ , and Fe(CH 3 OH) 3 3+ ) governed catalytic performance. The Brønsted-to-Lewis acidity hierarchy (Al 3+  > Fe 3+  > Cu 2+ ) correlated with metal electronegativity but was predominantly modulated by ligand electron transfer. Fukui function analysis highlighted distinct electronic behaviors of H + and Cu 2+ , enabling cooperative catalysis that shifted the rate-limiting step from initial dehydration (39.87 and 39.13 kcal/mol) to glucoside-fructoside isomerization (33.87 and 35.87 kcal/mol). These insights underscore the superiority of bifunctional catalysts in reducing energy barriers and optimizing reaction pathways. By integrating process intensification with acid-site engineering, this work establishes a strategy to reconcile yield-concentration trade-offs in high-solid biomass conversion, offering a scalable route for sustainable levulinate production.