Optimization of second‐generation lactic acid from corn stover by alkaline catalysis in subcritical water reaction medium

The optimization of hydrothermal lactic acid (LA) production from corn stover using Ca(OH)2 as catalyst was investigated. Initial studies with a 9 g L−1 glucose solution, and different Ca(OH)2 concentrations, in the range of 0.025–0.1 M, identified 0.075 M as the optimal concentration. Ca(OH)2 acted both as catalyst and reactant, neutralizing LA, and other organic acids produced, primarily acetic and formic acids. Excess Ca(OH)2 was required to maintain the alkaline environment necessary for LA production, achieving 3.5 g LA L−1 with a 40% yield. Lactic acid production from corn stover was studied at temperatures ranging from 90 to 260 °C with 0.075 M Ca(OH)2 and a biomass loading of 5 wt%. Higher temperatures increased LA production reaching 5.9 g LA L−1 with 15.1% of yield at 260 °C. The lower value of the yield compared with that obtained from glucose was attributed to the lower OH−/sugar monomer molar ratio, which was 3 for glucose, but only 0.65 for corn stover. To counteract organic acid formation, Ca(OH)2 concentrations up to 1.5 M were tested. The OH−/sugar monomer molar ratio emerged as a key design parameter, with an optimum value of 2.61, resulting in 12.4 g LA L−1 and 32% yield. This ratio compensates Ca(OH)2 consumption in neutralization reactions and minimizes excess base, which primarily remained in the solid residue owing to the low solubility of Ca(OH)2 in water. This approach also achieves minimal mass intensity, with 5.8 kg of reactants kg−1 of LA, with unnecessary reactant consumption.

This work was supported by the Agencia Estatal de Investigación (grant numbers PID2022-136385OB-I00, TED2021-129311B-I00), the Junta de Castilla y León and the European Regional Development Fund (grant number BU027P23). A.E. Illera's postdoctoral contract was funded by BU027P23. H. Candela's pre-doctoral contract was funded by the Junta de Castilla y León and the European Social Fund (ORDEN EDU/1009/2024). P. Barea's pre-doctoral contract is funded by JCyL and the European Social Fund (ORDEN EDU/1868/2022).