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

Abstract 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.