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.