One-pot lactic acid production using subcritical water as a sustainable approach in the valorization of corn stover

Abstract

Corn stover is an agricultural waste obtained after the corn grain harvest, formed by the stalks, leaves and cobs of the plant. Its production worldwide accounts with more than 1 billion tons per year, and it is usually considered as feed for livestock. However, due to its interesting composition of cellulose and hemicellulose fractions, corn stover can be considered as a valuable biomass for the production of biofuels or value-added chemicals. Among the different possibilities, the production of lactic acid was chosen due to its broad application in the food and chemical industries, as a food preservative or flavoring, or as a precursor of polylactic acid, respectively. In this regard, a one-pot subcritical water treatment was proposed for the production of lactic acid from corn stover. Under subcritical conditions, water ionic product increases to a great extent, acquiring Brønsted acid properties, which combined with the Lewis acidity provided by the lanthanide Yb3+, makes a synergistic effect and favors the lactic acid production route from the pentoses and hexoses present in the biomass. The effect of temperature was tested from 180 to 260 °C. It was determined that a minimum temperature of 220 °C was required to successfully hydrolyze the cellulose and hemicellulose fractions into convertible monosaccharides. This temperature was selected as the optimum one, which yielded a 22 % lactic acid yield after just one hour, and showed a low HMF production, showing the high selectivity of the process at this temperature. One-pot subcritical water treatment of corn stover proved to be a sustainable solution for the valorization of the agricultural waste.