Investigation of Selectivity and Yield in the Hydrothermal Lewis Acid-Catalyzed Conversion of Corn Stover to Lactic Acid

This study optimizes lactic acid (LA) production from corn stover using a Lewis acid catalyst. Initially, an equimolar mixture of Al(III):Sn(II) was used as catalyst at 190 °C with 5 wt. % biomass. Increasing catalyst concentration led to more LA, with optimal results at 16 mM, between 5 and 32 mM. Low catalyst concentration produced mainly dehydration products, furfural, and HMF from corn stover sugars. Higher catalyst concentration increased LA but led to degradation of dehydration products into levulinic and formic acids, reducing LA selectivity. Al(III) was essential for LA formation, while Sn(II) was less effective due to low solubility of Sn(II) species, shown by Sn(II) in the solid residue. 16 mM of Al(III) yielded the highest LA levels, 7.4 g/L and 20.7% yield. Increasing temperature to 210 °C accelerated LA production with the lowest energy consumption at the maximum of LA production (0.47 kWh/g LA), though longer treatments caused LA degradation. AlCl3 has been identified as an ideal catalyst for biomass conversion to LA, being cheap and low in toxicity.