Insights into the selectivity to glyceric acid and lactic acid from biomass: Reaction mechanism and temperature effect

Abstract

The disclosure of the underlying reaction mechanism on biomass valorization is of significant importance but remains challenging. We here unveil the complicated reaction mechanism on the coproduction of glyceric and lactic acids from biomass (glucose as representative) in the presence of cheap but robust CaO. Interestingly, it was discovered that the selectivity to glyceric acid and lactic acid from glucose was temperature-tunable. Glyceric acid was preferentially produced at ≤70 °C, while lactic acid generation was favorable at >90 °C. Glyceric and lactic acids could be coproduced with almost 1:1 ratio at moderate reaction temperature (∼80 °C). Isotopic tracing experiment verified that at moderate temperature, glyceric acid and equivalent lactic acid were predominantly originated from glyceraldehyde and dihydroxyacetone intermediates (generated after C3-C4 cleavage), respectively. However, both glyceraldehyde and dihydroxyacetone intermediates were transformed mainly towards glyceric acid at temperature ≤70 °C, but predominantly afforded lactic acid at high temperature. Kinetic studies in combination with DFT calculation verified that glyceric acid production from both glyceraldehyde and dihydroxyacetone intermediates showed lower apparent activation energy than that for lactic acid production. This well explained the selectivity to glyceric and lactic acids via temperature control. The generated alkali in CaO aqueous solution was the major active species for glyceric acid production, while both dissolved Ca(OH)2 and solid CaO contributed to lactic acid formation. The insights into the complicated reaction mechanism will provide useful clues in biomass valorization with efficient carbon utilization.