Oligomer-first mechanism in the transformation of biomass derivatives selectively to produce D-lactic acid

Enhancing carbon utilization in biomass conversion makes a big challenge on sustainability. Herein, a novel catalytic system, MgO-D-lactic acid (D-LaA) was adopted, and a new oligomer-first mechanism was discovered for xylose transformation to a mixture very rich in D-LaA in a unique way. Supported by C-13 NMR and DFT calculation, it was discovered that xylose oligomerized to C-10/C-15 through coupling of C-C bond via aldol condensation catalyzed by MgO species. The resultant oligomers were further split into C-3 intermediates via selective C-C cleavage by Mg(OH)(H2O)(3)(+) catalytic species. Besides, MgO-D-LaA catalytic system could also enable the reutilization of C-2 by-product, glycolaldehyde, which was oligomerized to C-4/C-6 and further contributed to LaA production. These two factors synergistically made a significant breakthrough on carbon utilization for hemicellulose valorization, enabling the outstanding LaA yield up to 70.2 C-mol% with 73.1% ee value for D-LaA. The proposed oligomer mechanism comprehensively explained the origin of excess LaA yield beyond 60% via the conventional 3 + 2 pathway. This work made a significant breakthrough for the improvement of carbon utilization in xylose transformation to LaA, and allowed for the sufficient use of carbon atoms in xylose.

Automated summary

This study utilized the MgO-D-lactic acid catalytic system and discovered a new mechanism for the conversion of xylose, leading to enhanced carbon utilization in hemicellulose valorization.