Selective glycerol to lactic acid conversion via a tandem effect between platinum and metal oxides with abundant acid groups†

Abstract

Phasing out petrochemical-based thermoplastics with bio-plastics produced in an energy efficient and environmentally friendly way is of paramount interest. Among them, polylactic acid (PLA) is the flagship with its production accounting for 19% of the entire bioplastics industry. Glycerol electrolysis for producing the monomer lactic acid, while co-generating green H₂, represents a promising approach to boost the production of PLA, yet the reaction selectivity has been a bottleneck. Here, we report a combined electrochemical and chemical route using a tandem Pt/C-γ-Al₂O₃ multicomponent catalyst which can achieve a glycerol-to-lactic acid selectivity of 61.3 ± 1.2%, among the highest performance reported so far. Combining an experimental and computational mechanistic analysis, we suggest that tuning the acidic sites on the catalyst surface is crucial for shifting the reaction towards the dehydration pathway, occurring via dihydroxyacetone intermediate. Within the tandem effect, Pt is the active site to electrochemically catalyze glycerol to dihydroxyacetone and glyceraldehyde, while the γ-Al₂O₃ provides the required acidic sites for catalyzing dihydroxyacetone to the pyruvaldehyde intermediate, which will then go through Cannizzaro rearrangement, catalyzed by the OH⁻ ions to form lactic acid. This catalytic synergy improves the selectivity towards lactic acid by nearly two-fold. A selectivity descriptor (ΔG_GLAD* − ΔG_DHA*) from density functional theory calculations was identified, which could be used to screen other materials in further research. Our findings highlight the promise of tandem electrolysis in the development of strategies for selective electrochemical production of high-value commodity chemicals from low value (waste) precursors.