Mild electrocatalytic hydrogenation of lactic acid to lactaldehyde and propylene glycol

Reduction of fermentation-derived lactic acid (LA) offers a renewables-based pathway to propylene glycol (PG), a large-scale commodity chemical, currently manufactured by the oxidation of petroleum-derived propene. Complementing our previously described catalytic hydrogenation of LA to PG, we now report electrocatalytic hydrogenation (ECH) of LA in an aqueous electrolyte using constant current electrolysis. A reticulated vitreous carbon (RVC) electrode serves to agglomerate, support, and supply current to a 5% Ru/C powder catalyst, the same catalyst used in the classical hydrogenations. The ECH conditions are mild (ambient pressure, 70 °C vs 1500 psi H2, 150 °C) relative to the chemical hydrogenation. More surprisingly, the major electrohydrogenation product is lactaldehyde (LAL), with small quantities of PG also formed. Variable current studies in the range of 10–100 mA show an increase in product yields and a shift in selectivity toward PG with increasing current. Experiments carried out with different acids as electrolytes reveal a distinct effect of the anion on the yields of the two products. In situ ATR-FTIR studies of the ECH of LA point to a chelating bidentate carboxylate adsorption mode for lactate on the Ru surface and offer insight into the effects of electrolyte anions on surface adsorption and reactivity.