Selective Oxidation of 1,2-Propanediol in Alkaline Anion-Exchange Membrane Electrocatalytic Flow Reactors: Experimental and DFT Investigations

Electrocatalytic oxidation of polyhydric alcohols represents an important route for coproduction of biorenewable chemicals and energy. However, the governing factors leading to high product selectivity remain unclear. Herein, we investigate the selective oxidation of 1,2-propanediol (PDO) to pyruvate or lactate in electrocatalytic reactors over carbon-supported platinum (Pt/C) and gold (Au/C) anode catalysts. PDO-fed alkaline anion-exchange membrane fuel cells successfully cogenerated electricity and valuable chemicals with peak power densities of 46.3 mW cm(-2) on Pt/C and 10.0 mW cm(-2) on Au/C. Pt/C was highly selective for primary alcohol group oxidation to lactate (86.8%) under fuel cell conditions, but Au/C yielded significant amounts of pyruvate, a product that has previously eluded heterogeneous catalytic studies on Au. Sequential oxidation of lactate to pyruvate was not observed on Au/C but did occur slowly on Pt/C. The electrode potential dependent product distribution was investigated, and it was revealed that pyruvate selectivity on Au/C was sensitive to anode potential and could be varied from 20 to 56%. On the basis of observed product distributions and linear sweep voltammetry of intermediate products, we propose that the intermediates hydroxyacetone and pyruvaldehyde, which are not stable in high pH electrolyte, can be further oxidized to pyruvate on Au/C only if they are trapped within the thick liquid diffusion layer of the carbon cloth supported catalyst layer. Density functional theory (DFT) calculations of reaction energies identified the most favorable reaction intermediates and provided insight into the likely reaction pathways.