Halide Adsorption Enhances Electrochemical Hydrogenolysis of 5-Hydroxymethylfurfural by Suppressing Hydrogenation

Reductive upgrading of 5-hydroxymethylfurfural (HMF), a biomass-derived platform molecule, to 2,5-dimethylfuran (DMF), a biofuel with an energy density 40% greater than that of ethanol, involves hydrogenolysis of both the aldehyde (C=O) and the alcohol (C-OH) groups of HMF. It is known that when hydrogenation of the aldehyde occurs to form 2,5-bis(hydroxy-methyl)furan (BHMF), BHMF cannot be further reduced to DMF. Thus, aldehyde hydrogenation must be suppressed to increase the selectivity for DMF production. Previously, it was shown that on a Cu electrode hydrogenolysis occurs mainly through proton-coupled electron transfer (PCET), where a proton from the solution and an electron from the electrode are transferred to the organic species. In contrast, hydrogenation occurs not only through PCET but also through hydrogen atom transfer (HAT), where a surface-adsorbed hydrogen atom (H*) is transferred to the organic species. This study shows that halide adsorption on Cu can effectively suppress HAT by decreasing the steady-state H* coverage on Cu during HMF reduction. As HAT enables only aldehyde hydrogenation, a striking suppression of BHMF is observed, thereby enhancing DMF production. We discuss how the identity and concentration of the halide, along with the reduction conditions (i.e., potential and pH), affect halide adsorption on Cu and identify when optimal halide coverages are achieved to maximize DMF selectivity. Our experimental results are presented alongside computational results that elucidate how halide adsorption affects the adsorption energy of hydrogen and the steady-state H* coverage on Cu, which provide an atomic-level understanding of all experimentally observed effects.

Automated summary

This study demonstrates that halide adsorption on Cu electrode can effectively suppress the hydrogen atom transfer (HAT) reaction during the reductive upgrading of 5-hydroxymethylfurfural (HMF), leading to enhanced production of 2,5-dimethylfuran (DMF). The identity and concentration of the halide, as well as the reduction conditions, were found to affect the halide adsorption on Cu, and optimal halide coverages were identified to maximize the selectivity for DMF. Experimental and computational results provide a detailed understanding of the effects of halide adsorption at the atomic level.