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

Abstract

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(hydroxy­methyl)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.

Steering Selectivity in Electrocatalytic Furfural Reduction via Electrode–Electrolyte Interface Modification

“…Given that Br – was coadsorbed over Cu, and it was previously reported to enhance HMF hydrogenolysis to DMF, we further conducted catalytic tests using BTA­(NTf 2 ) and HBr as the alternative to BTAB (Figure S23). The catalytic results reveal that Cu–Br – delivers a lower FE for FA (15.8%) and a higher FE for MF (58.0%) compared to the values of Cu (FA: 38.8%, MF: 42.3%), suggesting that Br – may also impede the FF-to-FA process.…”

“…Since the RDS of the overall process in our work involves a PCET process, it is possible that the addition of the last two hydrogens to FurCH* proceeds via either a PCET or HAT mechanism if they are not the RDS. In addition, there is still a debate on whether the addition of the first hydrogen atom to the oxygen , or carbon in FF’s carbonyl group. Seeking evidence from in situ spectroscopy to understand the reaction pathway should be the focus of future work.…”

“…This is not unreasonable, since electrocatalytic FF-to-FA and FF-to-MF conversions follow distinct pathways, unlike in thermocatalytic hydrogenolysis where FA is an intermediate to MF. , Regarding electrocatalytic FF-to-FA conversion (Figure b, left panel), it requires hydrogenation of the aldehyde group in FF mostly via a hydrogen atom transfer (HAT) process using adsorbed hydrogen atoms (H*), or a 2H + /2e – -involved proton-coupled electron transfer (PCET) process . While for electrocatalytic FF-to-MF conversion (Figure b, right panel), it necessitates hydrogenolysis of the aldehyde group in FF predominantly via a 4H + /4e – -involved PCET process. , The reactivity of the HAT process is influenced by the H* coverage on the electrode surface, which is facilitated by a decrease of electrolyte’s pH (which corresponds to the higher concentration of hydronium ions) and also affected by catalyst’s activity [which determines the reduction reactivity of H + (or H 2 O) to H*]. , In contrast, a PCET process was reported to be proportional to the concentration of hydronium ions and thus is favorable in a strong acid solution. , Hence, FF-to-FA conversion via the HAT process is preferentially conducted in a nonacidic solution, in a way to suppress FF-to-MF conversion (that is, via the PCET process), while FF-to-MF conversion is prone to be performed in acidic solution. , …”

“…26,27 In contrast, a PCET process was reported to be proportional to the concentration of hydronium ions and thus is favorable in a strong acid solution. 25,28 Hence, FF-to-FA conversion via the HAT process is preferentially conducted in a nonacidic solution, in a way to suppress FF-to-MF conversion (that is, via the PCET process), while FF-to-MF conversion is prone to be performed in acidic solution. 15,16 As a result, it remains difficult to steer FA and MF selectivities under the same reaction conditions.…”

Unraveling Two Pathways for Electrocatalytic Acetonitrile Reduction

“…Through organothiol modification, Chadderdon et al found two competing mechanisms for the reduction of furfural on Cu, where the hydrogenation and hydrogenolysis are produced by ECH, and the coupling product is generated by DER . Choi et al found that halide adsorption promotes electrochemical hydrogenolysis of furfural on Cu, supporting that hydrogenation is only created by ECH, while hydrogenolysis occurs mainly by DER . In contrast, Zou et al prefer hydrogenation on Cu as an ECH process and hydrogenolysis as a mixed DER/ECH process .…”