The electrochemical oxidation of biomass platform 5-hydroxymethyl furfural (HMF) to 2,5-furandicarboxylic acid (FDCA), is an important reaction in the emerging area of renewably powered biomass valorization. To this end, the efficiency of catalytic systems still needs to be improved to render these systems economically competitive. In the electrochemical HMF oxidation reaction (HMFOR), the utilization of well-defined active sites is critical for fundamental insights into their function and subsequently rationally turning them to maximize catalytic activity. However, heterogeneous catalysis of HMF conversation often feature poorly defined catalytic surfaces, hampering the community's complete understanding of the reaction mechanism. Herein, we report the use of reticular chemistry to design a conductive metal-organic framework-based electrocatalytic model system with well-defined M-O4 active sties for HMFOR to better understand its mechanism. The activity of MOFs bearing Ni-O4 (Ni-CAT) and Co-O4 (Co-CAT) active sites was analyzed with electrochemical and operando spectroscopic techniques to elucidate the reaction mechanism occurring on the surface. Electrochemical experiments reveal that Co-CAT has an earlier onset potential for enabling HMFOR, relative to other known catalysts thus far, while the Ni-CAT shows faster kinetics for the conversion of HMF to FDCA. Also, Ni-CAT exhibits efficient charge transfer properties and intrinsically high kinetic towards HMFOR. It achieved FDCA yields of 98.7% with 86.8% faradic efficiency. Infrared spectroscopy captures the surface-bound aldehyde group as the key intermediate in the catalytic cycle, which forms once M(II\III) oxidation occurs. This work illustrates the advantage of utilizing molecularly defined active sites coupled with operando spectroscopy to provide fundamental insights into a variety of electrosynthetic reactions and paves the way for future catalyst design.

Conductive metal-organic frameworks with molecularly defined M-O4 active sites as highly active biomass valorization electrocatalysts

ACS Fall 2021

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