Electrocatalysis poses a number of possible advantages for the valorization of biomass-derived feedstocks, most critically its amenability to direct conversion in acidic aqueous media. Furanic biomass derivatives, such as furfural, are substrates with a number of value-added chemical outlets via partial oxidation, most notably furoic acid (FA), a potential precursor to 2,5-furandicarboxylic acid (FDCA). Pairing such partial oxidations with H2 evolution or other reduction reactions (e.g. CO2) in an electrochemical cell presents an opportunity to perform electrolysis at lowered voltages, while coproducing products that are more valuable than O2. Here, we have utilized differential reactor studies with online electrochemical mass spectrometry (OLEMS), as well as in situ infrared spectroscopy attenuated total reflectance-surface-enhanced infrared reflection-absorption spectroscopy (ATR-SEIRAS), to probe the oxidative reaction pathways of furfural on platinum catalysts in acidic electrolyte. We find furfural electro-oxidation selectivity to depend on potential, with the largest shifts corresponding to the transition of Pt to Pt-oxide. Below 1.2 VRHE, FA and 5-hydroxyfuroic acid are the primary products. At higher potential, selectivity shifts predominantly toward 5-hydroxy-furan-2­(5H)-one (HFN), with the appearance of maleic acid (MA) as well. ATR-SEIRAS and OLEMS indicate that decomposition and overoxidation to CO2 occurs via decarbonylated or decarboxylated ring intermediates, while MA is less easily activated toward further oxidation once it is formed. Significant oxidative currents are only achieved at potentials where the surface is cleared of CO, which is derived from spontaneous decarbonylation of furfural on the metallic Pt surface. As potential is increased, selectivity to C4 and C5 oxygenates over CO2 is then promoted by a high steady-state surface coverage of organic intermediates that inhibit rapid adsorption and addition of oxygen from the discharge of water. Based on these findings, we propose a reaction pathway and directions for the design of more active and selective electrocatalysts.

The drive to reduce consumption of fossil resources, coupled with expanding capacity for renewable electricity, invites the exploration of new routes to utilize this energy for the sustainable production of fuels, chemicals, and materials. Biomass represents a possible source of platform precursors for such commodities due to its inherent ability to fix CO 2 in the form of multi-carbon organic molecules. Electrochemical methods for the valorization of biomass are thus intriguing, but there is a need to objectively evaluate this field and define the opportunity space by identifying pathways suited to electrochemistry. In this contribution we offer a comprehensive, critical review of recent advances in lowtemperature (liquid phase), electrochemical reduction and oxidation of biomass-derived intermediates (polyols, furans, carboxylic acids, amino acids, and lignin), with emphasis on identifying the state-of-the-art for each documented reaction. Progress in computational modeling is also reviewed. We further suggest a number of possible reactions that have not yet been explored but which are expected to proceed based on established routes to transform specific functional groups. We conclude with a critical discussion of technological challenges for scale-up, fundamental research needs, process intensification opportunities (e.g., by pairing compatible oxidations and reductions), and new benchmarking standards that will be necessary to accelerate progress toward application in this still-nascent field.

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The electrochemical oxidation of furfural and down-pathway intermediate and product species (furoic acid, 2­(5H)-furanone, 5-hydroxy-2­(5H)-furanone, and maleic acid) is investigated on platinum electrodes via attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) with modulation excitation spectroscopy (MES). Density functional theory (DFT) calculations are performed to further illuminate the elementary reaction mechanism and the surface orientations of key reaction intermediates. Evidence from these techniques suggests that oxidation selectivity of the furanic compounds is governed by a balance between several properties of the catalyst. Propensity toward C–C cleavage steps yields self-limiting accumulation of adsorbed CO at low potentials and promotes the formation of C4 products and CO2 at higher potentials. Affinity for surface carboxylates such as furoate and maleate species also leads to their accumulation on the surface at higher potentials. It is thus suggested that correlations between the various surface oxygenate binding energies represent a challenge in balancing accumulation of strongly bound intermediates against a need for the surface to bind hydroxyl to initiate O-addition steps.

Electrocatalysis poses many potential advantages for distributed scale biomass valorization. Furanic biomass derivatives such as furfural and 5-hydroxymethylfurfural (HMF) can provide access to a number of value-added chemicals by partial oxidation, including furoic acid, maleic acid, and 2,5-furandicarboxylic acid (FDCA). Here, we have utilized differential reactor studies with online electrochemical mass spectrometry (OLEMS), as well as in situ attenuated total reflectance-surface-enhanced infrared reflection-absorption spectroscopy (ATR-SEIRAS), to probe these reaction pathways on various metal electrodes. Experimental insights are combined with computational results to piece together plausible mechanisms. In particular we find tradeoffs between decarbonylation activity on Pt-group electrodes and and self-assembly of furoate intermediates on noble metals to be critical selectivity and rate determining steps. Strategies for improved catalyst design will be discussed.