Pd-M Bimetallic Electrocatalysts for Selective Oxidation of Multi-Functional Biorenewable Molecule 5-Hydroxymethylfurfural (HMF)

Conversion of biomass-derived compounds into renewable chemicals is a critical step towards our sustainable future. 5-hydroxymethylfurfural (HMF) is considered an important biorenewable compound as it readily synthesized from cellulose¹ and sugars² and can be transformed into building-blocks for fine chemicals, such as its oxidation product 2,5-furandicarboxylic acid (FDCA), a potential precursor for polymer manufacturing.³ An efficient catalyst for converting HMF to FDCA must be able to oxidize the alcohol and aldehyde functional groups of HMF without affecting the furan ring structure. Therefore, HMF serves as a model molecule for studying the catalytic processing of multi-functional molecules typically derived from biomass. Significant progress has been made in the selective oxidation of HMF in traditional heterogeneous catalytic systems, which often require external oxygen at elevated pressure and temperature. A promising alternative route is to use electrocatalytic systems, where electrochemical potential is the driving force of oxidation and reactions can be performed at ambient conditions.⁴ Also, electrochemical systems uniquely give rise to fuel cells, which can simultaneously produce chemical products and renewable electricity.

We explored the electrocatalytic oxidation of HMF in alkaline media over carbon supported bimetallic nanoparticle catalysts. Combined bulk electrolysis in flow reactors and three-electrode cyclic voltammetry studies demonstrated the synergistic effects of alloying in Pd–M catalysts for the selective formation of FDCA. Results from electrolysis product analysis at various electrode potentials revealed the catalyst effect on the competitive oxidation of the alcohol and aldehyde functional groups of HMF. Aldehyde oxidation was very facile on Au/C catalyst, which resulted in high selectivity to 5-hydroxymethyl-2-furan-carboxylic acid (HFCA) at low potentials, and high electrode potentials were required to further oxidize the alcohol group to form FDCA. HMF oxidation on Pd/C followed two competitive routes to FDCA and the pathway was dependent on electrode potential. Oxidation of aldehyde groups occurred much slower on Pd/C than on Au/C at low potentials, but was greatly enhanced at increased potentials or by alloying with Au. Pd–M bimetallic catalysts yielded FDCA at lower potentials than monometallic catalysts and the product distribution was dependent on the electrode potential and the composition of the metal surface. Bimetallic catalysts take advantage of both single components with efficient alcohol and aldehyde group oxidation, resulting in enhanced HMF conversion rate and selectivity to the desired di-acid product.

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