Electro-oxidation of furfural on gold is limited by furoate self-assembly

Highlights

• Electro-oxidation of furfural on Au/C in acidic electrolyte efficiently produces furoic acid.

• Initial furoic acid production is an order of magnitude faster on Au/C than previously seen on Pt/C.

• Kinetic studies, ATR-SEIRAS, and DFT show self-inhibition by assembly furoate product.

Abstract

Processing of biomass-derived compounds with electrocatalysis has shown promise to directly couple the production of valuable feedstocks with the storage of renewably produced electricity. One potential route of electrocatalytic conversion is the partial oxidation of furfural to furoic acid (FA), a precursor to 2,5-furandicarboxylic acid (FDCA). We have utilized differential electrochemical reactor studies along with infrared spectroscopy (ATR-SEIRAS) experiments and density functional theory (DFT) calculations to probe the oxidative reaction pathways of furfural on gold catalysts in acidic electrolyte. We find furfural electro-oxidation activity (~2 µA/cm²_Au at 1.0 V_RHE) to be an order of magnitude higher than that observed on Pt/C. 96 ± 6% Faradaic efficiency to FA is achieved at 0.8 V_RHE. Product desorption is rate limiting, and spectroscopic evidence indicates that the most abundant intermediate is surface furoate. Deeper oxidation products observed with dilution of furfural suggest that self-assembly of the furoate species contributes to selectivity.

Introduction

Furoic acid (FA) has recently gained attention as an alternative biomass-derived precursor to 2,5-furan-dicarboxylic acid (FDCA), a promising bio-renewable monomer produced mainly from 5-(hydroxymethyl)furfural (HMF) [1], [2], [3], [4], [5]. FA, which is commonly used as a preservative, fungicide, and pharmaceutical precursor, can be derived from furfural, a product of the acid-catalyzed hydrolysis of hemicellulose sources such as sugar cane [6]. The conventional methods for FA production require either the addition of a strong base (e.g. NaOH) to disproportionate furfural into both FA and furfuryl alcohol (Cannizarro reaction) [7] or the addition of a chemical oxidizer (e.g. O₂, H₂O₂), typically in the presence of a catalyst and at elevated temperature and pressure [8], [9], [10], [11]. Electrochemical methods offer a unique opportunity to convert furfural directly to FA without the need to alkalize the acidic aqueous feedstock, provide an oxidizing agent, or perform costly extra separations (Scheme 1) [12]. Electrocatalytic oxidation processes typically operate at lower reaction temperatures and pressures as they are activated via an applied potential—ideally utilizing renewable electricity resources—and are paired with reduction processes that can co-produce H₂ [13], [14], [15], [16] and/or fuels [17], [18], [19], [20]. Combined with methods of FA carboxylation (a supplemental means of sequestering CO₂), the resulting FDCA monomer has the potential to be carbon neutral [1]. In recent work, we illustrated that the electrocatalytic partial oxidation of furfural to FA in acidic electrolyte is achievable on Pt/C, albeit with moderate selectivity to FA below 1.0 V_RHE and low overall activity (<1 µA/cm²_Pt) [21], [22].

Noble metal catalysts generally serve as stable anodes under acidic conditions. Thus, studying partial oxidations on noble metals serves as a beneficial starting point to develop fundamental mechanistic understanding. Furan oxidation catalyzed by supported Au and Au-alloys has been found to be selective and active towards carboxylic acid (FA and FDCA) production, either using O₂ as the oxidant [9], [23], [24], [25] or via electrochemical oxidation in basic electrolytes [26], [27]. The present study investigates the activity and Faradaic efficiency of the electro-oxidation of furfural to FA on Au in acid electrolyte, which is most relevant to real feedstocks generated by biomass hydrolysis or pyrolysis. The side products 5-hydroxy-furan-2(5H)-one (HFN) and maleic acid (MA) were found to be minor products at high overpotential, consistent with previous electrochemical studies on other surfaces in acidic electrolyte (Scheme 1) [28], [29]. DFT calculations and experiments using ATR-SEIRAS suggest that FA readily oxidizes to a surface bound furoate species, the desorption of which is found to be rate-limiting and potential-dependent. A proposed mechanism of catalyst deactivation due to the buildup of furoate is discussed.