Electrochemical transformation of biomass-derived furanic compounds for valuable chemical production and innovative process development

Abstract

It is critical to combat global warming and stabilize the global temperature to a manageable level of +1.5C by curbing carbon dioxide (CO2) emissions, eventually achieving zero net emissions by 2050. This target requires significant renewable and clean energy deployment in all industry sectors. Biomass represents a potential source of platform precursors for commodities due to its inherent capability to fix CO2 in the form of organic compounds with multi-carbons. Electrochemical conversions offer a sustainable way to turn these renewable carbon chemicals into valuable products. Powered by renewable sources of electricity (from wind or solar), these products can be made with zero carbon footprints, thus impacting future distributed manufacturing of green chemicals. In my Ph.D. studies, I mainly focus on the electrochemical transformation of biomass-derived compounds. I offered a comprehensive study of low temperature and liquid phase electrochemical reduction and oxidation of biomass-derived furanic compounds [e.g., furfural and 5-hydroxymethylfurfural (HMF)], with an emphasis on understanding electrochemical mechanisms, identifying reaction pathways, designing efficient electrocatalysts, and developing cost-effective processes. Electrocatalytic hydrogenation (ECH) provides a sustainable and environmentally friendly approach to producing chemicals and biofuels by operating at mild conditions with “clean” electrons as the reducing agents, instead of dealing with fossil-derived H2 at elevated temperature/pressure. Through tailoring the local environments, including H/D composition and local H3O+ and H2O content, I have studied the ECH of furfural on Pb electrodes in acid conditions and elucidated the detailed pathways toward three key products: furfuryl alcohol (FA), 2-methylfuran (MF), and hydrofuroin. Various strategies, including isotope labeling, electrokinetics, electrode/electrolyte interface modification, and DFT calculations were implemented for the mechanistic understanding of reductive pathways. The formation of FA is highly relied on the local H2O content and follows a Langmuir-Hinshelwood mechanism. The furfural-to-MF path depends on the local H3O+ concentration and follows an Eley-Rideal route. Hydrofuroin is favorably produced through the self-coupling of ketyl radicals in the electrolyte, which are formed from outer-sphere, single-electron transfers. Furthermore, the ECH can be coupled with an electrocatalytic oxidation (ECO) reaction to develop paired electrolyzers. This system can realize the co-generation of valuable chemicals on both cathode and anode. More importantly, the substitution of thermodynamically unfavorable and kinetically sluggish oxygen evolution reaction (OER) not only enables the production of valuable chemicals from the anode, but also results in a remarkable decrease in the cell voltage, leading to a sudden drop in energy consumption. I first demonstrated divergent paired electrolysis of HMF, and have achieved a combined FE of >160% in the simultaneous generation of cathodic 2,5-bis(hydroxymethyl)furan (BHMF) and anodic 2,5-furandicarboxylic acid (FDCA) with long-term operational stability (24 h). TEMPO-mediated (on carbon cloth) and TEMPO-free (on NiFeOx) systems with different membranes for ECO of HMF were demonstrated and compared. Moreover, I have developed electrolyzers with different configurations: three-compartment flow cells with reference electrodes, membrane electrode assembly (MEA)-based flow cells with zero-gap configurations, and electrocatalytic-thermocatalytic tandem reactors with multiple reactions simultaneously occurred. All those cell configurations have their respective advantages for paired electrolysis. Although the successful demonstration of paired electrolysis systems, the cell voltages of > 1.0 V and current densities <100 mA cm−2 are still far from practical applications. Then, I considered using a unique electrocatalytic dehydrogenation (EOD) reaction to replace OER. This EOD can produce H2 and valuable 2-fuoric acid on Cu-based anodes at an ultra-low onset potential of ~0.1 VRHE. Experimental and computational studies further investigated the EOD kinetics on Cu surfaces and differentiated its pathway with the conventional ECO and the non-Faradaic, solution-phase Cannizzaro pathways. Besides, the rational design of Cu-based bimetal catalysts through a galvanic replacement method not only significantly increased EOD activity, but also improved catalysts durability. Moreover, I developed an EOD-hydrogen evolution reaction (HER) paired system for bipolar H2 production. It enables a high-current-density and ultra-low cell voltage H2 evolution from both cathode and anode in the zero-gap flow cells. After developing porous catalysts and optimizing systems, I have achieved a maximum industrial-relevant current density of 300 mA cm−2 at the cell voltage of <0.5 V and a doubled H2 production rate with its anodic and cathodic faradaic efficiency (FE) of ~100%. These cell voltages are much lower than that of conventional HER-ECO (>0.9 V, on Ni-based catalysts) and HER-OER (>1.23 V) paired systems.