Electrocatalysis for selective oxidation of biomass-derived compounds
Producing chemicals from biomass feedstocks is a potential route for reducing our dependence on unsustainable petroleum resources. However, unlike petrochemicals, biomass-derived chemicals are highly oxygenated, and there is a need to develop new processes to efficiently upgrade them to useful chemicals and products. One approach is to perform the desired transformations using electrochemistry. Electrochemical reactions are usually conducted at mild temperatures, which is advantageous for the conversion of thermally-unstable bioderived compounds. Additionally, electroanalytical methods can reveal mechanistic information about the complex processes occurring at electrode/electrolyte interfaces. Electrochemical cells are uniquely capable of integrating sustainable chemical production and renewable energy conversion. Alcohol-fed fuel cells can convert chemical energy to electricity, and may also generate value-added chemical products. Electrolytic cells can utilize renewable energy directly by employing sunlight-absorbing semiconductor photoelectrodes, or indirectly by using renewable electricity to drive non-spontaneous reactions. Electrochemistry may play key roles in our future chemical and energy landscapes, yet fundamental and applied research is needed to develop these technologies.
In this work, the electrochemical oxidation of biomass-derived compounds to valuable chemicals was explored. Specifically, this research focused on overcoming challenges related to catalyst activity and product selectivity for the electrocatalytic oxidation of bioderived polyols and polyfunctional molecules. Selective oxidation of vicinal diols under electrochemical conditions was studied using 1,2-propanediol (PDO) as a model compound. Potential-dependent pathways and selectivity trends were elucidated for PDO oxidation with carbon-supported Pt and Au nanoparticle electrocatalysts in anion-exchange membrane fuel cells. Electrochemical conversion of a polyfunctional substrate, namely 5-hydroxymethylfurfural (HMF), was studied in electrolytic flow cells. HMF was selectively oxidized to 2,5-furandicarboxylic acid (FDCA), an important precursor for biobased polymers, using carbon-supported palladium and gold bimetallic nanoparticle electrocatalysts. Tuning anode potential and catalyst composition were critical for achieving high selectivity to FDCA. Efficient photoelectrochemical oxidation of HMF to FDCA was demonstrated using a homogeneous electrocatalyst, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), together with a heterostructured photoanode. Modifying bismuth vanadate semiconductor films with electrodeposited cobalt phosphate simultaneously enhanced TEMPO oxidation photocurrent and suppressed the undesired oxygen evolution reaction.