(212g) Photoelectrochemical Oxidation of Biorenewable Alcohols By Nitroxyl Radical Catalysts

Transforming primary alcohols to acids is a fundamental synthetic process of interest in both the academic and industrial worlds, but it typically requires stoichiometric oxidants which are costly and generate potentially toxic waste. On the other hand, conversion of biomass-derived chemicals in a photoelectrochemical (PEC) cell is one of the most environmentally-friendly chemical production routes available, by taking advantage of renewable carbon sources from the earth, renewable photons from the sun, and renewable electrons from wind, geothermal, or photovoltaic sources.

Stable nitroxyl radicals such as TEMPO are a well-known class of homogeneous oxidation catalysts with widespread use in industrial and laboratory applications.¹ Furthermore, TEMPO is easily oxidized to its active form, an oxoammonium cation, at moderate electrode potentials and is very selective for Câ??OH or C=O oxidations. It was recently reported that the overpotential required to drive TEMPO oxidation can be significantly reduced in a PEC reactor, where energy is harvested from visible light with an earth-abundant bismuth vanadate (BiVO₄) semiconductor electrode.² However, external electrical energy was still required to enhance electron-hole separation and achieve reasonable photocurrents. Additionally, the electron efficiency for TEMPO oxidation was <100% owing to competition with water oxidation. Extensive work has been done to improve the performance of BiVO₄ photoanodes for solar water splitting including nanostructuring, doping, and the addition of an electrocatalyst layer, but optimization to promote TEMPO oxidation has not been explored.

Previously, we have studied selective electrocatalytic oxidation of alcohols and 5-hydroxymethylfurfural (HMF) on supported noble metal catalysts.^3-4 Now, we have investigated electrochemical and PEC systems for efficient TEMPO-mediated oxidation of key biomass-derived chemicals including HMF, glycerol, and ethylene glycol. BiVO₄ modified with electrocatalyst layers facilitated TEMPO oxidation at overpotentials >400 mV lower than unmodified BiVO₄, while significantly suppressing water oxidation. The novel photoelectrocatalytic system achieved nearly 100% efficiency to TEMPO-mediated oxidations and bulk electrolysis gave high yields of organic acid products. In this talk, factors that tune the competition between TEMPO and water oxidation, including photoanode structure, applied potential, and other reaction conditions will be discussed.

References:

(1) Rafiee, M.; Miles, K. C.; Stahl, S. S., JACS (2015), 137 14751â??14757.

(2) Cha, H. G.; Choi, K. S., Nature Chem. (2015), 7 328â??333.

(3) Chadderdon, D. J. et al., Green Chem. (2014), 16 3778â??3786.

(4) Chadderdon, D. J. et al., ACS Catal. (2015), 5 6926â??6936.