Radical Redox Mediator Free Photoelectrochemical Biomass Valorization

Abstract

Over the past decade, researchers have begun to re-explore the idea of replacing water oxidation with more kinetically facile oxidation reactions for photoelectrochemical solar H2 production.1–3 Alternate photo-oxidation reactions can be employed as a means of chemical valorization, in addition to providing electrons for H2 production from water while reducing the losses associated with water oxidation. In particular, the oxidation of 5-hydroxymethylfurfural (HMF)—which is a key intermediate derived from the acid catalyzed poly-dehydration of 6 carbon monosaccharides—has gained significant attention. This is because a number of its oxidation products, such as 2,5-Furandicarboxylic acid (FDCA), may be employed as monomers in the synthesis of renewable polyesters, and FDCA in particular, has also been identified by the US Department of Energy as one of 12 priority chemicals for establishing the green chemistry industry of the future.4 As a result, solar powered chemical transformations of HMF have recently gained attention. However, to date, the selective oxidation of HMF using either photo-, or electrochemical means, has been limited by the need to employ the radical redox mediator (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO). This additional redox-mediating reagent poses a problem by increasing system complexity, and by behaving as a competitive light absorber in photoelectrochemical (PEC) systems —which reduces the solar conversion efficiency. Recently, our group has investigated TEMPO free, direct PEC oxidation of HMF using metal oxide photoelectrodes. In this presentation, we show that the direct oxidation of HMF can be carried out on sol-gel processed WO3 electrodes without the need to use an additional redox shuttle such as TEMPO. Figure 1 depicts a linear sweep voltammogram under 1 sun illumination of a sol-gel processed WO3 electrode in the presence and absence of HMF. We observe that the addition of HMF to an aqueous buffered solution improves photocurrent densities by 33%, and the onset potential is favorably shifted by -50 mV. These properties highlight this reaction’s promise as a replacement for water oxidation in an H2 producing PEC cell. In addition, we report the oxidation product distribution as well as efforts to control product selectivity via solution processed heterogeneous catalysts and the optimization of the electrolyte solution. Figure 1: Linear sweep voltammograms of a WO3 electrode in a pH 4 0.1M KPi buffered solution with (red), and without (black), 5mM HMF in solution. The illumination intensity was 1 sun AM1.5G. Dotted lines represent the linear sweep voltammograms in the absence of illumination.