Sustainable Electrochemistry: From Lignin Processing to Paired Electrolysis Reactions

Abstract

When evaluating the sustainability of an electrochemical synthesis, the source and efficient use of electricity, the generation of waste byproducts, and the source of chemicals used should be considered in the development of a greener process. Constant current electrochemical oxidation reactions increase functionality of a molecule but are limited in scope based on oxidation potential. This shortcoming disappears when using a chemical oxidant, which can select based on steric hindrance, chirality, chelation control, and other factors. However, a major pitfall of any stoichiometric chemical oxidation reaction is the generation of a stoichiometric reduction byproduct, oftentimes a metal waste product. To circumvent this problem, chemical oxidants can be recycled directly at the anode while generating only H­2 gas at the cathode and can be powered by a simple photovoltaic power supply.1,2 From a green chemistry standpoint, the substrates used in anodic oxidations should have their origins from renewable feedstocks. Lignin, a biopolymer found in wood, has been disassembled into small monomers and converted into substrates for synthesis of heterocyles and quinones.3 Ideally, to maximize efficiency of these and other electrochemical processes, one wants to perform a useful oxidation at the anode simultaneously with a useful reduction at the cathode. Using this idea of a paired electrolysis, electrochemical oxidation of lignin-derived substrates has been coupled with CO2 reduction. In this presentation, the recycling of a variety of chemical oxidants at an electrode using simple photovoltaics as the source of electricity will be discussed. Furthermore, breakdown of lignocellulosic biomass into chemical building blocks for use in paired electrolyses will be explored (Figure 1).