Speciation of potential-dependent fouling on copper foil electrodes during electrochemical hydrogenation and hydrogenolysis of furfural in strong acid

Abstract

The electrochemical transformation of sustainable feedstocks such as biomass derived species can help to electrify the chemical industry and reduce reliance on petroleum. Furfural (FF), one such biomass derived species, can be electrochemically reduced to form furfuryl alcohol (FA) and 2-methylfuran (MF) which are used to produce thermally stable molds and resins, and a fuel candidate or additive, respectively. Using Cu electrodes allows for selective production of FA or MF, dependent on the pH of solution, with MF requiring a low pH. The highly acidic conditions to produce MF also drive homogeneous side reactions and foul the Cu electrode with time. To better understand the nature of the build up on the electrodes, accelerated fouling conditions of high initial concentrations of FF and very low pH were investigated. At concentrations of 200 mM FF in 0.5 M H₂SO₄, we observed fouling of the Cu electrode during electrochemical hydrogenation and hydrogenolysis (ECH) of FF. The fouling of the electrode was also shown to be potential dependent, with poly(furfuryl alcohol) (pFA) forming at −560 mV RHE while an amorphous carbon resembling soot was observed more significantly at −700 mV RHE. The fouling of the Cu electrodes was shown to increase the polarization resistance during FF reduction, hindering the rates of reactions to FA and MF. Through control studies, it was found that the fouling is electrochemically-driven and requires the starting reactant FF to be present. By investigating and understanding the fouling of Cu electrodes during the electrochemical hydrogenation and hydrogenolysis of FF, catalysts can be better designed to inhibit fouling and be used for longer durations or for more cycles before requiring a treatment to regain activity or replacement. Additionally, while this work shows the detriment of carbon fouling on the electrode, pFA is a desirable product. Process intensification through development of a FF to pFA electrochemical step could be achieved through electroreduction with high concentration of FF in 0.5 M H₂SO₄.