Oxidation of alcohols in photocatalytic hydrogen production: from sacrifice to valorization

Abstract

Photocatalytic hydrogen production, even if it should ideally be performed from pure water (i.e. water splitting), benefits from the presence of an easily oxidized reagent, either inorganic (classically sulfite/sulfide) or organic (classically methanol) that scavenges photoproduced holes and alleviates the process form the kinetically hindered, multi-electron process of water oxidation to molecular oxygen. Even if pioneering works of the photocatalytic reaction between alcohols and water examined the outcome of the oxidation branch of the reaction, the use of these reagents passed through a period in which reporting only hydrogen evolution became common practice, assuming total oxidation and taking the consumption of the organic as a sacrifice for hydrogen production. However, in more recent years, the oxidation outcome of the reaction has regained attention, mainly because of the interest in coupling photocatalysis with biomass utilization. Thus, the valorization of biomass-derived alcohol hole scavengers has become an interesting topic in photocatalysis research. Here, we highlight some recent works on this topic, selecting those that have received more attention in the last 2–5 years: polyol (glycerol, glucose) valorization, transformations of furfuryl alcohol and 5-hydroxymethyl furfural, and C-C coupling reactions starting from alcohols. In our opinion, these represent promising niches for the application of photocatalytic processes.