Realising the ambivalent nature of H2O2 in oxidation catalysis – its dual role as an oxidant and a substrate

Abstract

H₂O₂ is a desirable terminal oxidant due to its good atom economy with H₂O being the only by-product when used productively. Its relative stability is advantageous in transport and storage, meaning that catalysts can both activate and direct its oxidising power towards selective oxidation of organic substrates. Wasteful disproportionation of H₂O₂ (into H₂O and O₂) is a well-recognised challenge and receives little, if any, attention in catalyst design. Nevertheless, understanding how H₂O₂ reacts during catalysed oxidations is essential to avoid inefficient use of H₂O₂, and, more importantly, hazardous conditions in which large amounts of O₂ are released by disproportionation. Reaction progress monitoring is an essential component in catalyst development, typically focusing on substrate conversion/product yield. In this frontier article, we advocate for multi-spectroscopic reaction progress monitoring in which all reaction components, including the oxidant and O₂, are tracked over the course of catalysed reactions to establish comprehensive time resolved mass balances. This approach provides insight into the reaction pathways that lead to disproportionation and the species responsible for it. We discuss selected cases to highlight the range of pathways possible and how these impact efforts towards reaction optimisation through catalyst design. In particular, the paradigm that the catalyst responsible for substrate oxidation is a distinct species from that responsible for H₂O₂ disproportionation, e.g., catalyst degradation products, is likely often incorrect. Rather, various pathways are possible, e.g., the same catalyst intermediate engages in both H₂O₂ and substrate oxidation. Various reaction pathways with respect to H₂O₂ consumption are discussed in the case studies. Our conclusion is that it is useful to consider that H₂O₂, in addition to being an oxidant, can compete with the intended organic substrate. This aspect is particularly important in efforts to elucidate reaction mechanisms and when redesigning catalysts rationally to improve performance, especially for use on large reaction scales where safety is paramount.