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

TL;DR

This paper explores how hydrogen peroxide (H2O2) can both act as an oxidant and a substrate in catalytic reactions, emphasizing the need to monitor its behavior to improve catalyst design and safety.

Contribution

The paper introduces a new approach to catalyst development by advocating for multi-spectroscopic monitoring of H2O2 and reaction byproducts.

Findings

H2O2 can act as both an oxidant and a substrate, competing with organic substrates in catalytic reactions.

Tracking all reaction components, including H2O2 and O2, is essential for understanding reaction pathways and improving catalyst design.

The same catalyst intermediate can be involved in both H2O2 oxidation and substrate oxidation, challenging previous assumptions.

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.

Activation of catalysts by H2O2 generates the intermediate species that react with the desired target organic functional group. The oxidation of H2O2 itself is often overlooked despite it being central to efforts to optimise catalysts by design.

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