Low-Temperature Heterogeneous Oxidation Catalysis and Molecular Oxygen Activation

ABSTRACT Many natural oxidations are relevant to the origin and running of life but usually proceeded under mild conditions using molecular oxygen (O2) in the air as the sole oxidant and enzymes as the catalysts. In modern society, catalysis plays an essential role in many industries, such as chemical and pharmaceutical industries. However, most heterogeneous catalytic reactions need high operational reaction temperature and pressure. Research interest is redirected to green catalysis in recent years, e.g., running catalytic reactions under mild conditions, employing green solvents and green oxidants such O2, particularly air. One question always exists: can these industrial catalytic processes be ultimately run similar to the natural oxidation processes in efficiency and operation conditions? For many catalytic oxidation reactions, the greatest challenge lies in activating molecular oxygen under mild conditions. Therefore, a molecular-level understanding of the interactions of O2 molecules with catalysts or substrates is necessary and crucial. In this review, we discuss the activation of O2 to different active species (e.g., O2 2− or O2 2−) and their participation in low-temperature (≤300 oC) catalytic oxidation reactions. The challenges, recent progress, and trends in some low-temperature oxidation reactions are discussed and highlighted. The early studies on the activation of oxygen on various catalysts mainly paid attention to the interaction between the molecular oxygen and the oxygen vacancies of metal oxides. In contrast, recent studies try to fully understand the generation, measurement, and catalytic roles of the various active oxygen species. Therefore, the design of catalysts that can facilely activate O2 at low temperatures is of importance. With such catalysts, it is possible to reduce the high energy consumption, improve the selectivity of catalytic oxidations, and ultimately realize the industrial oxidation reactions at conditions as mild as possible to that of many natural oxidation processes. Finally, from the current body of knowledge, we propose future directions that can effectively utilize O2 for solving practical problems at low temperatures and help understand the oxidation catalysis at the molecular level.