Current progress on catalytic oxidation of toluene: a review

Volatile organic compounds (VOCs) have boiling points ranging from 50 ℃ to 260 ℃ at room temperature. These compounds are emitted from complex sources, including automobile manufacturing, fine chemicals, the pharmaceutical industry, interior decoration, and the plastics industry. VOCs serve as important precursors of secondary organic aerosols and ozone. However, some VOCs discharged into the atmosphere undergo photochemical reactions, resulting in the formation of photochemical smog. This byproduct is irritating, teratogenic, and carcinogenic, causing considerable harm to human health and posing a serious threat to the environment. Among VOCs, aromatic hydrocarbons originate from a wide range of sources relevant to human life. These compounds have been a constant hot spot in the field of VOC removal due to their unique aromatic ring structure, highly toxic effects, and challenging treatment. VOC removal mainly comprises source reduction, process management, and end–to–end treatment. End–to–end treatment methods involve adsorption, photocatalytic, plasma decomposition, membrane separation, and catalytic oxidation. Catalytic oxidation can effectively convert VOCs into carbon dioxide and water at relatively low temperatures, making it one of the most effective methods for VOC treatment. Efficient catalyst development is the key point for catalytic oxidation methods. This review highlights progress in catalyst development for the catalytic oxidation of toluene, focusing on the characteristics of noble-metal catalysts, such as noble-metal species, particle size, alloying systems, and support properties. While transition-metal oxide catalysts are abundant and inexpensive, they exhibit low catalytic performance for toluene oxidation. Therefore, discussions on transition-metal oxide catalysts include synthesis or calcination temperature, atomic substitution (isovalent and heterovalent substitutions), surface modification (noble- and transition-metal doping), and in situ surface treatment (chemical etching and surface modification). Although studies on toluene degradation using spinel- and perovskite-type oxide catalysts and metal-organic framework-based catalysts are limited, a brief summary of their characteristics during catalytic oxidation of toluene is provided, focusing on the relationship between the catalyst microstructure and performance. Furthermore, the kinetic model for the catalytic oxidation of toluene was briefly evaluated, and the corresponding reaction mechanism and kinetics were analyzed. This review aims to contribute to the development of efficient catalysts for the complete or selective oxidation of toluene.