Impact of the Oxygen Vacancies on Copper Electronic State and Activity of Cu‐Based Catalysts in the Hydrogenation of Methyl Acetate to Ethanol

Hydrogenation of Carboxylic Acids, Esters, and Related Compounds over Heterogeneous Catalysts: A Step toward Sustainable and Carbon-Neutral Processes

The catalytic hydrogenation of esters and carboxylic acids represents a fundamental and important class of organic transformations, which is widely applied in energy, environmental, agricultural, and pharmaceutical industries. Due to the low reactivity of the carbonyl group in carboxylic acids and esters, this type of reaction is, however, rather challenging. Hence, specifically active catalysts are required to achieve a satisfactory yield. Nevertheless, in recent years, remarkable progress has been made on the development of catalysts for this type of reaction, especially heterogeneous catalysts, which are generally dominating in industry. Here in this review, we discuss the recent breakthroughs as well as milestone achievements for the hydrogenation of industrially important carboxylic acids and esters utilizing heterogeneous catalysts. In addition, related catalytic hydrogenations that are considered of importance for the development of cleaner energy technologies and a circular chemical industry will be discussed in detail. Special attention is paid to the insights into the structure-activity relationship, which will help the readers to develop rational design strategies for the synthesis of more efficient heterogeneous catalysts.

Active Sites and Interfacial Reducibility of CuxO/CeO2 Catalysts Induced by Reducing Media and O2/H2 Activation

The development of a highly efficient and stable catalyst for preferential oxidation of CO for the commercialization of proton-exchange membrane fuel cells has been a result of continuous effort. The main challenge is the simultaneous control of abundant active sites and interfacial reducibility over the catalyst Cu_xO/CeO₂. Here, we report a strategy to modulate porosity, active sites, and O-vacancy sites (O_V) by reducing media and O₂/H₂ activation. O₂-pretreated CeO₂-supported Cu catalysts unequivocally demonstrate the low-temperature activity owing to the excess concentrations of Cu⁺ and Cu²⁺ as well as the relative population of Ce³⁺ and O-vacancy sites at the surface. O₂ activation improves the Cu²⁺ diffusion into the CeO₂ lattice to generate the synergistic effect and induces the formation of electron-enriched Cu²⁺-O_V-Ce³⁺ sites, which accelerate the activation and dissociation of CO/O₂ and the formation of reactive oxygen species during catalysis. Density function theory (DFT) calculations reveal that CO adsorbs on Cu₂O {110} and CuO {111} with relatively optimal adsorption energy and longer C-Cu lengths in contrast to that on Cu {111}, favoring the adsorption and desorption of CO. These are crucial for ongoing CO oxidation, producing CO₂ by the Mars-van Krevelen mechanism.