Nanostructured Catalysts toward Efficient Propane Dehydrogenation

Propylene serves as one of the most significant compounds in the chemical industry. Propane dehydrogenation (PDH), an "on purpose" propylene production technology is developing. Pt- and CrOx-based catalysts are widely applied in commercialized PDH processes, and both exhibit high activity and propylene yields. However, as an intensively endothermic process, PDH requires operation at high temperatures (generally above 500 degrees C), which restricts the C3H6 selectivity and catalyst structure stability on account of coking side reactions, particle sintering, and so forth. Nanostructured catalysts (NCs) based on metals and/or metal oxides with tunable geometric and electronic properties play significant roles because such features intrinsically influence the adsorption of propyl intermediates on the catalyst surface. However, thermodynamical metastability of these NCs results in grand challenges in their structure-controlled preparation. The regulation of material structure and reaction performance at the molecular and atomic levels has attracted extended attention over the past few years. This Account describes our recent advances in controllable regulation of metal and oxide NCs toward efficient propane dehydrogenation. As a structure-insensitive reaction, the dehydrogenation of propane can occur on an individual active site, while larger ensembles of active sites also induce structure-sensitive side reactions, leading to C-C cracking and coke deposition. This paper is aimed at delivering general fundamentals for rational design of NCs in PDH reactions. We start with the catalytic kinetics on the active sites regarding the adsorption of key propyl intermediates on the surface. In subsequent sections, we present the effective regulation strategies for metal and oxide NCs by promoter and support effects. Upon metal NCs, coke deposition and nanoparticles (NPs) sintering tend to occur, which can be suppressed with the increase of geometric separation and charge density of surface active sites by changing alloy compositions, ordered intermetallic alloys, single-atom catalysts, core-shell, and metal-oxide interface structures. Notably, the confinement approach of embedding active sites in zeolite frameworks significantly inhibits the sintering of metal NPs. As alternatives to metals, metal oxides exhibit lower cost but higher barriers of C-H activation and coking inclination. The C-H bond cleavage has been promoted by inducing intrinsic defect sites, such as oxygen vacancies, hydroxyls, and hydrides on the surface and heterogeneous doping in the bulk. Importantly, the structures of the submonolayer/monolayer triggered by spontaneous dispersion and confinement in mesoporous materials significantly improve the oxide activity and stability. All of these strategies have been essential for the efficient PDH reactions. Moreover, the challenges and perspectives are also discussed. It is hoped that the deliberate manipulation of nanostructured catalysts to regulate the reaction mechanism will hold the key to efficient alkane conversion.