Well-Defined Supported ZnOx Species: Synthesis, Structure, and Catalytic Performance in Nonoxidative Dehydrogenation of C3–C4 Alkanes

Abstract

Zinc oxide (ZnO) is a multipurpose material and finds its applications in various fields such as rubber manufacturing, medicine, food additives, electronics, etc. It has also been intensively studied in photocatalysis due to its wide band gap and environmental compatibility. Recently, heterogeneous catalysts with supported ZnO_x species have attracted more and more attention for the dehydrogenation of propane (PDH) and isobutane (iBDH) present in shale/natural gas. The olefins formed in these reactions are key building blocks of the chemical industry. These reactions are also of academic importance for understanding the fundamentals of the selective activation of C–H bonds. Differently structured ZnO_x species supported on zeolites, SiO₂, and Al₂O₃ have been reported to be active for nonoxidative dehydrogenation reactions. However, the structure–activity–selectivity relationships for these catalysts remain elusive. The main difficulty stems from the preparation of catalysts containing only one kind of well-defined ZnO_x species.

In this Account, we describe the studies on PDH and iBDH over differently structured ZnO_x species and highlight our approaches to develop catalysts with controllable ZnO_x speciation relevant to their performance. Several methods, including (i) the in situ reaction of gas-phase metallic Zn atoms with OH groups on the surface of supports, (ii) one-pot hydrothermal synthesis, and (iii) impregnation/anchoring methods, have been developed/used for the tailored preparation of supported ZnO_x species. The first method allows precise control of the molecular structure of ZnO_x through the nature of the defective OH groups on the supports. Using this method, a series of ZnO_x species ranging from isolated, binuclear to nanosized ZnO_x have been successfully generated on different SiO₂-based or ZrO₂-based supports as demonstrated by complementary ex/in situ characterization techniques. Based on kinetic studies and detailed characterization results, the intrinsic activity (Zn-related turnover frequency) of ZnO_x was found to depend on its speciation. It increases with an increasing number of Zn atoms in a Zn_mO_n cluster from 1 to a few atoms (less than 10) and then decreases strongly for ZnO_x nanoparticles. The latter promote the formation of undesired C₁–C₂ hydrocarbons and coke, resulting in lower propene selectivity in comparison with the catalysts containing only ZnO_x species ranging from isolated to subnanometer Zn_mO_n clusters. In addition, the strategy for improving the thermal stability of ZnO_x species and the consequences of mass-transport limitations for DH reactions were also elucidated. The results obtained allowed us to establish the fundamentals for the targeted preparation of well-structured ZnO_x species and the relationships between their structures and the DH performance. This knowledge may inspire further studies in the field of C–H bond activation and other reactions, in which ZnO_x species act as catalytically active sites or promoters, such as the dehydroaromatization of light alkanes and the hydrogenation of CO₂ to methanol.