The Fundamentals for Efficient Non-oxidative Propane Dehydrogenation over ZrO2-Based Catalysts

Abstract

The activation of C–H bonds in light alkanes efficiently is a challenging yet crucial aspect of heterogeneous catalysis. This process is essential for converting abundant hydrocarbon feedstocks into valuable products. The non-oxidative propane dehydrogenation to propene (PDH) has attracted widespread attention due to the presence of cheap propane in shale and has become the basis of an important on-purpose technology to bridge the gap between propene production and demand. It is also an important model reaction for studying the fundamentals of C–H bond activation. Compared to traditional oil-based cracking processes, the PDH reaction has the following advantages: (1) abundant propane recourses, mainly from shale gas and refinery plants, (2) high selectivity to propene (above 90%), and (3) the composition of the products is simple and easy to separate. Currently, commercial PDH processes rely on the Catofin and Oleflex technologies developed by CB&I Lummus and UOP Company, which apply PtSn/γ-Al₂O₃ and K–CrO_x/γ-Al₂O₃ catalysts, respectively. However, Pt-based catalysts are expensive and Cr(VI)O_x-based catalysts are toxic, limiting their application to a certain degree. Therefore, the search for environmentally friendly and cost-effective PDH catalysts has become a key topic of ongoing research.

In this Account, we will summarize the research progress on the development of ecofriendly and cost-efficient bulk ZrO₂-based catalysts for PDH reaction in our collaborative group during the last ten years. Their productivity and propene selectivity are very close to those of commercial-like CrO_x-based catalysts. These alternative-type PDH catalysts were first introduced by us. We have also elucidated the fundaments relevant to controlling their activity and product selectivity. Our novel concept inspired other research groups to develop catalysts based on other typically nonreducible metal oxides. This Account will mainly focus on the structural regulations of ZrO₂-based catalysts, which influence the C–H bond activation pathways as well as propene selectivity, catalyst activity, on-stream stability, and durability in the PDH reaction. First, the mechanistic aspects of propene and byproduct formation are briefly described to guide catalyst development. Second, we present the strategies used to regulate the PDH performance of ZrO₂-based catalysts and provide molecular level details of propene and hydrogen formation. Our approaches were aimed at (1) controlling the crystallite size, phase composition, and morphology of bare ZrO₂, (2) constructing binary MZrO_x catalyst systems, such as LaZrO_x, YZrO_x, CrZrO_x, and GaZrO_x, and (3) introducing metal or metal oxide components on the surface of ZrO₂-based materials. Furthermore, the effects of operating conditions such as reaction temperature, catalyst treatment temperature and duration, kind of reducing agent, and H₂ co-feeding on catalyst performance are discussed. The comparison between ZrO₂-based catalysts and other bulk metal oxide catalysts such as Al₂O₃ is also discussed in terms of catalytic performance, active site, and regulation strategies. Finally, our personal views on strategies to improve the PDH performance of metal oxide-based catalysts are provided. The achievements summarized in this Account are expected to inspire further developments of catalysts used not only for efficient C–H bond activation but also for various hydrogenation reactions.