Effect of Diffusion Constraints and ZnOx Speciation on Nonoxidative Dehydrogenation of Propane and Isobutane over ZnO-Containing Catalysts

Heterogeneously catalyzed gas–solid-phase reactions generally suffered from diffusion limitations in large-scale processes or in academic studies when zeolites were used as catalysts or supports. Here, we elucidated the effects of diffusion of reactants/products in nonoxidative propane (PDH) and isobutane dehydrogenation (iBDH) reactions on the performance of catalysts possessing differently structured ZnOx species on (S-1), dealuminated beta (deAl beta), and ZrO2. The catalysts were prepared through physically mixing ZnO and the support. Force-field molecular dynamics simulations revealed that the effectiveness factor η is larger than 0.99 in the PDH reaction over all catalysts and in the iBDH reaction over the ZnO-deAl beta catalyst, thus suggesting that mass transport limitations do not play any significant role. However, the iBDH reaction over S-1-based catalysts suffers from some diffusion limitations (0.35 < η < 0.9). Such conditions are favorable for cracking reactions responsible for isobutene selectivity loss. To compare intrinsic catalyst activity in the PDH and iBDH reactions over the ZnOx/S-1 catalyst, molecular-level insights into individual reaction pathways were derived from density functional theory calculations. The nature of active ZnOx sites was investigated by X-ray absorption spectroscopy and was established to depend on the kind of support material. Binuclear ZnOx species are formed inside small S-1 pores or on the surface of ZrO2, while three-dimensional multinuclear ZnOx clusters are generated in the β zeolite with larger pores. The latter show higher Zn-related activity in the PDH reaction under conditions free of any diffusion constraints. The developed ZnO–deAl beta showed the space–time yield of propene or isobutene formation of 2 kgC3H6 kgcat–1 h–1 or 6.3 kgi-C4H8 kgcat–1 h–1 at 550 °C and about 70–80% equilibrium alkane conversion with an olefin selectivity of about 90%. The activity values are higher than those reported for the state-of-the-art non-noble metal oxide catalysts tested at the same or even higher temperatures.