Effect of Diffusion Constraints and ZnO<i><sub>x</sub></i> Speciation on Nonoxidative Dehydrogenation of Propane and Isobutane over ZnO-Containing Catalysts
Dan Zhao, Mingbin Gao, Xinxin Tian, Dmitry E. Doronkin, Shanlei Han, Jan‐Dierk Grunwaldt, Uwe Rodemerck, David Linke, Mao Ye, Guiyuan Jiang, Haijun Jiao, Evgenii V. Kondratenko
Abstract
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 ZnO x species on (S-1), dealuminated beta (deAl beta), and ZrO 2 . 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 ZnO x /S-1 catalyst, molecular-level insights into individual reaction pathways were derived from density functional theory calculations. The nature of active ZnO x sites was investigated by X-ray absorption spectroscopy and was established to depend on the kind of support material. Binuclear ZnO x species are formed inside small S-1 pores or on the surface of ZrO 2, while three-dimensional multinuclear ZnO x 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 kg C 3 H 6 kg cat –1 h –1 or 6.3 kg i-C 4 H 8 kg cat –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.