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

ACS catalysis 13, 3356 - 3369 (2023). doi:10.1021/acscatal.2c05704
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 ZnOx 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 kgC$_3$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.
Published by ACS, Washington, DC