Correlating the Structural Evolution of ZnO/Al2O3 to Spinel Zinc Aluminate with its Catalytic Performance in Propane Dehydrogenation

Zn-based Al₂O₃-suported materials have been proposed as inexpensive and environmentally friendly catalysts for the direct dehydrogenation of propane (PDH), however, our understanding of these catalysts’ structure and deactivation routes is still limited. Here, we correlate the catalytic activity for PDH of a series of Zn-based Al₂O₃ catalysts with their structure and structural evolution. To this end, three model catalysts are investigated. (i) ZnO/Al₂O₃ prepared by atomic layer deposition (ALD) of ZnO onto γ-Al₂O₃ followed by calcination at 700 °C, which yields a core-shell spinel zinc aluminate/γ-Al₂O₃ structure. (ii) Zinc aluminate spinel nanoparticles (Zn_xAl_yO₄ NPs) prepared via a hydrothermal method. (iii) A reference core-shell ZnO/SiO₂ catalyst prepared by ALD of ZnO on SiO₂. The catalysts are characterized in detail by synchrotron X-ray powder diffraction (XRD), Zn K-edge X-ray absorption spectroscopy (XAS), and ²⁷Al solid state nuclear magnetic resonance (ssNMR). These experiments allowed us to identify tetrahedral Zn sites in close proximity to Al sites of a zinc aluminate spinel phase (Zn_IV–O–Al_IV/VI linkages) as notably more active and selective in PDH relative to the supported ZnO wurtzite phase (Zn_IV–O– Zn_IV linkages) in ZnO/SiO₂. The best performing catalyst, 50ZnO/Al₂O₃ gives 77% selectivity to propene (gaseous products based) at 9 mmol C₃H₆ gcat−1 h⁻¹ space time yield (STY) after 3 min of reaction at 600 °C. On the other hand, the core-shell ZnO/Al₂O₃ catalyst shows an irreversible loss of activity over repeated PDH and air-regeneration cycles, explained by Zn depletion on the surface due to its diffusion into subsurface layers or the bulk. ZnxAlyO₄ NPs gave a comparable initial selectivity and catalytic activity as 50ZnO/Al₂O₃. With time on stream, Zn_xAl_yO₄ NPs deactivate due to the formation of coke at the catalyst surface, yet the extend of coke deposition is lower than for the ZnO/Al₂O₃ catalysts, and the activity of Zn_xAl_yO₄ NPs can be regenerated almost fully using calcination in air.