Correlating the Structural Evolution of ZnO/Al <sub>2</sub> O <sub>3</sub> to Spinel Zinc Aluminate with its Catalytic Performance in Propane Dehydrogenation
Manouchehr Nadjafi, Agnieszka Kierzkowska, Andaç Armutlulu, René Verel, Alexey Fedorov, Paula M. Abdala, Christoph R. Müller
Abstract
We correlate the catalytic activity for propane dehydrogenation (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; (ii) zinc aluminate spinel nanoparticles (ZnxAlyO4 NPs) prepared via a hydrothermal method; and (iii) ZnO/SiO2 prepared by ALD of ZnO on SiO2. The catalysts are characterized by synchrotron X-ray powder diffraction (XRD), Zn K-edge X-ray absorption spectroscopy (XAS), and 27Al solid-state nuclear magnetic resonance (ssNMR). We identify tetrahedral Zn sites in close proximity to Al sites of a zinc aluminate spinel phase (ZnIV–O–AlIV/VI linkages) as more active and selective in PDH relative to the supported ZnO wurtzite phase (ZnIV–O– ZnIV linkages) in ZnO/SiO2. 50ZnO/Al2O3 gives 77% selectivity to propene at 9 mmol C3H6 gcat–1 h–1 space-time yield after 3 min of reaction at 600 °C. The ZnO/Al2O3 catalyst shows an irreversible loss of activity over repeated PDH and air-regeneration cycles attributed to Zn depletion on the surface, while the activity loss of ZnxAlyO4 NPs due to coke deposition can be recovered by air regeneration.