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

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.