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

Abstract

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 (ZnxAlyO4 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 27 Al 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 (ZnIV-O-AlIV/VI linkages) as notably more active and selective in PDH relative to the supported ZnO wurtzite phase (ZnIV-O-ZnIV 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, ZnxAlyO4 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 ZnxAlyO4 NPs can be regenerated almost fully using calcination in air.

Visualizing Structural and Chemical Transformations of an Industrial Cu/ZnO/Al2O3 Pre‐catalyst during Activation and CO2 Reduction

“…While the interaction of Cu with ZnO has been frequently studied in model catalysts, less attention has been devoted to the role of Al 2 O 3 , which is typically considered as a structural promoter. However, recent work shows that Al 2 O 3 may be also involved in activation of CO 2 , [18a] promoting the reverse water gas shift reaction over the methanol synthesis, [23] and hindering the reduction of cationic Zn via forming spinel‐like structures [7b,24] . Our in situ TEM results not only validate the formation of a mixed Al−Zn oxide and the beneficial effect in stabilizing the cationic Zn, but also uncover the co‐existence of different alumina morphologies (film, rod, and island/cluster) in the industrial catalyst.…”

Advanced design and development of catalysts in propane dehydrogenation

“…The relationship between the catalytic performance and structural properties was evaluated for catalyzing the PDH process based on Al 2 O 3 supported Zn-based catalysts. 115 The authors found that the exposed active Zn species on the surface of ZnO/Al 2 O 3 catalysts quickly deactivated during the reaction-regeneration process, while the spinel type of Zn x Al y O 4 catalyst with a stabilized structure could almost recover the entire activity after being carbonized in air. Chen et al investigated the performance of HZSM-5 zeolite supported Zn with various contents.…”

Particle size effect of SiO2-supported ZnO catalysts in propane dehydrogenation

“…Tremendous efforts have been devoted to the design of a transitional metal oxide catalyst containing earth-abundant elements. To date, a series of metal oxide catalysts, including ZrO 2 , 11,12 GaO x , 13,14 VO x , [15][16][17][18][19] ZnO, [20][21][22][23][24][25][26][27][28] isolated FeO x , 29,30 and CoO x , 31 have been investigated. In metal oxide-catalysed PDH reactions, the coordinatively unsaturated acidic sites of metal oxides are considered catalytically active sites.…”