Hydroxyl-Poor Al2O3 Avoids the Formation of ZnAl2O4 Spinel for Propane Dehydrogenation

Zinc-based catalysts offer the advantages of high catalytic activity, low cost, and low toxicity, which are deemed as promising alternatives for Pt- and CrO_x-based catalysts toward propane dehydrogenation (PDH). However, ZnO/Al₂O₃ is prone to form the ZnAl₂O₄ spinel phase at high temperatures, which limits the further utilization of Zn-based propane dehydrogenation catalysts. Here, the reason for the formation of ZnAl₂O₄ is investigated by changing the calcination atmosphere. XRD, Raman, XPS, UV–vis, and H₂-FTIR characterizations and density functional calculations show that hydroxyl-rich Al₂O₃ promotes the formation of the ZnAl₂O₄ spinel phase. In order to avoid the formation of ZnAl₂O₄ spinel, a sol–gel method was employed to synthesize hydroxyl-poor Al₂O₃, which inhibited ZnAl₂O₄ formation and enabled Zn species to mainly exist in the form of ZnO nanoclusters after calcination. As a result, hydroxyl-poor Al₂O₃-supported ZnO exhibited better PDH performance than the case with hydroxyl-rich Al₂O₃ supports. Combined with quantitative XPS calculations, ZnO was shown to be a more efficient active center for ZnO/Al₂O₃ systems in the PDH reaction.