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 CrOx-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.