Role of oxygen vacancies employing GaxZr1-xO2-y / ZrO2 in isobutene generation from ethanol

This study advances the understanding of oxygen vacancies (Vo) in heterogeneous catalysis, focusing on isobutene synthesis from ethanol. Ga-doped ZrO2 was investigated and compared with Zn-doped ZrO2. Characterization techniques, including EPR, XRD, XPS, H2O dissociation, and Raman spectroscopy, revealed that Ga substitutes Zr in the m-ZrO2 lattice, generating more Vo than Zn. DFT calculations supported these findings, showing that incorporating two Ga atoms into monoclinic ZrO2 leads to spontaneous defect formation, with energies more negative than those for Zn-doped ZrO2. The isobutene synthesis from ethanol involves three steps: the acetaldehyde generation from ethanol, acetone synthesis from this aldehyde, and isobutene formation from acetone, the rate-limiting step (rls). Catalytic tests and CO2-TPD showed that the Lewis basicity of the ZrO2-based catalysts is not relevant for the isobutene formation. Experiments with Ga-doped t-ZrO2 (tetragonal) highlighted the relevance of ZrO2′s structure in the isobutene synthesis. It was observed that Ga-doped catalysts generate much more isobutene from acetone when compared with Zn-based catalysts. Moreover, the isobutene yield shows a linear correlation with Ga at. %, and consequently, with the Vo concentration. The superior performance of Ga-doped ZrO2 compared to Zn-doped ZrO2 might be attributed not only to Ga's greater ability to generate Vo but also to the formation of a Ga(2)-Vo atomic ensemble, proposed by the DFT studies, which likely participates in the rls of this synthesis. This study offers new insights into the role of Vo in catalytic processes, providing perspectives for improving isobutene synthesis from ethanol and advancing the design of Vo-rich catalysts.