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

Abstract

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.