The role of oxygen vacancies and Zn in isobutene synthesis from ethanol employing Zn, Zr-based catalysts

Abstract

Zn, Zr-based catalysts were prepared by dry impregnation and employed in the synthesis of isobutene from ethanol in the presence of water. The catalysts were characterized by means of several techniques such as XPS, XRD, N₂ physical adsorption, HRTEM, NH₃-TPD, EPR, CO₂-TPD, XRF, ethanol-TPD, and CO₂/H₂-TPSR. The mechanism of isobutene synthesis from ethanol was examined using density functional theory (DFT) calculations. This work shows that employing the wetness impregnation procedure makes it possible to replace Zr⁴⁺ on m-ZrO₂ with Zn²⁺, generating oxygen vacancies and Zr³⁺ (cus) species on the ZrO₂ surface. It was observed that the Zn solubility limit in m-ZrO₂ is reached above 6 at% Zn. The Zn²⁺ and oxygen vacancies on the surface of m-ZrO₂ drastically changed the catalytic behavior of this oxide. They promote the generation of acetaldehyde from ethanol and also acetone from this aldehyde. Moreover, the Zn²⁺ and oxygen vacancies play a pivotal role in the rate-limiting step of this cascade reaction, which involves the conversion of acetone into isobutene and more specifically acetaldehyde enolate species condensation. This phenomenon was very well described by the linear correlation obtained between the Zn concentration on the m-ZrO₂ surface and the yield of isobutene. Finally, this paper presents new perspectives for the generation of isobutene from ethanol, shedding light on the mechanism of this rather complex process.