High-efficiency CO2 utilization in isobutane dehydrogenation over ZnAl hydrotalcite derivatives: optimizing calcination temperature and unlocking the reaction mechanism

Abstract

CO₂-assisted oxidative dehydrogenation of isobutane (CO₂-ODHB) offers a promising route for the production of isobutene (i-C₄H₈) using CO₂. In this study, a series of ZnAlOx oxide catalysts were obtained by calcination–reduction of the ZnAl hydrotalcite precursor and evaluated in the CO₂-ODHB reaction. The conversion of isobutane (i-C₄H₁₀) and CO₂ as well as isobutene selectivity increased first and then decreased with enhancing calcination temperature, and the catalyst calcined at 600 °C achieved the best dehydrogenation activity and stability with an i-C₄H₁₀ conversion of ca. 59% and an i-C₄H₈ selectivity of above 90%. The effect of calcination temperature in the range from 550 °C to 700 °C on the structure and physicochemical properties of the catalysts was investigated through supportive experiments and comprehensive characterization. The increasing calcination temperature can result in the phase transformation from bulk amorphous ZnAlOx composite metal oxide to ZnAl₂O₄ spinel, reducing the specific surface area and hindering the reduction of Zn²⁺ ions by inducing strong Zn–Al interaction. The optimum dehydrogenation performance of the catalyst calcined at 600 °C is related to the moderate oxygen vacancies and CO adsorption as well as the balance establishment of moderate weak acidic sites and abundant basic sites, which depends on the Zn–Al interaction. The CO₂-ODHB mechanism suggested that i-C₄H₁₀ and CO₂ are mainly activated on the surface of ZnAlO_x composite metal oxide and subsequently react via a Mars–van Krevelen mechanism.