High-Efficiency CO2 Utilization in Isobutane Dehydrogenation over ZnAl Hydrotalcite Derivatives: Optimizing Calcination Temperature and Unlocking 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 ZnAl hydrotalcite precursor and evaluated in 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 oC achieved the best dehydrogenation activity and stability with the i-C4H10 conversion of ca. 59% and i-C4H8 selectivity above 90%. The effect of calcination temperature in the range from 550 oC to 700 oC on the structure and physicochemical properties of catalysts was investigated through supportive experiments and comprehensive characterization. The increasing calcination temperature can result in the phase transformation from bulk amorphous ZnAlOx composition metal oxide to ZnAl2O4 spinel, which reducing special 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 oC 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 depending on the Zn-Al interaction. The CO2-ODHB mechanism was suggested that the i-C4H10 and CO2 are mainly activated on the surface of ZnAlOx composition metal oxide and subsequently reacting via a Mars-van Krevelen mechanism.