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 ZnAlOx composite metal oxide and subsequently react via a Mars–van Krevelen mechanism.