Isobutane dehydrogenation over chromia alumina catalysts prepared from MIL-101: Insight into chromium species on activity and selectivity

Various mesoporous chromia alumina catalysts were prepared by five different methods based on a metal-organic framework MIL-101 and their catalytic performances over isobutane dehydrogenation were investigated. The highly dispersed chromium species were produced on catalyst KCrAl-I₁ with largest specific surface area of 198 m²·g⁻¹ prepared with aluminium isopropoxide (Al(i-OC₃H₇)₃) by ultrasonic impregnation method. However, the catalyst KCrAl-I₂ synthesized by stirring impregnation possessed crystalline α-Cr₂O₃ phase, which was poorly dispersed. Two types of Cr-rich and Al-rich Cr_xAl_2–xO₃ solid solutions, designated as CrAl-I and CrAl-II phase, were formed over the catalysts KCrAl-I₃ (prepared by Al(i-OC₃H₇)₃ with nitric acid regulation), KCrAl-C₄ (prepared by aluminium chloride hexahydrate) and KCrAl-N₅ (prepared by aluminium nitrate nonahydrate). Catalytic evaluation results revealed that KCrAl-I₁ exhibited the high isobutane conversion due to its highly dispersed chromium species. However, KCrAl-I₃, KCrAl-C₄ and KCrAl-N₅ showed the higher isobutene selectivity (95.2 %–96.4%) on account of the formation of chromia alumina solid solutions in the catalysts. Moreover, the solid solution over the chromia alumina catalysts could greatly suppress the coke formation.