Metal-organic framework Cr-MIL-101-assisted synthesis of highly dispersed chromium oxide incorporated silica for efficient n-hexane dehydrogenation to n-hexenes

Mesoporous CrO_x-SiO₂ with highly dispersed chromium sites was synthesized by encapsulating silica into the metal-organic framework Cr-MIL-101 for the dehydrogenation of n-hexane. The optimal SiO₂ content is 27.1 wt% for CrO_x-SiO₂ catalyst, where the octahedral structure of Cr-MIL-101 is preserved, Cr atoms are uniformly distributed, a large specific surface area (395 m²/g) is obtained, and abundant surface oxygen vacancies are observed. 89.7 wt% selectivity to n-hexenes and 28.9 wt% n-hexane conversion are achieved on the CrO_x-27.1wt%SiO₂ catalyst in the absence of H₂. The characterization and in-situ experimental results reveal that the proper distance between the active chromium sites (16.7 Cr/nm² based on the highly dispersed Cr atoms) on the CrO_x-27.1wt%SiO₂ catalyst is responsible for the high n-hexenes selectivity. This critical distance depends on the (i) molecular size and (ii) adsorption geometry of n-hexane, (iii) structural properties of the catalyst. The specially designed redox reaction and density functional theory calculation (DFT) at active oxidative chromium sites show that chromium sites accompanied by oxygen vacancies exhibit high dehydrogenation activity for n-hexane conversion. This work gives a novel idea to synthesis high-performance dehydrogenation catalyst for n-hexane and long-chain alkanes.