Ethane Dehydrogenation with CO₂as a soft oxidant over a Cr-TUD-1 catalyst

A Cr-TUD-1 catalyst was directly synthesized under hydrothermal conditions using triethanolamine as a template for mesopore formation, tetraethylorthosilicate as a silica precursor, and Cr(NO3)3 as a chromium source. Dehydrogenation of ethane to ethylene (DHE) in the presence of CO2 as soft oxidant was investigated over the Cr-TUD-1 catalyst, which had a nanosponge structure and a surface area of ∼650 m2 g-1, and in which Cr6+ species were dominantly distributed in a form of monochromate. During DHE under N2, Cr6+ underwent partial reduction to Cr3+/Cr° with concomitant formation of large-sized Cr2O3 and CrCx particles. As a result, the Cr-TUD-1 catalyst was deactivated within 2 h due to both the diminish of chromium active sites and the formation of coke. Replacement of N2 with CO2 enhanced the catalyst stability and produced little coke, which was attributed to enhanced inter-convertibility of Cr6+ and highly dispersed Cr3+ and a reverse Boudouard reaction between coke and CO2. Aggregated large Cr2O3 nanoparticles could not be readily oxidized to Cr6+ by CO2. The addition of O2 to CO2 further enhanced catalyst lifetime because the oxygen contributed to the redispersion of sintered Cr2O3 nanoparticles to form isolated Cr6+ species.