Oxidative Conversion Of Methane Over ReOx/Alumina Catalysts

Abstract

ReOx/alumina samples have been studied as catalysts for the oxidative conversion of methane. It was shown that the reaction of methane with an oxidized and then evacuated at this temperature (973 K, 1 h) samples leads to the formation of primarily C2H6, C2H4 and CO2, and then only CO and C6H6. The formation of ethane and ethylene in the initial stage of the reaction of methane with ReOx / alumina samples is the result of the reaction of oxidative condensation of methane with the formation of ethane, followed by its dehydrogenation to ethylene. The reaction proceeds with the participation of O-• ion-radical type (V-type defect) centers of the ReOx / aluminum-oxide structure (CH4 + [O-•] = CH3 • + OH- , 2CH3 • = C2H6). CO2 is formed by the oxidation of methane with surface oxygen forms (O2 2- and / and O2 -•) oxide structure. Direct oxidation of methane to methanol and further splitting of the latter to CO and H2 in our case are not excluded: CH4 + ReOx/Al2O3 = CO + 2H2 + ReOx-1/Al2O3. It was shown that catalysts preliminarily oxidized at 973 K for 1 h in oxygen and then evacuated at the same temperature for 1 h are characterized by ESR spectra belonging to the paramagnetic Re6+ ion with 5d1 unpaired electron and strong Re=O bond. The ESR spectra of this ion are characterized by a hyperfine structure (A║ = 48.3 mT) due to the interaction of an unpaired electron with 185.187Re magnetic nuclei having the spin of I = 5/2 and are easily observed at room temperature for all samples. The ESR signals of these centers disappear after the interaction of samples evacuated at high temperature (973 K) with methane. It was shown that high-temperature contact of this sample with methane leads to the formation of centers that catalyze the oxidative dehydrocyclization of methane. The degree of oxidation of rhenium ions in these samples is less than 6+, and these ions are coordinatively unsaturated. To maintain the activity of the catalyst, its regeneration is required. Activation of the catalyst is achieved by short-term heat treatment in oxygen flow followed by purging with an inert gas (nitrogen, argon).