We describe the remarkable performance of a new catalyst for the chemical looping (CL-) epoxidation of ethylene, performed at atmospheric pressure and without any promoters added to either the catalyst or the feed gas. To undertake the CL-epoxidation of ethylene, silver was used as the catalyst, supported on either the perovskite SrFeO3 or Ce-modified SrFeO3. Here, the oxygen for the reaction is supplied to the silver catalyst from the active solid support, not from the gas stream. When the support has been reduced and depleted of oxygen, it is regenerated in a separate step with air, which makes the process cyclic and closes a chemical loop. Thus, there is no need to co-feed gaseous oxygen along with the ethylene feed, an important improvement in safety. Two methods were used to synthesise Ce-modified materials, employing either (i) the mechanical mixing of powdered CeO2 and the solid precursors of the perovskite, or (ii) the impregnation of a solution of cerium nitrate into solid particles of SrFeO3. In both cases, the materials were calcined to produce a mixture of CeO2 and SrFeO3. Both CeO2-SrFeO3 materials surpassed the unmodified SrFeO3 for CL-epoxidation. For the CeO2-SrFeO3 prepared by mechanical mixing, the production of ethylene oxide was stable over 15 cycles, giving 60% selectivity at 10% conversion of C2H4. In contrast, the material prepared by impregnation gave up to 85% selectivity but only in the first cycle of reduction, with the performance degrading over subsequent cycles. The reported results are better than the 50% selectivity achieved for the classical epoxidation using pure silver as the catalyst and feeds of gaseous ethylene and oxygen, without reaction promoters.
Chemical looping epoxidation (CLE) of ethylene to ethylene oxide (EO) presents an exciting alternative to the incumbent technology of direct epoxidation of ethylene with O2(g). In CLE, the reaction is still catalysed by Ag but oxygen is provided as Olattice from a solid metal oxide, eliminating the need and limitations of using O2(g). Here, the influence of catalyst preparation in CLE is investigated. The Ag catalyst was impregnated on a solid oxide, SrFeO3, which was used as the donor of Olattice in CLE.Temperature programmed reduction in H2 indicated that O-species participating in CLE can be attributed to the removal of the first monolayer of oxygen in SrFeO3. By changing the temperature of calcination in Ag-SrFeO3 preparation, the size of Ag particles was varied, resulting in a simultaneous increase of selectivity for EO (up to 60%) and conversion of C2H4 (up to 10%). Then, an assessment of the effects of impurities (carbonates, hydroxides), which deposit over time on the Ag-SrFeO3 surface, was performed, showing that impurities deteriorate the catalyst performance in CLE. Finally, the doping of the SrFeO3 with Ce to the A-site of the perovskite was carried out, and a substantial improvement of the conversion of C2H4 was achieved, reaching 15%, while maintaining the 60% selectivity for EO.
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