Understanding the reaction-induced restructuring of CoOx species in silicalite-1 to control selectivity in non-oxidative dehydrogenation of propane

Abstract

Non-oxidative dehydrogenation of propane (PDH) is an important route for large-scale on purpose propene production. Although cobalt-based catalysts are promising alternatives to currently used platinum- or chromium oxide-based catalysts, their further developments are hindered by the uncertainties related to the kind of the active sites involved in the desired and side reactions. To contribute to closing such a gap, we systematically investigate the role of oxidized CoO_x and metallic Co⁰ species in the PDH reaction over catalysts based in Silicalite-1 with supported CoO_x species differing in their redox properties. C₃H₈ pulse experiments with sub-millisecond and second resolution at pulse sizes of about 13 and 2200 nmol, respectively, combined with in-depth catalyst characterization and PDH tests at different propane conversions enabled us to understand how the reaction-induced reduction of CoO_x affects product selectivity. Propane readily reacts with CoO_x to yield propene, carbon oxides and water. The formed Co⁰ species show high activity to coking and cracking reactions. However, if the size of such species is below 2 nm, these undesired reactions are significantly hindered due to the coverage of the active sites by carbon-containing species. The remaining uncovered surface Co⁰ sites selectively dehydrogenate propane to propene. The best-performing catalyst showed higher activity than a commercial-like K-CrO_x/Al₂O₃ and operated durable in a series of 10 dehydrogenation/regeneration cycles under industrial relevant conditions. The space time yield of propene formation of 0.97 kg·h^–1·kg_cat^–1 was achieved at 550 °C, 52% equilibrium propane conversion and 95% propene selectivity.