Insights into the unusual role of chlorine in product selectivity for direct hydrogenation of CO/CO2 to short-chain olefins

Abstract

Light olefins (C2=-C4=) are the key intermediates for the petrochemical industries, and their synthesis from hydrogen-lean (H2/CO < 1) Bio-derived syngas having impurities, such as, Chlorine, CO2 through Fischer-Tropsch route (FTO) is very challenging as Cl is considered poison for the catalysts. Herein, we report the synthesis of Fe-Cu-K catalysts, using chloride salts and nitrate salts for the single-step conversion of hydrogen-lean syngas (H2/CO = 0.8) with an optimum CO2 content (16%) to light olefins with 70–75% selectivity at CO conversion 65–90%. The utilisation of such syngas is currently out of the scope of available research data in FTO. The kinetic analysis shows that even a small amount of Chlorine in the catalyst system controls the hydrogen adsorption on the catalyst surface-active iron carbide sites and boosts up the selectivity for short-chain olefins in the direct hydrogenation of CO/CO2.In-situDRIFT studies and DFT-calculations on Fe5C2 (100) indicates that associative adsorption of CO/CO2 and H2 occurs on the chlorided catalyst with controlled H2-adsorption that results in the selective syngas to olefins through an unorthodox alkyl route.