Selective Light Hydrocarbon Production from CO₂ Hydrogenation over Na/ZnFe₂O₄ and CHA-Zeolite Hybrid Catalysts

Abstract

Carbon dioxide hydrogenation to value-added fuels and chemicals has been studied widely as a means to recycle the most-troublesome greenhouse gas. The reaction produces hundreds of different chemicals, and therefore, selectivity control toward specific desired products is of paramount importance. In this study, a hybrid catalyst system consisting of Na/ZnFe2O4 (ZFO) and a CHA-zeolite (SSZ-13 or SAPO-34) is developed to maximize C-2-C-4 light hydrocarbon production. Utilizing the compact 3.8 & Aring; pore size of CHA-zeolites, the Na/ZnFe2O4 catalyst-produced long-chain hydrocarbons are efficiently shortened to C-2-C-4 hydrocarbons with over 55% selectivity in the hybrid systems. Notably, ZFO + SAPO-34 shows a preference for light olefins, while ZFO + SSZ-13 uniquely enhances selectivity for C-3 products. The difference is attributed to the much stronger acid sites present in SSZ-13 than in SAPO-34, which promote hydrogenation of olefins and the ethylene-to-propane conversion reaction in particular. Further modification of SSZ-13 with steam treatment leads to the dealumination of its framework and an enhanced activity of the ethylene-to-propane reaction, yielding 32.8% of C-3-selectivity. Accordingly, a hybrid catalytic system combining a CO2 Fischer-Tropsch catalyst with a CHA-zeolite is a promising route to produce light hydrocarbons from CO2 hydrogenation more selectively than single catalysts. This work also demonstrates that acidity control could be a powerful tool to manipulate the reaction pathway that occurs on zeolite catalysts.