CO2 hydrogenation using bifunctional catalysts based on K-promoted iron oxide and zeolite: influence of the zeolite structure and crystal size

Abstract

In the present manuscript, the influence of the zeolite structure and crystal size on bifunctional tandem catalysts combining K-promoted iron oxide (K/Fe₃O₄) with different zeolites has been studied for the CO₂ hydrogenation reaction at 320 °C and 25 bar. First, to evaluate the influence of the zeolite structure on CO₂ conversion, three different zeolite topologies have been evaluated (BEA, MFI and CHA) with similar Si/Al molar ratios. The combination of K/Fe₃O₄ with MFI maximizes the formation of aromatic products, while its combination with CHA and BEA increases the C₁–C₄ gas fractions, with a high olefin selectivity. In addition, aromatics and aliphatic hydrocarbons are present in the condensed liquids of the tandem catalysts containing BEA, while no aromatics are observed for those with CHA. These different product selectivities can be ascribed to the different consecutive reactions within the three zeolites, where MFI favors the aromatization of alkenes, and BEA and CHA favor oligomerization/cracking reactions leading to an increase of light olefin yield. Second, the evaluation of the nanosized form of the three proposed zeolite frameworks has also been carried out. The reduction of the particle size allows for increasing light olefin selectivity in all cases as compared to zeolites with larger crystals. Shorter intracrystalline diffusion paths facilitate the egression of light olefins before being involved in consecutive oligomerization reactions. In the particular case of the tandem catalysts with nanosized MFI zeolites, the C₂–C₄ fraction and its olefinicity are increased while maintaining the overall aromatic selectivity comparable to that obtained with micron-sized MFI zeolites.