(654b) Direct Synthesis of Branched Hydrocarbons from CO2 Hydrogenation over Composite Catalysts in a Single Reactor

Two common approaches for CO₂ hydrogenation to long chain hydrocarbons (CO₂-to-HCs) are modified Fischer-Tropsch synthesis and methanol-mediated pathways, both of which combine multifunctional catalysts to achieve hydrocarbon (HC) synthesis via oxygenate intermediates, including CO and methanol (MeOH), respectively. The methanol-mediated pathway provides better controlled of the HC distribution in the product slates via methanol-to-hydrocarbon (MTH) chemistry by selecting the zeolite component and reaction conditions. An important challenge for further development of CO₂-to-HCs processes via the methanol-mediated pathway is the mismatch between reaction conditions of MeOH synthesis and MTH, where MeOH synthesis is thermodynamically favorable at low temperatures and commonly performed in the 200 – 250 °C range, while higher temperatures (> 300 °C) are typically used for MTH reactions.

In this study, hydrogenation of CO­₂ to HCs using a composite catalyst system in a single reactor under mild reaction conditions was investigated, by combining MeOH synthesis, MeOH dehydration and dimethyl ether homologation catalysts. This combination alleviates the mismatch in operating conditions of various reactions and provide opportunity to integrate these processes into a single reactor. High selectivity to isoparaffins, which are versatile precursors to sustainable aviation fuels, was achieved. The use of Cu/BEA significantly improved both the conversion of oxygenates and the HC yield compared to HBEA by facilitating H-incorporation in the C-chain growth during the homologation reaction. A study of catalyst composition and reaction conditions revealed that nearly complete conversion of oxygenates was achieved using a stacked-bed configuration, providing high C-selectivity to C₄₊ HCs (34.1% among all products and 95.5% in products excluding CO). The ability to completely convert oxygenates to HCs and achieve high CO-free selectivity to desired HC products is unprecedented in CO₂-to-HC reports, and advantageous for downstream separation, as CO can be easily separated and recycled to improve overall HC production in an integrated process.