Influence of Recycle Operation on the Catalytic Hydrogenation of CO₂ to Long-Chain Hydrocarbons

The catalytic hydrogenation of CO2 to long-chain hydrocarbons (CO2-based Fischer–Tropsch synthesis) may become an important industrial process for the production of sustainable hydrocarbons for the chemical industry or fuel applications in the future. Several questions regarding the scale-up of the process remain unsolved, though, as there have scarcely been studies beyond lab-scale. Recycle operation might be necessary in a technical application to achieve high reactant conversions. However, as not only unconverted reactants but also light hydrocarbons are recycled into the reactor, the product composition might be significantly altered. In this study, we investigated the influence of recycle operation in a bench scale recycle reactor setup (10–20 g catalyst) for a potassium promoted, alumina supported iron catalyst at 300 °C, 10 bar, (H2/CO2)in = 3, and fresh feed space velocities ranging from 1800 to 7200 mLN h–1 g–1. An increase in reactant conversion could be clearly observed under recycle conditions which can be well described with previously developed models. The product composition, however, was only slightly affected. The data indicates a slight increase of the average molecular weight under recycle conditions which may be caused not only by secondary reactions of linear 1-alkenes but also by more favorable synthesis conditions. Among secondary reactions of linear 1-alkenes, there was only convincing evidence for hydrogenation (especially for ethene) and double-bond-shift