Sustainable Fuels from CO 2 -Rich Synthesis Gas via Fischer–Tropsch Technology

CO2-containing synthesis gas is a relevant feedstock for the production of synthetic fuels using Fischer–Tropsch synthesis (FTS). We report the role of CO2 in CO2, CO, and H2 mixed feeds over a cobalt-based catalyst at 220 °C and 21 bar in a packed bed reactor and define the process boundary conditions where CO2 switches from an inert to a reactive gas in FTS. The C5+ selectivity remains above 78% even for CO2-rich synthesis gas with 75% CO2/(CO + CO2). Using 13CO2 isotopic labeling, the increase in methane selectivity is attributed to both CO and CO2 methanation, which is limited by maintaining a H2/CO outlet ratio below 10 and an outlet CO partial pressure above 0.2 bar, respectively. CO and CO2 are proposed to adsorb on the same Co sites, and with CO adsorption being more favorable, we postulate that sufficient CO prevents CO2 adsorption and reaction. These results overturn the prevailing assumption that C5+ selectivity is dictated by the CO2 partial pressure or the CO2/CO ratios. In situ modulated diffuse reflectance infrared Fourier transform spectroscopy confirms a positive relationship between CO surface coverage and CO partial pressure. From DFT and microkinetic modeling, enhanced CO and CO2 methanation could be attributed to a lower surface coverage and a higher H2 surface coverage. This work identifies boundaries for efficient cobalt-catalyzed mixed-feed FTS for synthetic fuel production.