Fischer–Tropsch synthesis: Pore size and Zr promotional effects on the activity and selectivity of 25%Co/Al2O3 catalysts

Abstract

The effects of pore size (10.8 and 25nm) and zirconium loading (0–5%) on Fischer–Tropsch synthesis (FTS) performance of 25%Co/Al2O3 catalysts were systematically studied at a CO conversion level of ∼50% under industrially relevant conditions. The catalysts were characterized by adsorption and X-ray spectroscopic techniques in order to understand the relationships between the catalyst physicochemical properties and the FTS performance parameters. Unpromoted wide pore 25%Co/Al2O3 displayed greater stability (CO% rate loss over 150h of testing: 5.2 versus 27.6%) and 40% higher activity than the narrow pore catalyst. Addition of 1–5% Zr improved the initial activity of the 25%Co/Al2O3 catalysts by 25–71% regardless of support pore size, but the catalyst deactivation rates increased. The activity improvements by Zr for the wide pore and narrow pore catalysts are due to different reasons. XRD, hydrogen-chemisorption, and XANES results suggested that Zr primarily increased Co dispersion for the wide pore catalyst whereas it mainly increased Co reduction for the narrow pore catalysts. The smaller mean Co cluster size observed with the wide pore Co catalyst resulted from a relatively higher fraction of Co residing inside the pores as compared to the narrow pore catalyst.The unpromoted wide pore catalyst exhibited lower CH4 selectivity (5.9–6.3 versus 8.3–8.9%) and higher C5+ selectivity (88–89.2 versus 80–82%) compared with the narrow pore one. Zr addition decreased CH4 selectivity and increased C5+ selectivity for the narrow pore 25%Co/Al2O3 catalyst, but the opposite trend was observed with the wide pore Co catalyst. Explanations for the selectivity trends (including olefins and CO2) are provided based on the effects of Co size and location, and the resulting changes in H2/CO mass transport within pores of the Co/Al2O3 catalysts with and without Zr promoter.