Fischer-Tropsch synthesis on cobalt and ruthenium. Metal dispersion and support effects on reaction rate and selectivity

Metal dispersion and support effects on Fischer-Tropsch synthesis rate and selectivity were studied at conditions that favor the information of C 5+ hydrocarbons (> 80% selectivity). On Ru, these effects are minor for the supports (SiO 2, Al 2O 3, TiO 2) and the dispersion range (0.0009–0.60) tested. Site-time yields are similar (1.25–1.95 × 10 −2 s −1) on all Ru catalysts (476 K, 560 kPa, H 2/CO = 2.1). On Co, hydrocarbon synthesis rates are also proportional to metal dispersion (0.0045–0.095) and independent of the metal oxide support (SiO 2, Al 2O 3, TiO 2, and ZrO 2-modified SiO 2 and TiO 2). Site-time yields (1.6–3.0 × 10 −2 s −1) are independent of Co dispersion and support (473 K, 2000 kPa, H 2/CO = 2.1). Dispersion and support influence C 5+ selectivity slightly on both Co and Ru catalysts; these changes reflect transport-enhanced secondary reactions and not modifications of intrinsic chain growth kinetics. Specifically, transport restrictions imposed by the physical structure of the support and by a high site density within catalyst pellets increase the residence time and the readsorption probability of reactive a-olefins and lead to higher C 5+ yields and more paraffinic products.