oa Fischer-Tropsch synthesis on Co/Al₂O₃ catalyst: Effects of carbidization and reduction conditions

Alumina supported cobalt (Co) catalysts are the preferred choice for Fischer-Tropsch synthesis (FTS) when using natural gas as raw material, which is the case of Qatar. It is thus of economical interest to find ways of increasing activity and/or selectivity of these catalysts. One way of modifying catalytic activity, consists of transformation of the active Co phase crystallites. A shift from the abundance of the face-centered cubic (fcc) Co to hexagonal-close packed (hcp) Co phase has been found to increase catalyst activity [1]. This can be accomplished through the carbidization-reduction step with Co carbide as an intermediate. The catalyst was prepared by incipient wetness impregnation of alumina support (15% Co/Al₂O₃). Its fcc form was obtained by reducing calcined catalyst in pure hydrogen at 375°C for 10 h. The hcp form was subsequently prepared by carbidization with pure CO for 14 h at 220°C, followed by reduction in pure hydrogen at 220 or 250°C for 9 h. The catalyst was tested in a fixed-bed reactor at 220°C, 20 bar, H₂/CO ratio of 2 and a space velocity of 3.5 NL/g-cat/h. The figure shows thermogravimetric results during the first reduction step which decreases weight of the sample due to the Co(II,III) → Co⁰ transformation and weight gain during the carbidization step. After 14 h of carbidization, the stoichiometric Co₂C amount is obtained. Temperature programmed hydrogenation and oxidation of this sample showed evidence for the presence of Co₂C and absence of free carbon. CO conversion during FTS decreased from 59% (H₂ reduction) to 55% or 48% after carbidization-reduction pretreatment (at 220 and 250°C respectively); suggesting the need for future studies on alternative reduction/carbidization conditions. Methane selectivity increased from 8% for H₂ reduced catalyst to 11% after both carbidization-reduction pretreatments, probably due to presence of surface and/or bulk carbides. [1] Karaca H. et al. Journal of Catalysis 277 (2011) 14.