Preparation of Co/Al2O3 catalyst for Fischer–Tropsch synthesis: Combination of impregnation method and homogeneous precipitation method

Abstract

Co(20wt.%)/γ-Al2O3 catalysts were prepared by four methods, i.e., impregnation method (Imp), two kinds of precipitation methods using aqueous solution of ammonia (Pa) or ammonium oxalate (Pao) as the precipitant and homogenous precipitation method (HP). The prepared Co/Al2O3 catalysts were characterized by XRD, H2-TPR, H2 chemisorption and TEM-EDS, and the catalytic activity for Fischer–Tropsch (FT) synthesis was evaluated with the continuously stirred tank reactor. For the Co/Al2O3 catalysts prepared by the Imp and Pa methods, the reducibility of Co and the surface area of Co metal were low, but the intrinsic activity per active Co metal species (TOF) was high. On the other hand, for the Co/Al2O3 catalysts prepared by the Pao and HP methods, the reducibility of Co and the surface area of Co metal were high, but the TOF was low. Thus, the overall reaction rate increased in the following order: Pao<HP<Pa<Imp. Combination of the Imp and HP methods was also examined for the preparation of Co/Al2O3 catalysts; that is to say, part of Co was loaded by the HP method, followed by loading with the rest of Co by the Imp method. Co/Al2O3 catalysts prepared by this two-step procedure (Co(x, Imp)/Co(y, HP)/Al2O3, x+y=20) showed higher activity than those prepared by either the Imp method (Co(20, Imp)/Al2O3) or the HP method (Co(20, HP)/Al2O3), and the Co(15, Imp)/Co(5, HP)/Al2O3 catalyst showed the best performance. The high activity of the Co(x, Imp)/Co(y, HP)/Al2O3 catalyst is originated from the coexistence of two different kinds of Co species, i.e., easily reducible Co species derived from the HP method and highly active Co species derived from the Imp method. These two kinds of Co species cooperatively promoted the reduction and the dispersion of Co metal to enhance the overall catalytic activity.