Fischer-Tropsch synthesis over cobalt/montmorillonite promoted with different interlayer cations

Abstract

The effect of the montmorillonite (MMT) interlayer cations on the performance of Co/MMT catalysts for Fischer-Tropsch (FT) synthesis was comparatively investigated in this work. By ion-exchanging the natural Na-type MMT (Na-MMT) with K+, Mg2+, Zr4+, Fe3+, Ni2+, Cu2+, Zn2+, and Ce3+, respectively, MMTs with different interlayer cations were obtained after calcination at 500°C for 4h. The natural Na-MMT, Ca-MMT, and the ion-exchanged MMTs were loaded with 20wt.% Co by the incipient impregnation method. The materials were characterized by XRD, SEM/EDS, N2 adsorption–desorption at low temperature, H2-TPR, and NH3-TPD techniques. Results indicate that the textural properties of the MMTs were strongly dependent on the interlayer cations. Moreover, an enhanced acidity, which slightly depends on the interlayer cation, was observed for the ion-exchanged MMTs. Remarkably, the reduction behavior of the Co-supported samples was strongly dependent on the MMT interlayer cations. In a fixed-bed reactor, the Co/MMT catalysts were evaluated for FT synthesis under the conditions of 1.0MPa, 235°C, H2/CO=2, and W/F=5.02ghmol−1. Results indicate that the CO conversion was decreased in the order of Co/Zr-MMT>Co/Ce-MMT>Co/Ca-MMT≈Co/Fe-MMT>Co/Ni-MMT>Co/Mg-MMT≈Co/K-MMT>Co/Na-MMT>Co/Cu-MMT≈Co/Zn-MMT. Moreover, the product selectivity over Co/Zr-MMT, Co/Ce-MMT, Co/Mg-MMT, Co/Fe-MMT, and Co/Ni-MMT, i.e., 40–50wt.% of C4–C12 hydrocarbons and 7–20wt.% of C21+ hydrocarbons, was significantly different from that over Co/SiO2. The reaction results were well explained based on the characterization results of the samples. The reduction behavior of Co/MMT catalysts was revealed to be responsible for the significantly varied CO conversions while the acidity of the MMTs played the crucial role in determining the product distribution. Considering the simple ion-exchange method, high activity, and high selectivity to C4–C12 hydrocarbons, Co/Zr-MMT can be an important FT catalyst for selectively synthesizing liquid fuels.