Fischer–Tropsch synthesis over Co/montmorillonite—Insights into the role of interlayer exchangeable cations

Abstract

To develop an efficient catalyst for controlling the product distribution of the Fischer–Tropsch (FT) synthesis, the natural Na-type montmorillonite (Na-MMT) was modified by ion-exchanging the interlayer Na + with selected cations, NH 4 +, Co 2+, and Al 3+. The degree of the interlayer Na + replaced by the selected cations was regulated by changing the ion-exchange conditions. A 20 wt.% Co/MMT catalyst was prepared by the incipient impregnation method. The materials were characterized by XRD, SEM/EDS, N 2 adsorption–desorption at low temperature, H 2-TPR, and NH 3-TPD techniques, and the results indicate that the MMT interlayer structure was well reserved for all the ion-exchanged MMTs. Moreover, acidic sites, rich micropores and open mesopores were created after exchanging the Na-MMT with NH 4 +, Co 2+, or Al 3+. The Co-supported catalysts were investigated for the FT synthesis in a fixed-bed reactor under the conditions of 1.0 MPa, 235 °C, H 2/CO = 2, and W/F = 5.02 g h mol −1. The results show that the Co/Na-MMT is an inefficient catalyst for the FT synthesis. In contrast, the Co/ion-exchanged MMT showed sharply increased CO conversion. The CO conversion was dependent on the ion-exchange conditions used, however. Moreover, the product selectivity over Co/MMT, i.e., 40–47% C 4–C 12 hydrocarbons and 13–20% C 21 + hydrocarbons, significantly deviates from the Anderson–Schulz–Flory distribution in the FT synthesis over the typical Co/SiO 2 catalyst. Based on the catalysts characterization, the difference of the FT activity as a result of interlayer cation can be explained as the different reduction behavior of the catalysts. Due to the cracking of the long-chain FT hydrocarbons, a narrow product distribution over the Co/MMT catalyst was observed, which correlates well with the NH 3-TPD results.