Insights into Structural and Chemical Properties of Activated Montmorillonite for Fischer-Tropsch Synthesis over Supported Cobalt Catalysts

Abstract

A Na-type montmorillonite (Na-MMT) activated with HNO3 at elevated temperatures for different times was investigated for the selective synthesis of liquid fuels via the Fischer-Tropsch synthesis over 20 wt.% Co-supported catalysts under the conditions of 1.0 MPa, 235 °C, H2/CO=2, and W/F=5.02 g·h·mol-1. Although Co/Na-MMT is not a suitable catalyst for the FT synthesis, high CO conversion was achieved over Co/activated MMT catalysts, which is similar to that of typical Co/SiO2 catalyst of the FT synthesis. Moreover, the product selectivity over Co/activated MMT, i.e., clearly higher selectivity of C4-C12 hydrocarbons and obviously lower selectivity of C21 + hydrocarbons than those over Co/SiO2, was significantly deviated from the Anderson-Schulz-Flory distribution of the FT synthesis. Among the catalysts tested, Co/Acid-MMT-12/80 was the best catalyst to control the product distribution of the FT synthesis. The materials were characterized by XRD, FTIR, SEM, N2 adsorption-desorption at low temperature, H2-TPR, and NH3-TPD techniques. Depending on the activation conditions, the acidity and textural properties of the activated MMT were significantly changed due to the following events occurred in different degrees: the leaching of the substituted cations, the exchanging of H+ with the interlayer Na+, and the destruction of the MMT interlayer structure. Ultimately, the MMT laminar structure was completely destructed after activation of Na-MMT at 100 °C for 24 h. The reaction results were well explained based on these characterization data. The content of Na+ was revealed as the main factor to control the FT activity, and the acidity of the activated MMT was responsible for the narrowed product distribution. Considering the simple procedure for the activation of MMT, Co/activated MMT is a promising catalyst for selectively synthesizing liquid fuels via the FT route provided the MMT is activated under suitable conditions.