Effective activation of montmorillonite and its application for Fischer-Tropsch synthesis over ruthenium promoted cobalt

• Montmorillonite (MMT) in a high concentrated suspension was activated with HNO3.
• Reduction of Co/MMT was affected by the addition of Ru and activation extent of MMT.
• High selectivity to liquid FT hydrocarbon and low selectivity to wax was achieved.
• Co-Ru/acid-MMT is a promising FT catalyst for selectively synthesizing liquid fuels.

The natural Na-type montmorillonite (Na-MMT) suspended in the 20 wt.% HNO3 aqueous solution at concentrations between 2-20 wt.% was activated at 70–104 °C for a period, respectively. The thus obtained materials were abbreviated as acid-MMT-m-t/T, where m, t and T are the concentration of Na-MMT suspension (wt.%), time (h), and temperature (°C) during the acid activation, respectively. The Co-Ru/acid-MMT (20 wt.% Co and 0.1 wt.% Ru) was prepared by the incipient impregnation method. The materials were characterized by XRD, FT-IR, XRF, N2 adsorption-desorption at low temperature, H2-TPR, H2-TPD, oxygen titration, and NH3-TPD techniques. In comparison to the results of the Na-MMT, the structural, textural, and acidic properties of the acid-MMTs were clearly changed, the extent of which is strongly dependent on the acid activation conditions. For the first time, the acid-MMT with desired properties were obtained using a high ratio of Na-MMT suspension, such as 10 wt.%, leading to a significantly improved process efficiency of the acid activation. The structural and compositional changes of acid-MMTs gave significant impacts on the reduction behavior of Co-Ru/acid-MMT catalysts. Moreover, the addition of Ru to Co/acid-MMTs decreased the peak temperatures of the two-step reduction of Co3O4 to Co via the CoO intermediate, leading to an increased extent of reduction of cobalt. The catalysts were comparatively investigated for Fischer-Tropsch (FT) synthesis in a fixed-bed reactor under the conditions of H2/CO = 2, W/F = 5.0 g · h · mol− 1, 235 °C, and 1.0 MPa. Results showed that the CO conversion was decreased in the order of Co-Ru/acid-MMT-2-12/104 > Co-Ru/acid-MMT-10-12/104 > Co-Ru/acid-MMT-20-12/104 ≫ Co-Ru/acid-MMT-2-12/70 > Co-Ru/acid-MMT-2-4/70 > Co/acid-MMT-2-4/70. Moreover, all of the Co-Ru/acid-MMT catalysts showed clearly higher selectivity of C5-C12 hydrocarbons and lower selectivity of C21+ hydrocarbons compared with Co/SiO2 catalyst. The Co-Ru/acid-MMT-2-12/70 showed about 46% C5-C12 selectivity at a reasonable CO conversion level.

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