Effect of Sodium on the Physicochemical and Catalytic Properties of Co–Al2O3/SiO2–ZSM-5 Hybrid Catalysts in Fischer–Tropsch Synthesis

The effect of sodium and the method of its introduction on the physicochemical and catalytic properties of hybrid cobalt–zeolite catalysts in Fischer–Tropsch synthesis (FTS) was studied. Hybrid catalysts containing Co–Al2O3/SiO2 and ZSM-5 zeolite were prepared by mixing the components followed by extrusion. XRD, H2-TPR, NH3-TPD, and IR spectroscopy of adsorbed pyridine showed that the location of sodium determines its effect on catalyst structure and activity. The introduction of sodium into the Co–Al2O3/SiO2 catalyst was shown to increase the Co0 particle size from 9.0 to 14.0 nm and enhance the activity: CO conversion increases from 76.4 to 85.3%, while the C5+ productivity increases to 114.5 kg/(mcat3 h). In contrast, the introduction of sodium into the zeolite decreases the total acidity from 640 to 589 μmol/g and results in a substantial decrease in activity, with CO conversion decreasing from 85.6 to 66.2%, C5+ productivity decreasing by 35%, and the fractional composition shifting toward a higher proportion of high-molecular-weight C19+ hydrocarbons, from 8.3 to 15.6%. The residual sodium content in the zeolite was found to be a critical parameter determining the efficiency of the hybrid catalyst.