A series of core–shell catalysts with cobalt nanoparticles coated by silica shells were prepared to provide an insight into the effects of the shell thickness on the Fischer–Tropsch synthesis. The catalysts displayed uniform silica shell thicknesses in the range of 4.3–18.2 nm as ascertained by TEM. From the H₂ chemisorption results, increasing the shell thickness did not reduce the number of active sites due to the similar active cobalt surface areas. Even though the reducibility determined by H₂-TPR decreased rapidly with the increase in shell thickness, the catalytic activity was not evidently reduced. The hydrocarbon products shifted to shorter chains and the C₁₅–C₁₈ selectivity had a volcano-type dependence as the shell thickness increased, which is probably because thicker shells contribute to more severe diffusion limitations of the reactants.

Highly siliceous ZSM-22 was synthesized under hydrothermal conditions with ZSM-22 seeds. Characterization by FT-IR and ¹H and ²⁹Si NMR spectroscopy confirms that due to the existence of defective sites in the siliceous ZSM-22, acidic silanol groups including vicinal silanol and silanol nests are detected. Acidic silanol groups are considered as the acid sites of siliceous ZSM-22. As a result, siliceous ZSM-22 shows much lower acid site density and lower acid strength than silica–alumina ZSM-22 which contains hydroxyl groups bridging Si and Al as acid sites. In n-dodecane isomerization, Pt/siliceous ZSM-22 shows much higher isomer selectivity than Pt/silica–alumina ZSM-22. Furthermore, a lower loading amount of Pt on siliceous ZSM-22 (about 0.2 wt% Pt) can lead to high isomer selectivity. Owing to the acidic silanol groups and the low acid site density of siliceous ZSM-22, a good balance between metal and acid function in Pt/siliceous ZSM-22 is achieved, which results in a lower allowable Pt loading value and higher isomer selectivity.

Internal diffusion limitations in Fischer–Tropsch catalysts strongly affects their catalytic activities and product selectivities.