Selective Hydrocracking of Fischer–Tropsch Waxes to High-quality Diesel Fuel Over Pt-promoted Polyoxocation-pillared Montmorillonites

Polyoxocation ([AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]⁷⁺ and [Zr₄(OH)₁₄(H₂O)₁₀]²⁺)-pillared montmorillonites were prepared by ion exchange, and they were used as solid acid catalysts, in comparison to some other solid acids, for the selective hydrocracking of Fischer–Tropsch waxes to diesel-ranged hydrocarbons. XRD patterns and elemental analyses proved that the polyoxocations were successfully introduced into the interlayer region of the montmorillonites. N₂ adsorption–desorption measurement indicated that polyoxocation-pillared montmorillonites have large BET surface areas (>230 m² g⁻¹), high thermal stability (>673 K), and large pores (comparing to those in zeolites). FT-IR spectra of chemisorbed pyridine indicated that polyoxocation-pillared montmorillonites possess both Lewis and Bronsted acid sites on the solid surface. Temperature-programmed desorption of ammonia indicated that the acidity strength of the polyoxocation-pillared montmorillonites was weaker than those of H–ZSM-5, H–Y, WO₃/ZrO₂, and Al₂O₃–SiO₂. In the hydrocracking of Fischer–Tropsch (F–T) waxes, the acidity strength of the solid acids in bifunctional catalysts greatly influences the product composition. Pt-promoted H–Y afforded a high yield of gasoline-ranged hydrocarbons (>90%) while Pt-promoted H–ZSM-5 afforded a larger amount of gas products due to its strong solid acid sites. On the other hand, among various catalysts, Pt-modified polyoxocation-pillared montmorillonites afforded the highest yield of diesel-ranged hydrocarbons (>70%) due to the appropriately weak acid strength, high thermal stability, large BET surface area, and large pore size. Pt is necessary for the hydrocracking of F–T waxes because it enables hydrogenation/dehydrogenation. However, a high Pt loading on the catalyst produces more light hydrocarbons due to the stimulation of hydrogenolysis. High hydrogen pressure improved the selectivity for diesel-ranged hydrocarbons but decreased the conversion of F–T waxes due to the suppression of alkane dehydrogenation. The hydrocracking of F–T waxes at a low temperature with a large amount of catalyst for longer reaction time increased the yield of diesel-ranged hydrocarbons.