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

摘要

Polyoxocation ([AlO4Al_(12)(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 die-sel-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 m~2 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/dehy-drogenation. 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.