Middle distillate fuels from Fischer-Tropsch wax using anion-modified zirconia catalysts.

摘要

Platinum-promoted anion-modified zirconia catalysts, such as tungstate-modified zirconia and sulfate-modified zirconia were studied in the hydrocracking and hydroisomerization of long-chain alkanes in Fischer-Tropsch waxes for producing high yields of middle distillates (C10-C 20) transportation fuels.; Model compounds such as n-C24 and n-C36 were used as reactants and the reactions were carried out in a microautoclave reactor system. To compare the catalytic performances of various catalysts, a set of particular reaction conditions, such as 200°C, 300-psi initial hydrogen pressure, 25-minute reaction time and 4:1(wt) reactant to catalyst ratio were applied. For catalyst samples with platinum (Pt), the Pt concentration was chosen as 0.5 wt%.; In the conversion of long-chain normal paraffins, Pt-promoted tungstated zirconia (PtWZr) has a high selectivity to hydroisomerization and Pt-promoted sulfated zirconia (PtSZr) is a very strong cracking catalyst. Either PtWZr or PtSZr alone gives a low yield of middle distillate products. It is found that a hybrid catalyst system consisting of PtWZr and SZr effectively combines the functions of the two components and leads to increased yields of middle distillates under the same reaction conditions. The increased reactivity and selectivity to middle distillates of PtWZr/SZr suggests active beneficial interactions between the components. The performance of PtWZr/SZr hybrid catalysts is affected by the component ratio. For a given reaction condition, there is an optimum component ratio for a maximum yield of middle distillates. Hydrogen pressure is an important factor affecting the reactivities and selectivities of the two types of anion-modified zirconia catalysts. Higher yields of middle distillates are obtained from n-C36 with PtWZr/SZr catalysts at 300-psi hydrogen pressure than are obtained at 100-psi hydrogen pressure.; PtWZr/zeolite hybrid catalysts were also prepared and tested in the conversion of n-C36. Mordenite, Y zeolite and Beta zeolite with similar Si/Al ratios were used as the second component. The hybrid catalysts have a higher reactivity in the production of middle distillates than the individual components.; Characterization tests, such as SEM, EDX, XRD, TPD, BET and CO chemisorption showed that the hybrid catalysts prepared in this research are physical combination of their components.; This research demonstrated a novel approach of using hybrid anion-modified zirconia catalysts for producing middle distillate fuels. The work furnishes a valuable foundation for further investigations.