(119b) Hydrocracking of Cobalt-Based Fischer-Tropsch Wax Over Pt/ZSM-5 and Pt/SiO2-Al2O3 Catalysts

Hydrocracking is a key step in the industrial Coal-to-Liquid (CTL) process as it converts heavy hydrocarbons produced by Fischer-Tropsch synthesis over iron and cobalt catalysts into high quality transportation fuels such as gasoline (C5-C11) and diesel (C12-C19). However, the choice of catalyst and process conditions play decisive roles in controlling the hydrocracking efficiency, impacting both conversion and selectivity. In this contribution, we studied the operating conditions over two 0.5%Pt catalysts supported on a molecular sieve, ZSM-5, and an amorphous silica-alumina (Davison) carrier. Both catalysts (0.5%Pt/ZSM-5 and 0.5%Pt/SiO2-Al2O3) were prepared by an ion-exchange method. The catalysts were tested in a fixed-bed reactor under a wide range of process conditions (e.g., 250-340 oC, 400-700 psig, and H2/wax ratios of 5-30 and liquid hourly space velocities (LHSV) of 1-30). Wax conversion, product selectivities and yields (including degrees of hydrocracking and isomerization on the catalysts with the condition variables) were determined. Both catalysts displayed higher activity at higher temperatures, pressures and H2/wax ratios. Although increasing LHSV resulted in relatively low wax conversion, higher selectivities to products in the diesel range were obtained. The optimized LHSV for formation of diesel over Pt/ZSM5 is 5. The 0.5%Pt/ZSM-5 catalyst was found to be more active than the 0.5%Pt/SiO2-Al2O3 catalyst; however, it appears that the SiO2-Al2O3 support produced a greater fraction of hydrocarbons in the diesel range. The carbon number of the hydrocarbon that began to participate in wax hydrocracking was lower for the ZSM-5 supported catalyst compared with the SiO2-Al2O3 supported catalysts (13 versus 15-16). Furthermore, hydrocraking and isomerization reactions were observed to take place simultaneously on both catalysts, but the isomeration reaction strongly depends on hydrocarbon chain length. The isomerization reaction rate peaks at about 10, and little isomerization occurred at chain lengths greater than 18.