Direct synthesis of isoparaffin from synthesis gas under supercritical conditions

To produce isoparaffins from synthesis gas directly, modified Fischer–Tropsch (FT) synthesis was carried out under supercritical conditions using n-butane as a medium. One-step FT synthesis using a hybrid catalyst consisting of Co/SiO2, HZSM-5 and Pd/SiO2 was carried out. Introduction of supercritical-phase n-butane increased light isoparaffins significantly and suppressed the formation of the by-product, methane. Under supercritical-phase butane, hydrogenolysis and isomerization reactions were promoted. Due to the fact that the optimum temperatures for FT and HZSM-5 catalysts are different, 513 K and over 573 K, respectively, two-step FT synthesis was also carried out to optimize the reaction temperatures. The first-step reaction used Co/SiO2 catalyst containing small amount of HZSM-5 for FT synthesis at 513 K, and the second-step reaction used a hybrid catalyst containing Pd/SiO2 and zeolite for hydrogenolysis and isomerization of hydrocarbons at 573 K. Introduction of supercritical n-butane increased the isoparaffin selectivity, and decreased the methane selectivity significantly. The production of heavy hydrocarbons C9+ was inhibited in both gas and supercritical phase. The isoparaffin selectivity in the gas phase decreased with time-on-stream, but very stable for the supercritical-phase reaction. Because water and heavy hydrocarbons were removed from active sites on zeolite and the zeolite acidity was promoted in the supercritical medium, the selectivity of isoparaffin was considered stable. Among zeolites added to the hybrid catalyst in the second-step reactor, HZSM-5 and H-beta zeolite were suitable for producing light isoparaffins. These results indicated that two-step FT synthesis under supercritical n-butane was superior for producing light isoparaffins from synthesis gas directly.