Selective synthesis of middle isoparaffins via a two-stage Fischer-Tropsch reaction: Activity investigation for a hybrid catalyst

The activity of hybrid catalysts for the direct isoparaffin synthesis, especially C₄-C₆ with a very high octane number, from Fischer-Tropsch synthesis (FTS) in a two-stage reactor was investigated. The hybrid of Co/SiO₂ and H-ZSM-5 was used as the first-stage reaction catalyst, and different types of H zeolites alone or their hybrid with Pd/SiO₂ were used as the second-stage reaction catalysts. The obtained results demonstrated that the carbon number of products could terminate at 6 and isoparaffin hydrocarbons could be synthesized in very high selectivity from synthesis gas compared with that in a single-stage reaction using a physically mixed Co/SiO₂ and H-ZSM-5 catalyst. The first-stage Co/SiO₂ + H-ZSM-5 catalyst made the products mainly distribute in the C₁-C₁₁ range and considerably increase in the corresponding isoparaffins because of successive cracking, as well as isomerization, of the primary FTS products on H-ZSM-5. The selectivities of isoparaffins with much lower carbon number were further raised through the hydrocracking and isomerization of the first-stage reaction products on the second-stage H-zeolite catalysts in the presence of hydrogen. In addition, the effect of the reaction temperature suggested that an appropriate temperature combination of the first- and second-stage reactions was necessary. Using Co/SiO₂ + H-ZSM-5 and H-Beta + Pd/SiO₂ as the first- and second-stage catalysts, respectively, with a suitable temperature selection for the first- and second-stage reactor, a selectivity of 64.4% for C ₄-C₆ isoparaffins was achieved. With the hybrid of H-ZSM-5 in the first-stage catalyst, heavier hydrocarbon deposition was removed near Co/SiO₂ and the catalyst life was stabilized; with the hybrid of Pd/SiO₂ in the second-stage catalyst, the acid sites of H zeolites were maintained and strengthened by means of supposed hydrogen spillover. As a result of the overall effect, the lifetime of the catalysts was effectively extended.