Selective Fischer–Tropsch synthesis over an Al 2O 3 supported cobalt catalyst in supercritical hexane

Abstract

Supercritical fluids (SCFs) offer several advantages as reaction media for catalytic reactions. These advantages include the ability to manipulate the reaction environment through simple changes in pressure to enhance solubility of reactants and products, to eliminate interphase transport limitations, and to integrate reaction and separation unit operations. Benefits derived from the SCF-phase Fischer–Tropsch synthesis (SCF-FTS) involve the gas-like diffusivities and liquid-like solubilities, which together combine the desirable features of the gas- and liquid-phase FT synthesis routes. In this paper, FT synthesis under SCF hexane conditions is examined in a continuous, high-pressure reactor by employing a traditional Co catalyst (15% Co–0.5% Pt/Al 2O 3). Steady-state operation was quickly achieved under SCF conditions and the SCF-FT process has a marked effect on the hydrocarbon product distribution with a shift to higher carbon number products owing to enhanced heat and mass transfer from the catalyst surface. In addition, an obvious difference in the olefin content was observed where the 1-olefin content in the SCF phase was always higher than in the gas phase. Based on the experimental observations, a mechanistic explanation is provided for the difference of the reaction behavior under supercritical and gas-phase environments. Enhanced olefins readsorption and increased availability of active sites in the supercritical state contribute to the increased olefin selectivity and chain growth probability in the supercritical phase. In addition, the effect of pressure tuning in the supercritical phase reaction was investigated as well as the effect of the supercritical medium on heat transfer and temperature distribution within the reactor.