Reaction and deactivation kinetics for Fischer-Tropsch synthesis on unpromoted and potassium-promoted iron catalysts

Rate data for Fischer-Tropsch synthesis on unsupported Fe and Fe/K catalysts were obtained at 10 atm total pressure, 200-240 degrees C and H-2 and CO partial pressures of 0.9-1.4 and 0.4-0.9 arm, respectively Based on a simple power-law expression, reaction orders with respect to H-2 and CO are 1.4 and -0.42 for the unpromoted Fe catalyst and 0.60 and -0.05 for the K-promoted Fe catalyst. A mechanistically-based rate model for FT synthesis is shown to be consistent with the kinetic data obtained for the Fe catalyst. Deactivation rate data were obtained during synthesis on the same catalysts at 10 atm total pressure, 250-280 degrees C and H-2 and CO partial pressures of 0.6-1.4 and 0.3-0.9 atm, respectively, for periods of 24-65 h; from a fit of these data to a generalized power-law expression (GPLE) of the form r(d) = k(d)'(a - a(infinity))(m), values of the deactivation order m are found to be unity for both catalysts. Activation energies for deactivation are 108 and 58 kJ/mole for Fe and Fe/K catalysts, respectively. A deactivation model involving parallel paths is proposed, i.e, simultaneous conversion of (a) atomic to polymeric to graphitic carbon and (b) active carbon-rich carbides to inactive carbon-poor carbides. (C) 1999 Elsevier Science B.V. All rights reserved.