Single-event microkinetic model for Fischer-Tropsch synthesis on iron-based catalysts

A single-event microkinetic (SEMK) model was developed for Fischer-Tropsch synthesis and applied to experimental data obtained on an iron-based catalyst in a temperature range from 523 to 623 K, total pressures from 0.6 to 2.1 MPa, and H-2/CO inlet ratios from 2 to 6 mol/mol. The use of the single-event concept allowed a significant reduction of the number of adjustable parameters. The single-event pre-exponential factors were calculated based on statistical thermodynamics. The reaction enthalpies, as well as initial guesses for the activation energies, were obtained through the unity bond index-quadratic exponential potential (UBI-QEP) method. Ten activation energies of the kinetically relevant reaction families and four atomic chemisorption enthalpies remained as adjustable parameters, the latter corresponding to so-called catalyst descriptors. The SEMK model describes well the product distribution over a wide range of operating conditions with physically sound kinetic parameters. The reductive elimination toward n-alkanes and the beta-hydride elimination involved in the formation of 1-alkenes with activation energies amounting to 117.8 and 96.3 kJ/mol are the two most kinetically significant steps determining the product distribution. In particular, the symmetry effects specifically accounted for by the single-event concept appeared critical in the interpretation of the deviations from the Anderson-Schulz-Flory distribution at low carbon numbers. Because of its fundamental character, the SEMK model developed here can be easily applied to describe Fischer-Tropsch synthesis over other catalysts.