Detailed Kinetics of the Fischer–Tropsch Synthesis on Cobalt Catalysts Based on H-Assisted CO Activation

Abstract

A complete mechanistic kinetic model of the Fischer–Tropsch synthesis (FTS) over a Co/Al2O3 state-of-the-art catalyst is developed here under conditions relevant to industrial operation. On the basis of the most recent findings on the reaction mechanism, here described according to the H-assisted CO dissociation theory and the alkyl chain growth mechanism, and on the basis of the latest indications available on the rate determining step involved in the CO activation process, rate expressions for all the steps leading to CO and H2 consumption and n-paraffins, α-olefins and H2O formation are derived. Such expressions are functions of the molar fraction of CO, H2 and olefins in the liquid phase surrounding the catalyst pellets, that in turn are related to the gas-phase pressure and composition, and of the surface coverage of the adsorbed species. Thermodynamic and kinetic parameters involved in the model are estimated by non-linear multi-response regression on a complete set of FTS experimental data collected in a lab-scale tubular reactor at steady-state conditions (P = 8–25 bar, T = 210–235 °C, H2/CO feed ratio = 1.8–2.7 mol mol−1, GHSV = 2,000–7,000 cm3(STP) h−1 gcat−1). Both the experimental CO conversion and the hydrocarbon distribution (up to N = 50) in FTS are well predicted by the model with 13 adaptive parameters, without the need of introducing any empirical parameter. Analysis of the model offers insight in the rates of the elementary steps associated with the reaction mechanism and in the surface coverages of the adsorbed species. Such information explains the peculiar reactivity observed over cobalt-based Fischer–Tropsch catalysts, and can provide guidelines for the design of more active and selective catalytic materials.