Microkinetics of oxygenate formation in the Fischer-Tropsch reaction

Microkinetics simulations are presented on the intrinsic activity and selectivity of the Fischer-Tropsch reaction with respect to the formation of long chain oxygenated hydrocarbons. Two different chain growth mechanisms are compared: the carbide chain growth mechanism and the CO insertion chain growth mechanism. The microkinetics simulations are based on quantum-chemical data on reaction rate parameters of the elementary reactions steps of the Fischer-Tropsch reaction available in the literature. Because the overall rate constant of chain growth remains too low the CO insertion chain growth mechanism is not found to produce higher hydrocarbons, except for ethylene and acetaldehyde or the corresponding hydrogenated products. According of the carbide mechanism available quantum-chemical data are consistent with high selectivity to long chain oxygenated hydrocarbon production at low temperature. The anomalous initial increase with temperature of the chain growth parameter observed at such conditions is reproduced. It arises from the competition between the apparent rate of C-O bond activation to produce "CHx" monomers to be inserted into to the growing hydrocarbon chain and rate of chain growth termination. The microkinetics simulations data enable analysis of selectivity changes as a function of critical elementary reaction rates as rate of activation of C-O bond of CO, insertion rate of CO into the growing hydrocarbon chain or rate constant of methane formation. Simulations show that changes in catalyst site reactivity affect elementary reaction steps differently. This has opposing consequences for oxygenate production selectivity, so that an optimizing compromise has to be found. The simulation results are found to be consistent with most experimental data available today. It is concluded that Fischer-Tropsch type catalysis has limited scope to produce long chain oxygenates with high yield, but there is an opportunity to improve yield of C2 oxygenates.