(178f) Molecular and Microkinetic Modeling of Supercritical Fluid and Liquid-Phase Fischer-Tropsch Synthesis

Due to the specific and tunable properties of supercritical fluids (SCFs), they have been considered as unique media for different technical processes of extraction, separation, particle formation, and chemical reactions. In the particular case of Fischer-Tropsch synthesis (FTS), experiments have shown that utilizing supercritical solvent in this heterogeneous process would result in several distinguished improvements. Accelerating the conversion of reactants, providing more favorable product distributions, as well as preventing catalyst deactivation are some of the mentioned advantages.[1] While previous experimental studies have discussed that all these known enhancements (such as better heat distribution, improved mass transfer, selective solubility of coking products, etc.[2]) relate to solution thermodynamics and transport, not many investigations have considered the solvent effect(s) on heterogeneous kinetics of FTS reactions. In this study, we have analyzed the influence of hexane solvent on the reaction rates of FTS process over Co(0001) catalyst using different theoretical approaches of molecular dynamics and quantum mechanics. By adopting an existing microkinetic model[3] and modifying it, potential of mean forces (PMF) and density functional theory (DFT) calculation have been performed in order to estimate the equilibrium conditions and rate parameters of individual elementary reactions under different phase conditions. Ultimately, these kinetic and thermodynamic parameters determined from molecular modeling and simulation are used as input into a micro-kinetic model, to characterize the differences in catalysis in SCF and liquid hexane versus conventional gas-phase processing. Microkinetic modeling results include yield and selectivity versus reaction time and net rate and sensitivity analyses of the elementary reaction mechanisms in all the considered reaction phases. The simulation results show that the SCF and liquid phase solvent not only have significant effects on the thermodynamics of the adsorption/desorption reactions, but also they may alter the kinetics of surface reactions toward selecting certain reaction pathways.

Keywords: Fischer-Tropsch synthesis, supercritical fluid, microkinetic modeling, PMF, DFT