(560fa) DFT Modeling of Liquid Solvent Effects on the Catalytic Surface Reactions in Fischer-Tropsch Synthesis

Authors: 
Asiaee, A., University of Mississippi
Benjamin, K. M., South Dakota School of Mines and Technology
For decades, the process of Fischer-Tropsch synthesis (FTS) has been studied and applied to produce valuable hydrocarbon products, such as gasoline, from synthesis gas (Syngas). The early commercial designs of this heterogeneous catalytic process were developed under gaseous-phase conditions. Later the process was industrialized under liquid-phase conditions to overcome the drawbacks of gaseous process, such as catalyst deactivation and high yield of methane, as one of the main undesired products. Majority of the literature discussed that the observed enhancements in the liquid-phase FTS are principally due to a better heat distribution in the reactor and the selective solvation properties of the liquid solvent. We have performed first-principle density functional theory (DFT) calculations to study the elementary reactions of FTS on a flat cobalt catalyst surface. A rigorous methodology has been applied from both energetic and entropic aspects to evaluate the reaction rate parameters of the elementary surface reactions to address the challenges in determining the dominant mechanism of reactants’ activation and hydrocarbon chain growth*, which have been a matter of debate between the computational and the experimental communities in this field. Ultimately, using the continuum-based conductor-like screening model (COSMO), liquid hexane solvent was included in our DFT study. We found that presence of the liquid solvent also affects the surface reactions; particularly in the intermediate reactions with oxygenate species**. The solvent effects on the kinetics and thermodynamics of these elementary reactions enhanced additional chain growth pathways besides the dominant mechanism, resulting in a product distribution that is in agreement with experimental observations.

References:

*Asiaee, A. and Benjamin, K. M. Molecular Catalysis 436 (2017) 218-227

** Asiaee, A. and Benjamin, K. M. Molecular Catalysis 436 (2017) 210-217