Coarse-Grained Molecular Dynamics Simulation of Cobalt Nanoparticle in the n‑Octacosane–Water Mixture: The Effect of Water Concentration and Nanoparticle Size

The Fischer–Tropsch synthesis (FTS) is central in the Gas-to-Liquids (GTL) process, which is commercially used in the production of environmentally benign transportation fuels from synthesis gas. However, several factors can impede the performance of FTS reactors and thereby increase the overall GTL cost, among which is water in excess concentrations. Water is a byproduct of the FTS process and is present in varying amounts. In order to gain a better understanding of the behavior of catalyst nanoparticles (NPs) inside the produced wax–water mixtures at reaction conditions and realistic scales and sizes, one needs to incorporate coarse-grained approaches in the context of FTS. The present study focuses on coarse-grained (CG) Molecular Dynamics (MD; CGMD) simulations of n-octacosane (n-C28)–water mixture at low-temperature FTS conditions (473.15 K) and considers various water mole fractions with the inclusion of different-sized Co NPs using the MARTINI force field. The Co NPs are suspended inside these bulk mixtures thereby resembling a slurry phase type of reactor. Water mole fractions below and above the solubility of water in n-C28 as predicted by MARTINI were used so as to better capture the phase behavior of the produced wax–water mixtures. Our results show that, for the NP sizes examined, water in small concentrations does not aggregate on the Co NP surface. For water concentrations above the calculated solubility in n-C28, water forms a cluster that fully encapsulates the NP, and for excess water, the NPs are completely immersed in the aqueous phase. This behavior bears implications in the mobility of the NPs examined, which appears to increase as a function of water concentration. Calculations of Co NPs self-diffusion coefficient show that the smaller nanoparticle moves faster, and that mobility drops as nanoparticle size increases. For high water mole fractions, that is, well-exceeding water solubility in bulk n-C28, an increasing effect in the mobility of the examined Co NPs was observed. The computational approach using the MARTINI force field and the results presented showcase that CGMD simulations can be employed to study the hydrodynamic effects on the NP mobility in FTS-related processes.