Molecular dynamics simulation of the n-octacosane-water mixture confined in hydrophilic and hydrophobic mesopores: The effect of oxygenates

Abstract

Wax and water mixtures are the main products of the Gas-To-Liquids (GTL) process through the Fischer-Tropsch synthesis (FTS), along with relatively small quantities of oxygenated compounds such as alcohols. It has been established that water production in high concentrations leads to phase segregation inside the catalyst support pores. Excess water conditions are considered responsible for sintering of catalytic nanoparticles, which reduces catalyst lifetime and increases GTL operational cost. Apart from the effects of increased water concentration in wax – water FTS mixtures, the presence of oxygenates has not been studied at the molecular level. Moreover, the effect of the surface support chemistry on the wax – water mixture inside catalyst pores is essential in understanding mixture phase behavior. All these open questions are of significant interest for the petrochemical industry. The present study focuses on simulating by means of Molecular Dynamics (MD) the n-octacosane (n-C28) – water mixture inside titanium dioxide (TiO2) and silicon carbide (SiC) mesopores at low-temperature FTS conditions (473.15 K). The presence of small alcohols such as methanol and ethanol on the n-C28 – H2O mixture's phase behavior was also evaluated. The united atom TraPPE (TraPPE-UA) force field was used for n-C28 and the alcohols, while the SPC/E was used for water. Catalyst support atoms were modeled as Lennard-Jones spheres (charged or neutral) at fixed positions. Our simulations show that inside TiO2, water molecules organize into two discrete layers on the TiO2 surface, with n-C28 occupying the pore center; mixture phase separation is completely reversed inside the SiC pore. In both cases, alcohol addition does not influence either n-C28 or H2O distribution inside the pores. Furthermore, we examine the hydrogen bonds formed between water molecules and how these are influenced upon alcohol addition and confinement. Finally, through the calculation of lateral diffusion coefficients, we provide insights on the mobility of the individual n-C28 – H2O mixture components inside the pores considered.