Molecular Simulation of n-Octacosane–Water Mixture in Titania Nanopores at Elevated Temperature and Pressure

Abstract

The transport properties of wax and water mixtures under confinement and particularly inside catalyst nanopores is a topic of significant interest for the petrochemical industry. These mixtures are the products of the Gas-To-Liquids (GTL) process through the Fischer−Tropsch (FT) route, which experienced an increasing number of commercially viable applications over the past decades. Under reaction conditions, water is produced in high concentrations, leading to phase segregation inside the catalyst nanopores and water-assisted sintering of catalytic nanoparticles, reducing catalyst lifetime and increasing GTL operational cost. It is thus important to understand the wax−water liquid−liquid equilibrium (LLE) at reaction conditions, as it determines the maximum allowable amount of water in the FT wax. Furthermore, elucidating the phase behavior of wax−water mixture inside the nanopores, by explicit incorporation of wall effects, is essential in revealing the role of confinement on mixture phase behavior. The present study focuses on simulating the phase behavior of the n-octacosane (n-C 28 )−water mixture inside TiO 2 nanopores. Molecular Dynamics (MD) simulations with realistic molecular models were employed, highlighting the importance of confinement on the mixture transport properties, particularly in the excess water regime. Even though phase segregated mixtures retain their structural properties compared to their bulk counterparts, significant deviations arise in terms of density profiles inside the nanopore. Water molecules organize into two discrete layers on the TiO 2 surface, shielding n-C 28 from the nanopore walls. Octacosane's self-diffusion is not influenced by confinement; water on the other hand is severely hindered by the TiO 2 nanopore surface, with its diffusivity bearing a strong dependence on the distance from the nanopore center.

Water-wax behaviour in porous silica at low temperature Fischer-Tropsch conditions

“…This will arise both from the presence of remaining n-C28 slowing down the motion of water molecules within the water-rich surface layer and the confinement effect of the water-rich surface layer between the pore surface and the oil-rich phase in the centre of the pore. These results are in excellent agreement with the observation from the simulation of water-C28 equilibrium in a 5.5 nm width TiO2 pore at 200 C reported by Papavasileiou et al [25] who showed that even under conditions close to full water saturation with water mole fraction of 0.9655 in the pore, the water molecules remained in a phase separated layer adjacent to the pore surface, with n-C28 molecules present in the pore centre. The restriction of the motion of water molecules in the surface layer adjacent to the pore surface was also observed in that work.…”

“…It was predicted that the capillary condensation of water could greatly influence catalyst performance under the reaction conditions by causing significant reduction in surface area available for reactants due to the low solubility of CO and H2 in water [23,24]. Papavasileiou et al [25] studied the phase behaviour of a water and n-octacosane (n-C28) mixture in TiO2 nanopores using molecular dynamics simulation. They reported that n-C28 was isolated from the surface by water which, as the surface-wetting phase, showed a density profile and diffusivity significantly different from those of bulk water as a function of distance from the pore wall [25].…”

“…Papavasileiou et al [25] studied the phase behaviour of a water and n-octacosane (n-C28) mixture in TiO2 nanopores using molecular dynamics simulation. They reported that n-C28 was isolated from the surface by water which, as the surface-wetting phase, showed a density profile and diffusivity significantly different from those of bulk water as a function of distance from the pore wall [25].…”

“…In future work, modifications to the pore size and surface chemistry of the SiO2 studied here will be considered. n-C28 is chosen to model the FT wax as in previous studies [25,26]. The system is studied using three operando NMR techniques: 1 H NMR spectroscopy, 1 H pulsed-field gradient (PFG) NMR, and 1 H spin-lattice relaxation time (T1) measurements.…”

Recent advances in theoretical investigation of titanium dioxide nanomaterials. A review

“…The threshold radius was observed to be 50 Å for a length of 300 Å [60]. The phase behavior of the n-octacosane (n-C 28 )−water mixture inside TiO 2 nanopores was considered with classical MD simulations [61]. Three model systems having different TiO 2 (101) anatase pore sizes of 34, 40, and 55 Å were examined (Fig.…”