Propane simulated in silica pores: Adsorption isotherms, molecular structure, and mobility

Abstract

Molecular dynamics simulations were conducted for bulk propane in contact with fully protonated slitshaped silica pores. The effective pore width was of either 0.8 or 2.7 nm. The temperature was set at 343, 368, and 373K. The TraPPE-UA and CLAYFF force fields were implemented to model propane and silica, respectively. Each individual simulation yields the density of confined fluid as a function of the bulk pressure. For a given temperature, adsorption isotherms were estimated by repeating the simulations at various bulk pressures. The results qualitatively agree with available experimental data; namely, at fixed temperature the excess sorption is found to show a maximum near the pressure at which the pores fill; at fixed pressure the excess adsorption is found to decrease as the temperature increases and as the pore width expands. At equilibrium, pronounced layering was observed for propane near the pore surface, especially in the narrower pore and at the highest densities considered. The propane molecules at contact with silica tend to lay with their CH 3-CH 3 vector parallel to the pore surface. The mean square displacement as a function of time was used to quantify the self-diffusion coefficient of confined propane as a function of temperature, pressure and pore width. These results will be useful for enhancing the interpretation of experimental data.

N-octane diffusivity enhancement via carbon dioxide in silica slit-shaped nanopores – a molecular dynamics simulation

“…6,7 We previously conducted extensive molecular dynamics (MD) simulations to study propane adsorption, structure and diffusion in slit-shaped silica pores at sub-, near-, and super-critical conditions. 8 The results were qualitatively consistent with the experimental adsorption isotherms reported by Gruszkiewicz et al, 9 and the SANS data reported by Rother et al 10 We recently conducted MD simulations for the structure and dynamics of CO2-butane mixtures confined within slit-shaped silica pores. 11 Preferential adsorption of carbon dioxide near the -OH groups on the surface was observed, where the adsorbed CO2 molecules tend to interact simultaneously with more than one -OH group.…”

“…To obtain the planar self-diffusion coefficient Ds we calculated the mean square displacement (MSD) following established procedures. 8 To investigate the effect of CO2 loading on the mobility of confined fluids, 7 different systems were simulated whose details can be found in Table 1. As the molecular density increases, the pressure in the pore is also expected to increase.…”

“…The effective pore volume was estimated in ~200.15 nm 3 . 8,14 Periodic boundary conditions were applied in all directions. Because of periodic boundary conditions, the systems considered are composed by silica slabs that are infinitely long along the X and Y directions, and separated along the Z direction by the slitshaped pore.…”

Molecular dynamics simulation study of meso-confined propane in TiO2

“…In mesoporous materials, propane diffusion shows a stronger dependence on loading and at higher loadings is found to have diffusion coefficients in silica aerogel (76 Â 10 À10 m 2 /s [25]) comparable to the present study. In a recent MD simulation study on propane diffusion in silica slit nanopores (2.8 nm) a much higher value of 2690 Â 10 À10 m 2 /s has been obtained for in plane diffusion [53]. The shape and size of the nanopores in this study are however quite different from our study and thus a direct comparison is difficult.…”

Grand Canonical Monte Carlo Simulation of Nitrogen Adsorption in a Silica Aerogel Model

“…However, he treated the composite as a crystal, which may not work for pure silica aerogel. Le [17] constructed a silica cylindrical channel and simulated propane adsorption through the MD method. Only qualitative agreement with experimental results was obtained.…”