Influence of Zeolite Surface in the Sorption of Methane from Molecular Dynamics

We report results of molecular dynamics simulations on the transient and equilibrium surface adsorption of methane in a model Si-LTA zeolite structure with explicit silanol terminal groups. Using probability occupancy maps, 2D occupational densities, and free energy profiles, we extract a microscopic picture of the adsorption process, showing surface x preferential zones of interaction around the partially exposed oxygen atoms of the surface silanol groups due to attractive silanol guest attractions. The global effect of the loading on the uptake process is observed as a decrease in the free energy barrier as the loading increases and an increase in the surface permeability, calculated either from the initial uptake transient process or from equilibrated trajectories. The order of magnitude of the obtained surface permeability suggests a barrier that becomes relevant to the overall mass transfer for crystal sizes in the range of a few unit cells. Our results support experimental evidence that ascribe surface resistances to severe crystal structure deviations, causing an almost complete blockage of the surface available entering windows.