Molecular Dynamics Simulation of Pore-Size Effects on Gas Adsorption Kinetics in Zeolites

Strong gas-mineral interactions or slow adsorption kinetics require a molecular-level understanding of both adsorption and diffusion for these interactions to be properly described in transport models. In this combined molecular simulation and experimental study, noble gas adsorption and mobility is investigated in two naturally abundant zeolites whose pores are similar in size (clinoptilolite) and greater than (mordenite) the gas diameters. Simulated adsorption isotherms obtained from grand canonical Monte Carlo simulations indicate that both zeolites can accommodate even the largest gas (Rn). However, gas mobility in clinoptilolite is significantly hindered at pore-limiting window sites, as seen from molecular dynamics simulations in both bulk and slab zeolite models. Experimental gas adsorption isotherms for clinoptilolite confirm the presence of a kinetic barrier to Xe uptake, resulting in the unusual property of reverse Kr/Xe selectivity. Finally, a kinetic model is used to fit the simulated gas loading profiles, allowing a comparison of trends in gas diffusivity in the zeolite pores.

Automated summary

Strong gas-mineral interactions and slow adsorption kinetics affect gas mobility in zeolite pores, especially at pore-limiting window sites. This study combines molecular simulations and experiments to investigate noble gas adsorption and mobility in two zeolites. The results show that even the largest gas (Rn) can be accommodated in both zeolites, but gas mobility is significantly hindered in clinoptilolite. The experimental results confirm the presence of a kinetic barrier to Xe uptake, resulting in reverse Kr/Xe selectivity. A kinetic model is used to compare gas diffusivity in the zeolite pores.