Molecular mechanism of selective adsorption and separation of n-butane/i-butane on MFI zeolite

MFI zeolite is one of the most attractive molecular sieve materials for selective adsorption and separation of C4 hydrocarbon isomers. Grand Canonical Monte Carlo and molecular dynamics simulations were used to explore the correlation between the channel structure, adsorption thermodynamics, and kinetic behavior of butane isomers. The results of single component and binary competitive adsorption isotherms of n-butane/i-butane at 273 K, 303 K, 343 K, and 373 K showed excellent low pressure loading and adsorption selectivity for n-butane on MFI zeolite. Density distribution maps and Radial Distribution Function data revealed different preferential adsorption sites for n-butane and i-butane molecules in MFI zeolite. Because of van der Waals repulsion, the adsorption energy of i-butane is less than that of n-butane, making i-butane less apt to be adsorbed in the sinusoidal and straight channels but preferentially in the cross channel of MFI zeolite. Additionally, the mean square displacement and the calculated diffusion coefficient showed that the diffusion rate of n-butane is faster than that of i-butane by more than two orders of magnitude. These findings clarify the molecular mechanism of selective adsorption and separation of n-butane/i-butane on rationally designed MFI zeolite-based materials.