Siting and segregation effects of simple molecules in zeolites MFI, MOR, and BOG

The effect of the spatially heterogeneous nature of zeolite pores on molecular arrangement and adsorption is investigated. Three zeolites (MFI, MOR, and BOG) are chosen to represent structures containing dissimilar pore shapes and sizes. Four simple adsorbates (argon, methane, CF₄, and SF₆) are chosen as probes of varying size and interaction energy. Direct Monte Carlo integration of the partition function is used to determine free energies, potential energies and entropies of adsorption in spatially distinct regions of the zeolites at low loadings. These calculations lead to intuitive explanations of siting seen in one- and two-component grand canonical Monte Carlo simulations of adsorption. Both simulation techniques are implemented using the same potential model in identical zeolites. Owing to the similar nature of the two MFI channels, only small differences in adsorption free energies between channels are seen and segregation effects are minimal. Conversely, the very heterogeneous nature of MOR leads to substantial free energies differences and pronounced segregation. Confinement effects in the MOR side pockets produce a large entropic penalty for adsorption. The dissimilar 10-ring and 12-ring channels in BOG also lead to substantial segregation under many conditions. The siting distributions for the single-component systems are explained using filling and packing effects. The binary data is explained by introducing four qualitative models that can be used to classify the observed segregation.