Adsorption and Diffusion of Alkanes in Na-MOR: Modeling the Effect of the Aluminum Distribution

We investigated the adsorption and the diffusion of alkanes in the sodium-exchanged zeolite Mordenite (Na-MOR) using molecular simulations. MOR-type zeolite consists of main channels (6.5 x 7 angstrom) oriented along the z crystallographic axis that are connected to small side pockets (3.4 x 4.8 angstrom). It is well-known that the adsorption of alkanes in Na-MOR strongly depends on the precise location of the framework Al atoms either in the main channel or the side pockets (Calero and co-workers, Angew. Chem, Int. Ed. 2007, 46, 276). We found that this effect can be characterized by a single-order parameter: the number of framework Al in the main channel divided by the number of framework Al in the side pocket (M/S ratio). For any M/S ratio, the adsorption isotherm follows from a linear interpolation between the reference isotherms. This enabled us to predict adsorption isotherms for any distribution of the Al framework atoms or estimate the M/S ratio for a given isotherm. We found that the same model can predict the effect of the M/S ratio on the self-diffusion coefficient, the Maxwell-Stefan diffusion coefficient, and the accessible micropore volume.