Intracrystalline Diffusion of Linear and Branched Alkanes in the Zeolites TON, EUO, and MFI

Diffusion constants D and activation energies for diffusion E a were obtained for linear and branched alkanes inside the zeolites TON, EUO, and MFI via molecular dynamics simulations. Molecules with carbon numbers in the range n = 7−30 were studied in the dilute limit. The zeolites used have channels formed by 10-member silicate rings, but the diameter and connectivity of the channels differ between zeolites. Because the zeolites' channel features are different, it was observed that the influences of channel structure on molecular transport, or lattice effects, were also different between zeolites. For linear alkanes in the relatively uniform channels of TON, lattice effects were least pronounced so the highest values for D were observed and D scaled as 1/ n . Furthermore, E a ≈ 1 kcal/mol at small n and decreased to nearly zero for large n . The features of the channels in EUO and MFI were such that lattice effects were observed for linear alkanes. Generally, D was lower than in TON and decreased faster than 1/ n; in EUO, E a increased with n, while in MFI the dependence of E a on n was anisotropic. Monomethyl-branched alkanes diffused slower than their linear counterparts. Branched molecules are effectively bulkier making interactions with the channel walls more influential. Lattice effects were observed for branched molecules in all the zeolites; especially in the trends of D with branch position.