Mixture diffusion in zeolites studied by MAS PFG NMR and molecular simulation

Abstract

A combination of magic-angle spinning (MAS) and pulsed field gradient (PFG) NMR has been used to determine the self-diffusivities in MFI zeolite of n-butane, D_nC4, in mixtures with iso-butane in which the total loading, q, is maintained constant at 4 molecules per unit cell. When the loading of iC4 in the mixture, q_iC4, is increased from 0 to 2, the diffusivity D_nC4is observed to decrease dramatically by two orders of magnitude. Snapshots obtained from molecular simulations indicate that the iC4 molecules are preferentially located at the intersections between the straight channels and zig-zag channels of MFI; these intersections serve as traffic junctions. At q_iC4 = 2, the molecular traffic along the straight channels is brought to a virtual stand-still because of the obstructive influence of slow-diffusing iC4 ensconced at these junctions. Molecular dynamics (MD) simulations of D_nC4 in nC4/iC4 mixtures show good qualitative agreement with the observed experimental results. In sharp contrast, both experimental measurements and MD simulations, of D_nC4 and D_iC4 in wide pore FAU zeolite yield D_nC4 ≈ D_iC4 and there is no influence of mixture composition on component diffusion.