Molecular dynamics simulations were used (table 55) to obtain a detailed understanding of the self-diffusion mechanism of methanol in the zeolite NaY system. A new force-field term was derived in order to describe the interaction between the methanol molecules and the extra-framework cations. From the simulations, it was found that the diffusive behavior in the high-temperature range consisted of a combination of short- and long-range motions at low and intermediate loadings. This type of motion was characterized by an activation energy that decreased as the loading increased. At low loadings, a short-range diffusive behavior was also observed which was based upon a surface-mediated mechanism. The short-range behavior corresponded to motion only at the scale of an FAU super-cage, whereas the long-range behavior involved inter-cage diffusion. For the saturation loading, of 96 methanol molecules per unit cell, only short-range motions within the same super-cage predominated.

Diffusion of Methanol in Zeolite NaY:  a Molecular Dynamics Study. D.F.Plant, G.Maurin, R.G.Bell: Journal of Physical Chemistry B, 2007, 111[11], 2836–44