Computer Simulation of Incommensurate Diffusion in Zeolites:  Understanding Window Effects

Dedicated molecular simulation techniques afford the study of the abnormal adsorption and diffusion of linear alkanes in ERI-, CHA-, and LTA-type zeolites. The exceptionally slow diffusion rates required the development of a combination of rare-event transition-state theory techniques and the configurational-bias Monte Carlo algorithm. The diffusion coefficients computed by this novel method agree well with the nondisputed rates determined experimentally for LTA-type sieves. The computed rates corroborate the nonmonotonic variation of the diffusion rate with alkane chain length published by Gorring, that is, the rate increases by orders of magnitude when the molecular and cage shape are no longer commensurate, so a molecule ends up stretched across a cage tethered at opposite windows. The simulations corroborate this “window effect” for both ERI- and CHA-type sieves and suggest that it is characteristic for all sieves with windows approximately 0.4 nm across. They predict that it also occurs for LTA-type sieves provided that the n -alkane is long enough to exceed the diameter of the LTA-type supercage.