(341d) Sorption Kinetics in MFI- the First Hydrophobic Zeolite

Although preceded by several silica clathrasils, MFI, first synthesized and named ZSM-5 by Mobil Corp. in 1972 (U.S. Patent 3,702,886) is probably the first hydrophobic zeolite. The Al free analog (silicalite) was synthesized by Union Carbide at about the same time. In independent publications in 1978^(1,2) the structures of these materials were shown to be the same.

During the next decade numerous applications of these materials emerged both as catalysts and selective adsorbents. Since then the molecular sieve properties and the sorption kinetics have been studied in considerable detail. Some of these results are reviewed and discussed in the light of more recent data. The pore size is about 5.5À which is large enough to admit molecules as large as cyclohexane or o-xylene, albeit with significant hindrance. Diffusion in a silicalite crystal is non – isotropic, with almost equal diffusivities in both transverse directions (straight and sinusoidal channels) and a significantly lower diffusivity in the direction of the longest axis (z). Microscale measurements (PFG NMR and QENS) show generally good agreement but measurements at greater length scales often yield lower diffusivity values, apparently due to the presence of structural defects leading to dislocation of the pores. However, CO₂ (a small fast diffusing molecule)shows the opposite effect with higher apparent diffusivities measured over the length scale of the crystal, possibly due to preferential transport along the twin planes.

Recent studies by IFM and frequency response have revealed a transition from intra-crystalline diffusion control in large silicalite crystals to surface resistance control in small (micron sized) crystals. It has also been shown that while the intra-crystalline diffusivity is reasonably constant the surface resistance can vary by orders of magnitude between different crystals within the same sample.

It has recently been suggested that surface resistance may lead to a difference in the average diffusional path length between adsorption and desorption. However, a recent MD study shows that this hypothesis is incorrect; the mean path length and the path length distribution are the same for adsorption and desorption with or without a surface barrier, in conformity with the Principle of Microscopic Reversibility.

1. G.T. Kokotailo, S.L.Lawton, D.H.Olson, W.M. Meier Nature 272, 437 (1978)
2. E.M.Flanigen, J.M.Bennet, E.W.Grose, J.P.Cohen, R.L.Patton, R.M.Kirchner, J.V.Smith Nature 271, 512 (1978)