Analytical Estimate of the Entering Probability of Molecules into Crystalline Nanoporous Materials

This paper considers the probability that a molecule at the surface of a nanoporous material, e.g., a zeolite, will be able to enter the intracrystalline space. A relation is derived between the entering probability and the self-diffusion coefficient for the case of an ideal surface. It is known that the flux through the surface is limited mainly by a certain energy difference, ΔE, (Barrer, R. J. Chem. Soc., Faraday Trans. 1990, 86, 1123) and, for real crystals, the fraction of open pores b ≤ 1. With the help of the relation presented here, it is possible to separate the two different effects responsible for transport resistance caused by the surface. This opens new perspectives for the characterization of real surfaces. Furthermore, it is shown that ΔE must be less than 50% of the heat of adsorption and, surprisingly, can even practically vanish in special cases. The relation is tested using molecular dynamics simulations of ethane adsorbing into silicious zeolite LTA. The results of the simulations show...