Diffusion in silicalite

Table 6.9 Energetics and predictions from modeling molecular diffusion in silicalite.

June et al. (12) used TST as an alternative method to investigate Xe diffusion in silicalite. Interactions between the zeolite oxygen atoms and the Xe atoms were modeled with a 6-12 Lennard-Jones function, with potential parameters similar to those used in previous MD simulations (11). Simulations were performed with both a rigid and a flexible zeolite lattice, and those that included flexibility of the zeolite framework employed a harmonic term to describe the motion of the zeolite atoms, with a force constant and bond length data taken from previous simulations (26). 

Other flexible framework calculations of methane diffusion in silicalite have been performed by Catlow et al. (64, 66). A more rigorous potential was used to simulate the motion of the zeolite lattice, developed by Vessal et al. (78), whose parameters were derived by fitting to reproduce the static structural and elastic properties of a-quartz. The guest molecule interactions were taken from the work of Kiselev et al. (79), with methane treated as a flexible polyatomic molecule. Concentrations of 1 and 2 methane molecules per 2 unit cells were considered. Simulations were done with a time step of 1 fs and ran for 120 ps. 

Hernandez and Catlow (86) recently reported an investigation of n-butane and of n-hexane diffusion in silicalite the work is similar to that of June et al. (85). Many calculation details were the same as in the earlier work, including the assumption of identical Lennard-Jones coefficients of intermolecular dispersion and repulsion. Simulations were performed at different loadings for butane, namely, 2, 4, 5.3, and 8 molecules per unit cell. In addition, simulations were performed at a constant loading and variable temperature (200, 300, and 400 K) for both butane and hexane. These calculations were performed for 1000 ps, twice the length of those of June et al. The zeolite framework was held rigid. 

In an MD study of methane sorption and diffusion in silicalite, Nicholas et al. (67) identified favorable sites for sorption. From the MD calculations, the time-averaged position of the center of mass of the methane molecule was plotted. Energy minimization calculations were then performed, locating the methane molecule at positions where the MD calculations predicted they spent the most time. Each channel intersection region was found to contain two sites that are minima for methane-zeolite interactions. These two sites are separated by a translation parallel to the straight channel... 

Fig. 14 FR response curves of CO2 diffusion in silicalite-1 at 273 K and 2.0 Torr. a The fundamental frequency response data b the first ( , ), third ( , o), fifth ( , 0), seventh (V, V) and ninth (A, A) harmonic frequency data fitted by the theoretical model (lines) and K = 0.27. Note 1 Torr = 133.33 Pa...

Around a value of the gas-phase fraction of 2-methylpentane of about 0.83, the influence of the acid sites on the n-hexane diffusivity is not dominant anymore in comparison to the pore occupation of slow-diffusing 2-methyl-pentane. Figure 14 shows the dependence of the diffusivities of both components versus the concentration of adsorbed 2-methylpentane in terms of molecules per unit cell. The diffusivities of n-hexane in silicalite-1 and H-ZSM-5 become nearly equal when the concentration of 2-methylpentane reaches approximately 2.75 molecules per unit cell. For 2-methylpentane we And that the self-diffusivity in silicalite-1 becomes very close to the value in H-ZSM-5 at the same loading. 

Figure 5 Characteristic FR ftinctions for ethane diffusing in silicalite-l.

Characteristic function vs frequency curves (points) of n-butane diffusion in silicalite-I fitted by theoretical lines calculated in silicalite-l fitted by theoretical lines calculated from Equation (6) I and II indicate the contribution of the first and second terms, respectively. Temperatures (a) 298 K, (b) 323 K and (c) 348 K. Pressure 1.5 Torr. 

Shen, D. and Rees, L.V.C., Analysis of bimodal frequency-response behaviour of p-xylene diffusion in silicalite-1, J. Chem. Soc., Faraday Trans., 91, 2027-2033, 1995. 

Table 1. Energetics and Predictions from Modeling Molecular Diffusion in Silicalite.

June, R.L. Bell, A.T. Theodorou, D.N. Transition-state studies of xenon and SFe diffusion in silicalite. J. Phys. Chem. 1991, 95, 8866-8878. 

---