Diffusion silicalite

Uptake rate measurements with large oriented crystals have been used by Caro [24] to demonstrate the nonisotropy of silicalite. Diffusion coefficients for the longitudinal and transverse directions differed by a factor of about three. 

Force field without Coulomb term 38, 39 Structural changes of silicalite diffusion of H2O in natrolite and of CH4 in various zeolites 21-23... 

As described in the previous section, the silica-alumina catalyst covered with the silicalite membrane showed exceUent p-xylene selectivity in disproportionation of toluene [37] at the expense of activity, because the thickness of the sihcahte-1 membrane was large (40 pm), limiting the diffusion of the products. In addition, the catalytic activity of silica-alumina was not so high. To solve these problems, Miyamoto et al. [41 -43] have developed a novel composite zeohte catalyst consisting of a zeolite crystal with an inactive thin layer. In Miyamoto s study [41], a sihcahte-1 layer was grown on proton-exchanged ZSM-5 crystals (silicalite/H-ZSM-5) [42]. The silicalite/H-ZSM-5 catalysts showed excellent para-selectivity of >99.9%, compared to the 63.1% for the uncoated sample, and independent of the toluene conversion. 

The quantification of the extra-framework titanium species in titanium silicalites of MFI structure, TS-1, was performed using either XANES at the Ti K-edge or XPS Ti (2p) photolines. In addition, two different framework sites, [Ti(OH)(OSi)3] and [Ti(OSi)4], were characterized in dehydrated samples using Diffuse Reflectance UV-visible, multiple scattering analysis of EXAFS, H and Si NMR spectroscopies. 

Methane (continued) silicalite, 42 66-70 zeolite A, 42 64-65 vibrational spectra, 42 207-210 ZSM-5 self-diffusion coefficients, 39 369-371... 

Examples of rate-selective adsorption are demonstrated using silicalite adsorbent for separation of Ciq-Cm n-paraffins from non- -paraffins [40, 41] and Ciq-Ch mono-methyl-paraffins from non-n-paraffins [42-45]. Silicalite is a ten-ringed zeolite with a pore opening of 5.4A x 5.7 A [22]. In the case of -paraffins/non-n-paraffins separation [40, 41], n-paraffins enter the pores of silicalite freely, but non-n-paraffins such as aromatics, naphthenes and iso-paraffins diffuse into the pores more slowly. However, the diffusion rates of both normal -paraffins and non-n-paraffins increase with temperature. So, one would expect to see minimal separation of n-paraffins from non-n-paraffins at high temperatures but high separation at lower temperature. 

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

UV-Visible diffuse reflectance spectra also show that vanadium is mainly present as V in V-containing silicalite samples. TPR and XPS results are in agreement with this conclusion. In addition, XPS data indicate that V is homogeneously dispersed in VSU545, whereas in VSill 17 part of the vanadium is segregated on the external surface of the silicalite samples. 

Xenon has been considered as the diffusing species in simulations of microporous frameworks other than faujasite (10-12, 21). Pickett et al. (10) considered the silicalite framework, the all-silica polymorph of ZSM-5. Once again, the framework was assumed to be rigid and a 6-12 Lennard-Jones potential was used to describe the interactions between Xe and zeolite oxygen atoms and interactions between Xe atoms. The potential parameters were slightly different from those used by Yashonath for migration of Xe in NaY zeolite (13). In total, 32 Xe atoms were distributed randomly over 8 unit cells of silicalite at the beginning of the simulations and calculations were made for a run time of 300 ps at temperatures from 77 to 450 K. At 298 K, the diffusion coefficient was calculated to be 1.86 X 10 9 m2/s. This... 

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). 

There are experimental results that show the anisotropic nature of diffusion of methane in silicalite (24, 77). From a stochastic jump model of the diffusion process, Karger et al. (24) found that the ratio of the rate of diffusion in the direction of the two channel systems should not be less than 4.4 times that in the orthogonal direction ... 

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. 

We now consider the diffusion of other hydrocarbons. Most calculations have been performed for n-alkanes, up to and including n-hexane, but alkenes and alkynes have also been considered. Calculations involving larger molecules have been mainly restricted to silicalite. 

Nowak et al. (63) presented a comparative study of the diffusivities of rigid models of methane, ethane, and propane in silicalite. (The details of the calculation are reported in the preceding section.) The calculated diffusion coefficients decreased as the length of the carbon chain increased, and the effect was found to be far more pronounced for ethane than propane. The calculated diffusivities, in units of 108 m2/s, were 0.62, 0.47, and 0.41 for methane, ethane, and propane, respectively. The ethane value is in satisfactory agreement with PFG-NMR measurements [0.38 (77), 0.3 (80), 0.4 (42) for silicalite. The value for propane, however, was calculated to be almost an order of magnitude larger than the NMR results of Briscoe et al. (80). [The agreement with the value of Caro et al. (71) is better, but still an overestimation.]... 

Dumont and Bougeard (68, 69) reported MD calculations of the diffusion of n-alkanes up to propane as well as ethene and ethyne in silicalite. Thirteen independent sets of 4 molecules per unit cell were considered, to bolster the statistics of the results. The framework was held rigid, but the hydrocarbon molecules were flexible. The internal coordinates that were allowed to vary were as follows bond stretching, planar angular deformation, linear bending (ethyne), out-of-plane bending (ethene), and bond torsion. The potential parameters governing intermolecular interactions were optimized to reproduce infrared spectra (68). 

The value of the diffusion coefficient for ethene in silicalite (1.29 X 10-8 m2/s) is an order of magnitude larger than that from a similar MD study... 

Nicholas et al. (67) have performed MD calculations of propane in sili-calite in which the propane molecule is given complete flexibility. The calculations, which have been detailed previously for methane diffusion, employed a large simulation box with multiple sets of adsorbates to ensure good statistics. The framework was kept fixed and data were collected over a 40-ps run. The results predict diffusion coefficients in very good agreement with the values of Caro et al. (71). The calculated values for a concentration of 4 and 12 propane molecules per silicalite unit cell are 0.12 and 0.005 X 10 8 m2/s, respectively. These values for propane are far lower than those of Nowak et al. (63), the reason for this is that Nicholas et al. used flexible adsorbate molecules, whereas Nowak et al. used rigid ones. 

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