Faujasite calculation

Hill, J.-R., Freeman, C. M., Delley, B., 1999, Bridging Hydroxyl Groups in Faujasite Periodic vs Cluster Density Functional Calculations , J. Phys. Chem. A, 103, 3772. 

The calculation permits a transformation of the 29Si NMR intensities to give distributions of aluminum atoms in faujasite with implications for the numbers of strong and weak acid sites available. 

Table II. Comparison of Calculated Maximum Probability Distributionsa of Silica Atoms in Faujasite Having 0, 1, 2, 3, and 4 Nearest Neighbor 0A1 Groups with Experimental Distributions from Reference 2...

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... 

The diffusion coefficient was estimated to be 4 x 10 9 m2/s. Experimental values for benzene in faujasites range from 10 10 to 10"13 m2/s, depending on the measurement technique (24, 97). PFG-NMR measurements are the closest to the MD value, which was admitted by the authors to be a crude estimate (mainly on grounds of a short simulation and inflexible molecules). The simulation time was too short to permit a calculation of the residence times of the benzene at either the cation or the window site or inside a particular cage. The cage residence times were estimated to be at least an order of magnitude longer than those for methane in NaY zeolite (43). 

Bull et al. (97) reported a systematic 2H-NMR and MD study of siliceous faujasite. MD calculations were performed for 1 molecule of benzene adsorbed in a single unit cell of faujasite. Full framework flexibility was incorporated, using potential parameters from MSI s cff91 force field (5). Simulations were performed for diffusion at 298, 350,400, and 450 K, using a time step of 1 fs for a 25-ps calculation run (following 5 ps of equilibration). 

The calculations led to predictions of adsorption sites for the nonpolar compounds that are in good agreement with those determined experimentally. The cation site is preferred over the window site. The activation barrier for movement between two cation sites was calculated to be 30 kJ/ mol and that for movement between a cation and a window site 43 kJ/mol. Experimental measurements of activation barriers to diffusion of benzene in faujasites are between 17 and 27 kJ/mol (24). The calculations provide strong support for the mechanism of surface-mediated diffusion for all guest molecules in the limit of infinite dilution and 0 K. The MEPs show that molecules slide along the wall of the supercage, with the plane of the aromatic ring almost parallel to the pore wall. 

In Faujasites. Bezus et al. (49) reported in 1978 statistical calculations on the low-coverage adsorption thermodynamics of methane in NaX zeolite (Si/Al = 1.48). As for single-atom adsorbates described earlier, the agreement between their calculated values and a range of experimental values was excellent. Allowing for different orientations of the molecule, they calculated a value of 17.9 kJ/mol for the isosteric heat of adsorption at 323 K. Experimental values available for comparison at that time (134-136) ranged from 17.6 to 18.8 kJ/mol. Treating the methane molecule as a hard-sphere particle, with a radius of 2 A, resulted in a far lower heat of adsorption (12.6 kJ/mol). Further calculations (99) yielded heats of adsorption of 19.8 and 18.1 kJ/mol for methane in NaX and NaY zeolites, respectively. 

Fig. 7. Predicted minimum energy sites of methane in faujasite from Monte Carlo calculations. The methane molecules are shown as large dark circles within the faujasite cavity. Reprinted with permission from Nature, Ref. 46. Copyright 1988 Macmillan Magazines Limited.

Bates et al. (172-174) considered the energetics, locations, and conformations of alkanes ranging from butane to decane in a variety of different all-silica zeolites. Calculations similar to those described already were performed for alkanes in mordenite, zeolite rho, faujasite, ferrierite, and zeolite A. A linear increase in the calculated heat of adsorption with increasing carbon number was found for all zeolites. Less experimental information is available to compare with the calculated heats of adsorption, and thus the performance of the technique and parameters cannot be subjected to quite the same scrutiny as the results for silicalite (111). Nonetheless, where... 

NMR Investigations of the Framework Cations of Various Faujasite-Type Zeolites and Their Interpretation by Model Calculations... 

Freude et al. (169) carried out a systematic study of the relative amounts of four- and six-coordinated Al in thermally treated zeolite Y, using wide-line and MAS 27A1 NMR at 16 and 70.34 MHz, respectively. Table XV gives the results calculated per one faujasitic supercage ( of the unit cell). It is evident that loss of 27A1 line intensity takes place in treated zeolites in comparison with the parent material, evidently due to extra-framework Al being in an environment of low symmetry. 

Pfeifer et al. (263) conclude from their measurements of T, and T2 versus temperature in samples with controlled water contents that the lifetime of sorption complexes of water is 3.5 x 10-9 sec at 50°C with nonlocalized cations and at - 10°C with localized ones. Water was found to be bound more strongly in faujasites with higher Si/Al ratios, which agrees with model calculations by Dempsey (282) of the electrostatic fields around cations. At higher coverages the mobility of H20 is independent of the Si/Al ratio and is two orders of magnitude lower than in bulk water. 

Barrer and Gibbons (1965) did calculations for the interaction potentials of C02 and NH3 moving along the axes running through the center of the 12-ring window of faujasite-type 26-hedral cage of zeolite X. The results are shown in Tables VII and VIII. The qualitative comparison with experimental... 

The transport and adsorption properties of hydrocarbons on microporous zeolites have been of practical interest due to the important properties of zeolites as shape-selective adsorbents and catalysts. The system of benzene adsorbed on synthetic faujasite-type zeolites has been thoroughly studied because benzene is an ideal probe molecule and the related role of aromatics in zeolitic catalysts for alkylation and cracking reactions. For instance, its mobility and thermodynamic properties have been studied by conventional diffusion 1-6) and adsorption 7-9) techniques. Moreover, the adsorbate-zeolite interactions and related motion and location of the adsorbate molecules within the zeolite cavities have been investigated by theoretical calculations 10-15) and by various spectroscopic methods such as UV (16, 17), IR 17-23), neutron 24-27), Raman 28), and NMR 29-39). 

In order to obtain more specific qualitative information for above acid attack dealu-mination in USY zeolite, the pople s CNDO/2 calculations were made on a model of the six ring cluster with a TtO,(OH)i structure unit (T represents A1 atom or Si atom) in Fig, 1 (ref. 4, ref. 7) which simu-lates the Sr and Si sites of faujasite zeolite. 

It is of interest to compare this figure with the heat of immersion of aluminum-rich faujasite. For a compound with composition NaggAlggSij 20384 one calculates -122.5 kJ/mol (4), which is mainly hydration energy of intra-channel sodium ions. 

The lower energy of the more open structures relates to the decrease in Madelung energy. However, as Figure 2 shows, local topological effects also play a role. See, for instance, the difference in energy calculated between zeolite A and faujasite. 

Results of Lattice Energy Minimization Calculations. Relative lattice energies of faujasite, mordenite, silicalite and sodalite were compared. For the framework and cation positions of faujasite and sodalite the same data were used as before, from Hseu (18) and Olson (19), and Baerlocher (20) and Chao (21), respectively. For mordenite and sodalite the data of Meier (22) and Mortier (23 ) (on mordenite) and Baerlocher and Meier (24) (on sodalite) were used. 

The starting unit cells for faujasite and mordenite have the chemical composition Nax[AlxSi g xOgg]. For sodaite and ZSM—5 we used S 12 24 an< Nax[AlxSig6 xOi92]. The framework and cation positions were allowed to relax under constant pressure. Parameter values used and details of the calculation can be found in (25). 

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