Diffusion chabazites

The results of experimental studies of the sorption and diffusion of light hydrocarbons and some other simple nonpolar molecules in type-A zeolites are summarized and compared with reported data for similar molecules in H-chabazite. Henry s law constants and equilibrium isotherms for both zeolites are interpreted in terms of a simple theoretical model. Zeolitic diffusivitiesy measured over small differential concentration steps, show a pronounced increase with sorbate concentration. This effect can be accounted for by the nonlinearity of the isotherms and the intrinsic mobilities are essentially independent of concentration. Activation energies for diffusion, calculated from the temperature dependence of the intrinsic mobilitieSy show a clear correlation with critical diameter. For the simpler moleculeSy transition state theory gives a quantitative prediction of the experimental diffusivity. 

The theoretical lines in Figure 2 are calculated assuming constant values of D0 with the derivative d In p/d In c calculated from the best fitting theoretical equilibrium isotherm (Equation 8). The theoretical lines give an adequate representation of the experimental data suggesting that the concentration dependence of the diffusivity is caused by the nonlinearity of the relationship between sorbate activity and concentration as defined by the equilibrium isotherm. The diffusivity data for other hydrocarbons showed similar trends, and in no case was there evidence of a concentration-dependent mobility. Similar observations have been reported by Barrer and Davies for diffusion in H-chabazite (7). 

Although the theoretical models presented in this paper are simple idealizations of complex systems, the theory provides a useful understanding of many aspects of the sorption and diffusion of simple nonpolar molecules in type-A zeolites and in H-chabazite. The extent to which such theories are applicable to other systems has not yet been investigated. 

The cage or window effect was proposed by Gorring (48) to explain the nonlinear effect of chain length observed in hydrocracking of various n alkanes over T zeolite, chabazite (CHA) and erionite (ERI). Thus, when a nC22 alkane is cracked over erionite, there are two maxima in the size distribution of the product molecules at carbon numbers of 4 and 11 and a minimum at carbon number of 8. The diffiisivities of n-alkanes also change in a similar periodic manner by over two orders of magnitude between the minimum at C8 and the maxima. This shows that for diffusion, and hence for shape selective effects, not only the size but also the structure of the reactant and product molecules need to be considered. 

It has already been mentioned that zeolites are shape selective with respect to molecular adsorption. This property relates to their micropores stmcture. The zeolite framework shows a limited flexibility, which is essential. For instance, Yashonath et al. have shown in their classical dynamic simulations study of molecular diffusion within zeolite micropore that the zeolite framework flexibility affects significantly diffusion when the molecules have a size comparable with the micropore size. To get an idea of the order of magnitude of this flexibility, one can consider the hybrid semi-empirical DFT periodic study of chabazite zeolite of Ugliengo et al. V They introduced in the unit cell of chabazite Br0nsted acidic sites which are known to induce an increase of the volume of around 10 This increase of the volume relates with the difference of volume between a Si04 tetraheron and a... 

Anionic frameworks have been proposed for chabazite. gmelinite 5 and erionite. In this paper we suggest an anionic framework for levynite, and compare, on the basis of the proposed frameworks, the ease and degree of anisotropy of molecule diffusion, and the possible molecular sieve behaviour, for the four zeolites. It has already been shown that diverse intracrystalline channel systems can arise in structures such as analcite, nosean-sodaUte minerals, cancrinite, faujasite, and Linde Sieve A.9> 10... 

Two additional conditions must be satisfied for these dimensions rigorously to determine molecular sieve action. First, when water is removed by heat and evacuation, the rings must keep their stereochemical configuration and secondly, cations must not be so located as to block these rings. The first condition is approximately fulfilled the latter is not in all cases. Thus, in aluminous, and therefore cation-rich, synthetic near-chabazites, sodium ions are so numerous, and so placed, as to prevent molecule diffusion, except of small polar molecules like water. 3... 

Fig. 5(0).— The orientation of diffusion channels in chabazite, in relation to hexagonal and rhombic unit cells.

The only information for sorbate-zeolite systems from which Equation 2 can be discussed was obtained by Barrer and Fender (7). They studied intrinsic and tracer diffusion of water in chabazite, heulandite, and gmelinite, using H2O and D2O. The activity correction ... 

The correspondence with the law is illustrated in Figure 4(7) for the intrinsic diffusion of water in chabazite, gmelinite, and heulandite. 

As expected from the diffusion theory, the larger the particles, the slower the sorption A becomes smaller for constant V. This is illustrated in Figure 6 (8) for sorption of propane at 200°C in chabazite. 

Interval Method. The quantity 0 of sorbate initially in the crystal can be varied systematically by steps from zero to near saturation. At each value of 0 a small extra amount, dQ, is then sorbed, and the value of Da obtained can be regarded as constant over each interval 0 + Thus Da is found as a function of o- An interval method was used for several diffusing species in chabazite by Barrer and Brook (4), in which dQ was often considerable, compared with o- Thus, an integral value of Da was obtained over the interval 0 to Qr. Some results are given in Table IVa, in which the areas A were those determined by a flow method. Da appears to decrease rather strongly with increasing average concentra-... 

E for tracer diffusion of monovalent cations in chabazite (3) does not seem to be very different from E for water in the same zeolite ... 

From the industrial viewpoint, particular interest attaches to the migration of large molecules in zeolites, especially of paraffins. In Ca-chabazite and Ca-A, diffusion is regulated by apertures through which n-para fin chains can pass, with something of a squeeze, but not iso-paraflBns, aromatics, or naphthenes. There are 2 factors which may reduce... 

The diffusion coefficients of nonpolar molecules in zeolites can be changed greatly by the addition of controlled amounts of small polar molecules. These are sorbed very strongly and are immobile at the temperature of the subsequent runs with the nonpolar sorbates. Moderated diffusion was studied first in 1954 (11) for Ho, O2, N2, Ar, and C2H6 in crystals of mordenite and chabazite. The moderators were H2O, NH.3, and CH3NH2. These measurements were extended subsequently to O2, N2, and Ne diffusing in Na-, (Ca,Na)-, and (K,Na)-A moderated with controlled amounts of NH3 (28). 

As an illustration of the current state of the art for electronic spectroscopy of transition metal ions in zeolites, refer to the recent review by Schoonheydt of Cu2+ in different zeolites [56]. Schoonheydt shows that experimental measurement of diffuse reflectance spectra (and in the case of Cu2 + EPR spectra) must be combined with theoretical calculations if a complete interpretation is to be made. The exact frequencies of the d-d transitions in the electronic spectrum of Cu2+ are independent of the zeolite structure type, the Si Al ratio, and the co-exchanged cations, but depend solely on the local coordination environment. Figure 20 shows the diffuse reflectance spectrum of dehydrated Cu-chabazite the expanded portion reveals the three d-d transitions in the region around 15000 cm l. 

Figure 20. Diffuse reflectance UV-VIS spectrum of dehydrated CuNa-chabazite. Reproduced with permission from reference 59.

For the high-silica chabazite, the barriers to migration were studied and, as expected, diffusion of cations can be ruled out for anhydrous Ca-chabazite at reasonable temperatures. 

For about 20 years Cu(II) has been used as a probe to study the coordination sites of zeolites. Attention has been primarily given to zeolites X and Y and, to a lesser extent, zeolite A, Chabazite and Mordenite. The techniques used were X-ray diffraction (XRD), electron spin resonance (ESR) and diffuse reflectance spectroscopy (DRS) (1-11). The results of these techniques are not directly comparable, because e.g. small Cu-loadings have to be used to obtain... 

In this way, Barrer and Riley 46) and Barrer and Brook 47) obtained apparent diffusion coefficients that were decreasing with concentration for propane, butane, and other gases in chabazites. However, the initial slopes frequently cannot be measured accurately if only very small crystals are available. In one case, it was found that diffusion in the macropores of pellets controls the rate of sorption 48) in another case at least the rate of desorption seems to be influenced by the transfer through the phase boundary, as different diffusivities were reported for sorption and desorption 49). The determination of diffusivities from sorption rates is thus impeded by several difficulties. The results that have been obtained thus far can be generalized only with respect to a few points ... 

The rates of exchange of DgO against H2O in larger single crystals of chabazite, gmelinite, and heulandite have been measured by Barrer and Fender (50). The resulting binary diffusion coefficients have been compared with the diffusion coefficients of pure HgO in the same solids. The relation... 

This system is the Na-K chabazite system, which was studied in detail by them (20). The results computed using the extended theory of Brooke and Rees disagree badly with their experimental results. The Helfferich-Plesset equation disagrees just as badly. Brooke and Rees have themselves suggested that in addition to the variation of ionic activities, the variation of ionic diffusion coefficients with concentration must be considered, and perhaps water transport. 

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