Zeolite diffusion water

Single Crystals. Naturally-occurring zeolites are sometimes found as large single crystals. Tiselius (34, 35) used this feature to study diffusion in zeolites. Diffusion of water in heulandite crystals was followed by an... 

Durable changes of the catalytic properties of supported platinum induced by microwave irradiation have been also recorded [29]. A drastic reduction of the time of activation (from 9 h to 10 min) was observed in the activation of NaY zeolite catalyst by microwave dehydration in comparison with conventional thermal activation [30]. The very efficient activation and regeneration of zeolites by microwave heating can be explained by the direct desorption of water molecules from zeolite by the electromagnetic field this process is independent of the temperature of the solid [31]. Interaction between the adsorbed molecules and the microwave field does not result simply in heating of the system. Desorption is much faster than in the conventional thermal process, because transport of water molecules from the inside of the zeolite pores is much faster than the usual diffusion process. 

Citric acid separation from fermentation broth employs the full allotment of Sorbex beds in addition to the four basic Sorbex zones. The process utilizes a resin instead of a zeolite based adsorbent. The resin is a nonionic cross-linked polystyrene polyvinyl benzene formulation. Operating temperatures for this process are sufficient to overcome diffusion limitations with a corresponding operating pressure to maintain liquid-phase operation. The desorbent consists of water blended with acetone. Subsequent processing steps remove the desorbent from the desired extract product citric acid. 

Illustrative examples of substances which can behave as porous hosts in one of the above ways are also given. For instance, water readily forms open ice lattices which incorporate guests in clathrate hydrates of types I and II (see later text). Ordinary ice also possesses considerable porosity so that, as shown in Table I, He and Ne can readily diffuse through it. Ice below 0°C is zeolite-like in that it has a permanent, somewhat porous structure which (unlike the open-ice frameworks of the clathrate hydrates) does not require guest molecules for stabilization. 

Karger (14) has made the only successful use of this technique thus far. His results for the water-13X system (Figures 4 and 5) are compared with the results of relaxation time measurements below. He has also made a detailed theoretical analysis (15) of this zeolite situation which indicates that in some cases diffusion information may be extracted even if... 

Since the presentation of this model new data have appeared which allow various tests and new conclusions. The diffusion coefficients of Karger (14), together with Equation 1 and the median jump time from the relaxation data at room temperature yield a jump distance of 2.7 A for the zeolitic water as compared with 2.2 A in bulk water (see Table III for a data summary). One might be tempted to explain the jump distance in terms of some geometrical constant of the zeolite structure such as the distance between Sn and Sm ionic sites (40), but with the cages full of... 

Thus the mobility picture for water fully loaded into NaX zeolite fits together at least semiquantitatively. Further tests of the model based on newer NMR techniques have been proposed (9) and are in progress. Diffusion and other NMR data (45, 45) indicate on the one hand that the water mobility is greater at lower than maximum loadings, but on the other hand that the picture is not that simple. A theory based on fractionally filled cages in equilibrium with filled cages seems to be required. 

Additional dividends from NMR will most likely continue to lie in the area of diffusion and kinetics. Newer NMR techniques here are the ultra-slow motion (25) and rotating frame relaxation (26) techniques which allow measurements of very long jump times. Application of these techniques to the exchange region has been reported for water on NaX in this region they offer a means of deducing second moments of the tightly bound species (9, 52). The CIDNP technique should be applicable to the study of radical reactions on surfaces and in zeolites (58). 

Figure 5. Experimental self-diffusion coefficient of water molecules in NaX as a function of reciprocal temperature, (a) Mean diameter of the zeolite crystals 3 pm ...

Unlike ordinary zeolites, silicalite is organophilic and hydrophobic and can remove from water a variety of dissolved organic compounds. Both ZSM-5 and silicalite display remarkable shape selectivity because of the geometry of the channels only certain reactants may enter and diffuse through the crystals, and only certain products may diffuse out of the intracrystalline space. 

Relaxation times T, and T2 depend on the motion of molecules which contain the nuclei (236) and their measurement often leads to the various kinetic parameters for the adsorbed molecules, the knowledge of which is essential for the understanding of the mechanism of many zeolite-mediated processes. The diffusion coefficient of the reactants and products in a catalytic reaction, which can be determined from NMR, is often rate limiting. Relaxation studies can also determine surface coverage by the sorbed species and provide information about the distribution of adsorption energy between the different sites on the surface of a catalyst. For these reasons a great deal of NMR work has been done with adsorbed species in zeolites in the course of the last twenty years. From the applied viewpoint the emphasis is on water and hydrocarbons as guest molecules from the fundamental viewpoint species such as Xe, SF6, H2, CH4, and NH3 are of special interest. 

The first spectrum could be recorded 25 s after admission of alcohol to the catalyst. For all the zeolite samples of various crystallite sizes (Table I) at 296 K, the adsorption was complete within 25 s for sec- and isobutyl alcohols. The dehydration process of these alcohols in the zeolitic pores was, however, slower. For a given alcohol (/ -, sec-, or iso-) the kinetics of water elimination were identical for catalysts of different crystallite sizes. This firmly establishes the absence of any diffusion limitation for dehydration for these three alcohols. 

---