Zeolite studies

Dynamic Structural Change of Pd Induced by Interaction with Zeolites Studied by Means of Dispersive and Quick XAFS 427... 

Taarit, Y. Ben Vedrine, J.C. Coudrier, G. Praliaud, H., Eds., Catalysis by Zeolites, Studies in Surface Science and Catalysis 5, Elsevier Sci. Publ. Co. Amsterdam, 1980,... 

Jobic, H. and Methivier, A. (2005) Intracrystalline diffusion in zeolites studied by neutron scattering techniques. Oil Gas Sci. Technol., 60 (5) 815-830. 

Figure 13.14 was published as Figure 11 from the article Intracrystalline Diffusion in Zeolites Studied by Neutron Scattering Techniques by H. Jobic and A. Methivier, Oil and Gas Science and Technology - Rev. IFP, Vol. 50 (2005),... 

Table 3. Catalytic properties of zeolites studied in thylation. =250°C, methanol aniline=3 1(mol), WHSV=1 h...

In this paper we report the use of supported heteropoly acid (silicotungstic acid) and supported phosphoric acid catalysts for the acylation of industrially relevant aromatic feedstocks with acid anhydrides in the synthesis of aromatic ketones. In particular, we describe the acylation of thioanisole 1 with iso-butyric anhydride 2 to form 4-methyl thiobutyrophenone 3. The acylation of thioanisole with acetic anhydride has been reported in which a series of zeolites were used as catalysts. Zeolite H-beta was reported to have the highest activity of the zeolites studied (41 mol % conversion, 150°C) (2). 

The clearest and most direct experimental evidence from zeolite studies for the existence of a free carbenium ion intermediate obtained by any means is summarized in Fig. 14 (113). We followed the dimerization of styrene to form cis and trans isomers in a series of low-temperature MAS NMR experiments. Identification of the dimeric products was further... 

Sorption capacity is one of the major properties used for industrial applications of zeolites. H. Lee reviews the aspects of zeolites used as adsorbents. The other papers in the section deal with the theory of sorption and diffusion in porous systems, the variation of sorption behavior upon modification, and the variation of crystal parameters upon adsorption. NMR and ESR studies of sorption complexes are reported. H. Resing reviews the mobility of adsorbed species in zeolites studied by NMR. 

Table I lists the results of spectroscopic studies of iV,iV-dimethylaniline in zeolite samples 1, 2, 3, and 4. The spectral data show that the correlation of the number of different types of sites depends on the temperature of preliminary thermal activation, and the amount of different types of sites depends on the composition of crystals for various samples of zeolites treated at one temperature. Among the zeolites studied, sample 3 differs from samples 1,2, and 4 by its activity in the dark. The crystal compositions show that this sample has the highest degree of exchange of Na+ for NH4+.

Materials. The zeolites studied are summarized in Table I. The unit cells of the starting materials contain, in the dehydrated form ... 

Carbon dioxide adsorption causes changes in the sizes of all the zeolites studied similar to the variation observed for faujasite-type zeolites after water adsorption (Figure 6). For all zeolites, an increase in the adsorption of carbon dioxide leads to contraction this reached a minimum in the adsorption range 3-5.5 mM/gram. The final length of the pellets is below the initial value up to a relative pressure of p/ps 0.7 for zeolites CaA, CaY, and NaY while for NaA and NaX the contraction passes to an expansion, reaching 0.11% of the initial length at p/ps = 0.66 for NaX and 0.32 for NaA. 

It is unexpected that the catalytic activity and the proton acidity do not depend on the lanthanum content. This result cannot be related to the schemes of hydrolysis of the zeolitic rare earth cations reviewed in Ref. 13. On the other hand, acidity measurements in solution (15) have shown that in the lanthanum zeolites studied in this work the La3+ ions have replaced the NH4+ ions and have not formed a lanthanum compound (13). Finally, the variations in the sodium content of these lanthanum zeolites do not seem to be the dominant factor in contrast to the alkaline earth zeolites (26). 

Influence of Temperature. Data concerning the thermal stability of the catalytic activity are given in Figure 2 and Table I. The thermal stability of the starting materials Na-8.7 and D.Na-5.4 is discussed first. The limit of stability of the Na-8.7 sample appears to be higher than for the NaHY zeolites studied previously (3, 6, 27, 28). Nevertheless, this sample cannot be considered ultrastable since neither its structural data nor the thermal stability of its OH groups are characteristic of ultrastable zeolites (17). This increase in the stability may be explained by dry air heating and subsequent rehydration. 

That these increases are due to uptake of nitrogen and not traces of water or other nonvolatile impurities within the vacuum system is supported by l) the variability of the magnitude of the effect among the four zeolites studied, 2) the increase of the size of the residues with increasing content of... 

The silica/alumina mole ratios of the zeolites studied were as follows ZSM-5, 62 large crystal ZSM-5, 75 high silica ZSM-5, 1670 ZSM-11, 78 and dealuminized mordenite, 61. Aluminum analysis was by atomic absorption spectroscopy. Zeolite crystallite sizes were generally less than O.ly, except for the large crystal ZSM-5 sample which was larger than in. 

Figure 4.11 shows an example of how ZSM-5 is applied as a catalyst for xylene production. The zeolite has two channel types - vertical and horizontal - which form a zigzag 3D connected structure [62,63]. Methanol and toluene react in the presence of the Bronsted acid sites, giving a mixture of xylenes inside the zeolite cages. However, while benzene, toluene, and p-xylene can easily diffuse in and out of the channels, the bulkier m- and o-xylene remain trapped inside the cages, and eventually isomerize (the disproportionation of o-xylene to trimethylbenzene and toluene involves a bulky biaryl transition structure, which does not fit in the zeolite cage). For more information on zeolite studies using computer simulations, see Chapter 6. 

The mechanism of HS formation is less obvious. The stabilizing effect of TMA on high silica sodalite units(22) may have some bearing. The destabilization of D4R and D6R at high Na/Si ratios in solution (high pH ) has been observed(16). HS forms as higher pH than the other zeolites studied here. This may lead to a mechanism Involving SnR. 

Irradiation at 4 K gives a silver species designated as Ag°(B) in all of the hydrated A-zeolites studied. This particular silver species is characterized by a silver isotropic hyperfine coupling of about 1480 MHz and a g-factor of 1.991. For comparison the isotropic coupling constant of the silver atom in the gas phase is 1979 MHz, so species Ag°(B) shows considerable interaction with its environment. If the A-zeolites are dehydrated by heating to 400° C no silver species are formed by irradiation. Presumably the silver atoms that are formed are more mobile in the absence of water and form dimers or multimers. In addition, more prominent spectra of Ag2+ are seen upon irradiation of the dehydrated A-zeolites. Thus it seems that species Ag°(B) is probably associated with some adjacent water molecules. 

Irradiation was therefore carried out on silver doped A-zeo-lites at 77 K and a new silver atom species was detected with a different hyperfine splitting than that of the one observed at 4K. This species is designated as Ag°(A) with an isotropic hyperfine splitting of about 1985 MHz which is very close to that of the free atom value. Species Ag°(A) is the dominant species formed by irradiation at 77 K in all of the A-zeolites studied. In addition, species Ag°(B) is also visible at 77 K. However, upon warming above 77 K species Ag°(B) decays to apparently yield Ag°(A). 

In the case of zeolites, studies have also been performed to distinguish sites located at the external surface from those present in the cavities, and also to distinguish the position in different cavities for zeolites with a complex pore structure. This can be performed using molecules having different molecular sizes but similar chemical behavior. Some of the useful molecules are shown in Scheme 3.6. 

Eic M., Micke A., KoCirik M., Jama M. and Zik6nov6 A. Diffusion and immobilization mechanisms in zeolites studied by ZLC chromatography. Adsorption... 

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