Faujasite discussion

Purely parallel reactions are e.g. competitive reactions which are frequently carried out purposefully, with the aim of estimating relative reactivities of reactants these will be discussed elsewhere (Section IV.E). Several kinetic studies have been made of noncompetitive parallel reactions. The examples may be parallel formation of benzene and methylcyclo-pentane by simultaneous dehydrogenation and isomerization of cyclohexane on rhenium-paladium or on platinum catalysts on suitable supports (88, 89), parallel formation of mesityl oxide, acetone, and phorone from diacetone alcohol on an acidic ion exchanger (41), disproportionation of amines on alumina, accompanied by olefin-forming elimination (20), dehydrogenation of butane coupled with hydrogenation of ethylene or propylene on a chromia-alumina catalyst (24), or parallel formation of ethyl-, methylethyl-, and vinylethylbenzene from diethylbenzene on faujasite (89a). 

Extremely high ion exchange affinities are however sometimes observed for alkali metals (e.g. Cs) and transition metal ion complexes in clay minerals and zeolites. The objective of this paper is to give an account of the factors which are involved in these high selectivity phenomena. The discussion will be focussed mostly on montmorillonites and faujasites as representatives of the phyllosilicate and tectosilicate groups. 

Under the same conditions (batch or GL-PTC) discussed for CHg-acidic compounds, primary aromatic amines also react with DMC. In this case, although the reaction yields selectively the mono-A-methylated amines with no dimethylated by-products, sizable amounts of methyl carbamates (ArNHCOgMe) are formed. ° Much better results can be gathered in the presence of zeolites, particularly alkali metal exchanged Y and X faujasites. These aluminosilicates posses pseudospheri-cal cavities (supercavities) of 11-8 A in diameter, which can be accessed through channels whose size is 7.4 kP ... 

The most commonly employed crystalline materials for liquid adsorptive separations are zeolite-based structured materials. Depending on the specific components and their structural framework, crystalline materials can be zeoUtes (silica, alumina), silicalite (silica) or AlPO-based molecular sieves (alumina, phosphoms oxide). Faujasites (X, Y) and other zeolites (A, ZSM-5, beta, mordenite, etc.) are the most popular materials. This is due to their narrow pore size distribution and the ability to tune or adjust their physicochemical properties, particularly their acidic-basic properties, by the ion exchange of cations, changing the Si02/Al203 ratio and varying the water content. These techniques are described and discussed in Chapter 2. By adjusting the properties almost an infinite number of zeolite materials and desorbent combinations can be studied. 

Co2(CO)g has been used to obtain encapsulated cobalt clusters in Y-faujasite, which have been used as model catalysts for methane homologation [152]. The gas phase adsorption of Co2(CO)8 under N2 rendered predominately encaged Co4(CO)i2 species whereas Co,s(CO)iis was obtained when the impregnation of Co2(CO)8 was carried out under a CO/H2 atmosphere [152, 155], Samples were oxidized at 80°C, subsequently reduced at 400 °C and then structurally characterized by EXAFS. Clusters of two and three cobalt atoms were formed from encaged Co4(CO)i2 and COis(CO)iis, respectively. Higher methane conversion and selectivity to C2+ products in the CH4 homologation reaction have been obtained for the two atoms-size cluster sample the results were discussed using a DFT model [152]. 

There are three different kinds of octane catalysts in current use. Some are based in part on an active non-zeolite matrix composed of a porous silica/alumina component. Others are based on low cell size (2.425-2.428 nm) ultra stable faujasite (USY), a catalyst composition developed in 1975 (2) for the purpose of octane enhancement. A third catalyst system makes use of a small amount (1-2%) of ZSM-5 as an additive. While the net effect in all cases is an increase in the measured octane number, each of the three catalytic systems have different characteristic effects on the composition and yield of the gasoline. The effects of the ZSM-5 component on cracking is described in other papers of this symposium and will not be discussed here. 

Based on the Knoevenagel reactions discussed above, we have studied the condensation of a series of benzaldehydes 4a-e with ethyl acetoacetate on a series of X, Y and sodium germanium faujasites. 

Most of the published information regarding surface acidity and its relation to catalytic activity has involved zeolites of the faujasite structure as found in zeolites X and Y. A smaller number of investigations of mor-denite have been reported. This discussion will concentrate on studies of these two types of zeolites because their acidic and catalytic properties have been most widely investigated, and because they are both of significant industrial importance. 

Nature of acidic sites. The location of the acidic hydroxyl groups in the faujasite structure has been the subject of numerous investigations and much discussion. The results of adsorption experiments with several molecules led Eberly (170) to conclude that the 3550-cm-1 hydroxyl absorption band represented hydroxyl groups located in the hexagonal prisms of the faujasite framework [(Si sites (171)], where they were relatively inac-... 

Infrared spectral studies of rare earth (RE) ion-exchanged faujasites have been reported by Rabo et al. (214), Christner et al. (217), Ward (211, 212), and Bolton (218). Distinct hydroxyl absorption bands are observed at 3740, 3640, and 3522 cm-1 after calcination at temperatures in the range of 340° to 450°C. As previously discussed, the hydroxyl groups at 3740 cm-1 are attributed to silanol groups either located at lattice termination sites or arising from amorphous silica associated with the structure. The hydroxyl groups that form the 3522 cm-1 band are nonacidic to pyridine or piperidine and are thought to be associated with the rare earth cations. 

The use of traditional and new techniques to elucidate the structure of synthetic faujasites with different silica alumina ratios, dealuminated by steaming and chemical treatment, and with and without faulting will be described. The migration and fixation of cations and the role of aluminum in the dealumination of the zeolite will be discussed. 

Following the pioneering works of Ito and Fraissard (57) and Ripme-ester (58), 129Xe NMR of xenon adsorbed on zeolite has proven sensitive probe of its local environment due to its chemical inertness and excellent sensitivity (59). In this work, we used 11 and 13C NMR measurements of the adsorbed benzene in conjunction with 129Xe NMR and adsorption isotherm measurements of the co-adsorbed xenon to study the homogeneous adsorption behavior of benzene on faujasite-type zeolites with various Si/Al ratios. Detailed macroscopic and microscopic adsorption states of the benzene in various NaX and NaY zeolites are discussed in terms of NMR linewidths and chemical shifts and are compared with results obtained from other studies. 

The resurgence of interest in the field of zeolite chemistry, which has been stimulated by the appreciation of their enormous potential as catalysts, has led to the application of several sophisticated physical methods in the study of their structural properties. Important advances have already been made using high resolution, solid state NMR (1,2) and electron microscopy (3), and in this paper we discuss the scope and limitations of neutron diffraction studies with powder samples, with some specific applications to zeolite-A and synthetic faujasite. 

We discuss the combined use of high-resolution electron microscopy, electron diffraction, optical diffractometry and computer graphics for investigating zeolitic structure. Particular attention is given to twinned faujasitic materials and to intergrowth structures in ZSM-5 and ZSM-11 catalysts. 

It is also of interest to compare the relative peak intensities for these three materials. The differences between the peak intensities for Na-X and Y and ZK-4 as a function of composition have previously been discussed (22). Gallium sodalite with Si/Ga - 1.26 has a different intensity pattern to either faujasite or ZK-4. A comparison of the ratios of I(Si-4T)/I(Si-3T) and I(Si-3T)/I(Si-2T) for the three compounds shows the differences. The corresponding values are 1.23, 1.74, 1.25 and 3.18, 2.16, 1.98 for ZK-4, Na-X and gallium sodalite, respectively. Further discussion of the Si/Ga distribution in gallium sodalite requires data over a range of composition and this not yet available. 

The cationic forms of zeolites are experimentally investigated rather well by various spectroscopic techniques, including IR, ESR, NMR, and UV VIS spectroscopy. Structural interpretation of the spectroscopic data requires quantum-chemical computations of the optical and magnetic properties of 3d-element ions in different environments. For this purpose, Mikheikin et al. (98, 99) have applied a simple crystal-field theory to discuss the optical and magnetic properties of ions with a dn shell (n = 1,. 9) in the crystal field of low symmetry. The results could be used to discuss optical and ESR spectra of faujasites containing Cu2+, Co2+, Cr3+, Fe3+, Ni2+, and Mn2 + ions and their complexes with various molecules (H20, NH3, CH3OH, C2FI5OH, etc.). 

At the same time, specific properties (primarily the Si/Al ratio) of a zeolite should be taken into account when discussing the mechanism of its dehy-droxylation. It is quite possible that the mechanism typical of H forms of faujasites would be completely improper for high-silica-containing zeolites. Thus, in their studies of dehydroxylated forms of ZSM-5 zeolite by means of IR spectroscopy of molecular hydrogen adsorbed at low temperatures, Kazansky et al. (72, 76) have demonstrated that the Uytterhoeven-Cristner-Hall scheme seems valid in this case. 

The assignment of the three component peaks in Njg spectra of H-ZSM-5 samples in Figures 14 and 15 is different from the one discussed in section 4.1 for faujasites. This assignment was made after a careful IR study of pyridine chemisorbed on the same samples. Table 6 reports the relative intensities of the three component peaks for the samples of Figure 14 and Table 4. 

The skeletal isomerization of tetrabydrodicyclopentadiene into adamantane is an example of a very complex rearrangement diat is commercially carried out over strong Lewis acids with a hydride transfer initiator. The reaction can be catalyzed by rare earth (La, Ce, Y, Nd, Yb) exchanged faujasites (Scheme 1) in a Hj/HCl atmosphere at 25(yX3. Selectivities to adamantane of up to 50% have been reported, when a metal fimction, such as Pt, capable of catalyzing hydrogenation is added [54]. Initially acid catalyzed endo- to exo- isomerization of tetrahydro-dicyclopentadiene takes place and then a series of 1,2 alkyl shifts involving secondary and tertiary carbonium ions leads eventually to adamantane[55]. The possible mechanistic pathways of adamantane formation from tetrahydro-dicyclopentadiene are discussed in detail in ref [56]. 

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