Zeolites cationic forms

Benzene alkylation over Y zeolites has been studied as a function of olefin, olefin aromatic ratio, temperature, and zeolite cation form. The rate has been modeled, and the rate-limiting process has been quantified as product desorption. 

Minachev et al. [76] studied oxidative dehydrogenation of cyclohexane on zeolite cationic forms at 300-475 °C, the main reaction product of which is cyclohexene. Cyclohexadiene and C02 are also formed, and at long-term contacts benzene is detected. Cyclohexene yield and selectivity of the reaction depend on zeolite structure and composition, reaction temperature and oxygen cyclohexane ratio in the reaction mixture. Among alkaline cationic forms of zeolite, the highest cyclohexene yield (21%) is observed for NaA zeolite (66% selectivity). 

Molybdenum/zeolite catalysts prepared by impregnating zeolites with ammonium hepiamolybdate solution generally give rise to poor dispersion of molybdenum [2]. In contrast, ion exchange would be an ideal method for loading active metal species onto supports. Few cationic forms are available as simple salts of molybdenum of high oxidation... 

For cationic zeolites Richardson (79) has demonstrated that the radical concentration is a function of the electron affinity of the exchangeable cation and the ionization potential of the hydrocarbon, provided the size of the molecule does not prevent entrance into the zeolite. In a study made on mixed cationic zeolites, such as MgCuY, Richardson used the ability of zeolites to form radicals as a measure of the polarizing effect of one metal cation upon another. He subsequently developed a theory for the catalytic activity of these materials based upon this polarizing ability of various cations. It should be pointed out that infrared and ESR evidence indicate that this same polarizing ability is effective in hydrolyzing water to form acidic sites in cationic zeolites (80, 81). 

The existence of a variety of adsorption sites in the cationic form of Y-type zeolites is also evident from ESR spectra of the superoxide ion as shown in Fig. 28. Here, only the low-field maxima are shown. At least... 

IR extinction coefficients as a criterion for chemical activation upon adsorption propene interaction with cationic forms of y zeolite... 

IMECs of the IR bands of propene adsorbed by the different cationic forms of zeolites were calculated by modifying the Lambert-Beer law in the following way to describe the adsorption on solid wafers ... 

Figure 1. Propene adsorption by different Figure 2 IR spectra of propene adsorbed cationic forms of zeolite Y. on ZnY at room temperature (initial...

Table 1. Absolute intensities A (IMECs) of different bands of propene in the gas phase and adsorbed in different cationic forms of Y zeolite.

Tang, S.L.Y., McGarvey, D.J. and Zholobenko, V.L. (2003). Photooxidation and dark thermal oxidation of 1-butene on cationic forms of zeolite Y a spectroscopic study. Phys. Chem. Chem. Phys. 5, 2699-2705... 

Specific structure-directing effects of some organic bases or cations When in the procedure BT Pr N is replaced by other organics, various pentasil-type zeolitic precursors are formed. It appears that specific zeolites are formed only when quaternary ammonium salts are used, their nature (structure) being essentially dependent on the length of the alkyl chains pure ZSM-8, ZSM-5 and ZSM-11 are obtained respectively with Et N+, Pr N+ and Bu N cations. TG data indicate that the latter fill nearly completely the zeolitic channel system (Table VIII). 

In this way, fairly stable acidic REY and REX zeolites are formed containing not only protons but also oxidic RE species. High-silica zeolites are more stable in acidic solutions and can be exchanged directly by protons from acidic solutions. However, a certain structure dealumination is often observed for these materials as well and typically not all cations are replaced by protons. 

This table shows that it is diflicult, even in a model system, to present a simple view of the nature of the adsorption site because of the number of different parameters involved in the stabilization of OJ. For zeolites the problem is apparently more diflicult than for oxides, since not only do the framework ions and the exchanged cations form two distinct types of adsorption sites but the latter can migrate within the zeolite structure. It is difficult to obtain a full description of the coordination of the exchanged cations and so far there has been no systematic study on this point. 

Figure 24 reports 13C MAS spectra of the ferf-butyl cation (43) and the methylcyclopentyl cation 17 (45) on the solid metal halides A1C13 and AlBr3 the asymmetry parameters, CSAs, and isotropic shifts (Table III) are unambiguous for the species indicated. Repeated attempts in various laboratories to observe the ferf-butyl cation as a persistent species in a zeolite have thus far been unsuccessful. Detailed theoretical work will be required to determine whether or not the ferf-butyl cations are local minima (i.e., true intermediates) on typical reaction pathways in zeolites. The ease with which these cations form in true superacids (liquid or solid) should be contrasted with the history of negative observations in zeolites. 

Kerr, Plank, and Rosinski reported the preparation and catalytic properties of aluminum-deficient zeolite Y materials 35). Topchieva and co-workers studied the catalytic properties of cationic forms of aluminum-deficient Y zeolites, the aluminum deficiency being effected by the H4EDTA method 36-40). They found that up to 50% aluminum removal increased both stability and cumene cracking activity maximum activity was observed at the 50% removal level. Increased catalytic cracking activity was observed by Eberly and Kimberlin for mordenites from which about 80% aluminum had been removed (. 1). Weiss et al. removed over 99% of the aluminum from a hydrogen mordenite and found the zeolite retained catalytic activity of the type induced by Bronsted acids 42). Although the initial activity of this material was lower than that of more aluminum-rich mordenites, the aging rate was markedly reduced, and in a relatively short time the aluminum-deficient catalyst was the most active. 

Palladium ions were reduced by hydrogen at room temperature. The zeolite thus formed has hydroxyl groups identical to those found in de-cationated Y zeolites and probably has a Bronsted acid character. Furthermore, hydrogen reduction produces metallic palladium almost atomically, dispersed within the zeolite framework as demonstrated by our IR, volumetric, and x-ray (23) results. Palladium atoms are located near Lewis acid sites which have a strong electron affinity. Electron transfer between palladium atoms and Lewis acid sites occurs, leaving some palladium atoms as Pd(I). Reduction by hydrogen at higher temperatures leads to a solid in which metal palladium particles are present. The behavior of these particles for CO adsorption seems to be identical to that of palladium on other supports. 

A number of cationic forms of zeolite type A were found to have high activity and selectivity in the hydration of ethylene to ethanol (27, 28). At 220° C the activity decreased in the order LaY > CaY > MgA > CdA > ZnA > AgA > SrA > CaA CeA. Minachev et al. found a one-step reaction in the formation of sec-butylbenzene from benzene and ethylene over ion-exchanged zeolites (29). 

The reactions of chlorobenzene and benzaldehyde with ammonia over metal Y zeolites have been studied by a pulse technique. For aniline formation from the reaction of chlorobenzene and ammonia, the transition metal forms of Y zeolites show good activity, but alkali and alkaline earth metal forms do not. For CuY, the main products are aniline and benzene. The order of catalytic activity of the metal ions isCu> Ni > Zn> Cr> Co > Cd > Mn > Mg, Ca, Na 0. This order has no relation to the order of electrostatic potential or ionic radius, but is closely related to the order of electronegativity or ammine complex formation constant of metal cations. For benzonitrile formation from benzaldehyde and ammonia, every cation form of Y zeolite shows high activity. 

Wu et al. (5) recently interpreted the thermal decomposition mechanism of tetramethylammonium-exchanged Y zeolite. The order of occurrence of the gaseous decay products is (mole %) (CH N (50), CH4 (11), (CH3)20 (10), CO (9), CH3OH (6), H2 (4), C4H8 (4), C2H4 (trace), for the decomposition carried out at 275°C under vacuum. At this temperature, a displacement reaction of water nucleophile on the tetramethyl cation, forming methanol and trimethylamine, is proposed ... 

The rate decay time constant is independent of cation form of the zeolite in the ethene system (Figure 4) although the alkylation activity of the three forms is considerably different (Figure 2). This indicates that the active site within the zeolite (at least for deactivation) is the same for all three cation forms as expected from our current picture of active sites for acid-catalyzed reactions in these zeolites (8, 18, 19). The three catalysts should have different numbers of active sites because of their individual response to activation at 823°K, but these sites should be similar thus M2 should be independent of cation form, Mi should depend on it. 

Si MAS NMR Chemical Shifts for the Single Signal Observed in Various Cationic Forms of Zeolite A"... 

Lefrancois and Malbois (227) determined the types of acidity present on H-mordenite and various cationic forms by obtaining infrared spectra of pyridine adsorbed on the zeolite. H-mordenite activated at 400° contained both Br0nsted and Lewis acid sites. Upon addition of water, the band due to Lewis-bound pyridine disappeared and the Br0nsted site concentration increased. Removal of the added water by evacuation restored some of the Lewis acid sites. 

As a numerical example, consider a partially dealuminated H-Y zeolite that contains 32 Alf/u.c., all of which are isolated, and 8 extraframework Al cations/u.c. This example is similar to the case of the partially dealuminated H-ZSM-20 zeolite in Figure 6 if one assumes that most of the extraframework Al is present in the cationic form. If the extraframework Al is complexed such that each Al has an equivalent charge of +2, 16 protons would be required to balance the framework charge. Here it is assumed that the cationic extraframework... 

A Ni bifunctional catalyst supported in a homoionic natural clinoptilolite will be used, as an example, to further explain the methodology of preparation of this type of catalysts [18]. As was previously commented, the thermal reduction of zeolites previously exchanged with metals is the method currently used for the preparation of bifunctional catalysts for hydrocarbon conversion. To produce these catalysts, synthetic [19,20] and natural zeolites [18,21-23] are used. During this procedure, the zeolite is exchanged with the cationic form of the metal that will be used as the catalyst and afterward the obtained exchanged zeolite is reduced in a H2 atmosphere at about 450°C and maintained at this temperature for about 2h [18,19-23],... 

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