Synthetic faujasite

We have examined the rate constants for disproportionation and isomerization for a variety of zeolites, using a commercial-type feed containing 70% m-xylene and 30% o-xylene in a fixed-bed flow reactor. The results, listed in Table I, show the exceptionally low disproportionation/isomerization selectivity of ZSM-5 relative to synthetic faujasite. Synthetic mordenite and ZSM-4 have intermediate selectivities. 

It has been suggested that the reason for this difference is the different site density. According to this proposal, the large concentration of acid sites in synthetic faujasite (ca. 5 meq/g) favors the bimolecular disproportionation reaction relative to the monomolecular isomerization. By contrast, ZSM-5 has a low acid site concentration, typically less than 0.5 meq/g. 

Figure 1. Comparison of the relative disproportionation versus isomerization selectivities of HZSM-5 and synthetic faujasite (8). Feed m-xylene. Temperature 300°C.

The correlation between selectivity and intracrystalline free space can be readily accounted for in terms of the mechanisms of the reactions involved. The acid-catalyzed xylene isomerization occurs via 1,2-methyl shifts in protonated xylenes (Figure 3). A mechanism via two transalkylation steps as proposed for synthetic faujasite (8) can be ruled out in view of the strictly consecutive nature of the isomerization sequence o m p and the low activity for disproportionation. Disproportionation involves a large diphenylmethane-type intermediate (Figure 4). It is suggested that this intermediate can form readily in the large intracrystalline cavity (diameter. 1.3 nm) of faujasite, but is sterically inhibited in the smaller pores of mordenite and ZSM-4 (d -0.8 nm) and especially of ZSM-5 (d -0.6 nm). Thus, transition state selectivity rather than shape selective diffusion are responsible for the high xylene isomerization selectivity of ZSM-5. 

We find from separate e qperiments with ZSM-5 catalyst that the intrinsic rate constant for isomerization, kj, is much faster than that for disproportionation, kD at 482°C, kj/kD > 7000, i.e. kj/kD is much faster than it is for the synthetic faujasite catalyst. 

The frameworks of zeolites are formed by fully connected Si04 and A104 tetrahedra linked by shared oxygen atoms as shown in Figure 4.1 (top) for a Faujasite-type zeolite. Faujasite is a zeolitic mineral, which can be found in nature. Synthetic Faujasite-type zeolites are of particular importance in zeolite catalysis as we will see below. 

Sherry, H.S. (1966) The ion-exchange properties of zeolites. I. Univalent ion exchange in synthetic faujasite. f Phys. Chem., 70 (4), 1158-1168. 

Si/Al ratios of synthetic faujasites obtained from NMR agree quite well with that obtained from X-ray fluorescence [209]. Comparison of bulk versus framework Si/... 

Dempsey, E., Kuhl, G.H., and Olson, D.H. (1969) Variation of the lattice parameter with aluminum content in synthetic faujasites. Evidence for ordering of the framework ions. 

Figure 7. Diagram of synthetic faujasite cages showing cation positions (SI, SF, Sir, SII, sill) and pore openings...

Faujasite-type zeolite structures have maximum symmetry Fd3m, and all the 192 T atoms per unit cell of the A structure are symmetrically equivalent. The observed Si/Al ratios of synthetic faujasite-type species vary within a range from slightly over 1 up to 2.5 (and occasionally above). Unmodified species thus normally contain between 48 and almost 96 A1 atoms per unit cell. The almost continuous range in A1 content does not by itself rule out any kind of Si, A1 order. Discontinuities in the plot of the cell dimensions against the number of A1 atoms per unit cell have been reported by several investigators (11, 12). The observed discontinuity at around 64 Al, in particular, has been related to Si, A1 ordering (12). Full details and references on faujasite-type zeolite structures can be found in the comprehensive and critical review by Smith (13). 

Here we link these different results and interpretations together by dielectric measurements on a series of synthetic faujasites with four differ-... 

Olefin Separation. U.O.P. s Olex Process. U.O.P. s other hydrocarbon separation process developed recently—i.e., the Olex process—is used to separate olefins from a feedstock containing olefins and paraffins. The zeolite adsorbent used, according to patent literature 29, 30), is a synthetic faujasite with 1-40 wt % of at least one cation selected from groups I A, IIA, IB, and IIB. The Olex process is also believed to use the same simulated moving-bed operation in liquid phase as U.O.P. s other hydrocarbon separation processes—i.e., the Molex and Parex processes. 

The number of reactions catalyzed by zeolites is continually increasing. Synthetic faujasites containing transition metal cations are active for the complete oxidation of H2, CO, C2H4, NH3 (9). Upon interaction of NH3 with 02 over CrY and AgY, N20 and N2 are formed (9). Mahida et al. (21) investigated the oxidation of propylene over Cu2+Y. Depending on the temperature and on the water vapor content in the reaction mixture,... 

We studied the effect of C02 on synthetic faujasites for the disproportionation of toluene (56, 62, 76) and for the alkylation of benzene with olefins (77) in great detail. Lapidus et al. (78) investigated the conversion of isobutylene over NaX and NaY zeolites in the presence of C02. Over NaY, the conversion could be increased by adding C02 to the olefin, and C5 and C7 hydrocarbons were formed. Over NaX the effect is less pronounced (Table II). Addition of N2 had no appreciable effect. 

Infrared Spectroscopic Study of the Isotopic Exchange of Lattice Hydroxyls in Synthetic Faujasites... 

In contrast to previous work (12), in the present paper the D2 exchange is followed continuously with the infrared spectrometer at reaction temperature. Samples were selected to compare the ability for deuteration of all the types of hydroxyl groups reported in synthetic faujasites. 

Experimental and Corrected Values of29Si Chemical Shifts in Synthetic Faujasites, and Widths at Half-Height of Simulated Si(nAl) Signals ... 

High-Resolution 29Si MAS NMR Peak Intensities in Synthetic Faujasites Determined by Computer Simulation, with Si/Al Ratios Determined from Spectref... 

The obvious case to be considered first is that of synthetic faujasites, which come in a range of compositions, and for which a considerable amount of spectral information is available. Evidence of Si, A1 ordering in zeolites X and Y is provided by the presence of discontinuities in the plot of the (cubic) lattice parameter versus the Si/Al ratio (60), which indicates stepwise rather than gradual change in Si, A1 distribution. This effect is even more pronounced in synthetic faujasitic gallosilicates (61). 

A new method of achieving such substitution in synthetic faujasites has been reported by Beyer and Belenykaja (205). It involves passing silicon tetrachloride vapor through a bed of dehydrated zeolite at elevated temperatures. Klinowski et al. (206-209) and Hays et al. (210) applied 29Si and 27A1 MAS NMR to the study of this remarkable reaction in a number of zeolites. 

West (231) observed no 23Na resonance in dehydrated synthetic faujasites, suggesting that the EFG at the cationic site is larger than in hydrated samples because of the displacement of the cations away from their high-symmetry positions. The signal appeared when seven H20 molecules per cage were present. Fully hydrated Na-X and Na-Y had Tf of 100 and 140 fj.sec, respectively, while in dehydrated samples much faster transverse relaxation was observed. 

It is known that the 29Si NMR chemical shift in zeolites is sensitive to the type of the exchangeable cation (56), which indicates the presence of interactions between cations and the framework. In particular, the substitution of Na+ by Li+ in zeolite A and in synthetic faujasite moves the 29Si resonances ca. 4 ppm downfield in both cases. Melchior et al. (235) have used this effect to study the location of cations in a series of partially exchanged zeolites (Li,Na)-A. They found that the average 29Si chemical shift is not a... 

Where Cz characterizes Xe-surface collisions and p, is the probability factor depending only on the particular structure under consideration. The importance of Eq. (27) lies in the fact that its various terms can be determined separately. For example, ds + SE is obtained by extrapolation of the plot of chemical shift to zero pressure. It is found that 8B is negligible in zeolites Na-Y and H-Y, possibly because of motional averaging of electric field effects on the large faujasite supercages, but it becomes important in alkaline earth-exchanged forms. Zeolites Ca-A, Na-X, Na-Y, H-Y Ca-Y, L, mordenite, and ZSM-5 have been studied using the method because of the aperture size, xenon is not sorbed on zeolite Na-A. In synthetic faujasites, the chemical shift is a linear function of pressure and varies between 58 to 110 ppm (from Xe gas... 

The model has been tested for a wide variety of gas-zeolite combinations. Gases of increasing complexity were considered Ar(non-polar), (quadrupole moment, no dipole moment), NgO (quad-rupole moment, small dipole moment), and NHg (large dipole moment, small quadrupole moment). The zeolites tested were all in the synthetic faujasite family however, they ranged from the cation-rich zeolite X to the cation-poor zeolite Y. Cation geometries considered in the tests were those typical of the dehydrated zeolite form and those typical of the hydrated geometry (associated with NHg adsorption). Two forms of representative cations were considered, Li and Na+. 

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. 

Faujasite is a rare aluminosilicate mineral. Its synthetic high-aluminia counterpart, Linde X, was synthesized by Milton (2), and Linde Y, a high-silica/alumina synthetic faujasite, was synthesized by Breck (3). The framework of faujasite-type zeolites has been well established by Bergerhoff et al. (4) and Broussard and Shoemaker ( 5). 

Si-MAS-NMR spectra of the synthetic faujasites indicate a much lower concentration of non-framework Al when dealuminated with SiCl4 than in the case of the hydrothermally treated samples. The difference between the Si/Al ratio as obtained by 29Si-MAS-NMR and elemental analysis is much smaller, indicating that much of the Al is completely removed from the lattice. 

Beyerlein et al. (33) studied the catalytic properties of a series of ultrastable synthetic faujasites dealuminated by steaming and by acid extraction to determine catalytic acidity as a function of framework characteristics. They found that carbonium-ion activity in isobutane conversion is proportional to framework-Al content, and comparing results obtained by using hydrothermally and AHF-dealuminated synthetic faujasite, they found that the steamed material, which contains extra-framework Al, gave a large increase in carbonium-ion activity compared with the AHF-treated material, which had a relatively clean framework. This indicates that strong acidity exhibited by mildly steamed synthetic faujasite, while directly related to framework-Al content, depends on a balance between framework and extra-framework Al, and that this extraframework Al contributes greatly towards catalytic performance. 

Cotterman et al. (34) showed that hexadecane-cracking activity of AFS and USY zeolites appeared to be a function of total Al content, independent of method of dealumination, implying that hexadecane cracking occurs over both framework- and extra-framework-acid sites. Hence, extra-framework material in mildly steamed synthetic faujasite, USY, makes a significant contribution to catalyst activity, as previously reported (32). Gasoline selectivity is influenced by both the method of dealumination and steam treatment, and depends on both framework-acid sites and the presence of extra-framework material. 

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