Synthetic faujasites

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. 

Figure 14 Structural elements of zeolites (a) chabazite (b) synthetic crystal Linde A (c) faujasite, synthetic Linde X and Linde Y, all with similar structure. Straight lines Join the centers of neighbouring tetrahedra. Each vertex represents thus an aluminum or silicon atom.

Zeolites employed in the manufacture of the FCC catalyst are synthetic versions of naturally occurring zeolites called faujasites. There are about 40 known natural zeolites and over 150 zeolites that have been synthesized. Of this number, only a few have found commercial applications. Table 3-1 shows properties of the major synthetic zeolites. 

Faujasite is a naturally occurring mineral, having a specific crystalline, alumina-silicate structure, used in the manufacturing of the FCC catalyst. Zeolite faujasite is a synthetic form of the mineral. 

Natural zeolites may bear the name of the mineral (mordenite, faujasite, ferrier-ite, silicalite), or sometimes that of the discoverer, e.g. Barrerite after Professor Barrer, or the place where they were found, e.g. Bikitaite from Bikita, Zimbabwe. Synthetic zeolites are usually named after the industry or university where they were developed, e.g. VPI comes from Virginia Polytechnic Institute, and ZSM stands for Zeolite Socony Mobil. 

The linking pattern of two zeolites is shown in Fig. 16.24. They have the /I-cage as one of their building blocks, that is, a truncated octahedron, a polyhedron with 24 vertices and 14 faces. In the synthetic zeolite A (Linde A) the /3-cages form a cubic primitive lattice, and are joined by cubes. j3-Cages distributed in the same manner as the atoms in diamond and linked by hexagonal prisms make up the structure of faujasite (zeolite X). 

One of the most promising techniques for studying transition metal ions involves the use of zeolite single crystals. Such crystals offer a unique opportunity to carry out single crystal measurements on a large surface area material. Suitable crystals of the natural large pore zeolites are available, and fairly small crystals of the synthetic zeolites can be obtained. The spectra in the faujasite-type crystals will not be simple because of the magnetically inequivalent sites however, the lines should be sharp and symmetric. Work on Mn2+ in hydrated chabazite has indicated that there is only one symmetry axis in that material 173), and a current study in the author s laboratory on Cu2+ in partially dehydrated chabazite tends to confirm this observation. 

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. 

The nomenclature of zeolites is rather arbitrary and follows no obvious rules because every producer of synthetic zeolites uses his/her own acronyms for the materials. However, as mentioned before, at least the structure types of the different zeolites have a unique code. For example, FAU represents Faujasite-type zeolites, LTA Linde Type A zeolites, MFI Mobile Five, and BEA Zeolite Beta. The structure commission of the International Zeolite Association (IZA) is the committee granting the respective three-letter codes [4], Some typical zeolites, which are of importance as catalysts in petrochemistry, will be described in the following sections. 

Schwochow, F. and Heinze, G. (1973) Production of synthetic zeolites of faujasite strucmre. US Patent 3,720,755. 

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-... 

Tn the course of experimentation with formulations of silica, alumina, and A various alkali metal oxides in attempts to prepare new synthetic zeolites, a formulation containing cesium replacing some of the sodium in a typical faujasite preparation yielded a new crystalline zeolitic product which showed a typically cubic powder diagram having a body-centered pattern of... 

Tphe study of the properties of zeolites, either synthetic or natural, has A received a great deal of attention in recent years. Among the synthetic zeolites, the faujasites X and Y types have been most frequently and thoroughly studied. A summary of the advances in this area is found in a recent review by Sherry (1). 

Synthetic Na-faujasites with different degrees of NH4+ exchange and with different Si/Al ratios in the lattice were used as starting materials. Compositions of the samples investigated were ... 

Egerton and Stone (29), taking into account that synthetic sodalite zeolites did not adsorb CO molecules, concluded that CO does not enter the sodalite cages of the Y zeolites. However, the strong electric fields present in zeolites could also produce changes in the adsorptive properties of the solids thus the energies associated with the cationic sites in crystalline zeolites must be considered. From our IR results, we concluded that CO molecules were located in the volume of the sodalite cages. Thus, the steric effect alone cannot explain the different adsorptive properties exhibited by sodalite and faujasite. 

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. 

In these experiments, synthetic zeolites of the faujasite-type without binding substance were used. Calcium and nickel-calcium samples in ionic form were obtained by ion exchange under conditions ensuring stability of the crystal structure (5). Platinum addition was carried out by ion exchange with Pt(NH3)6Cl4 (6). 

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. 

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