Linde-A zeolite

An alternative hypothesis, developed from studies of the synthesis of Linde A zeolite carried out by Kerr (5) and Ciric (6), pointed to growth occurring from solution. The gel was believed to be at least partially dissolved in solution, forming active aluminosilicate species as well as silicate and aluminate ions. These species linked to form the basic building blocks of the zeolite structure and returned to the solid phase. Aiello et al. (7) followed the synthesis from a highly alkaline clear aluminosilicate solution by electron microscopy, electron diffraction, and x-ray diffraction. These authors observed the formation of thin plates (lamellae) of amorphous aluminosilicates prior to actual crystal formation. 

Figure 2.9(a) The Linde A zeolite on one side of the P-surface. The water-alkali structure is on the other side, (b) The faujasite structure on one side of the D-surface. 

Shape selectivity and catalyst deactivation. A serious problem in catalytic cracking and other refinery operations is catalyst deactivation by coking. Coke forms on the catalyst from bulky molecules such as polyalkyl benzenes and polycyclic aromatics that are slow or unable to escape from the catalyst [57], These molecules, in turn are formed mainly from cracked olefins. Coking is severe in zeolites with window-and-supercage structure (chabazite, erionite, Linde A). Zeolites like ZMS-5, with straight channels and no supercages, are much less affected because the formation of bulky coke precursors is sterically inhibited [58]. 

Fig. 8. a Isothermal DTA trace showing the endotherm connected with the Na-Pl Na-P2 transformation b DTA curve of Ba-exchanged Linde A zeolite, showing the sharp endotherm connected with lattice collapse (reproduced by permission from [35])... 

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). 

OlefinSiv A process for isolating isobutene from a mixture of C4-hydrocarbons by chromatography over a zeolite molecular sieve. Developed by the Linde Division of the Union Carbide Corporation, as one of its IsoSiv family of processes. 

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. 

Garcia-Sanchez, A., Garcia-Perez, E., Dubbeldam, D., Krishna, R., and Calero, S. (2007) A simulation study of alkanes in Linde type A zeolites. Adsorp. Sci. Technol, 25, 417-427. 

A Linde NaY zeolite without binder was ion exchanged in an ammonia-cal solution of PdCl2 which provides exchangeable (Pd(NH3)4)2+ cations. The solution was stirred at room temperature for 24 hr and then filtered. The zeolite was washed with ammonia solution to eliminate Cl ions. The desired exchange level was readily obtained by allowing the zeolite to equilibrate in a solution where a suitable amount of palladium has been introduced. Chemical analysis for palladium and sodium showed the composition of the calcined sample to be Pdtf.sNaiQ.sHn.sAlfieSiiaeCW (10 wt % of Pd proton concentration determined by difference). 

The influence of exchangeable monovalent cations on the framework vibrations for the hydrated zeolites Linde A and X has been investigated. An approximately linear relationship is found between the frequency of some absorption bands and the inverse of the sum of the cation and framework oxygen ionic radii. It is proposed that the shift in framework vibrations is largely caused by those cations which are strongly interacting with the zeolite framework. Thus the linear relationship indicates that these monovalent cations are all similarly sited in the zeolite lattice. This is consistent with the presently available x-ray analyses on some of these zeolites. Since Rb + and Cs + are only partially exchangeable in both Linde A and Linde X, these cations deviate from this linear relationship. 

The object of the present investigation was to study systematically the effect of monovalent cations on the lattice vibrations in the synthetic zeolites Linde A and Linde X. It was reasoned that mid-infrared spectroscopy might yield information on cation siting in these zeolites. 

Cation Siting in Linde A. At the time this work was completed, x-ray studies on hydrated NaA (3, 4) and hydrated KA (5) had shown that 8 of the 12 exchangeable cations per unit cell are firmly bound to the zeolite framework and would therefore be expected to have the major influence on the lattice vibrations. These cations are sited in front of the sodalite... 

The exchangeable monovalent cations have a marked influence on the framework vibrations of hydrated Linde A and X. For some vibrational modes the frequency shifts appear to give a quantitative measure of the interaction between cations and lattice. A regularity is found for Li+, Na+, Ag+, K+, and T1+ exchanged forms which implies a similar distribution of cation sites for both zeolites. It is further deduced that in the Cs+ and Rb+ exchanged forms there is only a relatively weak interaction between the cations and the zeolite framework. This technique can be readily extended to study cation siting in other zeolites in both hydrated and dehydrated forms. 

In Linde A and sodalite syntheses the signal grew to about 20 times its initial intensity. In other systems, such as faujasite, the increase was somewhat smaller. The increase seemed to depend upon the Si/Al ratio of the resultant zeolite crystals—i.e., the smallest increase occurred for mordenite crystallizations having an Si/Al ratio of 5 (for Linde A and sodalite Si/Al = 1). No Fe3+ phosphorescence was observed in the liquid phase of the gel. In three experiments carried out under identical conditions Fe3+ phosphorescence studies of the growth kinetics gave identical results (induction periods equal within 5%, Fe3+ intensity increase on crystallization equal within 10%). 

Three Co(II)Y zeolites with different cobalt concentrations were prepared from a Linde NaY zeolite (lot no. 13544-76) by conventional ion-exchange. A cation analysis of the Co(II)Y zeolites indicated concentrations of 0.8, 5, and 16 Co2+ ions per unit cell. 

We further illustrate the approach by reference to studies (331) of two related zeolite structures zeolite ZK-4 (isostructural with Linde A) and the highly siliceous analogue of sodalite known as TMA-sodalite. As was shown earlier (Sections III,A and III,D), the structure of zeolite A consists of a cubic array of / -cages linked through double four-membered rings so as to form larger polyhedral a-cages. The sodalite structure (Fig. 7) consists of a dense,... 

The ion exchange properties of zeolite ZSM-5 are of interest for several reasons. The high Sic>2/Al203 ratio of this material means low ion exchange capacity and the ion selectivity patterns of this unique, siliceous zeolite should be quite different from those of low SiC /A Oj zeolites such as Linde A and synthetic... 

Dehydration of the synthetic zeolite Linde A, Na12[(A102)i2(Si02)i2]-27H20, leaves cubic microcrystals in which the A104 and Si04 tetrahedra are linked together... 

Figure 2.48. Representations of zeolite structures. Shown are molecular and crystal representations of a polyhedron (A) formed from 24 Si04 tetrahedra. Also shown is the three- dimensional array of LTA, Linde A [Nai2(Ali2Sii204g)]-27H20 formed from interlocking Si04 and AIO4 polyhedra of (pore size, B 4.1 A). Reprinted from Greenwood, N. N. Earnshaw, A. Chemistry of the Elements, 2nd ed.. Copyright 1998, with permission from Elsevier.

FIGURE 25.18 Main zeolite structures. SOD sodalite LTA Linde type A zeolite FAU faujasite, zeolites X or Y MOR mordenite LTL Linde type L zeolite MFI ZSM-5. (From Ozin, G.A., Kuperman, A., and Stein, A., Angew. Chem. Int. Ed. Engl., 28, 359, 1989.)... 

Examples of zeolites sitting close to or on one side of periodic minimal surfaces are shown in Figs. 2.9(a),(b) the zeolite known as Linde-A on the P-surface and faujasite on the D-surface. Other examples are zeolite N in which the D-surface partitions the ZK5 and sodalite structures, and also paulingite which is described by the P-surface. 

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