Faujasites

A new dimension to acid-base systems has been developed with the use of zeolites. As illustrated in Fig. XVIII-21, the alumino-silicate faujasite has an open structure of interconnected cavities. By exchanging for alkali metal (or NH4 and then driving off ammonia), acid zeolites can be obtained whose acidity is comparable to that of sulfuric acid and having excellent catalytic properties (see Section XVIII-9D). Using spectral shifts, zeolites can be put on a relative acidity scale [195]. An important added feature is that the size of the channels and cavities, which can be controlled, gives selectivity in that only... 

Zeolites (section C2.13) are unique because they have regular pores as part of their crystalline stmctures. The pores are so small (about 1 nm in diameter) that zeolites are molecular sieves, allowing small molecules to enter the pores, whereas larger ones are sieved out. The stmctures are built up of linked SiO and AlO tetrahedra that share O ions. The faujasites (zeolite X and zeolite Y) and ZSM-5 are important industrial catalysts. The stmcture of faujasite is represented in figure C2.7.11 and that of ZSM-5 in figure C2.7.12. The points of intersection of the lines represent Si or A1 ions oxygen is present at the centre of each line. This depiction emphasizes the zeolite framework stmcture and shows the presence of the intracrystalline pore stmcture. In the centre of the faujasite stmcture is an open space (supercage) with a diameter of about 1.2 nm. The pore stmcture is three dimensional. 

Figure C2.12.4. Typical polyhedra found in zeolites (a) sodalite cage found in sodalite, zeolite A or faujasite (b) cancrinite or a-cage found in cancrinite, erionite, offretite or gmelinite (c) the 5-ring polyhedron found in ZSM-5 and ZSM-11 (d) the large cavity of the faujasite stmcture and (e) the a-cage fonning the large cavity in zeolite A.

Damour A A 1942 Description de la faujasite, nouvelle espece minerals Ann. Mines 4 395-9... 

The separation of fmctose from glucose illustrates the interaction between the framework stmcture and the cation (Fig. 5) (50). Ca " is known to form complexes with sugar molecules such as fmctose. Thus, Ca—Y shows a high selectivity for fmctose over glucose. However, Ca—X does not exhibit high selectivity. On the other hand, K—X shows selectivity for glucose over fmctose. This polar nature of faujasites and their unique shape-selective properties, more than the molecular-sieving properties, make them most useful as practical adsorbents. 

Fig. 5. Fructose—glucose separation on faujasite adsorbents, (a) Ca—Y adsorbent (b) Ca—X adsorbent (c) K—X adsorbent.

Fig. 4. Model of the ciystal structure of zeolites X, Y, and the mineral faujasite. At the tight is shown the tetrahedral arrangement of tmncated octahedra surrounding one large cavity. On the left the packing model of zeohte X is shown, containing three types of Na cations.

Fig. 14. Schematic representation of the stmcture of a faujasitic 2eohte. I and II indicate cation positions.

Another catalytically important zeohte is ZSM-5 (81). There is a three-dimensional network of pores in this zeohte, represented in Figure 16. A set of straight parallel pores is intersected by a set of perpendicular zigzag pores. These pores are smaller than those of the faujasites (Fig. 15). ZSM-5 is classified as a medium pore zeohte, the faujasites ate large pore zeohtes, and zeohte A (Table 2) is a small pore zeohte. 

The diffusion, location and interactions of guests in zeolite frameworks has been studied by in-situ Raman spectroscopy and Raman microscopy. For example, the location and orientation of crown ethers used as templates in the synthesis of faujasite polymorphs has been studied in the framework they helped to form [4.297]. Polarized Raman spectra of p-nitroaniline molecules adsorbed in the channels of AIPO4-5 molecular sieves revealed their physical state and orientation - molecules within the channels formed either a phase of head-to-tail chains similar to that in the solid crystalline substance, with a characteristic 0J3 band at 1282 cm , or a second phase, which is characterized by a similarly strong band around 1295 cm . This second phase consisted of weakly interacting molecules in a pseudo-quinonoid state similar to that of molten p-nitroaniline [4.298]. 

Figure 10.4. Outline structures of (a) zeolite A, (b) its homologue faujasite, (c) the channel network of the tubular zeolite ZSM-5.

L. A. Luke and J. V. Brunnock, Separation of naphthenic and paraffinic hydrocarbons up to C from hydrocarbon mixtures by gas cliromatography on faujasite molecular sieves , Ger. Ojfen. 1 908418 (1968). 

Zeolite, or more properly, faujasite, is the key ingredient of the FCC catalyst. It provides product selectivity and much of the catalytic activity. The catalyst s performance largely depends on the nature and quality of the zeolite. Understanding the zeolite structure, types, cracking mechanism, and properties is essential in choosing the right catalyst to produce the desired yields. 

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. 

Soda Y Zeolite is a crystallized form of Y-faujasite before any ion exchanges occur. 

Ultra Stable Y is a hydrothermally treated Y-faujasite which has a unit cell. size at or below 24.50 °A and exhibits superior hydrothermal stability over Soda Y faujasite. 

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). 

The framework structures and pore cross-sections of two types of zeolites are shown. (Top) A Faujasite-type zeolite has a three-dimensional channel system with pores of at least 7.4 A in diameter. A pore is formed by 12 oxygen atoms in a ring. (Bottom) ZSM-5 zeolite has interconnected channels running in one direction, with pores 5.6 A in diameter. ZSM-5 pores are formed by 10 oxygen atoms in a ring. Reprinted with permission from Chemical Engineering Progress, 84(2), February 1988, 32. 

Figure 4.2. The structure of Faujasite, a more open, larger pore zeolite. Larger molecules can enter this structure, which is more open, and slightly less regular than HZSM5 (Figure 4.1). Nevertheless, there are still many important molecules which cannot enter the pores of this zeolite, one of the most accessible of the class.

Zeolites are used in various applications such as household detergents, desiccants and as catalysts. In the mid-1960s, Rabo and coworkers at Union Carbide and Plank and coworkers at Mobil demonstrated that faujasitic zeolites were very interesting solid acid catalysts. Since then, a wealth of zeolite-catalyzed reactions of hydrocarbons has been discovered. Eor fundamental catalysis they offer the advantage that the crystal structure is known, and that the catalytically active sites are thus well defined. The fact that zeolites posses well-defined pore systems in which the catalytically active sites are embedded in a defined way gives them some similarity to enzymes. 

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. 

With over 600 currently known zeolites and new ones discovered every year, it is useful to have a general classification of structures endorsed by the lUPAC. In this system each structure has three letters, for example EAU for faujasites, MFI for ZSM-5 and MOR for mordenite. Within a given structure there can still be many different zeolites, as the composition may vary. 

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