Zeolite faujasitic

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. 

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. 

Catalytic cracking with faujasite zeolites X and y zeolites... 

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). 

Kieger, S., Delahay, G. and Coq, B. (2000) Influence of co-cations in the selective catalytic reduction of NO by NH3 over copper exchanged faujasite zeolites, Appl. Catal. B 25, 1. 

Sarkar N, Das K, Nath DN, Bhattacharyya K (1994) Twisted charge transfer processes of Nile red in homogeneous solutions and in faujasite zeolite. Langmuir 10(l) 326-329... 

Keywords basicity, faujasite, zeolite, MBOH, methylacetylene, H2S. 

In all cases, the high activity and selectivity of this zeolite as compared to other three-dimensional zeolites, such as beta or dealuminated Faujasite zeolite, is related to the easier and/or faster diffusion of the products and to minimization of undesired consecutive reactions. 

New Zeolitic Structures. Multiply twinned faujasitic zeolites (typically zeolite-Y) have recently been shown (30, 31) to be capable, by recurrent twinning on 111 planes, to generate a new, hexagonal zeolite in which tunnels replace the interconnected cages of the parent cubic structure. 

Figure 10a, Scheme of the aluminosilicate framework of a typical faujasitic zeolite Si/Al ratio of LI8 (arbitrarily chosen to illustrate the ordering among tetrahedral sites) before (left half) and after (right half) exposure to SiCls, which dealuminates the zeolite (see Figure 10b),... 

Figure 10b, Magic anglespinning 29Si-NMR spectra of a typical faujasitic zeolite (Si/Al ratio 2.61) before and after dealumination by exposure to SiClk vapor...

Zeolites are crystalline porous solids with pore dimensions at the molecular level. Some zeolite types, such as the faujasites (zeolite X and Y or their hexagonal isomer EMT), possess large supercages with an internal diameter of approximately 1.2 nm, connected by pores with a diameter of approximately 0.75 nm. A metal complex will be confined in the supercage, when its size exceeds 0.8 nm. 

Early attempts to utilize the high acid activity of faujasite zeolite catalysts for direct xylene isomerization suffered from low selectivity. Considerable improvement was obtained first by using a large pore zeolite (7) catalyst and subsequently in several process modifications that use ZSM-5 as catalyst (2). In the following we will show how these selectivity differences can be related to structural differences of the various zeolites. 

The pores of zeolites can be regarded as extensions of their surfaces zeolites have an external surface, i.e., the surface of the zeolite crystallites, and an internal surface, i.e., the surface of their channels and/or cages. In total, the surface areas of zeolites are remarkably large. One gram of a typical Faujasite zeolite expresses a geometric surface area of about 1100 m2/g (specific surface area). The contribution of the external surface area to this number is almost negligible (about 5 m2 g 1 for 1 pm crystallites), and almost the complete surface area is due to the surface of the micropores. 

Erising, T. and Leflaive, P. (2008) Extraframework cation distributions in X and Y faujasite zeolites a review. Micropor. Mesopor. Mat., 114, 27-63. 

For a zeohtic catalyst where Pt, Pd or other transition metal might be present to provide metal activity, STEM can be used to determine whether the metal is agglomerated and to what extent, whether the metal is in the zeolite or present on the geometric exterior or whether the metal is associated with the zeolite or binder. As an example of the utility of the technique. Figure 4.15 shows the growth of Pt clusters for fresh and spent faujasite zeolite catalyst. After time under reaction conditions, the Pt clusters have grown from Inm to 2nm. The clusters have remained in the channels of the faujasite. Pt agglomeration can be concluded as the deactivation mechanism. 

Zeolites can be ion-exchanged with cations or impregnated with various metals to modify their performance for use in applications such as separations, adsorption and catalysis. For example, faujasite zeolites exchanged with Na, Li, K, Ca, Rb, Cs, Mg, Sr, Ba, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Pd, Ag, Cd, In, Pt, H, Pb, La, Ce, Nd, Gd, Dy and Yb have been made and studied due to their use in separation and catalysis [135]. The ability to determine the distributions of these cations in the zeolitic structure is one of the key parameters needed in understanding adsorption mechanisms and molecular selectivities. Little has compiled an excellent reference... 

Kraikul, N., Rangsunvigit, P., and Kulprathipanja, S. (2005) Study on the adsorption of 1,5-1, 5- and 2,5-dimethyl-naphthalene on a series of alkaline and alkaline earth ion-exchanged faujasite zeolites. Adsorption, 12, 317. 

The faujasite zeolite in the UOP Parex process has some finite affinity for aU the aromatic species in the mixed xylene feed, indicated by the fact that selectivities between the components are typically less than five. Because the adsorbent has the tendency to adsorb all aromatic species in the feed to some extent, the fundamental variable dictating the adsorption zone operation is the ratio of zeolitic selective pore volume circulated past the feedpoint by the stepping action of the rotary valve per the volume of aromatics conveyed to the adsorption chambers. Typically this ratio is set to obtain a certain target recovery of p-xylene. 

The protons released are presumably available to compensate for the loss of the charge balancing cations within the zeolite. In conventional syntheses, the phtha-lonitrile condensation normally requires the nucleophilic attack of a strong base on the phthalonitrile cyano group [176, 177]. This function is presumably accommodated by the Si-O-Al (cation) basic sites within the ion-exchanged faujasite zeolites [178, 179]. The importance of this role is perhaps emphasized by the widespread use of alkali metal exchanged faujasites, particularly the more basic NaX materials of higher aluminium content [180, 181] as hosts for encapsulated phthalocyanine complexes. 

In the preparation of faujasite zeolite-supported Pt-Re catalysts, bimetallic PtRe clusters have been reported to be predominantly formed when a carbonyl rhenium precursor (Re2(CO)io) is contacted with zeolite in which platinum has been previously introduced and reduced. The preexisting Pt clusters may act as nucleation sites. After reduction, these Pt-Re systems show a high selectivity to CH4 in the hydrogenolysis of n-heptane [58]. 

For faujasitic zeolites presenting a high A1 content, an easy stepwise decarbonyl-ation of [Mo(CO)6] occurs by heating under vacuum, the temperature of CO evolution depending on both the Lewis acidity of the cation and the Si/Al raho [39, 40,... 

The active component for olefin oxidation is Pd2+, while Cu2+ acts as a promoter for the reoxidation of Pd. The sequence of ion exchange of Pd + and Cu2+ on the faujasite zeolite influences the catalytic performance. Best results seem to be obtained when Pd + is introduced in the second step of the ion exchange as it will then be located mainly at the more easily accessible cation sites II and/or III [23], The amount of exchanged Pd + determines the catalytic activity of Pd +Cu +Y, provided that Cu2+ is present in sufficient amounts to assure fast regeneration of Pd2+. A Pd/Cu atomic ratio of four is required here. Increasing acidity in Pd +Cu +NaY results in a decrease of both the activity and selectivity in the olefin oxidation [26]. 

FIGURE 7.6 Zeolite frameworks built up from sodalite units (a) sodalite (SOD), (b) zeolite A (LTA), and (c) faujasite (zeolite X and zeolite Y)... 

In Fig. 2.1.6.6, the FTIR spectra of the Jacobsen ligand (a), the Jacobsen catalyst (bj, and the immobilized manganese salen complex in the cages of dealuminated faujasite zeolite (c) are compared. While spectra a and b have been measured using the standard KBr technique, the spectrum c of the ship in a bottle catalyst has been recorded using a self-supported wafer. The bands at wavenumbers 1466 cm, 1434 cm" , 1399 cm" and 1365 cm" in spectrum c can be assigned to the... 

Similarly, our forcefield works equally well for unsaturated halocarbons. For example, calorimetric heats of adsorption for trichloroethylene in the same three faujasite zeolites are in excellent agreement with our (N.V.T) Monte Carlo simulations [16]. Our results at "zero" loading suggest, unlike hydrocarbons, an analogy between the adsorption processes of saturated and unsaturated halocarbons. 

The forcefield has been successfully extended to treat fluorocarbons and chlorofluorocarbons in faujasite zeolites [13]. (N,V,T) Monte Carlo simulations on the adsorption of a series of fluoro-, chlorofluoro- and hydrofluoro-carbons (CF CF,C1, CF,C1, CFC1, CHFj) in siliceous Y and NaY zeolites were performed and compared with available calorimetric data on the same host-guest systems. They predict adsorption heats with good accuracy (Table 3), while yielding a first validation of our forcefield parameters. 

Zeolites. Unlike silica and clay, zeolites possess interior structures that are uniform and well defined in shape and size. In spite of this, inhomogeneity in the microenvironment around a guest included in a faujasite zeolite may arise for two reasons variation in the occupancy number within a cage and the presence of sites of varying microenvironment. Even at low loading levels, the... 

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