Zeolite Framework Structure

In the direct ammoxidation of propane over Fe-zeolite catalysts the product mixture consisted of propene, acrylonitrile (AN), acetonitrile (AcN), and carbon oxides. Traces of methane, ethane, ethene and HCN were also detected with selectivity not exceeding 3%. The catalytic performances of the investigated catalysts are summarized in the Table 1. It must be noted that catalytic activity of MTW and silicalite matrix without iron (Fe concentration is lower than 50 ppm) was negligible. The propane conversion was below 1.5 % and no nitriles were detected. It is clearly seen from the Table 1 that the activity and selectivity of catalysts are influenced not only by the content of iron, but also by the zeolite framework structure. Typically, the Fe-MTW zeolites exhibit higher selectivity to propene (even at higher propane conversion than in the case of Fe-silicalite) and substantially lower selectivity to nitriles (both acrylonitrile and acetonitrile). The Fe-silicalite catalyst exhibits acrylonitrile selectivity 31.5 %, whereas the Fe-MTW catalysts with Fe concentration 1400 and 18900 ppm exhibit, at similar propane conversion, the AN selectivity 19.2 and 15.2 %, respectively. On the other hand, Fe-MTW zeolites exhibit higher AN/AcN ratio in comparison with Fe-silicalite catalyst (see Table 1). Fe-MTW-11500 catalyst reveals rather rare behavior. The concentration of Fe ions in the sample is comparable to Fe-sil-12900 catalyst, as well as... 

Numerous new developments and applications of solid state NMR techniques have emerged. Multidimensional NMR methods are able to probe connectivity patterns of zeolite framework structures and solve ambiguities in line assignments [27], high-resolution techniques for quadrupolar nuclei have been developed [31-34], and powerful double-resonance methods permit the study of spatial... 

A tutorial review of 170 solid state NMR has recently been published by Ashbrook and Smith [103]. Although it has been early envisaged that nO solid state NMR can provide useful information on the zeolite framework structure [104, 105], the number of publications employing this nucleus has remained scarce compared to 27A1 or 29Si which is at least in part due to the need for... 

Gibbs energy minimization has also predicted negative isobaric expansion coefficients for certain crystalline zeolite framework structures, which subsequently were confirmed experimentally [6], Many solids show negative thermal expansion at very low temperatures, including even some alkali halides (Barron and White (Further reading)). Many other solids on heating expand in some directions and contract in others. 

One of the most signiflcant variables affecting zeolite adsorption properties is the framework structure. Each framework type (e.g., FAU, LTA, MOR) has its own unique topology, cage type (alpha, beta), channel system (one-, two-, three-dimensional), free apertures, preferred cation locations, preferred water adsorption sites and kinetic pore diameter. Some zeolite characteristics are shown in Table 6.4. More detailed information on zeolite framework structures can be found in Breck s book entitled Zeolite Molecular Sieves [21] and in Chapter 2. 

The above redox behaivor of copper ions in zeolites is very distinct from those on other supports or in aqueous solution and is a specific phenomenon observed only on the zeolite. Die copper (I) ion in zeolites is fairly stable, as mentioned above, whereas the copper(II) ion supported on silica gel is readily and directly reduced to copperfO). The difficulty of generating Cu in zeolites may prevent the formation of copper particles. This may be the reason why we need a zeolite framework structure and why silica gel is a poor support for reacti(Mi.26... 

The cation plays a prominent structure-directing role in zeolite crystallization. The unique structural characteristics of zeolite frameworks containing polyhedral cages (62, 63) have led to the postulate that the cation stabilizes the formation of structural subunits which are the precursors or nucleating species in crystallization. The many zeolite compositions and complex cation base systems studied allow a test of the structuredirecting role of the cation and the cation templating concept. Table I summarizes the cation base systems from which zeolites have been synthesized. The systems used before 1969 are indicated to illustrate the number and complexities of new cation systems investigated since that time. Table II presents a summary of zeolite framework structure types, the cation systems in which they have been formed, and a proposal for a cation specificity for the formation of each framework type. A similar... 

An XRD of the deteriorated Cu-ZSM5 showed almost the same pattern, including intensities and widths at half maximum, as a fresh catalyst. Table 2 shows the BET surface area and the amount of the CO adsorption of the deteriorated Cu-ZSM5. The BET surface area was also unchanged before and after deterioration. From these results, it can be concluded that the zeolite framework structure was not destroyed in the deteriorated Cu-ZSM5. 

Immersion calorimetry is a very useful technique for the surface characterization of solids. It has been widely used with for the characterization of microporous solids, mainly microporous carbons [6]. The heat evolved when a given liquid wets a solid can be used to estimate the surface area available for the liquid molecules. Furthermore, specific interactions between the solid surface and the immersion liquid can also be analyzed. The appropriate selection of the immersion liquid can be used to characterize both the textural and the surface chemical properties of porous solids. Additionally, in the case zeolites, the enthalpy of immersion can also be related to the nature of the zeolite framework structure, the type, valence, chemistry and accessibility of the cation, and the extent of ion exchange. This information can be used, together with that provided by other techniques, to have a more complete knowledge of the textural and chemical properties of these materials. 

There exist several ways to treat the zeolite framework structure in modeling. One can take into account either the periodicity of the full lattice or only a small part of the lattice, the latter sometimes being called the cluster approach. The cluster approach is often used in quantum chemistry studies because it requires less computer time. As long as specific properties connected with the framework topology (e.g., the dimensions of the channels) are not dominating the outcome of the calculations, this approach can provide valuable informa-... 

The structure of many zeolites contains cages with trapped cation templates inside. E.M. Flanigen summarized the zeolite framework structures with an SBU cage and corresponding cation template. The results are shown in Tables 3.4 and 3.5.[231... 

Closely related zeolite framework structures often form under very similar conditions, and this can lead to the formation of stacking faults or intergrowth structures. For example, both ZSM-5 (MFI) and ZSM-11 (MEL) contain penlasil sheets. The only difference between the two is the linkage between adjacent sheets (they are related by a center of inversion in MFI and by a mirror plane in MEL, see sections 2.2.7 and 2.2.8), and it is not uncommon for an occasional stacking fault to occur [23]. If substantial domains of two framework types are formed and these domains share a common face, the material is referred to as an intergrowth. 

Figure 6.17 An example of a zeolite framework structure (faujasite). The aluminosilicate framework surrounds void spaces which contain the charge balancing cations. Access to these voids is restricted by the size of the apertures between pores. (Adapted from [204].)... 

Figure 9.1 Diagram showing the way zeolite framework structures are built up from...

The particular complexities of zeolite framework structures have spawned a number of theoretical approaches directed towards their prediction and refinement. In this section we shall concentrate principally on some recent applications of atomistic simulation methods to the study of zeolite framework structure. However, it is appropriate first to give a brief survey of other theoretical advances in this field, which have for the most part been concerned with developing an understanding of framework structure from an empirical standpoint. Fuller details of these topics may be found in the previous reviews of Freeman et al. (1992) and Price et al. (1992). 

The complex relationships among the family of zeolite structures discussed here aptly illustrate the difficulties in identifying zeolite framework structures on the basis of a general similarity in x-ray powder diffraction patterns. 

Zeolites (seetion C2.131 are unique because they have regular pores as part of their crystalline structures. 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 structures are built up of linked SiO and AlO tetrahedra that share 0 ions. The faujasites (zeolite X and zeolite Y) and ZSM-5 are important industrial catalysts. The structure 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 structure and shows the presence of the intracrystalline pore structure. In the centre of the faujasite structure is an open space (supercage) with a diameter of about 1.2 nm. The pore structure is three dimensional. 

At least in principle, 2D NMR techniques can be used to establish connectivities in the solid-state, and for crystalline three-dimensional framework structures (in contrast to the case of molecular crystals), these connectivities could be used to define the three-dimensional lattice itself. In the present section, we examine the potential of 2D 39Si MAS NMR measurements involving scalar coupling interactions to establish three-dimensional Si-O-Si lattice connectivities in zeolite framework structures. 

Zeolites are widely used in heterogeneous catalysis. In principal, their highly controllable porous structures have great potential for use as enantioselective catalysts. A considerable research effort has been devoted to the development of chiral zeolites [32]. Only zeolite beta and titanosiUcate ETSIO exist in chiral form [33, 34], although it is very difficult to obtain zeolite in enantiopure form [32]. Zeolites are typically synthesized in the presence of surfactant templates, which are removed by high-temperature calcination, a process that invariably destroys the chiral conformation of such assemblies [32]. Low enantioselectivities attributable to the chiral zeolite framework structure have been observed by Davis and Lobo [35] for the ring opening of trans-stilbene oxide with water. 

Solid tate 2D refocused INADEQUATE Si Si double quantum and 2D Si H) HETCOR (dipolar-mediated) NMR spectra have been used by Chmelka et al to characterize two as-synthesized zeolites, ITW and MTT. The obtained results, in combination with synchrotron X-ray diffraction analyses, allowed the authors to establish the Si-O- Si site eonneetivities providing insight on the complicated order and disorder within the silieeous zeolite framework structures. 

About 38 different zeolite framework structures have been identified, including both natural and synthetic forms. Detailed reviews have been given by Breck, Barrer, Meier,and Smith. The Atlas of Zeolite Structures prepared by Meier and Olson contains numerous stereoscan pictures and is especially useful for quick reference. The present discussion is therefore limited to a brief review of the structures of some of the more important commercial zeolite adsorbents. 

FIGURE 1.4. (a) Secondary building units and (b) commonly occurring polyhedral units in zeolite framework structures. (From ref. 19, copyright John Wiley Sons, Inc., 1974 reprinted with permission.)... 

EXAFS metal carbonyl clusters met clusters bimetallic clusters semiconductor clusters structures of metal, metal carbonyl, and semiconductor clusters interaction of these clusters with zeolite framework Structures are derived from coordination number and distance of metal-metal, metal-support, and metal-adsorbate contributions useful for characterization of metal-support interface interpretation of EXAFS data ould be based on the spectra of well-diar-acterized standards of molecular analogues of the supported spe-des that have been diaracterized by X-ray crystallography. 

The XRD patterns of the Cu/beta catalyst as a function of temperature/time interval are plotted in Fig. 5.2. At temperatures below 800 °C (Scan number = 16 in Fig. 5.2), the change in intensity of all the diffraction peaks that correspond to the beta zeolite framework structure follows nearly the same trend as seen on the parent beta zeolite. Above this temperature however, the intensity of all peaks decreases rapidly. The zeolite crystalline structure completely collapses and... 

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