Beta framework structure

Electron crystallography offers an alternative approach in such cases, and here we describe a complete structure determination of the structure of polymorph B of zeolite beta [3] using this technique. The clear advantage of electron microscopy over X-ray powder diffraction for elucidating zeolite structures when they only occur in small domains is demonstrated. In order to test the limit of the structural complexity that can be addressed by electron crystallography, we decided to re-determine the structure of IM-5 using electron crystallography alone. IM-5 was selected for this purpose, because it has one of the most complex framework structures known. Its crystal structure was solved only recently after nine years of unsuccessful attempts [4],... 

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 isomorphous replacement of aluminum by gallium in the framework structure of zeolites (beta, MFI, offretite, faujasite) offers new opportunities for modified acidity and subsequently modified catalytic activity such as enhanced selectivity toward aromatic hydrocarbons [249,250]. The Ga + ions in zeolites can occupy tetrahedral framework sites (T) and nonframework cationic positions. 

Figure 2.15 Framework structures of the germanosilicates Beta C (left) and ITQ-21 (right), each of which possesses three-dimensionally connected large pore (12MR) channel systems. These germanosilicates are characterised by the presence of D4Rs in their frameworks D4Rs are favoured by the presence of germanium in framework cation positions.

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. 

Other microporous titanosilicates with the MFI/MEL, ZSM-48, BETA and ZSM-12 framework structures have been synthesized but Uttle evidence has been shown for these materials which demonstrates real incorporation of the transition metal into the framework. 

The XRD scans of the parent beta zeolite support as a function of temperature/ scan interval are compiled in Fig. 5.1. The initial increase of the intensity from room temperature to 253 °C (Scan numbers = 1-6 in Hg. 5.1) for the peak at 20-7.5° can be attributed to dehydration of water from the zeolite [38]. The peak maintains its intensity until 415 °C (Scan number = 9 in Fig. 5.1). Above this temperature, a slow decline in the peak intensity can be seen, suggesting a gradual loss of crystallinity. The trend is slightly different for the diffraction at 20-22.8° and other peaks. Their intensity remains relatively constant until the temperature reaches 600 °C (Scan number = 13 in Fig. 5.1). Thereafter, a slow decrease in the intensity becomes noticeable. All these reflection peaks (including the one at 20-7.5°) however, remain even after 1 h dwell at 970 °C. This indicates that the material still maintains its framework structure. Indeed, a final XRD scan of the sample when it is cooled to room temperature reveals that the zeolite structure remains relatively intact, although it does show about a 40 % loss in intensity compared to the XRD pattern before the heat treatment. Nevertheless, the beta zeolite sample used in this study has good thermal stability. 

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... 

The rest of the catalysts shown in Table 5.1 all consist of small-pore zeolites with 8-ring openings but a variety of framework structures such as CHA, LEV, ERI, and DDR. When fresh, all the catalysts exhibit excellent SCR activity achieving nearly 100 % NOx conversion at 250 °C similar to the Cu/beta and the Cu/ZSM-5 catalysts. These smaU-pore zeolite supported Cu SCR catalysts however, exhibit much higher hydrothermal stability. After 750 °C/24 h or 900 °C/1 h aging, they still achieve very high NOx conversion efficiency. In fact, no obvious performance degradation occurs for most of the catalysts. 

As has been confirmed by XRD, the framework of montmorillonite has been partly destroyed due to the calcination under high temperature. Most diffraction peaks of montmorillonite are faint. After hydrothermal crystallization the characteristic Bragg reflections for zeolite Beta structure at 7.7° and 22.42° 20 are detected in the composite, indicating the presence of the Beta phase. 

These microporous crystalline materials possess a framework consisting of AIO4 and SiC>4 tetrahedra linked to each other by the oxygen atoms at the comer points of each tetrahedron. The tetrahedral connections lead to the formation of a three-dimensional structure having pores, channels, and cavities of uniform size and dimensions that are similar to those of small molecules. Depending on the arrangement of the tetrahedral connections, which is influenced by the method used for their preparation, several predictable structures may be obtained. The most commonly used zeolites for synthetic transformations include large-pore zeolites, such as zeolites X, Y, Beta, or mordenite, medium-pore zeolites, such as ZSM-5, and small-pore zeolites such as zeolite A (Table I). The latter, whose pore diameters are between 0.3... 

As Ti is incorporated in the silicate lattice, the volume of the unit cell expands (consistent with the flexible geometry of the ZSM-5 lattice) (75), but beyond a certain limit, it cannot expand further, and Ti is ejected from the framework, forming extraframework Ti species. Although no theoretical value exists for such a maximum limit in such small crystals, it depends on the type of silicate structure (MFI, beta, MCM, mordenite, Y, etc.) and the extent of defects therein, the latter depending to a limited extent on the preparation procedure. Because of the metastable positions of Ti ions in such locations, they can expand their geometry and coordination number when required (for example, in the presence of adsorbates such as H20, NH3, H2O2, etc.). Such an expansion in coordination number has, indeed, been observed recently (see Section II.B.2). The strain imposed on such 5- and 6-fold coordinated Ti ions by the demand of the framework for four bonds with tetrahedral orientation may possibly account for their remarkable catalytic properties. In fact, the protein moiety in certain metalloproteins imposes such a strain on the active metal center leading to their extraordinary catalytic properties (76). 

The majority of the titanium ions in titanosilicate molecular sieves in the dehydrated state are present in two types of structures, the framework tetrapodal and tripodal structures. The tetrapodal species dominate in TS-1 and Ti-beta, and the tripodals are more prevalent in Ti-MCM-41 and other mesoporous materials. The coordinatively unsaturated Ti ions in these structures exhibit Lewis acidity and strongly adsorb molecules such as H2O, NH3, H2O2, alkenes, etc. On interaction with H2O2, H2 + O2, or alkyl hydroperoxides, the Ti ions expand their coordination number to 5 or 6 and form side-on Ti-peroxo and superoxo complexes which catalyze the many oxidation reactions of NH3 and organic molecules. 

The most commonly employed crystalline materials for liquid adsorptive separations are zeolite-based structured materials. Depending on the specific components and their structural framework, crystalline materials can be zeoUtes (silica, alumina), silicalite (silica) or AlPO-based molecular sieves (alumina, phosphoms oxide). Faujasites (X, Y) and other zeolites (A, ZSM-5, beta, mordenite, etc.) are the most popular materials. This is due to their narrow pore size distribution and the ability to tune or adjust their physicochemical properties, particularly their acidic-basic properties, by the ion exchange of cations, changing the Si02/Al203 ratio and varying the water content. These techniques are described and discussed in Chapter 2. By adjusting the properties almost an infinite number of zeolite materials and desorbent combinations can be studied. 

Zeolite catalysts play a vital role in modern industrial catalysis. The varied acidity and microporosity properties of this class of inorganic oxides allow them to be applied to a wide variety of commercially important industrial processes. The acid sites of zeolites and other acidic molecular sieves are easier to manipulate than those of other solid acid catalysts by controlling material properties, such as the framework Si/Al ratio or level of cation exchange. The uniform pore size of the crystalline framework provides a consistent environment that improves the selectivity of the acid-catalyzed transformations that form C-C bonds. The zeoHte structure can also inhibit the formation of heavy coke molecules (such as medium-pore MFl in the Cyclar process or MTG process) or the desorption of undesired large by-products (such as small-pore SAPO-34 in MTO). While faujasite, morden-ite, beta and MFl remain the most widely used zeolite structures for industrial applications, the past decade has seen new structures, such as SAPO-34 and MWW, provide improved performance in specific applications. It is clear that the continued search for more active, selective and stable catalysts for industrially important chemical reactions will include the synthesis and application of new zeolite materials. 

In Beta zeolites synthesized using TEA, only a part of the TEA is compensating the charge of the framework aluminum, while the other is "occluded" in the structure. The proportion of the occluded TEA increases when increasing the Si/Al ratio of the zeolite, and can be removed at lower temperature than the TEA associated with framework Al. 

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