Pentasil unit

ZSM-5 is a catalyst now widely used in the industrial world. Its structure is generated from the pentasil unit depicted in Figure 7.11 (as are the others of this group). These units link into chains, which join to make layers. Appropriate stacking of these layers gives the various pentasil structures. Both ZSM-5 and ZSM-11 are characterized by channels controlled by 10-ring windows with diameters of about 550 pm. The pore systems in these zeolites do not link big cavities, but they do contain intersections where larger amounts of free space are available for molecular... 

FIGURE 7.11 A pentasil unit together with a slice of the structure of ZSM-5, illustrating a linked chain of pentasil units highlighted. 

Figure 1.23. (a) Pentasil unit (building block of ZSM-5 and ZSM-11) (b) chain of pentasil units (c) chains are interlinked to form the framework with 5.5 A openings (tetrahedral sites are shown by circles). 

Fig. 10.14. a) Pentasil structural unit of ZSM-5 b) partial ZSM-5 structure showing a pentasil unit. 

A number of structural features (cages, channels, chains, sheets) arc common to several different zeolite framework types, so designations such as a-cavity and (i-cage, pentasil unit, crankshaft and double crankshaft chain, and 4.82 sheet or net have crept into common usage. To help the reader, some of these subunits are shown in Figs. 1, 2 and 3. In these drawings, oxygen atoms have been omitted for clarity. 

Figure 2 Schematic representation of (a) the pentasil unit, (b) a chain formed by interlocking pentasil units, and the structures of (c) zeolite ZSM-11 and (d) zeolite ZSM-5.

It is convenient to visualize zeolites in terms of secondary building units (SBUs). Several of the SBUs currently recognized by the IZA are depicted in Fig. 3 along with other useful structural units, the sodalite and pentasil cages. The sodalite unit is... 

One of the earliest direct bonuses of imaging zeolitic catalysts by HRTEM was the discovery (10) that the nominally phase-pure ZSM-5 (structure code MFI) contained sub-unit-cell coherent intergrowths of ZSM-11 (MEL). It soon became apparent (46) that, depending on the mode of synthesis of these and other pentasil (zeolitic) catalysts, some specimens of ZSM-5 contained recurrent (regular) intergrowths of ZSM-11. It also emerged that intergrowths of offretite and erionite are features of both nominally phase-pure erionite and of pure offretite and of many members of the so-called ABC-6 family of zeolites (47). 

Infrared spectroscopy has been used to help solve or determine the structure of zeolites. The technique is particularly useful for identifying the presence of double four- and six-rings as well as five-membered pentasil rings. In the structural characterization of beta zeolite, Newsam and coworkers used a variety of techniques including IR, electron microscopy (TEM), X-ray diffraction (XRD) and sorption data to solve the stacked, faulted structure [57]. The presence of IR absorption bands at 1232 and 560cm indicated that the structure contained five-member pentasil building units. 

Van Bekkum and coworkers extensively studied the IR spectra of zeolites with and without five-member pentasil ring structural units. They concluded that the presence of pentasil structural units in a zeolite will give rise to two vibrational bands around 1230 and 550cm . They proposed that the bands around 1230 and 550cm are related to five-member ring chains and blocks, respectively [92]. 

As mentioned in Section 4.2 earlier in this chapter, infrared spectroscopy was used to provide information about the structural units present in UZM-5. The framework IR spectrum of a UZM-5 sample is shown in Figure 4.18. The characteristic vibrational bands for double four-rings (D4R) and pentasil rings (S5) are present. This provided some valuable information about the types of linkage units present and combined with data from other techniques such as XRD and TEM allowed the structure of UZM-5 to be solved. 

Vedrine and coworkers studied vibrational bands for different types of zeoUtes with different particle sizes [95]. They concluded that during the synthesis of ZSM-type zeolite that the presence of vibrational bands at 550 and 450 cm indicate that a ZSM-type zeolite may have formed. Absence of the 550cm" band indicates that such a structure has not formed. The 550cm" band is characteristic of five-member pentasil rings which are a structural unit of ZSM-type zeohtes. 

Figure 2.31 Characteristic structural units in ZSM-5. (a) [58] Unit (b) pentasil chain and its linkage within chains in ZSM-5...

Methane catalytic conversion into carbon and hydrogen was examined over nickel-containing pentasil zeolites in a vacuum-circulation laboratory unit [4] at a catalyst/feed mass ratio of 5.0 and within a temperature range of 743 - 843 K. 

Pentasil MFI (ZSM-5) type materials exhibiting different crystallite sizes and AEL (SAPO-11) type materials have been studied. Structural investigations using X-ray diffraction techniques, developed for polycrystalline powdered samples, allow one to determine precisely the structure and crystallinity of the samples and the effect of adsorbates (e.g., p-xylene, n-hexane, etc) on the framework topology (monoclinic towards orthorhombic for MFI) and unit-cell dimensions. 

Some zeolites have much more complex unit cells and lattice structures than zeolites A and X, each of which happens to have a single, unique lattice site. A commercially important example having a very complex unit cell is zeolite ZSM-5, one end member of a series of pentasil zeolites, the other being zeolite ZSM-11 [10,11]. The pentasil building unit consists entirely of five-membered rings (Fig. 2a). Joining these units in columns (Fig. 2b) and then... 

The framework of terranovaite (Fig. 1), characterized by a pentasil chain, can be described by the interconnection of the polyhedral subunits 4 6, 4 5 and 5 6. The 4 6 unit has been found in laumontite and boggsite the 4 5 unit has been found in brewsterite, heulandite group zeolites and in synthetic SSZ-23 and SSZ-33 the 5 6 unit has been found in gottardiite, boggsite and in synthetic EU-1. The net of terranovaite projected onto the be plane (Fig. 1) is equivalent to that of many other pentasil zeolites (ferrierite, boggsite, ZSM-5, ZSM-11, theta-1), while the net projected onto the ab plane is equivalent to that of AIPO4-41 [9]. 

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