Structure of zeolites

Zeolites are crystalline aluminosilicates that develop uniform pore structure having minimum channel dijuneter of 0.3 to 1.0 nm. The size depends primarily upon the type of zeolite. Zeolites provide high activity and unusual selectivity in a variety of acid-catalyzed reactions. Most of the reactions are caused by the acidic nature of zeolites. This section will discuss the acidic properties of zeolites. 

The structure of zeolites consists of a three-dimensional framework of Si04 or AIO4 tetrahedra, each of which contains a silicon or aluminum atom in the center. The oxygen atoms are shared between adjoining tetrahedra, which can be present in various ratios and arranged in a variety of ways. The framework thus obtained contains pores, channels, and cages, or interconnected voids. 

The frameworks of zeolites used most frequently as adsorbent or catalyst are shown in Figs. 3.60-3.63. The A1 or Si atoms are located at the intersection of lines that represent oxygen bridges. The X and Y zeolites are structually and topologically related to the mineral faujasite and frequently referred to as faujasite-type zeolites. The two materials differ chemically by their Si/Ai ratios, which arc 1 — 1.5 and 1.5 —3.0 for X and Y zeolite, respectively. In faujasites, large cavities of 1.3 nm in diameter (supercages) are connected to each other thmugh apertures of 1.0 nm. 

In type A zeolite (Fig. 3.61), large cavities are connected through apertures of 

The mordenite pore structure (Fig. 3.62) consists of elliptical and noninterconnect-ed channels parallel to the r-axis of the orthorhombic structure. Their openings are limited by twelve-membered rings (0.6 —0.7 nm). ZSM-5 zeolite (Fig. 3.63) shows a unique pore structure that consists of two intersecting channel systems one straight and the other sinusoidal and perpendicular to the former (Fig. 3.63). Both channel systems have ten-membered-ring elliptical openings (ra. 0.55 A in diameter) 

---

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.

The most important undesired metallic impurities are nickel and vanadium, present in porphyrinic structures that originate from plants and are predominantly found in the heavy residues. In addition, iron may be present due to corrosion in storage tanks. These metals deposit on catalysts and give rise to enhanced carbon deposition (nickel in particular). Vanadium has a deleterious effect on the lattice structure of zeolites used in fluid catalytic cracking. A host of other elements may also be present. Hydrodemetallization is strictly speaking not a catalytic process, because the metallic elements remain in the form of sulfides on the catalyst. Decomposition of the porphyrinic structures is a relatively rapid reaction and as a result it occurs mainly in the front end of the catalyst bed, and at the outside of the catalyst particles. 

Other exciting applications involved using parallel tempering in connection with available experimental data. For example, Falcioni and Deem [57] used X-ray data to refine structures of zeolites, and Haliloglu et al. [58] refined NMR structural data for proteins (in particular using residual dipolar coupling constraints). 

On the intergrowth structure of zeolite crystals as revealed by wide field and confocal fluorescence microscopy of the template removal processes... 

The influence of alkali cations on the structure of zeolite precursor gels investigated by positron lifetime spectroscopy... 

FIGURE 14.2 The structure of zeolites derived from the sodalite structure shown in (a). 

Figure 11. The truncated octahedron building block (also termed sodalite cage,f or p-cage ) (a) tetrahedral atoms (usually Si or Al) are located at the corners of the polygons with oxygen atoms halfway between them. Illustration of the linkage, through double four-membered rings, of two truncated octahedra (b) and the structure of zeolite-A (c).

Figure 12. The structure of zeolite-A formed by linking truncated octahedra through double four-membered rings (a), the sodalite structure formed by direct face-sharing of four-membered rings in the neighboring truncated octahedra (b), and the faujasite structure formed by linking the truncated octahedra through double six-membered rings (c).

High-resolution EM also showed that the synthetic zeolitic catalyst ZSM-23 (MTT) is a recurrently twinned version of the synthetic zeolite theta-1 (TON) (51). It is noteworthy that the elucidation of the structures of zeolite beta, for a long time an enigma and problematic for X-ray crystallographers, came only through the application of HRTEM (50). 

Quantum Chemical Calculations on the Electronic Structure of Zeolite Clusters... 

This chapter shows that zeolite L is a very suitable host for the arrangement of a wide variety of chromophores. The structure of zeolite L is such that the formation of non-fluorescent dimers inside the channels can be prohibited and chromophores can be aligned in a certain direction. We have shown that this host-guest system can be used to make very efficient nanoscale two-directional photonic antenna systems. A broad spectral absorption range can be achieved by using several different cationic and neutral dyes. 

STRUCTURES OF ZEOLITES AND MESOPOROUS CRYSTALS DETERMINED BY ELECTRON DIFFRACTION AND HIGH-RESOLUTION ELECTRON MICROSCOPY... 

Structures of zeolites and mesoporous crystals determined by electron diffraction and high-resolution electron microscopy... 

Many structures of zeolites can be described by certain structural units called building units, such as double 4 or 6 membered rings (D4R or D6R), structural sheets and rods. The main requirement for stmcture analysis of zeolites at present is not the refinement of electron charge distribution in the unit cell but the determination of the framework t5q)e stmctures, manner of arrangements of secondary building units or characterizing their lattice defects. 

Case study 1 Fine structures of zeolites, intergrowths and surface structures. 

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. 

D. L. (2008) Solving the crystal structures of zeolites using electron diffraction data. 1. The use of potential-density histograms. Acta Crystallogr. A, A64, 284-294. 

Omegna, A., van Bokhoven, J.A., and Prins, R. (2003) Flexible aluminum coordinabon in alumino-sihcates. Structure of zeolite H-USY and amorphous silica-alumina. J. Phys. 

Bonilla, G., Tsapatsis, M., Vlachos, D.G., and Xomeritakis, G. (2001) Fluorescence confocal optical microscopy imaging of the grain bormdary structure of zeolite... 

Zeolites are crystalline aluminosilicates with porous, framework structures made up of linked [Si04] and [A104] tetrahedra that form channels and cages of discrete size [24]. The framework structures of zeolites bear a net negative charge, which must be balanced by positively charged species, typically alkali or alkaline earth metal cations these cations maybe exchanged for one another under appropriate experimental conditions. Zeolites are capable of... 

As for the cage structure of zeolites the role played by the presence of Na, and... 

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. 

Zeolitic materials have been prominent amongst those so far studied by high resolution powder diffraction using synchrotron X-rays [36]. High definition synchrotron PXD data has been helpful in a number of framework structure determinations and has facilitated studies of planar faulting (see below). Successful Rietveld refinements of the framework structures of zeolite ZSM-11 [37, 38] and silica-ZSM-12 [39], and of the complete structures of zeolite Y containing cadmium sulfide [40] and cadmium selenide [41] clusters have been described. 

---