Structure Types of Zeolites

Industrial applications of zeolites cover a broad range of technological processes from oil upgrading, via petrochemical transformations up to synthesis of fine chemicals [1,2]. These processes clearly benefit from zeolite well-defined microporous structures providing a possibility of reaction control via shape selectivity [3,4] and acidity [5]. Catalytic reactions, namely transformations of aromatic hydrocarbons via alkylation, isomerization, disproportionation and transalkylation [2], are not only of industrial importance but can also be used to assess the structural features of zeolites [6] especially when combined with the investigation of their acidic properties [7]. A high diversity of zeolitic structures provides us with the opportunity to correlate the acidity, activity and selectivity of different structural types of zeolites. 

Chemistry of Zeolites and Related Porous Materials Table 2.1 Structure types of zeolites... 

The SBUs for various structure types of zeolites given in the database of zeolite structures141 are summarized in Table 2.2. One type of framework can comprise several SBUs. For example, the LTA framework contains five types of SBUs, including 4, 8, 4-2, 4-4, and 6-2 units, any of which can be used to describe its framework structure. In some instances, combinations of SBUs have been encountered, i.e., the framework cannot be generated by only one type of SBU. Examples include LOV, MEP, and other clathrasil-type frameworks. 

Studying the polymerization state of the silicate in the solution during the formation of zeolites is a very important and complex issue. After years of study, it has been found that some correlations exist between the polymerization state of the silicate in the solution and the structure type of zeolites, while the polymerization state and its distribution could be affected by many factors such as alkalinity, nature of the cation, organic templates, temperature, and pressure. These parameters could partially determine the condensation rate between polymerized silicate and aluminate, which will finally affect the formation of zeolite structure. 

The continuous effort of numerous academic and industrial laboratories around the world has resulted in recent years in successful synthesis of a number of new porous materials including new structural types of zeolites and zeolypes, siliceous and non-siliceous mesoporous molecular sieves, mesoporous zeolite single crystals, and micro/mesoporous or micro/macroporous composite materials of different chemical compositions. As a consequence of the success of basic research in this area, zeolites have found new industrial applications. 

On the basis of the examples reviewed above, it can be concluded that heterogeneous catalysis of the Fischer Indole Synthesis provides a practical and environmentally friendly alternative to the acids traditionally employed. Although it has not yet been possible to demonstrate unambiguously the use of a zeolite to effect the shape-selective formation of a single indole isomer, new structural types of zeolite and related materials continue to be synthesized, so that catalysts offering pore access and thus enhanced activity combined with shape selectivity remain a realistic research goal. 

Zeolites containing phosphorus in the tetrahedral site in the framework have been synthesized. Phosphorus incorporation in a variety of structural types of zeolite frameworks has been achieved analcime, phillipsite, chabazite, Type A zeolite, Type L zeolite, and Type B ( ) zeolite. The syntheses and properties of some of the new aluminosilicophos-phate zeolites are described. The synthesis technique involves gel crystallization where incorporation of phosphorus is accomplished by controlled copolymerization and coprecipitation of all the framework component oxides, aluminate, silicate, and phosphate, into a relatively homogeneous gel phase. Subsequent crystallization of the gel is carried out at temperatures in the region of 80° to 210°C. Proof and mechanism of framework substitution of phosphorus is based on electron microprobe analysis, infrared spectroscopy, and other characterization. 

The framework structures and pore cross-sections of two types of zeolites are shown. (Top) A Faujasite-type zeolite has a three-dimensional channel system with pores of at least 7.4 A in diameter. A pore is formed by 12 oxygen atoms in a ring. (Bottom) ZSM-5 zeolite has interconnected channels running in one direction, with pores 5.6 A in diameter. ZSM-5 pores are formed by 10 oxygen atoms in a ring. Reprinted with permission from Chemical Engineering Progress, 84(2), February 1988, 32. 

Figure 1. Trends in the relationship between SDA size and the types of zeolite structures formed. [Adapted from (5).]...

Using 3G and 4-31G type basis sets (39-41), ab initio quantum chemical calculations have been carried out for several small structural units of zeolites, with a variety of observed and hypothetical Si-Al distributions (29-32). The results of these studies can be summarized in a series of hypothetical Si - A1 exchange reactions within these structural units. The calculated internal energy changes for the reactions involving two neighbouring tetrahedra, are as follows ... 

Many types of zeolites are known but only a rather small number of zeolites are used in catalysis. In this section, the most important zeolites will be introduced. We will focus on the most commonly used types which are Zeolite X, Zeolite Y, ZSM-5, and Zeolite Beta. Apart from these, a couple of other zeolites, e.g., Mordenite or Zeolite L, are also used for specific reactions but they are produced on a smaller scale. Most of these zeolites have a remarkable thermal stability and can be heated to a temperature of 600°C without structural damage some of them resist even temperatures of 800 to 1000°C. 

The nomenclature of zeolites is rather arbitrary and follows no obvious rules because every producer of synthetic zeolites uses his/her own acronyms for the materials. However, as mentioned before, at least the structure types of the different zeolites have a unique code. For example, FAU represents Faujasite-type zeolites, LTA Linde Type A zeolites, MFI Mobile Five, and BEA Zeolite Beta. The structure commission of the International Zeolite Association (IZA) is the committee granting the respective three-letter codes [4], Some typical zeolites, which are of importance as catalysts in petrochemistry, will be described in the following sections. 

In situ crystallization begins with a precursor synthesis sol consisting of Si, Al, Na, H2O and a structure-directing agent (or template) [7]. The composition of the sol is dependent on the type of zeolite being synthesized and the final desired... 

The zeolites are another structure type of alumino silicate minerals. The eighteenth century identification of this mineral group was made on a few... 

The advantages of shape selective catalysis are alreacfy ejq)loited in a number of industrial processes [11-14]. Astonishingly, virtually all these processes rely on a single structural type of catalyst, viz. zeolite ZSM-5 in various modifications, or its titanium containing analogue TS-1 [15]. It is, moreover, noteworthy that many of these processes convert and/or produce mononuclear aromatic compounds. It is not surprising, therefore, that a vast scientific literature exists on shape selective reactions of benzene derivatives in zeolite ZSM-5. 

Meier, W. M. Olson, D. H.(1978)/lt/fls of Zeolite Structure Types, Int. Zeolite Assoc., Polycrystal Book Serv., Pittsburgh, Pennsylvania. 

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... 

Most of the published information regarding surface acidity and its relation to catalytic activity has involved zeolites of the faujasite structure as found in zeolites X and Y. A smaller number of investigations of mor-denite have been reported. This discussion will concentrate on studies of these two types of zeolites because their acidic and catalytic properties have been most widely investigated, and because they are both of significant industrial importance. 

Reference cites the literature from which the crystal data, atomic coordinates, and displacement factors were obtained. In many cases there are multiple refinements of the same zeolitic material, but because of space limitations not all refinements could be included. We would be appreciative if authors and users would inform us of any errors or omissions. A listing of the references for isotypic species can be found in the Atlas of Zeolite Framework Types (Baerlocher, McCusker and Olson (2007)). A list of references to structure analyses of zeolites with different cations, up to 1982, is given in the Compilation of Extra Framework Sites in Zeolites, Mortier (1982). 

W. M. Meier and D. H. Olson, Atlas of Zeolite Structure Types", International Zeolite Association, Zurich, 1978. 

---