Structure of Zeolites and Other Chemical Systems

Chemical Structure of Zeolites and Other Chemical Systems 

This discovery was made in the 1850s, and it was the first ion exchange water-softening process utilized commonly. The ion exchangers used today in home softening units are closely related in structure and exchange properties, but are more stable for longterm use. 

Lest you be muttering, So out with phosphate pollution, in with zeolite pollution , zeolites seem to be one of the few things we can add to the ecosystem without negative consequences. The very structures of zeolites make them thermodynamically unstable, and they degrade readily to more stable aluminosilicates that are naturally occurring clays. But that raises other interesting questions If they are metastable. why do they form, rather than their more stable decomposition products How can wc synthesize them  

Before we can understand how these molecular interactions can take place, we must understand the structures of zeolites. Important Tor at least a century, the use of structural information to understand chemistry is more important now than ever before. The determination of chemical structures is a combination or careful experimental technique and of abstract reasoning. Because we have seen pictures of tinker-toy molecules all our lives in TV commercials and company logos, it is almost impossible for us to realize that it has not been long in terms of human history since arguments were made that such structures could not be studied (or even could not exist ) because it was impossible to see atoms (jif they existed). The crystallographer s ability to take a crystal in hand and to determine the arrangement of invisible atoms (Fig. 1.1) is a 

4 when you buy un ordinary 5-10-5 fertilizer, you arc buying nitrogen (5%, expressed as N), phosphate (10%, expressed as P203), and potassium (5%, expressed as KzO), 

1 The structure of the synthetic zeoliie ZSM-S (a) microscopic ciyslals (b) an electron micrograph of the area marked in (a) (c the crystal stiuciurc of ZSM-5 related to the electron micrograph. [Courtesy of J. M. Thomas. Royal Insiiiuie of Chemistry.l 

Although it is not possible for the chemist to absolutely control the movement of individual atoms or molecules in zeolite structures, the nature of the structure itself results in channels that direct the molecular motions (Fig. 1.3). Furthermore, the sizes and shapes of the channels determine which molecules can form most readily, and which can leave readily. A molecule that cannot leave (Fig. I.4 is apt to react further. This may have important consequences A catalyst IZSM-S) that is structurally related to boggsite is used in the alkylation of toluene by methanol to form poro-xylene. The methanol can provide methyl groups to make all three (ortho, meta, and para) 

Zeolites may be used in purely inorganic catalysis, however. One reaction that may be used to reduce air pollution from mixed nitrogen oxides, NO, in the industrial production of nitric acid is catalytic reduction by ammonia over zeolitic catalysts  

A related catalytic removal of NO from automobile exhaust may come about from the reaction  

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Chemical Structure of Zeolites and Other Chemical Systems 3 Chemical Reactivity 5 Conclusion 7... 

An iron-exchanged mordenite was also studied by Meisel et al. (182), who incorporated Fe3+ into the zeolite structure. Upon calcination at temperatures greater than 500 K the appearance of Fe2+ was noted in the Mdss-bauer spectrum, and for calcination temperatures higher than 770 K the formation of a-Fe203 was observed to take place inside the mordenite. For the iron- mordenite system, it can now be seen that the Mossbauer effect provides information about the chemical state, symmetry, interaction strength with the support, and location on the support of the resonant iron ions. This information enhances the understanding of the catalytic activity of this and other zeolites (178). 

A much more attractive and popular methodology to solve the electronic structure problem of complex chemical systems such as zeolites and other microporous materials is based on the DFT. DFT is not based on wave functions, but on electron density function (electron probability density function, electron density, charge density), p(r). This is the main conceptual difference between the approaches discussed earlier and the DFT methodologies. The attractiveness of these methods is partially owing to the fact that a wave function cannot be experimentally measured. It is still debated whether a wave function is just a convenient mathematical model or a true physical entity. On the contrary, the electron density is a directly measurable property that can be obtained from, for example. X-ray diffraction... 

It seems that the zeolites have been well screened in a qualitative sense, for their catalytic properties. This paper is concerned with the quantitative aspects of catalytic reaction rates in zeolites. The question whether the model of coupled surface adsorption and reaction is still meaningful in the case of zeolite catalysis was already raised by Weisz and Frilette (4) when they wrote In conventional surface catalysis the termination of a three-dimensional solid structure is considered to be the locus of activity. For these zeolites the concept of surface loses its conventional meaning.. . It is the purpose of the present article to examine critically some possibile models representing equilibrium and rate phenomena in gas-zeolite systems, in order to obtain an understanding of the kinetics of chemical reactions in zeolites. Sorption equilibria, on the one hand, and rates of sorption/desorption, exchange, and catalytic reaction on the other hand are closely related and therefore have to be represented in terms of the same model. 

Recently we successfully obtained in situ molecular. A.FM images of pyridine base species, pyridine and 5-picoline, adsorbed on cleaved (010) surfaces of natural zeolites. stilbite and heulandite [3-5]. These adsorption systems possessed three adsorption phases one physically adsorbed, and two chemically adsorbed. One of the latter two adsorption phases consists of monolayer of molecules randomly adsorbed, and the other formed a well-ordered (quasi-)hexagonal array. The present paper compares the adsorption characteristics of these adsorption systems in terms of the array and orientation structure of the adsorbed molecules as determined, for the first time, by AFM. 

Diffusion in micropore is assumed to be driven by the chemical potential gradient of the adsorbed species, instead of the concentration gradient. This is not a general rule, but it has been shown in many systems (Ruthven, 1984) that the chemical potential gradient is the proper description for the driving force of diffusion in zeolite, especially zeolites A, X, Y. Diffusion in other zeolites, and molecular sieve particles, there are still some discrepancies in the description of the diffusion. Solid structure and properties of the diffusing molecule may all contribute to these discrepancies. 

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