Zeolite chemistry applications

Zeolite chemistry is an excellent example of how a three-dimensional surface can alter the course of chemical reactions, selecting for one product out of a host of potential candidates. In addition to the many commercial applications that they have found, shape-selective zeolites have provided the basis for a rich new area of catalytic science and technology, one expected to spawn yet more materials, knowledge, and applications. 

Refs. [i] Rolison DR (1994) The intersection of electrochemistry with zeolite science. In Jansen JC, Stacker M, Karge HG, Weitkamp / (eds) Advanced zeolite science and applications. Elsevier, Amsterdam [ii] Walcarius A (2003) Implication of zeolite chemistry in electrochemical science and applications of zeolite-modified electrodes. In Auerbach SM (ed) CRC handbook of zeolite science and technology. CRC press, Boca Raton, pp 721-784 [Hi] Cejka J, van Bekkum H (2005) Zeolites and ordered mesoporous materials progress and prospects. Elsevier, Amsterdam, p 157... 

One of the most exciting aspects of zeolite chemistry is our increasing understanding of the functionality of framework structure, chemisorption, and mass transport as related to chemical behavior. The research in this area has resulted in numerous advances during the past 2 years, many of which are presented in this text. It is expected that the applications and interest in crystalline molecular sieves will continue to expand rapidly. 

The resurgence of interest in the field of zeolite chemistry, which has been stimulated by the appreciation of their enormous potential as catalysts, has led to the application of several sophisticated physical methods in the study of their structural properties. Important advances have already been made using high resolution, solid state NMR (1,2) and electron microscopy (3), and in this paper we discuss the scope and limitations of neutron diffraction studies with powder samples, with some specific applications to zeolite-A and synthetic faujasite. 

Among the numerous theoretical approaches applied to the zeolite reactivity problem, we focus our attention mainly on calculations, which use recently developed ab initio molecular dynamics techniques. After a very brief overview of the main features of this methodology, we discuss some applications taken from the modeling of zeolite chemistry the characterization of the catalytic sites, the protonation of a water molecule and the mechanism of the protolytic reactions of alkanes. 

There is an already impressive literature on the application of various first principles and semi-empirical approaches to aspects of zeolite chemistry (see, e.g., [104-107]). Even a cursory overview of this aspect of zeolite modeling and simulation would be beyond the scope of the present paper. However, two recent development areas are noted. 

This conversational and somewhat subjective overview of computational approaches in zeolite chemistry has illustrated that the field is very diverse, and expanding rapidly. Modeling and simulation at the atomistic or electronic structural level clearly contribute at various levels to practical zeolite research and development programs. Characterization and zeolite physical and chemical property prediction are the most prominent application domains at present. 

To succeed in the above, the techniques of synthetic intrazeolitic chemistry need to be further developed. The consequences of each step of a procedure should be clear to the chemist, so that simple and stoichiometric products are produced. In this regard, zeolite chemistry is a young science with much of a "gold rush" mentality (rush to application) to it, with more fundamental work set aside for much later. 

In the field of transition metal catalysis, zeolites may offer opportunities for uniform active sites. With the discovery of both aluminosilicate and aluminophosphate, zeolites with a variety of transition-metal ions in tetrahedral firework positions may offer new possibilities. On the basis of existing zeolite chemistry dealing vrith aluminum hydrolysis and the formation of adsorption adducts in the zeolite pores, chemists may envision strategies aimed at the activation of tetrahedral transition metal ions, either by lattice oxide replacement or by the application of strong donor ligands. The demonstrated... 

Silica materials have been studied extensively because of the structural flexibility of silica (through Si04 tetrahedral connections), easy control of hydrolysis and polymerization of silica species, high thermal stability of silica framework, easy modification of the silica surface, and well known silica and zeolite chemistry. Amorphous silica is also the main inorganic component for certain natural materials obtained from bioassembly, such as diatoms. Various mesoporous silica materials have been reported, which are very important for both fundamental research and applications. 

The process of adsorption and interaction of probe molecules such as ammonia, carbon monoxide as well as the whole spectrum of organic reactant molecules with zeolite catalysts has been the subject of numerous experimental and computational studies. These interaction processes are studied using several computational methods involving force fields (Monte Carlo, molecular dynamics emd energy minimization) or quantum chemical methods. Another paper [1] discusses the application of force field methods for studying several problems in zeolite chemistry. Theoretical quantum chemical studies on cluster models of zeolites help us to understand the electronic and catalytic properties of zeolite catalysts. Here we present a brief summary of the application of quantum chemical methods to understand the structure and reactivity of zeolites. 

From Mossbauer spectroscopy, the environment of Fe2+ ions in zeolites has been deduced (34). In summary, research in recent years on zeolites has been concerned with those materials of specific commercial importance. There seems to be a direct relation between the level of scientific interest and the area of major application. This, of course, may be prompted in part by the availability of materials since some zeolite minerals are rare and difficult to obtain. Many zeolites and many potentially interesting aspects of zeolite chemistry and zeolite properties have been neglected so far. 

Other Miscellaneous Methods.- The three methods which have been described in the previous sections are all techniques which have already found considerable application, or have shown, the potential to be important methods in the future. In this section some other methods which have been applied to the problems of zeolite chemistry will be described briefly, but this must by necessity be a selection from a wide range of calculation types, as the field sometimes gives the appearance of generating as many novel techniques as there are investigators. 

Perhaps the best way to look at the applications of the computational methods is to consider the questions of zeolite chemistry as falling into a number of different areas of interest. For this reason, the discussion of the calculations will be ordered according to the objectives of the study, as opposed to looking at the application of a method in general. 

Research has shown that a solute with a diameter closer to that of zeolite pore dimension showed higher adsorption (close fit mechanism). An important consideration for applying zeolites in drinking water treatment practice is that their size exclusion and close fit adsorption mechanism makes them effective for the removal of specific solutes. [4]. Because the Si Al ratio allows for tuning of the surface properties and the resultant electrostatic double layer such membranes could also be tuned for specific ion-selective applications, but further work is needed to fully understand the connection between zeolite chemistry and membrane performance. Osewe (2014) investigated the dissipation half-life of malathion as affected by the largest window opening for FAU, MOR, and ZSM-5s zeolites [16]. 

A. Walcarius, Implication of Zeolite Chemistry in Electrochemical Science and Applications of... 

Nowadays zeolites have to be defined in line with the lUPAC recommendations as a group of solids based on either aluminosilicates up to silica polymorphs, aluminophosphates or metallosilicates or phosphates with a well-defined microporous structure. As zeolite science and application is still a rapidly growing area, the reader may refer to a recently published book "Introduction to Zeolite Science and Practice" [9] for more detailed information. This book represents the latest comprehensive review on the different fields of zeolite chemistry. 

The overwhelming success of using theoretical methods to predict and explain experimental results has encouraged and stimulated the application of computers to new areas of research in zeolite chemistry. The capability of current computational techniques used in zeolite studies includes molecular modelling of the framework structure, simulation of X-ray diffraction data, predictions of the physical and chemical properties of zeolite crystals, their stability at different temperatures and pressures leading to prediction of new structures, the adsorption and diffusion of sorbed molecules, calculating vibrational properties of sorbed and sorbent molecules and predicting the reaction pathways in catalytic reactions. 

From the point of view of catalytic activity and in terms of selectivity and waste production, zeolites are the most traditional microporous catalysts with clear advantages over the conventionally used homogeneous acid catalysts. Zeolites find applications in the petrochemical and fine chemical industries, and also in gas separation, purification, and ion exchange [13-15] in addition, zeolites are used in pharmaceutical industry, sensing microsystems, nanotechnology, the intensification process, and green chemistry. 

Electronegativity and the Periodic Table Experimental Data Evaluation and Quality Control Factual Information Databases Inorganic Chemistry Databases Inorganic Compound Representation Internet-based Computationai Chemistry Tools Lanthanides and Actinides Materiais Properties Online Databases in Chemistry Structural Chemistry Application of Mathematics Symmetry in Chemistry X-Ray Crystallographic Analysis and Semiempirical Computations Zeolites Applications of Computational Methods. 

Density functional techniques that use plane waves as basis sets for the valence electrons and describe the atomic cores by pseudopotentials or other special techniques only became applicable to zeolites recently. This approach had already been standard in solid state physics for many years, but only recent advances in computer technology and pseudopotential theory made it possible to apply it to solids with unit cells as large as those of zeolites. Moreover, the computational efficiency of plane wave methods is coupled to the DPT approximation and the accuracy needed in chemistry is reached only with gradient-corrected functionals which emerged only in recent years. The striking advantage of plane wave basis sets is the very efficient analytical calculation of the forces on the nuclei (only Hellmann-Feynman terms contribute) which allows structure optimizations and makes even molecular dynamics (MD) runs possible by the Car-Parrinello method. This type of technique is expected to make a major contribution to zeolite chemistry over the next few years. 

Isomorphous substitution is an important way to modify zeolite properties for practical applications and has achieved considerable interest in the field of zeolite chemistry. The thermodynamics of this process have been considered by Barrer [112]. The... 

Walcarius A (2003) Implication of zeolite chemistry in electrochemical science and applications of zeolite-modified electrodes. In Auerbach SM, Carrado KA, Dutta PK (eds) Handbook of zeolite science and technology. Marcel Dekker Inc., New York, NY, pp 721-783... 

There are only a few Mossbauer nuclei which are interesting in zeolite chemistry and, thus, candidates for application of Mossbauer spectroscopy in soUd-state ion exchange. However, among them is one of the most important elements, viz., iron, which has also attracted much attention in zeoUte chemistry as a key component of possible catalyst formulations. Mossbauer spectroscopy proved to be exceptionally successful in discriminating Fe + and Fe + cations residing on extra-framework sites after introduction of iron via soHd-state ion exchange. Moreover, Mossbauer spectroscopy provides information about the various coordinations of Fe + and Fe in zeoUte lattices (cf. Sect 5.3.4). 

---