Zeolites and Related Molecular Sieves

At present, the oil refining industry is faced with important challenges, such as the processing of heavier and more contaminated crudes, the increasing demand for higher quality transportation fuels with reduced emissions of contaminants, and the need for more petrochemical feedstocks (e.g. olefins, aromatics). In this context, there is no doubt that zeolites (and related molecular sieves) can help refiners to achieve the new goals. Recent advances in zeolite synthesis and post-synthesis modifications are expected to contribute to the development of improved catalysts and processes. 

Crystalline molecular sieve zeolites, and related molecular sieve materials that are not technically zeolites, eg, silicalite, AlP04s, SAPOs, etc. 

Catalysis by zeolites is a rapidly expanding field. Beside their use in acid catalyzed conversions, several additional areas can be identified today which give rise to new catalytic applications of zeolites. Pertinent examples are oxidation and base catalysis on zeolites and related molecular sieves, the use of zeolites for the immobilization of catalytically active guests (i.e., ship-in-the-bottle complexes, chiral guests, enzymes), applications in environmental protection and the development of catalytic zeolite membranes. Selected examples to illustrate these interesting developments are presented and discussed in the paper. 

Finally, after fifty years of intensive investigation of synthesis, properties and applications of zeolites and related molecular sieves resulting in a number of industrial applications, it is clear that this research area is still very topical and many interesting and useful materials and processes will certainly be developed in future. 

The addition of small amount of certain oxyanions of group V-VII elements (such as phosphate, arsenate, perchlorate, chlorate etc.) enhance the nucleation and crystallization of zeolites and related molecular sieves to a varying extent. The effect of is very general in nature and applicable to different zeolite structures. Liquid state Si and P NMR experiments indicate reversible interaction of these promoter oxyanions with template enclathrated Q silicate-water structure at the onset of crystallization, and a catalytic role of the promoter in enhancing the crystallization rate of zeolites and related molecular sieves. 

The use of microporous solid catalysts such as zeolites and related molecular sieves has an additional benefit in organic synthesis. The highly precise organization and discrimination between molecules by molecular sieves endows them with shape-selective properties [12] reminiscent of enzyme catalysis. The scope of molecular sieve catalysis has been considerably extended by the discovery of ordered mesoporous materials of the M41S type by Mobil scientists [13,14]. Furthermore, the incorporation of transition metal ions and complexes into molecular sieves extends their catalytic scope to redox reactions and a variety of other transition metal-catalyzed processes [15,16]. 

The latter is analogous to the hydrothermal synthesis of zeolites and related molecular sieves (see later). Redox metal ions can be incorporated into acidic clays or zeolites by ion exchange, and oxoanions can be similarly exchanged into hydrotalcite-like anionic clays [30]. 

Alternatively, redox metal ions can be incorporated into framework positions of zeolites and related molecular sieves by hydrothermal synthesis or post-synthesis modifications [15]. A suitable choice of molecular sieve, with an appropriate pore... 

One approach to creating heterogeneous oxidation catalysts with novel activities and selectivities is to incorporate redox metals, by isomorphous substitution, into the lattice framework of zeolites and related molecular sieves. Site-isolation of redox metals in inorganic lattices prevents the dimerization or oligomerization of active oxometal species which is characteristic of many homogeneous oxometal complexes and leads to their deactivation in solution. We coined the term redox molecular sieves to describe such catalysts . The first and most well-known example is titanium silicalite (TS-1) which has been shown to catalyze a variety of systhetically useful oxidations with H202. ... 

Thomas fM, Bell RG, Catlow CRA (1997) Zeolites and related molecular sieves. In Erd G, Knozinger H, Weitkamp I (eds) Handbook of heterogeneous catalysis, vol 1. Wiley, Weinheim, p 286... 

Crystalline porous solids such as zeolites also offer excellent opportunities for novel supported organometallic complexes and metal particles. The molecular chemistry of the synthesis of zeolites and related molecular sieves is still obscure the expansion of the class of molecular sieves is proceeding rapidly on an empirical basis with... 

In the case of silica materials, zeolites and related materials, and mesoporous molecular sieves (MMS), the spectrum in the middle IR region, that is, 250 to 4000cm"1 can be classified in three regions ... 

Nanoporous materials like zeolites and related materials, mesoporous molecular sieves, clays, pillared clays, the majority of silica, alumina, active carbons, titanium dioxides, magnesium oxides, carbon nanotubes and metal-organic frameworks are the most widely studied and applied adsorbents. In the case of crystalline and ordered nanoporous materials such as zeolites and related materials, and mesoporous molecular sieves, their categorization as nanoporous materials are not debated. However, in the case of amorphous porous materials, they possess bigger pores together with pores sized less than 100 nm. Nevertheless, in the majority of cases, the nanoporous component is the most important part of the porosity. 

It is a mass transfer between a mobile, solid, or liquid phase, and the adsorption bed packed in a reactor. To carry out adsorption, a reactor, where a dynamic adsorption process will occur, is packed with an adsorbent [2], The adsorbents normally used for these applications are active carbons, zeolites and related materials, silica, mesoporous molecular sieves, alumina, titanium dioxide, magnesium oxide, clays, and pillared clays. 

Zeolites and related materials, pillared clays, and mesoporous molecular sieves (MMSs) are the most important acid catalysts. In this section, the catalytic properties of these materials are explained [12-22],... 

In contrast with the widespread application of zeolites as solid acid catalysts (see earlier), their use as solid base catalysts received scant attention until fairly recently [121]. This is probably because acid-catalyzed processes are much more common in the oil refining and petrochemical industries. Nonetheless, basic zeolites and related mesoporous molecular sieves can catalyze a variety of reactions, such as Knoevenagel condensations and Michael additions, which are key steps in the manufacture of flavors and fragrances, pharmaceuticals and other specialty chemicals [121]. Indeed, the Knoevenagel reaction of benzaldehyde with ethyl cyanoacetate (Fig. 2.36) has become a standard test reaction for solid base catalysts [121]. 

All the books and reviews on zeolites which have been published deal mainly with X and Y. Even so, two books, one on all aspects of zeolites except catalysis and the other on catalysis over zeolites and related topics, are essential texts for all workers on non-faujasitic zeolites. Other useful books and reviews, including the Proceedings of the 3rd and 4th International Molecular Sieve Conferences and the Symposium on Zeolites at Szeged, have appeared. 

Since 1950s, there have been three traditional fields of application for molecular sieves and porous materials 1) separation, purification, drying and environment treatment process 2) petroleum refining, petrochemical, coal and fine chemical industries 3) ion-exchange, detergent industry, radioactive waste storage, and treatment of liquid waste. In addition to the traditional application fields, zeolites and related porous materials may also find applications in new areas such as microelectronics and molecular device manufacture. 

Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Zeolite Molecular Sieves. Chemistry of Zeolites and Related Porous Materials - Synthesis and Structure Rureh Xu, Wenqin Pang, Jihong Yu, Qisheng Huo and Jiesheng Chen Science Press, Beijing, 1978. 

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