Molecular sieve catalysis

Martens J A and Jacobs P A 1999 Phosphate-based zeolites and molecular sieves Catalysis and Zeolites, Fundamentals and Applications ed J Weitkamp and L Puppe (Berlin Springer) pp 53-80... 

The early use and success of molecular sieve catalysis was spurred by the dramatic improvement in activity selectivity for catalytic cracking of vacuum gas oil achieved by using the faujasite based catalysts in comparison to the previously used amorphous SiOj/AUOj. These catalysts had a factor of about 10 -10 higher catalytic activity than the amorphous SiOj/AfrOj catalysts [42]. Paraffin, C4 to C8 isomerization [43] was one of the first successful non-petroleum processing applications using zeolite catalysts. The complexity of tailoring zeolite catalysts, however, is well illustrated by the fact that is only four years back that Shell has developed the first zeolite based process for isomerization of n-butene to isobutene [44]. 

PHYSICAL ASPECTS OF MOLECULAR SIEVE CATALYSIS FOR CHEMICAL SYNTHESIS. 

Chemical functionalities of molecular sieves Physical aspects of molecular sieve catalysis... 

In 1992, researchers of the Mobil Oil Company introduced a new concept in the synthesis of mesoporous materials. They used supramolecular arrays of surfactant molecules as templating agents in order to obtain mesostructured silicates or alumosilicates which retain after calcination an ordered arrangement of pores with diameters between 2 and 10 nm and a narrow pore size distribution comparable to that of zeolites. These materials called M41S phases give access to the regime of the mesopores which is very interesting for different kinds of new size selective applications, e.g., molecular sieves, catalysis and nanocomposites [1]. 

In addition, in the field of dewaxing (gas oils, HDC residues, lubricating oil, etc.), synthesis of novel molecular sieves with better adsorption and separation abilities is highly desired. In the past 20 years, thanks to the discovery of many molecular sieves with new compositions and structural features [secondary building units (SBUs) and pores], there have appeared a number of new application fields for molecular sieves, such as basic catalysis, extra-large microporous molecular sieve catalysis, redox catalysis, asymmetric catalysis, and dual- and multi-functional catalysis.1-201 All of these will lay a further solid foundation for the development of molecular sieves in catalysis, adsorption, and separation. 

Studies of Molecular Sieve Catalysis by Diffuse Reflectance IR Spectroscopy... 

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]. 

Srinivas, D. R. Srivastava P. Ratnasamy. Transesterifications over titanosilicate molecular sieves. Catalysis Today 200, 96, 127—133. 

A Report on Molecular Sieve Catalysis, Bull. F-1578, Union Cartnde Corp., Linde Div., New York (1967)... 

Kozhevnikov, I.V., Sinnema, A., Jansen, R.J.J., Pamin, K., Van Bekkum, H. 1995. New acid catalyst comprising heteropoly acid on a mesoporous molecular sieve. Catalysis Letters 30(l ) 241-252. 

In parallel to developments in the field of electronics, nanostmctured materials have been developed by materials scientists and chemists also. The concept of nanocrystalline structures emerged in the field of materials science, and polycrystals with ultrafine grain sizes in the nanometer range have been produced. These nanophase materials have been shown to have significant modifications of their mechanical properties compared with the coarse-grain equivalent materials. The huge surface area of nanoporous materials has attracted much attention for applications in chemistry such as molecular sieves, catalysis, and gas sensing. This has motivated intense research aimed at the fabrication of materials with a well-controlled composition and nanoscale stracture, such as synthetic zeolites. 

However, Lewis acid sites not only modify the Bronsted acid sites. These species will also act themselves in various ways in molecular sieve catalysis. Together with the adjacent framework oxygen atoms, Lewis acid sites will act as Lewis acid-base pairs, and may polarize bonds in reacting molecules, possibly enhancing their chemical reactivity [27,28]. For molecules that are already polar, this polarization could also be sufficient to catalyze a chemical transformation, e.g., in the reaction of alcohols [51]. Lewis acid sites also act as hydride or anion receptors in a variety of reactions. Thus, in most cases the character of an acid-base pair site will be more pronounced in the case of Lewis than in the case of Bronsted acid sites [52,53]. 

Davis ME. New vistas in zeolite and molecular sieve catalysis. Acc Chem Res 1993 26 111-5. 

Davis, M.E., 1994. Introduction to large pore molecular sieves. Catalysis Today 19 (1), 1—5. 

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