Shape-Selective Catalysis with Zeolites and Molecular Sieves

Abstract

The types of shape selective catalysis that occur in zeolites and molecular sieves are reviewed. Specifically, primary and secondary acid catalyzed shape selectivity and encapsulated metal ion and zero valent metal particle catalyzed shape selectivity are discussed. Future trends in shape selective catalysis, such as the use of large pore zeolites and electro-and photo-chemically driven reactions, are outlined. Finally, the possibility of using zeolites as chiral shape selective catalysts is discussed.Zeolites are crystalline aluminosilicates that are constructed from TO4 tetrahedra (T = tetrahedral atom, e.g., Si, Al); each apical oxygen atom is shared with an adjacent tetrahedron. Thus, the ratio of O/T is always equal to 2. A S1O4 unit in a framework (structure) is neutral since an oxygen atom bridges two Τ atoms and shares electron density with each ( Figure 1). However, since Al is +3, for every aluminum containing tetrahedron there is a net -1 charge which must be balanced by a cation.The tetrahedra are coordinated such that the zeolites have open framework structures with high surface areas. Access to the cavities is possible through voids of various sizes which are of the size of small molecules. Because of these unique properties, zeolites are able to be shape and size selective in catalytic molecular rearrangements.Approximately 70 distinct structures of zeolites and molecular sieves are known (2). There are natural zeolites, synthetic analogues of natural zeolites, and synthetic zeolites with no natural counterparts. Their pore sizes range from 4 Â to 13 À as illustrated in Figure 2. Zeolites with pores that are comprised of eight T-atoms (and eight oxygen atoms) are consider small pore zeolites. They have free diameters of 3.0-4.5 À, e.g., zeolite A. Medium pore zeolites have pores formed by ten T-atom rings with 4.5-6.0 Â free diameter, e.g., ZSM-5. Zeolites with twelve or more T-atoms in rings that make-up the pores are consider large pores zeolites. They have free diameters of 8.0 A or more, e.g., zeolite X and Y (3). To date, no zeolite (aluminosilicate) exists with pores larger than ~ 8 À. In addition to zeolites, there are now many non-aluminosilicate molecular sieves. Referring to Figure 1, if tetrahedra containing aluminum and phosphorous are connected in a strict Al/P = 1, a neutral framework is obtained. The aluminophosphate or AIPO4 is the "3-5" analogue of the "4-4" pure S1O2. Also, other elements can be incorporated into the AIPO4 framework

Impact of long-range electrostatic and dispersive interactions on theoretical predictions of adsorption and catalysis in zeolites

“…The inclusion of dispersion corrections was found to be essential to predicting this outcome. This study highlights the importance of including long-range dispersion interactions, especially for theoretical predictions of shape selective catalysis [136].…”

Zeolite-based photocatalysts

“…Among the advantages of photosensitiser encapsulation, one that has been rarely exploited is the possibility to introduce shapeselectivity. This effect, well established in thermal catalysis by zeolites, [56][57][58] consists in modifying the product distribution observed in solution merely by geometrical constraints arising from the different shape and molecular size of reagents, products and transition states. Those molecules whose dimensions allow diffusion within the pores of the zeolite can react with the internal active sites, while those others whose size is too big to permit access to the interior of the pores cannot react.…”

Synthesis strategies in the search for hierarchical zeolites

“…Due to the presence of uniform pores within the micropore range, zeolites were soon recognized to exhibit shape selectivity properties, 2,3 being able to discriminate between compounds depending on their size and shape. [4][5][6] Accordingly, zeolites have found important applications in separation processes as highly selective adsorbents. 7,8 The presence of Al atoms in the zeolite framework generates negative charges in the structure that require external cations to be balanced, providing zeolites with ion-exchange capacity.…”