Shape-Selective Catalysis in Zeolites

Abstract

ScopeAmong the primary tasks of catalysis is the control of the selectivity in chemical reactions. In heterogeneously catalyzed reactions in zeolites and zeolite-related microporous solids, this can, inter alia, be achieved by exploiting the phenomenon of shape-selective catalysis. In a very simplified manner, shape-selective catalysis can be described as the combination of catalysis with the molecular sieve effect. Shape selectivity effects can occur, if the sizes and shapes of reactants, of products, of transition states or of reaction intermediates are similar to the dimensions of the pores and cavities of the zeolite.The first examples of shape-selective catalysis in zeolites were reported more than 35 years ago by Weisz and Frilette from the Mobil laboratories [1,2]. Since those days, research in the field of shape-selective catalysis has expanded both at universities and in industrial laboratories in an impressive manner. This has resulted in numerous examples of shape selectivity effects in the course of catalytic conversions over zeoli tic catalysts, which are nowadays no longer restricted to acid-catalyzed reactions as described in the early days. Rather, there are also examples for shape-selective catalysis on metals, on redox active sites, on basic guests in zeolites, and even for shape-selective photochemistry in a zeoli tic environment. Moreover, from the intensive research done in the field of shapeselective catalysis during the last decades, several large-scale commercial applications in the refining and the petrochemical industries have emerged, demonstrating the usefulness and importance of the phenomenon of shape-selective catalysis.In the past, numerous review papers on shape-selective catalysis, its principles and its commercial applications have been published [e. g., 3 -19]. In view of the vast amount of work that has been done in this field in recent years, it is obvious that a review on shape-selective catalysis cannot be exhaustive. Rather, the discussion has to be restricted to selected topics. In the present contribution, emphasis will be placed on a thorough discussion of the classical and of more recent principles of shape-selective catalysis and on how these principles can be used to rationalize the observed catalytic behavior in selected examples. In addition, it is the aim of the present contribution to discuss the available methods for tailoring the shape-selective properties of zeolite catalysts and for their characterization by catalytic methods. Finally, an attempt will be under-

Molecular Simulations of Zeolites: Adsorption, Diffusion, and Shape Selectivity

“…It is the only form of shape selectivity that occurs irrespective of the extent of mass transfer limitations between gas and adsorbed phases. 433 In the reaction scheme of Figure 51, the transition state for hydroisomerization is a dialkylcyclopropyl cation. An example of transition state selectivity is the inability of TON-type zeolites to form (and hydrocrack) RR-dimethylalkanes.…”

“…Whether the exterior zeolite surface has a sufficiently regular structure to yield product distributions different from amorphous aluminosilicates continues to be a subject for debate. Reflecting the lack of agreement on the relevance of the exterior surface to shape selective catalysis, it has been given a plethora of names, such as pore mouth catalysis, 456 keylock mechanism, 417,433 and exterior surface shape selectivity. 433 The relative importance of the zeolite exterior surface is difficult to determine experimentally, as one has to selectively deactivate the exterior surface.…”

“…433 Figure 57 gives the results of the contribution of the zeolite to the free energy of formation of some of the key reaction intermediates. These results give the free energy difference with respect to n-decane in various zeolites.…”

A new view of diffusion in nanoporous materials

“…Simultaneously, guest-diffusion in nanoporous host materials is among the rate limiting processes in their technological application [1] for upgrading by heterogeneous catalysis [2], selective adsorption [3] and membrane separation [4]. The scientific progress in diffusion studies with nanoporous materials was accompanied by a series of surprises.…”

High-pressure synthesis of a polyethylene/zeolite nano-composite material

“…Their large-scale applications are, among others, fine gas separation, gas storage and catalysis [1][2][3][4][5][6][7][8][9] . More emerging applications of these materials, once properly functionalized, are also in sight in diverse fields such as nano-electronics, nanosensoring and nano-photonics 10,11 .…”