Organic catalysis over zeolites: A perspective on reaction paths within micropores

The framework topology of zeolite beta

Zeolites have proved to be valuable technical catalysts in petrochemistry and in oil processing. The characteristic properties of zeolites, such as acidity, shape-selectivity and thermal stability also enable them to be used for highly selective synthesis in the fields of chemical intermediates and fine chemicals. This interesting area of application has grown continuously in recent years. The present article summarizes the various standard types of reaction involved in organic syntheses which can be catalyzed by zeolites; these include, inter alia, electrophilic and nucleophilic substitution reactions, isomerization of double bonds and carbon skeletons, as well as addition, elimination and hydrogenation reactions.

Zeolites: Catalysts for Organic Syntheses

Zeolite beta, first described in 19671, is an active catalyst and a useful sorbent1 Sorption1,2 and catalytic data3,4 suggest that the zeolite could possess a three-dimensional 12-ring pore system Such a pore system suggests technological potential similar to that of faujasite framework materials, but until now the structure of this zeolite has eluded determination Powder X-ray diffraction patterns comprise both sharp and broad features, indicative of an extensively faulted structure Here we determine the structure of zeolite beta by high-resolution electron microscopy, electron diffraction and computer-assisted modelling The zeolite is an intergrown hybrid of two distinct but closely related structures Both are constructed from the same centrosymmetrlcal tertiary building unit arranged in layers, and both possess three-dimensional 12-ring pore systems One end member, polymorph A, forms an enantiomorphic pair, with symmetries P4122 and P4322, with a = 124 A and c = 265 A Polymorpb B, in which the stacking of layers alternates in handedness, is achiral with space group P1 ¯, and a ≃ b = 124 A, c = 145 A, α ≃ β = 73°, γ ≃ 90° The high density of stacking faults in zeolite beta materials arises because successive layers must interconnect in either a left- or a right-handed fashion, and both modes of linkage occur with almost equal probability

Two new three-dimensional twelve-ring zeolite frameworks of which zeolite beta is a disordered intergrowth

The tetrahedral framework structure of zeolite beta is disordered along (001). The disordered structure is related by a/3 and/or b/3 displacements on (001) planes to three ordered polytype structures with triclinic, monoclinic, and tetragonal symmetry. Three mutually perpendicular 12-ring channel systems are characteristic of the three ordered polytypes and the disordered beta structure. The proposed framework structures are consistent with the known diffraction, sorption an cation exchange properties of zeolite beta.

M.K. Rubin

Papers

The framework topology of ZSM-12: A high-silica zeolite

The framework topology of ZSM-23: A high silica zeolite