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

Abstract

Zeolite beta, first described in 1967¹, is an active catalyst and a useful sorbent¹. Sorption^1,2 and catalytic data^3,4 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 = 12.4 Å and c = 26.5 Å. Polymorpb B, in which the stacking of layers alternates in handedness, is achiral with space group P1 ¯, and a ≃ b = 12.4 Å, c = 14.5 Å, 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.