Evidence for semi-regularly ordered sequences of mirror and inversion symmetry planes in ZSM-5/ZSM-11 shape-selective zeolitic catalysts

Abstract

The combined use of high-resolution electron microscopic imaging, electron diffraction, optical diffraction and computer graphics for characterizing intergrowth structures of ZSM-5 and ZSM-11 is described. In particular, a specific preparation is shown to consist mainly of the ZSM-5 framework structure but with single insertions of strips of ZSM-11 at an average spacing of ca. 66 8, along [loo]. Evidence is also presented for the occurrence of an intergrowth in which one slab of ZSM-5 is related to an adjoining slab such that one of these is rotated through 90" around a [OOl] axis. An idealized atomic model for this intergrowth is proposed.ZSM-5 and ZSM-1 1 are commercially important shape-selective highly siliceous zeolite catalysts'-6 which have closely related structure^."^ Identical basic building units made up of five-membered rings are joined along [OOl] to form chains (see shaded region of fig. l), which pack laterally along [OlO]. The ZSM-5 structure is generated when adjacent (100) planes or sheets are related t o one another by inversion (i), and the ZSM-11 structure when the (100) sheets are related by mirror symmetry. The net result is that in ZSM-5 two orthogonal sets of intersecting ten-membered aperture channels occur; those that run along [OlO] are straight with nearly circular diameter of ca. 5.5 A, whilst those along [loo] follow a sinusoidal course and possess an elliptical cross-section (ca. 5.1 A X 5.5 A). In ZSM-11 the two orthogonal sets of channels are equivalent, straight and elliptical (5.1 A x 5.5 A).A consequence of these features is that whereas the cavities formed at the intersections of the channels in ZSM-5 are equivalent, there are two distinct types in ZSM-11: one almost identical to that occurring in ZSM-5, the other with ca. 30% additional volume. This duality of cavity geometry is of particular significance in determining the recise nature of the shape-selective catalytic characteristics of the two materials.The nature of the subtle differences between structures as close1 akin as ZSM-5 and ZSM-11 has led to the suggestion, by Kokotailo and Meier! that an almost infinite variety of related compounds (intermediate between ZSM-5 and ZSM-11) could be formed by regular, and specific kinds of, sequencing of mirror and inversion planes. A number of possible systems have been described," and one example, symbolized by criiiiiidiiiiicr, is shown in fig. 1. Hitherto there has been no direct evidence that demonstrates the occurrence of any of these intergrowths. Recently,

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