Synthesis and Structure of RUB-58: An Almost Ordered, Highly Crystalline Member of the ZSM-48 Family of Zeolites

Abstract

ZSM-48 describes a family of disordered high-silica zeolites with a one-dimensional pore system of straight 10-ring channels. Although many different synthesis procedures and various organic structure-directing agents have been tried, all ZSM-48-type materials produced to date have proven to be highly disordered. A few structural models were proposed by Schlenker et al. ( Zeolites 1985, 5, 355−358) and by Lobo and van Koningsveld ( J. Am. Chem. Soc. 2002, 124, 13222−12230) to describe the framework topology and the disorder of the real structure, but it has not been possible to refine the structure of a ZSM-48-type material using experimental diffraction data. RUB-58 was synthesized under hydrothermal conditions at 160 to 200 °C in the system SiO2/H2O/ethylenediamine/pyridine. The product consists of spherelike aggregates of small, needlelike, colorless crystals and has an X-ray powder diffraction (XPD) pattern similar to those published for ZSM-48-type materials. The pattern, however, contains a much larger number of sharp reflections indicative of a more ordered, crystalline material. The reflections could be indexed with the unit cell parameters a = 14.2095(3) Å, b = 20.0874(5) Å, c = 8.3752(2) Å in the space group Pna21, a sub-subgroup of Imma, which is the highest possible symmetry of the ordered ZSM-48 polymorph A (see the Database of Disordered Zeolite Structures). The structure analysis was performed in successive steps: (I) analysis of the average structure, which showed that the framework structure of RUB-58 can be described in terms of polymorph A and that the pyridine molecules occupy defined positions in the elliptical 10-ring channels; (II) refinement of a faulted structure using two separate, but identical polymorph A-type framework structures, with one shifted by exactly 0.5 of the unit cell along the channel direction (volume ratio: 92/8); (III) analysis of the structure of guest-free RUB-58 to examine the geometry of the channels in the empty framework (elliptical cross section nearly identical to that of the as-made material); and (IV) development of convincing disordered models to describe the real structure of RUB-58.