Ordered silicon vacancies in the framework structure of the zeolite catalyst SSZ-74

Abstract

Physico-chemical characterization of the high-silica zeolite catalyst SSZ-74 (ref. 1) suggested that it, like the related materials TNU-9 (ref. 2) and IM-5 (ref. 3), has a multidimensional 10-ring channel system. Such pore systems are ideal for many petrochemical applications, and indeed SSZ-74 has been shown to be a good catalyst for a wide variety of reactions. The elucidation of its framework structure, however, proved to be difficult. Comparable problems were encountered with TNU-9 and IM-5, which were synthesized with related structure-directing agents. Their framework structures, which are the two most complex ones known, both have 24 Si atoms in the asymmetric unit, and were finally solved by combining high-resolution powder diffraction data with information derived from high-resolution electron microscopy images. Therefore, a similar approach, using the powder charge-flipping algorithm to combine the two types of data and molecular modelling to help to locate the structure-directing agent, was applied to SSZ-74. This procedure eventually revealed a most unusual 23-Si-atom framework structure (|(C(16)H(34)N(2))(4)&Si(92)(4)O(184)(OH)(8)]) with ordered Si vacancies.

The charge-flipping algorithm in crystallography

“…Both the phases and the envelope can be used as additional information in the charge-flipping iteration. The complete flow chart of the method is complicated, and details can be found in the original works (Baerlocher et al, 2007a;Baerlocher et al, 2008;). In general terms, the envelope is used to eliminate the density outside the envelope.…”

“…In practice, the known phases are fixed for a certain number of iteration cycles, and then released to give the model more freedom to develop, and to correct possible errors in the supposedly known phases. Using this combination of charge flipping, histogram matching, structure envelope and known phases, some of the most complex zeolite structures could be elucidated (Baerlocher, Gramm et al, 2007;Baerlocher et al, 2008;Koyama et al, 2008;Sun et al, 2009).…”

Structure determination of the zeolite IM-5 using electron crystallography

“…There are only 3 possible space groups corresponding to these reflection conditions: Cmc2 1 , C2cm and Cmcm [11]. These three space groups have the same systematic absences and can not be distinguished from diffraction Structure determination of the zeolite IM-5 using electron crystallography data.…”

“…Recently X-ray powder diffraction has been combined with electron crystallography to determine the structures of several complex zeolites: TNU-9 [9], IM-5 [10], SSZ-74 [11] and ITQ-37 [12]. The advantages of electron crystallography as a complementary tool to powder diffraction are twofold: (1) electrons interact much more strongly with atoms than do X-rays, so polycrystalline materials down to 10 nm behave like single crystals with electrons on a TEM, and (2) the crystallographic structure factor phases, which are lost in a diffraction experiment, can be obtained from high resolution transmission electron microscopy (HRTEM) images.…”

“…Several complex zeolite structures have been solved by combining Xray powder diffraction data and electron crystallography using this new approach [10][11][12]. One example is that of the high-silica zeolite catalyst IM-5, whose synthesis was first reported in 1998 [31].…”