VPI-8: A high-silica molecular sieve with a novel `pinwheel` building unit and its implications for the synthesis of extra-large pore molecular sieves
- California Inst. of Technology, Pasadena, CA (United States)
Relatively large (up to 4 {mu}m), needle-like crystals of the high-silica molecular sieve VPI-8 are synthesized by hydrothermal methods and are subsequently calcined to remove the pore-filling organic TEA{sup +} (tetraethylammonium cation). Numerous physicochemical techniques are used to determine and characterize the structure of organic-free VPI-8. The structure is discussed with the use of two ordered topologies (la and lb) and an average structure (lc) that accounts for the possibility of structural disorder in one of the T-atoms (Sil). The symmetry and space group of the ordered structure type la (tetragonal, P4{bar O} (no. 81)) is supported by electron diffraction data and the Rietveld refinement of the synchrotron X-ray powder data based on la converges with the lowest agreement factors. The pore system of VPI-8 consists of one-dimensional channels containing 12-membered T-atom rings (12MRs) that run down the c-axis. HRTEM images along [001] confirm the general structure topology (the possible disorder occurs only along c). The structure of VPI-8 contains five unique T-atoms in atomic ratios of 1:4:4:4:4 per unit cell. These number densities are consistent with the data obtained from {sup 29}Si solid-state NMR spectra. The main feature of the VPI-8 structure is a novel `pinwheel` framework building unit that has not been observed before in microporous materials and that seems well-suited for forming extra-large pore molecular sieves. 23 refs., 13 figs., 2 tabs.
- OSTI ID:
- 374492
- Journal Information:
- Journal of the American Chemical Society, Vol. 118, Issue 31; Other Information: PBD: 7 Aug 1996
- Country of Publication:
- United States
- Language:
- English
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