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

Relatively large (up to 4 μm), 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⁺ (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 (1a and 1b) and an average structure (1c) that accounts for the possibility of structural disorder in one of the T-atoms (Si 1). The symmetry and space group of the ordered structure type 1a (tetragonal, P4 (no. 81)) is supported by electron diffraction data and the Rietveld refinement of the synchrotron X-ray powder data based on 1a 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 ²⁹Si 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. Hypothetical framework models of extra-large pore molecular sieves containing one-dimensional 14MR (2), 16MR (3), 18MR (4), and 20MR (5) channel systems based on this new building unit are presented and discussed.