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 (Si1). The symmetry and space group
of the ordered structure type 1a (tetragonal,
P4̄ (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.