Synthesis and crystal structure of as-synthesized and calcined pure silica zeolite ITQ-12

The small-pore pure silica zeolite ITQ-12 has been synthesized with fumed silica as the silica source in the presence of 1,3,4-trimethylimidazolium hydroxide and hydrofluoric acid under hydrothermal conditions at 448 K. Rietveld refinement using synchrotron X-ray diffraction data of the calcined ITQ-12 product taken at 298 K confirms the proposed topology, framework type code ITW, which can be described by a monoclinic unit cell [Si₂₄O₄₈] having Cm symmetry. Unit cell parameters are a = 10.3360(4), b = 15.0177(6), and c = 8.8639(4) Å, β = 105.356(3) ų. For as-synthesized ITQ-12, the occluded fluoride anion is located inside the double four-membered ring, while the flat 1,3,4-trimethylimidazolium cation lies on the equatorial plane of the slit-shaped [4⁴5⁴6 ⁴8⁴] cage, with its longest dimension in the [010] direction. The monoclinic unit cell |[(C₆N₂H ₁₁)⁺₂F^-₂|[Si ₂₄O₄₈], having Cm symmetry, has parameters a = 10.4478(3), b = 14.9854(4), and c = 8.8366(3) Å, β = 105.935(2)°, and cell volume V= 1330.34(7) ų at 298 K. Cooperative structure-directing effects during the crystallization of ITQ-12 are discussed in terms of the structure of the as-made material.