Synthesis, characterization and crystal structure of EMS-2 – a novel microporous stannosilicate

EMS-2, a novel mixed tetrahedral–octahedral microporous stannosilicate, has been synthesized under hydrothermal conditions using Na and K ions as structure directing agents. EMS-2 has been characterized by laboratory and synchrotron X-ray powder diffraction, 23Na, 29Si and 119Sn MAS NMR, elemental, thermogravimetric and scanning electron microscopy analyses. The powder diffraction pattern of EMS-2 has been indexed with a monoclinic unit cell with parameters: a = 10.289 Å, b = 15.938 Å, c = 9.182 Å and β = 104.6° and a space group C2/m. EMS-2 is the synthetic tin-counterpart of the rare zirconosilicate mineral lemoynite. The structure of EMS-2 consists of “silicate slabs” of SiO4 tetrahedra, parallel to [0 1 0], joined together by isolated SnO6 octahedra. It has 10-membered ring channels, with pore openings of 6.47 × 4.61 Å, which host cations and water molecules. NMR confirms the unique occurrence (in stannosilicates) of three distinct silicon environments, Si(2Si, 2Sn), Si(3Si, 1Sn) and Si(4Si), and the high Na coordination symmetry foreseen by the diffraction structure. To our knowledge, EMS-2 is the only reported stannosilicate containing Si(4Si) sites.