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

摘要

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 A,b = 15.938 A,c = 9.182 A and beta = 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 [010],joined together by isolated SnO6 octahedra.It has 10-membered ring channels,with pore openings of 6.47 x 4.61 A,which host cations and water molecules.NMR confirms the unique occurrence (in stan-nosilicates) 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.