Structural characterization of the new porous sodium silicate Mu-11 by 29Si and 23Na solid-state NMR

Abstract

A variety of solid-state NMR techniques were applied in order to assign resonances in the ²⁹Si and ²³Na NMR spectra of a new crystalline silicate to crystallographically distinct silicon and sodium sites, respectively. Mu-11 is a zeolite-like material with a 1D channel system occupied by water and sodium cations whose framework contains both bridging and non-bridging oxygens. The observed ²⁹Si isotropic chemical shifts did not allow a direct assignment. A preliminary assignment based on the electronegativities calculated for the four ligands around each silicon was supported by correlations between the extent of the distortion from an ideal tetrahedron and the measured chemical shift anisotropy for the different Si sites. ¹H–²⁹Si cross polarization experiments confirmed the assignments. The two crystallographically different sodium atoms were resolved by ²³Na 3Q-MAS NMR, and an assignment for these two sites is proposed.