X-ray powder diffraction and NMR-spectroscopic investigations on a porous zincosilicate related to the zeolite VPI-7 (VSV)

Abstract

Structural investigations on a new porous zincosilicate phase, VPI-7#, which is related to VPI-7 were carried out using high resolution X-ray powder diffraction and multinuclear NMR experiments ( 29Si CP MAS and 23Na DOR). The combination of these complementary techniques allowed for the identification of VPI-7# and an understanding of the structural differences between VPI-7# and VPI-7. VPI-7# crystallizes in the triclinic space group P1 with cell parameters a = 40.12(2) A ̊ , b = 10.296(5) A ̊ , c = 10.246(5) A ̊ , α = 90.74(2) °, β = 91.66(2) ° and γ = 87.89(2) °. Taking into account the closely related unit cell parameters and the similarity of the chemical shifts of the 29Si NMR signals in the spectra of VPI-7# and VPI-7 the framework topology of the two zincosilicates is the same. Based on a preliminary Rietveld refinement the Zn-ordering in the zincosilicate framework of VPI-7# is also the same as in VPI-7. 23Na DOR experiments of VPI-7 and VPI-7# show an increase in the number of symmetrically inequivalent, however, well ordered Na-sites. This leads to the conclusion, that the ordering scheme of the nonframework constituents Na and H 2O is different. The conclusion is based on experimental facts as well as crystal chemical reasoning since the number of atoms in the asymmetric unit (216 framework atoms in general sites) by far exceeds the potential of a Rietveld analysis.