X-RAY-POWDER DIFFRACTION AND NMR-SPECTROSCOPIC INVESTIGATIONS ON A POROUS ZINCOSILICATE RELATED TO THE ZEOLITE VPI-7 (VSV)

Structural investigations on a new porous zincosilicate phase, VPI-7#, which is related to VPI-7 were carried out using high resolution X-ra y powder diffraction and multinuclear NMR experiments (Si-29 CP MAS an d Na-23 DOR). The combination of these complementary techniques allowe d for the identification of VPI-7# and an understanding of the structu ral differences between VPI-7# and VPI-7. VPI-7# crystallizes in the t riclinic space group P1 with cell parameters a = 40.12(2) Angstrom, b = 10.296(5) Angstrom, c = 10.246(5) Angstrom, alpha = 90.74(2)degrees, beta = 91.66(2)degrees and gamma = 87.89(2)degrees. Taking into accou nt the closely related unit cell parameters and the similarity of the chemical shifts of the Si-29 NMR signals in the spectra of VPI-7# and VPI-7 the framework topology of the two zincosilicates is the same. Ba sed on a preliminary Rietveld refinement the Zn-ordering in the zincos ilicate framework of VPI-7# is also the same as in VPI-7. Na-23 DOR ex periments of VPI-7 and VPI-7# show an increase in the number of symmet rically inequivalent, however, well ordered Na-sites. This leads to th e conclusion, that the ordering scheme of the nonframework constituent s Na and H2O is different. The conclusion is based on experimental fac ts as well as crystal chemical reasoning since the number of atoms in the asymmetric unit (216 framework atoms in general sites) by far exce eds the potential of a Rietveld analysis.