SOLID-STATE NMR-STUDY OF THE TRANSFORMATION OF VPI-5/MCM-9 INTO ALPO4-8/SAPO-8

The removal of water at 100-degrees-C (or under specific conditions) from VPl-5/MCM-9 materials results in significant structural changes. The VPl-5/MCM-9 products transform into AlPO4-8/SAPO-8. This transformation is partially reversible and strongly depends on the evacuation and rehydration treatments, the heating rate, and probably on lattice defect concentrations. From the knowledge of the VPl-5 structure and of the structural model recently proposed for AlPO4-8, we have reassigned all the P-31 n.m.r. lines characterizing both materials and explained further the effect of water on the dramatic changes occurring in their P-31 and Al-27 n.m.r spectra. P-31 and Al-27 n.m.r. have been used to investigate systematically the transformation of VPl-5 into AlPO4-8 and to determine which specific T sites undergo modifications. A mechanism accounting for the transformation is proposed together with a model allowing the determination of the degree of VPl-5/AlPO4-8 conversion by computing the relative intensities of five specific P-31 n.m.r. lines that characterize the transition intermediate structures. P-31 n.m.r. proved more sensitive than did conventional X-ray diffraction to detect short-range structural changes in these open structures.