Solid-state n.m.r. study of the transformation of VPI-5/MCM-9 into AIPO₄-8/SAPO-8

The removal of water at 100 °C (or under specific conditions) from VPI-5/MCM-9 materials results in significant structural changes. The VPI-5/MCM-9 products transform into AIPO₄-8/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 VPI-5 structure and of the structural model recently proposed for AIPO₄-8, we have reassigned all the ³¹P n.m.r. lines characterizing both materials and explained further the effect of water on the dramatic changes occurring in their ³¹P and ²⁷Al n.m.r. spectra. ³¹P and ²⁷Al n.m.r. have been used to investigate systematically the transformation of VPI-5 into AIPO₄-8 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 VPI-5/AIPO₄-8 conversion by computing the relative intensities of five specific ³¹P n.m.r. lines that characterize the transition intermediate structures. ³¹P n.m.r. proved more sensitive than did conventional X-ray diffraction to detect short-range structural changes in these open structures.