Further studies on aluminophosphate molecular sieves. Part 2.—VPI-5 and related aluminophosphate materials

VPI-5 and AlPO4-H2 are constructed from the same type of chain building unit which contains octahedral Al atoms (coordinated with four framework oxygens and two water molecules). The bonded water molecules are believed to stabilize the chain building unit and play an important role in the crystallization of these materials. This chain building unit limits the possibilities of incorporating silicon into the VPI-5 framework and synthesizing aluminosilicate or pure-silica versions of the VPI-5 structure. VPI-5 and AlPO4-H2 can irreversibly transform to AlPO4-8 and AlPO4-tridymite, respectively, via similar bond rearrangements. Several new aluminophosphates with X-ray powder diffraction reflections similar to some of those for VPI-5 are synthesized and their physicochemical properties do not match those for VPI-5. Strong evidence is provided to suggest that previous claims that VPI-5 and AlPO4-H1 are the same phase are unwarranted