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

Abstract

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.