Thermal and hydrothermal stability of molecular sieve VPI-5 by in situ x-ray powder diffraction

The framework stability of the molecular sieve VPI-5 is studied at various conditions of temperature, pressure, and humidity. Dehydration of VPI-5, by either rapid heating or pressure reduction, leads to a contraction in a and an expansion in c (hexagonal cell). These changes are reversible upon rehydration. The transformation of VPI-5 into AlPO{sub 4}-8 depends on the rate at which water is removed from the pores. Slow heating results in the irreversible formation of AlPO{sub 4}-8 from VPI-5 at about 100C. However, VPI-5 rapidly heated to at least 550C can withstand exposure to steam at these elevated temperatures with little loss of structural integrity.