THERMAL AND HYDROTHERMAL STABILITY OF MOLECULAR-SIEVE VPI-5 BY INSITU 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 AlPO4-8 depends on the rate at which water is removed from the pores. Slow heating results in the irreversible formation of AlPO4-8 from VPI-5 at about 100-degrees-C. However, VPI-5 rapidly heated to at least 550-degrees-C can withstand exposure to steam at these elevated temperatures with little loss of structural integrity.