Migration of molybdenum into intracrystalline cavities in molybdate-impregnated NaY zeolite

Abstract

Molybdenum-containing Y-type zeolites have been prepared by thermal decomposition of molybdate-impregnated samples according to a nonconventional procedure which uses a constantrate decomposition at very low water vapor pressure (10 N m −2). The combined results of X-ray diffraction, mid-infrared, surface area, and sorption (water and n-hexane) capacity measurements show that zeolite structure was largely retained when the impregnated material was activated at temperatures below 623 K. However, a progressive loss of crystallinity and a parallel decrease of the surface Mo Si ratio, as obtained from the X-ray photoelectron spectra, was observed at higher temperatures. Furthermore, on heating at higher temperatures (793 K), the electron spin resonance spectra indicated that a fraction of the Mo(V) ions are essentially isolated in the sodalite cages or hexagonal prisms of the zeolite. A mechanism which involves “vaporization” of MoO 3 from the external surface of the zeolite by water vapor is proposed to describe the redistribution of molybdenum in the impregnated precursor.