V(IV) Species, Location and Adsorbate Interactions in VH-SAPO-42 Studied by ESR and Electron Spin-Echo Modulation Spectroscopies

Abstract

Vanadium-incorporated aluminophosphate microporous molecular sieve VH-SAPO-42 has been studied by electron spin resonance(ESR) and electron spin-echo modulation (ESEM) spectroscopies to determine the vanadium location and interaction with various adsorbate molecules. The results are interpreted in terms of V(IV) ion location and coordination geometry. As-synthesized VH-SAPO-42 contains only vanadyl species with distorted octahedral or trigonal bipyramidal coordination. Vanadium incorporated into H-SAPO-42 occupied extra-framework site. After calcinations in O2 and exposure to moisture, only species A is observed with reduced intensities. Species A is identified as a VO(H2O)22+ complex coordinated to three framework oxygen atoms bonded to aluminum. When hydrated VH-SAPO-42 is dehydrated at elevated temperature by calcination, species A loses its water ligand and transforms to VO2+ ions coordinated to three framework oxygens (species B). Species B reduces its intensities significantly after treatment with O2 at high temperature, thus suggesting oxidation of V4+ to V5+. When dehydrated VH-SAPO-42 makes contact with D2O at room temperature, the ESR signal of species A is regained. The species is assumed as a VO(Of)3(D2O)2 by considering three framework oxygens. Adsorption of deuterated methanol on dehydrated VH-SAPO-42 results in another new vanadium species D, which is identified as a VO(CD3OH)2 complex. When deuterated ethylene is adsorbed on dehydrated VH-SAPO-42, another new vanadium species E identified as a VO(C2D4)2+, is observed. Possible coordination geometries of these various complexes are discussed.