Location and adsorbate interactions of vanadium in VAPO-5 molecular sieve studied by electron spin resonance and electron spin echo modulation spectroscopies

Large-pole vanadium-incorporated aluminophosphate molecular sieve VAPO-5 ha s been studied by electron spin resonance (ESR) and electron spin echo modu lation (ESEM) spectroscopies to determine the vanadium location and interac tion with various adsorbate molecules. As-synthesized VAPO-5 contains two p aramagnetic vanadyl species A and B with distorted octahedral coordination. After calcination in O-2 and exposure to moisture, only species A is obser ved with reduced intensity. Species A is identified as a VO(H2O)(3)(2+) com plex coordinated to two framework oxygens bonded to aluminum. Species B is observed only in as-synthesized VAPO-5. Species B is suggested to be simila r to species A in which one of the water ligands is replaced by hydroxyl in teracting with a protonated amine. When calcined, hydrated VAPO-5 is dehydr ated at elevated temperature, species A loses its water ligands and transfo rms to VO2+ ions coordinated to two framework oxygens (species C). Species C reduces its intensity significantly after treatment with Or at high tempe rature, thus suggesting oxidation of V4+ to V5+. High-temperature H-2 treat ment of a previously oxidized sample shows more than one vanadium species. Weak P-31 ESEM is observed in both as-synthesized and dehydrated VAPO-5. Si mulation of the P-31 ESEM spectrum of dehydrated VAPO-5 shows one phosphoru s nucleus at 4.4 Angstrom from the vanadium ion. Strong Al-27 ESE modulatio ns are observed in both as-synthesized and dehydrated VAPO-5, which suggest s that the vanadium ion is located at sites nearer to framework aluminum th an to phosphorus. These observations lead to the conclusion that vanadium i ncorporated into AlPO4-5 occupies a framework phosphorus site probably as a localized defect. When dehydrated VAPO-5 makes contact with D2O at room te mperature, the ESR signal of species A is observed. This species is identif ied as a VO(D2O)(3)(2+) complex from the D-2 ESEM spectrum. Adsorption of N D3 on VAPO-5 generates a new vanadium species F identified as a VO(ND3)(2)( 2+) complex. Adsorption of deuterated methanol on dehydrated VAPO-5 results in another new vanadium species G, which is identified as a VO(CD3OH)(2)(2 +) complex. When deuterated ethylene is adsorbed on dehydrated VAPO-5, anot her new vanadium species H identified as VO(C2D4)(1)(2+) is observed. Possi ble coordination geometries of these various complexes are discussed.