Location and Adsorbate Interactions of Vanadium in VAPO-5 Molecular Sieve Studied by Electron Spin Resonance and Electron Spin Echo Modulation Spectroscopies

Abstract

Large-pore vanadium-incorporated aluminophosphate molecular sieve VAPO-5 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. As-synthesized VAPO-5 contains two paramagnetic vanadyl species A and B with distorted octahedral coordination. After calcination in O2 and exposure to moisture, only species A is observed with reduced intensity. Species A is identified as a VO(H2O)32+ complex coordinated to two framework oxygens bonded to aluminum. Species B is observed only in as-synthesized VAPO-5. Species B is suggested to be similar to species A in which one of the water ligands is replaced by hydroxyl interacting with a protonated amine. When calcined, hydrated VAPO-5 is dehydrated at elevated temperature, species A loses its water ligands and transforms to VO2+ ions coordinated to two framework oxygens (species C). Species C reduces its intensity significantly after treatment with O2 at high temperature, thus suggesting oxidation of V4+ to V5+. High-temperature H2 treatment of a previously oxidized sample shows more than one vanadium species. Weak 31P ESEM is observed in both as-synthesized and dehydrated VAPO-5. Simulation of the 31P ESEM spectrum of dehydrated VAPO-5 shows one phosphorus nucleus at 4.4 Å from the vanadium ion. Strong 27Al ESE modulations are observed in both as-synthesized and dehydrated VAPO-5, which suggests that the vanadium ion is located at sites nearer to framework aluminum than to phosphorus. These observations lead to the conclusion that vanadium incorporated into AlPO4-5 occupies a framework phosphorus site probably as a localized defect. When dehydrated VAPO-5 makes contact with D2O at room temperature, the ESR signal of species A is observed. This species is identified as a VO(D2O)32+ complex from the 2D ESEM spectrum. Adsorption of ND3 on VAPO-5 generates a new vanadium species F identified as a VO(ND3)22+ complex. Adsorption of deuterated methanol on dehydrated VAPO-5 results in another new vanadium species G, which is identified as a VO(CD3OH)22+ complex. When deuterated ethylene is adsorbed on dehydrated VAPO-5, another new vanadium species H identified as VO(C2D4)12+ is observed. Possible coordination geometries of these various complexes are discussed.