Paramagnetic dioxovanadium(iv) molecules inside the channels of zeolite BEA – EPR screening of VO2 reactivity toward small gas-phase molecules

Abstract

Interaction of small gas-phase molecules (NO, N₂O, O₂, CO) with VO₂ radicals inside the channels of a dealuminated SiBEA zeolite was investigated by means of electron paramagnetic resonance (EPR), infrared (IR), and mass (QMS) spectroscopies to provide direct insights into the chemistry of a unique paramagnetic state of vanadium – VO₂ molecules. A facile way of forming VO₂ inside the channels of SiBEA via thermal reduction of VO₂⁺ precursor cations was shown. Dioxovanadium(IV) was identified based on its unusual EPR signal which, as compared with the typical monooxovanadium(IV) (VO²⁺ cation), is featured by rhombic symmetry and a positive A_iso value leading to a hyperfine splitting as large as 32 mT. VO₂ molecules exhibit reducing properties transforming N₂O and O₂ into vanadium intrachannel cage adducts comprising of reactive oxygen species (O⁻ and O₂⁻, respectively). Interaction with CO led to its oxidation to CO₂, while paramagnetic NO acted as a scavenger for VO₂ radicals producing diamagnetic adducts. The observed reactivity was rationalized in terms of spin-pairing, electron transfer, and oxygen transfer processes. As a result new chemical pathways of vanadium reactivity were demonstrated which were not observed so far either in the homogeneous molecular systems or supported vanadium materials.