Structure and Adsorbate Interactions of Vanadium in a Vanadium Silicate (VS-1) Molecular Sieve

Abstract

A medium pore vanadium silicate molecular sieve VS-1 with the ZSM-5 structure was studied by electron spin resonance (ESR) and electron spin−echo modulation (ESEM) spectroscopies to determine the vanadium structure and interaction with various adsorbate molecules. The VS-1 material is compared with ion-exchanged VO2+-ZSM-5 and impregnated V/silicalite-1 materials. As-synthesized VS-1 contains two vanadyl species with distorted octahedral coordination. Deviating from earlier assignments, we assign one of these species to VO(H2O)2+ based on ESEM results. The other species is suggested to be a similar complex in which the water molecule is replaced by a hydroxyl ion interacting with a tetrapropylammonium ion. The VO2+ ions oxidize to V5+ on calcination and then form a V5+···H2O complex after room-temperature hydration. Calcined, hydrated VS-1, after dehydration at elevated temperature, shows vanadyl species with square pyramidal geometry involving four framework oxygens. High-temperature H2 or CO treatment o...