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

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 VO 2+ -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(H 2 O) 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 VO 2+ ions oxidize to V 5+ on calcination and then form a V 5+ ···H 2 O 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 H 2 or CO treatment of a previously oxidized sample shows vanadyl species that are suggested to be VO 2+ ions with weak interaction with H 2 or CO molecules. When dehydrated VS-1 is contacted with D 2 O at room temperature, a vanadyl species identified as VO(D 2 O) 1 2+ from 2 D ESEM data is observed. Adsorption of C 2 D 4 on VS-1 generates a new vanadium species, VO(C 2 D 4 ) 1 2+ . When deuterated ammonia is adsorbed on dehydrated VS-1, another new vanadium species VO(ND 3 ) 1 2+, is observed. Adsorption of deuterated methanol on dehydrated VAPO-5 resulted in another new vanadium species.