Location and Adsorbate Interactions of V(IV) Species in VH-SAPO-34 Studied by EPR and Electron Spin-Echo Modulation Spectroscopies

Abstract

Vanadium-doped H-SAPO-34 samples were prepared by a high-temperature solid-state reaction between SAPO-34 and the paramagnetic V(IV) species and characterized carefully by EPR and Electron Spin-Echo Modulation(ESEM) studies. The paramagnetic vanadium species generated in both V2O5 and VOSO4 of SAPO-34 have the same narrow range of g value for vanadium species assigned to VO2+ inferred from the isotropic EPR spectrum at 293 K. The EPR and ESEM data indicate that the V(IV) species exist as a vanadyl ion either as [V(IV)]O2+ or V4+. The [V(IV)]O2+ species seems to be more probable because SAPO-34 having a low negative framework charge35 and more positively charged species like V4+ can not be easily stabilized. Tetravalent vanadium ion in vadium-doped H- SAPO-34 can only be observed at the temperature lower than 77 K, while the vanadyl ion, VO2+ in the activated sample of VH-SAPO-34 can produce the ion even at room temperature. After the adsorption of methanol, ethanol, propanol or ethene to the VH-SAPO-34, only one molecule coordinate to [V(IV)]O2+ was observed in EPR and ESEM spectra.