Oxidative dehydrogenation of isobutane to isobutene by pyrovanadates, M2V2O7, where M(II) = Mn, Co, Ni, Cu and Zn, and Co2VO4 and ZnV2O4: The effect of gold nanoparticles

Abstract

The pyrovanadates M2V2O7, where MII=Mn, Co, Ni, Cu and Zn, have been investigated for the oxidative dehydrogenation of isobutane from 300 to 450°C, both with and without 5wt% gold nanoparticles. Reduction of the pyrovanadate compounds was generally observed, yielding products that contained V(IV) and/or V(III), and depended on both M(II) and the presence of gold nanoparticles. The most stable pyrovanadate was Ni2V2O7. Products identified by XRD were M2VO4, MVO3, MV2O4, or V2O3. The conversion of isobutane always increased with temperature, reaching 11% and 16% in the absence and presence of gold nanoparticles, respectively, at 450°C. Selectivities to isobutene were as high as 40–50%. Vanadium 2p3/2 XPS studies indicated that all catalysts exhibited surface V(V) and V(IV), while V(III) was present in those catalysts that generated V(III)-containing products. This suggests that catalytic activity depends on the rate of reoxidation of the lower oxidation states to V(V). Gold 4f7/2 XPS studies always indicated the presence of Au(0) and Au(I), and for some catalysts Au(III). The highest yields of isobutene correlate with the lowest Au(I) content. The reduction products Co2VIVO4 and ZnVIII2O4 were compared in their activities with Co2V2O7 and α-Zn2V2O7. The reduced phase Co2VO4 proved to be a good catalyst, comparable to the best 5wt% Au/M2V2O7 compositions.