Comparative study of physico-chemical, acid-base and catalytic properties of vanadium based catalysts in the oxidehydrogenation of n-butane: effect of the oxide carrier

Vanadium based catalysts with the following compositions: 30%V2O5-70%MgO, 30%V2O5-70%SiO2 and 30%V2O5-40%MgO-30%SiO2 were prepared by the wet impregnation method using MgO and/or SiO2 as support. The structure, texture and morphology of the active phase strongly depend on the choice of support: for 30%V2O5-70%MgO, thin particles of Mg3V2O8 were identified with TEM (associated to EDS) together with MgO particles while large V2O5 particles beside silica particles bearing vanadium species on their surface (EDS) were observed in the case of the 30%V2O5-70%SiO2. For the 30%V2O5-40%MgO-30%SiO2, three crystalline phases were detected by XRD: Mg3V2O8, Mg2SiO4 and MgO. Mg3V2O8 and Mg2SiO4 were also identified with TEM-EDS with a silica-rich phase corresponding to the silica support bearing Mg and V species on its surface. The acid-base properties of the prepared materials were evaluated by the isopropanol decomposition test. The 30%V2O5-70%SiO2 displays only acidic properties (formation of propene) that can be assigned to the V2O5 phase and/or to the vanadium species dispersed on silica; the 30%V2O5-70%MgO presents mainly basic properties (formation of acetone) while the 3-components catalyst (30%V2O5-40%MgO-30%SiO2) displays both acid and basic properties. The catalytic activity was investigated in the oxidehydrogenation of n-butane. The selectivity to reaction products (butenes, COx, cracking, isobutane) is affected by the nature of surface vanadium and oxygen species, acid-base character and the operating conditions. The best selectivities to butenes (with low COx production) were obtained on the catalysts containing magnesium, probably a consequence of basic properties of MgO and highly dispersed magnesium-vanadium mixed phase. The decrease of contact time favors the dehydrogenation of n-butane to butenes especially for the V2O5-SiO2 catalyst.