III International Conference

PP-II-82During the stage of combined hydrolysis of V-Mg-alkoxides it was found that thegelation time increases as V loading in sample grows. At V concentrations > 15 wt.% thegelation does not occur. From the other hand, in case of adding vanadium isopropoxide to aas-formed Mg(OH) 2 suspension, it was possible to obtain V-Mg-hydroxide gel within thewide range of V concentrations. Drying of these V-Mg-gels in supercritical conditionsresulted in formation of aerogel V-Mg-hydroxides with a surface area of 1000-1100 m 2 /g,regardless to concentration of introduced vanadium. Synthesized aerogel V-Mg-hydroxidesare characterized by quite narrow pore size distribution, with a maximum near 4 nm. Thedirect heat treatment of V-Mg-hydroxides in the temperature programmed regime in air leadsto formation of aerogel VO x /MgO catalysts with surface area of 350-430 m 2 /g, even if theordinary used vacuum dehydration step is just skipped.Synthesized aerogel VO x /MgO samples were studied by the set of physico-chemicalmethods. The absence of V 2 O 5 phase was suggested for all examined samples. By XRD andFTIR data, the formation of poorly crystallized orthovanadate phase was assumed, althoughXANES and Raman-spectroscopy data do not evidence this assumption. As follows fromTEM images, the aerogel VO x /MgO samples are characterized by quite uniform distributionof active component throughout the MgO support. According to SEM data, the secondarystructure of aerogel V-Mg-hydroxides and VO x /MgO samples is represented by 2-5 µm sizedplatelets with a thickness of 0.1-0.4 µm.Aerogel VO x /MgO catalysts were studied in comparative ODHP tests along withreference samples prepared on the basis of nanoscale MgO-AP support by mechanical mixingand impregnation methods. At all other parameters kept the same, the aerogel VO x /MgOsamples turned out to be significantly more active as compared with similar catalysts preparedby conventional ways. It was also shown that the replacement of O 2 by N 2 O in the ODHPreaction gives the considerable benefit in selectivity towards C 3 H 6 , however the yield ofpropylene does not change. This is as well accompanied with more intensive process of cokeformation on the surface of aerogel VO x /MgO catalysts.Financial support by the Russian Foundation for Basic Research (Grant 06-03-32540) isacknowledged with gratitude.References1. R. Vidal-Michel, K.L. Hohn, , J.Catal., 2004, V. 221, P. 127-136.2. V.V.Chesnokov, A.F.Bedilo, D.S.Heroux, I.V.Mishakov, and K.J.Klabunde, J. Catal., 2003, Vol.218, P. 438-446.3. C. Pak, A.T. Bell, T.D. Tilley, J. Catal., 2002, V. 206, P. 49-59.313