Structure of flame-made vanadia/silica and catalytic behavior in the oxidative dehydrogenation of propane

摘要

Vanadia/silica particles with a specific surface area up to 330 m ~2 g~(-1) and a V_2O_5 content up to 50 wt.% or V surface density up to 27.6 V nm~(-2) were prepared by flame spray pyrolysis. The catalysts were characterized by nitrogen adsorption, X-ray diffraction, temperature-programmed reduction, Raman spectroscopy, and ~(51)V MAS NMR and tested in the oxidative dehydrogenation (ODH) of propane. Depending on vanadia content, different vanadia species were formed. The as-prepared flame-made catalysts showed dominantly isolated monomeric VO_x surface species for V loadings exceeding even the typical "monolayer coverage" (2 V nm~(-2)) of classic wet-impregnated materials. The stability of these VO_x species depended on temperature and V surface density. Catalysts with 3.3 V nm~(-2) were stable up to 500 °C and those with 2 V nm~(-2) up to 600 °C. Catalysts loaded with 3-25 wt.% V_2O_5 were tested for the ODH of propane. Catalysts containing ≥15 wt.% V_2O_5 showed structural rearrangement of the VO_x species during the catalytic tests inducing a transition from monomeric to crystalline vanadia. The turnover frequency of flame-made catalysts decreased with increasing vanadia loading, indicating a higher activity of monomeric VO_x species compared to crystalline V_2O_5. The conversion (global activity), however, showed a maximum for the 20 wt.% V_2O_5/SiO_2 (4.6 V nm~(-2)) catalyst. The selectivity to propene depended mainly on propane conversion and only to a lesser extent on the structure of the VO_x species. Highest selectivity (55%) was achieved for the low loaded catalysts. With increasing vanadia loading, formation of CO_x increased and the product ratio of CO_2/CO decreased. Highest propene yield was measured for catalysts with relative high V surface density containing both monomeric VO_x and crystalline V_2O_5. Flame spray pyrolysis proved to be a very versatile method for synthesis of V_2O _5/SiO_2 catalysts with high dispersion of isolated VO _x species at high surface density.