The effect of loading of vanadia on silica in the oxidation of butane

The effect of vanadium loading on the structure and the catalytic behavior of silica-supported vanadium oxide catalysts in the selective oxidation of butane was investigated in a continuous flow reaction system using catalysts containing loadings of 0.53, 0.58, or 6.4 wt% vanadium supported on Davison or Cabosil silica. The lower loading samples were found to be more selective for oxidative dehydrogenation. At 520°C, with a feed of He/O₂/C₄H₁₀ = 88/8/4 at a total flow rate of 100 ml/min, the total dehydrogenation selectivity on the 0.58 wt% V sample was 65 and 50% at butane conversions of 5 and 20%. respectively, whereas they were 42 and 5% on the 6.4 wt% V samples. The activation energies for the oxidation of butane were about 110 kJ/mol for the lower loading samples and 164 kJ/mol for the higher loading samples. At 520°C the activity per vanadium was comparable for the different samples. Laser Raman spectra of fresh catalysts indicated that only a well-dispersed vanadia species existed on the 0.58 wt% V sample, while crystalline V₂O₅ was also formed on the 6.4 wt% V samples. The ratio of well-dispersed to crystalline vanadia species on the fresh catalysts was higher for the Cabosil silica-supported sample than the Davison silica-supported sample. After reaction, the spectra of the 0.58 wt% V samples did not indicate any changes. However, significantly more crystalline V₂O₅ was found on the 6.4 wt% V samples on both silica supports, indicating that aggregation of vanadia had occurred during reaction. The higher total dehydrogenation selectivity on the 0.58 wt% V samples was attributed to the presence of the well-dispersed vanadia, while the presence of crystalline V₂O₅ species on the higher loading samples contributed to the production of total oxidation products.