Vanadium dispersion and catalytic activity of Pd/VOx/SBA-15 catalysts in the Wacker oxidation of ethylene

Transmission electron microscopy (TEM), X-ray diffractometry (XRD), in situ diffuse reflectance ultra violet – visible (UV–Vis) spectroscopy, and temperature-programmed reduction by hydrogen (H 2 -TPR) were used to identify the vanadia forms in vanadium-containing SBA-15 preparations (VO x /SBA-15). Wacker type supported Pd/VO x /SBA-15 catalysts were obtained by introducing Pd into VO x /SBA-15 samples using conventional impregnation method. The activity of the catalysts was tested in the gas phase partial oxidation of ethylene by O 2 in the presence of H 2 O (Wacker oxidation). VO x /SBA-15 sample was obtained by micelle-templated synthesis using vanadium-containing synthesis gel. The vanadium became incorporated in the silica structure from the gel in near to atomic dispersion. This catalyst was quite active in ethylene oxidation to CO 2 but had low Wacker activity. Isolated, polymeric and bulk vanadia species were identified in the VO x /SBA-15 prepared by wet impregnation/calcination method. The specific surface area of the sample was found to be smaller than that of the neat SBA-15 support because some pores were blocked by vanadia agglomerates. The corresponding Pd/VO x /SBA-15 catalyst showed high selectivity for acetaldehyde formation but the activity was relatively low due to low accessible active surface. A third VO x /SBA-15 sample was obtained by applying directed surface reaction between silanol groups of dehydrated SBA-15 and anhydrous solution of vanadyl acetylacetonate. Large number of accessible Pd/VO x sites were present in the corresponding Pd/VO x /SBA-15 catalyst. Latter catalyst induced ethylene oxidation to acetaldehyde with high yield at temperatures ≤160 °C and with good yield to acetic acid at temperatures ≥160 °C.