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The present study concerns preparation of Pd/V2O5/TiO2 catalysts and activity of the catalysts in selective gas phase oxidation of ethylene by O2 in presence of water at atmospheric pressure and in the temperature range of 100–200 °C. The influence of palladium and vanadia loading and the partial pressures of the reactants on the yield of oxidation products (acetaldehyde and acetic acid) were examined. The surface-bound vanadia forms were identified by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), and FT-Raman spectroscopy. The best activities were achieved with catalysts having near to nominal monolayer vanadia coverage of the support. Time-on-stream catalytic tests and chemical analysis of the fresh and used catalysts proved the structural and catalytic stability of the Pd/V2O5/TiO2 preparations. It was shown that under the conditions of Wacker-oxidation primary product acetaldehyde could be further oxidized to CO2, whereas no consecutive oxidation of product acetic acid proceeded.

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.

This paper concerns the Wacker oxidation of ethylene by oxygen in the presence of water over supported Pd/VO x catalysts. High surface area porous supports were obtained from layer-structured materials, such as, montmorillonite (MT), laponite (smectites), and hydrotalcite (layered double hydroxide, LDH) by pillaring. Before introduction of Pd, supports MT and LDH were pillared by vanadia. The laponite was used in titania-pillared form (TiO2–LAP) as support of Pd/VO x active component. Acetaldehyde, acetic acid and CO2 were the products with yields and selectivities, depending on the reaction conditions and the properties of the applied catalyst. Under comparable conditions the pillared smectite catalysts gave higher AcH yield than the pillared LDH catalyst. UV–Vis spectroscopic examination suggested that the pillared smectites contained polymeric chains of VO4, whereas only isolated monomeric VO4 species were present in the pillared LDH. The higher catalytic activity in the Wacker oxidation was attributed to the more favorable redox property of the polymeric than of the monomeric vanadia. The V3+ ions in the polymeric species can reduce O2 to O2− ions, whereas the obtained V5+ ions are ready to pass over O to Pd0 to generate PdO whereon the oxidation of the ethylene proceeds.