Heterogeneous catalytic Wacker oxidation of ethylene over oxide-supported Pd/VOx catalysts: The support effect

摘要

This paper concerns the selective oxidation of ethylene (EE) to acetaldehyde (AL) and acetic acid (AA) by oxygen in the presence of steam over non-supported Pd/V2O5 catalyst and over Pd/V2O5 catalysts supported by SiO2, TiO2, γ- Al2O3, and α-Al2O3. A flow-through microreactor was applied at atmospheric pressure in the temperature range of 150-200 oC. The WHSV of EE was 0.17 or 0.84 h-1. The vanadia content of the supported catalysts was 17 wt%, whereas their Pd content was 0.8 wt %. The reducibility of vanadia was determined using temperature-programmed reduction by hydrogen (H2-TPR). Applying ultraviolet-visible (UV-vis) spectroscopy and X-ray diffractometry (XRD) different vanadia species were identified over different supports. In the Pd/V2O5/α-Al2O3 catalyst the vanadia had the same structure than in the Pd/V2O5 preparation. Even the low surface area α-Al2O3 support affects the Wacker oxidation activity of the catalyst. Vanadia, deposited on the surface of TiO2 or γ-Al2O3 forms easily reducible polymeric species. In interaction with Pd this polymeric species is responsible for the total oxidation EE to CO2. Palladium, bound to the surface of bulk V2O5 or to monomeric vanadate-like species on silica, forms Pd/VOx redox pairs, which are active and selective catalytic centers of the Wacker reaction. The Wacker mechanism was verified by test reactions, where one of the four components, such as Pd, V2O5, O2, or H2O, was left out from the reacting system. In absence of any of the components no selective catalytic partial EE oxidation proceeded, indicating that the Wacker mechanism could not operate.