Synthesis, characterization and catalytic testing of metal tungstates as catalysts for activation of lower alkanes
The electronic and structural properties of vanadium-containing phases govern the formation of isolated active sites at the surface of these catalysts for selective alkane oxidation. This concept is not restricted to vanadium oxide. The deliberate use of hydrothermal techniques can turn the typical combustion catalyst manganese oxide into a selective catalyst for oxidative dehydrogenation of propane (ODP). Nanostructured, crystalline MnWO4 serves as the support that stabilizes a defect-rich MnOx surface phase. Oxygen defects can be reversibly replenished and depleted at the reaction temperature. Terminating MnOx zigzag chains on the (010) crystal planes are supposed to bear structurally site-isolated oxygen defects that account for the unexpectedly good performance of the catalyst in propane activation. The mechanism of C-H activation in selective oxidation reactions of short-chain alkane molecules over transition metal oxides is critically affected by the balance of acid-base and redox sites at the surface of the catalyst. Using the example of manganese tungstate we discuss how the relative abundance of these sites can be controlled via synthetic techniques. Phase-pure catalysts composed of the thermodynamic stable monoclinic MnWO4 phase have been prepared by hydrothermal synthesis. Variation of the initial pH value resulted in rod-shaped nano-crystalline MnWO4 catalysts composed of particles with varying aspect ratio. The synthesis products have been analysed by transmission electron microscopy, X-ray diffraction, infrared and photoelectron spectroscopy. In-situ Raman spectroscopy was used to investigate the dissolution-re-crystallization processes occurring under hydrothermal conditions. Ethanol oxidation was applied to probe the surface functionalities in terms of acid-base and redox properties. Changes in the aspect ratio of the primary catalyst particles are reflected in the product distribution induced by altering the fraction of acid-base and redox sites exposed at the surface of the catalysts in agreement with the proposed mechanism of particle growth by re-crystallization during ageing under hydrothermal conditions. Phase-pure CoWO4 catalysts were successfully prepared by hydrothermal synthesis and tested in ODP. Higher initial pH value in the synthesis gel gives rise to a higher Co/W ratio in the near surface region as detected by XPS. Furthermore, there exists a correlation between the specific propane consumption rate and the Co/W ratio. CoWO4 enriched in tungsten in the near surface region shows low selectivity to propene, and the selectivity to conversion trajectory is similar to that of mesoporous tungsten oxide. However, the catalyst with the highest Co/W ratio shows the best selectivity to propene at around 9 % propane conversion. Moreover, the surface termination by a Co chain structure on the (010) crystal planes is confirmed by HRTEM. FT-IR spectroscopy exhibits a band at 3387 cm-1 attributed to surface Co bridging hydroxyl groups. The terminating oxo-hydroxy layer in the catalyst precursor is proposed to develop a Co-rich defective surface responsible for high activity and selectivity of CoWO4 in ODP.
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