Hydrothermal synthesis of bi-functional nanostructured manganese tungstate catalysts for selective oxidation

Abstract

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 using 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 using 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.

Selective oxidation of cyclohexane: Ce promotion of nanostructured manganese tungstate

“…A manganese tungstate with a pure monoclinic wolframite-type structure was obtained in good agreement with the literature [20,31,33]. The BET specific surface area of the MnWO 4 prepared in this work (Table 1) has a BET specific surface area that is about half the values commonly reported in the literature [19,20]. Despite this difference, the TEM micrographs ( Fig.…”

“…Despite this difference, the TEM micrographs ( Fig. 2a) show that MnWO 4 in the shape of nanorods with ~100-250 nm length and ~30-50 nm width was in fact formed by the hydrothermal synthesis [19,31]. The crystallinity of the MnWO 4 nanorods was also confirmed by TEM and SAED ( Fig.…”

“…Nanostructured MnWO 4 samples were prepared according to the hydrothermal method described in the literature [19,20,31]. Optimum temperature and pH conditions were selected from the literature in order to obtain highly crystalline anisotropic nanorods with the best surface properties for oxidation reactions.…”

“…A study of cyclohexane oxidation by air over unsupported Mn oxides showed the highest cyclohexane conversion was achieved for calcination at 400ºC, which was attributed to the higher surface Mn 4+ concentration promoting oxygen mobility and increased oxygen adsorption capacity [18]. The use of Mn containing mixed-metal oxides in oxidation reactions has been studied frequently in the literature [19][20][21][22][23].…”

Concluding remarks: progress toward the design of solid catalysts

“…For example, manganese tungstate catalysts for selective oxidation of short-chain alkanes prepared by Li et al 6 were intended to have a balance of acid-base and redox sites. Preparation of these catalysts was guided by Raman spectra recorded during the synthesis, as well as high-resolution transmission electron microscopy and field emission scanning electron microscopy images, X-ray diffraction data, X-ray photoelectron spectra, and porosity data characterizing the resulting materials.…”