Mesoporous Silicates with Spherical Morphology Modified with Vanadium Highly Active in Oxidation of Cyclohexene with H2O2

Abstract

Vanadium-containing mesoporous molecular sieves have been prepared by hydrothermal treatment at 373 K. These materials showed spherical morphology with a narrow particle size distribution between 2 and 4 lm. The techniques used for their physicochemical characterization were: XRD, AAS, N 2 physisorption, SEM, TEM and DR-UV-Vis spectroscopy. All the materials presented high specific surface area ([900 m 2 /g), characteristic of MCM-41 materials. A well-defined mesoporous structure was observed by TEM measures although there was no onedimensional ordering of pores characteristic of such materials. Additionally, secondary mesoporosity domains were determined in the BJH size distribution. The sample synthesized with the highest content of V presented marked differences in their structural characteristics, which were attributed to the blockage of channels by the presence of nano-clusters and/or V x O y nano-oxides. From the DR-UV-Vis analysis, a successful incorporation of V ions to silica structure in tetrahedral coordination with oxygen of the network could be inferred. The catalytic activity of these materials was evaluated in the test reaction of cyclohexene oxidation using H 2 O 2 as oxidizing agent, showing a high conversion of about 93% respect to the maximum, resulting dominant the radicalary mechanism over the direct oxidation mechanism. Apparently, the isolated V ions incorporated into the silica structure would be responsible for the high catalytic activity of these materials.

Nature of active vanadium nanospecies in MCM-41 type catalysts for olefins oxidation

“…Thus, Raman band of 1018 cm À1 corresponds to the stretching frequency of a terminal of V]O group bonded to the silicate MCM-41 [6,33]. In addition, the Raman band at 269 cm À1 can be ascribed to the VeOeV bending mode and the bands centered around 335 and 920 cm À1 may correlate with the VeOeV stretching vibrations [6,33]; these bands are increased with increasing content of V. Finally, the absence of typical bands to 995, 703 and 530 cm À1 implies that no crystalline V 2 O 5 is formed in the samples, which is in accordance with XRD [25] and UVeVis-RD results. This suggests that the vanadium species in V-MMS(x) are isolated or, at least, highly dispersed on the surface.…”

“…Finally, the total conversion of H 2 O 2 was measured by iodometric titration and H 2 O 2 efficiency was calculated as the percentage of this reactive converted to total oxidized products. Table 1 summarizes the structural and morphological properties of the V-MMS synthesized with different V contents, reported by us elsewhere [25]. All materials exhibited MCM-41 type mesoporous structure, high surface areas between 900 and 1400 m 2 /g and a narrow pore size distribution around to 2.4 nm.…”

“…Specific surface area values of V-MMS(60) and V-MMS(240) samples are very similar (~1375 m 2 /g, respectively) possibly due to their very low vanadium content (<0.15 wt %). However, the V-MMS (20) sample, synthesized with the highest content of vanadium, shows the lower surface area (934 m 2 /g) probably due to the partial blocking of mesopores by V-nanoclusters and/or nano-oxides (<5 nm) [25].…”

“…The vanadium-containing mesoporous molecular sieves (V-MMS), having various Si/V atomic ratios, were prepared by direct hydrothermal synthesis according to the procedure described by us elsewhere [25]. The color of the final catalysts was white, which turns to yellow when the materials are exposed at atmospheric humidity.…”

“…In a previous work, we reported the hydrothermal synthesis of vanadium-modified mesoporous silicates, its structural and morphological characterization and its catalytic testing in the cyclohexene oxidation by employing H 2 O 2 as an oxidant agent [25]. In the present work, we investigated in detail the nature of vanadium nanospecies formed on these mesoporous silicates through a synergistic study of Ultravioletevisible diffuse reflectance spectroscopy (UVeVis-DRS), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR).…”