Cosolvent and Local Environment Effects of Vanadium Incorporation on MCM-41 Catalysts for Selective Oxidation Reactions

Many oxidative and reductive processes have made extensive use of supported vanadium oxide catalysts. Understanding surface-active speciation and support interactions in these materials is critical to the development of heterogeneous catalysts. In this context, the vanadium incorporated on Mobil Composition of Matter No. 41 (V-MCM-41) catalysts were prepared with ethanol (AM) and without ethanol (NAM) at room temperature. The chemical and structural properties of the V-MCM-41 (AM) and V-MCM-41 (NAM) materials were studied by X-ray diffraction (XRD), N2 physisorption, inductively coupled plasma-optical emission spectrometry (ICP-OES), diffuse reflectance UV–vis spectroscopy (DRUV–vis), temperature-programmed reduction with hydrogen (H2-TPR), X-ray photoelectron spectroscopy (XPS), 51V NMR analysis, and high-resolution transmission electron microscopy (HRTEM) analysis. The TEM images showed that the V-MCM-41(AM) sample pores were arranged radially with nanosized particles. Further, the elemental analysis reports confirmed that the majority of the vanadium species were successfully loaded onto the pristine siliceous materials. However, the local environment of the vanadium and support interaction was entirely different due to the influence of ethanol in the synthesis medium. The NMR and DRUV–vis results showed that the incorporated vanadium oxide species were highly dispersed on both catalysts with an octahedral coordination environment under hydrated conditions. The detailed investigation from the XPS analysis combined with the TPR results confirmed that the fraction of the V5+ ion on the mesoporous nanospherical V-MCM-41 (AM) catalyst was significantly larger than the bulk V-MCM-41 (NAM) catalyst. The efficiency of catalysts was tested for diphenylmethane (DPM) oxidation reaction using CO2-free air as an oxidant, and catalytic experiments revealed that nanospherical V-MCM-41 (AM) showed a higher catalytic activity than bulk V-MCM-41 (NAM). Differences in catalytic activity were mainly encountered by the concentration of surface-active sites, which were influenced by the radial arrangements of nanospherical particles and the diffusion capability of vanadium oxides. The Raman and NMR studies also showed that a V5+ ion in a distorted coordination environment with a terminal V═O bond is required to activate the O2 molecule for the selective oxidation reaction. The calculated activation energies for V-MCM-41 (AM) and V-MCM-41 (NAM) were found to be 15 and 35 kJ/mol, respectively. This research paves the way to developing the nanosized MCM-41 material for a variety of shape-selective reactions and storage applications.