Insights into Redox Dynamics of Vanadium Species Impregnated in Layered Siliceous Zeolitic Structures during Methanol Oxidation Reactions

Abstract

Supported transition metal catalysts have been extensively applied to oxidative and reductive processes. The understanding of surface speciation and active site‐support interactions in these materials play a substantial role in developing improved heterogeneous catalysts. Herein, a series of impregnated 3D ferrierite and 2D ITQ‐6 siliceous supports with variable loading of vanadium oxide was prepared. Chemical and structural properties of the materials were studied by X‐ray diffraction, N2 physisorption, inductively coupled plasma – optical emission spectrometry, X‐ray absorption, Fourier transform infrared and diffuse reflectance UV‐vis spectroscopies, and temperature‐programmed reduction with H2. Reactivity of the catalyst surface, associated with the incidence of isolated silanol groups, was found to be more effective when vanadium oxides were better dispersed and stabilized than increases in surface area. Differences in activation and the oxidation state dynamic behavior of active sites were then probed by methanol oxidation as a model reaction monitored by in situ FTIR spectroscopy and XANES/MS. By applying isothermal periods of reaction under non‐oxidizing atmosphere and regeneration of catalysts by O2, it was found that, even at distinct rates, all types of sites are accessible during reaction, since a complete reduction to V4+ was observed. However, reoxidation of sites to V5+ is limited and sensitive to the different vanadium species on the surface, and probably, the determinant factor of the distinct V5+/V4+ equilibrium reached for the catalysts when the reaction is carried out under constant oxidizing atmosphere.

Redox Dynamics of Active VOx Sites Promoted by TiOx during Oxidative Dehydrogenation of Ethanol Detected by Operando Quick XAS

“…Wu et al 22 investigated supported VO x /Al 2 O 3 catalyst after exposure to methanol with XPS under UHV conditions and detected V 3+ and V 4+ species. Vieira et al 19 investigated zeolite-supported vanadia catalysts in the methanol ODH by in situ time-resolved V K-edge XAS and concluded that V 4+ is the main intermediate involved in the reaction. The observed rate of V 4+ reoxidation by oxygen, however, was slower than V 5+ reduction by methanol, which contradicts the literature and might be related to the large volume of the in situ cell, which is not ideal for kinetic studies.…”

“…12−14 The oxidation of the partially reduced metal intermediate (M (x−n)+ ) by molecular oxygen (Step 2 in Scheme 1) is assumed to be very rapid 8,9,12,15,16 and, therefore, the concentration of the M (x−n)+ intermediate during the ODH process should be low. The mechanism of alcohol oxidation over supported VO x species was examined by many groups using multiple in situ spectroscopic techniques (Fourier transform infrared (FT-IR) spectroscopy, 2,9,15−19 Raman spectroscopy, 11,18,20 diffuse reflectance UV−vis (DR UV−vis) spectroscopy, 18,20 X-ray photoelectron spectroscopy (XPS), 9,15,17 and X-ray absorption spectroscopy (XAS) 19,21 ). Although several reaction mechanisms were proposed based on the nature of the observed surface reaction intermediates, the redox activity of vanadium and titanium species is difficult to assess under operando conditions and in a quantitative manner.…”

Kinetic Coupling of Redox and Acid Chemistry in Methanol Partial Oxidation on Vanadium Oxide Catalysts

“…59 At the 2-D monolayer, polyvanadates resemble the surface of crystalline domains with trigonal bipyramidal environments. These three structures and their coexistence have been previously probed with the following: Raman spectroscopy, where Raman band shifts for the V�O features (1008 to 1031 cm −1 ) and the emergence of broad V−O−V features (600−1000 cm −1 ) indicate the structural transition of supported VO x from predominantly monomeric to polymeric domains; 37,40,60 X-ray adsorption, where measured coordination shells and bond lengths were used to define the structures of VO x domains on ferrierite-type zeolites; 61 and high-resolution transmission electron microscopy, where above the 2-D monolayer VO x loading, the formation of overlayers in conjunction with bulk crystallite clusters was imaged for VO x supported on anatase TiO 2 . 36 To quantify the surface densities ρ s of VO x domains and the lattice oxygen atoms, we first define the footprint of the various VO x domains dispersed on the surface.…”

A Review and Experimental Revisit of Alternative Catalysts for Selective Oxidation of Methanol to Formaldehyde

“…The redox dynamics of V impregnated into 3D Ferrierite and 2D ITQ-6 siliceous supports with 1-10 wt% V were investigated by Vieira et al [176], who found that dispersion and stabilization of V oxides were improved by isolated silanol groups on the surface before impregnation. XRD, XAS, and DR-UV-Vis spectroscopy identified V to be highly dispersed at 1 wt% and 5 wt% V in Si-ITQ-6 with monomeric and polymeric species or oxide clusters while 10 wt% in Si-ITQ-6 and Si-FER showed formation V 2 O 5 clusters on the surface.…”

“…EXAFS and XANES showed complete reduction of V 5+ to V 4+ and back by isothermal periods of non-oxidizing atmosphere followed by regeneration with O 2 . All V species were concluded to be active in the oxidation of MeOH albeit with different rates due to differences in the V 5+ /V 4+ equilibrium under oxidant conditions as reoxidation was limited [176]. In a study by Zhang et al [177] on co-electrospun VtiO x hollow nanofibers, a yield of 90% MF was reached at 150 • C and was attributed to a high surface content of V 4+ [177].…”