The mechanism for the controlled synthesis of highly dispersed vanadia supported on silica SBA-15

Abstract

The mechanism for the controlled synthesis of highly dispersed vanadia supported on mesoporous silica SBA-15 (VO x /SBA-15) is elucidated using X-ray photoelectron spectroscopy (XPS), Raman and diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS). The synthesis consists of functionalization of SBA-15 using 3-aminopropyltrimethoxysilane (APTMS) followed by HCl treatment, anion exchange of decavanadate (V 10 O 28 6-) and calcination to yield the final material. Spectroscopic analysis yields that APTMS is grafted onto the surface via hydroxyl groups. Under ambient conditions, protonated APTMS on SBA-15 is largely hydrolyzed and present in its bidentate and tridentate form. Detailed XPS analysis reveals a strong influence of surface water on the resulting structure of APTMS on SBA-15. Under dry conditions, the protonated APTMS molecules on SBA-15 adopt the monodentate and bidentate form. During the anion exchange reaction, decavanadate is incorporated intactly into the pores, without any changes in the structure of the organic framework. In the calcination step, decavanadate is decomposed and vanadia is anchored onto the surface of SBA-15 via silanol groups resulting in highly dispersed surface vanadia species, while the organic resiues are fully removed from the pores of the silica matrix. d

The influence of water on the dispersion of vanadia supported on silica SBA-15

“…The temporal changes of the binding energies of the V 2p3/2 components can be well described by a first order exponential decay and were used to determine their values at t = 0. Charging of the powder samples was accounted for by setting the peak of the Si 2p signal to 103.6 eV [19]. For the hydrated (air-exposed) sample the C 1s signal from adventitious carbon was located at 284.7 eV in excellent agreement with the common reference value of 284.7±0.5 eV [25].…”

“…Therefore, we have prepared vanadium oxide model catalysts via controlled grafting/anion exchange, which consist of highly dispersed vanadium oxide supported on the inner pores of the well-ordered mesoporous matrix of silica SBA-15 [18][19][20]. Only recently, we have demonstrated the excellent catalytic properties of this catalyst system in the propane partial oxidation to acrylic acid as well as methanol partial oxidation to formaldehyde [21,22].…”

Structure of molybdenum oxide supported on silica SBA-15 studied by Raman, UV–Vis and X-ray absorption spectroscopy

“…SBA-15 was synthesized as described in the literature [37,38] and functionalized with 3-aminopropyltrimethoxysilane (APTMS) according to a procedure developed by Hess et al [34,39]. For the subsequent ion exchange 2.0 g of functionalized SBA-15 was suspended in 60 ml distilled water.…”

“…As the molybdenum oxide precursor is held electrostatically within the channels of the mesoporous matrix in a prearranged geometry, this method allows one to precisely control the amount and dispersion of the metal oxide introduced into SBA-15 within a broad range of densities. Recently, by such an approach new insight into the structure of silica SBA-15 supported vanadium oxide catalysts has been obtained [32][33][34][35][36].…”

Nature of dispersed vanadium oxide: influence of the silica support structure and synthesis methods

“…A detailed description of the setup as well as the experimental procedures has been given elsewhere. 31 Briefly, the binding energy scale of the XPS system was calibrated using Au4f 7/2 = 84.0 eV and Cu2p 3/2 = 932.67 eV from foil samples. To account for charging of the powder samples the peak of the Si2p signal was set to 103.6 eV.…”