Figure 3 - uploaded by Anne Davidson
Content may be subject to copyright.
29 Si MAS NMR and 1 H– 29 Si CP MAS NMR spectra of V 0.45 Siβ: (a, a ) as prepared, (b, b ) after calcination at 573 K.
Source publication
A dealuminated zeolite β has been contacted with solutions of ammonium metavanadate to incorporate vanadium as tetrahedral
V species which are not removed under treatment with aqueous solution of ammonium acetate. The effects of calcination and
rehydration on the environment of the V species have been studied by diffuse reflectance UV-visible and 5...
Context in source publication
Context 1
... relative intensity of the 29 Si MAS NMR peak at −102 ppm to that at −110 ppm (Q 4 sites, signal of Si(OSi) 4 ) indicates that about 25% of the framework Si correspond to Q 3 sites, wearing an hydroxyl group. A significant amount of these OH groups are con- sumed upon vanadium introduction ( figure 3(a)). ...
Similar publications
24 wt% of iron has been successfully grafted as amorphous iron oxide over mesoporous channels of SBA‐15 and 10 wt% alumina coated SBA‐15 (AlSBA‐15) using selective extraction deposition (SED) technique. The synthesized materials have been characterized using XRD, N2 adsorption studies, SEM‐EDS, ²⁷Al MAS NMR, XPS, H2‐TPR, and HR‐TEM techniques. ²⁷Al...
Citations
... The bands at the wavenumber of 3335 cm − 1 and 1646 cm − 1 and 3622 cm − 1 and 1629 cm − 1 , respectively for the synthetic and natural zeolite, were responsible for the stretching and bending vibrations of the hydroxyl group of water molecules. Similar results for natural zeolites were obtained by Dzwigaj et al. (2000), in their research, FTIR signals at the levels of 3620 and 3650 cm − 1 . The next band at the wavenumber of 966 cm − 1 (synthetic zeolite) and 1005 cm − 1 was characteristic of the asymmetric Si-O-Al stretching vibrations. ...
... The type of vanadium species in the VAPO catalysts were further studied by UV-vis spectroscopy (see Figure 3). In general, the absorbance spectra of V5APO and V7.5APO samples present a band at 230 nm that can be assigned to tetrahedral isolated V species, attributed to oxygentetrahedral V 5+ charge transfer from vanadyls (V 5+ =O) [29,35], and also to V 4+ charge transfer band of VO 2+ species [36]. The V7.5APO and V10APO samples present a new band located at 365 nm that is The type of vanadium species in the VAPO catalysts were further studied by UV-vis spectroscopy (see Figure 3). ...
... The V7.5APO and V10APO samples present a new band located at 365 nm that is The type of vanadium species in the VAPO catalysts were further studied by UV-vis spectroscopy (see Figure 3). In general, the absorbance spectra of V5APO and V7.5APO samples present a band at 230 nm that can be assigned to tetrahedral isolated V species, attributed to oxygen-tetrahedral V 5+ charge transfer from vanadyls (V 5+ =O) [29,35], and also to V 4+ charge transfer band of VO 2+ species [36]. The V7.5APO and V10APO samples present a new band located at 365 nm that is attributed to tetrahedral vanadium species in a polymeric state [37]. ...
The catalytic activity of a series of vanadium aluminophosphates catalysts prepared by sol-gel method followed by combustion of the obtained gel was evaluated in glycerol conversion towards methanol. The materials were characterized by several techniques such as X-ray diffraction (XRD), UV-vis, Fourier-transform infrared (FTIR), Raman and X-ray photoelectron (XPS) spectroscopies. The amount of vanadium incorporated in aluminophosphates framework played an important role in the catalytic activity, while in the products distribution the key role is played by the vanadium oxidation state on the surface. The sample that contains a large amount of V4+ has the highest selectivity towards methanol. On the sample with the lowest vanadium loading the oxidation path to dihydroxyacetone is predominant. The catalyst with higher content of tetrahedral isolated vanadium species, such V5APO, is less active in breaking the C–C bonds in the glycerol molecule than the one containing polymeric species.
... The type of vanadium species in the VAPO catalysts were further studied by UV-vis spectroscopy (see Figure 3). In general, the absorbance spectra of V5APO and V7.5APO samples present a band at 230 nm that can be assigned to tetrahedral isolated V species, attributed to oxygentetrahedral V 5+ charge transfer from vanadyls (V 5+ =O) [29,35], and also to V 4+ charge transfer band of VO 2+ species [36]. The V7.5APO and V10APO samples present a new band located at 365 nm that is The type of vanadium species in the VAPO catalysts were further studied by UV-vis spectroscopy (see Figure 3). ...
... The V7.5APO and V10APO samples present a new band located at 365 nm that is The type of vanadium species in the VAPO catalysts were further studied by UV-vis spectroscopy (see Figure 3). In general, the absorbance spectra of V5APO and V7.5APO samples present a band at 230 nm that can be assigned to tetrahedral isolated V species, attributed to oxygen-tetrahedral V 5+ charge transfer from vanadyls (V 5+ =O) [29,35], and also to V 4+ charge transfer band of VO 2+ species [36]. The V7.5APO and V10APO samples present a new band located at 365 nm that is attributed to tetrahedral vanadium species in a polymeric state [37]. ...
... A broad band at 3509 cm -1 shows the presence of H-bonded hydroxyls that are stable even at 673 K. It is believed that these groups form hydroxyl nests created during zeolite dealumination [39]. The respective silanols terminating the nests are monitored by the shoulder at 3710 cm -1 . ...
... The lower panels represent the OD modes and the X-axis scale is shifted by the theoretical isotopic shift factor. Thus, if the experimental shift followed strictly the theory of the harmonic oscillator, the OH and OD bands should be exactly one below the other.The spectrum of [Si]BEA contains one sharp band at 3738 cm -1 associated with internal Si-OH species[1][2][3]39] (Fig. 1A, spectrum a). A shoulder around 3750 cm -1 is ...
In this review, some of the advantages of the use of isotopically labelled molecules for FTIR characterisation of porous materials are summarised. The most important sites determining the properties of these materials are the hydroxyl groups and the incorporated cations. D H exchange is very useful for characterisation of hydroxyls and here some new possibilities of the technique are shown. The OH OD isotopic shift factor, i, of isolated hydroxyls is about 0.737-0.738, a value higher than the theoretical one (0.728). These experimental deviations are mainly due to anharmonicity. The anharmonicity slightly decreases when the OH groups participate in weak H-bonding with adsorbed molecules, which should lead to a decrease of i. However, contrary to the expectations, i additionally increases. This is attributed to the lower acidity of the OD groups as compared to the respective OH groups. It is shown that i of hydroxyls that are H-bonded to framework oxygen is also higher as compared to isolated ones. The use of D H exchange also allows reassignment of some phenomena, as occurrence of Fermi resonance. Another important characteristic of mesoporous materials is the coordination state of guest cations. The number of coordinative vacancies is well determined by the utilisation of CO isotopic mixtures. Here the principles of this technique are considered and the advantages of CO-13 C 18 O isotopic mixtures as compared to CO-13 CO are demonstrated. Finally, other applications of labelled molecules for determination of the structures of species formed on porous materials are briefly reviewed.
... A broad band at 3509 cm -1 shows the presence of H-bonded hydroxyls that are stable even at 673 K. It is believed that these groups form hydroxyl nests created during zeolite dealumination [39]. The respective silanols terminating the nests are monitored by the shoulder at 3710 cm -1 . ...
... The lower panels represent the OD modes and the X-axis scale is shifted by the theoretical isotopic shift factor. Thus, if the experimental shift followed strictly the theory of the harmonic oscillator, the OH and OD bands should be exactly one below the other.The spectrum of [Si]BEA contains one sharp band at 3738 cm -1 associated with internal Si-OH species[1][2][3]39] (Fig. 1A, spectrum a). A shoulder around 3750 cm -1 is ...
The speciation of vanadium in initial aqueous NH4VO3 solutions as a function of pH and concentration, in supernatant and in wet solids were investigated by ⁵¹V static and MAS NMR. Two series of VSiBeta zeolite catalysts were prepared by a two-step postsynthesis procedure at pH = 2.5 and 6. ⁵¹V static and MAS NMR, ⁵¹V 3Q MAS NMR, DR UV–vis, XPS and EPR allowed determining the state of vanadium in both series of catalysts. The catalytic activity of VSiBeta catalysts in SCR of NO strongly depended on the state of the vanadium present. The V-single site V1.0SiBeta(I) and V1.4SiBeta(I) catalysts with isolated pseudo-tetrahedral V(V) were active in SCR of NO with NH3 process, with maximum NO conversion about 75 % at 773 K for V1.4SiBeta(I). In contrast, V1.0SiBeta(II) and V7.5SiBeta(I) catalysts containing pseudo-octahedral V(V) were much less active in SCR of NO and high amount of undesired N2O was produced.
Two series of V-containing Beta zeolites have been prepared by contacting of the aqueous NH 4 VO 3 solution with two SiBeta zeolites at pH = 2.5 and 6, respectively. Because of the presence in NH 4 VO 3 solution at pH = 2.5 mainly monomeric VO 2⁺ ions, vanadium have been easily incorporated into framework of SiBeta zeolite as pseudo-tetrahedral non hydroxylated (SiO) 3 V = O and hydroxylated (SiO) 2 (OH)V = O species. The moderate nucleophilicity of the basic vanadyl oxygen of the latter species play important role in methanol oxidation toward formaldehyde. The selectivity toward formaldehyde on this series of V x SiBeta(I) increases with the amount of pseudo-tetrahedral hydroxylated (SiO) 2 (HO)V = O species, which act as either redox or acidic and basic centres. In contrast at pH = 6, the aqueous NH 4 VO 3 solution is expected to contain both mononuclear and polynuclear V ions, thus it is more difficult to incorporate V species in SiBeta at this condition as shown by FT-IR and NMR data. The absence of pseudo-tetrahedral V species in framework position of V 0.6 SiBeta(II) is probably responsible for lack of activity of this catalyst in methanol oxidation. The appearance of this species in the series of V x SiBeta(II) zeolites for high V content leads to their activity in methanol oxidation toward formaldehyde.
Vanadium (V) containing all silica Beta zeolites (VSi-Beta) were directly synthesized in a dry-gel-conversion route, in which the gel was prepared from acidic cohydrolysis of silica precursor and ammonium metavanadate. Besides the framework isolated V species, VSi-Beta contained tetrahedral [V2O7]4- and (VO3)nn- species that were bonded to the framework with a high dispersion. These V species resisted the over-oxidation of fructose and efficiently catalyzed the oxidation of 5-hydroxymethylfurfural (HMF) into 2,5-diformylfuran (DFF). As a result, VSi-Beta demonstrated high activity in the one-pot and one-step conversion of fructose into DFF by using atmospheric oxygen (O2) in the presence of sulfuric acid, affording a high yield of 86.3% and a maximum turnover number (TON) of 270. The catalyst can be facilely recovered by filtration and reused with well reusability. Various other carbohydrates (glucose, sucrose, inulin, raffinose, maltose and starch) were also effectively converted into DFF by using VSi-Beta.
Surface hydroxyl groups are active centers in many catalytic reactions and can play an important role during catalyst preparation. In this chapter, the application of infrared spectroscopy for identification and characterization of surface OH groups is reviewed. The potential of other techniques is also briefly described. The vibrational signature of various types of hydroxyls is discussed; however, the amount of information that can be gathered from the hydroxyl spectra itself is limited. In contrast, application of probe molecules allows a profound characterization of hydroxyl species. Two scenarios are considered: the formation of H-bonds between hydroxyl groups and probe molecules, and chemical reactions between hydroxyl groups and molecules or ions (such as protonation of basic probe molecules, exchange reactions, redox processes). Means to explore the accessibility and location of hydroxyl groups are introduced. The properties of OD and OH groups are compared, and the application of H/D exchange as a diagnostic reaction is discussed. Finally, the hydroxyl population on materials of practical interest is analyzed.
