Characterization of Silica-Supported Vanadium(V) Complexes Derived from Molecular Precursors and Their Ligand Exchange Reactions

Rice, Gordon; Scott, Susannah L.

doi:10.1021/la960679d

Cited by 73 publications

(93 citation statements)

References 31 publications

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“…Spectral subtraction of the support spectrum revealed the band also to be weakly present at lower vanadium oxide loadings, giving support for the literature assignment to a V-O-Si vibration [55,56,81,82]. A relative decrease of the IR band intensity upon re-hydration would have supplied additional evidence for the correctness of this assignment, but unfortunately, the band was obscured by the enhanced intensity of the interfering support Si-O stretching band around 1000 cm À1 .…”

supporting

confidence: 64%

“…Furthermore, the broadness of the 915 cm À1 Raman band was attributed to thermal movement of the bound oxygen molecule. Most papers in the literature, however, suggest a Raman band in this area to be a V-O-support vibration [55,56,67,81,82,[85][86][87][88][89]. Our data of the dehydrated catalysts show that the Raman band at 915 cm À1 is present at all vanadium oxide loadings, and since the EXAFS results exclude di-or polymeric species, this eliminates the spectral assignment to a V-O-V vibration.…”

mentioning

confidence: 48%

“…of pure dehydrated silica and is generally assigned to a Si-O(H) stretching vibration [37,38,42,81,82]. Obviously, the band intensity not only decreases with increasing vanadium oxide loading and upon dehydration, but also (partly) re-appears upon hydroxylation.…”

Section: Discussionmentioning

confidence: 99%

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Hydration effects on the molecular structure of silica-supported vanadium oxide catalysts: A combined IR, Raman, UV–vis and EXAFS study

Keller

Visser

Soulimani

et al. 2007

Vibrational Spectroscopy

125

130

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The effect of hydration on the molecular structure of silica-supported vanadium oxide catalysts with loadings of 1-16 wt.% V has been systematically investigated by infrared, Raman, UV-vis and EXAFS spectroscopy. IR and Raman spectra recorded during hydration revealed the formation of V-OH groups, characterized by a band at 3660 cm À1. Hydroxylation was found to start instantaneously upon exposure to traces of water, reflecting a very high sensitivity of the supported vanadium oxide catalysts for H 2 O. Further hydration resulted in the appearance of a V-O-V vibration band located around 700 cm À1 pointing to the formation of di-or polymeric species. EXAFS analysis at 77 K indicated structural changes as the oxygen coordination changed from four to five. Moreover, a VÁ Á ÁV contribution was detected for the hydrated species. The IR, Raman and UV-vis data suggested a pyramidal anchoring of the dehydrated VO x species, whereas, the EXAFS data pointed to the presence of single V-O-Si bonded VO x species. This difference is attributed to water condensation effects at 77 K during EXAFS acquisition, resulting in a partial re-hydroxylation of the dehydrated samples, as confirmed by complementary IR and Raman analysis. as well as literature led to a reaction scheme in which a monomeric VO x species anchored by three Si-O-V bonds to the silica support (pyramidal-type structure) is transformed into a monomeric VO x species anchored by one Si-O-V bond (umbrella-type structure) by partial hydration of the catalyst material. This results in the formation of both V-O-H and Si-O-H bonds. At higher water pressures, larger vanadium oxide clusters are formed due to full hydration of the catalyst surface and a de-attachment of the vanadium oxide from the support surface. The results of this study provide evidence, that an umbrella-type structure (i.e., Si-O-V O(OH) 2 ) could be present under catalytic conditions where H 2 O is a reaction product (e.g., partial oxidation of methanol to formaldehyde and oxidative dehydrogenation of alkanes). In other words, both the pyramidal ((Si-O) 3 -V O) and the umbrella (Si-O-V O(OH) 2 ) model can exist at a support surface, their relative ratio depending on the hydration degree of the catalyst material. This study also illustrates that a corroborative characterization requires the use of multiple spectroscopic techniques applied at the same samples under almost identical measuring conditions. #

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supporting

confidence: 64%

mentioning

confidence: 48%

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Hydration effects on the molecular structure of silica-supported vanadium oxide catalysts: A combined IR, Raman, UV–vis and EXAFS study

Keller

Visser

Soulimani

et al. 2007

Vibrational Spectroscopy

125

130

View full text Add to dashboard Cite

show abstract

“…Degussa Aerosil™-200 (surface area 183 m 2 /g) was rehydrated then partially dehydroxylated in vacua at 200°C, after which treatment the hydroxyl content is reproducible at 2.6 OH/nm 2 (14). All subsequent manipulations were performed in situ in the absence of inert gases or solvents, using standard breakseal and high vacuum techniques.…”

Section: Experimental Preparation Of Silica-supported Titanium Catalystmentioning

confidence: 99%

“…Using techniques previously described to quantify gaseous products of surface reactions (13,14), we measured the quantity of epoxide formed by reaction of the tert-butylperoxotitanium complex 2 with an excess of cyclohexene in the absence of terf-butylhydroperoxide (i.e., stoichiometric gas phase reaction conditions). Complex 2, with four tert-butylperoxo ligands per TiOTi unit, generated (4.11 ± 0.08) equiv.…”

Section: Epoxidation Stoichiometrymentioning

confidence: 99%