336
J. Handzlik
Fig. 19 Example models of SP molybdacyclobutane species differently located on the silica surface. Adapted with permission from [119]. Copyright (2016) American Chemical Society
CrO x /SiO 2 and MoO x /SiO 2 , the catalytic activity of WO x /SiO 2 is often attributed to
well-dispersed surface metal oxide species [84, 85, 121–123, 126].
The structure of the surface W(VI) species in oxidized WO x /SiO 2 systems under
dehydrated conditions was determined mainly from in situ Raman, UV-vis, XANES
and EXAFS studies [32, 33, 84, 100, 121]. It was postulated that isolated fourcoordinate dioxo W(VI) species and five-coordinate monooxo W(VI) species coexist
on the silica surface and their ratio is a function of temperature. On the other hand,
surface oligomeric W(VI) species were sometimes proposed [127, 128]. At higher
tungsten loadings, crystalline WO 3 nanoparticles are also present [32, 84, 121, 127,
128].
Following the previous computational works on silica-supported chromium(VI)
[7] and molybdenum(VI) [8] oxide species, Guesmi et al. [9] performed DFT (PBE)
studies of isolated tungsten(VI) oxide species on hydroxylated silica surface represented by the slab model of Tielens et al. [13]. Thermodynamic stability of mono-, di,
tri- and tetragrafted monomeric tungsten(VI) species at different degrees of hydration was determined for a wide range of temperatures (Fig. 20). It was predicted
that hydrated species with W–OH groups might be present only at very low temperatures. In more typical conditions, five-coordinate monooxo W(VI) species was
found to be most stable; however, this tetragrafted species is statistically disfavoured
due to the low probability to have four silanols, required for grafting, in one nest.
For this reason, the digrafted dioxo W(VI) species is expected to dominate over the
more thermodynamically preferred monooxo species. The presence of both dioxo
and monooxo species was additionally confirmed by the computed W=O stretching frequencies, being close to the corresponding experimental Raman data. Finally,
a comparison between the tungsten oxide species and other group VI metal oxide
species on silica confirmed a greater similarity with the Mo species than with the Cr
species.
In recent experimental works [85, 121–123], activation of the WO x /SiO 2 catalyst
for alkene metathesis was extensively investigated. It was postulated that the isolated
dioxo W(VI) species are the active site precursors. Similar to the MoO x /SiO 2 system,
J. Handzlik
Fig. 19 Example models of SP molybdacyclobutane species differently located on the silica surface. Adapted with permission from [119]. Copyright (2016) American Chemical Society
CrO x /SiO 2 and MoO x /SiO 2 , the catalytic activity of WO x /SiO 2 is often attributed to
well-dispersed surface metal oxide species [84, 85, 121–123, 126].
The structure of the surface W(VI) species in oxidized WO x /SiO 2 systems under
dehydrated conditions was determined mainly from in situ Raman, UV-vis, XANES
and EXAFS studies [32, 33, 84, 100, 121]. It was postulated that isolated fourcoordinate dioxo W(VI) species and five-coordinate monooxo W(VI) species coexist
on the silica surface and their ratio is a function of temperature. On the other hand,
surface oligomeric W(VI) species were sometimes proposed [127, 128]. At higher
tungsten loadings, crystalline WO 3 nanoparticles are also present [32, 84, 121, 127,
128].
Following the previous computational works on silica-supported chromium(VI)
[7] and molybdenum(VI) [8] oxide species, Guesmi et al. [9] performed DFT (PBE)
studies of isolated tungsten(VI) oxide species on hydroxylated silica surface represented by the slab model of Tielens et al. [13]. Thermodynamic stability of mono-, di,
tri- and tetragrafted monomeric tungsten(VI) species at different degrees of hydration was determined for a wide range of temperatures (Fig. 20). It was predicted
that hydrated species with W–OH groups might be present only at very low temperatures. In more typical conditions, five-coordinate monooxo W(VI) species was
found to be most stable; however, this tetragrafted species is statistically disfavoured
due to the low probability to have four silanols, required for grafting, in one nest.
For this reason, the digrafted dioxo W(VI) species is expected to dominate over the
more thermodynamically preferred monooxo species. The presence of both dioxo
and monooxo species was additionally confirmed by the computed W=O stretching frequencies, being close to the corresponding experimental Raman data. Finally,
a comparison between the tungsten oxide species and other group VI metal oxide
species on silica confirmed a greater similarity with the Mo species than with the Cr
species.
In recent experimental works [85, 121–123], activation of the WO x /SiO 2 catalyst
for alkene metathesis was extensively investigated. It was postulated that the isolated
dioxo W(VI) species are the active site precursors. Similar to the MoO x /SiO 2 system,