the hydration process is reversible: we could observe both the
ReOx-b and ReOx-a forms by dehydrating the material again.
Precedent to such species exist in the literature, as for instance
in the case of [Re2O7(H2O)2] that features two rhenium-
coordinated water molecules, as shown by X-ray diffraction
studies.18 Most interestingly, as in the 10ReSi case, further
hydration of this compound leads to formation of aqueous
perrhenic acid. This hypothesis is not contradictory with the
XAS data, in so far as a weak interaction with water is not
expected to modify the XANES nor the EXAFS spectra,
especially when the structure remains disordered.17 Other
structures implying silanols or specific sites of silica gel (cyclic
structures, as an example) may also be considered.
Fig. 2 (a) Experimental XANES spectrum of 10ReSi, O2, 350 1C and
(b) experimental XANES spectrum of solvated ReO4À in water (aqueous
perrhenic acid); in inset: Fourier transform of the EXAFS signal.
XANES and EXAFS spectra were recorded at the Re LI edge and Re
LIII edge, respectively, on SAMBA Beamline, SOLEIL in Qu-EXAFS
configuration.w16
In conclusion, our new ReOx/SiO2 catalyst prepared using a
unique sol–gel wet route exhibits a markedly lower redox
potential that finely tunes the redox/acidic properties towards
highly efficient and selective conversion of methanol into
DMM. Most interestingly, these features have been correlated
to the observation of an unprecedented structure in which the
ReO4 tetrahedron is in weak interaction with water. Our
current investigations are targeted at the determination of
the actual structure of this new form of supported rhenium
oxide, an intermediÀate between fully dehydrated RSi–OReO3
and aqueous ReO4 anion.
at 350 1C under O2 flow, shown in Fig. 2a, is essentially similar
to that of perrhenic acid in aqueous solution (Fig. 2b).
The presence of a pre-edge peak of high intensity indicates
that the rhenium atom stands as ReVII in tetrahedral coordi-
nation. This is confirmed by the analysis of the first shell of the
EXAFS spectrum as it can be fitted with 4 oxygen neighbours.
No contribution from neighbouring Re is visible in the Fourier
transform of the EXAFS spectrum (in inset, Fig. 2), indicating
that ReOx moieties are essentially isolated.17 As a consequence,
the new ReOx-b structure evidenced by Raman spectroscopy is
not consistent with Re oligomers.
The work was supported by the ANR SAXO project (Grant
number ANR-07-BLAN-0265-01). We thank the Ministry of
Higher Education and Research (PhD grant for AY) and the
CNRS for financial support.
The Raman frequencies and the wet pathway chemistry
used for the preparation make a peculiar interaction of the
oxorhenate phase with water possible. An in situ Raman
investigation of the gradual hydration of 10ReSi at 175 1C
was thus run to clarify this point (Fig. 3).
Notes and references
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c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 4285–4287 4287