Characterisation of rhenium oxide-alumina metathesis catalysts

Interactions Involving Lewis Acidic Aluminum Sites in Oxide-Supported Perrhenate Catalysts

“…37,38 In several EXAFS studies of γ-alumina-supported perrhenate, no such interactions have been reported. 11,39À42 However, in some cases the spectra were obtained for airexposed samples, 11,39 and supported perrhenates are known to undergo rapid hydrolysis by atmospheric moisture. 12 The air-free XANES of perrhenate on γ-alumina at the Re L 1 -edge was reported to be consistent with "AlOReO 3 " structure, although spectra of model compounds were not recorded for comparison.…”

“…The origins of these support-dependent reactivities lie in structural and/or electronic differences between the supported perrhenate sites. Despite much investigation, ,− little is known about the nature of these differences. On the basis of their vibrational spectra, a single structure with local C 3 v symmetry (S-OReO 3 , where S is a support cation) has been proposed to describe the perrhenate sites on γ-alumina, titania, and zirconia. , They are believed to arise by anchoring perrhenate at surface hydroxyl sites during calcination, as shown for silanol-containing supports in eq .  SiOH + NH 4 ReO 4 →  SiOReO 3 + NH 3 false↑ + .25em normalH 2 normalO false↑ In support of monomer structures, no characteristic Re–O–Re vibrations ,, or diffraction lines for extended rhenium oxide phases (e.g., Re 2 O 7 ) have been detected at low Re loadings.…”

“…Lewis acid–base interactions between grafted metal complexes and solid supports can be difficult to detect by vibrational spectroscopy but may be observed using X-ray absorption spectroscopy. Recently, we demonstrated that CH 3 ReO 3 supported on amorphous silica–alumina interacts with both a Lewis acidic Al site and a neighboring Lewis basic O atom of the support. , In several EXAFS studies of γ-alumina-supported perrhenate, no such interactions have been reported. ,− However, in some cases the spectra were obtained for air-exposed samples, , and supported perrhenates are known to undergo rapid hydrolysis by atmospheric moisture . The air-free XANES of perrhenate on γ-alumina at the Re L 1 -edge was reported to be consistent with “AlOReO 3 ” structure, although spectra of model compounds were not recorded for comparison .…”

Metathesis of Alkenes

“…However, certain authors concluded from spectroscopic (IR) and activity studies that dimeric Re − O − Re species are also present at higher loadings [70][71][72][73][74], in spite of the fact that two coupled surface rhenium monomers have a very similar structure to that of Re 2 O 7 , which is volatile above 300 • C. Also, XRD of catalysts with a Re 2 O 7 content of up to 18 wt.% indicated the absence of Re 2 O 7 crystals on the Re 2 O 7 /Al 2 O 3 catalyst surface [56,75]. However, certain authors concluded from spectroscopic (IR) and activity studies that dimeric Re − O − Re species are also present at higher loadings [70][71][72][73][74], in spite of the fact that two coupled surface rhenium monomers have a very similar structure to that of Re 2 O 7 , which is volatile above 300 • C. Also, XRD of catalysts with a Re 2 O 7 content of up to 18 wt.% indicated the absence of Re 2 O 7 crystals on the Re 2 O 7 /Al 2 O 3 catalyst surface [56,75].…”