Surface ReO_x Sites on Al₂O₃ and Their Molecular Structure–Reactivity Relationships for Olefin Metathesis

Abstract

Supported ReO x /Al 2 O 3 catalysts were investigated for propylene metathesis as a function of surface rhenia loading and extensively characterized with in situ UV−vis, Raman, IR, XANES/EXAFS, and isotopic 18 O− 16 O exchange studies. The experimental studies were complemented with DFT calculations using realistic models of the alumina surface. The surface ReO x sites were found to be isolated surface dioxo (O) 2 ReO 2 species, which represent the most stable surface rhenia structures on alumina as shown by DFT. Two distinct surface ReO 4 species, however, were found to be present and only slightly differ in their bridging Re−O−Al bond lengths brought about by anchoring at different sites of the Al 2 O 3 support. The deformed surface ReO 4 −I species preferentially anchor at more basic μ 1 Al IV and μ 1 Al VI sites and are difficult to activate for propylene metathesis. The surface ReO 4 −II species are formed at more acidic μ 2 Al VI and μ 3 Al VI sites and are the catalytic active sites for propylene metathesis. The surface ReO 4 −II sites were readily activated by propylene while the deformed surface ReO 4 −I sites were almost not affected by propylene, with only a few sites being activated. The steady-state propylene metathesis reaction rates are much higher for the surface ReO 4 −II sites than the deformed surface ReO 4 −I sites. The formation of the less reactive deformed surface ReO 4 −I species could be blocked by occupation of the μ 1 Al IV sites with sacrificial surface TaO x species that resulted in catalysts exclusively containing the more active surface ReO 4 −II sites on alumina. This is the f irst study to demonstrate that the surface ReO 4 −II sites are the precursors for the catalytic active sites for propylene metathesis by supported ReO 4 /Al 2 O 3 catalysts and to molecularly design olefin metathesis catalysts that exclusively contain isolated surface ReO 4 −II sites.

Direct Conversion of Glycerol to Allyl Alcohol Over Alumina‐Supported Rhenium Oxide

“…The Raman spectrum (Figure ) of the fresh catalyst exhibited four bands at 989, 971, 939, and 334 cm −1 corresponding to the symmetric stretching vibration of distorted solvated ReO 4 − /Al 2 O 3 ν s (Re=O), the symmetric stretching of bare ReO 4 − ν s (Re=O), the asymmetric stretching ν as (Re=O), and the δ s (O−Re‐O) bending vibration, respectively –. These results, together with the absence of signal in the 400 to 800 cm ‐1 range (stretching vibration of Re 2 O 7 ), suggest that isolated ReO x species were formed on alumina (Figure S4) ,,…”

Bridging the Gap between Industrial and Well‐Defined Supported Catalysts

“…However, the complexity of industrial catalysts prevents as ystematic analysis of these data. [472] In addition, it is well-known that Re 2 O 7 /Al 2 O 3 is active towards functionalized olefins such as oleates upon pre-activation with SnMe 4 . [444] Although the role of the Lewis acid sites is not known, the surface species present in Re 2 O 7 /Al 2 O 3 corresponds to isolated perrhenate species coordinated to adjacent Al sites.…”

Eine Brücke zwischen industriellen und wohldefinierten Trägerkatalysatoren

“…B. Al 2 O 3 und SiO 2 ‐Al 2 O 3 . Während die Rolle der Lewis‐Säure‐Zentren nicht bekannt ist, entsprechen die in Re 2 O 7 /Al 2 O 3 vorliegenden Oberflächenspezies isolierten Perrhenatspezies, die an benachbarte Al‐Zentren koordiniert sind . Experimentelle und rechnerische Studien weisen darauf hin, dass die aktivsten Zentren vermutlich Re VII ‐Oxo‐Spezies entsprechen, die an Lewis‐saure dreifach und vierfach koordinierte Zentren (Al III ‐ bzw.…”

“…Experimentelle und rechnerische Studien weisen darauf hin, dass die aktivsten Zentren vermutlich Re VII ‐Oxo‐Spezies entsprechen, die an Lewis‐saure dreifach und vierfach koordinierte Zentren (Al III ‐ bzw. Al IV ) gebunden sind, während andere Kombinationen von Al‐Zentren inaktiv sind (die römische Zahl im Index entspricht der Koordinationszahl der Al 3+ ‐Oberflächenzentren im Ausgangsmaterial) . Ferner ist bekannt, dass Re 2 O 7 /Al 2 O 3 nach Voraktivierung mit SnMe 4 gegenüber funktionalisierten Olefinen wie Oleaten aktiv ist.…”

“…An zwei Al IV ‐Zentren gebundenes MeReO 3 liefert sehr wenig aktive oder inaktive Metallzentren. Wie für Re 2 O 7 /Al 2 O 3 gefunden, spielen Al III ‐Zentren eine entscheidende Rolle bei der hohen Aktivität von Metathesekatalysatoren auf Re‐Basis mit Aluminiumoxidträgern. Damit ist es nicht überraschend, dass die Aktivität von Re 2 O 7 /Al 2 O 3 und MeReO 3 /Al 2 O 3 von der Voraktivierungstemperatur abhängt; die höchste Aktivität wird für eine Aktivierungstemperatur von etwa 700 °C gefunden, bei der die höchste Dichte von Al III ‐Zentren erzeugt wird …”