Role of Tricoordinate Al Sites in CH₃ReO₃/Al₂O₃ Olefin Metathesis Catalysts

Abstract

Re2O7 supported on γ-alumina is an alkene metathesis catalyst active at room temperature, compatible with functional groups, but the exact structures of the active sites are unknown. Using CH3ReO3/Al2O3 as a model for Re2O7/Al2O3, we show through a combination of reactivity studies, in situ solid-state NMR, and an extensive series of DFT calculations, that μ-methylene structures (Al-CH2-ReO3-Al) containing a Re═O bound to a tricoordinated Al (AlIII) and CH2 bound to a four-coordinated Al (AlIVb) are the precursors of the most active sites for olefin metathesis. The resting state of CH3ReO3/Al2O3 is a distribution of μ-methylene species formed by the activation of the C-H bond of CH3ReO3 on different surface Al-O sites. In situ reaction with ethylene results in the formation of Re metallacycle intermediates, which were studied in detail through a combination of solid-state NMR experiments, using labeled ethylene, and DFT calculations. In particular, we were able to distinguish between metallacycles in TBP (trigonal-bipyramidal) and SP (square-pyramidal) geometry, the latter being inactive and detrimental to catalytic activity. The SP sites are more likely to be formed on other Al sites (AlIVa/AlIVa). Experimentally, the activity of CH3ReO3/Al2O3 depends on the activation temperature of alumina; catalysts activated at or above 500 °C contain more active sites than those activated at 300 °C. We show that the dependence of catalytic activity on the Al2O3 activation temperature is related to the quantity of available AlIII-defect sites and adsorbed H2O.

Bridging the Gap between Industrial and Well‐Defined Supported Catalysts

“…[306] Although the m-methylene species exchanges with ethylene and propylene (Scheme 36 c), [499] thus indicating that this site is an active site precursor,the alkylidene has not yet been observed by solid-state NMR spectroscopy.H owever, the in situ solid-state NMR spectrum of the catalyst measured under an ethylene pressure provides aspectral signature that can be assigned to am etallacyclobutane in at rigonal bipyramidal (TBP) geometry as the most abundant reaction intermediate.Asquare-pyramidal (SP) metallacycle is also observed, but not the alkylidene species (Scheme 36 c). [502] MeReO 3 /Al 2 O 3 shows catalytic properties similar to Re 2 O 7 /Al 2 O 3 activated with SnMe 4 .B oth catalysts are active in the self-metathesis of both ethylene and ethyl oleate,while Re 2 O 7 /Al 2 O 3 is not active towards these olefins in the absence of activators. [444,448,449,504] Solid-state NMR studies on SnMe 4 / Re 2 O 7 /Al 2 O 3 and MeReO 3 /Al 2 O 3 obtained using various labeling strategies indicate that m-methylene sites are formed in both cases.…”

“…Detailed descriptions of the alumina supports [305,306] and MeReO 3 /Al 2 O 3 [501,502] showed that the Al sites coordinated to MeReO 3 are essential for the formation of the alkylidene and metallacyclobutane intermediates.T he most active m-methylene Re sites contain aR e =Ob ound to as trongly Lewis acidic tricoordinated Al site (Al III ), and the m-methylene fragment bound to af our-coordinated Al site (Al IV ). The reverse combination of Al sites (m-methylene fragment bound to Al III and Re=Ob ound to Al IV )f orms inactive Re sites because of the very strong coordination of the m-methylene to the Al III site (Scheme 36 b).…”

“…Reviews not surprising that the activity of Re 2 O 7 /Al 2 O 3 and MeReO 3 / Al 2 O 3 depends on the pre-activation temperature;the highest activity is found for an activation temperature of about 700 8 8C, [502,503] at which the highest density of Al III sites is generated. [306] Although the m-methylene species exchanges with ethylene and propylene (Scheme 36 c), [499] thus indicating that this site is an active site precursor,the alkylidene has not yet been observed by solid-state NMR spectroscopy.H owever, the in situ solid-state NMR spectrum of the catalyst measured under an ethylene pressure provides aspectral signature that can be assigned to am etallacyclobutane in at rigonal bipyramidal (TBP) geometry as the most abundant reaction intermediate.Asquare-pyramidal (SP) metallacycle is also observed, but not the alkylidene species (Scheme 36 c).…”

Material descriptors for photocatalyst/catalyst design

“…However, for many complicated industrial catalytic reactions, both the structures of the active sites and the reaction mechanisms are still unknown. Worse still, real world systems are usually determined by interconnected reaction network instead of only one single rate‐determining step in the reaction (Theopold, ; Valla et al, ). In such cases, one cannot define a unique chemical potential or descriptor for all the possible reaction pathways. Due to inherent deficiencies in DFT, methodological improvements are required to provide more accurate descriptions of many systems, such as strongly correlated ones (Cohen, Mori‐Sánchez, & Yang, ). Finally, proposed descriptors, as well as the tuning methods used, must be experimentally verified.…”

Eine Brücke zwischen industriellen und wohldefinierten Trägerkatalysatoren

“…Ausführliche Beschreibungen der Aluminiumoxidträger und von MeReO 3 /Al 2 O 3 zeigten, dass die an MeReO 3 koordinierten Al‐Zentren für die Entstehung von Alkyliden‐ und Metallacyclobutan‐Zwischenprodukten notwendig sind. Die aktivsten μ‐Methylen‐Re‐Zentren enthalten ein an ein stark Lewis‐saures, dreifach koordiniertes Al‐Zentrum (Al III ) gebundenes Re=O und das an ein vierfach koordiniertes Al (Al IV ) gebundene μ‐Methylen‐Fragment.…”