Ligand Exchange-Mediated Activation and Stabilization of a Re-Based Olefin Metathesis Catalyst by Chlorinated Alumina

Abstract

Extensive chlorination of γ-AlO results in the formation of highly Lewis acidic surface domains depleted in surface hydroxyl groups. Adsorption of methyltrioxorhenium (MTO) onto these chlorinated domains serves to activate it as a low temperature, heterogeneous olefin metathesis catalyst and confers both high activity and high stability. Characterization of the catalyst reveals that the immobilized MTO undergoes partial ligand exchange with the surface, whereby some Re sites acquire a chloride ligand from the modified alumina while donating an oxo ligand to the support. More specifically, Re L-edge EXAFS and DFT calculations support facile ligand exchange between MTO and Cl-AlO to generate [CHReOCl] fragments that interact with a bridging oxygen of the support via a Lewis acid-base interaction. According to IR and solid-state NMR, the methyl group remains intact, and does not evolve spontaneously to a stable methylene tautomer. Nevertheless, the chloride-promoted metathesis catalyst is far more active and productive than MTO/γ-AlO, easily achieving a TON of 100 000 for propene metathesis in a flow reactor at 10 °C (compared to TON < 5000 for the nonchlorinated catalyst). Increased activity is a consequence of both a larger fraction of active sites and a higher intrinsic activity for the new sites. Increased stability is tentatively attributed to a stronger interaction between MTO and chlorinated surface regions, as well as extensive depletion of the Brønsted acidic surface hydroxyl population. The reformulated catalyst represents a major advance for Re-based metathesis catalysts, whose widespread use has thus far been severely hampered by their instability.

Novel tungsten-incorporated mesoporous silicates synthesized via evaporation-induced self-assembly: Enhanced metathesis performance

“…In this context, on-purpose propylene technologies such as olefin metathesis and methanol-to-olefins technologies have attracted considerable attention [2,3]. For the heterogeneous ethylene + butene metathesis reaction, the most frequently reported catalysts contain W, Mo or Re as the active species [4][5][6][7][8][9][10][11][12][13][14][15].…”

Bridging the Gap between Industrial and Well‐Defined Supported Catalysts

“…MeReO 3 is only active in metathesis when in contact with molecular or solid Lewis acids . The catalytic activity of supported MeReO 3 is related to the presence of Lewis acidic sites in the support . The structures of the surface sites of MeReO 3 /Al 2 O 3 were investigated in detail by using a combination of IR and solid‐state NMR spectroscopy, EXAFS, and DFT calculations using a periodic model of γ‐alumina.…”

Eine Brücke zwischen industriellen und wohldefinierten Trägerkatalysatoren

“…MeReO 3 ist nur nach Kontakt mit molekularen oder festen Lewis‐Säuren bei der Metathese aktiv . Die katalytische Aktivität von trägerfixiertem MeReO 3 steht mit dem Vorhandensein von Lewis‐sauren Zentren in dem Träger in Verbindung . Die Struktur der Oberflächenzentren von MeReO 3 /Al 2 O 3 wurde mit einer Kombination aus IR‐ und Festkörper‐NMR‐Spektroskopie, EXAFS und DFT‐Berechnungen mit einem periodischen Modell von γ‐Aluminiumoxid ausführlich untersucht.…”