The metathesis of α-olefins over supported Re-catalysts in supercritical CO₂

Abstract

At 35 • C, in the presence of supercritical carbon dioxide (80-150 bar) as a solvent, a-olefins (RCH=CH 2 , R = C4-C6) undergo highly selective self-metathesis catalyzed by supported Re-oxide (7%). To the best of our knowledge, this is the first procedure for the metathesis of alkenes, in which heterogeneous catalysts are combined with the use of dense CO 2 . The intrinsic eco-compatibility and the unique physicochemical properties of this medium offer both environmental and synthetic advantages: not only conventional toxic solvents (e.g. n-heptane and toluene) can be replaced, but the reaction is faster. For instance, after 2 h, the average conversion of 1-octene is 67% and 40% in scCO 2 and n-heptane, respectively. The product of self-methatesis, 7-tetradecene, can be isolated in yields up to 68%. At 90 bar, the reaction is rather sensitive to the mole fraction of the olefin (in scCO 2 ); though, the enhancement of the pressure (and the density) of the supercritical medium does not induce significant effects on either the rate or the selectivity of the process. The nature of the catalytic support also greatly affects the reaction outcome: Re-oxide shows good activity if dispersed over g-Al 2 O 3 , while silica-based systems are ineffective.

Continuous-flow alkene metathesis: the model reaction of 1-octene catalyzed by Re2O7/γ-Al2O3 with supercritical CO2 as a carrier

“…This product distribution matched our previous findings for the same reaction carried out under batch-CO 2 systems. 7,10 Although preliminary, these results proved that unprecedented reaction conditions obtained by the combination of a CF-scCO 2 reactor with Al 2 O 3 -supported Re 2 O 7 catalysts, were suitable for metathesis processes.…”

“…At 35°C and 90 bar, the maximum CF-conversion for the selfmetathesis of 1-octene (23%) is far lower than that (65-70%) obtained for the same reaction in the static mode. 7,10 Also, a CO 2 pressure dependence is observed for the CF-metathesis, while the corresponding batch process is insensitive to pressure effects. The reaction environment is therefore greatly differentiated when CF-dynamic or static conditions are used.…”

“…Re 2 O 7 /γ-Al 2 O 3 catalysts were prepared by a wet-impregnation technique already reported by us. 7 Accordingly, two catalytic samples were obtained starting from NH 4 ReO 4 (555 mg, 2.1 mmol) and γ-Al 2 O 3 (5 g) in both cases. They were labeled as Re/Al WI1 and Re/Al WI2 , and they were characterized by optical ICP, TEM, and N 2 -physisorption.…”

Self-Metathesis of 1-Octene Using Alumina-Supported Re2O7 in Supercritical CO2

“…(Scheme 3) [10,28,29]. In a previous paper [30] we have observed that alumina is the best support for Re-oxide for the metathesis of 1-octene in scCO 2 . Under these conditions, the most significant aspect was that scCO 2 could not only act as a solvent for the reaction, but it also improved the overall efficiency (Table 2) [30].…”

“…In a previous paper [30] we have observed that alumina is the best support for Re-oxide for the metathesis of 1-octene in scCO 2 . Under these conditions, the most significant aspect was that scCO 2 could not only act as a solvent for the reaction, but it also improved the overall efficiency (Table 2) [30]. At 35°C, the reaction proceeded with an average conversion of over 30% higher in the supercritical medium than in classical liquid solvents such as n-heptane or toluene.…”