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

The metathesis of alkenes represents a paramount archetype of green synthesis with reduced emissions of hazardous wastes to the environment. Nonetheless, the replacement of conventional media for the metathesis reaction - typically hydrocarbons or light chlorinated compounds – with safer and greener solvents, is a largely unexplored area. We report here that in the presence of heterogeneous catalysts such as Re2O7 supported on -Al2O3, not only the self-metathesis of -olefins occurs efficiently in supercritical carbon dioxide (scCO2), but it also takes place faster than in classic media (n-heptane, toluene and n-hexane). The reaction has been investigated under both batch and continuous-flow conditions. Batch experiments showed that at 35 °C, in the presence of Re-oxide/-Al2O3, the self-metathesis of 1-olefins (RCH=CH2, R=C4-C6) proceeded with a conversion over 30% higher on average, in scCO2 (90 bar) than in a conventional solvent such as n-heptane. For instance, after 2 h, the average conversion of 1-octene was 67%, 40% and 36% in scCO2, n-heptane and toluene, respectively. The product of self-methatesis, 7-tetradecene, was isolated in yields up to 68%. Continuous-flow (c.-f.) experiments were carried out in a plug-flow reactor filled with a catalytic bed of Re-oxide supported on -Al2O3. scCO2 and n-hexane were used as solvents for the self-metathesis of 1-octene. Compared to the hydrocarbon medium, the supercritical phase allowed residence times up to five-fold lower to reach similar reaction conversions (40-50%). Alike to batch conditions, c.-f. experiments proved that scCO2 as a solvent could improve both productivity and yield for the alkene metathesis.