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

The self-metathesis of -olefins has been accomplished according to a procedure never previously reported, based on the combined use of heterogeneous catalysts (supported Re-oxides) and of supercritical carbon dioxide as the solvent. The reaction outcome is affected by the nature of the catalytic support (-Al2O3 and silica), and by the properties of compressed CO2 which offers advantages in terms of replacement of conventional toxic solvents (e.g. n-heptane and toluene), of favourable reaction rates, and of easier recovery of final products. Keywords: metathesis, Re-supported catalysts, supercritical CO2. 1. Introduction The metathesis of olefins is one of the best and more general carbon-carbon bond forming methodologies. The reaction has also been acknowledged as the archetype Green Chemistry process, for clean productions of pharmaceuticals and polymer with reduced environmental emissions of hazardous wastes. In this field however, notwithstanding the enormous efforts for the development of new catalysts able to extend the scope of the metathetic process, the replacement of conventional solvents, typically hexane, toluene, and dichloromethane, 3, with alternative safer and greener media, have been poorly, if any, investigated. In particular, to the best of our knowledge, the combined use of heterogeneous catalysts and CO2 solvent has never been claimed for the metathesis of olefins. We report here that in presence of Re2O7 supported on -Al2O3, not only the self-metathesis of -olefins occurs efficiently in supercritical carbon dioxide (scCO2), but interestingly, the reaction is more rapid than in n-heptane and toluene. 2. Experimental The catalysts were prepared through conventional impregnation methods of aqueous solutions of NH4ReO4 over both -Al2O3 and silica, followed by calcination. The metathesis of 1-alkenes was performed in dense CO2 at pressure of 80-150 bars. Since previous methodologies were not available, a new Schlenk apparatus was arranged ad hoc, to allow multiple operations under an inert (N2) atmosphere: i) the activation of the catalyst at a high-temperature (550 °C); ii) the charge of the catalyst and of the reactant olefins (RCH=CH2, R = C4-C6) in a stainless-steel autoclave; iii) the reaction step under CO2 pressure. The method was also compared to conventional procedures carried out at atmospheric pressure, under liquid-phase conditions, using both n-heptane and toluene as solvents. 3. Results and discussion Table 1 reports the results for the metathesis of 1-octene carried out at 35 °C, in presence of Re2O7 supported on both -Al2O3 and silica, and with different solvents. In all cases, regardless of the conditions used, the formation of the product of self-metathesis (7-tetradecene, 1a) is accompanied by different co-products identified as isomers of 1-octene (iso: 2-, 3-, and 4-octene) and linear olefins C9-C13. Data indicate that: i) Only the catalyst prepared on γ-Al2O3 is active for the transformation (entries 1-2, and 4). The different surface acidity between alumina and silica mostly account for this result. ii) In scCO2, the average reaction conversion is over 30% higher than in n-heptane and toluene. Plausible reasons are based on the gas-like diffusivity and viscosity and the liquid-like density of scCO2, which may improve the mass transfer (with respect to conventional liquids). iii) In all cases, the self-metathesis selectivity was of 95-98%. Conclusions Overall, the combined use of heterogeneous and scCO2 appears a viable alternative for the replacement of conventional liquid-phase methods ordinarily used in the metathesis of olefins.