Greenhouse Gas Conversion by Homogeneous Salen Catalyst Systems under Very Mild Reaction Condition

CONCLUSION

Homogeneous salen catalyt systems were investigated for the conversion of greenhouse gas into cyclic carbonate under room temperature and atmospheric pressure. Salen complexes were more active than salophen complexes. Neither electron withdrawing nor donating group showed lower conversion efficiency. Co-catalysts such as moisture or ammonium salt increased the conversion efficiency dramatically. These results have potential to decrease carbon dioxide emitted from fossil fuel combustions under mild condition.

 

EXPERIMENTAL

Propylene oxide was heated at reflux over a mixture of KOH and CaH2 and fractionally distilled under a nitrogen atmosphere. CO2 was purified by passing through a column packed with 4 molecular sieve before use.

Modified salen ligands were synthesized by a single-step Mannich condensation between the phenylene diamine, ethylene diamine, formaldehyde and the substituted phenol.8,9 The synthesized salen compounds are shown in Scheme 1 the structures of synthetic compounds are confirmed with the spectroscopic data.8,9

The Schiff base aluminum complexes were prepared by the reaction of the corresponding free bases with Et2AlCl according to the literature methods.8,9 These complexes are all sensitive to air or moisture and should be stored in a nitrogen atmosphere. The coupling reaction with epoxide was carried out in a 50 ml test tube with CO2 bubbling at 50 ml/min (Scheme 2).

In a typical procedure, to a test tube (50 ml) were added salen catalyst (5 mol%) and n-Bu4NY (5 mol%) under dry nitrogen. Propylene oxide (50 ml) and water (5 mol%) were introduced and CO2 gas was bubbled for 24 h at 50 ml/min. The remaining mixture was degassed and fractionally distilled under reduced pressure to afford pure cyclic carbonates.