Solvent-free cycloaddition of CO₂ and propylene oxide to cyclic carbonates using different ligand metal complexes

ABSTRACT

Tetradentate ligand metal complexes salen-Co(III)OBzF₅, salen-CH₃Co(III)OBzF₅, salen-CH₃Co(III)Br, and salen-ClCo(III)Br were synthesized. These complexes were used as catalysts for the chemical fixation of CO₂ and propylene oxide to cyclic carbonates. Tetradecyltrimethylammonium bromide, hexadecyltrimethylammonium bromide, [PPN]⁺Cl⁻ ([PPN]⁺ = bis(triphenylphosphine)iminium), and tetrabutyl ammonium bromide were used as co-catalysts in the reactions. Additionally, the catalytic activity of naphthen-CoSO₄ and salen-CoSO₄ was also explored using the above co-catalysts. The conversion of propylene oxide to cyclic carbonate was found to increase with increase in temperature and pressure of carbon dioxide. It was noted that cyclic carbonate product could be obtained at a temperature as low as 60°C and pressure of 20 bar in just 2 h. Further, it was observed that introduction of electron-donating methyl substituent increased the nucleophilic character of the catalyst and reaction conversion. On the contrary, electron-withdrawing substituents like chlorine decreased the cyclic carbonate conversion, thereby making it a weaker nucleophilic catalyst.

KEYWORDS:

• Carbon dioxide
• propylene oxide
• cyclic carbonate
• catalyst
• co-catalysts
• ligand metal complexes

Funding

This work was supported in part by All India Council of Technical Education, New Delhi (India) under Project No. 8023/RID/RPS/068/11/12.