Solventless synthesis of cyclic carbonates by direct utilization of CO2 using nanocrystalline lithium promoted magnesia

Abstract

Cyclic carbonates are industrially important chemicals. In this work, an efficient synthesis of cyclic carbonate was achieved by cyclization of epoxide with CO2 using nanocrystalline lithium promoted magnesia (Li-MgO), without using any co-catalyst or solvent. A series of Li-MgO were prepared by gel combustion method and well characterized. Li-MgO forms active F-centers (crystallographic defect) due to the difference in valence state of lithium (Li+) and magnesium (Mg2+) and acts as an active site for CO2 activation. In the synthesis of 4-(chloromethyl)-1,3-dioxolan-2-one from epichlorohydrin, 0.75% (w/w) Li-MgO was the most active catalyst for CO2 fixation into cyclic carbonate with excellent conversion (∼98%) and selectivity (100%), at 130 °C and 3 MPa of CO2 pressure. The catalyst showed structural stability and was reused for three cycles without loss of activity. The current synthesis protocol is 100% atom-efficient and thus was extended to a variety of substrates. Langmuir- Hinshelwood-Hougen-Watson (LHHW) type of mechanism was proposed and kinetics studied. Both reactants are strongly adsorbed making the overall reaction zero order with an apparent activation energy of 15.14 kcal/mol.