Identification and tuning of active sites in selected mixed metal oxide catalysts for cyclic carbonate synthesis from epoxides and CO₂

The reaction of CO₂ with epoxides to produce cyclic carbonate is one of the important transformations of CO₂ which has commercial applications. In this study, several mixed metal oxides were explored as potential catalysts for CO₂ cycloaddition reaction with epoxides. The catalytic activities of these catalysts were correlated with their physicochemical properties. It is found that the catalytic activity in terms of cyclic carbonate yield has a good correlation with the number of weak and medium acid-base sites irrespective of type of metal oxides used indicating that a combination of acidity and basicity is an important factor for this reaction. Among, different mixed metal oxides, Mn-Ba and Sn-Ni mixed metal oxides were found to be effective solid catalysts for the synthesis of cyclic carbonate from epoxide and CO₂. These mixed metal oxides showed higher activity compared to individual oxides and physical mixture of respective metal oxides. Different compositions of mixed metal oxides and the effect of calcination temperature of the catalyst on cycloaddition of epoxide and CO₂ were studied. Mn-Ba oxide and Sn-Ni oxide catalysts with metal ratios of 4.3:1 and 1.5:1 respectively, showed better activity compared to other compositions. The effect of various reaction parameters was studied to know the influence on the conversion and yield. Under optimized reaction conditions, Mn-Ba and Sn-Ni mixed oxides gave 96.0 and 90.2% yield respectively for propylene carbonate. Thus, mixed oxides were shown to be highly efficient catalysts with good recyclability for propylene oxide and CO₂ reaction and also can be applied to some other epoxides effectively to make cyclic carbonates.