Nickel–cobalt hydroxide catalysts for the cycloaddition of carbon dioxide to epoxides

Nickel and cobalt bimetallic hydroxides, in which both metal cations present the same oxidation state, were synthesized and characterized by XRD, TGA, TEM, XPS, and ICP-OES. The catalytic activity in the cycloaddition of carbon dioxide with epoxides was evaluated in a binary system, using tetrabutylammonium bromide as cocatalyst. All materials were active catalysts for the production of cyclic carbonates in the initial conditions, providing conversions and selectivities up to 75% and 93%, respectively. The influence of the parameters time, temperature, catalyst amount, pressure, and cocatalyst type was examined. Using the catalyst with the molar ratio 50:50 of Ni and Co (NiCo-50), a series of epoxides was converted to the respective carbonates under the optimal conditions, providing excellent conversions and selectivity. Alkylic propylene oxide and butylene oxide presented 84% and 83% of conversion, respectively. For epoxides containing electron-withdrawing substituents, the conversions were quantitative (> 99%) and highly selective to the cyclic carbonate. Reuse experiments were conducted, and showed that NiCo-50 can be recovered and reused at least five times, without decline in conversion and selectivity.