Catalytic coupling of CO2 and epoxides by lignin-based catalysts: A combined experimental and theoretical study

A new strategy for the construction of an amine-modified lignin has been developed through the Mannich reaction. The lignin-based catalyst, (PEI(70000)-SL/ZnBr2), acted as a green and reusable heterogeneous catalyst for the catalytic conversion of CO2 and epoxides into cyclic carbonates. This catalyst exhibited excellent conversion and selectivity in the CO2 cycloaddition reaction due to the synergistic effects of the hydroxyls (-OH) and metal (Zn2+) as Lewis acid sites, and organic amines (-NH2) and halogen anions (Br-) as nucleophiles. Some lignin model compounds were used to study the relationship between structure and activity in order to evaluate the synergistic effects during the cycloaddition reaction. Functional groups, such as Zn2+,-OH and-NH2, in adjacent positions of the lignin-based catalyst yielded the cyclic carbonates in high efficiencies and selectivities. Furthermore, DFT calculation and the experimental results revealed that Zn2+ enhanced the ring-opening process via SN1 mechanism, with the energy barrier of the ring-opening process of 18.9 kcal mol(-1) due to the cooperation of the Zn2+ with phenolic hydroxyl groups. These findings do not only provide a novel design concept for biomass-based catalysts but also accomplished the transformation of CO2 into valuable products.