Encapsulation of Ru Complexes into Poly(ionic liquid) Enables the Methanation of CO₂under Mild Conditions

In this work, the vapor-phase methanation of CO2 at a low temperature was achieved under the catalysis of [Ru]@P(ILn-POSS)-Cl-A (n = 2, 3, 4) hybrids, wherein the molecular low-valent Ru complexes were encapsulated into the imidazolium chloride-decorated porous poly(ionic liquid) skeleton. Mechanistic studies suggest that it follows the cascade hydrogenation of CO2 without the formation of CO intermediates and the presence of a chloride ligand at the Ru center is crucial for [Ru]@P(ILn-POSS)-Cl-A to reach a high activity. Besides, the textural structure of [Ru]@P(ILn-POSS)-Cl-A can be adjusted by the numbers of IL units (n) in the monomer, thereby affecting the activity and catalyst durability. As such, the activity is positively correlated with the specific surface area rather than the adsorption capacity of CO2, and thus, [Ru]@P(IL2-POSS)-Cl-A provided the best TONCH4 of 81 at 160 °C. On the other hand, the high density of the IL unit in [Ru]@P(IL4-POSS)-Cl-A is conductive to prevent the aggregation of the active Ru species, enabling satisfactory catalyst durability. As a proof of concept, the catalytic activity could be further improved by the strategy of a molecular fence, wherein the molecular Ru sites in [Ru-NMP]@P(IL2-POSS)-Cl-A were highly dispersed and stabilized with N-methylpyrrolidone.