Efficient Acid–Base Bifunctional Catalysts for the Fixation of CO₂ with Epoxides under Metal‐ and Solvent‐Free Conditions

Abstract

A series of acid-base bifunctional catalysts (ABBCs) that contain one or two Brønsted acidic sites in the cationic part and a Lewis-basic site in the anionic part are used as efficient catalysts for the synthesis of cyclic carbonates by cycloaddition of CO(2) to epoxides, without the use of additional co-catalyst or co-solvent. The effects of the catalyst structures and various reaction parameters on the catalytic performance are investigated in detail. Almost complete conversion can be achieved in 1 h for propylene oxide using [{(CH(2))(3)COOH}(2) im]Br under mild reaction conditions (398 K and 2 MPa). Furthermore, the catalyst can be recycled over five times without substantial loss of catalytic activity. This protocol is found to be applicable to a variety of terminal epoxides, producing the corresponding cyclic carbonates in good yields and high selectivities. A synergistic effect of the acidic and the basic sites as well as suitable hydrogen-bonding strength of ABBCs are considered crucial for the reaction to proceed smoothly. The activities of the ABBCs increase remarkably with increasing carboxylic-acid chain length of the cation. This metal- and solvent-free process thus represents an environmentally friendly process for BTC-catalyzed conversion of CO(2) into value-added chemicals.

Hydrogen Bonds: A Structural Insight into Ionic Liquids

“…In our group acid-base bifunctional ILs were developed for the synthesis of cyclic carbonates. [132][133][134][135] The enhancement of catalytic performance is presumably attributable to a synergistic effect of the two functional groups to the epoxide ring, that is, abduction polarization of the oxygen atom by hydrogen bonds of the Brønsted acidic center and nucleophilic activation of the carbon atom by the Lewis basic center (halide anion). Therefore, the epoxy ring was opened easily.…”

Ionic Liquids: The Synergistic Catalytic Effect in the Synthesis of Cyclic Carbonates

“…The carboxylic acid group, which is a Brønsted acid and a hydrogen bonding donor, could accelerate the ring opening of epoxides and showed a co-operative effect with halide anions. The acid-base bifunctional imidazolium(im) or methylimidazolium(mim) ILs with a carboxylic acid group were synthesized (Scheme 2) [71]. They were used to catalyze the reaction of propylene oxide and CO 2 ( Entries 13-14).…”

Insight into the catalytic activity for a series of synthesized and newly designed phosphonium-based ionic liquids on the fixation of carbon dioxide

“…The combination of different cations and anions can achieve the IL materials with the specific task in diverse areas. Moreover, introducing the functional group will further refine their properties and improve ability, which has been testified in abovementioned section and other literatures [27][28][29]. Is there any other functional group with better catalytic activity except for the ones investigated in experiment?…”

“…Sun et al [27] introduced hydroxyl group into traditional imidazolium-based ILs to form the hydroxyl-functionalized ILs, which have presented satisfied catalytic activity with product conversion as high as 99 % at 125 °C and 2.0 MPa. Later, Sun et al [28] brought one or two carboxyl group(s) into imidazolium-based IL achieving even higher catalytic activity. As compared with well-studied imidazolium-based IL, little attention has been paid on the phosphonium-based ILs.…”