Immobilization of Ionic Liquid on a Covalent Organic Framework for Effectively Catalyzing Cycloaddition of CO2 to Epoxides

Transforming CO₂ into value-added chemicals has been an important subject in recent years. The development of a novel heterogeneous catalyst for highly effective CO₂ conversion still remains a great challenge. As an emerging class of porous organic polymers, covalent organic frameworks (COFs) have exhibited superior potential as catalysts for various chemical reactions, due to their unique structure and properties. In this study, a layered two-dimensional (2D) COF, IM4F-Py-COF, was prepared through a three-component condensation reaction. Benzimidazole moiety, as an ionic liquid precursor, was integrated onto the skeleton of the COF using a benzimidazole-containing building unit. Ionization of the benzimidazole framework was then achieved through quaternization with 1-bromobutane to produce an ionic liquid-immobilized COF, i.e., BMIM4F-Py-COF. The resulting ionic COF shows excellent catalytic activity in promoting the chemical fixation of CO₂ via reaction with epoxides under solvent-free and co-catalyst-free conditions. High porosity, the one-dimensional (1D) open-channel structure of the COF and the high catalytic activity of ionic liquid may contribute to the excellent catalytic performance. Moreover, the COF catalyst could be reused at least five times without significant loss of its catalytic activity.     

Restrictive Regulation of Ionic Liquid Quaternary Ammonium Salt in SBA-15 Pore Channel for Efficient Carbon Dioxide Conversion

Herein, the synthesis of a new type of catalyst, SBA−M (Schiff complex of different metal types grafted on SBA-15) based on a quaternization reaction, is described. Various amounts of ionic liquid were grafted into the pore channels of SBA-15 using the post-grafting method, which allowed the ionic liquid to be grafted into the pore channels restrictively. Notably, over six cycles, SBA−Mn (0.2) has been shown to maintain its catalytic activity and stability. In addition, a reaction mechanism for the cycloaddition of CO₂ with epoxides based on density-functional theory is proposed. The cycloaddition reaction of CO₂ and epoxides is an efficient way of carbon fixation. It is demonstrated that the metal coordinated with the oxygen atom of the epoxides and that a halogen attacked the carbon of epoxides. Moreover, theoretical calculations and synthesis strategy provide a new approach for CO₂ conversion.     

Bifunctional Catalysts Containing Zn(II) and Imidazolium Salt Ionic Liquids for Chemical Fixation of Carbon Dioxide

Zn(II) can efficiently promote the catalytic performance of imidazolium salt ionic liquids (imi-ILs) for the chemical fixation of CO₂ into epoxides. To obtain sustainability, immobilized bifunctional catalysts containing both imi-ILs and Zn(II) were prepared using bimodal mesoporous silica (BMMs) as carrier, through grafting of Zn(OAc)₂ and 1-(trimethoxysilyl)propyl-3-methylimidazolium chloride (Si-imi) separately in the nanopores. The catalysts, named as BMMs−Zn&ILs, were identified as efficient catalysts for cycloaddition reaction of CO₂ into epoxides under solvent-free conditions. BMMs−Zn&ILs showed good catalytic activity, which increased with the increase of the molar ratio of Zn(II) to Si-imi. As a comparison, different catalytic systems including homogeneous imi-IL, BMMs-ILs and BMMs−Zn were studied to demonstrate different cooperation behaviors. Furthermore, the kinetics studies of homogeneous and heterogeneous bifunctional catalysts were employed to confirm the differences, as well as to support the proposed cooperative catalysis mechanism in the nanopores.     

Polymerised ionic liquid crystals bearing imidazolium and bipyridinium groups

Novel kinds of polymerised ionic liquid crystals (PILCs) bearing imidazolium and bipyridinium groups were synthesised and characterised in this work. Some bromo-polyesters were synthesised in an esterification chain reaction using 2,3-dibromosuccinyl dichloride, isophthaloyl chloride and 4,4′-biphenol. The polyester imidazolium bromides (PIBs) and polyester dipyridinium bromides (PDBs) were obtained by a quaternisation reaction between the bromo-polyesters, N-methylimidazole and 4-4′-bipyridine, respectively. The polyester imidazolium tetrafluoroborates (PITs) and polyester dipyridinium tetrafluoroborates (PDTs) were synthesised using the corresponding PIBs and PDBs in an anion-exchange reaction. The chemical structures, liquid crystalline properties and molecular weights of these polymers were characterised by multiple experimental techniques. All the PILCs including PIBs, PDBs, PITs and PDTs display smectic A phase on heating and cooling cycles. The liquid-crystalline properties of bromo-polyesters are influenced by the length of flexible spacer and composition of polyester backbones, while those of PILCs are determined by the ionic groups as additional important influent factors. In comparison with those of the mother bromo-polyesters, the d-spacing of PILCs reduced slightly due to Im⁺–Br^?, Dp⁺–Br^?, Im⁺–BF₄^? and Dp⁺–BF₄^? ion pairs in the polymer systems. Monocationic imidazolium salts display weaker interionic and intermolecular interaction, higher mobility and lower viscosity than dicationic dipyridinium salts.     

Encapsulation of an Ionic Metalloporphyrin into a Zeolite Imidazolate Framework in situ for CO2 Chemical Transformation via Host–Guest Synergistic Catalysis

It is a great challenge to rationally integrate multiple reactive sites into a composite material with confined nanospace, which can be applied as a nanoreactor to facilitate targeting catalytic reaction. In this work, an ionic metalloporphyrin has been encapsulated in situ into ZIF‐8 for a solvent‐free synthesis of cyclic carbonates from CO₂ and epoxides without any co‐catalyst under 1 atm CO₂.     

Activation of Hydrogen Peroxide by Ionic Liquids: Mechanistic Studies and Application in the Epoxidation of Olefins

Imidazolium‐based ionic liquids that contain perrhenate anions are very efficient reaction media for the epoxidation of olefins with H₂O₂ as an oxidant, thus affording cyclooctene in almost quantitative yields. The mechanism of this reaction does not follow the usual pathway through peroxo complexes, as is the case with long‐known molecular transition‐metal catalysts. By using in situ Raman, FTIR, and NMR spectroscopy and DFT calculations, we have shown that the formation of hydrogen bonds between the oxidant and perrhenate activates the oxidant, thereby leading to the transfer of an oxygen atom onto the olefin demonstrating the special features of an ionic liquid as a reaction environment. The influence of the imidazolium cation and the oxidant (aqueous H₂O₂, urea hydrogen peroxide, and tert‐butyl hydrogen peroxide) on the efficiency of the epoxidation of cis‐cyclooctene were examined. Other olefinic substrates were also used in this study and they exhibited good yields of the corresponding epoxides. This report shows the potential of using simple complexes or salts for the activation of hydrogen peroxide, owing to the interactions between the solvent medium and the active complex.     

Efficient DMF‐Catalyzed Coupling of Epoxides with CO2 under Solvent‐Free Conditions to Afford Cyclic Carbonates

To develop a simple, low‐molecule, and cost‐effective organocatalyst for the coupling of epoxides with CO₂, we have screened this coupling reaction in different organic solvents and found that DMF is an efficient organic catalyst for the coupling of epoxides with CO₂ to give cyclic carbonates in high yield. In some cases, the catalytic activity of DMF can be significantly increased by the addition of catalytic amount of H₂O.     

Acetic acid functionalized ionic liquid systems: An efficient and recyclable catalyst for the regioselective ring opening of epoxides with NaN3

In the present study, an environmentally benign, efficient, and solvent-free procedure was developed for the synthesis of 1,2-azidoalcohols by the regioselective ring opening of some epoxides with sodium azide (NaN₃) in the presence of an acetic acid functionalized imidazolium salt [Cmmim]BF₄ or [Cmmim]Br as a green and Brønsted acidic ionic liquid (BAIL) catalyst under mild and neutral reaction conditions at 60 °C. The remarkable features of this procedure are excellent regioselectivity, simple work-up procedure, high yields of products, short reaction times, and ease of recyclability of ionic liquids.     

N‐Heterocyclic Olefin–Carbon Dioxide and –Sulfur Dioxide Adducts: Structures and Interesting Reactivity Patterns

Depending on the amount of methanol present in solution, CO₂ adducts of N‐heterocyclic carbenes (NHCs) and N‐heterocyclic olefins (NHOs) have been found to be in fully reversible equilibrium with the corresponding methyl carbonate salts [EMIm][OCO₂Me] and [EMMIm][OCO₂Me]. The reactivity pattern of representative 1‐ethyl‐3‐methyl‐NHO–CO₂ adduct 4 has been investigated and compared with the corresponding NHC–CO₂ zwitterion: The protonation of 4 with HX led to the imidazolium salts [NHO–CO₂H][X], which underwent decarboxylation to [EMMIm][X] in the presence of nucleophilic catalysts. NHO–CO₂ zwitterion 4 can act as an efficient carboxylating agent towards CH acids such as acetonitrile. The [EMMIm] cyanoacetate and [EMMIm]₂ cyanomalonate salts formed exemplify the first C?C bond‐forming carboxylation reactions with NHO‐activated CO₂. The reaction of the free NHO with dimethyl carbonate selectively led to methoxycarbonylated NHO, which is a perfect precursor for the synthesis of functionalized ILs [NHO–CO₂Me][X]. The first NHO‐SO₂ adduct was synthesized and structurally characterized; it showed a similar reactivity pattern, which allowed the synthesis of imidazolium methyl sulfites upon reaction with methanol.     

Bifunctional Onium and Potassium Iodides as Nucleophilic Catalysts for the Solvent-Free Syntheses of Carbonates,Thiocarbonates, and Oxazolidinones from Epoxides

The catalytic potential of organo-onium iodides as nucleophilic catalysts is aptly demonstrated in the synthesis of cyclic carbonates from epoxides and carbon dioxide (CO₂), as a representative CO₂ utilization reaction. Although organo-onium iodide nucleophilic catalysts are metal-free environmentally benign catalysts, harsh reaction conditions are generally required to efficiently promote the coupling reactions of epoxides and CO₂. To solve this problem and accomplish efficient CO₂ utilization reactions under mild conditions, bifunctional onium iodide nucleophilic catalysts bearing a hydrogen bond donor moiety were developed by our research group. Based on the successful bifunctional design of the onium iodide catalysts, nucleophilic catalysis using a potassium iodide (KI)-tetraethylene glycol complex was also investigated in coupling reactions of epoxides and CO₂ under mild reaction conditions. These effective bifunctional onium and potassium iodide nucleophilic catalysts were applied to the solvent-free syntheses of 2-oxazolidinones and cyclic thiocarbonates from epoxides.     

聚合物负载季鏻盐催化 CO2 与环氧化物的环加成反应

 制备了聚合物负载的季鏻盐催化剂, 并用于 CO₂ 与环氧氯丙烷环加成反应中. 采用红外光谱、热分析、原子吸收光谱和扫描电子显微镜等手段测定了催化剂的结构、热性能、磷元素含量和表面形貌等. 考察了 CO₂ 压力、温度、催化剂用量和反应时间等对环加成反应性能的影响. 结果表明, 在催化剂用量为 0.09 g, CO₂ 压力为 4.5 MPa, 于 150 ^oC 反应 6 h 时, 3-氯甲基环碳酸酯收率可达 97.7%, 选择性大于 99%. 且催化剂易分离回收, 重复使用 5 次后产物收率和选择性没有明显下降. 同时探讨了该催化剂上 CO₂ 环加成合成环碳酸酯的可能机理.     

介孔聚离子液体的可控合成及在常压CO2环加成反应中应用

采用刚性的离子液体聚合单体双-（3-乙烯基-1-咪唑）亚甲基双溴盐（[C₁DVIM]Br）,以聚乙二醇（PEG）为溶剂,能够简单快捷制备出高比表面积的介孔聚离子液体.通过调节PEG的分子量大小,即可有效控制所得聚离子液体的孔结构.介孔聚离子液体由于具有典型的聚阳离子骨架、较高的比表面积以及丰富的卤素位,作为非金属多相催化剂在常压下氧化苯乙烯为底物的CO₂环加成反应中表现出优异的催化活性和良好的回收稳定性,循环使用5次后催化性能基本保持不变.此外,该催化材料还表现出良好的底物兼容性,可以有效转化很难反应的脂肪类环氧化合物.     

Polystyrene-bound diethanolamine based ionic liquids for chemical fixation of CO2

Polystyrene-bound diethanolamine based ionic liquids (PS-DHEEAB and PS-THEAB) were synthesized and applied for the chemical fixation of CO₂ into cyclic carbonates without any additional co-catalyst and solvent. The effect of the catalysts with different number of hydroxyl group in the cation of the IL on the reaction was systematically investigated. Highest activity and selectivity were achieved in the presence of polystyrene supported diethanolamine ethyl bromide (PS-DHEEAB) in comparison with other catalysts employed. The catalyst was tough in stability and also found to be extended to a variety of terminal epoxides and aziridines. The relationship between high catalytic reactivity and the –OH functional groups was proposed.     

An imidazolium ionic liquid having covalently attached an oxime carbapalladacycle complex as ionophilic heterogeneous catalysts for the Heck and Suzuki-Miyaura cross-coupling

An oxime carbapalladacycle, analogous to that used as catalyst in homogeneous phase, has been derivatized to increase its ionophilicity by introducing an imidazolium group covalently attached through a chain at the complex. The resulting complex is soluble in 1-butyl-3-methylimidazolium ionic liquid (bmimPF₆) and not extractable by ether. The catalytic activity of this palladium complex in bmimPF₆ is, however, unsatisfactory and only increases marginally in bmimPF₆/supercritical CO₂. This limitation has been overcome by supporting this imidazolium palladium complex on high surface area Al/MCM-41 aluminosilicate, whereby a solid active catalyst for the Suzuki cross-coupling has been obtained. Reusability and stability over reuse for this Al/MCM-41-supported catalyst have been studied.     

The Use of Sustainable Transition Metals for the Cycloaddition of Epoxides and CO2 under Mild Reaction Conditions

Following green chemistry principles, we present a one pot synthesis of zinc curcumin complex as a homogeneous, sustainable catalyst for cyclic carbonate (CC) synthesis from CO₂ and a variety of terminal and internal epoxides under 1 bar CO₂ at 60 °C using DMSO as a solvent. Zinc curcumin ( Zn-Cur ) complex was identified via ¹H/¹³C NMR and ATR-FTIR spectroscopies, elemental analysis, ionic conductivity, and quantitative silver nitrate test to confirm a 1 : 1 molar ratio of metal to the ligand. Interestingly, Zn-Cur showed an outstanding performance in catalysing the synthesis of renewable CCs including dimeric pinene and limonene carbonates. In this context, other metal complexes, including magnesium (Mg^II), calcium (Ca^II), and nickel (Ni^II) complexes showed good to excellent catalytic activity. Furthermore, a plausible reaction mechanism was investigated by density functional theory (DFT) calculations. The reaction was found to be overall exergonic by 39 kJ/mol and the rate determining step was CO₂ insertion, which is in good agreement with the experimental findings.     

Design and synthesis of fused-ring chiral ionic liquids from amino acid derivatives

Six novel imidazolium salts, which contain a chiral moiety as well as a fused-ring system, have been designed, synthesized, and fully characterized. The synthesis of these ionic liquids is concise and practical due to the commercial availability of the starting materials. These imidazolium compounds were readily prepared from 1-methyl-2-imidazoliumcarboxaldehyde and chiral amino alcohols. Salts that contain the PF₆ anion were solids, but salts with the NTf₂ anion were liquids at room temperature. We envision that these new chiral imidazolium compounds can serve as effective reaction media as well as chiral catalysts for asymmetric reactions, which are presently being investigated in our lab.     

Synthesis of Cyclic Carbonate Catalyzed by DBU Derived Basic Ionic Liquids

In this work, 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU), 1,5‐diazabicyclo[4.3.0]‐5‐nonene (DBN), and imidazole (MIM)‐derived bromide ionic liquids (ILs) were synthesized and used to catalyze the cycloaddition reactions of carbon dioxide (CO₂) with several kinds of epoxides to form cyclic carbonates. The DBU derived bromide ionic liquid system was found to have the best catalytic activity among all the tested ILs. The influences of reaction conditions (including temperature, pressure and reaction time) on the reaction of CO₂ to propylene oxide (PO) were studied to show the best conditions of 120 °C, 1 MPa, 2.5 h catalyzed by 2 mol% DBU‐derived bromide ionic liquid, with the conversion of PO and the selectivity of propylene carbonate (PC) reaching 99% and 99%, respectively. Under the optimum reaction conditions, the ionic liquid system could be reused at least five times without decrease in selectivity and conversion. NMR spectroscopy and DFT calculations were used to reveal the hydrogen‐bond interaction between ionic liquids and reagent, based on which the reaction mechanism was proposed.     

Click Ionic Liquids: A Family of Promising Tunable Solvents and Application in Suzuki–Miyaura Cross‐Coupling

A series of click ionic salts 4 a – 4 n was prepared through click reaction of organic azides with alkyne‐functionalized imidazolium or 2‐methylimidazolium salts, followed by metathesis with lithium bis(trifluoromethanesulfonyl)amide or potassium hexafluorophosphate. All salts were characterized by IR, NMR, TGA, and DSC, and most of them can be classified as ionic liquids. Their steric and electronic properties can be easily tuned and modified through variation of the aromatic or aliphatic substituents at the imidazolium and/or triazolyl rings. The effect of anions and substituents at the two rings on the physicochemical properties was investigated. The charge and orbital distributions based on the optimized structures of cations in the salts were calculated. Reaction of 4 a with PdCl₂ produced mononuclear click complex 4 a‐Pd , the structure of which was confirmed by single‐crystal X‐ray diffraction analysis. Suzuki–Miyaura cross‐coupling shows good catalytic stability and high recyclability in the presence of PdCl₂ in 4 a . TEM and XPS analyses show formation of palladium nanoparticles after the reaction. The palladium NPs in 4 a are immobilized by the synergetic effect of coordination and electrostatic interactions with 1,2,3‐triazolyl and imidazolium, respectively.     

Aza‐Crown Ether Complex Cation Ionic Liquids: Preparation and Applications in Organic Reactions

Aza‐crown ether complex cation ionic liquids (aCECILs) were devised, fabricated, and characterized by using NMR spectroscopy, MS, thermogravimetric differential thermal analysis (TG‐DTA), elemental analysis and physical properties. These new and room‐temperature ILs were utilized as catalysts in various organic reactions, such as the cycloaddition reaction of CO₂ to epoxides, esterification of acetic acid and alcohols, the condensation reaction of aniline and propylene carbonate, and Friedel–Crafts alkylation of indole with aldehydes were investigated carefully. In these reactions, the ionic liquid exhibited cooperative catalytic activity between the anion and cation. In addition, the aza‐[18‐C‐6HK][HSO₄]₂ was the best acidic catalyst in the reactions of esterification and Friedel–Crafts alkylation under mild reaction conditions.     

Improving CO2 selectivity in polymerized room-temperature ionic liquid gas separation membranes through incorporation of polar substituents

Solid, polymer membranes fabricated from room-temperature ionic liquid monomers containing oligo(ethylene glycol) or nitrile-terminated alkyl substituents tethered to imidazolium cations were found to exhibit ideal CO₂/N₂ and CO₂/CH₄ separation factors significantly greater than those with comparable length n-alkyl substituents, with similar CO₂ permeability. Polymers containing these functional groups exhibited CO₂/N₂ gas separation performance exceeding the “upper bound” of a “Robeson Plot”.