Catalytic Strategies for the Cycloaddition of Pure, Diluted, and Waste CO2 to Epoxides under Ambient Conditions

Polymeric frustrated Lewis pairs in CO2/cyclic ether coupling catalysis

Frustrated Lewis pairs (FLPs) are now ubiquitous as metal-free catalysts in an array of different chemical transformations. In this paper we show that this reactivity can be transferred to a polymeric system, offering advantageous opportunities at the interface between catalysis and stimuli-responsive materials. Formation of cyclic carbonates from cyclic ethers using CO2 as a C1 feedstock continues to be dominated by metal-based systems. When paired with a suitable nucleophile, discrete aryl or alkyl boranes have shown significant promise as metal-free Lewis acidic alternatives, although catalyst reuse remains illusive. Herein, we leverage the reactivity of FLPs in a polymeric system to promote CO2/cyclic ether coupling catalysis that can be tuned for the desired epoxide or oxetane substrate. Moreover, these macromolecular FLPs can be reused across multiple reaction cycles, further increasing their appeal over analogous small molecule systems.

Utilization of CO2-Available Organocatalysts for Reactions with Industrially Important Epoxides

Recent knowledge in chemistry has enabled the material utilization of greenhouse gas (CO2) for the production of organic carbonates using mild reaction conditions. Organic carbonates, especially cyclic carbonates, are applicable as green solvents, electrolytes in batteries, feedstock for fine chemicals and monomers for polycarbonate production. This review summarizes new developments in the ring opening of epoxides with subsequent CO2-based formation of cyclic carbonates. The review highlights recent and major developments for sustainable CO2 conversion from 2000 to the end of 2021 abstracted by Web of Science. The syntheses of epoxides, especially from bio-based raw materials, will be summarized, such as the types of raw material (vegetable oils or their esters) and the reaction conditions. The aim of this review is also to summarize and to compare the types of homogeneous non-metallic catalysts. The three reaction mechanisms for cyclic carbonate formation are presented, namely activation of the epoxide ring, CO2 activation and dual activation. Usually most effective catalysts described in the literature consist of powerful sources of nucleophile such as onium salt, of hydrogen bond donors and of tertiary amines used to combine epoxide activation for facile epoxide ring opening and CO2 activation for the subsequent smooth addition reaction and ring closure. The most active catalytic systems are capable of activating even internal epoxides such as epoxidized unsaturated fatty acid derivatives for the cycloaddition of CO2 under relatively mild conditions. In case of terminal epoxides such as epichlorohydrin, the effective utilization of diluted sources of CO2 such as flue gas is possible using the most active organocatalysts even at ambient pressure.

Efficient synthesis of cyclic carbonates from CO2 under ambient conditions over Zn(betaine)2Br2: experimental and theoretical studies

It is very interesting to synthesize high value-added chemicals from CO₂ under mild conditions with low energy consumption. Here, we report that a novel catalyst, Zn(betaine)₂Br₂, can efficiently promote the cycloaddition of CO₂ with epoxides to synthesize cyclic carbonates under ambient conditions (30 °C, 1 atm). DFT calculations provide important insights into the mechanism, particularly the unusual synergistic catalytic action of Zn²⁺, Br^- and NR⁴⁺, which is the critical factor for the outstanding performance of Zn(betaine)₂Br₂. The unique features of the catalyst are that it is cheap, green and very easy to prepare.

Improving the Thermal Properties of Polycarbonate via the Copolymerization of a Small Amount of Bisphenol Fluorene with Bisphenol A

Polycarbonate is an attractive transparent plastic with high mechanical/thermal properties. A family of copolycarbonates of bisphenol-A (BPA), 9, 9-bis (4-hydroxyphenyl) fluorene (BHPF), and diphenyl carbonate (DPC) were prepared by a transesterification polymerization. The weight-average molecular weight of the polycarbonates ranges from 65,000 to 107,000 g/mol; the copolycarbonates showed $T_g$ and $T_{d-5\%}$ from 63-70°C and 100-105°C higher than the control, respectively. Meanwhile, the processing properties of polycarbonate remain unchanged. These properties endow the polymers with potential for use as high-temperature resistance materials.

Boosting Electrochemical Carbon Dioxide Reduction on Atomically Dispersed Nickel Catalyst

Single-atom catalysts (SACs) with metal–nitrogen (M–N) sites are one of the most promising electrocatalysts for electrochemical carbon dioxide reduction (ECO₂R). However, challenges in simultaneously enhancing the activity and selectivity greatly limit the efficiency of ECO₂R due to the improper interaction of reactants/intermediates on these catalytic sites. Herein, we report a carbon-based nickel (Ni) cluster catalyst containing both single-atom and cluster sites (NiNx-T, T = 500–800) through a ligand-mediated method and realize a highly active and selective electrocatalytic CO₂R process. The catalytic performance can be regulated by the dispersion of Ni–N species via controlling the pyrolysis condition. Benefitting from the synergistic effect of pyrrolic-nitrogen coordinated Ni single-atom and cluster sites, NiNx-600 exhibits a satisfying catalytic performance, including a high partial current density of 61.85 mA cm⁻² and a high turnover frequency (TOF) of 7,291 h⁻¹ at −1.2 V vs. RHE, and almost 100% selectivity toward carbon monoxide (CO) production, as well as good stability under 10 h of continuous electrolysis. This work discloses the significant role of regulating the coordination environment of the transition metal sites and the synergistic effect between the isolated single-site and cluster site in enhancing the ECO₂R performance.

Anion-Afforded Functions of Ionic Metal-Organic Frameworks: Ionochromism, Anion Conduction, and Catalysis

The exchangeable counterions in ionic metal-organic frameworks (IMOFs) provide facile and versatile handles to manipulate functions associated with the ionic guests themselves and host-guest interactions. However, anion-exchangeable stable IMOFs combining multiple anion-related functions are still undeveloped. In this work, a novel porous IMOF featuring unique self-penetration was constructed from an electron-deficient tris(pyridinium)-tricarboxylate zwitterionic ligand. The water-stable IMOF undergoes reversible and single-crystal-to-single-crystal anion exchange and shows selective and discriminative ionochromic behaviors toward electron-rich anions owing to donor-acceptor interactions. The IMOFs with different anions are good ionic conductors with low activation energy, the highest conductivity being observed with chloride. Furthermore, integrating Lewis acidic sites and nucleophilic guest anions in solid state, the IMOFs act as heterogeneous and recyclable catalysts to efficiently catalyze the cycloaddition of CO₂ to epoxides without needing the use of halide cocatalysts. The catalytic activity is strongly dependent upon the guest anions, and the iodide shows the highest activity. The results demonstrate the great potential of developing IMOFs with various functions related to the guest ions included in the porous matrices.

Cycloaddition of di-substituted epoxides and CO2 under ambient conditions catalysed by rare-earth poly(phenolate) complexes

Lanthanum complex 1/TBAI is the first catalyst to achieve the cycloaddition of 1,2-disubstituted epoxides with 1 bar CO2 at room temperature. A DFT study discloses that the poly(phenolato) ligand plays a key role in the product dissociation step.

Unusual DBU-catalyzed decarboxylative formation of allylic thioethers from vinyl cyclic carbonates and thiols

Vinyl cyclic carbonates undergo an exo-cyclic attack by thiol nucleophiles under DBU catalysis to form allylic thioether products in moderate to good yields through a decarboxylative process under attractive process conditions.

A highly stable Zn9-pyrazolate metal–organic framework with metallosalen ligands as a carbon dioxide cycloaddition catalyst

A three-dimensional (3D) metal–organic framework constructed from unprecedented Zn9O2(OH)2(pyz)12 (pyz = pyrazolate) clusters and Ni(salen)-derived linkers was reported.

Facile synthesis, structure and properties of CO2-sourced poly(thioether-co-carbonate)s containing acetyl pendants via thio-ene click polymerization

Synthetic transformation of CO2 into di- and trivinyl carbonates allows for photo-initiated thio-ene polymerization leading to sequence controlled poly(thioether-carbonate)s.

Optimizing the mobility of active species in ionic liquid/MIL-101 composites for boosting carbon dioxide conversion

The mobility of active species has been optimized based on ionic liquid/metal–organic framework (MOF) composites for efficient CO2 chemical fixation.

A metal-free hydroxyl functionalized quaternary phosphine type ionic liquid polymer for cycloaddition of CO2 and epoxides

Quaternary phosphine type hypercrosslinked polymer catalysts were successfully prepared using the Friedel-Crafts alkylation reactions, which benefit from the synergistic effects between the Brønsted acidity of the hydroxyl group and nucleophilicity of the Br^- group as well as the CO₂ capture of the ionic liquids. The as-obtained metal-free catalysts facilitate the cycloaddition and synthesis of high value-added fine chemicals.

Study on Selected Metal-Organic Framework-Based Catalysts for Cycloaddition Reaction of CO2 with Epoxides: A Highly Economic Solution for Carbon Capture and Utilization

The level of carbon dioxide in the atmosphere is growing rapidly due to fossil fuel combustion processes, heavy oil, coal, oil shelter, and exhausts from automobiles for energy generation, which lead to depletion of the ozone layer and consequently result in global warming. The realization of a carbon-neutral environment is the main focus of science and academic researchers of today. Several processes were employed to minimize carbon dioxide in the air, some of which include the utilization of non-fossil sources of energy like solar, nuclear, and biomass-based fuels. Consequently, these sources were reported to have a relatively high cost of production and maintenance. The applications of both homogeneous and heterogeneous processes in carbon capture and storage were investigated in recent years and the focus now is on the conversion of CO2 into useful chemicals and compounds. It was established that CO2 can undergo cycloaddition reaction with epoxides under the influence of special catalysts to give cyclic carbonates, which can be used as value-added chemicals at a different level of pharmaceutical and industrial applications. Among the various catalysts studied for this reaction, metal-organic frameworks are now on the frontline as a potential catalyst due to their special features and easy synthesis. Several metal-organic framework (MOF)-based catalysts were studied for their application in transforming CO2 to organic carbonates using epoxides. Here, we report some recent studies of porous MOF materials and an in-depth discussion of two repeatedly used metal-organic frameworks as a catalyst in the conversion of CO2 to organic carbonates.

Synthesis of Diverse Polycarbonates by Organocatalytic Copolymerization of CO2 and Epoxides: From High Pressure and Temperature to Ambient Conditions

Organophosphazenes combined with triethylborane (TEB) were selected as binary organocatalyts for the copolymerization of CO₂ and epoxides. Both the activity and selectivity were highly dependent on the nature of phosphazenes. 2,4,6-Tris[tri(1-pyrrolidinyl)-iminophosphorane]-1,3,5-triazine (C₃ N₃ -Py-P₃ ) with a relatively low basicity (pK_a =26.5 in CD₃ CN) and a bulky molecular size (φ=1.3 nm) exhibited an unprecedented efficiency (TON up to 12240) and selectivity (>99 % polymer selectivity and >99 % carbonate linkages) toward copolymerization of CO₂ and cyclohexene oxide (CHO), and produced CO₂ -based polycarbonates (CO₂ -PCs) with high molar masses (M_n up to 275.5 kDa) at 1 MPa of CO₂ and 80 °C. Surprisingly, this binary catalytic system achieved efficient CO₂ /CHO copolymerization with TOF up to 95 h^-1 at 1 atm pressure and room temperature.

POSS and imidazolium-constructed ionic porous hypercrosslinked polymers with multiple active sites for synergistic catalytic CO2 transformation

In this work, we reported a facile one-pot approach to construct polyhedral oligomeric silsesquioxane (POSS) and imidazolium-based ionic porous hypercrosslinked polymers (denoted as iPHCPs) with multiple active sites towards efficient catalytic conversion of carbon dioxide (CO₂) to high value-added cyclic carbonates. The targeted iPHCPs were synthesized from a rigid molecular building block octavinylsilsesquioxane (VPOSS) and a newly-designed phenyl-based imidazolium ionic crosslinker through the AlCl₃-catalyzed Friedel-Crafts reaction. The desired multiple active sites come from the mixed anions including free Cl^- and Br^- anions, and in situ formed Lewis acidic metal-halogen complex anions [AlCl₃Br]^- within imidazolium moieties and POSS-derived Si-OH groups during the synthetic process. The typical polymer iPHCP-12 possesses a hierarchical micro-/mesoporous structure with a high surface area up to 537 m² g^-1 and shows a fluffy nano-morphology. By virtue of the co-existence of free nucleophilic Cl^- and Br^- anions, the metal complex anion [AlCl₃Br]^- with both electrophilic and nucleophilic characters and electrophilic hydrogen bond donor (HBD) Si-OH groups, iPHCP-12 is regarded as an efficient recyclable heterogeneous catalyst for synergistic catalytic conversion of CO₂ with various epoxides into cyclic carbonates under mild conditions. The present work provides a succinct one-pot strategy to construct task-specific ionic porous hypercrosslinked polymers from easily available modules for the targeted catalytic applications.

Metal-Cocatalyst Interaction Governs the Catalytic Activity of MII-Porphyrazines for Chemical Fixation of CO2

Chemical fixation of CO₂ to produce cyclic carbonates can be a green and atomic efficient process. In this work, a series of porphyrazines (Pzs) containing electron-withdrawing groups and central M^II ions (where M = Mg, Zn, Cu, and Co) were synthesized and investigated as catalysts for the cycloaddition of CO₂ to epoxides. Then, the efficiency of the Pzs was tested by varying cocatalyst type and concentration, epoxide, temperature, and pressure. Mg^IIPz bearing trifluoromethyl groups (1) showed the best conversion, producing, selectively, 78% of propylene cyclic carbonate (PCC), indicating that a harder and stronger Lewis acid is more effective for epoxide activation. Moreover, cocatalyst variation showed a notable effect on the reaction yields. Spectrophotometric titrations, MALDI-TOF mass spectra, and theoretical calculations suggest poisoning of the catalyst when tetrabutylammonium chloride (TBAC) and large amounts of tetrabutylammonium bromide (TBAB) were used in the system. The same was not observed for tetrabutylammonium iodide (TBAI), indicating that the metal-cocatalyst interaction may govern the reaction rate. In addition, two rare examples of crystalline structures were obtained, proving the distorted square pyramidal geometry with water molecule as axial ligand. This is one of the first studies reporting Pzs as catalysts for the chemical fixation of CO₂, and we believe that the intricate balance between cocatalyst concentration and conversion efficiency shown here may aid future studies in the area.

Borane catalyzed polymerization and depolymerization reactions controlled by Lewis acidic strength

Triphenylborane catalyzes the polymerization of anhydrides and epoxides along with the block co-polymerization of anhydride/epoxides with epoxides/CO₂. Switching to the more Lewis acidic borane, tris(pentafluorophenyl)borane, the carbonate block of the polymer could be selectively depolymerized to yield the corresponding cyclic carbonate opening the door towards chemical repurposing of the polymer.

A Bio-Based Alginate Aerogel as an Ionic Liquid Support for the Efficient Synthesis of Cyclic Carbonates from CO2 and Epoxides

In this work, the ionic liquid [Aliquat][Cl] was supported into alginate and silica aerogel matrices and applied as a catalyst in the cycloaddition reaction between CO2 and a bio-based epoxide (limonene oxide). The efficiency of the alginate aerogel system is much higher than that of the silica one. The method of wet impregnation was used for the impregnation of the aerogel with [Aliquat][Cl] and a zinc complex. The procedure originated a well-defined thin solvent film on the surface of support materials. Final materials were characterised by Fourier Transform Infrared Spectroscopy, N2 Adsorption–Desorption Analysis, X-ray diffraction, atomic absorption and Field Emission Scanning Microscopy. Several catalytic tests were performed in a high-pressure apparatus at 353.2 K and 4 MPa of CO2.