Imidazolium- and Triazine-Based Porous Organic Polymers for Heterogeneous Catalytic Conversion of CO2 into Cyclic Carbonates

Building a Co-Salen-Immobilized Porous Organic Polymer Catalyst for CO2 Cycloaddition

The CO2-epoxide addition to cyclic carbonate is of great significance but usually requires high temperatures and CO2 pressures. Herein, a spirobifluorene-based porous organic polymer catalyst is designed with a Co-salen complex immobilized on the backbone (ST-CoSalen-POP) to enable CO2 fixation under mild conditions. ST-CoSalen-POP possesses a high Co-loading content (9.35 wt%), a large pore volume, and high CO2 adsorption capacity. This catalyst achieves a 96% yield in the cycloaddition of CO2 with epoxides at 25 °C and 0.1 MPa CO2 pressures. Moreover, ST-CoSalen-POP also offers the advantages of wide adaptability over epoxide substrates and high structural stability for three consecutive cycles. This study presents a novel catalytic approach for promoting CO2 fixation, offering a significant advancement in the efficient synthesis of fine chemicals.

Imidazolium and Pyridinium-Based Ionic Porous Organic Polymers: Advances in Transformative Solutions for Oxoanion Sequestration and Non-Redox CO2 Fixation

The rapid pace of industrialization has led to a multitude of detrimental environmental consequences, including water pollution and global warming. Consequently, there is an urgent need to devise appropriate materials to address these challenges. Ionic porous organic polymers (iPOPs) have emerged as promising materials for oxoanion sequestration and non-redox CO2 fixation. Notably, iPOPs offer hydrothermal stability, structural tunability, a charged framework, and readily available nucleophilic counteranions. This review explores the significance of pores and charged functionalities alongside design strategies outlined in existing literature, mainly focusing on the incorporation of pyridinium and imidazolium units into nitrogen-rich iPOPs for oxoanion sequestration and non-redox CO2 fixation. The present review also addresses the current challenges and future prospects, delineating the design and development of innovative iPOPs for water treatment and heterogeneous catalysis.

Controllable synthesis of hollow mesoporous organosilica nanoparticles with pyridine-2,6-bis-imidazolium frameworks for CO2 conversion

A series of hard-template-derived hollow mesoporous organosilica nanoparticles (HMONs) with pyridine-2,6-bis-imidazolium frameworks have been described for the first time. As a part of the investigation, to evaluate the effects of the hard template nature, the Si/CTAB and organosilica/TEOS molar ratios, and the stepwise addition of precursors, four reaction conditions denoted as methods A-D were designed. In the presence of polystyrene latex as a hard template, the HMONs that we wished to synthesize were not yielded with a Si/CTAB molar ratio of 3 (method A), but we could synthesize the desired HMONs with a Si/CTAB molar ratio of 9 and an organosilica : TEOS ratio of 1 : 99 (method B). The ratio of organosilica to TEOS could be improved up to 2.5 : 97.5 if the precursor additions are made in a stepwise manner rather than by simultaneous additions (method C). Using sSiO2 as a hard template, a yolk-shell morphology was observed by adopting a Si/CTAB molar ratio of 3 (method D). The HMONs were modified by iodide ions and their activity was explored toward the coupling of CO2 with epoxides. Among the catalysts, I-HMON-L-C-2.5 exhibited excellent results under mild reaction conditions. Well-oriented pore sizes and short channel length facilitated easy mass transfer from one side and the integration of the interior hollow regions of the catalyst particles from the other side improved the CO2 retention time around pores where the imidazolium organocatalysts were located, which made I-HMON-L-C-2.5 an effective catalyst for title CO2 utilization. The catalyst was reused at least six times without exhibiting any changes in its activity. HMONs can also be used as solid CNC ligands for the preparation of copper catalysts for the click reaction between phenyl acetylene and benzyl azide.

Fabrication of imidazoline-linked cationic covalent triazine framework for enrichment of environmental estrogens

Imidazoline-linked cationic covalent triazine framework (IM-iCTF) was facilely prepared through the Debus-Radziszewski reaction, involving 4,4',4''-(1,3,5-triazine-2,4,6-triyl)trianiline, formaldehyde and methylglyoxal. The IM-iCTF was applied as a sorbent for cartridge solid-phase extraction (SPE). It provided good adsorption performance for estrogen and estrogen mimics including bisphenol F, bisphenol A, 7β-estradiol, bisphenol B and estrone. The adsorption isotherm, adsorption kinetic model, thermodynamic calculations and adsorption mechanism were investigated to reveal the adsorption behavior. The IM-iCTF was employed for the extraction of the estrogens and estrogen mimics from water, fish and shrimp (fish and shrimp samples were extracted with acetonitrile before the SPE). The analytes were then determined by high-performance liquid chromatography with diode array detection. The limits of detection were 0.008-0.05 ng mL-1 for water, 0.015-0.11 μg g-1 for fish, and 0.012-0.10 μg g-1 for shrimp samples. This research not only offers a new approach to construct cationic covalent triazine framework, but also provides a reliable strategy for the adsorption/enrichment trace level of organic pollutants.

Morphology Tuning via Linker Modulation: Metal-Free Covalent Organic Nanostructures with Exceptional Chemical Stability for Electrocatalytic Water Splitting

The development of synthetic routes for the formation of robust porous organic polymers (POPs) with well-defined nanoscale morphology is fundamentally significant for their practical applications. The thermodynamic characteristics that arise from reversible covalent bonding impart intrinsic chemical instability in the polymers, thereby impeding their overall potential. Herein, a unique strategy is reported to overcome the stability issue by designing robust imidazole-linked POPs via tandem reversible/irreversible bond formation. Incorporating inherent rigidity into the secondary building units leads to robust microporous polymeric nanostructures with hollow-spherical morphologies. An in-depth analysis by extensive solid-state NMR (1D and 2D) study on 1H, 13C, and 14N nuclei elucidates the bonding and reveals the high purity of the newly designed imidazole-based POPs. The nitrogen-rich polymeric nanostructures are further used as metal-free electrocatalysts for water splitting. In particular, the rigid POPs show excellent catalytic activity toward the oxygen evolution reaction (OER) with long-term durability. Among them, the most efficient OER electrocatalyst (TAT-TFBE) requires 314 mV of overpotential to drive 10 mA cm-2 current density, demonstrating its superiority over state-of-the-art catalysts (RuO2 and IrO2).

Ionic Conjugated Polymers as Heterogeneous Catalysts for the Cycloaddition of Carbon Dioxide to Epoxides to Form Carbonates under Solvent- and Cocatalyst-Free Conditions

The generation of cyclic carbonates by the cycloaddition of CO₂ with epoxides is attractive in the industry, by which CO₂ is efficiently used as C1 source. Herein, a series of catalysts were developed to efficient mediate the cycloaddition of CO₂ with epoxides to generate carbonates. The catalysts were easily synthesized via the amine-formaldehyde condensation of ethidium bromide with a variety of linkers. The newly prepared heterogeneous catalysts have high thermal stability and degradation temperatures. The surface of the catalysts is smooth and spherical in shape. The effect of temperature, pressure, reaction time and catalyst dosage on the cycloaddition of CO₂ with epoxide were investigated. The results show that the catalyst with 1,3,5-tris(4-formylphenyl)benzene as the linker can achieve 97.4 % conversion efficiency at the conditions of 100 °C, reaction time of 12 h, and the reaction pressure of 1.2 MPa in a solvent-free environment. Notably, the polymers serve as homogeneous catalysts during the reaction (reaction temperature above T_g ) and can be separated and recovered easily as homogeneous catalysts at room temperature. In addition, the catalyst is not only suitable for a wide range of epoxide substrates, but also can be recycled many times. Furthermore, DFT calculations show that the coordination between the electrophilic center of the catalyst and the epoxide reduces the energy barrier, and the reaction mechanism is proposed based on the reaction kinetic studies and DFT calculations.

Nitrogen-Rich Porous Organic Polymers with Supported Ag Nanoparticles for Efficient CO2 Conversion

As CO₂ emissions increase and the global climate deteriorates, converting CO₂ into valuable chemicals has become a topic of wide concern. The development of multifunctional catalysts for efficient CO₂ conversion remains a major challenge. Herein, two porous organic polymers (NPOPs) functionalized with covalent triazine and triazole N-heterocycles are synthesized through the copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. The NPOPs have an abundant microporous content and high specific surface area, which confer them excellent CO₂ affinities with a CO₂ adsorption capacity of 84.0 mg g^-1 and 63.7 mg g^-1, respectively, at 273 K and 0.1 MPa. After wet impregnation and in situ reductions, Ag nanoparticles were supported in the NPOPs to obtain Ag@NPOPs with high dispersion and small particle size. The Ag@NPOPs were applied to high-value conversion reactions of CO₂ with propargylic amines and terminal alkynes under mild reaction conditions. The carboxylative cyclization transformation of propargylic amine into 2-oxazolidinone and the carboxylation transformation of terminal alkynes into phenylpropiolic acid had the highest TOF values of 1125.1 and 90.9 h^-1, respectively. The Ag@NPOP-1 was recycled and used five times without any significant decrease in catalytic activity, showing excellent catalytic stability and durability.

Precise Introduction of Single Vanadium Site into Indium-Organic Framework for CO2 Capture and Photocatalytic Fixation

The capture and fixation of CO₂ under mild conditions is a cost-effective route to reduce greenhouse gases, but it is challenging because of the low conversion and selectivity issues. Metal-organic frameworks (MOFs) are promising in the fields of adsorption and catalysis because of their structural tunability and variability. However, the precise structural design of MOFs is always pursued and elusive. In this work, a metal-mixed MOF (SNNU-97-InV) was designed by precisely introducing single vanadium site into the isostructural In-MOF (SNNU-97-In). The single V sites clearly change the interactions between the MOF framework and CO₂ molecules, leading to a 71.3% improvement in the CO₂ adsorption capacity. At the same time, the enhanced light absorption enables SNNU-97-InV to efficiently convert CO₂ into cyclic carbonates (CCs) with epoxides under illumination. Controlled experiments showed that the promoted performance of SNNU-97-InV may be that the V═O site can more easily combine with CO₂ and convert them into an intermediate state under illumination, and the possible mechanism was thus speculated.

Preparation of a reusable and pore size controllable porous polymer monolith and its catalysis of biodiesel synthesis

A sulfonated porous polymer monolith (PPM-SO3H) has been prepared via the polymerisation of styrene (St) and divinyl benzene (DVB) with organic microspheres as pore-forming agents, followed by sulfonation with concentrated sulfuric acid. It was characterized by acid-base titration in order to determine its acid density, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, mercury intrusion porosimetry (MIP) and thermogravimetric analysis (TG). The PPM-SO3H showed an acid density of 1.89 mmol g-1 and pore cavities with an average diameter of 870 nm. The catalytic activity of PPM-SO3H in practical biodiesel synthesis from waste fatty acids was investigated and the main reaction parameters were optimized through orthogonal experiment. The best reaction conditions obtained for the optimization of methanol to oil ratio, catalyst concentration, reaction temperature and reaction time were 1 : 1, 20%, 80 °C and 8 h, respectively. PPM-SO3H showed excellent catalytic activity. In biodiesel synthesis, the esterification rate of PPM-SO3H is 96.9%, which is much higher than that of commercial poly(sodium-p-styrenesulfonate) (esterification rate 29.0%). The PPM-SO3H can be reused several times without significant loss of catalytic activity; the esterification rate was still 90.8% after 6 cycles. The pore size of this porous polymer monolith can be controlled. The dimension and shape of this porous polymer monolith were also adjustable by choosing a suitable polymerisation container.

Construction of porphyrin and viologen-linked cationic porous organic polymer for efficient and selective gold recovery

Recycling precious metals from electronic waste not only benefits environmental protection, but is also favorable for alleviating resource shortages. Ionic porous organic polymers, as one type of burgeoning material, are regarded as excellent adsorbents due to their high ion density, but their application in precious metal recovery is still very limited. Here, V-PPOP-Br, a highly stable and easy-to-build cationic porous organic polymer, was successfully prepared for the first time. By linking porphyrins with viologens, V-PPOP-Br obtained the characteristics of a hierarchical porous structure, a high ion density skeleton, and a rich nitrogen content, which gave it an ultrahigh adsorption capacity (Q_max = 792.22 mg g^-1) and rapid adsorption rate for Au(III). V-PPOP-Br also had an effective Au(III) recovery capability from SIM cards. Mechanism investigation confirmed that this remarkable adsorption performance was attributed to the interplay of ion exchange, redox reactions and coordination. Moreover, V-PPOP-Br had excellent recyclability and could maintain an ultrahigh adsorption efficiency of 81% after eight consecutive adsorption-desorption experiments. These excellent performances as well as the roughly calculated gold recycling economics ($37.37/g V-PPOP-Br) confirmed that it possesses promising potential as an ionic porous material for gold recovery.

Highly Efficient and Selective Gold Recovery Based on Hypercross-Linking and Polyamine-Functionalized Porous Organic Polymers

With the continuous increase of electronic products, there is an urgent need to effectively recover gold from e-waste and other secondary resources other than the original mine. Here, hypercross-linking and polyamine-functionalized porous organic polymers (Pc-POSS-POP) were designed and facially synthesized based on multiple azo-coupling polymerizations between 2,9,16,23-tetraaminophthalocyanine (H₂Pc(NH₂)₄) and octa(aminophenyl)-t8-silsesquioxane (OAPS) for the first time. The reaction requires no metal as a catalyst, thereby benefiting the purification of the product and the industrial scalability. Pc-POSS-POPs possess a hypercross-linking structure, highly conjugated frameworks, nitrogen-rich active sites, and extensively visible and near-infrared light absorption, which was utilized as an absorbent to retrieve Au (III). The results demonstrated that Pc-POSS-POPs have a high adsorption capacity (862.07 mg g^-1) and a rapid adsorption rate toward gold recycling. The maximum adsorption capacity could reach up to 1026.87 mg g^-1 as in the case of light irradiation. Due to the strong N coordination sites and the electronic interaction between the -NH₄⁺ groups of Pc-POSS-POPs and AuCl₄^-, Pc-POSS-POPs also exhibited excellent selectivity toward gold over several coordinated metals [Cr (VI), Co (II), Cd (II), Ni (II), and Hg (II)]. These properties together with the good regenerative ability and superior recyclability demonstrated that Pc-POSS-POPs possess promising potential as hypercross-linking polymers for capturing and recycling of Au (III).

Uniform Core-Shell Microspheres of SiO2@MOF for CO2 Cycloaddition Reactions

A SiO₂@MOF core-shell microsphere for environmentally friendly applications was introduced in this study. Several types of metal-organic framework core-shell microspheres were successfully synthesized. To achieve high stability and favorable catalytic performance, modification and coating methods were necessary for optimization. The improved SiO₂@MOF core-shell microspheres were used in the cycloaddition reaction of carbon dioxide and propylene oxide. Dispersion ability was enhanced by the addition of core-shell microspheres, which also produced high catalytic activity. Accompanied with tetrabutylammonium bromide as a co-catalyst, SiO₂@ZIF-67 had a maximum conversion of 97%, and the results revealed that SiO₂@ZIF-67 could be used for 5 reaction cycles while maintaining high catalytic performance. This recycling catalyst was also reacted with a series of terminal epoxides to form corresponding cyclic carbonates with high conversion rates, indicating that SiO₂@MOF core-shell microspheres exhibit promise in the field of catalysis.

Polyamine-functionalized imidazolyl poly(ionic liquid)s for the efficient conversion of CO2 into cyclic carbonates

The synergistic effect of polyamine groups and nucleophile (Br−) significantly improved the catalytic performance of N4-PIL-2, which can convert epoxides into cyclic carbonates with excellent yields and selectivity under ambient pressure.

N-Heterocyclic carbenes and their precursors in functionalised porous materials

Though N-heterocyclic carbenes (NHCs) have emerged as diverse and powerful discrete functional molecules in pharmaceutics, nanotechnology, and catalysis over decades, the heterogenization of NHCs and their precursors for broader applications in porous materials, like metal-organic frameworks (MOFs), porous coordination polymers (PCPs), covalent-organic frameworks (COFs), porous organic polymers (POPs), and porous organometallic cages (POMCs) was not extensively studied until the last ten years. By de novo or post-synthetic modification (PSM) methods, myriads of NHCs and their precursors containing building blocks were designed and integrated into MOFs, PCPs, COFs, POPs and POMCs to form various structures and porosities. Functionalisation with NHCs and their precursors significantly expands the scope of the potential applications of porous materials by tuning the pore surface chemical/physical properties, providing active sites for binding guest molecules and substrates and realizing recyclability. In this review, we summarise and discuss the recent progress on the synthetic methods, structural features, and promising applications of NHCs and their precursors in functionalised porous materials. At the end, a brief perspective on the encouraging future prospects and challenges in this contemporary field is presented. This review will serve as a guide for researchers to design and synthesize more novel porous materials functionalised with NHCs and their precursors.

Covalent Organic Frameworks for Simultaneous CO2 Capture and Selective Catalytic Transformation

Combination of capture and simultaneous conversion of CO2 into valuable chemicals is a fascinating strategy for reducing CO2 emissions. Therefore, searching for heterogeneous catalysts for efficient catalytic conversion of CO2 is of great importance for carbon capture and utilization. Herein, we report a metalloporphyrin-based covalent organic framework (Co(II)@TA-TF COF) that can capture CO2 and simultaneously convert it into cyclic carbonates under mild conditions. The COF was designed to possess micropores for the adsorption of CO2 and integrated with cobalt(II) porphyrin (Co(II)@TAPP) units as catalytic sites into the vertices of the layered tetragonal networks. The structure of the Co(II)@TA-TF COF is unique where Co(II)@TAPP units are alternately stacked along the z direction with a slipped distance of 1.7 Å, which gives an accessible space to accommodate small molecules, making it possible to expose catalytic sites to substrates within the adjacent stacked layers. As a result, this COF is found to be highly effective for the addition of CO2 and epoxides. Importantly, the Co(II)@TA-TF COF exhibited a dramatic size selectivity for substrates. In conjunction with its reusability, our results highlight the development of a new function of COFs for targeting simultaneous CO2 absorption and utilization upon complementary exploration of the structural features of skeletons and pores. Such promising catalytic performance of the COF makes it possible for its potential practical application.

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.

Azo-Functionalized Zirconium-Based Metal-Organic Polyhedron as an Efficient Catalyst for CO2 Fixation with Epoxides

Chemical fixation of CO₂ as C1 source at ambient temperature and low pressure is an energy-saving way to make use of the green-house gas, but it still remains a challenge since efficient catalyst with high catalytic active sites is required. Here, a novel monoclinic azo-functionalized Zr-based metal-organic polyhedron (Zr-AZDA) has been prepared and applied in CO₂ fixation with epoxides. The inherent azo groups not only endow Zr-AZDA with good solubilization, but also act as basic sites to enrich CO₂ showing efficient synergistic catalysis as confirmed by TPD-CO₂ analysis. XPS results demonstrate that the Zr active sites in Zr-AZDA possess suitable Lewis acidity, which satisfies both substrates activation and products desorption. DFT calculation indicates the energy barrier of the rate-determining step in CO₂ cycloaddition could be reduced remarkably (by ca. 60.9 %) in the presence of Zr-AZDA, which may rationalize the mild and efficient reaction condition employed (80 °C and 1 atm of CO₂ ). The work provides an effective multi-functional cooperative method for improvement of CO₂ cycloaddition.

A silsesquioxane-porphyrin-based porous organic polymer as a highly efficient and recyclable absorbent for wastewater treatment

Effective capture of pollutants from wastewater is crucial for protecting the environment and human health. An azo-based porous organic polymer (AzoPPOP) containing porphyrin and inorganics cage polyhedral oligomeric silsesquioxane units was synthesized via a catalyst-free coupling reaction. Results showed that AzoPPOP possess a high surface area, a hierarchically porous structure, good thermal stability, abundant adsorption sites, and an electronegative nature. Based on these properties, AzoPPOP had an extremely high adsorption capacity (1357.58 mg g^-1) for RhB, a fast adsorption rate, and good selectivity. Study of the mechanism revealed that in addition to electrostatic interactions, the high specific surface area, existence of -NH₂, and the strong π-π interaction between AzoPPOP and RhB also play important roles for the adsorption of RhB. AzoPPOP also displayed excellent adsorption properties for heavy metal ions (230.45, 192.24 and 162.11 mg g^-1 for Ag⁺, Hg²⁺, and Pb²⁺, respectively). More importantly, simulation of the purification experiment of waste water and the recycling regeneration experiment revealed that AzoPPOP has good high-level recyclability and could remove multi-pollutants in one pass through a simple adsorption column.

Carbon Dioxide Conversion Upgraded by Host-guest Cooperation between Nitrogen-Rich Covalent Organic Framework and Imidazolium-Based Ionic Polymer

The chemical conversion of CO₂ into value-added chemicals is one promising approach for CO₂ utilization. It is crucial to explore highly efficient catalysts containing task-specific components for CO₂ fixation. Here, a host-guest catalytic system was developed by integrating nitrogen-rich covalent organic framework (TT-COF) and imidazolium-based ionic polymer (ImIP), which serve as hydrogen-bonding donor and nucleophilic agent, respectively, for cooperatively facilitating the activation of the epoxides and subsequent CO₂ cycloaddition. The catalytic activity of the host-guest system was remarkably superior to those of ImIP, TT-COF, and their physical mixture. Furthermore, selective adsorption for CO₂ over N₂ rendered this catalytic system effective for the cycloaddition reaction of the simulated flue gas. The protocols for the unification of two catalytically active components provide new opportunities for the development of composite systems in multiple applications.

Building N-Heterocyclic Carbene into Triazine-Linked Polymer for Multiple CO2 Utilization

The development of new CO₂ detection technologies and CO₂ "capture-conversion" materials is of great significance due to the growing environmental crisis. Here, multifunctional triazine-linked polymers with built-in N-heterocyclic carbene (NHC) sites (designated as NHC-triazine@polymer) are presented for simultaneous CO₂ detection, capture, activation, and catalytic conversion. NHC-triazine@polymer were readily obtained through polymerization of cyanophenyl-substituted NHC. The obtained film-like polymers exhibited interesting CO₂ -triggered fluorescence "turn-on" response and CO₂ -sensitive reversible color change. Both NHC and triazine sites could act as efficient binding sites for CO₂ , and the CO₂ uptake of NHC and triazine reached 1.52 and 1.36 mmol g^-1 , respectively. Notably, after being captured by NHC, CO₂ was activated into a zwitterionic adduct NHC-CO₂ that could be easily transformed into cyclic carbonate in the presence of epoxides. Moreover, NHC-triazine@polymer were stable and active catalysts for the conversion of low-concentration CO₂ in a gas mixture (7 vol %) into cyclic carbonates as well as for hydrosilylation of CO₂ to formamides.

Recent Advances on Imidazolium-Functionalized Organic Cationic Polymers for CO2 Adsorption and Simultaneous Conversion into Cyclic Carbonates

The cycloaddition reaction of CO₂ with various epoxides to generate cyclic carbonates is one of the most promising and efficient approaches for CO₂ fixation. Typical imidazolium-based ionic liquids possessing electrophilic cations and nucleophilic halogen anions have been identified as excellent and environmentally friendly candidates for synergistically activating epoxides to convert CO₂ . Therefore, the feasible construction of a series of imidazolium-functionalized organic cationic polymers can bridge the gap between homogeneous and heterogeneous catalysis, thereby obtaining highly selective CO₂ adsorption and simultaneous conversion ability. This Review describes the recent advancements made with regard to the design and synthesis of this type of polymeric networks having imidazolium functionality. They are considered as an outstanding heterogeneous catalyst for the cycloaddition of CO₂ to epoxides. Based on the perspective from the design of building blocks to the synthesis of cationic polymers, the focus mainly lies on how to introduce imidazole units into the material backbone via a covalent linking approach and how to incorporate other active sites capable of activating CO₂ and/or epoxides into such polymeric materials.

Ionic-Liquid-Modified Click-Based Porous Organic Polymers for Controlling Capture and Catalytic Conversion of CO2

Capture and catalytic conversion of CO₂ into value-added chemicals is a promising and sustainable approach to relieve global warming and the energy crisis. Nitrogen-rich porous organic polymers (POPs) are promising materials for CO₂ capture and separation, but their application in the additive-free catalytic conversion of CO₂ into cyclic carbonates is still a challenge. Herein, a nitrogen-rich click-based POP (CPP) was developed for the cycloaddition reaction of CO₂ with epoxides in the absence of metal, solvents, and additives. The introduction of imidazolium-based ionic liquids on the CPP host backbone could modulate the porosity, CO₂ adsorption/desorption, CO₂ selectivity over N₂ , and catalytic activity in the chemical transformation. A tentative catalytic pathway was proposed to account for the superior catalytic activity of the catalytic systems, in which the incorporated ionic liquid and porous properties of CPP synergistically contributed to the catalytic reaction. This study provides a platform to understand the cooperative effects of porous properties and nucleophilic anions on the cycloaddition reaction of CO₂ with epoxides.

Metalated-bipyridine-based porous hybrid polymers with POSS-derived Si–OH groups for synergistic catalytic CO2 fixation

ZnBr₂ metalated-bipyridine porous hybrid polymers with POSS-derived Si–OH as “all-in-one” heterogeneous catalysts for synergistic catalytic CO₂ fixation.

A Covalent Triazine-Based Framework Consisting of Donor-Acceptor Dyads for Visible-Light-Driven Photocatalytic CO2 Reduction

Photocatalytic conversion of CO₂ into value-added chemical fuels is a promising approach to address the depletion of fossil energy and environment-related concerns. Tailor-making the electronic properties and band structures of photocatalysts is pivotal to improve their efficiency and selectivity in photocatalytic CO₂ reduction. Herein, a covalent triazine-based framework was developed containing electron-donor triphenylamine and electron-acceptor triazine components (DA-CTF). The engineered π-conjugated electron donor-acceptor dyads in DA-CTF not only optimized the optical bandgap but also contributed to visible-light harvesting and migration of photoexcited charge carriers. The activity of photocatalytic CO₂ reduction under visible light was significantly improved compared with that of traditional g-C₃ N₄ and reported covalent triazine-based frameworks. This study provides molecular-level insights into the mechanism of photocatalytic CO₂ reduction.

Pyridine-bridged bifunctional organocatalysts for the synthesis of cyclic carbonates from carbon dioxide

Hydroxyl- and carboxyl-functionalized imidazolium halides are used as efficient bifunctional organocatalysts for the synthesis of cyclic carbonates from CO2 and epoxides under mild reaction conditions. Control experiments suggest that the cycloaddition reaction is realized by the combination of the nucleophilic halide anions with hydroxyl and carboxyl groups as hydrogen bond donors. Moreover, the bifunctional organocatalysts can be easily recycled five times by simple filtration; however, a loss of activity was observed.

Hybrid Catalysts for CO2 Conversion into Cyclic Carbonates

The conversion of carbon dioxide into valuable chemicals such as cyclic carbonates is an appealing topic for the scientific community due to the possibility of valorizing waste into an inexpensive, available, nontoxic, and renewable carbon feedstock. In this regard, last-generation heterogeneous catalysts are of great interest owing to their high catalytic activity, robustness, and easy recovery and recycling. In the present review, recent advances on CO2 cycloaddition to epoxide mediated by hybrid catalysts through organometallic or organo-catalytic species supported onto silica-, nanocarbon-, and metal–organic framework (MOF)-based heterogeneous materials, are highlighted and discussed.

Ionic-Liquid-Functionalized UiO-66 Framework: An Experimental and Theoretical Study on the Cycloaddition of CO2 and Epoxides

A facile approach for modifying the UiO-66-NH₂ metal-organic framework by incorporating imidazolium-based ionic liquids (ILs) to form bifunctional heterogeneous catalysts for the cycloaddition of epoxides to CO₂ is reported. Methylimidazolium- and methylbenzimidazolium-based IL units (ILA and ILB, respectively) were introduced into the pore walls of the UiO-66-NH₂ framework through a condensation reaction to generate ILA@U6N and ILB@U6N catalysts, respectively. The resultant heterogeneous catalysts, especially ILA@U6N, exhibited excellent CO₂ adsorption capability, which makes them effective for cycloaddition reactions producing cyclic carbonates under mild reaction conditions in the absence of any cocatalyst or solvent. The significantly enhanced activity of ILA@U6N is attributed to the synergism between the coordinately unsaturated Lewis acidic Zr⁴⁺ centers and Br^- ions in the bifunctional heterogeneous catalysts. The size effect of the ILs on coupling between the epoxide and CO₂ was also studied for ILA@U6N and ILB@U6N. A periodic DFT study was performed to provide evidence of possible intermediates, transition states, and pathways, as well as to gain deeper insight into the mechanism of the ILA@U6N-catalyzed cycloaddition reaction between epichlorohydrin and CO₂ .

Experimental and theoretical study for CO2 activation and chemical fixation with epoxides

The synthesis of five-membered cyclic carbonates via catalytic cycloaddition reaction of CO₂ with epoxides is considered to be an effective technology for alleviation of the energy crisis and global warming. Various commercial organic bases and ionic salts were used as catalysts, while the relationship of catalytic activity and compound structure has been seldom explored. Herein, a facilely obtained binary catalytic system based on triethylamine/NBu₄Br was developed for CO₂ activation and chemical fixation. The highly efficient catalytic system showed outstanding conversion and above 99% selectivity under metal-free mild reaction conditions (100 °C, 1 atm) in one hour. The detailed process of CO₂ activation and chemical fixation was investigated at the molecular level by a series of experiments and theoretical calculation, which provided a mode for the design and synthesis of a highly efficient catalytic system for conversion of CO₂ under mild conditions.

NEt₃/NBu₄Br works as an excellent metal-free catalyst for CO₂ cycloaddition with epoxides and the detailed process of CO₂ activation by NEt₃ is first studied by theoretical calculation.

The design of a novel and resistant Zn(PZDC)(ATZ) MOF catalyst for the chemical fixation of CO2 under solvent-free conditions

Novel Zn(PZDC)(ATZ) with Lewis acid–base sites exhibited strong resistance to acids/alkalis and moisture and possessed high catalytic activity for CO₂ transformation.

Ag NPs decorated on a COF in the presence of DBU as an efficient catalytic system for the synthesis of tetramic acids via CO2 fixation into propargylic amines at atmospheric pressure

Ag NPs are decorated at the surface of a COF material TpPa-1 and the resulting Ag@TpPa-1 catalyzes efficiently for the synthesis of tetramic acids from a variety of propargylic amines using CO₂ as reagent.

A multifunctional triazine-based nanoporous polymer as a versatile organocatalyst for CO2utilization and C–C bond formation

Conversion of CO₂to cyclic carbonates, methanol and methane by using a nanoporous MNENP as a multifunctional metal-free organocatalyst.

Highly tunable periodic imidazole-based mesoporous polymers as cooperative catalysts for efficient carbon dioxide fixation

We designed new periodic imidazole-based mesoporous polymers for cooperative catalysis, revealing the structure–activity relationships in CO₂ cycloaddition.

Novel Covalent Triazine Framework for High-Performance CO2 Capture and Alkyne Carboxylation Reaction

Carbon dioxide capture and conversion have attracted extreme enthusiasm from the scientific community owing to global warming and environmental problems. However, conversion of CO₂ under atmospheric pressure is of great challenge because of the inertness of CO₂. Herein, we present a novel covalent triazine framework (CTF-DCE) prepared via ZnCl₂-catalyzed ionothermal trimerization reaction of di(4-cyanophenyl)ethyne, which displays a high Brunauer-Emmett-Teller surface area of 1355 m² g^-1 and an excellent CO₂ capture capacity of 191 mg/g at 273 K/1 bar. More importantly, silver species can be successfully fixed on the CTF matrix to produce a stable CTF-DCE-Ag heterogeneous catalyst for outstanding catalysis in the terminal alkyne carboxylation reactions under atmospheric pressure. CTF-DCE-Ag exhibited over sixfold higher turnover numbers than Ag@MIL-101. The recyclability test of the CTF-DCE-Ag catalyst demonstrated a great potential application in various environmental and energy-related applications.