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

Catalytic Strategies for the Cycloaddition of CO2 to Epoxides in Aqueous Media to Enhance the Activity and Recyclability of Molecular Organocatalysts

The cycloaddition of CO2 to epoxides to afford versatile and useful cyclic carbonate compounds is a highly investigated method for the nonreductive upcycling of CO2. One of the main focuses of the current research in this area is the discovery of readily available, sustainable, and inexpensive catalysts, and of catalytic methodologies that allow their seamless solvent-free recycling. Water, often regarded as an undesirable pollutant in the cycloaddition process, is progressively emerging as a helpful reaction component. On the one hand, it serves as an inexpensive hydrogen bond donor (HBD) to enhance the performance of ionic compounds; on the other hand, aqueous media allow the development of diverse catalytic protocols that can boost catalytic performance or ease the recycling of molecular catalysts. An overview of the advances in the use of aqueous and biphasic aqueous systems for the cycloaddition of CO2 to epoxides is provided in this work along with recommendations for possible future developments.

CO2 Valorization in Deep Eutectic Solvents

The deep eutectic solvent (DES) has emerged in recent years as a valuable medium for converting CO2 into valuable chemicals because of its easy availability, stability, and safety, and its capability to dissolve carbon dioxide. CO2 valorization in DES has evolved rapidly over the past 20 years. As well as being used as solvents for acid/base-promoted CO2 conversion for the production of cyclic carbonates and carbamates, DESs can be used as reaction media for electrochemical CO2 reduction for formic acid and CO. Among these products, cyclic carbonates can be used as solvents and electrolytes, carbamate derivatives include the core structure of many herbicides and pesticides, and formic acid and carbon monoxide, the C1 electrochemical products, are essential raw materials in the chemical industries. An overview of the application of DESs for CO2 valorization in recent years is presented in this review, followed by a compilation and comparison of product types and reaction mechanisms within the different types of DESs, and an outlook on how CO2 valorization will be developed in the future.

Bridging the Homogeneous and Heterogeneous Catalysis by Supramolecular Metal-Organic Cages with Varied Packing Modes

Integrating the advantages of homogeneous and heterogeneous catalysis has proved to be an optimal strategy for developing catalytic systems with high efficiency, selectivity, and recoverability. Supramolecular metal-organic cages (MOCs), assembled by the coordination of metal ions with organic linkers into discrete molecules, have performed solvent processability due to their tunable packing modes, endowing them with the potential to act as homogeneous or heterogeneous catalysts in different solvent systems. Here, the design and synthesis of a series of stable {Cu3} cluster-based tetrahedral MOCs with varied packing structures are reported. These MOCs, as homogeneous catalysts, not only show high catalytic activity and selectivity regardless of substrate size during the CO2 cycloaddition reaction, but also can be easily recovered from the reaction media through separating products and co-catalysts by one-step work-up. This is because that these MOCs have varied solubilities in different solvents due to the tunable packing of MOCs in the solid state. Moreover, the entire catalytic reaction system is very clean, and the purity of cyclic carbonates is as high as 97% without further purification. This work provides a unique strategy for developing novel supramolecular catalysts that can be used for homogeneous catalysis and recycled in a heterogeneous manner.

Functionalization of Magnetic UiO-66-NH2 with a Chiral Cu(l-proline)2 Complex as a Hybrid Asymmetric Catalyst for CO2 Conversion into Cyclic Carbonates

In this study, a chiral [Cu(l-proline)2] complex-modified Fe3O4@SiO2@UiO-66-NH2(Zr) metal-organic framework [Fe3O4@SiO2@UiO-66-NH-Cu(l-proline)2] via multifunctionalization strategies was designed and synthesized. One simple approach to chiralize an achiral MOF-structure that cannot be directly chiralized using a chiral secondary agent like 4-hydroxy-l-proline. Therefore, this chiral catalyst was synthesized with a simple and multistep method. Accordingly, Fe3O4@SiO2@UiO-66-NH2 has been synthesized via Fe3O4 modification with tetraethyl orthosilicate and subsequently with ZrCl4 and 2-aminoterephthalic acid. The presence of the silica layer helps to stabilize the Fe3O4 core, while the bonding between Zr4+ and the -OH groups in the silica layer promotes the development of Zr-MOFs on the Fe3O4 surface, and then the surfaces of the synthesized magnetic MOFs composite are functionalized with 1,2-dichloroethane and Cu(II) complex with 4-hydroxy-l-proline, [Cu(l-proline)2] to afford the magnetically chiral nanocatalyst. Multiple techniques were employed to characterize this magnetically chiral nanocatalyst such as Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDX), powder X-ray diffraction (PXRD), circular dichroism (CD), inductively coupled plasma (ICP), thermogravimetric analysis (TGA), vibrating-sample magnetometry (VSM), and Brunauer-Emmett-Teller (BET) analyses. Moreover, a magnetically chiral nanocatalyst shows the asymmetric CO2 fixation reaction under solvent-free conditions at 80 °C and in ethanol under reflux conditions with up to 99 and 98% ee, respectively. Furthermore, the reaction mechanism was illustrated concerning the total energy of the reactant, intermediates and product, and the structural parameters were analyzed.

Catalytic Domino Three-Component Synthesis of Functionalized Heterocycles from Carbon Dioxide

A catalytic domino, three-component reaction has been developed for the transformation of carbon dioxide into functionalized six-membered cyclic carbonates. The catalytic process combines an initial carboxylative cyclization of β-epoxy alcohols followed by an oxa-Michael reaction affording an unparalleled scope of heterocyclic structures. The wide range of functional groups, including free-alcohols, empowers the access to a range of products including C11-oxo-based bicyclic heterocycles. The versatility of these functionalized carbonates is further complemented by a series of synthetic diversifications. Control experiments are consistent with the first step of this domino process being promoted by a binary Lewis acid/base catalyst, while the second stage only requires catalytic base.

Enhanced cycloaddition between CO2 and epoxide over a bismuth-based metal organic framework due to a synergistic photocatalytic and photothermal effect

The cycloaddition reaction between CO2 and epoxide is an efficient way to convert CO2 into high value-added chemicals. Therefore, it is particularly important to develop efficient catalysts that can catalyze the reaction under mild conditions. In this work, a metal-organic framework (Bi-HHTP, consisting of bismuth (Bi) as metal dots and 2,3,6,7,10,11-hexahydroxy-triphenylene (HHTP) as organic linkers) with zigzagging corrugated topology was successfully synthesized, which shows excellent catalytic activity under visible light irradiation. Various characterizations suggest that the excellent activity is derived from the following reasons: (1) the abundant exposed Bi sites provide Lewis sites for adsorption of epoxides and CO2; (2) the free holes produced over Bi-HHTP under light irradiation which could oxidize epoxide, which consequently facilitateing the subsequent ring-opening reaction; and (3) the existence of synergistic photocatalytic and photothermal effect in Bi-HHTP. This study provides a new avenue of developing bismuth-based metal organic frameworks to promote the efficiency of cycloaddition of CO2 under mild conditions.

Coordination Synergy between Iridium Photosensitizers and Metal Nanoclusters Leading to Enhanced CO2 Cycloaddition under Mild Conditions

The achievement of photocatalytic CO2 and epoxide cycloaddition under mild conditions such as room temperature and atmospheric pressure is important for green chemistry, which can be achieved by developing coordination synergies between catalysts and photosensitizers. In this context, we exploit the use of coordinate bonds to connect pyridine-appended iridium photosensitizers and catalysts for CO2 cycloaddition, which is systematically demonstrated by 1H nuclear magnetic resonance titration and X-ray photoelectron spectroscopic measurements. It is shown that the hybrid Ir(Cltpy)2/Mn2Cd4 photocatalytic system with coordination synergy exhibits excellent catalytic performance (yield ≈ 98.2%), which is 3.75 times higher than that of the comparative Ir(Cltpy-Ph)2/Mn2Cd4 system without coordination synergy (yield ≈ 26.2%), under mild conditions. The coordination between the Mn2Cd4 catalyst and the Ir(Cltpy)2 photosensitizer enhances the light absorption and photoresponse properties of the Mn2Cd4 catalyst. This has been confirmed through transient photocurrent, electrochemical impedance, and electron paramagnetic tests. Consequently, the efficiency of cycloaddition was enhanced by utilizing mild conditions.

Photoelectrocatalytic Utilization of CO2 : A Big Show of Si-based Photoelectrodes

CO2 is a greenhouse gas that contributes to environmental deterioration; however, it can also be utilized as an abundant C1 resource for the production of valuable chemicals. Solar-driven photoelectrocatalytic (PEC) CO2 utilization represents an advanced technology for the resourcing of CO2 . The key to achieving PEC CO2 utilization lies in high-performance semiconductor photoelectrodes. Si-based photoelectrodes have attracted increasing attention in the field of PEC CO2 utilization due to their suitable band gap (1.1 eV), high carrier mobility, low cost, and abundance on Earth. There are two pathways to PEC CO2 utilization using Si-based photoelectrodes: direct reduction of CO2 into small molecule fuels and chemicals, and fixation of CO2 with organic substrates to generate high-value chemicals. The efficiency and product selectivity of PEC CO2 utilization depends on the structures of the photoelectrodes as well as the composition, morphology, and size of the catalysts. In recent years, significant and influential progress has been made in utilizing Si-based photoelectrodes for PEC CO2 utilization. This review summarizes the latest research achievements in Si-based PEC CO2 utilization, with a particular emphasis on the mechanistic understanding of CO2 reduction and fixation, which will inspire future developments in this field.

Engineering the Sulfide Semiconductor/Photoinactive-MOF Heterostructure with a Hollow Cuboctahedral Structure to Enhance Photocatalytic CO2-Epoxide-Cycloaddition Efficiency

Providing efficient electronic transport channels has always been a promising strategy to mitigate the recombination of photogenerated charge carriers. In this study, a heterostructure composed of a semiconductor/photoinactive-metal-organic framework (MOF) was constructed to provide innovative channels for electronic transport. Prepared using a previously reported method ( Angew. Chem., Int. Ed. 2016, 55, 15301-15305) with slight modifications to temperature and reaction time, the CuS@HKUST-1 hollow cuboctahedron was synthesized. The CuS@HKUST-1 heterostructure possessed a well-defined cuboctahedral morphology with a uniform size of about 500 nm and a hollow structure with a thickness of around 50 nm. The CuS nanoparticles were uniformly distributed on the HKUST-1 shell. Structural characterization in cooperation with density functional theory (DFT) calculations revealed that CuS can effectively transfer photogenerated electrons to HKUST-1. CuS@HKUST-1 hollow cuboctahedrons were first introduced to the photocatalytic cycloaddition reaction of CO2 with epoxides, demonstrating excellent photocatalytic activity and stability at mild conditions (room temperature, solvent-free, and 1 atm CO2 pressure). The high photocatalytic performance of the CuS@HKUST-1 hollow cuboctahedron could be attributed to (1) the unique hollow cuboctahedron morphology, which provided a large specific surface area (693.1 m2/g) and facilitated the diffusion and transfer of reactants and products; and (2) CuS@HKUST-1 providing electronic transport channels from CuS to HKUST-1, which could enhance the adsorption and activation of CO2. Cu2+ carrying surplus electrons can activate CO2 to CO2-. The charge separation and transfer in the photocatalytic process can also be effectively promoted. This work provides a cost-effective and environmentally friendly approach for CO2 utilization reactions under ambient conditions, addressing the critical issue of rising atmospheric CO2 levels.

Metal-Organic-Framework-Derived Bromine and Nitrogen Dual-Doped Porous Carbon for CO2 Photocycloaddition Reaction

The cycloaddition of CO2 with epoxides driven by light irradiation is an intriguing approach to preparing cyclic carbonates. However, it remains a great challenge to achieve high photocatalytic efficiency in the absence of a cocatalyst. Herein, we explored a metal-organic-framework (MOF)-templated pyrolysis strategy to prepare uniform bromine ions/nitrogen-codoped carbon materials (Br-CN) as low-cost photocatalysts for CO2 cycloaddition. The optimal catalyst Br-CN-1-550 can be used as a photocatalyst to catalyze CO2 cycloaddition, remarkably reducing the energy consumption. As a result of its benefits of high photothermal efficiency and rich nucleophilic sites (Br ions), BN-CN-1-550 affords a 9 times higher yield of 4-(chloromethyl)-1,3-dioxolan-2-one than that of the ZIF-8-derived CN under cocatalyst-free conditions and light irradiation (300 mW·cm-2 full-spectrum irradiation, 10 h). This strategy provides a cost-effective way to obtain cyclic carbonate under cocatalyst-free conditions.

Ligand-assisted formation of mesoporous Zn-N-C to realize superior catalytic activity in solvent-free CO2 cycloaddition reactions

Mesoporous nitrogen-doped carbon-anchored single atom Zn was synthesized through etching of ZIF-8 with 1,10-phenanthroline and subsequent pyrolysis based on the Kirkendall effect. The abundant pores and increased surface area promote CO2 adsorption and mass transfer, thus significantly improving the catalytic activity in solvent-free cycloaddition of epoxides with CO2.

Enhancing the photocatalytic performance of a rutile unit featuring a titanium-oxide cluster by Pb2+ doping

Titanium-oxide clusters (TOCs) are well-defined molecular models for TiO2 materials and provide the opportunity to study the structure-activity relationships of TiO2. Here, we report a new Pb-doped TOC, Ti12Pb2, which resembles a two-layer decker of the {TiTi6} structural units of rutile TiO2 with two Ti4+ ions replaced by two Pb2+ ions. Its electronic structure, photoresponse, and photocatalytic performances were investigated and compared with those of the Ti14 cluster, which is isostructural to Ti12Pb2. Our results indicate that Pb2+ does not affect the electronic structure, but it greatly enhances the photocatalytic activity by improving the charge-separation and interfacial charge-transfer properties of the TOC. The successful synthesis of Ti12Pb2 highlights the roles of closed-shell heterometal ions in the construction of new TOCs. Our mechanism may be an inspiration for understanding the structure-activity relationships of closed-shell heterometal-doped TiO2.

Cobalt porphyrin-based hypercrosslinked ionic polymers as biomimetic nanoreactors for CO2 conversion to cyclic carbonates

A simple and low-cost approach to construct one type of cobalt porphyrin-based hypercrosslinked ionic polymer with high specific surface areas, densely located ionic groups and highly dispersed cobalt sites has been demonstrated, which act as bifunctional catalysts for the solvent-additive-free conversion of CO2 into cyclic carbonates with outstanding biomimetic catalytic performance and good recyclability.

A cryptand-like Ti-coordination compound with visible-light photocatalytic activity in CO2 storage

A cryptand-like Ti-coordination compound, namely Ti12Cs, comprising two Ti6-salicylate cages and hosting two Cs+ ions, was synthesized by the solvothermal method. It exhibits strong visible-light absorption with an absorption band edge of 652 nm, attributed to the electron transition from salicylate ligands to Ti ions. Electrochemical impedance, visible-light transient photocurrent response, and photoluminescence spectra confirm that Ti12Cs has excellent visible-light response and charge-separation properties. Ti12Cs can be used as a heterogeneous and recyclable photocatalyst for CO2/epoxide cycloaddition, with high utilization efficiency of visible-light under mild conditions. The mechanism investigation points to a synergistic effect of photocatalysis and Lewis acid catalysis.

Two in one: aluminum porphyrin-based porous organic polymers containing symmetrical quaternary phosphonium salts for catalytic conversion of CO2 into cyclic carbonates

Based on the double activation models of epoxides, the design and synthesis of ionic porous organic polymers (iPOPs) is considered to be very attractive and promising but has remained a great challenge in recent decades owing to electrostatic interactions between charged groups. In this contribution, we developed a two-in-one strategy to fabricate metalloporphyrin-based iPOPs with unique nanostructures (named AlPor-QP@POP), which are composed of aluminum porphyrin units and three-dimensional quaternary phosphonium salts that work synergistically in the cycloaddition of CO2 with epoxides under mild conditions. The high symmetry of two monomers allows them to possess similar reactivity ratios and thus endows AlPor-QP@POP with densely located active sites, a large surface area and good CO2 capture capacity. More importantly, bifunctional AlPor-QP@POP has enormous potential to produce cyclic carbonates with simulated flue gas under ambient conditions. Moreover, AlPor-QP@POP can be readily recycled and efficiently reused more than ten times without an obvious decrease in catalytic activity. Finally, kinetic investigations and a comparative study have been conducted to understand the possible mechanism of CO2 catalytic cycloaddition.

Recyclable N-Heterocyclic Carbene Porous Coordination Polymers with Two Distinct Metal Sites for Transformation of CO2 to Cyclic Carbonates

Single-component catalysts with integrated multiple reactive centers could work in concert to achieve enhanced activity tailored for specific catalytic reactions, but they remain underdeveloped. Herein, we report the construction of heterogeneous bimetallic porous coordination polymers (PCPs) containing both porphyrin and N-heterocyclic carbene (NHC) metal sites via the coordinative assembly of the NHC functionalities. Three heterobimetallic PCPs (TIPP-Zn-Pd, TIPP-Cu-Pd and TIPP-Ni-Pd) have been prepared to verify this facile synthetic strategy for the first time. In order to establish a cooperative action toward the catalytic CO2 cycloaddition with epoxides, an additional tetraalkylammonium bromide functionality has also been incorporated into these polymeric structures through the N-substituent of the NHC moieties. The resulting heterogeneous bimetallic catalyst TIPP-Zn-Pd exhibits the best catalytic performance in CO2 cycloaddition with styrene oxide (SO) under solvent-free conditions at atmospheric pressure and is applicable to a wide range of epoxides. More importantly, TIPP-Zn-Pd works smoothly and is recyclable in the absence of a cocatalyst under 1.0 MPa of CO2 at 60 °C. This indicates that TIPP-Zn-Pd is quite competitive with the reported heterogeneous catalysts, which typically require a high reaction temperature above 100 °C under cocatalyst-free conditions. Thus, this work provides a new approach to design heterogeneous bimetallic PCP catalysts for high-performance CO2 fixation under mild reaction conditions.

Tetracosanuclear nickel complexes as effective catalysts for cycloaddition of carbon dioxide with epoxides

A series of well-defined tetracosanuclear nickel complexes 3-7 facilely produced by one-pot synthesis were active catalysts for cycloaddition of CO2 and cyclohexene oxide (CHO). These nickel complexes were doughnut-like supramolecular coordination complexes involving eight repeating units, and each of them contains one Schiff base ligand and three nickel(II) ions. Notably, the 24-nuclear nickel cluster complex 3 in combination with nucleophilic additives was the most efficient catalyst to mediate CO2 coupling with CHO to generate CO2-based cis-cyclohexene carbonates. In addition to CO2/CHO cycloaddition, complex 3 was also found to effectively couple CO2 with other alicyclic epoxides, generating the corresponding cyclic carbonates. Additionally, kinetic investigations for CO2 cycloaddition of CHO using 3 were reported.

Fabrication of Metal-Organic Framework-Based Mixed-Matrix Membranes by "Soft Spray" Technique

Metal-organic framework (MOF)-based mixed-matrix membranes (MMMs) represent a class of composite membranes that seamlessly integrate the properties of MOF fillers and polymer matrix into a hybrid system and have been widely used in countless advanced technologies. However, there remains a need for scalable and simple manufacturing techniques that can fabricate a MOF-based MMM with uniform dispersion. Herein, a series of MMMs with well-dispersed MOFs are constructed by a soft spray technique. In brief, by uniformly spraying metal ions onto the surface of a mixed solution containing polyvinylpyrrolidone (PVP) and organic ligands, a free-standing MMM is synthesized at the miscible liquid-liquid interface, facilitated by the dual function of metal ions. Moreover, soft spray technology can also introduce multifunctional materials into the MMM to customize performance. We have successfully introduced carbon black into a MOF-based MMM by soft spray, resulting in MMMs with excellent photothermal effects. The resulted MOF-based MMM exhibits favorable catalytic performance in the condensation reaction of benzaldehyde with primary amines, and the MOF-based MMM modified with carbon black significantly boosts the endothermic CO2 conversion. The work opens a new avenue for the development of MOF-based MMMs with a promising future.

Hydroxyl and amino dual-functionalized core-shell molecular sieves featuring hydrogen bond donor groups for efficient CO2 cycloaddition

In CO2 cycloaddition reactions, hydrogen bond donor (HBD) groups are considered environmentally friendly substitutes for metals to promote epoxide ring-opening through interactions with nucleophilic anions. A core-shell structured ILs-based catalyst (mSiO2@MCM-NH2-OH) with dual hydrogen bond donors (-OH and -NH2) was synthesized by copolymerization strategy. Through in-depth characterization, it has been demonstrated that the catalyst (mSiO2@MCM-NH2-OH) possesses multiple catalytic active sites including -OH, -NH2, Br- groups, and the synergistic effect of double HBD groups (-OH and -NH2) and Lewis base (Br-) significantly improved the catalytic activity. Meanwhile, the core-shell structure of the catalyst effectively prevents the loss of active components, which makes the yield remain at about 94 % after 10 cycles. Based on Density Functional Theory (DFT) calculations, a synergistic catalytic mechanism, which involves dual hydrogen-bond donors (-OH and -NH2) and Lewis bases (Br-) was proposed. The cooperative interaction between -OH/-NH2 and Br- reduced the ring-opening barrier of epoxide from 58.6 to 32.0 kcal mol-1 significantly, and thereby facilitated the CO2 cycloaddition reaction.

Diamide-linked imidazolyl Poly(dicationic ionic liquid)s for the conversion of CO2 to cyclic carbonates under ambient pressure

The ionic active centers and hydrogen-bond donors (HBDs) in heterogeneous catalytic materials are highly beneficial for enhancing the interaction between solid-liquid-gas three-phase interfaces and promoting effective fixation of carbon dioxide (CO2). Diamide-linked imidazolyl poly(dicationic ionic liquid)s catalysts PIMDILs (PMAIL-x and PBAIL-2) were synthesized through the copolymerization of diamide-linked imidazolyl dicationic ionic liquids (IMDILs) with divinylbenzene (DVB), which successfully enable the simultaneous construction of high-density and uniformly distributed ionic active centers (2.014-4.883 mmol g-1) and hydrogen-bond donors (HBDs). The as-synthesized PIMDILs present excellent catalytic activity in promoting the cycloaddition of CO2 with epoxides. PMAIL-2 could convert epichlorohydrin (ECH) with a quantitative conversion of 99.8 % (selectivity > 99 %) under ambient pressure. Furthermore, only a decrease in activity of 5 % was observed even after six cycles of recycling. The excellent conversions (>97.3 %) were achieved for various terminal substituted epoxides. The experimental and characterization results reveal that the high-density ionic active centers and amide HBDs can effectively activate the reaction substrates, their synergistic effect plays a crucial role at the catalyst interface. This work is expected to provide some useful insights for the rational construction of heterogeneous catalysts for CO2 conversion.

Halogen Bonding in the Decoration of Secondary Coordination Sphere of Zinc(II) and Cadmium(II) Complexes: Catalytic Application in Cycloaddition Reaction of CO2 with Epoxides

Three new zinc(II) complexes [Zn(H2L3)2(H2O)3] (Zn2), [Zn(H3L2a)(H2O)3]n (Zn3) (H3L2a = 2,4-diiodo-5-(2-(2,4,6-trioxotetrahydropyrimidin-5(2H)-ylidene)hydrazineyl)isophthalate) and [Zn(HL4)(DMF)(H2O)]n (Zn4) were synthesized by the reaction of Zn(II) salts with 5-(2-(2,4-dioxopentan-3-ylidene)hydrazineyl) isophthalic acid (H3L3), 2,4,6-triiodo-5-(2-(2,4,6-trioxotetrahydropyrimidin-5(2H)-ylidene)hydrazineyl) isophthalic acid (H5L2) (in the presence of NH2OH·HCl) and 5-(2-(2,4-dioxopentan-3-ylidene)hydrazineyl)-2,4,6-triiodoisophthalic acid (H3L4), respectively. According to the X-ray structural analysis, the intramolecular resonance-assisted hydrogen bond ring remains intact, with N···O distances of 2.562(5) and 2.573(5) Å in Zn2, 2.603(6) Å in Zn3, and 2.563(8) Å in Zn4. In the crystal packing of Zn3, the cooperation of I···O and I···I types of halogen bonds between tectons leads to a one-dimensional supramolecular polymer, while I···O interactions aggregate 1D chains of coordination polymer Zn4. These new complexes (Zn2, Zn3, and Zn4) and known [Zn(H3L1)(H2O)2]n (Zn1) (H3L1 = 5-(2-(2,4,6-trioxotetrahydropyrimidin-5(2H)-ylidene) hydrazineyl)isophthalate), {[Zn(H3L1)(H2O)3]·3H2O}n (Zn5), [Cd(H3L1)(H2O)2]n (Cd1), {[Cd(HL3)(H2O)2(DMF)]·H2O}n (Cd2), [Cd(H3L3)]n (Cd-3), {[Cd2(μ-H2O)2(μ-H2L4)2(H2L4)2]·2H2O}n (Cd4), and {[Cd(H3L1)(H2O)3]·4H2O}n (Cd5) were tested as catalysts in the cycloaddition reaction of CO2 with epoxides in the presence of tetrabutylammonium halides as the cocatalyst. The halogen-bonded catalyst Zn4 is the most efficient one in the presence of tetrabutylammonium bromide by affording a high yield (85-99%) of cyclic carbonates under solvent-free conditions after 48 h at 40 bar and 80 °C.

Hydrogenation of CO2 by a Bifunctional PC(sp3 )P Iridium(III) Pincer Complex Equipped with Tertiary Amine as a Functional Group

Reversible hydrogen storage in the form of stable and mostly harmless chemical substances such as formic acid (FA) is a cornerstone of a fossil fuels-free economy. In the past, we have reported a primary amine-functionalized bifunctional iridium(III)-PC(sp3 )P pincer complex as a mild and chemoselective catalyst for the additive-free decomposition of neat formic acid. In this manuscript, we report on the successful application of a redesigned complex bearing tertiary amine functionality as a catalyst for mild hydrogenation of CO2 to formic acid. The catalyst demonstrates TON up to 6×104 and TOF up to 1.7×104  h-1 . In addition to the practical value of the catalyst, experimental and computational mechanistic studies provide the rationale for the design of improved next-generation catalysts.

Samarium- and Ytterbium-Grafted Periodic Mesoporous Silica for Carbon Dioxide Capture and Conversion

Immobilized coordination compounds of Lewis acidic metals are powerful catalytic components of systems for the cycloaddition of CO2 to epoxides that do not require sophisticated coordination frameworks to harness the metal center and modulate its activity. Surface organometallic chemistry (SOMC) is a valuable methodology to prepare well-defined and site-isolated surface complexes and coordination compounds on metal oxides, with ligand environments easily adjustable to a targeted catalytic reaction. In this work, the SOMC methodology is applied to prepare SmII, YbII, and SmIII alkoxide surface complexes on periodic mesoporous (organo)silica of distinct pore symmetry/size for application in the CO2 cycloaddition reaction. The surface complexes are readily accessible by the grafting of the bis(trimethylsilyl)amide precursors LnII[N(SiMe3)2]2(THF)2 (Ln = Sm, Yb) and SmIII[N(SiMe3)2]3, followed by ligand exchange with alcohols (ethanol and neopentanol). The use of periodic mesoporous supports led to hybrid materials with relatively high surface areas and pore sizes, affording good performance in CO2 capture and in the cycloaddition of CO2 to epoxides under mild conditions (60-80 °C, 1-10 bar). In terms of catalytic performance, recyclability, and low amount of added nucleophile TBAX (X = Br, I), the most active materials prepared in this work compare well to a variety of previously reported SOMC-derived surface complexes and to other heterogeneous Lewis acids displaying more elaborate ligand environments.

Concerted Hydrosilylation Catalysis by Silica-Immobilized Cyclic Carbonates and Surface Silanols

Developing a method for creating a novel catalysis of organic molecules is essential because of the growing interest in organocatalysis. In this study, we found that cyclic carbonates immobilized on a nonporous or mesoporous silica support showed catalytic activity for hydrosilylation, which was not observed for the free cyclic carbonates, silica supports, or their physical mixture. Analysis of the effects of linker lengths and pore sizes on the catalytic activity and carbonate C=O stretching frequency revealed that the proximity of carbonates and surface silanols was crucial for synergistic hydrosilylation catalysis. A carbonate and silanol concertedly activated the silane and aldehyde for efficient hydride transfer. Density functional theory calculations on a model reaction system demonstrated that both the carbonate and silanol contributed to the stabilization of the transition state of hydride transfer, which resulted in a reasonable barrier height of 16.8 kcal mol-1. Furthermore, SiO2/carbonate(C4) enabled the hydrosilylation of an aldehyde with an amino group without catalyst poisoning, owing to the weak acidity of surface silanols, in sharp contrast to previously developed acid catalysts. This study demonstrates that immobilization on a solid support can convert inactive organic molecules into active and heterogeneous organocatalysts.

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.

CO2 Cycloaddition under Ambient Conditions over Cu-Fe Bimetallic Metal-Organic Frameworks

Carbon dioxide cycloaddition into fine chemicals is prospective technology to solve energy crisis and environmental issues. However, high temperature and pressure are usually required in the conventional cycloaddition reactions of CO2 with epoxides. Moreover, metal active sites play a vital role in the CO2 cycloaddition, but it is still unclear. Herein, we select the isostructural MOF-919-Cu-Fe and MOF-919-Cu-Al as models to promote the performance and clarify the effects of metal type on the CO2 cycloaddition. The MOF-919-Cu-Fe with exposed Fe and Cu Lewis acid sites reaches the CO2 cycloaddition with over 99.9% conversion and over 99.9% selectivity at room temperature and a 1 bar CO2 atmosphere, 3.0- and 52.6-fold higher than those of the MOF-919-Cu-Al with Al and Cu sites (33.8%) and the 1H-pyrazole-4-carboxylic acid, Fe, and Cu mixed system (1.9%), respectively. The proposed mechanism demonstrated that the exposed Fe3+ sites facilitate the ring opening of epoxide and CO2 activation to boost the CO2 cycloaddition reaction. This work provides a new insight to tune the catalytic sites of MOFs to achieve high performance for CO2 fixation.

Cu-catalyzed carboxylation of organoboronic acid pinacol esters with CO2

Chemical fixation of CO2 has received much attention. In particular, catalytic C-C bond formation with CO2 giving carboxylic acids is of great significance. Among the CO2 fixation methods, multiple carboxylation is one of the challenging subjects. Here we investigated the Cu-catalyzed carboxylation of a variety of boronic acid pinacol esters (C(sp2)-, C(sp3)-, and C(sp)-B compounds) with CO2, which efficiently provided the corresponding products, including aryl, alkenyl, alkyl, and alkynyl carboxylic acids. This carboxylation was also applicable to multiple CO2 fixation giving di- and tri-carboxylic acids under robust reaction conditions (totally 29 examples).

A robust and porous titanium metal-organic framework for gas adsorption, CO2 capture and conversion

A robust and porous titanium metal-organic framework (Ti-MOF; LCU-402) has been hydrothermally synthesized through combining a tetranuclear Ti2Ca2(μ3-O)2(μ2-H2O)1.3(H2O)4(O2C-)8 cluster and a tritopic 1,3,5-benzene(tris)benzoic (BTB) ligand. LCU-402 shows remarkable stability and permanent porosity for CO2, CH4, C2H2, C2H4, and C2H6 gas adsorption. Moreover, LCU-402 as a heterogeneous catalyst can smoothly convert CO2 under a simulated flue atmosphere into organic carbonate molecules by cycloaddition reactions of CO2 and epoxides, indicating that LCU-402 might be a promising catalyst candidate in practical applications. We are confident that the identification of a persistent titanium-oxo building unit would accelerate the development of new porous Ti-MOF materials.

Nickel(II) Complexes of Tripodal Ligands as Catalysts for Fixation of Atmospheric CO2 as Organic Carbonates

The fixation of atmospheric CO₂ into value-added products is a promising methodology. A series of novel nickel(II) complexes of the type [Ni(L)(CH₃ CN)₂ ](BPh₄ )₂ 1-5, where L=N,N-bis(2-pyridylmethyl)-N', N'-dimethylpropane-1,3-diamine (L1), N,N-dimethyl-N'-(2-(pyridin-2-yl)ethyl)-N'-(pyridin-2-ylmethyl) propane-1,3-diamine (L2), N,N-bis((4-methoxy-3,5-dimethylpyridin-2-ylmethyl)-N',N'-dimethylpropane-1,3-diamine (L3), N-(2-(dimethylamino) benzyl)-N',N'-dimethyl-N-(pyridin-2-ylmethyl) propane-1,3-diamine (L4) and N,N-bis(2-(dimethylamino)benzyl)-N', N'-dimethylpropane-1,3-diamine (L5) have been synthesized and characterized as the catalysts for the conversion of atmospheric CO₂ into organic cyclic carbonates. The single-crystal X-ray structure of 2 was determined and exhibited distorted octahedral coordination geometry with cis-α configuration. The complexes have been used as a catalyst for converting CO₂ and epoxides into five-membered cyclic carbonates under 1 atmospheric (atm) pressure at room temperature in the presence of Bu₄ NBr. The catalyst containing electron-releasing -Me and -OMe groups afforded the maximum yield of cyclic carbonates, 34% (TON, 680) under 1 atm air. It was drastically enhanced to 89% (TON, 1780) under pure CO₂ gas at 1 atm. It is the highest catalytic efficiency known for CO₂ fixation using nickel-based catalysts at room temperature and 1 atm pressure. The electronic and steric factors of the ligands strongly influence the catalytic efficiency. Furthermore, all the catalysts can convert a wide range of epoxides (ten examples) into corresponding cyclic carbonate with excellent selectivity (>99%) under this mild condition.

Cobalt complexes with α-amino acid ligands catalyze the incorporation of CO2 into cyclic carbonates

Arguably one of the largest research areas involving carbon dioxide (CO₂) fixation is the coupling of CO₂ to epoxides to form cyclic carbonates and polycarbonates. In this sense, there is an ever-increasing demand for the development of higher-performing catalytic systems that could counterbalance sustainability and energy efficiency in the production of cyclic carbonates. The use of abundant first-row transition metals combined with naturally occurring amino acids may be an ideal catalytic platform to fulfill this demand. Nevertheless, detailed information on the interactions between metal centers and natural products as catalysts in this transformation is lacking. Here a series of Co(III) amino acid catalysts operating in a binary system showed outstanding performance for the coupling reaction of epoxides and CO₂. Nine new complexes of the type trans(N)-[Co(aa)₂(bipy)]Cl (aa: ala, asp, lys, met, phe, pro, ser, tyr, and val) were used to explore the structure-activity relationship influenced by the complex outer coordination sphere, and its effect on the catalytic activity in the coupling reaction of CO₂ and epoxides.

Silver-Mediated Cascade Synthesis of Functionalized 1,4-Dihydro-2H-benzo-1,3-oxazin-2-ones from Carbon Dioxide

A conceptually novel catalytic domino approach is presented for the synthesis of highly functional 1,4-dihydro-2H-1,3-benzoxazine-2-one derivatives. Key to the chemoselectivity is a proper design of the precursor to override thermodynamically favored parasitic cyclization processes and empower the formation of the desired product through Thorpe-Ingold effects. The synthetic diversity of these CO₂ -based heterocycles is further demonstrated, and the isolation of a reaction intermediate supports an unusual ring-expansion sequence from an α-alkylidene, five-membered cyclic carbonate to a six-membered cyclic carbamate by N-induced isomerization.

Green Energy by Hydrogen Production from Water Splitting, Water Oxidation Catalysis and Acceptorless Dehydrogenative Coupling

In this review, we want to explain how the burning of fossil fuels is pushing us towards green energy. Actually, for a long time, we have believed that everything is profitable, that resources are unlimited and there are no consequences. However, the reality is often disappointing. The use of non-renewable resources, the excessive waste production and the abandonment of the task of recycling has created a fragile thread that, once broken, may never restore itself. Metaphors aside, we are talking about our planet, the Earth, and its unique ability to host life, including ourselves. Our world has its balance; when the wind erodes a mountain, a beach appears, or when a fire devastates an area, eventually new life emerges from the ashes. However, humans have been distorting this balance for decades. Our evolving way of living has increased the number of resources that each person consumes, whether food, shelter, or energy; we have overworked everything to exhaustion. Scientists worldwide have already said actively and passively that we are facing one of the biggest problems ever: climate change. This is unsustainable and we must try to revert it, or, if we are too late, slow it down as much as possible. To make this happen, there are many possible methods. In this review, we investigate catalysts for using water as an energy source, or, instead of water, alcohols. On the other hand, the recycling of gases such as CO2 and N2O is also addressed, but we also observe non-catalytic means of generating energy through solar cell production.

Aluminum Porphyrin-Based Ionic Porous Aromatic Frameworks Having High Surface Areas and Highly Dispersed Dual-Function Sites for Boosting the Catalytic Conversion of CO2 into Cyclic Carbonates

Multifunctionalization of porous organic polymers toward synergistic CO₂ catalysis has drawn much attention in recent decades, but it still faces many challenges. Herein, we develop a facile, simple, and efficient strategy to obtain a series of aluminum porphyrin-based ionic porous aromatic frameworks (iPAFs), which are considered excellent bifunctional catalysts for converting CO₂ into cyclic carbonates without any cocatalyst under mild and solvent-free conditions. By increasing the amounts of tetraphenylmethane fragments in the porphyrin backbones, the cooperative effect between Lewis acidic metal centers and nucleophilic ionic sites has been enhanced and then the significant improvement of catalytic activity can be achieved owing to the high surface areas (up to 719 m²·g^-1), abundant hierarchical micro-mesopores, and prominent CO₂ adsorption capacities (up to 1.8 mmol·g^-1 at 273 K) as well as highly dispersed dual-function sites. More fascinatingly, high-active AlPor-iPAF-3 enables CO₂ cycloaddition to perform with diluted CO₂ (15% CO₂ in 85% N₂, v/v) or under ambient conditions. Therefore, this postsynthetic modification procedure in combination with the framework dilution strategy provides a new approach to fabricating high-surface-area metalloporphyrin-based porous ionic polymers (PIPs) with hierarchical structures, which is conducive to improving the accessibility of multiple active sites around substrates.

Fabrication of Hierarchical, Porous, Bimetallic, Zeolitic Imidazolate Frameworks with the Incorporation of Square Planar Pd and Its Catalytic Application

Bimetallic zeolitic imidazolate frameworks (ZIFs) containing two different metal ions can exhibit superior performances when applied in heterogeneous catalysis. Herein, we present a facile one-pot synthesis method for PdCo-ZIFs with various Pd/Co ratios, where Pd(II) ions are successfully incorporated into the Co node sites of the ZIF structure. The local structure of the bimetallic ZIFs was comprehensively investigated by pore-structure, X-ray absorption fine structure, and in situ CO adsorption Fourier transform infrared analyses. The results demonstrated that the framework comprises different coordination geometries of Co (tetrahedral) and Pd (square planar) ions connected by the benzimidazolate ligand. Notably, the inherently nonporous, 2D Co-ZIF structure was transformed into a hierarchical porous structure, and the PdCo-ZIFs exhibited a significantly increased concentration of defects and distorted Co sites. Based on these results, the catalytic performances of the synthesized ZIFs in the cycloaddition of CO2 to epoxides were evaluated under a cocatalyst and solvent-free conditions. The PdCo-ZIFs exhibited significantly higher catalytic activity (maximum turnover frequency, TOF = 2501 h-1) than Co-ZIF (TOF = 65 h-1) and Pd-ZIF (no activity), which revealed that the undercoordinated Co sites with distorted structure are the active sites rather than the incorporated Pd ions. This study provides a facile one-pot method for synthesizing bimetallic ZIFs with mixed-coordination modes, hierarchical porous structures, and modified defect concentrations, which would expand the library of structurally diverse bimetallic ZIFs toward various applications.

Efficiency in Carbon Dioxide Fixation into Cyclic Carbonates: Operating Bifunctional Polyhydroxylated Pyridinium Organocatalysts in Segmented Flow Conditions

Novel polyhydroxylated ammonium, imidazolium, and pyridinium salt organocatalysts were prepared through N-alkylation sequences using glycidol as the key precursor. The most active pyridinium iodide catalyst effectively promoted the carbonation of a set of terminal epoxides (80 to >95% yields) at a low catalyst loading (5 mol%), ambient pressure of CO₂, and moderate temperature (75 °C) in batch operations, also demonstrating high recyclability and simple downstream separation from the reaction mixture. Moving from batch to segmented flow conditions with the operation of thermostated (75 °C) and pressurized (8.5 atm) home-made reactors significantly reduced the process time (from hours to seconds), increasing the process productivity up to 20.1 mmol_(product) h^-1 mmol_(cat)^-1, a value ~17 times higher than that in batch mode.

Organocatalytic Polymers from Affordable and Readily Available Building Blocks for the Cycloaddition of CO2 to Epoxides

The catalytic cycloaddition of CO₂ to epoxides to afford cyclic carbonates as useful monomers, intermediates, solvents, and additives is a continuously growing field of investigation as a way to carry out the atom-economic conversion of CO₂ to value-added products. Metal-free organocatalytic compounds are attractive systems among various catalysts for such transformations because they are inexpensive, nontoxic, and readily available. Herein, we highlight and discuss key advances in the development of polymer-based organocatalytic materials that match these requirements of affordability and availability by considering their synthetic routes, the monomers, and the supports employed. The discussion is organized according to the number (monofunctional versus bifunctional materials) and type of catalytically active moieties, including both halide-based and halide-free systems. Two general synthetic approaches are identified based on the postsynthetic functionalization of polymeric supports or the copolymerization of monomers bearing catalytically active moieties. After a review of the material syntheses and catalytic activities, the chemical and structural features affecting catalytic performance are discussed. Based on such analysis, some strategies for the future design of affordable and readily available polymer-based organocatalysts with enhanced catalytic activity under mild conditions are considered.

High-Surface-Area Metalloporphyrin-Based Porous Ionic Polymers by the Direct Condensation Strategy for Enhanced CO2 Capture and Catalytic Conversion into Cyclic Carbonates

Metalloporphyrin-based porous organic polymers (POPs) that behave as advanced biomimetic nanoreactors have drawn continuous attention for heterogeneous CO₂ catalysis in the past decades. Inspired by the double activation model of epoxides, the design and synthesis of metalloporphyrin-based porous ionic polymers (PIPs) are considered as one of the most promising approaches for converting CO₂ to cyclic carbonates under cocatalyst- and solvent-free conditions. To overcome the obstacle of poor reaction activity of ionic monomers or highly irregular stacking architecture, in this paper, we have proposed and demonstrated a modular bottom-up approach for constructing a series of high-surface-area metalloporphyrin-based PIPs in high yields by the direct condensation strategy, thus boosting the close contact of multiple active sites and achieving the enhanced CO₂ capture and catalytic conversion into cyclic carbonates with high turnover frequencies under mild conditions. These recyclable aluminum-porphyrin-based PIPs are featured with high surface areas, prominent CO₂ adsorptive capacities, rigid porphyrin skeletons, and flexible ionic pendants, as well as the matched amounts and spatial positions of metal centers and ionic sites, in which is demonstrated to be one of the quite competitive catalysts. Therefore, this strategy of introducing ionic components into the porphyrin frameworks as flexible side chains rather than main chains and adjusting the reactivity ratios of comonomers by structure-oriented methods, provides feasible guidance for the multifunctionalization of metalloporphyrin-based POPs, thereby increasing the accessibility of multiple active sites and improving their synergistic catalytic behavior.

Catalytic Properties of High Nitrogen Content Carbonaceous Materials

The influence of structural modifications on the catalytic activity of carbon materials is poorly understood. A collection of carbonaceous materials with different pore networks and high nitrogen content was characterized and used to catalyze four reactions to deduce structure-activity relationships. The CO₂ cycloaddition and Knoevenagel reaction depend on Lewis basic sites (electron-rich nitrogen species). The absence of large conjugated carbon domains resulting from the introduction of large amounts of nitrogen in the carbon network is responsible for poor redox activity, as observed through the catalytic reduction of nitrobenzene with hydrazine and the catalytic oxidation of 3,3',5,5'-tetramethylbenzidine using hydroperoxide. The material with the highest activity towards Lewis acid catalysis (in the hydrolysis of (dimethoxymethyl)benzene to benzaldehyde) is the most effective for small molecule activation and presents the highest concentration of electron-poor nitrogen species.