Delocalized electron effect on single metal sites in ultrathin conjugated microporous polymer nanosheets for boosting CO2 cycloaddition

Advancements and Future Prospectives of Single-Atom Catalysts in CO2 Cycloaddition to Carbonates

The conversion of CO2 and epoxides into cyclic carbonates represents a promising strategy for CO2 utilization and valorization, with applications spanning pharmaceuticals, agrochemicals, polymer manufacturing, and energy storage. This concept article provides a concise perspective on the advancements in CO2 cycloaddition reactions catalyzed by single-atom catalysts (SACs), encompassing photocatalysis, thermocatalysis, and photothermal catalysis. Despite significant progress in the field, challenges such as limited catalytic activity and low stability of SACs under reaction conditions remain significant obstacles to industrial implementation. Mechanistic insights into active species are emphasized to enable the rational design and optimization of catalytic systems. In addition, key industrial challenges, such as the elimination of co-catalysts, scalability limitations, and the development of cost-effective production methods, are critically examined. By bridging fundamental research and practical applications, this concept article seeks to guide future advancements in the sustainable production of cyclic carbonates through CO2 cycloaddition.

NH2-MIL-101(Fe) Nanocrystals Synthesized by the Ionic Liquid-Ethanol Interface for Efficient CO2 Fixation at Mild Conditions

In this study, an ionic liquid-ethanol interface strategy is proposed to synthesize NH2-MIL-101(Fe) nanocrystals at room temperature. The as-synthesized nanocrystals exhibit small crystal sizes, abundant ligand defects, and unsaturated metal sites. The NH2-MIL-101(Fe) nanocrystals present superior catalytic activity for the cycloaddition reaction of CO2 at mild conditions (room temperature and 1 bar CO2 pressure), much higher than the NH2-MIL-101 (Fe) microcrystals synthesized by the conventional solvothermal method. The conversion of propylene oxide catalyzed by NH2-MIL-101(Fe) nanocrystals achieves a 99% yield within 2.5 h, accompanied by a generation rate of carbonate production per gram of catalyst (Rcarbonate) of 52.8 mmol g-1 h-1. In contrast, the system employing NH2-MIL-101(Fe) microcrystals shows a much lower yield of 19.28% and a generation rate (Rcarbonate) of 10.28 mmol g-1 h-1. The NH2-MIL-101(Fe) nanocrystals were further used for the treatment of simulated industrial flue gas conditions with a volume ratio of N2/CO2 = 85:15. The results show the efficient conversion of CO2 at ambient temperature and pressure, even at relatively low CO2 concentrations. This work not only offers a facile, low energy-consumed, and environmentally benign interface method for the fabrication of MOF nanostructures but also provides high-performance systems for CO2 fixation and gas separation.

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.

Bifunctional chlorhexidine-based covalent organic polymers for CO2 capture and conversion without a co-catalyst

Two new cobalt/zinc-coordinated bifunctional covalent organic polymers (COP-Co and COP-Zn) based on chlorhexidine are prepared as heterogeneous catalysts for carbon dioxide (CO2) conversion. Due to the Cl- nucleophile and cobalt/zinc Lewis acid sites, COP-Co and COP-Zn can efficiently convert CO2 and epoxides into cyclic carbonates under mild conditions without a co-catalyst.

Symmetry-Broken Steered Delocalization State in a Single-Atom Photocatalyst

Single-atom catalysts (SACs) that feature uniform metal active sites with symmetry configurations hold great promise in photocatalysis, while their catalytic efficiency is often restricted by the insufficient inherent activity. Drawing inspiration from hard-soft acid-base theory, here we propose that the delocalized electronic state of single-atom centers can be selectively modulated by adjusting their coordination symmetry, thereby optimizing the adsorption and activation of the reactant molecules. By taking ceria-based Ru-SAC (Ru-CeO2) as an example, we show that after introducing symmetry breaking, the Ru-CeO2 with an asymmetric Ru-O4 configuration (named P-Ru-CeO2) exhibits highly delocalized electrons with a soft acidic nature, leading to a much higher photocatalytic performance than for pristine Ru-CeO2 and CeO2 counterparts. The corresponding inherent mechanism was systematically investigated by spectroscopy and theoretical studies. This work provides an effective strategy for the design and controllable modulation of atomically dispersed catalysts with symmetry-broken configurations, thereby advancing applications in photocatalysis.

Small Conjugated Organic Ligand-Modified Polyoxometalate-Based Hybrid Materials for Boosting Photocatalytic CO2 Reduction

Photocatalytic carbon dioxide (CO2) reduction to value-added chemicals is a multielectron transfer process, and the crucial step is the synthesis of photocatalysts. The introduction of small conjugated organic ligands can make the catalytic active site of the compound easier to be exposed in the reaction system and fully contact with the substrate, accelerating the photocatalytic reaction process. In this paper, we synthesized two isomorphic compounds, namely, {[Co(mtrz)3·(H2O)2]2·[SiW12O40]}·6H2O (1) and {[Ni(mtrz)3·(H2O)2]2·[SiW12O40]}·6H2O (2) (mtrz = 1-methyl-1,2,4-triazole). We found that compound 1 has a great photocatalytic performance through a series of experiments, with a CO reduction yield of 7364.92 μmol g-1 h-1 and a CO selectivity of 82.5%. Furthermore, the high catalytic activity can be maintained over four cycle experiments. The catalytic mechanism of its photocatalytic system is also elucidated, which provides an idea for realizing efficient catalytic reduction of CO2 to CO.

Atomically Dispersed Metal Catalysts for the Conversion of CO2 into High-Value C2+ Chemicals

The conversion of carbon dioxide (CO2) into value-added chemicals with two or more carbons (C2+) is a promising strategy that cannot only mitigate anthropogenic CO2 emissions but also reduce the excessive dependence on fossil feedstocks. In recent years, atomically dispersed metal catalysts (ADCs), including single-atom catalysts (SACs), dual-atom catalysts (DACs), and single-cluster catalysts (SCCs), emerged as attractive candidates for CO2 fixation reactions due to their unique properties, such as the maximum utilization of active sites, tunable electronic structure, the efficient elucidation of catalytic mechanism, etc. This review provides an overview of significant progress in the synthesis and characterization of ADCs utilized in photocatalytic, electrocatalytic, and thermocatalytic conversion of CO2 toward high-value C2+ compounds. To provide insights for designing efficient ADCs toward the C2+ chemical synthesis originating from CO2, the key factors that influence the catalytic activity and selectivity are highlighted. Finally, the relevant challenges and opportunities are discussed to inspire new ideas for the generation of CO2-based C2+ products over ADCs.

Metal-Semiconductor Heterojunction with Ohmic Contact Realizes Efficient Infrared-Light-Driven Photocatalysis

Efficient utilization of solar energy for photocatalytic applications, particularly in the infrared spectrum, is crucial for addressing environmental challenges and energy scarcity. Herein we present a general strategy for constructing efficient infrared-driven photocatalysts in a metal/semiconductor heterojunction with Ohmic contact, where metals with low work function as the infrared-light absorber and semiconductors with electron storage ability can overcome the unfavorable electron flowback. Taking the NixB/MO2 (M = Ce, Ti, Sn, Ge, Zr, etc.) heterojunction as an example, both experimental and theoretical investigations reveal that the formation of an Ohmic contact facilitates the transfer of hot electrons from NixB to MO2, which are stored by the ion redox pairs for the variable valence character of M. As expected, the heterojunction exhibits remarkable photocatalytic activity under infrared light (λ ≥ 800 nm), as evidenced by the efficient photofixation of CO2 to high-value-added cyclic carbonates. This study offers a general platform for designing infrared-light-driven photocatalysts.

Nitro-functionalization on MIL-53(Fe) for PCMX degradation: Elevating Fenton-like catalytic propelled by abundant reaction sites and iron cycle

To address the issue of excessive residues of 4-chloro-3,5-dimethylphenol (PCMX) in the water environment. In a one-step solvothermal process, iron-based metal-organic frameworks (Fe-MOFs) material MIL-53(Fe) undergoes a synthetic modification strategy. 2-Nitroterephthalic acid as an organic ligand reacted with Fe3+ in a solvothermal process lasting 18 h to yield the nitro-functionalized MIL-53(Fe)-NO2(18h). The objective was to augment the abundance of Fe central unsaturated coordination sites (Fe CUCs) and expedite the Fe(III)/Fe(II) redox cycle, thereby enhancing the heterogeneous Fenton-like treatment capability of pollutants. MIL-53(Fe)-NO2(18h) has excellent hydrogen peroxide (H2O2) catalytic activity and PCMX degradation across a broad pH spectrum (4.0∼8.0). Almost complete removal of PCMX was achieved within 30 min, while pseudo-first-order kinetic rate constants (kobs) increased 4.37 times over MIL-53(Fe). The confirmation of increased Fe CUCs abundance in MIL-53(Fe)-NO2(18h) was achieved through Lewis acidity, oxygen vacancies (OVs) signals, and Fe-O coordination characterization results. Density functional theory (DFT) calculations revealed that Fe CUCs in MIL-53(Fe)-NO2(18h) exhibits heightened affinity for H2O2 adsorption, showcasing stronger charge transfer and enhanced H2O2 dissociation ability. The Fe(III)/Fe(II) redox cycle, a driving force of Fenton-like reactions, was notably improved in the nitro-modified materials. These enhancements significantly expedited the Fenton-like process, resulting in the generation of increased amounts of reactive oxygen species (ROSs), with hydroxyl radicals (OH·) being pivotal components in degradation. The MIL-53(Fe)-NO2(18h)/H2O2 system has demonstrated versatility in treating a variety of emerging contaminants, achieving removal efficiencies exceeding 99.7% for other antibiotics and endocrine disruptors within 60 min. Furthermore, MIL-53(Fe)-NO2(18h) demonstrated outstanding reusability and adaptability in actual water environments. This study introduces a straightforward and environmentally friendly strategy for remediating environmental pollution using Fe-MOF-catalysed heterogeneous Fenton-like technology.

Self-condensation-assisted chemical vapour deposition growth of atomically two-dimensional MOF single-crystals

Two-dimensional metal-organic frameworks (MOFs) have a wide variety of applications in molecular separation and other emerging technologies, including atomically thin electronics. However, due to the inherent fragility and strong interlayer interactions, high-quality MOF crystals of atomic thickness, especially isolated MOF crystal monolayers, have not been easy to prepare. Here, we report the self-condensation-assisted chemical vapour deposition growth of atomically thin MOF single-crystals, yielding monolayer single-crystals of poly[Fe(benzimidazole)2] up to 62 μm in grain sizes. By using transmission electron microscopy and high-resolution atomic force microscopy, high crystallinity and atomic-scale single-crystal structure are verified in the atomically MOF flakes. Moreover, integrating such MOFs with MoS2 to construct ultrathin van der Waals heterostructures is achieved by direct growth of atomically MOF single-crystals onto monolayer MoS2, and enables a highly selective ammonia sensing. These demonstrations signify the great potential of the method in facilitating the development of the fabrication and application of atomically thin MOF crystals.

Ultrathin, Cationic Covalent Organic Nanosheets for Enhanced CO2 Electroreduction to Methanol

Metalloporphyrins and metallophthalocyanines emerge as popular building blocks to develop covalent organic nanosheets (CONs) for CO2 reduction reaction (CO2RR). However, existing CONs predominantly yield CO, posing a challenge in achieving efficient methanol production through multielectron reduction. Here, ultrathin, cationic, and cobalt-phthalocyanine-based CONs (iminium-CONs) are reported for electrochemical CO2-to-CH3OH conversion. The integration of quaternary iminium groups enables the formation of ultrathin morphology with uniformly anchored cobalt active sites, which are pivotal for facilitating rapid multielectron transfer. Moreover, the cationic iminium-CONs exhibit a lower activity for hydrogen evolution side reaction. Consequently, iminium-CONs manifest significantly enhanced selectivity for methanol production, as evidenced by a remarkable 711% and 270% improvement in methanol partial current density (jCH3OH) compared to pristine CoTAPc and neutral imine-CONs, respectively. Under optimized conditions, iminium-CONs deliver a high jCH3OH of 91.7 mA cm-2 at -0.78 V in a flow cell. Further, iminium-CONs achieve a global methanol Faradaic efficiency (FECH3OH) of 54% in a tandem device. Thanks to the single-site feature, the methanol is produced without the concurrent generation of other liquid byproducts. This work underscores the potential of cationic covalent organic nanosheets as a compelling platform for electrochemical six-electron reduction of CO2 to methanol.

Conjugated Microporous Polymers for Catalytic CO2 Conversion

Rising carbon dioxide (CO2) levels in the atmosphere are recognized as a threat to atmospheric stability and life. Although this greenhouse gas is being produced on a large scale, there are solutions to reduction and indeed utilization of the gas. Many of these solutions involve costly or unstable technologies, such as air-sensitive metal-organic frameworks (MOFs) for CO2 capture or "non-green" systems such as amine scrubbing. Conjugated microporous polymers (CMPs) represent a simpler, cheaper, and greener solution to CO2 capture and utilization. They are often easy to synthesize at scale (a one pot reaction in many cases), chemically and thermally stable (especially in comparison with their MOF and covalent organic framework (COF) counterparts, owing to their amorphous nature), and, as a result, cheap to manufacture. Furthermore, their large surface areas, tunable porous frameworks and chemical structures mean they are reported as highly efficient CO2 capture motifs. In addition, they provide a dual pathway to utilize captured CO2 via chemical conversion or electrochemical reduction into industrially valuable products. Recent studies show that all these attractive properties can be realized in metal-free CMPs, presenting a truly green option. The promising results in these two fields of CMP applications are reviewed and explored here.

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.

Pyrolysis Free Out-of-Plane Co-Single Atomic Sites in Porous Organic Photopolymer Stimulates Solar-Powered CO2 Fixation

Herein, a facile strategy is illustrated to develop pyrolysis-free out-of-plane coordinated single atomic sites-based M-POP via a one-pot Friedel Craft acylation route followed by a post-synthetic metalation. The optimized geometry of the Co@BiPy-POP clearly reveals the presence of out-of-plane Co-single atomic sites in the porous backbone. This novel photopolymer Co@BiPy-POP shows extensive π-conjugations followed by impressive light harvesting ability and is utilized for photochemical CO2 fixation to value-added chemicals. A remarkable conversion of styrene epoxide (STE) to styrene carbonate (STC) (≈98%) is obtained under optimized photocatalytic conditions in the existence of promoter tert-butyl ammonium bromide (TBAB). Synchrotron-based X-ray adsorption spectroscopy (XAS) analysis reveals the single atom coordination sites along with the metal (Co) oxidation number of +2.16 in the porous network. Moreover, in situ diffuse reflectance spectroscopy (DRIFTS) and electron paramagnetic resonance (EPR) investigations provide valuable information on the evolution of key reaction intermediates. Comprehensivecomputational analysis also helps to understand the overall mechanistic pathway along with the interaction between the photocatalyst and reactants. Overall, this study presents a new concept of fabricating porous photopolymers based on a pyrolysis-free out-of-plane-coordination strategy and further explores the role of single atomic sites in carrying out feasible CO2 fixation reactions.

Pyrazine-linked Iron-coordinated Tetrapyrrole Conjugated Organic Polymer Catalyst with Spatially Proximate Donor-Acceptor Pairs for Oxygen Reduction in Fuel Cells

Nitrogen-coordinated iron (Fe-N4 ) materials represent the most promising non-noble electrocatalysts for the cathodic oxygen reduction reaction (ORR) of fuel cells. However, molecular-level structure design of Fe-N4 electrocatalyst remains a great challenge. In this study, we develop a novel Fe-N4 conjugated organic polymer (COP) electrocatalyst, which allows for precise design of the Fe-N4 structure, leading to unprecedented ORR performance. At the molecular level, we have successfully organized spatially proximate iron-pyrrole/pyrazine (FePr/Pz) pairs into fully conjugated polymer networks, which in turn endows FePr sites with firmly covalent-bonded matrix, strong d-π electron coupling and highly dense distribution. The resulting pyrazine-linked iron-coordinated tetrapyrrole (Pz-FeTPr) COP electrocatalyst exhibits superior performance compared to most ORR electrocatalysts, with a half-wave potential of 0.933 V and negligible activity decay after 40,000 cycles. When used as the cathode electrocatalyst in a hydroxide exchange membrane fuel cell, the Pz-FeTPr COP achieves a peak power density of ≈210 mW cm-2 . We anticipate the COP based Fe-N4 catalyst design could be an effective strategy to develop high-performance catalyst for facilitating the progress of fuel cells.

Poly(ionic liquid)s for Photo-Driven CO2 Cycloaddition: Electron Donor-Acceptor Segments Matter

CO₂ cycloaddition with epoxides is a key catalytic procedure for CO₂ utilization. Several metal-based catalysts with cocatalysts are developed for photo-driven CO₂ cycloaddition, while facing difficulties in product purification and continuous reaction. Here, poly(ionic liquid)s are proposed as metal-free catalysts for photo-driven CO₂ cycloaddition without cocatalysts. A series of poly(ionic liquid)s with donor-acceptor segments are fabricated and their photo-driven catalytic performance (conversion rate of 83.5% for glycidyl phenyl ether) outstrips (≈4.9 times) their thermal-driven catalytic performance (17.2%) at the same temperature. Mechanism studies confirm that photo-induced charge separation is promoted by the donor-acceptor segments and can accelerate the CO₂ cycloaddition reaction. This work paves the way for the further use of poly(ionic liquid)s as catalysts in photo-driven CO₂ cycloaddition.

Vacancy-cluster-mediated surface activation for boosting CO2 chemical fixation

The cycloaddition of CO2 with epoxides towards cyclic carbonates provides a promising pathway for CO2 utilization. Given the crucial role of epoxide ring opening in determining the reaction rate, designing catalysts with rich active sites for boosting epoxide adsorption and C-O bond cleavage is necessary for gaining efficient cyclic carbonate generation. Herein, by taking two-dimensional FeOCl as a model, we propose the construction of electron-donor and -acceptor units within a confined region via vacancy-cluster engineering to boost epoxide ring opening. By combing theoretical simulations and in situ diffuse reflectance infrared Fourier-transform spectroscopy, we show that the introduction of Fe-Cl vacancy clusters can activate the inert halogen-terminated surface and provide reactive sites containing electron-donor and -acceptor units, leading to strengthened epoxide adsorption and promoted C-O bond cleavage. Benefiting from these, FeOCl nanosheets with Fe-Cl vacancy clusters exhibit enhanced cyclic carbonate generation from CO2 cycloaddition with epoxides.

Atomically Precise Integration of Multiple Functional Motifs in Catalytic Metal-Organic Frameworks for Highly Efficient Nitrate Electroreduction

Ammonia production plays a central role in modern industry and agriculture with a continuous surge in its demand, yet the current industrial Haber-Bosch process suffers from low energy efficiency and accounts for high carbon emissions. Direct electrochemical conversion of nitrate to ammonia therefore emerges as an appealing approach with satisfactory sustainability while reducing the environmental impact from nitrate pollution. To this end, electrocatalysts for efficient conversion of eight-electron nitrate to ammonia require collective contributions at least from high-density reactive sites, selective reaction pathways, efficient multielectron transfer, and multiproton transport processes. Here, we report a catalytic metal-organic framework (two-dimensional (2D) In-MOF In8) catalyst integrated with multiple functional motifs with atomic precision, including uniformly dispersed, high-density, single-atom catalytic sites, high proton conductivity (efficient proton transport channel), high electron conductivity (promoted by the redox-active ligands), and confined microporous environments. These eventually lead to a direct and efficient electrochemical reduction of nitrate to ammonia and record high yield rate, FE, and selectivity for NH₃ production. A novel "dynamic ligand dissociation" mechanism provides an unprecedented working principle that allows for the use of a high-quality MOF crystalline structure to function as highly ordered, high-density, single-atom catalyst (SAC)-like catalytic systems and ensures the maximum utilization of the metal centers within the MOF structure. Further, the atomically precise assembly of multiple functional motifs within a MOF catalyst offers an effective and facile strategy for the future development of framework-based enzyme-mimic systems.

Synergistic Effect between CO2 Chemisorption Using Amino-Modified Carbon Nitride and Epoxide Activation by High-Energy Electrons for Plasmon-Assisted Synthesis of Cyclic Carbonates

Photocatalytic CO2 cycloaddition is a promising approach for CO2 value-added processes. However, the efficiency of plasmon-assisted CO2 cycloaddition still needs to be improved and the reaction mechanism is unclear. Herein, g-C3N4/Ag (ACN-Ag) hybrids exhibited superior activity of CO2 cycloaddition by coupling a semiconductor into the plasmonic system, in which the ACN grafting amino group by the formation of carbon vacancies can enhance CO2 chemisorption; meanwhile, photo-generated electrons from ACN transfer to Ag to form high-energy electrons, which can activate propylene oxide, accelerating the ring-opening step. Importantly, photo-generated electron injection from ACN to Ag and the interaction between Ag nanoparticles and ACN were confirmed by single-particle photoluminescence spectroscopy. The wavelength-dependent activity demonstrated that the plasmon excitation is crucial for the reaction. Moreover, in situ single-particle PL quenching caused by propylene oxide and in situ electron paramagnetic resonance verified the activation of propylene oxide by ACN-Ag. This work is conducive to an in-depth understanding of the mechanism of CO2 cycloaddition at the single-particle level and provides guidance for the organic synthesis.

Self-assembled mononuclear complexes: open metal sites and inverse dimension-dependent catalytic activity for the Knoevenagel condensation and CO2 cycloaddition

To lessen the greenhouse effect, measures such as improving the recovery of crude oil and converting carbon dioxide (CO₂) into valuable chemicals are necessary to create a sustainable low-carbon future. To this end, the development of efficient new oil-displacing agents and CO₂ conversion has aroused great interest in both academia and industry. The Knoevenagel condensation and CO₂ cycloaddition are the key reactions to solve the above problems. Four Cu- or Zn-based molecular complexes built from different ligands possessing hydrophilic-hydrophobic layers and different dimensionalities were chosen as solid catalysts for this study. Structural analysis revealed the presence of hydrophilic-hydrophobic layers and open metal sites in the low-dimensional complexes. To obtain deep insight into the reaction mechanism, first-principles density functional theory (DFT) calculations were carried out. These calculations confirmed that in the Knoevenagel condensation reaction, the final formation of benzylidenemalononitrile is the rate-determining step (an energy barrier (ΔE) value of 73.2 kJ mol^-1). The zero-dimensional (0D) Cu molecular complex with unsaturated metal centers, hydrophilic and hydrophobic layers, exhibited higher catalytic activity (yield: 100%, temperature: room temperature, and time: 2 h) compared with one- and two-dimensional Cu complexes. In the presence of a 0D Zn complex co-catalyzed with Br^- in the CO₂ cycloaddition reaction, the ΔE value reduces to 35.5 kJ mol^-1 for the ring opening of styrene oxide (SO), which is significantly lower than Br^- catalyzed (80.9 kJ mol^-1) reactions. The roles of unsaturated metal centers, hydrophilic-hydrophobic layers and dimensionality in the Knoevenagel condensation and CO₂ cycloaddition were explained in the results of structure-activity relationships.

Strong Electronic Metal-Support Interaction between Iridium Single Atoms and a WO3 Support Promotes Highly Efficient and Robust CO2 Cycloaddition

The carbon dioxide (CO₂ ) cycloaddition of epoxides to cyclic carbonates is of great industrial importance owing to the high economical values of its products. Single-atom catalysts (SACs) have great potential in CO₂ cycloaddition by virtue of their high atom utilization efficiency and desired activity, but they generally suffer from poor reaction stability and catalytic activity arising from the weak interaction between the active centers and the supports. In this work, Ir single atoms stably anchored on the WO₃ support (Ir₁ -WO₃ ) are developed with a strong electronic metal-support interaction (EMSI). Superior CO₂ cycloaddition is realized in the Ir₁ -WO₃ catalyst via the EMSI effect: 100% conversion efficiency for the CO₂ cycloaddition of styrene oxide to styrene carbonate after 15 h at 40 °C and excellent stability with no degradation even after ten reaction cycles for a total of more than 150 h. Density functional theory calculations reveal that the EMSI effect results in significant charge redistribution between the Ir single atoms and the WO₃ support, and consequently lowers the energy barrier associated with epoxide ring opening. This work furnishes new insights into the catalytic mechanism of CO₂ cycloaddition and would guide the design of stable SACs for efficient CO₂ cycloaddition reactions.

Facet Engineering in Constructing Lewis Acid-Base Pairs for CO2 Cycloaddition to High Value-Added Carbonates

Cycloaddition of epoxides with CO₂ to synthesis cyclic carbonates is an atom-economic pathway for CO₂ utilization with promising industry application value, while its efficiency was greatly inhibited for the lack of highly active catalytic sites. Herein, by taking BiOX (X = Cl, Br) with layered structure for example, we proposed a facet engineering strategy to construct Lewis acid-base pairs for CO₂ cycloaddition, where the typical BiOBr with (010) facets expose surface Lewis acid Bi sites and Lewis base Br sites simultaneously. By the combination of in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and theoretical calculations, the oxygen atom of the epoxide is interacted with the Lewis acid Bi site to activate the ternary ring, then facilitates the attack of the carbon atom by the Lewis base Br site for the ring-opening of the epoxide, which is the rate-determining step in the cycloaddition reaction. As a result, the BiOBr-(010) with rich surface Lewis acid-base pairs showed a high conversion of 85% with 100% atomic economy in the synthesis of cyclic-carbonates without any cocatalyst. This study provides a model structure for CO₂ cycloaddition to high value-added long chain chemicals.

Unlocking Molecular Secrets in a Monomer-Assembly-Promoted Zn-Metalated Catalytic Porous Organic Polymer for Light-Responsive CO2 Insertion

Anthropogenic carbon dioxide (CO₂) emission is soaring day by day due to fossil fuel combustion to fulfill the daily energy requirements of our society. The CO₂ concentration should be stabilized to evade the deadly consequences of it, as climate change is one of the major consequences of greenhouse gas emission. Chemical fixation of CO₂ to other value-added chemicals requires high energy due to its stability at the highest oxidation state, creating a tremendous challenge to the scientific community to fix CO₂ and prevent global warming caused by it. In this work, we have introduced a novel monomer-assembly-directed strategy to design va isible-light-responsive conjugated Zn-metalated porous organic polymer (Zn@MA-POP) with a dynamic covalent acyl hydrazone linkage, via a one-pot condensation between the self-assembled monomer 1,3,5-benzenetricarbohydrazide (TPH) and a Zn complex (Zn@COM). We have successfully explored as-synthesized Zn@MA-POP as a potential photocatalyst in visible-light-driven CO₂ photofixation with styrene epoxide (SE) to styrene carbonate (SC). Nearly 90% desired product (SC) selectivity has been achieved with our Zn@MA-POP, which is significantly better than that for the conventional Zn@TiO₂ (∼29%) and Zn@gC₃N₄ (∼26%) photocatalytic systems. The excellent light-harvesting nature with longer lifetime minimizes the radiative recombination rate of photoexcited electrons as a result of extended π-conjugation in Zn@MA-POP and increased CO₂ uptake, eventually boosting the photocatalytic activity. Local structural results from a first-shell EXAFS analysis reveals the existence of a Zn(N₂O₄) core structure in Zn@MA-POP, which plays a pivotal role in activating the epoxide ring as well as capturing the CO₂ molecules. An in-depth study of the POP-CO₂ interaction via a density functional theory (DFT) analysis reveals two feasible interactions, Zn@MA-POP-CO₂-A and Zn@MA-POP-CO₂-B, of which the latter has a lower relative energy of 0.90 kcal/mol in comparison to the former. A density of states (DOS) calculation demonstrates the lowering of the LUMO energy (EL) of Zn@MA-POP by 0.35 and 0.42 eV, respectively, for the two feasible interactions, in comparison to Zn@COM. Moreover, the potential energy profile also unveils the spontaneous and exergonic photoconversion pathways for the SE to SC conversion. Our contribution is expected to spur further interest in the precise design of visible-light-active conjugated porous organic polymers for CO₂ photofixation to value-added chemicals.

Enzyme-Inspired Microenvironment Engineering of a Single-Molecular Heterojunction for Promoting Concerted Electrochemical CO2 Reduction

Challenges remain in the development of novel multifunctional electrocatalysts and their industrial operation on low-electricity pair-electrocatalysis platforms for the carbon cycle. Herein, an enzyme-inspired single-molecular heterojunction electrocatalyst ((NH_x )₁₆ -NiPc/CNTs) with specific atomic nickel centers and amino-rich local microenvironments for industrial-level electrochemical CO₂ reduction reaction (eCO₂ RR) and further energy-saving integrated CO₂ electrolysis is designed and developed. (NH_x )₁₆ -NiPc/CNTs exhibit unprecedented catalytic performance with industry-compatible current densities, ≈100% Faradaic efficiency and remarkable stability for CO₂ -to-CO conversion, outperforming most reported catalysts. In addition to the enhanced CO₂ capture by chemisorption, the sturdy deuterium kinetic isotope effect and proton inventory studies sufficiently reveal that such distinctive local microenvironments provide an effective proton ferry effect for improving local alkalinity and proton transfer and creating local interactions to stabilize the intermediate, ultimately enabling the high-efficiency operation of eCO₂ RR. Further, by using (NH_x )₁₆ -NiPc/CNTs as a bifunctional electrocatalyst in a flow cell, a low-electricity overall CO₂ electrolysis system coupling cathodic eCO₂ RR with anodic oxidation reaction is developed to achieve concurrent feed gas production and sulfur recovery, simultaneously decreasing the energy input. This work paves the new way in exploring molecular electrocatalysts and electrolysis systems with techno-economic feasibility.

Visible-Light-Driven Photocatalytic CO2 Reduction to CO/CH4 Using a Metal-Organic "Soft" Coordination Polymer Gel

The self-assembly of a well-defined and astutely designed, low-molecular weight gelator (LMWG) based linker with a suitable metal ion is a promising method for preparing photocatalytically active coordination polymer gels. Here, we report the design, synthesis, and gelation behaviour of a tetrapodal LMWG based on a porphyrin core connected to four terpyridine units (TPY-POR) through amide linkages. The self-assembly of TPY-POR LMWG with Ru^II ions results in a Ru-TPY-POR coordination polymer gel (CPG), with a nanoscroll morphology. Ru-TPY-POR CPG exhibits efficient CO₂ photoreduction to CO (3.5 mmol g^-1  h^-1 ) with >99 % selectivity in the presence of triethylamine (TEA) as a sacrificial electron donor. Interestingly, in the presence of 1-benzyl-1,4-dihydronicotinamide (BNAH) with TEA as the sacrificial electron donor, the 8e^- /8H⁺ photoreduction of CO₂ to CH₄ is realized with >95 % selectivity (6.7 mmol g^-1  h^-1 ). In CPG, porphyrin acts as a photosensitizer and covalently attached [Ru(TPY)₂ ]²⁺ acts as a catalytic center as demonstrated by femtosecond transient absorption (TA) spectroscopy. Further, combining information from the in situ DRIFT spectroscopy and DFT calculation, a possible reaction mechanism for CO₂ reduction to CO and CH₄ was outlined.

Isotype Heterojunction-Boosted CO2 Photoreduction to CO

Photocatalytic conversion of CO₂ to high-value products plays a crucial role in the global pursuit of carbon-neutral economy. Junction photocatalysts, such as the isotype heterojunctions, offer an ideal paradigm to navigate the photocatalytic CO₂ reduction reaction (CRR). Herein, we elucidate the behaviors of isotype heterojunctions toward photocatalytic CRR over a representative photocatalyst, g-C₃N₄. Impressively, the isotype heterojunctions possess a significantly higher efficiency for the spatial separation and transfer of photogenerated carriers than the single components. Along with the intrinsically outstanding stability, the isotype heterojunctions exhibit an exceptional and stable activity toward the CO₂ photoreduction to CO. More importantly, by combining quantitative in situ technique with the first-principles modeling, we elucidate that the enhanced photoinduced charge dynamics promotes the production of key intermediates and thus the whole reaction kinetics.

Near-Infrared Photothermal Catalysis for Enhanced Conversion of Carbon Dioxide under Mild Conditions

Enhanced conversion of carbon dioxide (CO₂) for cycloaddition with epoxide derivatives is highly desired in organic synthesis and green chemistry, yet it is still a challenge to obtain satisfactory activity under mild reaction conditions of temperature and pressure. For this purpose, an unexploited strategy is proposed here by incorporating near-infrared (NIR) photothermal properties into multicomponent catalysts. Through the electrostatic adsorption of Co- or Ce-substituted polyoxometalate (POM) clusters on the surface of graphene oxide (GO) with covalently grafted polyethyleneimine (PEI), a series of composite catalysts POMs@GO-PEI are prepared. The structural and property characterizations demonstrate the synergistic advantages of the catalysts bearing Lewis acids and bases and local NIR photothermal heating from the GO matrix for dramatically enhanced CO₂ cycloaddition. Noticeably, while the turnover frequency increases up to 2718 h^-1, the heterogeneous catalysts exhibit photothermal stability and recyclability. With this method, the onsite NIR photothermal transformation becomes extendable to more green reaction processes.

Charge-transfer regulated visible light driven photocatalytic H2 production and CO2 reduction in tetrathiafulvalene based coordination polymer gel

The much-needed renewable alternatives to fossil fuel can be achieved efficiently and sustainably by converting solar energy to fuels via hydrogen generation from water or CO₂ reduction. Herein, a soft processable metal-organic hybrid material is developed and studied for photocatalytic activity towards H₂ production and CO₂ reduction to CO and CH₄ under visible light as well as direct sunlight irradiation. A tetrapodal low molecular weight gelator (LMWG) is synthesized by integrating tetrathiafulvalene (TTF) and terpyridine (TPY) derivatives through amide linkages and results in TPY-TTF LMWG. The TPY-TTF LMWG acts as a linker, and self-assembly of this gelator molecules with Zn^II ions results in a coordination polymer gel (CPG); Zn-TPY-TTF. The Zn-TPY-TTF CPG shows high photocatalytic activity towards H₂ production (530 μmol g^-1h^-1) and CO₂ reduction to CO (438 μmol g^-1h^-1, selectivity > 99%) regulated by charge-transfer interactions. Furthermore, in situ stabilization of Pt nanoparticles on CPG (Pt@Zn-TPY-TTF) enhances H₂ evolution (14727 μmol g^-1h^-1). Importantly, Pt@Zn-TPY-TTF CPG produces CH₄ (292 μmol g^-1h^-1, selectivity > 97%) as CO₂ reduction product instead of CO. The real-time CO₂ reduction reaction is monitored by in situ DRIFT study, and the plausible mechanism is derived computationally.

Proton/Electron Donors Enhancing Electrocatalytic Activity of Supported Conjugated Microporous Polymers for CO2 Reduction

Metal phthalocyanines (MePc) hold great promise in electrochemical reduction of CO₂ to value-added chemicals, whereas the catalytic activity of MePc-containing polymers often suffers from a limited molecular modulation strategy. Herein, we synthesize an ultrathin conjugated microporous polymer sheath around carbon nanotubes by an ionothermal copolymerization of CoPc and H₂ Pc via the Scholl reaction. Given the H₂ Pc-mediated regulation in the synthesis, Co^II metal is well preserved in the form of single atoms on the polymer sheath of the carbon nanotubes. With the synergistic effect of H₂ Pc moieties as proton/electron donors, the composites can selectively reduce CO₂ to CO with a high Faradaic efficiency (max. 97 % at -0.9 V) in broad potential windows, exceptional turnover frequency (97 592 h^-1 at -0.65 V) and large current density (>200 mA cm^-2 ). It is thus desirable to develop a family of heterogeneous polymerized MePc with molecularly regulating electrocatalytic activity.

Atomically Dispersed High-Density Al-N4 Sites in Porous Carbon for Efficient Photodriven CO2 Cycloaddition

Highly active catalysts that can directly utilize renewable energy (e.g., solar energy) are desirable for CO₂ value-added processes. Herein, aiming at improving the efficiency of photodriven CO₂ cycloaddition reactions, a catalyst composed of porous carbon nanosheets enriched with a high loading of atomically dispersed Al atoms (≈14.4 wt%, corresponding to an atomic percent of ≈7.3%) coordinated with N (AlN₄ motif, Al-N-C catalyst) via a versatile molecule-confined pyrolysis strategy is reported. The performance of the Al-N-C catalyst for catalytic CO₂ cycloaddition under light irradiation (≈95% conversion, reaction rate = 3.52 mmol g^-1 h^-1 ) is significantly superior to that obtained under a thermal environment (≈57% conversion, reaction rate = 2.11 mmol g^-1 h^-1 ). Besides the efficient photothermal conversion induced by the carbon matrix, both experimental and theoretical analysis reveal that light irradiation favors the photogenerated electron transfer from the semiconductive Al-N-C catalyst to the epoxide reactant, facilitating the formation of a ring-opened intermediate through the rate-limiting step. This study not only provides an advanced Al-N-C catalyst for photodriven CO₂ cycloaddition, but also furnishes new insight for the rational design of superior photocatalysts for diverse heterogeneous catalytic reactions in the future.

Polyoxometalate Interlayered Zinc-Metallophthalocyanine Molecular Layer Sandwich as Photocoupled Electrocatalytic CO2 Reduction Catalyst

Developing efficient and robust heterogeneous metallophthalocyanine electrocatalysts for CO₂ reduction remains a challenge. Here, a general synthetic method of zinc-metallophthalocyanine (MPc) molecular layer/polyoxometalate (POM) sandwich lamellar material is developed, and thus improved performance of electrocatalytic and photocoupled electrocatalytic CO₂ reduction is achieved. The incorporation of POM could prevent the packing of MPc molecular layers from aggregation, which would be favorable to the exposure of active sites. The molecular layer sandwich catalyst presents superior CO₂ reduction activity, delivering the highest CO Faradaic efficiency (FE_CO) of 96.1% at -0.7 V vs RHE in dark field. Under light irradiation, over 93% FE_CO is achieved in a broad potential range from -0.6 to -0.9 V vs RHE with a maximum of 96.2%, and the carbon monoxide turnover frequency could exceed 2060 h^-1. Photoelectrochemical tests and luminescence characterizations reveal the molecular layer is beneficial for carrier separation during light irradiation; density functional theory calculations and electron paramagnetic resonance indicated a 2-fold enhancement of the external light field on the catalytic performance.

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.

Metal-organic frameworks as catalytic selectivity regulators for organic transformations

Selective organic transformations using metal-organic frameworks (MOFs) and MOF-based heterogeneous catalysts have been an intriguing but challenging research topic in both the chemistry and materials communities. Analogous to the reaction specificity achieved in enzyme pockets, MOFs are also powerful platforms for regulating the catalytic selectivity via engineering their catalytic microenvironments, such as metal node alternation, ligand functionalization, pore decoration, topology variation and others. In this review, we provide a comprehensive introduction and discussion about the role of MOFs played in regulating and even boosting the size-, shape-, chemo-, regio- and more appealing stereo-selectivity in organic transformations. We hope that it will be instructive for researchers in this field to rationally design, conveniently prepare and elaborately functionalize MOFs or MOF-based composites for the synthesis of high value-added organic chemicals with significantly improved selectivity.

Triptycene-supported bimetallic salen porous organic polymers for high efficiency CO2 fixation to cyclic carbonates

Triptycene units in bimetallic salen POPs are envisaged to support the alignment of bimetallic salen macrocycles in side walls of channels for exposing more metal active sites resulting in the high efficiency coupling reaction of epoxides with CO₂.

Integration of metalloporphyrin into cationic covalent triazine frameworks for the synergistically enhanced chemical fixation of CO2

Cobalt porphyrin as a Lewis acidic site was integrated into imidazolium-functionalized porous cationic covalent triazine frameworks for the cooperatively enhanced catalysis CO₂ cycloaddition to produce cyclic carbonates.