Imidazolatsysteme zur CO2 -Abscheidung und photochemischen Reduktion

Use of 5,10-Disubstituted Dibenzoazaborines and Dibenzophosphaborines as Cyclic Supports of Frustrated Lewis Pairs for the Capture of CO2

The reactivity of 5,10-disubstituted dibenzoazaborines and dibenzophosphaborines towards carbon dioxide was studied at the DFT, M06-2X/def2-TZVP, computational level. The profile of this reaction comprises of three stationary points: the pre-reactive complex and adduct minima and the transition state(TS) linking both minima. Initial results show that dibenzoazaborines derivatives are less suitable to form adducts with CO₂ than dibenzophosphaborine systems. The influence of the basicity on the P atom and the acidity on the B center of the dibenzophosphaborine in the reaction with CO₂ was also explored. Thus, an equation was developed relating the properties (acidity, basicity and boron hybridization) of the isolated dibenzophosphaborine derivatives with the adduct energy. We found that modulation of the boron acidity allows to obtain more stable adducts than the pre-reactive complexes and isolated monomers.

One-Step Access to Heteroatom-Functionalized Imidazol(in)ium Salts

Imidazolium salts have ubiquitous applications in energy research, catalysis, materials and medicinal sciences. Here, we report a new strategy for the synthesis of diverse heteroatom-functionalized imidazolium and imidazolinium salts from easily available 1,4-diaza-1,3-butadienes in one step. The strategy relies on a discovered family of unprecedented nucleophilic addition/cyclization reactions with trialkyl orthoformates and heteroatomic nucleophiles. To probe general areas of application, synthesized N-heterocyclic carbene (NHC) precursors were feasible for direct metallation to give functionalized M/carbene complexes (M = Pd, Ni, Cu, Ag, Au), which were isolated in individual form. The utility of chloromethyl function for the postmodification of the synthesized salts and Pd/carbene complexes was demonstrated. The obtained complexes and imidazolium salts demonstrated good activities in Pd- or Ni-catalyzed model cross-coupling and C-H activation reactions.

A High-Efficiency Hematite Photoanode with Enhanced Bonding Energy Around Fe Atoms

Hematite nanoarrays are important photoanode materials. However, they suffer from serious problems of charge transfer and surface states; in particular, the surface states hinder the increase in photocurrent. A previous strategy to suppress the surface state is the deposition of an Fe-free metal oxide overlayer. Herein, from the viewpoint of atomic bonding energy, it is found that the strength of bonding around Fe atoms in the hematite is the key to suppressing the surface states. By treating the surface of hematite with Se and NaBH₄ , the Fe₂ O₃ transforms to a double-layer nanostructure. In the outer layer, the Fe-O bonding is reinforced and the Fe-Se bonding is even stronger. Therefore, the surface states are inhibited and the increase in the photocurrent density becomes much faster. Besides, the treatment constructs a nanoscale p-n junction to promote the charge transfer. Improvements are achieved in onset potential (0.25 V shift) and in photocurrent density (5.8 times). This work pinpoints the key to suppressing the surface states and preparing a high-efficiency hematite nanoarray, and deepens our understanding of hematite photoanodes.

Engineering Heterostructured Nanocatalysts for CO2 Transformation Reactions: Advances and Perspectives

As a conceivable route to achieving anthropological carbon looping, carbon capture and utilization (CCU) technologies employ waste CO₂ as an accessible C1 building block to generate upgraded chemicals or fuels, thereby simultaneously remedying environmental issues and energy crises. However, efficient CO₂ conversion is disfavored by both its thermodynamics and its kinetics. Heterostructured materials with well-controlled interfaces have great potential for enhanced catalytic performance in various CO₂ transformation reactions, owing to the synergistic effects among components, numerous interfacial and/or surface active sites, increased CO₂ adsorption capacity, promoted charge transfer efficiency, and unique physicochemical properties. This Review highlights the state of the art in typical heterostructures, such as core-shell, yolk-shell, Janus, hierarchical (branched and hollow), and 2D/2D layered structures, applied for CO₂ conversion with various energy inputs (radiation, electricity, heat). Fabrication methods of different heterostructures and structure-composition-performance relationships are also discussed concisely. Finally, a brief summary and prospective research directions are provided. The motivation of this Review is to offer instructive information on the applicability of inorganic heterostructures for CO₂ transformation reactions, and it is hoped that further enlightening studies in this field could emerge in the future.

Reverse Semi-Combustion Driven by Titanium Dioxide-Ionic Liquid Hybrid Photocatalyst

Unprecedented metal-free photocatalytic CO2 conversion to CO (up to 228±48 μmol g-1 h-1) was displayed by TiO2@IL hybrid photocatalysts prepared by simple impregnation of commercially available P25-titanium dioxide with imidazolium-based ionic liquids (ILs). The high activity of TiO2@IL hybrid photocatalysts was mainly associated to (i) TiO2@IL red shift compared to the pure TiO2 absorption, and thus a modification of the TiO2 surface electronic structure; (ii) TiO2 with IL bearing imidazolate anions lowered the CO2 activation energy barrier. The reaction mechanism was postulated to occur via CO2 photoreduction to formate species by the imidazole/imidazole radical redox pair, yielding CO and water.

Recent Advances in Metal-Organic Frameworks for Photo-/Electrocatalytic CO2 Reduction

Considerable attention has been paid to the utilization of CO₂ , an abundant carbon source in nature. In this regard, porous catalysts have been eagerly explored with excellent performance for photo-/electrocatalytic reduction of CO₂ to high valued products. Metal-organic frameworks (MOFs), featuring large surface area, high porosity, tunable composition and unique structural characteristics, have been widely exploited in catalytic CO₂ reduction. This Minireview first reports the current progress of MOFs in CO₂ reduction. Then, a specific interest is focused on MOFs in photo-/electrocatalytic reduction of CO₂ by modifying their metal centers, organic linkers, and pores. Finally, the future directions of study are also highlighted to satisfy the requirement of practical applications.

Modulation of Polymeric Carbon Nitrides through Supramolecular Preorganization for Efficient Photocatalytic Hydrogen Generation

Supramolecular pre-assembly by design is an effective strategy to adapt the physicochemical properties of polymeric carbon nitrides (PCNs) to improve their solar conversion performance. A new supramolecular preorganization protocol, which employs H₂ O as the self-assembly medium and sodium persulfate as a modifier, is proposed to modulate the textural and photoelectronic features of PCNs for efficient visible-light H₂ evolution. Sodium persulfate is revealed to precisely tailor the final carbon nitride polymers with unusual porous layered structures and promoted charge separation and migration kinetics. As a result, the modulated PCNs with optimized structures show a greatly enriched activity for photocatalytic H₂ generation compared to the analogous materials derived from melamine without the modifier.

Atomically Dispersed Single Co Sites in Zeolitic Imidazole Frameworks Promoting High-Efficiency Visible-Light-Driven Hydrogen Production

As photocatalysis technology could transform renewable and clean solar energy into green hydrogen (H₂ ) energy through solar water splitting, it is regarded as the "Holy Grail" in chemistry field in the 21st century. Unfortunately, the bottleneck of this technique still lies in the exploration of highly active, cost-effective, and robust photocatalysts. This work reports the design and synthesis of a novel zeolitic imidazole framework (ZIF) coupled Zn_0.8 Cd_0.2 S hetero-structured photocatalyst for high-performance visible-light-induced H₂ production. State-of-the-art characterizations and theoretical computations disclose that the interfacial electronic interaction between ZIF and Zn_0.8 Cd_0.2 S, the high distribution of Zn_0.8 Cd_0.2 S on ZIF, and the atomically dispersed coordinately unsaturated Co sites in ZIF synergistically arouse the significantly improved visible-light photocatalytic H₂ production performance.

Imidazolium-Salt-Functionalized Covalent Organic Frameworks for Highly Efficient Catalysis of CO2 Conversion

The conversion of CO₂ into valuable chemicals is an ideal pathway for CO₂ utilization in industry, although the development of highly efficient catalysts remains a challenge. Herein, the design and synthesis of two covalent organic frameworks (COFs) functionalized with imidazolium salts were reported as catalysts for CO₂ conversion. The resultant COFs possessed highly crystalline structures, showed high stability and surface area, and contained dense catalytic active sites on the pore walls. They exhibited outstanding catalytic performances for the reaction of CO₂ with epoxides without any solvent or cocatalyst under mild conditions and afforded a record turnover number of 495 000. In addition, the COFs could serve as effective catalysts in the reductive reaction of CO₂ with amines. The results presented here thus demonstrate the exceptional potential of the functionalized COFs for various challenging CO₂ transformations.

A Facile and Versatile "Click" Approach Toward Multifunctional Ionic Metal-organic Frameworks for Efficient Conversion of CO2

Ionic metal-organic frameworks (IMOFs) that integrate synergistic Lewis-acid sites (intrinsic metal centers of the frameworks) and nucleophilic anions (halides encapsulated within pores) are intriguing platforms for the design of fully heterogeneous catalytic systems for cycloaddition of CO₂ to epoxides. A new, facile and versatile synthetic approach has been used to fabricate triazolium-based IMOFs for the first time. The approach makes use of azide-alkyne click chemistry and subsequent N-alkylation to post-synthetically create a cationic triazolium ring and introduce exchangeable counteranions at the same time. The IMOFs are efficient and recyclable heterogeneous catalysts for CO₂ conversion under mild and cocatalyst-free conditions. In particular, the click-accessible triazolium ring provides a handle to incorporate further functionality. The MIL-101-tzmOH-Br catalyst, which integrates hydrogen-bonding hydroxy groups besides metal centers and bromide anions, shows superior catalytic performance under mild conditions.

The Counterion Effect of Imidazolium-Type Poly(ionic liquid) Brushes on Carbon Dioxide Adsorption

Imidazolium-based poly(ionic liquid) brushes were attached to spherical silica nanoparticles bearing various functionalities by using a surface-initiated atom transfer radical polymerization ("grafting from" technique). A temperature-programmed desorption process was applied to evaluate and analyze the carbon dioxide adsorption performance of the synthesized polymer brushes. The confined structure of the surface-attached polymer chains facilitates gas transport and adsorption, leading to an enhanced adsorption capacity of carbon dioxide molecules compared with pure polymer powders. Temperature-programmed desorption profiles of the synthesized polymer brushes after carbon dioxide adsorption reveal that the substituent groups on the nitrogen atom at the 3-position of the imidazole ring, as well as the associated anions significantly affect the adsorption capacity of functionalized poly(ionic liquid) brushes. Of the tested samples, amine-functionalized poly(ionic liquid) brushes associated with hexafluorophosphate ions exhibit the highest carbon dioxide adsorption capacity of 2.56 mmol g^-1 (112.64 mg g^-1 ) at 25 °C under a carbon dioxide partial pressure of 0.2 bar.

Photocatalytic Reverse Semi-Combustion Driven by Ionic Liquids

The simple photolysis of CO₂ in aqueous solutions to generate CO and/or hydrocarbons and derivatives in the presence of a catalyst is considered to be a clean and efficient approach for utilizing CO₂ as a C1 building block. Despite the huge efforts dedicated to this transformation using either semiconductors or homogeneous catalysts, only small improvements of the catalytic activity have been achieved so far. This article reports that simple aqueous solutions of organic salts-denominated as ionic liquids-can efficiently photo-reduce CO₂ to CO without using photosensitizers or sacrificial agents. The system relies on the formation of the [CO₂ ]^.- intermediate through homolytic C-C bond cleavage in a cation-CO₂ adduct of imidazolium-based ionic liquids (ILs). The system continuously produced CO up to 2.88 mmol g^-1 of IL after 40 h of irradiation by using an aqueous solution of 1-n-butyl-3-methylimidazolium-2-carboxylate (BMIm.CO₂ ) IL, representing an apparent quantum yield of 3.9 %. The organophotocatalytic principles of our system may help to develop more simple and efficient organic materials for the production of solar fuels from CO₂ under mild conditions, which represents a real alternative to those based on semiconductors and homogeneous metal-based catalysts.

Robust Binding between Carbon Nitride Nanosheets and a Binuclear Ruthenium(II) Complex Enabling Durable, Selective CO2 Reduction under Visible Light in Aqueous Solution

Carbon nitride nanosheets (NS-C₃ N₄ ) were found to undergo robust binding with a binuclear ruthenium(II) complex (RuRu') even in basic aqueous solution. A hybrid material consisting of NS-C₃ N₄ (further modified with nanoparticulate Ag) and RuRu' promoted the photocatalytic reduction of CO₂ to formate in aqueous media, in conjunction with high selectivity (approximately 98 %) and a good turnover number (>2000 with respect to the loaded Ru complex). These represent the highest values yet reported for a powder-based photocatalytic system during CO₂ reduction under visible light in an aqueous environment. We also assessed the desorption of RuRu' from the Ag/C₃ N₄ surface, a factor that can contribute to a loss of activity. It was determined that desorption is not induced by salt additives, pH changes, or photoirradiation, which partly explains the high photocatalytic performance of this material.

Structure-Mediated Charge Separation in Boron Carbon Nitride for Enhanced Photocatalytic Oxidation of Alcohol

Boron carbon nitride (BCN) is a promising earth-abundant photocatalyst for solar energy conversion. However, the photocatalytic activities of BCN materials remain moderate because of the fast electron-hole recombination. Herein, an ordered BCN structure is fabricated by a facile one-step thermal treatment strategy. The ordered structure of BCN is directly evident from powder X-ray diffraction and high-resolution transmission electron microscopy. Importantly, it is found that the long-period ordered structure can intrinsically accelerate the separation and transfer kinetics of photogenerated charge carriers. Benefiting from these advantages, the ordered BCN structure exhibits remarkable performance for photoinduced selective oxidation of benzyl alcohol compared with the pristine BCN counterpart. This work highlights the important role of the crystal structure of light-harvesting materials in affecting electron-hole separation and at the same time points to the ample potential for improving the photocatalytic performance.

Electrocatalytic and Photocatalytic Reduction of CO2 to CO by Cobalt(II) Tripodal Complexes: Low Overpotentials, High Efficiency and Selectivity

The reduction of carbon dioxide (CO₂ ) has been considered as an approach to mitigate global warming and to provide renewable carbon-based fuels. Rational design of efficient, selective, and inexpensive catalysts with low overpotentials is urgently desired. In this study, four cobalt(II) tripodal complexes are tested as catalysts for CO₂ reduction to CO in a MeCN/H₂ O (4:1 v/v) solution. The replacement of pyridyl groups in the ligands with less basic quinolinyl groups greatly reduces the required overpotential for CO₂ -to-CO conversion down to 200-380 mV. Benefitting from the low overpotentials, a photocatalyst system for CO₂ -to-CO conversion is successfully constructed, with an maximum turnover number (TON) of 10 650±750, a turnover frequency (TOF) of 1150±80 h^-1 , and almost 100 % selectivity to CO. These outstanding catalytic performances are further elucidated by DFT calculations.

Imidazolium Ions with an Alcohol Substituent for Enhanced Electrocatalytic Reduction of CO2

Electrochemical reduction of CO₂ to CO is an attractive process for carbon capture and energy regeneration. Imidazolium ionic liquids (IMILs) are promising electrolyte catalytically active additives for this important reaction. Herein, we report functionalizing the imidazolium ion with a propanol substituent at the N site can significantly enhance the catalytic activity of IMILs, causing a positive shift of the onset potential for the CO₂ reduction by about 90 mV in an acetonitrile electrolyte. Theoretical calculations indicated that the propanol hydroxyl could bridge a local hydrogen-bonding chain as shortcut for proton transfer, leading to a dramatic decrease of the activation barrier for the catalytic reduction of CO₂ in IMIL.

Metal-Organic Framework-Templated Catalyst: Synergy in Multiple Sites for Catalytic CO2 Fixation

The types and quantities of active sites play a critical role in catalysis. Herein, ZnO nanoparticles encapsulated into N-doped porous carbon has been rationally prepared by the pyrolysis of a metal-organic framework (MOF) followed by a moderate oxidation treatment. The resulting catalyst exhibits excellent activity, selectivity, and recyclability in the CO₂ cycloaddtion reactions with epoxides owing to the synergy of multiple sites inherited from the MOF and generated by the oxidation process.

Bifunctional Ionic Liquids Derived from Biorenewable Sources as Sustainable Catalysts for Fixation of Carbon Dioxide

A series of highly efficient, bifunctional ionic liquids containing a quaternary alkyl ammonium cation and an amine anion were prepared from choline and amino acids, respectively. Nine ILs were synthesized, characterized, and applied as organocatalysts for the chemical fixation of carbon dioxide to form cyclic carbonates and quinazoline-2,4(1 H,3 H)-diones. A binary mixture of an IL and a co-catalysts generates deep eutectic solvents (DESs) and accelerates the rate of the cycloaddition reaction at atmospheric pressure and low temperature (70 °C). The presence of the hydroxyl functional group of choline and the free amine group of the amino acids in the ILs has a synergistic effect on the activation of the epoxide and carbon dioxide towards the cycloaddition reactions. These ILs are biodegradable and are synthesized from easily available biorenewable sources. Additionally, this catalytic method demonstrates ultimate environmental benignity because of the mild metal- and solvent-free conditions as well as the recyclability of the catalyst and co-catalyst.

Efficient Ionic-Liquid-Promoted Chemical Fixation of CO2 into α-Alkylidene Cyclic Carbonates

The efficient conversion of CO₂ into value-added chemicals under metal-free conditions is of significant importance from the viewpoint of sustainable chemistry. In this work, ionic liquids (ILs) with different properties were used to promote the reaction between CO₂ and propargylic alcohol for the synthesis of α-alkylidene cyclic carbonates. The protic IL 1,8-diazabicyclo-[5.4.0]-7-undecenium 2-methylimidazolide ([DBUH][MIm]) was prepared by simple neutralization of the superbase with a weak proton donor and could efficiently promote the reactions in high yields. After the reactions, the IL was separated from the reaction mixtures by simply adding water, and then reused after drying without an observable decrease in the catalytic activity and selectivity. NMR spectroscopy and detailed density functional theory analysis were used to propose a reaction mechanism. Both the cation and anion of the IL played a key synergistic role in promoting the reaction. These findings may be useful for the rational design of novel metal-free and recyclable routes for the reaction between CO₂ and propargylic alcohols.

Porous Ionic Polymers as a Robust and Efficient Platform for Capture and Chemical Fixation of Atmospheric CO2

Direct use of atmospheric CO₂ as a C₁ source to synthesize high-value chemicals through environmentally benign processes is of great interest, yet challenging. Porous heterogeneous catalysts that are capable of simultaneously capturing and converting CO₂ are promising candidates for such applications. Herein, a family of organic ionic polymers with nanoporous structure, large surface area, strong affinity for CO₂ , and very high density of catalytic active sites (halide ions) was synthesized through the free-radical polymerization of vinylfunctionalized quaternary phosphonium salts. The resultant porous ionic polymers (PIPs) exhibit excellent activities in the cycloaddition of epoxides with atmospheric CO₂ , outperforming the corresponding soluble phosphonium salt analogues and ranking among the highest of known metal-free catalytic systems. The high CO₂ uptake capacity of the PIPs facilitates the enrichment of CO₂ molecules around the catalytic centers, thereby benefiting its conversion. We have demonstrated for the first time that atmospheric CO₂ can be directly converted to cyclic carbonates at room temperature using a heterogeneous catalytic system under metal-solvent free conditions. Moreover, the catalysts proved to be robust and fully recyclable, demonstrating promising potential for practical utilization for the chemical fixation of CO₂ . Our work thereby paves a way to the advance of PIPs as a new type of platform for capture and conversion of CO₂ .

Cyanuric Acid-Based Organocatalyst for Utilization of Carbon Dioxide at Atmospheric Pressure

A organocatalytic system based on economical and readily available cyanuric acid has been developed for the synthesis of 2-oxazolidinones and quinazoline-2,4(1H,3H)-diones from propargylamines and 2-aminobenzonitriles under atmospheric pressure carbon dioxide. Notably, a low concentration of carbon dioxide in air was directly converted into 2-oxazolidinone in excellent yields without an external base. Through mechanistic investigation by in situ FTIR spectroscopy, cyanuric acid was demonstrated to be an efficient catalyst for carbon dioxide fixation.

Precise Formation of a Hollow Carbon Nitride Structure with a Janus Surface To Promote Water Splitting by Photoredox Catalysis

The precise modification of redox species on the inner and outer surfaces of hollow nanostructures is relevant in catalysis, surface science, and nanotechnology, but has proven difficult to achieve. Herein, we develop a facile approach to specifically fabricate Pt and Co₃O₄ nanoparticles (NPs) onto the interior and exterior surface of hollow carbon nitride spheres (HCNS), respectively, to promote the surface redox functions of the polymer semiconductors. The photocatalytic water splitting activities of HCNS with spatially separated oxidation and reduction centers at their nanodomains were enhanced. The origin of the enhanced activity was attributed to the spatially separated reactive sites for the evolution of H₂ and O₂ and also to the unidirectional migration of the electron and hole on the Janus surfaces, thereby preventing the unwanted reverse reaction of water splitting and decreasing charge recombination.