Light Triggered Pore Size Tuning in Photoswitching Covalent Triazine Frameworks for Low Energy CO2 Capture

Recently, photo switching porous materials have been widely reported for low energy costed CO₂ capture and release via simply remoted light controlling method. However, most reported photo responsive CO₂ adsorbents relied on metal organic framework (MOFs) functionalisation with photochromic moieties, and MOF adsorbents still suffered from chemically and thermally unstable issues. Thus, further metal free and highly stable organic photoresponsive adsorbents are necessary to be developed. CTFs, because of their high porosity and stability, have attracted great attention for CO₂ capture. Considering the high CO₂ uptake capacity and structural tunability of CTFs, it suggests high potential to fabricate the photoswitching CTF materials by the same functionalisation method as MOFs. Herein, the first series of photo switching CTFs were developed for low energy CO₂ capture and release. Apart from that, the CO₂ switching efficiency could be doubled either through the azobenzene numbers adjusting method or through the previously reported structural alleviation strategy. Furthermore, the pore size distribution of azobenzene functionalised PCTFs also could be tuned under UV exposure, which may contribute to the UV light induced decrease of CO₂ uptake capacity. These photoswitching CTFs represented a new kind of porous polymers for low energy costed CO₂ capture.     

Porous Cationic Covalent Triazine‐Based Frameworks as Platforms for Efficient CO2 and Iodine Capture

Porous cationic covalent triazine‐based frameworks (CTFs) with imidazolium salts as tectons were prepared via ionothermal synthesis. The high‐PF₆^?‐content CTF shows the CO₂ adsorption of 44.7 cm³ g^?1 and I₂ capture capacity of 312 wt %. The influence of counterion species and contents on the porosities, CO₂ adsorptions, and I₂ capture capacities of the CTFs has been investigated.     

Direct access to 1,4-benzothiazine 4,4-dioxides and 4-oxides via a domino reaction

The domino reactions of 2-fluoro benzensulfonyl acetonitrile and α-chloro oximes in the presence of Cs₂CO₃ in aprotic high boiling point solvents have been achieved to provide isoxazole?fused 4H-1,4-benzothiazine-4,4-dioxides via an unprecedented transition metal-free one-pot addition/cyclization process. The tunable synthesis of either isoxozolo-1,4-benzothiazin-4-oxides or their precursor 5-aminoisoxazoles can be controlled depending on the solvent selection. The observed products were characterized by means of (IR, ¹H, ¹³C NMR and HRMS) and physical methods.     

Desymmetrized Vertex Design toward a Molecular Cage with Unusual Topology

A novel low-symmetry organic molecular cage with distinctive geometry was successfully synthesized from 5,5′-(propane-2,2-diyl)bis(2-hydroxyisophthalaldehyde) and 1,2-cyclohexanediamine building blocks, through the desymmetrized vertex design strategy. Single-crystal X-ray crystallographic analysis shows that the cage contains asymmetrical and nonplanar windows, exhibiting an unprecedented C₂ symmetry and an efficient packing. The molecular cage structure was also characterized by FTIR, NMR, and MALDI-TOF. Quantum chemistry studies show that the cage structure contains rare intramolecular hydrogen-hydrogen (C−H⋅⋅⋅H−C) bonding interactions. The cage crystals exhibit high iodine vapor uptake (3.78 g g⁻¹), which is among the highest for porous molecular materials. The knowledge gained in this study would open new possibilities for the design and synthesis of molecular cages with novel topologies targeting a broad range of applications.     

A Robust 3D Cage‐like Ultramicroporous Network Structure with High Gas‐Uptake Capacity

A three‐dimensional (3D) cage‐like organic network (3D‐CON) structure synthesized by the straightforward condensation of building blocks designed with gas adsorption properties is presented. The 3D‐CON can be prepared using an easy but powerful route, which is essential for commercial scale‐up. The resulting fused aromatic 3D‐CON exhibited a high Brunauer–Emmett–Teller (BET) specific surface area of up to 2247 m² g^?1. More importantly, the 3D‐CON displayed outstanding low pressure hydrogen (H₂, 2.64 wt %, 1.0 bar and 77 K), methane (CH₄, 2.4 wt %, 1.0 bar and 273 K), and carbon dioxide (CO₂, 26.7 wt %, 1.0 bar and 273 K) uptake with a high isosteric heat of adsorption (H₂, 8.10 kJ mol^?1; CH₄, 18.72 kJ mol^?1; CO₂, 31.87 kJ mol^?1). These values are among the best reported for organic networks with high thermal stability (ca. 600 °C).     

Covalent Triazine Frameworks via a Low‐Temperature Polycondensation Approach

Covalent triazine frameworks (CTFs) are normally synthesized by ionothermal methods. The harsh synthetic conditions and associated limited structural diversity do not benefit for further development and practical large‐scale synthesis of CTFs. Herein we report a new strategy to construct CTFs (CTF‐HUSTs) via a polycondensation approach, which allows the synthesis of CTFs under mild conditions from a wide array of building blocks. Interestingly, these CTFs display a layered structure. The CTFs synthesized were also readily scaled up to gram quantities. The CTFs are potential candidates for separations, photocatalysis and for energy storage applications. In particular, CTF‐HUSTs are found to be promising photocatalysts for sacrificial photocatalytic hydrogen evolution with a maximum rate of 2647 μmol h⁻¹ g⁻¹ under visible light. We also applied a pyrolyzed form of CTF‐HUST‐4 as an anode material in a sodium‐ion battery achieving an excellent discharge capacity of 467 mAh g⁻¹.     

Ag Nanoparticles Supported on a Resorcinol‐Phenylenediamine‐Based Covalent Organic Framework for Chemical Fixation of CO2

Covalent organic frameworks are a new class of crystalline organic polymers possessing a high surface area and ordered pores. Judicious selection of building blocks leads to strategic heteroatom inclusion into the COF structure. Owing to their high surface area, exceptional stability and molecular tunability, COFs are adopted for various potential applications. The heteroatoms lining in the pores of COF favor synergistic host–guest interaction to enhance a targeted property. In this report, we have synthesized a resorcinol‐phenylenediamine‐based COF which selectively adsorbs CO₂ into its micropores (12 Å). The heat of adsorption value (32 kJ mol^?1) obtained from the virial model at zero‐loading of CO₂ indicates its favorable interaction with the framework. Furthermore, we have anchored small‐sized Ag nanoparticles (≈4–5 nm) on the COF and used the composite for chemical fixation of CO₂ to alkylidene cyclic carbonates by reacting with propargyl alcohols under ambient conditions. Ag@COF catalyzes the reaction selectively with an excellent yield of 90 %. Recyclability of the catalyst has been demonstrated up to five consecutive cycles. The post‐catalysis characterizations reveal the integrity of the catalyst even after five reaction cycles. This study emphasizes the ability of COF for simultaneous adsorption and chemical fixation of CO₂ into corresponding cyclic carbonates.     

Mixed-metal Ionothermal Synthesis of Metallophthalocyanine Covalent Organic Frameworks for CO2 Capture and Conversion

Phthalocyanines (PCs) are intriguing building blocks owing to their stability, physicochemical and catalytic properties. Although PC-based polymers have been reported before, many suffer from relatively low stability, crystallinity, and low surface areas. Utilizing a mixed-metal salt ionothermal approach, we report the synthesis of a series of metallophthalocyanine-based covalent organic frameworks (COFs) starting from 1,2,4,5-tetracyanobenzene and 2,3,6,7-tetracyanoanthracene to form the corresponding COFs named M-pPPCs and M-anPPCs, respectively. The obtained COFs followed the Irving–Williams series in their metal contents, surface areas, and pore volume and featured excellent CO₂ uptake capacities up to 7.6 mmol g⁻¹ at 273 K, 1.1 bar. We also investigated the growth of the Co-pPPC and Co-anPPC on a highly conductive carbon nanofiber and demonstrated their high catalytic activity in the electrochemical CO₂ reduction, which showed Faradaic efficiencies towards CO up to 74 % at −0.64 V vs. RHE.     

The Keto-Switched Photocatalysis of Reconstructed Covalent Organic Frameworks for Efficient Hydrogen Evolution

The keto-switched photocatalysis of covalent organic frameworks (COFs) for efficient H₂ evolution was reported for the first time by engineering, at a molecular level, the local structure and component of the skeletal building blocks. A series of imine-linked BT-COFs were synthesized by the Schiff-base reaction of 1, 3, 5-benzenetrialdehyde with diamines to demonstrate the structural reconstruction of enol to keto configurations by alkaline catalysis. The keto groups of the skeletal building blocks served as active injectors, where hot π-electrons were provided to Pt nanoparticles (NPs) across a polyvinylpyrrolidone (PVP) insulting layer. The characterization results, together with density functional theory calculations, indicated clearly that the formation of keto-injectors not only made the conduction band level more negative, but also led to an inhomogeneous charge distribution in the donor-acceptor molecular building blocks to form a strong intramolecular built-in electric field. As a result, visible-light photocatalysis of TP-COFs-1 with one keto group in the skeletal building blocks was successfully enabled and achieved an impressive H₂ evolution rate as high as 0.96 mmol g⁻¹ h⁻¹. Also, the photocatalytic H₂ evolution rates of the reconstructed BT-COFs-2 and -3 with two and three keto-injectors were significantly enhanced by alkaline post-treatment.     

Imidazolium Ionic Liquids,Imidazolylidene Heterocyclic Carbenes,and Zeolitic Imidazolate Frameworks for CO2 Capture and Photochemical Reduction

Imidazolium ionic liquids (ILs), imidazolylidene N‐heterocyclic carbenes (NHCs), and zeolitic imidazolate frameworks (ZIFs) are imidazolate motifs which have been extensively investigated for CO₂ adsorption and conversion applications. Summarized in this minireview is the recent progress in the capture, activation, and photochemical reduction of CO₂ with these three imidazolate building blocks, from homogeneous molecular entities (ILs and NHCs) to heterogeneous crystalline scaffolds (ZIFs). The developments and existing shortcomings of the imidazolate motifs for their use in CO₂ utilizations is assessed, with more of focus on CO₂ photoredox catalysis. The opportunities and challenges of imidazolate scaffolds for future advancement of CO₂ photochemical conversion for artificial photosynthesis are discussed.     

From CO2 activation to catalytic reduction: a metal-free approach

Over exploitation of natural resources and human activities are relentlessly fueling the emission of CO₂ in the atmosphere. Accordingly, continuous efforts are required to find solutions to address the issue of excessive CO₂ emission and its potential effects on climate change. It is imperative that the world looks towards a portfolio of carbon mitigation solutions, rather than a single strategy. In this regard, the use of CO₂ as a C1 source is an attractive strategy as CO₂ has the potential to be a great asset for the industrial sector and consumers across the globe. In particular, the reduction of CO₂ offers an alternative to fossil fuels for various organic industrial feedstocks and fuels. Consequently, efficient and scalable approaches for the reduction of CO₂ to products such as methane and methanol can generate value from its emissions. Accordingly, in recent years, metal-free catalysis has emerged as a sustainable approach because of the mild reaction conditions by which CO₂ can be reduced to various value-added products. The metal-free catalytic reduction of CO₂ offers the development of chemical processes with low cost, earth-abundant, non-toxic reagents, and low carbon-footprint. Thus, this perspective aims to present the developments in both the reduction and reductive functionalization chemistry of CO₂ during the last decade using various metal-free catalysts.

This review article documents the key developments in the metal-free catalytic reduction of CO₂ into various energy intensive chemicals and fuels, and reductive functionalization of CO₂ for the formation of new C–N bonds.     

Oxidative degradation of nitrogen-containing organic compounds: vaccum-ultraviolet (VUV) photolysis of aqueous solutions of 3-amino 5-methylisoxazole

The oxidative degradation of 3-amino 5-methyl isoxazole initiated by the VUV photolysis of water at 172 nm has been studied. Mineralization of CO₂, H₂O, NO ₃ ^– and NH ₄ ⁺ is more efficient when reductive conditions (argon saturated solutions) are favoured. Formation of compounds which cannot be completely oxidised to CO₂ is observed. Experiments performed under strictly oxidative conditions show higher yields of these inert compounds and, hence, incomplete mineralization. Cyanide was formed in concentrations lower than 5×10^–5 mol/l. In alkaline aqueous solutions, cyanide is completely transformed into CO ₃ ^2– , NH ₄ ⁺ and NO ₃ ^– during the irradiation time needed to mineralize the isoxazole. Therefore, cyanide does not present a potential risk for the use of the VUV photolysis for isoxazole degradation. Similarly, organic nitrogen is converted into both, NO ₃ ^– and NH ₄ ⁺ . The relative concentrations of the two ions depend on total irradiation time, oxygen saturation and reactor geometry. A sequence of reactions is proposed and discussed.Dedicated to Professor Dr. Dieter Klockow on the occasion of his 60th birthday     

Iron Coordination to Hollow Microporous Metal-Free Disalphen Networks: Heterogeneous Iron Catalysts for CO2 Fixation to Cyclic Carbonates

This work shows that a hollow and microporous metal-free N,N′-phenylenebis(salicylideneimine) (salphen) network (H-MSN) can be engineered by Sonogashira coupling of [tetraiodo{di(Zn-salphen)}] building blocks with 1,4-diethynylbenzene in the presence of silica templates and by successive Zn and silica etching. Iron(III) ions could be incorporated into the H-MSN to form hollow and microporous Fe–disalphen networks (H-MFeSN) with enhanced microporosity and surface area. The H-MFeSN showed efficient catalytic performance and recyclability in the CO₂ conversion to cyclic carbonates.     

Changing the Mechanism for CO2 Hydrogenation Using Solvent‐Dependent Thermodynamics

A critical scientific challenge for utilization of CO₂ is the development of catalyst systems that function in water and use inexpensive and environmentally friendly reagents. We have used thermodynamic insights to predict and demonstrate that the HCo^I(dmpe)₂ catalyst system, previously described for use in organic solvents, can hydrogenate CO₂ to formate in water with bicarbonate as the only added reagent. Replacing tetrahydrofuran as the solvent with water changes the mechanism for catalysis by altering the thermodynamics for hydride transfer to CO₂ from a key dihydride intermediate. The need for a strong organic base was eliminated by performing catalysis in water owing to the change in mechanism. These studies demonstrate that the solvent plays a pivotal role in determining the reaction thermodynamics and thereby catalytic mechanism and activity.     

Synthetic Control and Multifunctional Properties of Fluorescent Covalent Triazine‐Based Frameworks

Conjugated microporous polymers (CMPs), with the virtue of high porosity and optoelectronic activity, are attracting increasing research interest and have been used in various environmental and energy areas. Efficient synthesis and the exploitation of new functionalities are the research hotspots in the CMPs research area. Covalent triazine frameworks (CTFs) synthesized by CF₃SO₃H catalyzed trimerization reactions show properties quite alike to CMPs and this method avoids the use of noble metal catalysts. In this study, a series of novel fluorescent covalent triazine‐based frameworks (F‐CTFs) is prepared using different tetra‐cyano compounds as the starting monomers. Both porosity and fluorescence properties of the F‐CTFs can be adjusted by the monomer structure. Gas adsorption measurement reveals that F‐CTF1 with the largest surface area of 896 m² g⁻¹ shows the highest CO₂ uptake of 3.29 mmol g⁻¹ at 273 K and 1.13 bar among the polymers. Taking advantages of their large surface areas and strong fluorescence, these F‐CTFs could be used as efficient chemical sensing agents for various nitroaromatic compounds as well.

     

Organic Polymer Dots Photocatalyze CO2 Reduction in Aqueous Solution

Developing low-cost and efficient photocatalysts to convert CO₂ into valuable fuels is desirable to realize a carbon-neutral society. In this work, we report that polymer dots (Pdots) of poly[(9,9′-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-thiadiazole)] (PFBT), without adding any extra co-catalyst, can photocatalyze reduction of CO₂ into CO in aqueous solution, rendering a CO production rate of 57 μmol g⁻¹ h⁻¹ with a detectable selectivity of up to 100 %. After 5 cycles of CO₂ re-purging experiments, no distinct decline in CO amount and reaction rate was observed, indicating the promising photocatalytic stability of PFBT Pdots in the photocatalytic CO₂ reduction reaction. A mechanistic study reveals that photoexcited PFBT Pdots are reduced by sacrificial donor first, then the reduced PFBT Pdots can bind CO₂ and reduce it into CO via their intrinsic active sites. This work highlights the application of organic Pdots for CO₂ reduction in aqueous solution, which therefore provides a strategy to develop highly efficient and environmentally friendly nanoparticulate photocatalysts for CO₂ reduction.     

Self-Assembly of Polyoxometalate-Based Sub-1 nm Polyhedral Building Blocks into Rhombic Dodecahedral Superstructures

Self-assembly of subnanometer (sub-1 nm) scale polyhedral building blocks can yield some superstructures with novel and interesting morphology as well as potential functionalities. However, achieving the self-assembly of sub-1 nm polyhedral building blocks is still a great challenge. Herein, through encapsulating the titanium-substituted polyoxometalate (POM, K₇PTi₂W₁₀O₄₀) with tetrabutylammonium cations (TBA⁺), we first synthesized a sub-1 nm rhombic dodecahedral building block by further tailoring the spatial distribution of TBA⁺ on the POM. Molecular dynamics (MD) simulations demonstrated the eight TBA⁺ cations interacted with the POM cluster and formed the sub-1 nm rhombic dodecahedron. As a result of anisotropy, the sub-1 nm building blocks have self-assembled into rhombic dodecahedral POM (RD-POM) assemblies at the microscale. Benefiting from the regular structure, Br⁻ ions, and abundant active sites, the obtained RD-POM assemblies exhibit excellent catalytic performance in the cycloaddition of CO₂ with epoxides without co-catalysts. This work provides a promising approach to tailor the symmetry and structure of sub-1 nm building blocks by tuning the spatial distribution of ligands, which may shed light on the fabrication of superstructures with novel properties by self-assembly.     

Three-dimensional microporous and mesoporous covalent organic frameworks based on cubic building units

Covalent organic frameworks (COFs) have attracted extensive interest due to their unique structures and various applications. However, structural diversities are still limited, which greatly restricts the development of COF materials. Herein, we report two unusual cubic (8-connected) building units and their derived 3D imine-linked COFs with bcu nets, JUC-588 and JUC-589. Owing to these unique building blocks with different sizes, the obtained COFs can be tuned to be microporous or mesoporous structures with high surface areas (2728 m² g⁻¹ for JUC-588 and 2482 m² g⁻¹ for JUC-589) and promising thermal and chemical stabilities. Furthermore, the high selectivity of CO₂/N₂ and CO₂/CH₄, excellent H₂ uptakes, and efficient dye adsorption are observed. This research thus provides a general strategy for constructing stable 3D COF architectures with adjustable pores via improving the valency of rigid building blocks.

Two unusual cubic (8-connected) building units and their derived 3D imine-linked COFs based on bcu nets have been designed and synthesized, which demonstrates highly crystalline structures, excellent surface areas, and large pore sizes.     

Polymeric frustrated Lewis pairs in CO2/cyclic ether coupling catalysis

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

Polymeric frustrated Lewis pairs catalyse the coupling of epoxides and oxetanes with CO₂ with high selectivity under mild CO₂ pressures across multiple reaction cycles.     

Ferrocene-based hypercrosslinked polymers derived from phenolic polycondensation with unexpected H2 adsorption capacity

Metal-doped porous organic polymers often display unique properties for applications in gas uptake owing to the incorporation of the metal elements in the polymer networks. In this study, a series of novel ferrocene-based hypercrosslinked polymers were prepared by phenolic polycondensation (Fc-PR-HCPs). To generate the hypercrosslinked polymers, 1,1′-ferrocenedicarboxaldehyde (Fc(CHO)₂) and bisphenol A (BPA) were used as the building blocks. The maximum value of BET and micropore surface area is determined to be 1111.4 and 487.4 m²/g for Fc-PR-HCP3. A significant H₂ adsorption capacity of 3.11 wt% was achieved for Fc-PR-HCP3 at 77 K/1.0 bar, which was noted to be higher than the porous organic polymers with even higher BET surface area value. The high micropore surface area value and the adsorption sites (aromatic rings and metal ion-active sites) provided by two building blocks were used to explain the significant H₂ adsorption capacity successfully. Overall, the findings from this study indicate that Fc-PR-HCPs highlighted prospective applications in the field of H₂ capture.