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.     

Synthesis of Discrete CHA Zeolite Nanocrystals without Organic Templates for Selective CO2 Capture

Small-pore zeolites such as chabazite (CHA) are excellent candidates for the selective separation of CO₂; however, the current synthesis involves several steps and the use of organic structure-directing agent (OSDA), increasing their cost and energy requirements. We report the synthesis of small-pore zeolite crystals (aluminosilicate) with CHA-type framework structure by direct synthesis in a colloidal suspension containing a mixture of inorganic cations only (Na⁺, K⁺, and Cs⁺). The location of CO₂ molecules in the host structure was revealed by 3D electron diffraction (3D ED). The high sorption capacity for CO₂ (3.8 mmol g⁻¹ at 121 kPa), structural stability and regenerability of the discreate CHA zeolite nanocrystals is maintained for 10 consecutive cycles without any visible degradation. The CHA zeolite (Si:Al=2) reaches an almost perfect CO₂ storage capacity (8 CO₂ per unit cell) and high selectivity (no CH₄ was adsorbed).     

Semiconductor/Covalent-Organic-Framework Z-Scheme Heterojunctions for Artificial Photosynthesis

A strategy to covalently connect crystalline covalent organic frameworks (COFs) with semiconductors to create stable organic–inorganic Z-scheme heterojunctions for artificial photosynthesis is presented. A series of COF–semiconductor Z-scheme photocatalysts combining water-oxidation semiconductors (TiO₂, Bi₂WO₆, and α-Fe₂O₃) with CO₂ reduction COFs (COF-316/318) was synthesized and exhibited high photocatalytic CO₂-to-CO conversion efficiencies (up to 69.67 μmol g⁻¹ h⁻¹), with H₂O as the electron donor in the gas–solid CO₂ reduction, without additional photosensitizers and sacrificial agents. This is the first report of covalently bonded COF/inorganic-semiconductor systems utilizing the Z-scheme applied for artificial photosynthesis. Experiments and calculations confirmed efficient semiconductor-to-COF electron transfer by covalent coupling, resulting in electron accumulation in the cyano/pyridine moieties of the COF for CO₂ reduction and holes in the semiconductor for H₂O oxidation, thus mimicking natural photosynthesis.     

Relative Local Electron Density Tuning in Metal-Covalent Organic Frameworks for Boosting CO2 Photoreduction

The high local electron density and efficient charge carrier separation are two important factors to affect photocatalytic activity, especially for the CO₂ photoreduction reaction. However, the systematic studies on the structure-functional relationship regarding the above two factors based on precisely structure model are rarely reported. Herein, as a proof-of-concept, we developed a new strategy on the evaluation of local electron density by controlling the relative electron-deficient (ED) and electron-rich (ER) intensity of monomer at a molecular level based on three rational-designed vinylene-linked sp² carbon-covalent organic frameworks (COFs). As expected, the as-prepared vinylene-linked sp² carbon-conjugated metal-covalent organic framework (MCOFs) (VL-MCOF-1) with molecular junction exhibited excellent activities for CO₂-to-HCOOH conversion (283.41 μmol g⁻¹ h⁻¹) and high selectivity of 97.1 %, much higher than the VL-MCOF-2 and g-C₃₄N₆-COF, which is due to the synergistic effect of the multi-electronic metal clusters (Cu₃(PyCA)₃) (PyCA=pyrazolate-4-carboxaldehyde) as strong ER roles and cyanopyridine units as ED roles and active sites, as well as the boosted photo-induced charge separation efficiency of vinyl connection and increased light utilization ability. These results not only provide a strategy for regulating the electron-density distribution of photocatalysts at the molecular level but also offers profound insights for metal clusters-based COFs to effective CO₂ conversion.     

Isostructural Three‐Dimensional Covalent Organic Frameworks

Herein, we reported the designed synthesis of three isostructural three‐dimensional covalent organic frameworks (3D COFs) with ‐H, ‐Me, or ‐F substituents, which have similar crystallinity and topology. Their crystal structures were determined by continuous rotation electron diffraction (cRED), and all three 3D COFs were found to adopt a fivefold interpenetrated pts topology. More importantly, the resolution of these cRED datasets reached up to 0.9–1.0 Å, enabling the localization of all non‐hydrogen atomic positions in a COF framework directly by 3D ED techniques for the first time. In addition, the precise control of the pore environments through the use of different functional groups led to different selectivities for CO₂ over N₂. We have thus confirmed that polycrystalline COFs can be definitely studied to the atomic level as other materials, and this study should also inspire the design and synthesis of 3D COFs with tailored pore environments for interesting applications.     

Three‐Dimensional Anionic Cyclodextrin‐Based Covalent Organic Frameworks

Three‐dimensional covalent organic frameworks (3D COFs) are promising crystalline materials with well‐defined structures, high porosity, and low density; however, the limited choice of building blocks and synthetic difficulties have hampered their development. Herein, we used a flexible and aliphatic macrocycle, namely γ‐cyclodextrin (γ‐CD), as the soft struts for the construction of a polymeric and periodic 3D extended network, with the units joined via tetrakis(spiroborate) tetrahedra with various counterions. The inclusion of pliable moieties in the robust open framework endows these CD‐COFs with dynamic features, leading to a prominent Li ion conductivity of up to 2.7 mS cm⁻¹ at 30 °C and excellent long‐term Li ion stripping/plating stability. Exchanging the counterions within the pores can effectively modulate the interactions between the CD‐COF and CO₂ molecules.     

Multicomponent Synthesis of Imidazole-Linked Fully Conjugated 3D Covalent Organic Framework for Efficient Electrochemical Hydrogen Peroxide Production

The semiconducting properties and applications of three dimensional (3D) covalent organic frameworks (COFs) are greatly hampered because of their long-ranged non-conjugated skeletons and relatively unstable linkages. Here, a robust imidazole-linked fully conjugated 3D covalent organic framework (BUCT-COF-7) is synthesized through the one-pot multicomponent Debus-Radziszewski reaction of the saddle-shaped aldehyde-substituted cyclooctatetrathiophene, pyrene-4,5,9,10-tetraone, and ammonium acetate. The semiconducting BUCT-COF-7, as a metal-free catalyst, shows excellent two electron oxygen reduction reaction (ORR) activity in alkaline medium with high hydrogen peroxide (H₂O₂) selectivity of 83.4 %. When the BUCT-COF-7 as cathode catalyst is assembled into the electrolyzer, the devices showed high electrochemical production rate of H₂O₂ up to 326.9 mmol g⁻¹ h⁻¹. The accumulative amount of H₂O₂ could totally degrade the dye methylene blue via Fenton reaction for wastewater treatment. This is the first report about intrinsic 3D COFs for efficient electrochemical synthesis of H₂O₂, revealing the promising applications of fully conjugated 3D COFs in the environment-related field.     

Pore Geometry and Surface Engineering of Covalent Organic Frameworks for Anhydrous Proton Conduction

Developing new materials for anhydrous proton conduction under high-temperature conditions is significant and challenging. Herein, we create a series of highly crystalline covalent organic frameworks (COFs) via a pore engineering approach. We simultaneously engineer the pore geometry (generating concave dodecagonal nanopores) and pore surface (installing multiple functional groups such as −C=N−, −OH, −N=N− and −CF₃) to improve the utilization efficiency and host–guest interaction of proton carriers, hence benefiting the enhancement of anhydrous proton conduction. Upon loading with H₃PO₄, COFs can realize a proton conductivity of 2.33×10⁻² S cm⁻¹ under anhydrous conditions, among the highest values of all COF materials. These materials demonstrate good stability and maintain high proton conductivity over a wide temperature range (80–160 °C). This work paves a new way for designing COFs for anhydrous proton conduction applications, which shows great potential as high-temperature proton exchange membranes.     

The mechanism and kinetics of energy transfer processes in Xe–CCl4–M (M=CO,CO2) mixtures irradiated by xenon resonance light

The mechanism and kinetics of energy transfer from the Xe(6s[3/2]₁) resonance state to CO and CO₂ molecules have been investigated by XeCl(B–X) (λ_max=308 nm) fluorescence intensity measurements at stationary conditions in Xe–CCl₄–M systems. Steady-state analysis of the fluorescence intensity dependence on the xenon and M pressure at constant CCl₄ concentration shows that these processes occur in two- and three-body reactions: Xe(6s[3/2]₁⁰)+M→products; Xe(6s[3/2]₁⁰)+M+Xe→products. The two-body rate constants for above reactions have been found to be (0.7±0.2)×10⁻¹⁰ and (4.9±0.4)×10⁻¹⁰ cm³ s⁻¹ for CO and CO₂, respectively. The three-body rate constants have been found to be (3±1)×10⁻²⁹ and (2.4±0.3)×10⁻²⁸ cm⁶ s⁻¹ for CO and CO₂, respectively. It has been shown that the third order reaction is a very effective channel of xenon excited atoms decay at high xenon pressures (P(Xe)>50 Torr).     

Characterization of [M–H] cations,radicals and anions of glycine in the gas phase: a combined experimental and ab initio study

The gas phase structures of the [M–H] cations and anions of glycine have been studied by using a combination of ab initio calculations (at the MP2(FC)/6–31+G1 level of theory) and tandem mass spectrometry (MS/MS). It was found that the ab initio stability order for the anions is [H₂NCH₂CO₂]⁻ > [H₂NCHCO₂H]⁻ > [HNCH₂CO₂H]⁻. In contrast, the cations exhibit different behaviour, whereas [H₂NCHCO₂H]⁺ is predicted to be a stable structure, [H₂NCH₂CO₂]⁺ spontaneously fragments to the ion–molecule complex [H₂NCH₂⁺ ⋯ (OCO)] and the singlet [HNCH₂CO₂H]⁺ isomer is predicted to undergo a skeletal rearrangement to form [CH₂NHCO₂H]⁺. MS/MS spectra of [M–H]⁺ cations of various glycine isotopomers were obtained via: (i) collisional activation of electron impact generated cations and (ii) charge reversal of anions formed via HO⁻ negative ion chemical ionization. The resulting spectra were significantly different, suggesting different structures were involved. Neutralization–reionization experiments were performed on [M–H]⁻ anions in order to gain insights into the structures of the intermediate radicals.     

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.     

Maximizing Electroactive Sites in a Three-Dimensional Covalent Organic Framework for Significantly Improved Carbon Dioxide Reduction Electrocatalysis

Synthesis of functional 3D COFs with irreversible bond is challenging. Herein, 3D imide-bonded COFs were constructed via the imidization reaction between phthalocyanine-based tetraanhydride and 1,3,5,7-tetra(4-aminophenyl)adamantine. These two 3D COFs are made up of interpenetrated pts networks according to powder X-ray diffraction and gas adsorption analyses. CoPc-PI-COF-3 doped with carbon black has been employed to fabricate the electrocatalytic cathode towards CO₂ reduction reaction within KHCO₃ aqueous solution, displaying the Faradaic efficiency of 88–96 % for the CO₂-to-CO conversion at the voltage range of ca. ?0.60 to ?1.00 V (vs. RHE). In particular, the 3D porous structure of CoPc-PI-COF-3 enables the active electrocatalytic centers occupying 32.7 % of total cobalt-phthalocyanine subunits, thus giving a large current density (j_CO) of ?31.7 mA cm^?2 at ?0.90 V. These two parameters are significantly improved than the excellent 2D COF analogue (CoPc-PI-COF-1, 5.1 % and ?21.2 mA cm^?2).     

Photochemical In Situ Exfoliation of Metal–Organic Frameworks for Enhanced Visible-Light-Driven CO2 Reduction

Two novel two-dimensional metal–organic frameworks (2D MOFs), 2D-M₂TCPE (M=Co or Ni, TCPE=1,1,2,2-tetra(4-carboxylphenyl)ethylene), which are composed of staggered (4,4)-grid layers based on paddlewheel-shaped dimers, serve as heterogeneous photocatalysts for efficient reduction of CO₂ to CO. During the visible-light-driven catalysis, these structures undergo in situ exfoliation to form nanosheets, which exhibit excellent stability and improved catalytic activity. The exfoliated 2D-M₂TCPE nanosheets display a high CO evolution rate of 4174 μmol g⁻¹ h⁻¹ and high selectivity of 97.3 % for M=Co and Ni, and thus are superior to most reported MOFs. The performance differences and photocatalytic mechanisms have been studied with theoretical calculations and photoelectric experiments. This study provides new insight for the controllable synthesis of effective crystalline photocatalysts based on structural and morphological coregulation.     

Semiconductor/Covalent‐Organic‐Framework Z‐Scheme Heterojunctions for Artificial Photosynthesis

A strategy to covalently connect crystalline covalent organic frameworks (COFs) with semiconductors to create stable organic–inorganic Z‐scheme heterojunctions for artificial photosynthesis is presented. A series of COF–semiconductor Z‐scheme photocatalysts combining water‐oxidation semiconductors (TiO₂, Bi₂WO₆, and α‐Fe₂O₃) with CO₂ reduction COFs (COF‐316/318) was synthesized and exhibited high photocatalytic CO₂‐to‐CO conversion efficiencies (up to 69.67 μmol g^?1 h^?1), with H₂O as the electron donor in the gas–solid CO₂ reduction, without additional photosensitizers and sacrificial agents. This is the first report of covalently bonded COF/inorganic‐semiconductor systems utilizing the Z‐scheme applied for artificial photosynthesis. Experiments and calculations confirmed efficient semiconductor‐to‐COF electron transfer by covalent coupling, resulting in electron accumulation in the cyano/pyridine moieties of the COF for CO₂ reduction and holes in the semiconductor for H₂O oxidation, thus mimicking natural photosynthesis.     

Synthesis of Porous,Nitrogen‐Doped Adsorption/Diffusion Carbonaceous Membranes for Efficient CO2 Separation

A porous, nitrogen‐doped carbonaceous free‐standing membrane (TFMT‐550) is prepared by a facile template‐free method using letrozole as an intermediate to a triazole‐functionalized‐triazine framework, followed by carbonization. Such adsorption/diffusion membranes exhibit good separation performance of CO₂ over N₂ and surpassing the most recent Robeson upper bound. An exceptional ideal CO₂/N₂ permselectivity of 47.5 was achieved with a good CO₂ permeability of 2.40 × 10⁻¹³ mol m m⁻² s⁻¹ Pa⁻¹. The latter results arise from the presence of micropores, narrow distribution of small mesopores and from the strong dipole–quadrupole interactions between the large quadrupole moment of CO₂ molecules and the polar sites associated with N groups (e.g., triazine units) within the framework.     

Eutectic Molten Salt Synthesis of Highly Microporous Macrocyclic Porous Organic Polymers for CO2 Capture

The development of porous materials is of great interest for the capture of CO₂ from various emission sources, which is essential to mitigate its detrimental environmental impact. In this direction, porous organic polymers (POPs) have emerged as prime candidates owing to their structural tunability, physiochemical stability and high surface areas. In an effort to transfer an intrinsic property of a cyclotetrabenzoin-derived macrocycle – its high CO₂ affinity – into porous networks, herein we report the synthesis of three-dimensional (3D) macrocycle-based POPs through the polycondensation of an octaketone macrocycle with phenazine-2,3,7,8-tetraamine hydrochloride. This polycondensation was performed under ionothermal conditions, using a eutectic salt mixture in the temperature range of 200 to 300 °C. The resulting polymers, named 3D-mmPOPs, showed reaction temperature-dependent surface areas and gas uptake properties. 3D-mmPOP-250 synthesized at 250 °C exhibited a surface area of 752 m² g⁻¹ and high microporosity originating from the macrocyclic units, thus resulting in an excellent CO₂ binding enthalpy of 40.6 kJ mol⁻¹ and CO₂ uptake capacity of 3.51 mmol g⁻¹ at 273 K, 1.1 bar.     

N-Doped Porous Carbon Derived from Solvent-Free Synthesis of Cross-Linked Triazine Polymers for Simultaneously Achieving CO2 Capture and Supercapacitors

It is highly desirable to design advanced heteroatomic doped porous carbon for wide application. Herein, N-doped porous carbon (NPC) was developed via the fabrication of high nitrogen cross-linked triazine polymers followed by pyrolysis and activation with controllable porous structure. The as-synthesized NPC at the pyrolysis temperature of 700 °C possessed rich nitrogen content (up to 11.51 %) and high specific surface area (1353 m² g⁻¹), which led to a high CO₂ adsorption capability at 5.67 mmol g⁻¹ at 298.15 K and 5 bar pressure and excellent stability. When the activation temperature was at 600 °C, such NPC exhibited a superior electrochemical performance as anode for supercapacitors with a specific capacitance of 158.8 and 113 F g⁻¹ in 6 M KOH at a current density of 1 and 10 A g⁻¹, respectively. Notably, it delivered an excellent stability with capacity retention of 97.4 % at 20 A g⁻¹after 6000 cycles.     

Tuning the CO2 Hydrogenation Selectivity of Rhodium Single-Atom Catalysts on Zirconium Dioxide with Alkali Ions

Tuning the coordination environments of metal single atoms (M₁) in single-atom catalysts has shown large impacts on catalytic activity and stability but often barely on selectivity in thermocatalysis. Here, we report that simultaneously regulating both Rh₁ atoms and ZrO₂ support with alkali ions (e.g., Na) enables efficient switching of the reaction products from nearly 100 % CH₄ to above 99 % CO in CO₂ hydrogenation in a wide temperature range (240–440 °C) along with a record high activity of 9.4 mol_CO g_Rh⁻¹ h⁻¹ at 300 °C and long-term stability. In situ spectroscopic characterization and theoretical calculations unveil that alkali ions on ZrO₂ change the surface intermediate from formate to carboxy species during CO₂ activation, thus leading to exclusive CO formation. Meanwhile, alkali ions also reinforce the electronic Rh₁-support interactions, endowing the Rh₁ atoms more electron deficient, which improves the stability against sintering and inhibits deep hydrogenation of CO to CH₄.     

One-Dimensional Covalent Organic Frameworks for the 2e− Oxygen Reduction Reaction

Two-dimensional covalent organic frameworks (2D COFs) are often employed for electrocatalytic systems because of their structural diversity. However, the efficiency of atom utilization is still in need of improvement, because the catalytic centers are located in the basal layers and it is difficult for the electrolytes to access them. Herein, we demonstrate the use of 1D COFs for the 2e⁻ oxygen reduction reaction (ORR). The use of different four-connectivity blocks resulted in the prepared 1D COFs displaying good crystallinity, high surface areas, and excellent chemical stability. The more exposed catalytic sites resulted in the 1D COFs showing large electrochemically active surface areas, 4.8-fold of that of a control 2D COF, and thus enabled catalysis of the ORR with a higher H₂O₂ selectivity of 85.8 % and activity, with a TOF value of 0.051 s⁻¹ at 0.2 V, than a 2D COF (72.9 % and 0.032 s⁻¹). This work paves the way for the development of COFs with low dimensions for electrocatalysis.     

A Layered Cationic Aluminum Oxyhydroxide as a Highly Efficient and Selective Trap for Heavy Metal Oxyanions

Cationic framework materials, especially pure inorganic cationic frameworks that can efficiently and selectively capture harmful heavy metal oxyanions from aqueous solution are highly desired yet scarcely reported. Herein, we report the discovery of a 2D cationic aluminum oxyhydroxide, JU-111, which sets a new benchmark for heavy metal oxyanion sorbents, especially for Cr^VI. Its structure was solved based on 3D electron diffraction tomography data. JU-111 shows fast sorption kinetics (ca. 20 min), high capture capacity (105.4 mg g⁻¹), and broad working pH range (3–10) toward Cr^VI oxyanions. Unlike layered double hydroxides (LDHs), which are poorly selective in the presence of CO₃²⁻, JU-111 retains excellent selectivity for Cr^VI even under a large excess of CO₃²⁻. These superior features coupled with the ultra-low cost and environmentally benign nature make JU-111 a promising candidate for toxic metal oxyanion remediation as well as other potential applications.