From Supramolecular Organic Cages to Porous Covalent Organic Frameworks for Enhancing Iodine Adsorption Capability by Fully Exposed Nitrogen-Rich Sites

Deposition of Imidazole into Mesoporous Zirconium Metal-Organic Framework for Iodine Capture

Efficient capture of radioiodine is essential for the protection of the ecological environment and the sustainable development of nuclear energy generation. Metal-organic frameworks (MOFs) and their derivatives provide alternative potential for overcoming the limitations of traditional iodine adsorbents. Herein this work, imidazole (Im) is dispersed into a robust mesoporous zirconium MOF PCN-222 by the heat deposition method, forming a high-uniformity composite Im@PCN-222. The large pore size and marked chemical resistance skeleton of PCN-222 allow the incorporation of Im to reach 43 wt % while surviving the chemical corrosion of activated Im. The nearly saturated loading of Im significantly benefits the sorption capability toward iodine and gives rise to an exceptional adsorption capacity of 10.04 g g-1 and excellent recyclability. This value is higher than that of most of the MOFs and their derivatives. Further, Im was also successfully deposited onto the PCN-222 functionalized porous Al2O3 ceramic filtration membrane, which endowed the hybrid membrane with efficient iodine sorption and separation properties. This work highlights a new strategy for the fabrication of the MOF-based radioiodine adsorption composite as well as MOF composite-based filters.

Porous Organic Cage as an Efficient Platform for Industrial Radioactive Iodine Capture

Herein, we firstly develop porous organic cage (POC) as an efficient platform for highly effective radioactive iodine capture under industrial operating conditions (typically ≥150 °C), ≤150 ppmv of I2). Due to the highly dispersed and readily accessible binding sites as well as sufficient accommodating space, the constructed NKPOC-DT-(I-) (NKPOC=Nankai porous organic cage) demonstrates a record-high I2 uptake capacity of 48.35 wt % and extraordinary adsorption capacity of unit ionic site (~1.62) at 150 °C and 150 ppmv of I2. The I2 capacity is 3.5, 1.6, and 1.3 times higher than industrial silver-based adsorbents Ag@MOR and benchmark materials of TGDM and 4F-iCOF-TpBpy-I- under the same conditions. Furthermore, NKPOC-DT-(I-)Me exhibits remarkable adsorption kinetics (k1=0.013 min-1), which is 1.2 and 1.6 times higher than TGDM and 4F-iCOF-TpBpy-I- under the identical conditions. NKPOC-DT-(I-)Me thus sets a new benchmark for industrial radioactive I2 adsorbents. This work not only provides a new insight for effectively enhancing the adsorption capacity of unit functional sites, but also advances POC as an efficient platform for radioiodine capture in industry.

Halogen-functionalized covalent organic frameworks for photocatalytic Cr(VI) reduction under visible light

Covalent organic frameworks (COFs) are a type of novel organic catalysts which show great potential in the treatment of environmental contaminations. Herein, we synthesized three isoreticular halogen-functionalized (F, Cl and Br) porphyrin COFs for visible-light (420 nm ≤ λ ≤ 780 nm) photocatalytic reduction of Cr(VI) to Cr(III). Halogen substituents with tunable electronegativity can regulate the band structure and modulate the charge carrier kinetics of COFs. In the absence of any sacrificial reagent, the isoreticular COFs exhibited good photocatalytic reduction activity of Cr(VI). Particularly, the TAPP-2F showed nearly 100 % conversion efficiency and the highest reaction rate constants (k) on account of the strong electronegativity of F substituent. Experimental results and theoretical calculations showed that the conduction band (CB) potentials of COFs became more negative and charge carrier separation increased with the enhancement of electronegativity (Br < Cl < F), which could provide sufficient driving force for the photoreduction of Cr(VI) to Cr(III). The halogen substituents strategy for regulating the electronic structure of COFs can provide opportunities for designing efficient photocatalysts for environmental remediation. Meanwhile, the mechanistic insights reported in this study help to understand the photocatalytic degradation pathways of heavy metals.

N-Heteroatom Engineered Nonporous Amorphous Self-Assembled Coordination Cages for Capture and Storage of Iodine

Radioactive iodine isotopes from nuclear-related activities, present substantial risks to human health and the environment. Developing effective materials for the capture and storage of these hazardous molecules is paramount. Traditionally, nonporous solids were historically considered ineffective for adsorbing target species. In this study, we investigate the potential of four nonporous, amorphous, self-assembled coordination cages (C1, C2, C3, and C4) featuring varying numbers of nitrogen atoms within the core (pyridyl/triazine unit) and specific cavity sizes for iodine adsorption. These coordination cages demonstrate remarkable adsorption abilities for iodine in both vapor and solution phases, facilitated by enhanced electron-pair interactions. The cages exhibit high uptake capacities of up to 3.16 g g-1 at 75 °C, the highest among metal-organic cages and up to 434.29 mg g-1 in solution, highlighting the efficiency of these materials across different phases. Even at ambient temperature, they show significant iodine capture efficiency, with a maximum value of 1.5 g g-1. Furthermore, these robust materials can be recycled, enduring at least five reusable cycles without apparent fatigue. Overall, our findings present a "N-heteroatom engineering" approach for the development of recyclable amorphous containers for the capture and storage of iodine, contributing to the mitigation of nuclear-related risks.

An Alkyne-Bridged Covalent Organic Framework Featuring Interactive Pockets for Bromine Capture

The high degree of corrosivity and reactivity of bromine, which is released from various sources, poses a serious threat to the environment. Moreover, its coexistence with iodine forming an equilibrium compound, iodine monobromide (IBr) necessitates the selective capture of bromine from halogen mixtures. The electrophilicity of halogens to π-electron rich structures enabled us to strategically design a covalent organic framework for halogen capture, featuring a defined pore environment with localized sorption sites. The higher capture capacity of bromine (4.6 g g-1) over iodine by ~41 % shows its potential in selective capture. Spectroscopic results uncovering the preferential interaction sites are supported by theoretical investigations. The alkyne bridge is a core functionality promoting the selectivity in capture by synergistic physisorption, rationalized by the higher orbital overlap of bromine due to its smaller atomic size as well as reversible chemical interactions. The slip stacking in the structure has further promoted this phenomenon by creating clusters of molecular interaction sites with bromine intercalated between the layers. The inclusion of unsaturated moieties, i.e. triple bonds and the complementary pore geometry offer a promising design strategy for the construction of porous materials for halogen capture.

Optimizing Iodine Enrichment through Induced-Fit Transformations in a Flexible Ag(I)-Organic Framework: From Accelerated Adsorption Kinetics to Record-High Storage Density

Efficient capture and storage of radioactive I2 is a prerequisite for developing nuclear power but remains a challenge. Here, two flexible Ag-MOFs (FJI-H39 and 40) with similar active sites but different pore sizes and flexibility are prepared; both of them can capture I2 with excellent removal efficiencies and high adsorption capacities. Due to the more flexible pores, FJI-H39 not only possesses the record-high I2 storage density among all the reported MOFs but also displays a very fast adsorption kinetic (124 times faster than FJI-H40), while their desorption kinetics are comparable. Mechanistic studies show that FJI-H39 can undergo induced-fit transformations continuously (first contraction then expansion), making the adsorbed iodine species enrich near the Ag(I) nodes quickly and orderly, from discrete I- anion to the dense packing of various iodine species, achieving the very fast adsorption kinetic and the record-high storage density simultaneously. However, no significant structural transformations caused by the adsorbed iodine are observed in FJI-H40. In addition, FJI-H39 has excellent stability/recyclability/obtainability, making it a practical adsorbent for radioactive I2. This work provides a useful method for synthesizing practical radioactive I2 adsorbents.

Fine-tuning covalent organic frameworks for structure-activity correlation via adsorption and catalytic studies

In applications utilizing Covalent Organic Frameworks (COFs) for adsorption, the interplay between crystallinity (vis-à-vis surface area) and active sites still remains ambiguous. To address this, the present study introduces three isoreticular COFs-COP-N18 (covalent organic polymer with short-range order), COF-N18 (COF having long-range order), and COF-N27 (semicrystalline COF with pyridyl heteroatoms)-to explore this duality. Through systematic variations in structural order, pore volume, and pore-wall nitrogen content, we aim to establish a structure-activity relationship (SAR) for these COFs via adsorption and catalysis, using CO2 and I2 as probes. Our investigation highlights the positive influence of crystallinity, surface area, and pore volume in adsorption as well as catalysis. However, the presence of heteroatoms manifests complex behavior in CO2 adsorption and CO2 cycloaddition reactions with epoxides. COF-N18 and COF-N27 showed comparable CO2 uptake capacities at different temperatures (273, 293, and 313 K) and ∼1 bar pressure. Additionally, CO2 cycloaddition reactions were performed with substrates possessing different polarities (epichlorohydrin, 1,2-epoxydodecane) to elucidate the role of COF surface polarity. Further investigation into iodine adsorption was performed to understand the impact of COF structural features on the modes of adsorption and adsorption kinetics. Improvements in COF-crystallinity results in faster average iodine uptake rate at 80% (K80% = 1.79 g/h) by COF-N18. Whereas, heteroatom doping slows down iodine adsorption kinetics (0.35 g/h) by prolonging the adsorption process up to 72 h. Overall, this study advances our understanding of COFs as adsorbents and catalysts, providing key insights into their SAR while emphasizing structural fine-tuning as a key factor for impactful environmental applications.

Synthesis of Calix[4]arene-Based Porous Organic Cages and Their Gas Adsorption

Two crystalline large-sized porous organic cages (POCs) based on conical calix[4]arene (C4A) were designed and synthesized. The four-jaw C4A unit tends to follow the face-directed self-assembly law with the planar triangular building blocks such as tris(4-aminophenyl)amine (TAPA) or 1,3,5-tris(4-aminophenyl)benzene (TAPB) to generate a predictable cage with a stoichiometry of [6+8]. The formation of the large cages is confirmed through their relative molecular mass measured using MALDI-TOF/TOF spectra. The protonated molecular ion peaks of C4A-TAPA and C4A-TAPB were observed at m/z 5109.0 (calculated for C336H240O24N32: m/z 5109.7) and m/z 5594.2 (calculated for C384H264O24N24: m/z 5598.4). C4A-POCs exhibit I-type N2 adsorption-desorption isotherms with the BET surface areas of 1444.9 m2 ⋅ g-1 and 1014.6 m2 ⋅ g-1. The CO2 uptakes at 273 K are 62.1 cm3 ⋅ g-1 and 52.4 cm3 ⋅ g-1 at a pressure of 100 KPa. The saturated iodine vapor static uptakes at 348 K are 3.9 g ⋅ g-1 and 3.5 g ⋅ g-1. The adsorption capacity of C4A-TAPA for SO2 reaches to 124.4 cm3 ⋅ g-1 at 298 K and 1.3 bar. Additionally, the adsorption capacities of C4A-TAPA for C2H2, C2H4, and C2H6 were evaluated.

Fluorinated Nonporous Adaptive Cages for the Efficient Removal of Perfluorooctanoic Acid from Aqueous Source Phases

Per- and polyfluoroalkyl substances (PFAS) accumulate in water resources and pose serious environmental and health threats due to their nonbiodegradable nature and long environmental persistence times. Strategies for the efficient removal of PFAS from contaminated water are needed to address this concern. Here, we report a fluorinated nonporous adaptive crystalline cage (F-Cage 2) that exploits electrostatic interaction, hydrogen bonding, and F-F interactions to achieve the efficient removal of perfluorooctanoic acid (PFOA) from aqueous source phases. F-Cage 2 exhibits a high second-order kobs value of approximately 441,000 g mg-1 h-1 for PFOA and a maximum PFOA adsorption capacity of 45 mg g-1. F-Cage 2 can decrease PFOA concentrations from 1500 to 6 ng L-1 through three rounds of flow-through purification, conducted at a flow rate of 40 mL h-1. Elimination of PFOA from PFOA-loaded F-Cage 2 is readily achieved by rinsing with a mixture of MeOH and saturated NaCl. Heating at 80 °C under vacuum then makes F-Cage 2 ready for reuse, as demonstrated across five successive uptake and release cycles. This work thus highlights the potential utility of suitably designed nonporous adaptive crystals as platforms for PFAS remediation.

Efficient Removal of Iodine from Water by a Calix[4]pyrrole-Based Nanofilm

The efficient removal of radioactive iodine from an aqueous solution is largely dependent on the adsorbent materials employed. In this work, we report a calix[4]pyrrole-based nanofilm and its application for the rapid removal of iodine from water. The nanofilm was synthesized through a confined dynamic condensation of tetra hydrazide calix[4]pyrrole with 1,3,5-tri-(4-formylphenyl) aldehyde at the air/dimethyl sulfoxide (DMSO) interface. The thickness of the obtained nanofilm is ∼35 nm, enabling fast mass transfer and a high ratio of accessible binding sites for iodine. The pseudo-second-order rate constant of the nanofilm for iodine is ∼0.061 g g-1 min-1, 3 orders of magnitude higher than most reported adsorbent materials. Flow-through nanofiltration tests demonstrated that the nanofilm has an adsorption capacity of 1.48 g g-1, a high removal efficiency, and good reusability. The mechanism study revealed that the moieties of Schiff base, pyrrole, and aromatic rings play a key role for binding iodine. We believe this work provides not only a new strategy for the efficient removal of radioactive iodine from water but also new ideas for designing efficient iodine adsorbents.

Design and assembly of porous organic cages

Porous organic cages (POCs) represent a notable category of porous materials, showing remarkable material properties due to their inherent porosity. Unlike extended frameworks which are constructed by strong covalent or coordination bonds, POCs are composed of discrete molecular units held together by weak intermolecular forces. Their structure and chemical traits can be systematically tailored, making them suitable for a range of applications including gas storage and separation, molecular separation and recognition, catalysis, and proton and ion conduction. This review provides a comprehensive overview of POCs, covering their synthesis methods, structure and properties, computational approaches, and applications, serving as a primer for those who are new to the domain. A special emphasis is placed on the growing role of computational methods, highlighting how advanced data-driven techniques and automation are increasingly aiding the rapid exploration and understanding of POCs. We conclude by addressing the prevailing challenges and future prospects in the field.

Robust links in photoactive covalent organic frameworks enable effective photocatalytic reactions under harsh conditions

Developing heterogeneous photocatalysts for the applications in harsh conditions is of high importance but challenging. Herein, by converting the imine linkages into quinoline groups of triphenylamine incorporated covalent organic frameworks (COFs), two photosensitive COFs, namely TFPA-TAPT-COF-Q and TFPA-TPB-COF-Q, are successfully constructed. The obtained quinoline-linked COFs display improved stability and photocatalytic activity, making them suitable photocatalysts for photocatalytic reactions under harsh conditions, as verified by the recyclable photocatalytic reactions of organic acid involving oxidative decarboxylation and organic base involving benzylamine coupling. Under strong oxidative condition, the quinoline-linked COFs show a high efficiency up to 11831.6 μmol·g-1·h-1 and a long-term recyclable usability for photocatalytic production of H2O2, while the pristine imine-linked COFs are less catalytically active and easily decomposed in these harsh conditions. The results demonstrate that enhancing the linkage robustness of photoactive COFs is a promising strategy to construct heterogeneous catalysts for photocatalytic reactions under harsh conditions.

Recent progress in iodine capture by macrocycles and cages

The effective capture of radioiodine is vital to the development of the nuclear industry and ecological environmental protection. There is, therefore, a continuously growing research exploration in various types of solid-state materials for iodine capture. During the last decade, the potential of using macrocycle and cage-based supramolecular materials in effective uptake and separation of radioactive iodine has been demonstrated. Interest in the application of these materials in iodine capture originates from their diversified porous characteristics, abundant host-guest chemistry, high iodine affinity and adsorption capacity, high stability in various environments, facile modification and functionalization, and intrinsic structural flexibility, among other attributes. Herein, recent progress in macrocycle and cage-based solid-state materials, including pure discrete macrocycles and cages, and their polymeric forms, for iodine capture is summarized and discussed with an emphasis on iodine capture capacities, mechanisms, and design strategies.

Preparation of Novel Nitrogen-Rich Fluorinated Hyperbranched Poly(amide-imide) and Evaluation of Its Electrochromic Properties and Iodine Adsorption Behavior

In this study, we successfully synthesized a novel triacid monomer by means of the thermal cyclization reaction. Subsequently, a series of nitrogen-rich (A3+B2)-type fluorinated hyperbranched poly(amide-imide)s (denoted as PAI-1 and -2, respectively) were prepared by means of a one-pot method using this triacid monomer and a diamine monomer with a triphenylamine-carbazole unit as precursors. The degree of support of the prepared hyperbranched PAIs was found to be about 60% via 1H NMR calculations. Through X-ray photoelectron spectroscopy (XPS), it was found that the binding energies of C-N (398.4 eV) and -NH (399.7 eV) became lower under a current, while the binding energy peak of N+ appeared at 402.9 eV. In addition, the PAIs have good solubility and thermal stability (Tgs: 256-261 °C, T10%: 564-608 °C). Cyclic voltammetry (CV) analysis shows that the hyperbranched PAI films have good redox properties, and a range of values for the HOMO (4.83 to 4.85 eV) versus LUMO (1.85 to 1.97 eV) energy levels are calculated. The PAI films have excellent electrochromic properties: PAI-1 on coloration efficiency (CE) and transmittance change (ΔT, 852 nm) are 257 cm2/C and 62%, respectively, and have long-lasting redox properties (100 cycles). In addition, we conduct iodine adsorption tests using the structural features of PAIs with electron-drawing units, and the results show that PAI-1 had a high adsorption capacity for iodine (633 mg/g).

A Halogen-Bonded Organic Framework (XOF) Emissive Cocrystal for Acid Vapor and Explosive Sensing, and Iodine Capture

There is a strong and urgent need for efficient materials that can capture radioactive iodine atoms from nuclear waste. This work presents a novel strategy to develop porous materials for iodine capture by employing halogen bonding, mechanochemistry and crystal engineering. 3D halogen-bonded organic frameworks (XOFs) with guest-accessible permanent pores are exciting targets in crystal engineering for developing functional materials, and this work reports the first example of such a structure. The new-found XOF, namely TIEPE-DABCO, exhibits enhanced emission in the solid state and turn-off emission sensing of acid vapors and explosives like picric acid in nanomolar quantity. TIEPE-DABCO captures iodine from the gas phase (3.23 g g-1 at 75 °C and 1.40 g g-1 at rt), organic solvents (2.1 g g-1 ), and aqueous solutions (1.8 g g-1 in the pH range of 3-8); the latter with fast kinetics. The captured iodine can be retained for more than 7 days without any leaching, but readily released using methanol, when required. TIEPE-DABCO can be recycled for iodine capture several times without any loss of storage capacity. The results presented in this work demonstrate the potential of mechanochemical cocrystal engineering with halogen bonding as an approach to develop porous materials for iodine capture and sensing.

[M8L4]8+-Type Squares Self-Assembled by Dipalladium Corners and Bridging Aromatic Dipyrazole Ligands for Iodine Capture

Square-like metallamacrocyclic palladium(II) complexes [M₈L₄]⁸⁺ (1-7) were synthesized by reacting aromatic dipyrazole ligands (H₂L¹-H₂L³ with pyromellitic arylimide-, 1,4,5,8-naphthalenetetracarboxylic arylimide-, and anthracene-based aromatic groups, respectively) with dipalladium corners ([(bpy)₂Pd₂(NO₃)₂](NO₃)₂, [(dmbpy)₂Pd₂(NO₃)₂](NO₃)₂, or [(phen)₂Pd₂(NO₃)₂](NO₃)₂, where bpy = 2,2'-bipyridine, dmbpy = 4,4'-dimethyl-2,2'-bipyridine, and phen = 1,10-phenanthroline) in aqueous solutions via metal-directed self-assembly. Metallamacrocycles 1-7 were fully characterized by ¹H and ¹³C nuclear magnetic resonance spectroscopy and electrospray ionization mass spectrometry, and the square structure of 7·8NO₃^- was further confirmed via single crystal X-ray diffraction. These square-like metallamacrocycles exhibit effective performance for iodine adsorption.