Precise Modulation of Carbon Activity Sites in Metal-Free Covalent Organic Frameworks for Enhanced Oxygen Reduction Electrocatalysis.

Metal-free carbon-based materials have gained recognition as potential electrocatalysts for the oxygen reduction reaction (ORR) in new environmentally-friendly electrochemical energy conversion technologies. The presence of effective active centers is crucial for achieving productive ORR. In this study, we present the synthesis of two metal-free dibenzo[a,c]phenazine-based covalent organic frameworks (DBP-COFs), specifically JUC-650 and JUC-651, which serve as ORR electrocatalysts. Among them, JUC-650 demonstrates exceptional catalytic performance for ORR in alkaline electrolytes, exhibiting an onset potential of 0.90 V versus RHE and a half-wave potential of 0.72 V versus RHE. Consequently, JUC-650 stands out as one of the most outstanding metal-free COF-based ORR electrocatalysts report to date. Experimental investigations and density functional theory calculations confirm that modulation of the frameworks' electronic configuration allows for the reduction of adsorption energy at the Schiff-base carbon active sites, leading to more efficient ORR processes. Moreover, the DBP-COFs can be assembled as excellent air cathode catalysts for zinc-air batteries (ZAB), rivaling the performance of commercial Pt/C. This study provides valuable insights for the development of efficient metal-free organoelectrocatalysts through precise regulation of active site strategies.

Piezofluorochromism in Covalent Organic Frameworks: Pressure-Induced Emission Enhancement and Blue-Shifted Emission.

Achieving enhanced or blue-shifted emission from piezochromic materials remains a major challenge. Covalent organic frameworks (COFs) are promising candidates for the development of piezochromic materials owing to their dynamic structures and adjustable optical properties, where the emission behaviors are not solely determined by the functional groups, but are also greatly influenced by the specific geometric arrangement. Nevertheless, this area remains relatively understudied. In this study, a successful synthesis of a series of bicarbazole-based COFs with varying topologies, dimensions, and linkages was conducted, followed by an investigation of their structural and emission properties under hydrostatic pressure generated by a diamond anvil cell. Consequently, these COFs exhibited distinct piezochromic behaviors, particularly a remarkable pressure-induced emission enhancement (PIEE) phenomenon with a 16-fold increase in fluorescence intensity from three-dimensional COFs, surpassing the performance of CPMs and most organic small molecules with PIEE behavior. On the contrary, three two-dimensional COFs with flexible structures exhibited rare blue-shifted emission, whereas the variants with rigid and conjugated structures showed common red-shifted and reduced emission. Mechanism research further revealed that these different piezochromic behaviors were primarily determined by interlayer distance and interaction. This study represents the first systematic exploration of the structures and emission properties of COFs through pressure-treated engineering and provides a new perspective on the design of piezochromic materials.

Structural Modulation of Covalent Organic Frameworks for Efficient Hydrogen Peroxide Electrocatalysis.

The electrochemical production of hydrogen peroxide (H2O2) using metal-free catalysts has emerged as a viable and sustainable alternative to the conventional anthraquinone process. However, the precise architectural design of these electrocatalysts poses a significant challenge, requiring intricate structural engineering to optimize electron transfer during the oxygen reduction reaction (ORR). Herein, we introduce a novel design of covalent organic frameworks (COFs) that effectively shift the ORR from a four-electron to a more advantageous two-electron pathway. Notably, the JUC-660 COF, with strategically charge-modified benzyl moieties, achieved a continuous high H2O2 yield of over 1200 mmol g-1 h-1 for an impressive duration of over 85 hours in a flow cell setting, marking it as one of the most efficient metal-free and non-pyrolyzed H2O2 electrocatalysts reported to date. Theoretical computations alongside in situ infrared spectroscopy indicate that JUC-660 markedly diminishes the adsorption of the OOH* intermediate, thereby steering the ORR towards the desired pathway. Furthermore, the versatility of JUC-660 was demonstrated through its application in the electro-Fenton reaction, where it efficiently and rapidly removed aqueous contaminants. This work delineates a pioneering approach to altering the ORR pathway, ultimately paving the way for the development of highly effective metal-free H2O2 electrocatalysts.

Topological Isomerism in Three-Dimensional Covalent Organic Frameworks.

Although isomerism is a typical and significant phenomenon in organic chemistry, it is rarely found in covalent organic framework (COF) materials. Herein, for the first time, we report a controllable synthesis of topological isomers in three-dimensional COFs via a distinctive tetrahedral building unit under different solvents. Based on this strategy, both isomers with a dia or qtz net (termed JUC-620 and JUC-621) have been obtained, and their structures are determined by combining powder X-ray diffraction and transmission electron microscopy. Remarkably, these architectures show a distinct difference in their porous features; for example, JUC-621 with a qtz net exhibits permanent mesopores (up to ∼23 Å) and high surface area (∼2060 m2 g-1), which far surpasses those of JUC-620 with a dia net (pore size of ∼12 Å and surface area of 980 m2 g-1). Furthermore, mesoporous JUC-621 can remove dye molecules efficiently and achieves excellent iodine adsorption (up to 6.7 g g-1), which is 2.3 times that of microporous JUC-620 (∼2.9 g g-1). This work thus provides a new way for constructing COF isomers and promotes structural diversity and promising applications of COF materials.

Ab Initio Screening of Divalent Cations for CH4, CO2, H2, and N2 Separations in Chabazite Zeolite.

The efficient separation and adsorption of critical gases are, more than ever, a major focus point in important energy processes, such as CH4 enrichment of biogas or natural gas, CO2 separation and capture, and H2 purification and storage. Thanks to its physicochemical properties, cation-exchanged chabazite is a potent zeolite for such applications. Previous computational screening investigations have mostly examined chabazites exchanged with monovalent cations. Therefore, in this contribution, periodic density functional theory (DFT) calculations in combination with dispersion corrections have been used for a systematic screening of divalent cation exchanged chabazite zeolites. The work focuses on cheap and readily available divalent cations, Ca(II), Mg(II), and Zn(II), Fe(II), Sn(II), and Cu(II) and investigates the effect of the cation nature and location within the framework on the adsorption selectivity of chabazite for specific gas separations, namely, CO2/CH4, N2/CH4, and N2/H2. All the cationic adsorption sites were explored to describe the diversity of sites in a typical experimental chabazite with a Si/Al ratio close to 2 or 3. The results revealed that Mg-CHA is the most promising cation for the selective adsorption of CO2. These predictions were further supported by ab initio molecular dynamics simulations performed at 300 K, which demonstrated that the presence of CH4 has a negligible impact on the adsorption of CO2 on Mg-CHA. Ca(II) was found to be the most favorable cation for the selective adsorption of H2 and CO2. Finally, none of the investigated cations were suitable for the preferential capture of N2 and H2 in the purification of CH4 rich mixtures. These findings provide valuable insights into the factors influencing the adsorption behavior of N2, H2, CH4, and CO2 and highlight the crucial role played by theoretical calculations and simulations for the optimal design of efficient adsorbents.

Piezochromism in Dynamic Three-Dimensional Covalent Organic Frameworks.

Piezochromic materials with pressure-dependent photoluminescence tuning properties are important in many fields, such as mechanical sensors, security papers, and storage devices. Covalent organic frameworks (COFs), as an emerging class of crystalline porous materials (CPMs) with structural dynamics and tunable photophysical properties, are suitable for designing piezochromic materials, but there are few related studies. Herein, we report two dynamic three-dimensional COFs based on aggregation-induced emission (AIE) or aggregation-caused quenching (ACQ) chromophores, termed JUC-635 and JUC-636 (JUC=Jilin University China), and for the first time, study their piezochromic behavior by diamond anvil cell technique. Due to the various luminescent groups, JUC-635 has completely different solvatochromism and molecular aggregation behavior in the solvents. More importantly, JUC-635 with AIE effect exhibits a sustained fluorescence upon pressure increase (≈3 GPa), and reversible sensitivity with high-contrast emission differences (Δλem =187 nm) up to 12 GPa, superior to other CPMs reported so far. Therefore, this study will open a new gate to expand the potential applications of COFs as exceptional piezochromic materials in pressure sensing, barcoding, and signal switching.

Dissolution Behavior and Varied Mesoporosity of Zeolites by NH4 F Etching.

The mesopores formation in zeolite crystals has long been considered to occur through the stochastic hydrolysis and removal of framework atoms. Here, we investigate the NH4 F etching of representative small, medium, and large pore zeolites and show that the zeolite dissolution behavior, therefore the mesopore formation probability, is dominated by zeolite architecture at both nano- and sub-nano scales. At the nano-scale, the hidden mosaics of zeolite structure predetermine the spatio-temporal dissolution of the framework, hence the size, shape, location, and orientation of the mesopores. At the sub-nano scale, the intrinsic micropore size and connectivity jointly determine the diffusivity of reactant and dissolved products. As a result, the dissolution propensity varies from removing small framework fragments to consuming nanodomains and up to full digestion of the outmost part of zeolite crystals. The new knowledge will lead to new understanding of zeolite dissolution behavior and new adapted strategies for tailoring hierarchical zeolites.

Gating Effects for Ion Transport in Three-Dimensional Functionalized Covalent Organic Frameworks.

The development of bioinspired nano/subnano-sized (<2 nm) ion channels is still considered a great challenge due to the difficulty in precisely controlling pore's internal structure and chemistry. Herein, for the first time, we report that three-dimensional functionalized covalent organic frameworks (COFs) can act as an effective nanofluidic platform for intelligent modulation of the ion transport. By strategic attachment of 12-crown-4 groups to the monomers as ion-driver door locks, we demonstrate that gating effects of functionalized COFs can be activated by lithium ions. The obtained materials exhibit an outstanding selective ion transmission performance with a high gating ratio (up to 23.6 for JUC-590), which is among the highest values in metal ion-activated solid-state nanochannels reported so far. Furthermore, JUC-590 offers high tunability, selectivity, and recyclability of ion transport proved by the experimental and simulated studies.

Design and Synthesis of a Zeolitic Organic Framework.

The development of novel zeolite-like materials with large channel windows and high stability is of importance but remains a tremendous challenge. Herein, we report the first example of a 3D covalent organic framework with zeolitic network, namely the zeolitic organic framework (ZOF). By combining two kinds of tetrahedral building blocks with fixed or relatively free bond angles, ZOF-1 with the zeolitic crb net has been successfully synthesized. Its structure was determined by the single-crystal 3D electron diffraction technique. Remarkably, ZOF-1 shows high chemical stability, large pore size (up to 16 Å), and excellent specific surface area (≈2785 m2 g-1 ), which is superior to its analogues with the same network, including traditional aluminosilicate zeolites and zeolitic imidazole frameworks. This study thus opens a new avenue to construct zeolite-like materials with pure organic frameworks and will promote their potential applications in adsorption and catalysis for macromolecules.

Three-Dimensional Triptycene-Functionalized Covalent Organic Frameworks with hea Net for Hydrogen Adsorption.

Owing to the finite building blocks and difficulty in structural identification, it remains a tremendous challenge to elaborately design and synthesize three-dimensional covalent organic frameworks (3D COFs) with predetermined topologies. Herein, we report the first two cases of 3D COFs with the non-interpenetrated hea net, termed JUC-596 and JUC-597, by using the combination of tetrahedral and triangular prism building units. Due to the presence of triptycene functional groups and fluorine atoms, JUC-596 exhibits an exceptional performance in the H2 adsorption up to 305 cm3 g-1 (or 2.72 wt%) at 77 K and 1 bar, which is higher than previous benchmarks from porous organic materials reported so far. Furthermore, the strong interaction between H2 and COF materials is verified through the DFT theoretical calculations. This work represents a captivating example of rational design of functional COFs based on a reticular chemistry guide and demonstrates its promising application in clean energy storage.

Platelike MFI Crystals with Controlled Crystal Faces Aspect Ratio.

Zeolite crystals offering a short diffusion pathway through the pore network are highly desired for a number of catalytic and molecule separation applications. Herein, we develop a simple synthetic strategy toward reducing the thickness along the b-axis of MFI-type crystals, thus providing a short diffusion path along the straight channel. Our approach combines preliminary aging and a fluoride-assisted low-temperature crystallization. The synthesized MFI crystals are in the micrometer-size range along the a- and c-axis, while the thickness along the b-axis is a few tens of nanometers. The synthesis parameters controlling the formation of platelike zeolite are studied, and the factors controlling the zeolite growth are identified. The synthesis strategy works equally well with all-silica MFI (silicalite-1) and its Al- and Ga-containing derivatives. The catalytic activity of platelike ZSM-5 in the methanol-to-hydrocarbons (MTH) reaction is compared with a commercial nanosized ZSM-5 sample, as the platelike ZSM-5 exhibits a substantially extended lifetime. The synthesis of platelike MFI crystals is successfully scaled up to a kilogram scale.

Three-Dimensional Triptycene-Based Covalent Organic Frameworks with ceq or acs Topology.

The growth of three-dimensional covalent organic frameworks (3D COFs) with new topologies is still considered as a great challenge due to limited availability of high-connectivity building units. Here we report the design and synthesis of 3D triptycene-based COFs, termed JUC-568 and JUC-569, following the deliberate symmetry-guided design principle. By combining a triangular prism (6-connected) node with a planar triangle (3-connected) or another triangular prism node, the targeted COFs adopt non-interpenetrated ceq or acs topology, respectively. Both materials show permanent porosity and impressive performance in the adsorption of CO2 (∼98 cm3/g at 273 K and 1 bar), CH4 (∼48 cm3/g at 273 K and 1 bar), and especially H2 (up to 274 cm3/g or 2.45 wt % at 77 K and 1 bar), which is highest among porous organic materials reported to date. This research thus provides a promising strategy for diversifying 3D COFs based on complex building blocks and promotes their potential applications in energy storage and environment-related fields.

3D Thioether-Based Covalent Organic Frameworks for Selective and Efficient Mercury Removal.

Developing functionalized 3D covalent organic frameworks (3D COFs) is critical to broaden their potential applications. However, the introduction of specific functionality in 3D COFs remains a great challenge because most of the functional groups are not compatible with the synthesis conditions. Herein, for the first time 3D thioether-based COFs (JUC-570 and JUC-571) for mercury (Hg2+ ) removal from aqueous solution is reported. These 3D thioether-based COFs prepared by the bottom-up approach display high Hg2+ uptakes (972 mg g-1 for JUC-570 and 970 mg g-1 for JUC-571 at pH = 5), fast adsorption kinetics (distribution coefficient Kd value of 2.29 × 107  mL g-1 for JUC-570 and 2.07 × 107  mL g-1 for JUC-571), and favorable selectivity. In particular, JUC-570 is periodically decorated with isopropyl groups around imine bonds that markedly improve its chemical stability and effectively prevent the pore collapse, and thus endows high Hg2+ adsorption capacity (619 mg g-1 ) and excellent cycle performance even at pH = 1. This study not only puts forward a new route to construct stable functionalized 3D COFs, but also promotes their potential applications in areas related to the environment.

3D Hydrazone-Functionalized Covalent Organic Frameworks as pH-Triggered Rotary Switches.

The property expansion of 3D functionalized covalent organic frameworks (COFs) is important for developing their potential applications. Herein, the first case of 3D hydrazone-decorated COFs as pH-triggered molecular switches is reported, and their application in the stimuli-responsive drug delivery system is explored. These functionalized COFs with hydrazone groups on the channel walls are obtained via a multi-component bottom-up synthesis strategy. They exhibit a reversible E/Z isomerization at various pH values, confirmed by UV-vis absorption spectroscopy and proton conduction. Remarkably, after loading cytarabine (Ara-C) as a model drug molecule, these pH-responsive COFs show an excellent and intelligent sustained-release effect with an almost fourfold increase in the Ara-C release at pH = 4.8 than at pH = 7.4, which will effectively improve drug-targeting. Thus, these results open a way toward designing 3D stimuli-responsive functionalized COF materials and promote their potential application as drug carriers in the field of disease treatment.

Adsorption of Toluene and Water over Cationic-Exchanged Y Zeolites: A DFT Exploration.

In this study, density functional theory (DFT) calculations have been performed to investigate the adsorption mechanisms of toluene and water onto various cationic forms of Y zeolite (LiY, NaY, KY, CsY, CuY and AgY). Our computational investigation revealed that toluene is mainly adsorbed via π-interactions on alkalis exchanged Y zeolites, where the adsorbed toluene moiety interacts with a single cation for all cases with the exception of CsY, where two cations can simultaneously contribute to the adsorption of the toluene, hence leading to the highest interaction observed among the series. Furthermore, we find that the interaction energies of toluene increase while moving down in the alkaline series where interaction energies are 87.8, 105.5, 97.8, and 114.4 kJ/mol for LiY, NaY, KY and CsY, respectively. For zeolites based on transition metals (CuY and AgY), our calculations reveal a different adsorption mode where only one cation interacts with toluene through two carbon atoms of the aromatic ring with interaction energies of 147.0 and 131.5 kJ/mol for CuY and AgY, respectively. More importantly, we show that water presents no inhibitory effect on the adsorption of toluene, where interaction energies of this latter were 10 kJ/mol (LiY) to 47 kJ/mol (CsY) higher than those of water. Our results point out that LiY would be less efficient for the toluene/water separation while CuY, AgY and CsY would be the ideal candidates for this application.

Defect Formation, T-Atom Substitution and Adsorption of Guest Molecules in MSE-Type Zeolite Framework-DFT Modeling.

We used computational modeling, based on Density Functional Theory, to help understand the preference for the formation of silanol nests and the substitution of Si by Ti or Al in different crystallographic positions of the MSE-type framework. All these processes were found to be energetically favorable by more than 100 kJ/mol. We suggested an approach for experimental identification of the T atom position in Ti-MCM-68 zeolite via simulation of infrared spectra of pyridine and acetonitrile adsorption at Ti. The modeling of adsorption of hydrogen peroxide at Ti center in the framework has shown that the molecular adsorption was preferred over the dissociative adsorption by 20 to 40 kJ/mol in the presence or absence of neighboring T-atom vacancy, respectively.

Three-Dimensional Radical Covalent Organic Frameworks as Highly Efficient and Stable Catalysts for Selective Oxidation of Alcohols.

With excellent designability, large accessible inner surface, and high chemical stability, covalent organic frameworks (COFs) are promising candidates as metal-free heterogeneous catalysts. Here, we report two 3D radical-based COFs (JUC-565 and JUC-566) in which radical moieties (TEMPO) are uniformly decorated on the channel walls via a bottom-up approach. Based on grafted functional groups and suitable regular channels, these materials open up the application of COFs as highly efficient and selective metal-free redox catalysts in aerobic oxidation of alcohols to relevant aldehydes or ketones with outstanding turn over frequency (TOF) up to 132 h-1 , which has exceeded other TEMPO-modified catalytic materials tested under similar conditions. These stable COF-based catalysts could be easily recovered and reused for multiple runs. This study promotes potential applications of 3D functional COFs anchored with stable radicals in organic synthesis and material science.

Unlocking the Potential of Hidden Sites in Faujasite: New Insights in a Proton Transfer Mechanism.

Zeolite Y and its ultra-stabilized hierarchical derivative (USY) are the most widely used zeolite-based heterogeneous catalysts in oil refining, petrochemisty, and other chemicals manufacturing. After almost 60 years of academic and industrial research, their resilience is unique as no other catalyst displaced them from key processes such as FCC and hydrocracking. The present study highlights the key difference leading to the exceptional catalytic performance of USY versus the parent zeolite Y in a multi-technique study combining advanced spectroscopies (IR and solid-state NMR) and molecular modeling. The results highlight a hitherto unreported proton transfer involving inaccessible active sites in sodalite cages that contributes to the exceptional catalytic performance of USY.

Three-Dimensional Mesoporous Covalent Organic Frameworks through Steric Hindrance Engineering.

The development of three-dimensional (3D) covalent organic frameworks (COFs) with large pores and high surface areas is of great importance for various applications. However, it remains a major challenge due to the frequent structural interpenetration and pore collapse after the removal of guest species situated in the pores. Herein, we report for the first time a series of 3D mesoporous COFs through a general method of steric hindrance engineering. By placing methoxy and methyl groups strategically on the monomers, we can obtain non-interpenetrated 3D COFs of diamondoid structures with permanent mesopores (up to 26.5 Å) and high surface areas (>3000 m2 g-1), which are far superior to those of reported conventional COFs with the same topology. This work thus opens a new avenue to create 3D large-pore COFs for potential applications in adsorption and separation of large inorganic, organic, and biological molecules.

Three-Dimensional Large-Pore Covalent Organic Framework with stp Topology.

Three-dimensional (3D) covalent organic frameworks (COFs) are excellent crystalline porous polymers for numerous potential applications, but their building units and topological nets have been limited. Herein we report the design and synthesis of the first 3D large-pore COF with the stp topology constructed from a 6-connected triptycene-based monomer. The new COF (termed JUC-564) has a high specific surface area (up to 3300 m2 g-1), the largest pore size among 3D COFs (43 Å), and record-breaking low density among crystalline materials reported to date (0.108 g cm-3). The large pore size of JUC-564 was confirmed by the incorporation of a protein. This study expands the structural varieties of 3D COFs based on the deliberate symmetry-guided design principle as well as their applications for adsorption and separation of large biological molecules.