Hydrogen-bonded organic frameworks (HOFs), self-assembled from strategically pre-designed molecular tectons with complementary hydrogen-bonding patterns, are rapidly evolving into a novel and important class of porous materials. In addition to their common features shared with other functionalized porous materials constructed from modular building blocks, the intrinsically flexible and reversible H-bonding connections endow HOFs with straightforward purification procedures, high crystallinity, solution processability, and recyclability. These unique advantages of HOFs have attracted considerable attention across a broad range of fields, including gas adsorption and separation, catalysis, chemical sensing, and electrical and optical materials. However, the relatively weak H-bonding interactions within HOFs can potentially limit their stability and potential use in further applications. To that end, this Perspective highlights recent advances in the development of chemically and thermally robust HOF materials and systematically discusses relevant design rules and synthesis strategies to access highly stable HOFs.

Design Rules of Hydrogen-Bonded Organic Frameworks with High Chemical and Thermal Stabilities.

Water chestnut shell-derived N/S-doped porous carbons and their applications in CO2 adsorption and supercapacitor

The porous carbons have attracted tremendous interest for CO2 capture application thanks to their abundant availability and tunable physical properties. However, they suffer from insufficient CO2 utilization performance due to the poor active sites on their surface. Herein, we report for the first time the fabrication of urea-formaldehyde resin-derived N-rich porous carbon via two-step synthetic routes with physical carbonization followed by KOH activating. The optimized sample included 9.87 wt % N exhibit notable CO2 uptake capacities (5.42 and 3.53 mmol g–1 at 0 and 25 °C at 1 bar) and high selectivity efficiency of 23 between CO2 and N2 at ambient pressure. Overall, on the basis of the in-depth characterization techniques, the advanced textural properties and incorporation of nitrogen into carbon surfaces contribute to increasing the CO2 uptake capacity and selectivity efficiency. 

Efficient N-Doped Porous Carbonaceous CO2 Adsorbents Derived from Commercial Urea-Formaldehyde Resin

The high catalytic activity of metal-organic frameworks (MOFs) can be realized by increasing their effective active sites, which prompts us to perform the functionalization on selected linkers by introducing a strong Lewis basic group of fluorine. Herein, the exquisite combination of paddle-wheel [Cu2(CO2)4(H2O)] clusters and meticulously designed fluorine-funtionalized tetratopic 2',3'-difluoro-[p-terphenyl]-3,3″,5,5″-tetracarboxylic acid (F-H4ptta) engenders one peculiar nanocaged {Cu2}-organic framework of {[Cu2(F-ptta)(H2O)2]·5DMF·2H2O}n (NUC-54), which features two types of nanocaged voids (9.8 Å × 17.2 Å and 10.1 Å × 12.4 Å) shaped by 12 paddle-wheel [Cu2(COO)4H2O)2] secondary building units, leaving a calculated solvent-accessible void volume of 60.6%. Because of the introduction of plentifully Lewis base sites of fluorine groups, activated NUC-54a exhibits excellent catalytic performance on the cycloaddition reaction of CO2 with various epoxides under mild conditions. Moreover, to expand the catalytic scope, the deacetalization-Knoevenagel condensation reactions of benzaldehyde dimethyl acetal and malononitrile were performed using the heterogenous catalyst of NUC-54a. Also, NUC-54a features high recyclability and catalytic stability with excellent catalytic performance in subsequent catalytic tests. Therefore, this work not only puts forward a new solution for developing high-efficiency heterogeneous catalysts, but also enriches the functionalization strategies for nanoporous MOFs.

Fluorine-Functionalized NbO-Type {Cu2}-Organic Framework: Enhanced Catalytic Performance on the Cycloaddition Reaction of CO2 with Epoxides and Deacetalization-Knoevenagel Condensation.

Synthesis of potassium Bitartrate-derived porous carbon via a facile and Self-Activating strategy for CO2 adsorption application

Abstract The separation of C 2 H 2 /CO 2 is not only industrially important for acetylene purification but also scientifically challenging owing to their high similarities in physical properties and molecular sizes. Ultramicroporous metal-organic frameworks (MOFs) can exhibit a pore confinement effect to differentiate gas molecules of similar size. Herein, we report the fine-tuning of pore sizes in sub-nanometer scale on a series of isoreticular MOFs that can realize highly efficient C 2 H 2 /CO 2 separation. The subtle structural differences lead to remarkable adsorption performances enhancement. Among four MOF analogs, by integrating appropriate pore size and specific binding sites, [Cu(dps) 2 (SiF 6 )] (SIFSIX-dps-Cu, SIFSIX = SiF 6 2- , dps = 4.4’-dipyridylsulfide, also termed as NCU-100) exhibits the highest C 2 H 2 uptake capacity and C 2 H 2 /CO 2 selectivity. At room temperature, the pore space of SIFSIX-dps-Cu significantly inhibits CO 2 molecules but takes up a large amount of C 2 H 2 (4.57 mmol g −1 ), resulting in a high IAST selectivity of 1787 for C 2 H 2 /CO 2 separation. The multiple host-guest interactions for C 2 H 2 in both inter- and intralayer cavities are further revealed by dispersion-corrected density functional theory and grand canonical Monte Carlo simulations. Dynamic breakthrough experiments show a clean C 2 H 2 /CO 2 separation with a high C 2 H 2 working capacity of 2.48 mmol g −1 . 

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Fine pore engineering in a series of isoreticular metal-organic frameworks for efficient C2H2/CO2 separation

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Purification of C2H4 from a ternary C2H2/C2H6/C2H4 mixture by one-step adsorption separation is of prime importance but challenging in the petrochemical industry; however, effective strategies to design high-performance adsorbents are lacking. We herein report for the first time the incorporation of Lewis basic sites into a C2H6-selective MOF, enabling efficient one-step production of polymer-grade C2H4 from ternary mixtures. Introduction of amino groups into highly stable C2H6-selective UiO-67 can not only partition large pores into smaller cagelike pockets to provide suitable pore confinement but also offer additional binding sites to simultaneously enhance C2H2 and C2H6 adsorption capacities over C2H4. The amino-functionalized UiO-67-(NH2)2 thus exhibits exceptionally high C2H2 and C2H6 uptakes as well as benchmark C2H2/C2H4 and C2H6/C2H4 selectivities, surpassing all of the C2H2/C2H6-selective materials reported so far. Theoretical calculations combined with in situ infrared spectroscopy indicate that the synergetic effect of suitable pore confinement and functional surfaces decorated with amino groups provides overall stronger multipoint van der Waals interactions with C2H2 and C2H6 over C2H4. The exceptional performance of UiO-67-(NH2)2 was evidenced by breakthrough experiments for C2H2/C2H6/C2H4 mixtures under dry and wet conditions, providing a remarkable C2H4 productivity of 0.55 mmol g-1 at ambient conditions.

Immobilization of Lewis Basic Sites into a Stable Ethane-Selective MOF Enabling One-Step Separation of Ethylene from a Ternary Mixture.

Recent progress on porous MOFs for process-efficient hydrocarbon separation, luminescent sensing, and information encryption.

The rational design and synthesis of robust metal-organic frameworks (MOFs) based on novel organic building blocks are fundamental aspects of reticular chemistry. Beyond simply fabricating new organic linkers, however, it is important to elucidate structure-property relationships at the molecular level to develop high-performing materials. In this work, we successfully targeted a highly porous and robust cage-type MOF (NU-200) with an nbo-derived fof topology through the deliberate assembly of a cyclohexane-functionalized iron(II)-clathrochelate-based meta-benzenedicarboxylate linker with a Cu2(CO2)4 secondary building unit (SBU). NU-200 exhibited an outstanding adsorption capacity of xenon and a high ideal adsorbed solution theory (IAST) predicted selectivity for a 20/80 v/v mixture of xenon (Xe)/krypton (Kr) at 298 K and 1.0 bar. Our extensive computational simulations with grand canonical Monte Carlo (GCMC) and density functional theory (DFT) on NU-200 indicated that the MOF's hierarchical bowl-shaped nanopockets surrounded by custom-designed cyclohexyl groups─instead of the conventionally believed open metal sites (OMSs)─played a crucial role in reinforcing Xe-binding affinity. The optimally sized pockets firmly trapped Xe through numerous supramolecular interactions including Xe···H, Xe···O, and Xe···π. Additionally, we validated the unique pocket confinement effect by experimentally and computationally employing the similarly sized probe, sulfur dioxide (SO2), which provided significant insights into the molecular underpinnings of the high uptake of SO2 (11.7 mmol g-1), especially at a low pressure of 0.1 bar (8.5 mmol g-1). This work therefore can facilitate the judicious design of organic building blocks, producing MOFs featuring tailor-made pockets to boost gas adsorption and separation performances.

Banglin Chen

Papers

Fine pore engineering in a series of isoreticular metal-organic frameworks for efficient C2H2/CO2 separation

Fine pore engineering in a series of isoreticular metal-organic frameworks for efficient C2H2/CO2 separation

Immobilization of Lewis Basic Sites into a Stable Ethane-Selective MOF Enabling One-Step Separation of Ethylene from a Ternary Mixture.

Recent progress on porous MOFs for process-efficient hydrocarbon separation, luminescent sensing, and information encryption.

Creating Optimal Pockets in a Clathrochelate-Based Metal-Organic Framework for Gas Adsorption and Separation: Experimental and Computational Studies.