Ethylene/ethane separation in a stable hydrogen-bonded organic framework through a gating mechanism

A Microporous Hydrogen Bonded Organic Framework for Highly Selective Separation of Carbon Dioxide over Acetylene

The separation of acetylene (C2 H2 ) from carbon dioxide (CO2 ) is a very important but challenging task due to their similar molecular dimensions and physical properties. In terms of porous adsorbents for this separation, the CO2 -selective porous materials are superior to the C2 H2 -selective ones because of the cost- and energy-efficiency but have been rarely achieved. Herein we report our unexpected discovery of the first hydrogen bonded organic framework (HOF) constructed from a simple organic linker 2,4,6-tri(1H-pyrazol-4-yl)pyridine (PYTPZ) (termed as HOF-FJU-88) as the highly CO2 -selective porous material. HOF-FJU-88 is a two-dimensional HOFs with a pore pocket of about 7.6 Å. The activated HOF-FJU-88 takes up a high amount of CO2 (59.6 cm3  g-1 ) at ambient conditions with the record IAST selectivity of 1894. Its high performance for the CO2 /C2 H2 separation has been further confirmed through breakthrough experiments, in situ diffuse reflectance infrared spectroscopy and molecular simulations.

Recent Advances in Multifunctional Reticular Framework Nanoparticles: A Paradigm Shift in Materials Science Road to a Structured Future

Porous organic frameworks (POFs) have become a highly sought-after research domain that offers a promising avenue for developing cutting-edge nanostructured materials, both in their pristine state and when subjected to various chemical and structural modifications. Metal-organic frameworks, covalent organic frameworks, and hydrogen-bonded organic frameworks are examples of these emerging materials that have gained significant attention due to their unique properties, such as high crystallinity, intrinsic porosity, unique structural regularity, diverse functionality, design flexibility, and outstanding stability. This review provides an overview of the state-of-the-art research on base-stable POFs, emphasizing the distinct pros and cons of reticular framework nanoparticles compared to other types of nanocluster materials. Thereafter, the review highlights the unique opportunity to produce multifunctional tailoring nanoparticles to meet specific application requirements. It is recommended that this potential for creating customized nanoparticles should be the driving force behind future synthesis efforts to tap the full potential of this multifaceted material category.

Pore Modulation of Hydrogen-Bonded Organic Frameworks for Efficient Separation of Propylene

Developing hydrogen-bonded organic frameworks (HOFs) that combine functional sites, size control, and storage capability for targeting gas molecule capture is a novel and challenging venture. However, there is a lack of effective strategies to tune the hydrogen-bonded network to achieve high-performance HOFs. Here, a series of HOFs termed as HOF-ZSTU-M (M=1, 2, and 3) with different pore structures are obtained by introducing structure-directing agents (SDAs) into the hydrogen-bonding network of tetrakis (4-carboxyphenyl) porphyrin (TCPP). These HOFs have distinct space configurations with pore channels ranging from discrete to continuous multi-dimensional. Single-crystal X-ray diffraction (SCXRD) analysis reveals a rare diversity of hydrogen-bonding models dominated by SDAs. HOF-ZSTU-2, which forms a strong layered hydrogen-bonding network with ammonium (NH4 + ) through multiple carboxyl groups, has a suitable 1D "pearl-chain" channel for the selective capture of propylene (C3 H6 ). At 298 K and 1 bar, the C3 H6 storage density of HOF-ZSTU-2 reaches 0.6 kg L-1 , representing one of the best C3 H6 storage materials, while offering a propylene/propane (C3 H6 /C3 H8 ) selectivity of 12.2. Theoretical calculations and in situ SCXRD provide a detailed analysis of the binding strength of C3 H6 at different locations in the pearl-chain channel. Dynamic breakthrough tests confirm that HOF-ZSTU-2 can effectively separate C3 H6 from multi-mixtures.

Discriminatory Gate-Opening Effect in a Flexible Metal-Organic Framework for Inverse CO2 /C2 H2 Separation

Considering the significant application of acetylene (C2 H2 ) in the manufacturing and petrochemical industries, the selective capture of impurity carbon dioxide (CO2 ) is a crucial task and an enduring challenge. Here, a flexible metal-organic framework (Zn-DPNA) accompanied by a conformation change of the Me2 NH2 + ions in the framework is reported. The solvate-free framework provides a stepped adsorption isotherm and large hysteresis for C2 H2 , but type-I adsorption for CO2 . Owing to their uptakes difference before gate-opening pressure, Zn-DPNA demonstrated favorable inverse CO2 /C2 H2 separation. According to molecular simulation, the higher adsorption enthalpy of CO2 (43.1 kJ mol-1 ) is due to strong electrostatic interactions with Me2 NH2 + ions, which lock the hydrogen-bond network and narrow pores. Furthermore, the density contours and electrostatic potential verifies the middle of the cage in the large pore favors C2 H2 and repels CO2 , leading to the expansion of the narrow pore and further diffusion of C2 H2 . These results provide a new strategy that optimizes the desired dynamic behavior for one-step purification of C2 H2 .

Photo Responsive Electron and Proton Conductivity within a Hydrogen-Bonded Organic Framework

Rational design of crystalline porous materials with coupled proton-electron transfer has not yet been reported to date. Herein, we report a donor-acceptor (D-A) π-π stacking hydrogen-bonded organic framework (HOF; HOF-FJU-36) with zwitterionic 1,1'-bis(3-carboxybenzyl)-4,4'-bipyridinium (H2 L2+ ) as acceptor and 2,7-naphthalene disulfonate (NDS2- ) as donor to form a two-dimensional (2D) layer. Three water molecules were situated in the channels to connect with acidic species through hydrogen bonding interactions to give a 3D framework. The continuous π-π interactions along the a axis and the smooth H-bonding chain along the b axis provide the electron and proton transfer pathways, respectively. After 405 nm light irradiation, the photogenerated radicals could simultaneously endow HOF-FJU-36 with photoswitchable electron and proton conductivity due to coupled electron-proton transfer. By single-crystal X-ray diffraction (SCXRD) analyses, X-ray photoelectron spectroscopy (XPS), transient absorption spectra and density functional theory (DFT) calculations, the mechanism of the switchable conductivity upon irradiation has been demonstrated.

Effector-dependent structural transformation of a crystalline framework with allosteric effects on molecular recognition ability

Structurally flexible porous crystals that combine high regularity and stimuli responsiveness have received attracted attention in connection with natural allostery found in regulatory systems of activity and function in biological systems. Porous crystals with molecular recognition sites in the inner pores are particularly promising for achieving elaborate functional control, where the local binding of effectors triggers their distortion to propagate throughout the structure. Here we report that the structure of a porous molecular crystal can be allosterically controlled by local adsorption of effectors within low-symmetry nanochannels with multiple molecular recognition sites. The exchange of effectors at the allosteric site triggers diverse conversion of the framework structure in an effector-dependent manner. In conjunction with the structural conversion, it is also possible to switch the molecular affinity at different recognition sites. These results may provide a guideline for the development of supramolecular materials with flexible and highly-ordered three-dimensional structures for biological applications.

Revolutionizing Porous Liquids: Stabilization and Structural Engineering Achieved by a Surface Deposition Strategy

Facile approaches capable of constructing stable and structurally diverse porous liquids (PLs) that can deliver high-performance applications are a long-standing, captivating, and challenging research area that requires significant attention. Herein, a facile surface deposition strategy is demonstrated to afford diverse type III-PLs possessing ultra-stable dispersion, external structure modification, and enhanced performance in gas storage and transformation by leveraging the expeditious and uniform precipitation of selected metal salts. The Ag(I) species-modified zeolite nanosheets are deployed as the porous host to construct type III-PLs with ionic liquids (ILs) containing bromide anion , leading to stable dispersion driven by the formation of AgBr nanoparticles. The as-afforded type-III PLs display promising performance in CO2 capture/conversion and ethylene/ethane separation. Property and performance of the as-produced PLs can be tuned by the cation structure of the ILs, which can be harnessed to achieve polarity reversal of the porous host via ionic exchange. The surface deposition procedure can be further extended to produce PLs from Ba(II)-functionalized zeolite and ILs containing [SO4 ]2- anion driven by the formation of BaSO4 salts. The as-produced PLs are featured by well-maintained crystallinity of the porous host, good fluidity and stability, enhanced gas uptake capacity, and attractive performance in small gas molecule utilization.

Deep removal of trace C2H2 and CO2 from C2H4 by using customized potassium-exchange mordenite

Adsorptive separation using porous materials is a promising approach for separating alkynes/olefins due to its energy efficiency, while the deep removal of trace amounts of C₂H₂ and CO₂ from C₂H₄ is still very challenging for a commercial adsorbent. Herein, we report a low-cost inorganic metal cation-mediated mordenite (MOR) zeolite with the specific location and distribution of K⁺ cations acting as a goalkeeper for accurately controlling diffusion channels, as evidence of the experimental and simulation results. Deep purification of C₂H₄ from ternary CO₂/C₂H₂/C₂H₄ mixtures was first realized on K-MOR with exceptional results, achieving a remarkable polymer-grade C₂H₄ productivity of 1742 L kg^-1 for the CO₂/C₂H₂/C₂H₄ mixture. Our approach which only involves adjusting the equilibrium ions, is both promising and cost-effective, and opens up new possibilities for the use of zeolites in the industrial light hydrocarbon adsorption and purification process.

Simultaneous Control of Flexibility and Rigidity in Pore-Space-Partitioned Metal-Organic Frameworks

Flexi-MOFs are typically limited to low-connected (<9) frameworks. Here we report a platform-wide approach capable of creating a family of high-connected materials (collectively called CPM-220) that integrate exceptional framework flexibility with high rigidity. We show that the multi-module nature of the pore-space-partitioned pacs (partitioned acs net) platform allows us to introduce flexibility as well as to simultaneously impose high rigidity in a tunable module-specific fashion. The inter-modular synergy has remarkable macro-morphological and sub-nanometer structural impacts. A prominent manifestation at both length scales is the retention of X-ray-quality single crystallinity despite huge hexagonal c-axial contraction (≈ 30%) and harsh sample treatment such as degassing and sorption cycles. CPM-220 sets multiple precedents and benchmarks on the pacs platform in both structural and sorption properties. They possess exceptionally high benzene/cyclohexane selectivity, unusual C₃H₆ and C₃H₈ isotherms, and promising separation performance for small gas molecules such as C₂H₂/CO₂.

Charge-Assisted Hydrogen-Bonded Organic Frameworks with Inorganic Ammonium Regulated Switchable Open Polar Sites

Surface open polar sites within the voids of porous molecular crystals define the localized physicochemical environment for critical functions such as gas separation and molecular recognition. This study presents a new charge-assisted hydrogen bonding (H-bonding) motif, by exploiting inorganic ammonium (NH₄ ⁺ ) cations as H-bond donors, to regulate the assembly of C₂ -symmetric carboxylic tectons for building robust H-bonded frameworks with permanent ultra-micropores and open oxygen sites. Diverse building blocks are bridged by tetrahedral NH₄ ⁺ to expand distinctive H-bonded networks with varied pore architectures. Particularly, the open polar oxygen sites can be switched by altering NH₄ ⁺ sources to tune the deprotonation of carboxyl-containing tectons. The activated porous PTBA·NH₄ ·DMF preserves the pore architecture and open polar oxygen sites, exhibiting remarkably selective sorption of CO₂ (107.8 cm³ g^-1 ,195 K) over N₂ (11.2 cm³ g^-1 , 77 K) and H₂ (1.4 cm³ g^-1 , 77 K).

Rigid-Flexible Hybrid Porous Molecular Crystals with Guest-Induced Reversible Crystallinity

A weak CH/O hydrogen-bonded organic framework (HOF) with both rigidity and flexibility that could easily and reversibly switch from a non-crystalline to a crystalline phase was constructed. The specific solvent molecule acts as a "key" to control the crystallinity, while the highly rigid triangle macrocycle as the building block is the "lock". The introduction and removal of the "key" could influence the local flexibility of the whole framework and lead to switchable crystallinity. Furthermore, the obtained HOF exhibits excellent separation efficiency for benzene and cyclohexane (94.4 %).

Application of Hydrogen-Bonded Organic Frameworks in Environmental Remediation: Recent Advances and Future Trends

The hydrogen-bonded organic frameworks (HOFs) are a class of porous materials with crystalline frame structures, which are self-assembled from organic structures by hydrogen bonding in non-covalent bonds π-π packing and van der Waals force interaction. HOFs are widely used in environmental remediation due to their high specific surface area, ordered pore structure, pore modifiability, and post-synthesis adjustability of various physical and chemical forms. This work summarizes some rules for constructing stable HOFs and the synthesis of HOF-based materials (synthesis of HOFs, metallized HOFs, and HOF-derived materials). In addition, the applications of HOF-based materials in the field of environmental remediation are introduced, including adsorption and separation (NH3, CO2/CH4 and CO2/N2, C2H2/C2He and CeH6, C2H2/CO2, Xe/Kr, etc.), heavy metal and radioactive metal adsorption, organic dye and pesticide adsorption, energy conversion (producing H2 and CO2 reduced to CO), organic dye degradation and pollutant sensing (metal ion, aniline, antibiotic, explosive steam, etc.). Finally, the current challenges and further studies of HOFs (such as functional modification, molecular simulation, application extension as remediation of contaminated soil, and cost assessment) are discussed. It is hoped that this work will help develop widespread applications for HOFs in removing a variety of pollutants from the environment.

Hybrid Hydrogen-Bonded Organic Frameworks: Structures and Functional Applications

As a new class of porous crystalline materials, hydrogen-bonded organic frameworks (HOFs) assembled from building blocks by hydrogen bonds have gained increasing attention. HOFs benefit from advantages including mild synthesis, easy purification, and good recyclability. However, some HOFs transform into unstable frameworks after desolvation, which hinders their further applications. Nowadays, the main challenges of developing HOFs lie in stability improvement, porosity establishment, and functionalization. Recently, more and more stable and permanently porous HOFs have been reported. Of all these design strategies, stronger charge-assisted hydrogen bonds and coordination bonds have been proven to be effective for developing stable, porous, and functional solids called hybrid HOFs, including ionic and metallized HOFs. This Review discusses the rational design synthesis principles of hybrid HOFs and their cutting-edge applications in selective inclusion, proton conduction, gas separation, catalysis and so forth.

Insights into the Absorption of Hydrocarbon Gases in Phosphorus-Containing Ionic Liquids

The solubility of ethane, ethylene, propane, and propylene was measured in two phosphorus-containing ionic liquids, trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate, [P_6,6,6,14][DiOP], and 1-butyl-3-methylimidazolium dimethylphosphate, [C₄C₁Im][DMP], using an isochoric saturation method. The ionic liquid [C₄C₁Im][DMP] absorbed between 1 and 20 molecules of gas per 1000 ion pairs, at 313 K and 0.1 MPa, while [P_6,6,6,14][DiOP] absorbed up to 169 molecules of propane per 1000 ion pairs under the same conditions. [C₄C₁Im][DMP] had a higher capacity to absorb olefins than paraffins, while the opposite was true for [P_6,6,6,14][DiOP], with the former being slightly more selective than the later. From the analysis of the thermodynamic properties of solvation, we concluded that in both ionic liquids and for all of the studied gases the solvation is ruled by the entropy, even if its contribution is unfavorable. These results, together with density measurements, 2D NMR studies, and self-diffusion coefficients suggest that the gases' solubility is ruled mostly by nonspecific interactions with the ionic liquids and that the looser ion packing in [P_6,6,6,14][DiOP] makes it easier to accommodate the gases compared to [C₄C₁Im][DMP].

Probing sub-5 Ångstrom micropores in carbon for precise light olefin/paraffin separation

Olefin/paraffin separation is an important but challenging and energy-intensive process in petrochemical industry. The realization of carbons with size-exclusion capability is highly desirable but rarely reported. Herein, we report polydopamine-derived carbons (PDA-Cx, where x refers to the pyrolysis temperature) with tailorable sub-5 Å micropore orifices together with larger microvoids by one-step pyrolysis. The sub-5 Å micropore orifices centered at 4.1-4.3 Å in PDA-C800 and 3.7-4.0 Å in PDA-C900 allow the entry of olefins while entirely excluding their paraffin counterparts, performing a precise cut-off to discriminate olefin/paraffin with sub-angstrom discrepancy. The larger voids enable high C₂H₄ and C₃H₆ capacities of 2.25 and 1.98 mmol g^-1 under ambient conditions, respectively. Breakthrough experiments confirm that a one-step adsorption-desorption process can obtain high-purity olefins. Inelastic neutron scattering further reveals the host-guest interaction of adsorbed C₂H₄ and C₃H₆ molecules in PDA-Cx. This study opens an avenue to exploit the sub-5 Å micropores in carbon and their desirable size-exclusion effect.

Highly Selective Separation of C2H2/CO2 and C2H2/C2H4 in an N-Rich Cage-Based Microporous Metal-Organic Framework

The separation of acetylene (C2H2) from carbon dioxide (CO2) and the purification of ethylene (C2H4) from C2H2 are quite essential processes for the chemical industry. However, these processes are challenging due to their similar physical properties, including molecule sizes and boiling points. Herein, we report an N-rich cage-based microporous metal-organic framework (MOF), [Cd5(Tz)9](NO3) (termed as Cd-TZ, TZ stands for tetrazole), and its highly efficient separation of C2H2/CO2 and C2H2/C2H4. Single-component gas adsorption isotherms reveal that Cd-TZ exhibits high C2H2 adsorption capacity (3.10 mmol g-1 at 298 K and 1 bar). The N-rich cages in Cd-TZ can trap C2H2 with a higher isosteric heat of adsorption (40.8 kJ mol-1) than CO2 and C2H4 owing to the robust host-guest interactions between the noncoordinated N atoms and C2H2, which has been verified by molecular modeling studies. Cd-TZ shows a high IAST selectivity for C2H2/CO2 (8.3) and C2H2/C2H4 (13.3). The breakthrough simulations confirm the potential for separating C2H2/CO2 and the purification of C2H4 from C2H2.

Engineering HOF-Based Mixed-Matrix Membranes for Efficient CO2 Separation

Hydrogen-bonded organic frameworks (HOFs) have emerged as a new class of crystalline porous materials, and their application in membrane technology needs to be explored. Herein, for the first time, we demonstrated the utilization of HOF-based mixed-matrix membrane for CO2 separation. HOF-21, a unique metallo-hydrogen-bonded organic framework material, was designed and processed into nanofillers via amine modulator, uniformly dispersing with Pebax polymer. Featured with the mix-bonded framework, HOF-21 possessed moderate pore size of 0.35 nm and displayed excellent stability under humid feed gas. The chemical functions of multiple binding sites and continuous hydrogen-bonded network jointly facilitated the mass transport of CO2. The resulting HOF-21 mixed-matrix membrane exhibited a permeability above 750 Barrer, a selectivity of ~ 40 for CO2/CH4 and ~ 60 for CO2/N2, surpassing the 2008 Robeson upper bound. This work enlarges the family of mixed-matrix membranes and lays the foundation for HOF membrane development.

Polarity-Evolution Control and Luminescence Regulation in Multiple-Site Hydrogen-Bonded Organic Frameworks

Hydrogen-bonded organic frameworks (HOFs) have shown great potential in separation, sensing and host-guest chemistry, however, the pre-design of HOFs remains challenging due to the uncertainty of solvents' participation in framework formation. Herein, the polarity-evolution-controlled framework/luminescence regulation is demonstrated based on multiple-site hydrogen-bonded organic frameworks. Several distinct HOFs were prepared by changing bonding modes of building units via the evolution of electrostatic forces induced by various solvent polarities. High-polar solvents with strong electrostatic attraction to surrounding units showed the tendency to form cage structures, while low-polar solvents with weak electrostatic attraction only occupy hydrogen-bond sites, conducive to the channel formation. Furthermore, the conformation of optical building unit can be adjusted by affecting the solvent polarity, generating different luminescence outputs. These results pave the way for the rational design of ideal HOFs with on-demand framework regulation and luminescence properties.

Preparation of Ropivacaine Encapsulated by Zeolite Imidazole Framework Microspheres as Sustained-Release System and Efficacy Evaluation

The management of persistent postoperative pain still remains a clinical challenge currently. Although ropivacaine (RVC) is widely used for postoperative analgesia as a local anesthetic, the short half-life makes it difficult to achieve the desired duration of analgesia. Herein, a RVC sustained-release microspheres encapsulated by zeolite imidazole framework-8 (RVC@ZIF-8) was synthesized for the first time, which prolonged the sustained-release of RVC and decreased the resulting drug toxicity. RVC can continuously release in vitro for at least 96 h with high drug loading of 30.6 % and RVC@ZIF-8 had excellent biocompatibility and low cytotoxicity. In sciatic nerve block model, the sensory block time of RVC@ZIF-8 was significantly prolonged compared with RVC, achieving more than 72 h post injection and no inflammation or lesion were found. Based on high drug loading, ideal sustained-release and superior biological safety, RVC@ZIF-8 will be a novel delivery material for local anesthetic with potential application.

Multifunctional Porous Hydrogen-Bonded Organic Frameworks: Current Status and Future Perspectives

Hydrogen-bonded organic frameworks (HOFs), self-assembled from organic or metalated organic building blocks (also termed as tectons) by hydrogen bonding, π-π stacking, and other intermolecular interactions, have become an emerging class of multifunctional porous materials. So far, a library of HOFs with high porosity has been synthesized based on versatile tectons and supramolecular synthons. Benefiting from the flexibility and reversibility of H-bonds, HOFs feature high structural flexibility, mild synthetic reaction, excellent solution processability, facile healing, easy regeneration, and good recyclability. However, the flexible and reversible nature of H-bonds makes most HOFs suffer from poor structural designability and low framework stability. In this Outlook, we first describe the development and structural features of HOFs and summarize the design principles of HOFs and strategies to enhance their stability. Second, we highlight the state-of-the-art development of HOFs for diverse applications, including gas storage and separation, heterogeneous catalysis, biological applications, sensing, proton conduction, and other applications. Finally, current challenges and future perspectives are discussed.

Selective and Reversible Solvent Uptake in Tetra-4-(4-pyridyl)phenylmethane-based Supramolecular Organic Frameworks

The dynamic behavior of supramolecular organic frameworks (SOFs) based on the rigid tetra-4-(4-pyridyl)phenylmethane (TPPM) organic tecton has been elucidated through 3D electron diffraction, X-ray powder diffraction and differential scanning calorimetry (DSC) analysis. The SOF undergoes a reversible single-crystal-to-single-crystal transformation when exposed to vapours of selected organic solvents, moving from a closed structure with isolated small voids to an expanded structure with solvated channels along the b axis. The observed selectivity is dictated by the fitting of the guest in the expanded SOF, following the degree of packing coefficient. The effect of solvent uptake on TPPM solid-state fluorescence was investigated, evidencing a significant variation in the emission profile only in the presence of chloroform.

Exploring Multifunctional Hydrogen-Bonded Organic Framework Materials

Hydrogen-bonded organic framework (HOF) materials have provided a new dimension and bright promise as a new platform for developing multifunctional materials. They can be readily self-assembled from their corresponding organic molecules with diverse functional sites such as carboxylic acid and amine groups for their hydrogen bonding and aromatic ones for their weak π···π interactions to stabilize the frameworks. Compared with those established porous materials such as zeolites, metal-organic frameworks (MOFs), and covalent-organic frameworks (COFs), it is much more difficult to stabilize HOFs and thus establish their permanent porosities given the fact that hydrogen bonds are typically weaker than ionic, coordination, and covalent bonds. But it provides the uniqueness of HOF materials in which they can be easily recovered and regenerated through simple recrystallization. HOF materials can also be easily and straightforwardly processed and very compatible with the biomolecules, making them potentially very useful materials for industrial and biomedical applications. The reversible and weak bonding nature of the hydrogen bonds can be readily utilized to construct flexible porous HOF materials in which we can tune the temperature and pressure to control their porosities and, thus, their diverse applications, for example, on gas separations, gas storage, drug delivery, and sensing. Some specific organic functional groups are quite directional for the hydrogen bond formations; for example, carboxylic acid prefers to form a directional dimer, which has enabled us to readily construct reticular porous HOF materials whose pores can be systematically tuned. In this Account, we outline our journey of exploring this new type of porous material by establishing one of the first porous HOFs in 2011 and thus developing its diverse applications. We have been able to use organic molecules with different functional sites, including 2,4-diaminotriazine (DAT), carboxylic acid (COOH), aldehyde (CHO), and cyano (CN), to construct porous HOFs. Through tuning the pore sizes, introducing specific binding sites, and making use of the framework flexibility, we have realized a series of HOF materials for the gas separations of C₂H₂/C₂H₄, C₂H₄/C₂H₆, C₃H₆/C₃H₈, C₂H₂/CO₂, CO₂/N₂, and Xe/Kr and enantioselective separation of alcohols. To make use of optically active organic molecules, we have developed HOF materials for their luminescent sensing and optical lasing. Our research endeavors on multifunctional HOF materials have initiated extensive research in this emerging research topic among chemistry and materials sciences communities. We foresee that not only many more HOF materials will be developed but novel functions will be fulfilled beyond our imaginations soon.

A Flexible Hydrogen-Bonded Organic Framework Constructed from a Tetrabenzaldehyde with a Carbazole N-H Binding Site for the Highly Selective Recognition and Separation of Acetone

Rational design of hydrogen-bonded organic frameworks (HOFs) with multiple functionalities is highly sought after but challenging. Herein, we report a multifunctional HOF (HOF-FJU-2) built from 4,4',4'',4'''-(9H-carbazole-1,3,6,8-tetrayl)tetrabenzaldehyde molecule with tetrabenzaldeyde for their H bonding interactions and carbazole N-H site for its specific recognition of small molecules. The Lewis acid N-H sites allow HOF-FJU-2 facilely separate acetone from its mixture with another solvent like methanol with smaller pK_a value. The donor (D)-π-acceptor (A) aromatic nature of the organic building molecule endows this HOF with solvent dependent luminescent/chromic properties, so the column acetone/methanol separation on HOF-FJU-2 can be readily visualized.

Multi-Responsive Sensor Based on Porous Hydrogen-Bonded Organic Frameworks for Selective Sensing of Ions and Dopamine Molecules

Hydrogen-bonded organic frameworks (HOFs), as an emerging porous material, have attracted increasing research interest in fluorescence sensing due to their inherent fluorescence emission units with unique physicochemical properties. Herein, based on the organic building block 3,3',5,5'-tetrakis-(4-carboxyphenyl)-1,1'-biphenyl (H₄TCBP), the porous material HOF-TCBP was successfully synthesized using hydrogen bond self-assembly in a DMF solution. The fluorescence properties of the HOF-TCBP solution showed that when the concentration was high, excimers were easily formed, the PL emission was red-shifted, and the fluorescence intensity became weaker. HOF-TCBP showed good sensitivity and selectivity to metal ions Fe³⁺, Cr³⁺, and anion Cr₂O₇^2-. In addition, HOF-TCBP can serve as a label-free fluorescent sensor material for the sensitive and selective detection of dopamine (DA). HOF-based DA sensing is actually easy, low-cost, simple to operate, and highly selective for many potential interfering substances, and it has been successfully applied to the detection of DA in biological samples with satisfactory recoveries (101.1-104.9%). To our knowledge, this is the first report of HOF materials for efficient detection of the neurotransmitter dopamine in biological fluids. In short, this work widely broadens the application of HOF materials as fluorescent sensors for the sensing of ions and biological disease markers.

Energy‐Efficient Iodine Uptake by a Molecular Host⋅Guest Crystal

Recently, porous organic crystals (POC) based on macrocycles have shown exceptional sorption and separation properties. Yet, the impact of guest presence inside a macrocycle prior to adsorption has not been studied. Here we show that the inclusion of trimethoxybenzyl-azaphosphatrane in the macrocycle cucurbit[8]uril (CB[8]) affords molecular porous host⋅guest crystals (PHGC-1) with radically new properties. Unactivated hydrated PHGC-1 adsorbed iodine spontaneously and selectively at room temperature and atmospheric pressure. The absence of (i) heat for material synthesis, (ii) moisture sensitivity, and (iii) energy-intensive steps for pore activation are attractive attributes for decreasing the energy costs. ¹ H NMR and DOSY were instrumental for monitoring the H₂ O/I₂ exchange. PHGC-1 crystals are non-centrosymmetric and I₂ -doped crystals showed markedly different second harmonic generation (SHG), which suggests that iodine doping could be used to modulate the non-linear optical properties of porous organic crystals.

Tetrazolyl Porphyrin-Based Hydrogen-Bonded Organic Frameworks: Active Sites-Mediated Host-Guest Synergy for Advanced Antimicrobial Applications

Hydrogen-bonded organic frameworks (HOFs) with multiple functions and permanent pores have received widespread attention due to their potential applications in gas adsorption/separation, drug delivery, photocatalysis, proton conduction, and other fields. Herein, we constructed a three-dimensional (3D) HOF with 1D square channels by utilizing a dual-functional tetrazolyl porphyrin ligand bearing an active center of the porphyrin core and open sites of nitrogen atoms through π-π stacking and hydrogen-bonding interaction self-assembly. The structure exhibits both solvent resistance and thermal stability, and especially, maintains these after being transformed into nanoparticles. Meanwhile, the active sites exposed on the inner wall of the pores can interact well with the photoactive cationic dye molecules to form an effective host-guest (H-G) system, which can realize boosted photosensitized singlet oxygen (1O2) production under red light irradiation and synergistic sterilization toward Staphylococcus aureus (S. aureus) with an inhibition ratio as high as 99.9%. This work provides a valuable design concept for HOF-related systems in pursuit of promoted photoactivity.

A three dimensional graphdiyne-like porous triptycene network for gas adsorption and separation

Graphdiyne, an emerging carbon allotrope, has attracted many researchers devoted to the study of its synthesis and application. The utilization of graphdiyne in gas adsorption and separation has been predicted by computer simulation with many examples. In this work, the triangular basic unit of graphdiyne was introduced into a triptycene-based porous organic polymer to obtain a three dimensional graphdiyne-like porous triptycene network named G-PTN. With high surface area and a microporous structure, G-PTN exhibited convincing application potential for the storage of gas molecules, especially for the selective adsorption of acetylene over ethylene. Computational simulation proved the importance of the triptycene units and three dimensional structure to the selectivity, as well as the potential of graphdiyne units as selective binding sites, suggesting that through judicious design, new three-dimensional porous graphdiyne could be acquired with better gas adsorption and separation performance.

Prediction of the Ibuprofen Loading Capacity of MOFs by Machine Learning

Metal-organic frameworks (MOFs) have been widely researched as drug delivery systems due to their intrinsic porous structures. Herein, machine learning (ML) technologies were applied for the screening of MOFs with high drug loading capacity. To achieve this, first, a comprehensive dataset was gathered, including 40 data points from more than 100 different publications. The organic linkers, metal ions, and the functional groups, as well as the surface area and the pore volume of the investigated MOFs, were chosen as the model’s inputs, and the output was the ibuprofen (IBU) loading capacity. Thereafter, various advanced and powerful machine learning algorithms, such as support vector regression (SVR), random forest (RF), adaptive boosting (AdaBoost), and categorical boosting (CatBoost), were employed to predict the ibuprofen loading capacity of MOFs. The coefficient of determination (R²) of 0.70, 0.72, 0.66, and 0.76 were obtained for the SVR, RF, AdaBoost, and CatBoost approaches, respectively. Among all the algorithms, CatBoost was the most reliable, exhibiting superior performance regarding the sparse matrices and categorical features. Shapley additive explanations (SHAP) analysis was employed to explore the impact of the eigenvalues of the model’s outputs. Our initial results indicate that this methodology is a well generalized, straightforward, and cost-effective method that can be applied not only for the prediction of IBU loading capacity, but also in many other biomaterials projects.

Ultramicroporous Hydrogen-Bonded Organic Framework Material with a Thermoregulatory Gating Effect for Record Propylene Separation

Propane/propylene separation is one of the most challenging and energy-consuming but most important tasks in the petrochemical industry. Herein, a stable hydrogen-bonded organic framework (HOF-FJU-1) was tailor-made for highly efficient propylene separation from binary C₃H₆/C₃H₈ and even seven component CH₄/C₂H₄/C₂H₆/C₃H₆/C₃H₈/CO₂/H₂ mixtures. The temperature-controllable diffusion channels in HOF-FJU-1 have enabled the porous material to completely exclude propane to reach high-performance propylene purification under energy-efficient operation conditions. Single-crystal structural analysis revealed that the well-matched pore aperture of HOF-FJU-1 can exactly accommodate propylene molecules via multiple intermolecular interactions, exhibiting a very high propylene/propane selectivity of 616 at 333 K. The propylene purity and productivity are over 99.5% and 30.2 L kg^-1 from the binary C₃H₆/C₃H₈ (50/50) mixture at 333 K. Through a follow-up column separation of C₃H₆/C₂H₄ at 353 K, not only high-purity propylene (99.5%) but also ethylene (98.3%) can be readily collected from the seven component CH₄/C₂H₄/C₂H₆/C₃H₆/C₃H₈/CO₂/H₂ (31/10/25/10/10/1/13) cracking gas mixtures. The great potential of HOF-FJU-1 for the industrial propylene separation process has been further supported by the high stability of this porous material under different environments and straightforward processibility and regeneration feasibility.

An Ultrasensitive Picric Acid Sensor Based on a Robust 3D Hydrogen-Bonded Organic Framework

Hydrogen-bonded organic frameworks (HOFs), as a newly developed porous material, have been widely used in various fields. To date, several organic building units (OBUs) with tri-, tetra-, and hexa-carboxylic acid synthons have been applied to synthesize HOFs. To our knowledge, di-carboxylic acids have rarely been reported for the construction of HOFs, in particular, di-carboxylic acid-based HOFs with fluorescence sensing properties have not been reported. In this study, a rare example of a di-carboxylic acid-based, luminescent three-dimensional hydrogen-bonded organic framework has been successfully constructed and structurally characterized; it has a strong electron-rich property originated from its organic linker 9-phenylcarbazole-3,6-dicarboxylic acid. It represents the first example of HOF-based sensors for the highly selective and sensitive detection of PA (Picric acid) with reusability; the LOD is less than 60 nM. This work thus provides a new avenue for the fabrication of fluorescent HOFs sensing towards explosives.

Solvent-Triggered Fast and Visible Switching between Cage- and Channel-Type Hydrogen-Bonded Organic Frameworks

The inherent weak bonding nature of hydrogen-bonded organic frameworks (HOFs) performs like a double-edged sword in that it endows HOFs with superiority in processability and dynamicity but deactivates its on-demand controllability in the crystalline phase. Herein, based on the synergy of dynamic H-bonding interactions and the tailored low solubility in common organic solvents, reversible and fast topological transitions between cage- and channel-type HOFs were achieved upon immersing in the solution state. The aggregation-induced-emission character of the tecton facilitates the visualization of the elusive initial transition process with high sensitivity. In addition, the visible transition from cage- and channel-type HOFs to thermally stable crystalline phases is also achieved under thermal induction.

Thermodynamics-Kinetics-Balanced Metal-Organic Framework for In-Depth Radon Removal under Ambient Conditions

Radon (Rn), a ubiquitous radioactive noble gas, is the main source of natural radiation to human and one of the major culprits for lung cancer. Reducing ambient Rn concentration by porous materials is considered as the most feasible and energy-saving option to lower this risk, but the in-depth Rn removal under ambient conditions remains an unresolved challenge, mainly due to the weak van der Waals (vdW) interaction between inert Rn and adsorbents and the extremely low partial pressure (<1.8 × 10^-14 bar, <10⁶ Bq/m³) of Rn in air. Adsorbents having either favorable adsorption thermodynamics or feasible diffusion kinetics perform poorly in in-depth Rn removal. Herein, we report the discovery of a metal-organic framework (ZIF-7-Im) for efficient Rn capture guided by computational screening and modeling. The size-matched pores in ZIF-7-Im abide by the thermodynamically favorable principle and the exquisitely engineered quasi-open apertures allow for feasible kinetics with little sacrifice of sorption thermodynamics. The as-prepared material can reduce the Rn concentration from hazardous levels to that below the detection limit of the Rn detector under ambient conditions, with an improvement of at least two orders of amplitude on the removal depth compared to the currently best-performing and only commercialized material activated charcoal.

Nitro-Decorated Microporous Covalent Organic Framework (TpPa-NO2) for Selective Separation of C2H4 from a C2H2/C2H4/CO2 Mixture and CO2 Capture

A nitro-decorated microporous covalent organic framework, TpPa-NO₂, has been synthesized in a gram scale with a one-pot reaction. It can effectively selectively separate C₂H₄ from a C₂H₂/C₂H₄/CO₂ mixture and capture CO₂ from CO₂/N₂ based on ideal adsorption solution theory calculations and transient breakthrough experiments. Theoretical calculations illustrated that the hydrogen atoms of imine bonds, carbonyl oxygen, and nitro group show high affinity toward C₂H₂ and CO₂, playing vital roles in efficient separation.