One Scalable and Stable Metal-Organic Framework for Efficient Separation of CH4/N2 Mixture.

Separating CH4 from coal bed methane is of great importance but challenging. Adsorption-based separation often suffers from low selectivity, poor stability, and difficulty to scale up. Herein, a stable and scalable metal-organic framework [MOF, CoNi(pyz-NH2)] with multiple CH4 binding sites was reported to efficiently separate the CH4/N2 mixture. Due to its suitable pore size and multiple CH4 binding sites, it exhibits excellent CH4/N2 selectivity (16.5) and CH4 uptake (35.9 cm3/g) at 273 K and 1 bar, which is comparable to that of the state-of-the-art MOFs. Theoretical calculations reveal that the high density of open metal sites and polar functional groups in the pores provide strong affinity to CH4 than to N2. Moreover, CoNi(pyz-NH2) displays excellent structural stability and can be scale-up synthesized (22.7 g). This work not only provides an excellent adsorbent but also provides important inspiration for the future design and preparation of porous adsorbents for separations.

Bipolar Molecular Torque Wrench Modulates the Stacking of Two-Dimensional Metal-Organic Framework Nanosheets.

The precise modulation of nanosheet stacking modes introduces unforeseen properties and creates momentous applications but remains a challenge. Herein, we proposed a strategy using bipolar molecules as torque wrenches to control the stacking modes of 2-D Zr-1,3,5-(4-carboxylphenyl)-benzene metal-organic framework (2-D Zr-BTB MOF) nanosheets. The bipolar phenyl-alkanes, phenylmethane (P-C1) and phenyl ethane (P-C2), predominantly instigated the rotational stacking of Zr-BTB-P-C1 and Zr-BTB-P-C2, displaying a wide angular distribution. This included Zr-BTB-P-C1 orientations at 0, 12, 18, and 24° and Zr-BTB-P-C2 orientations at 0, 6, 12, 15, 24, and 30°. With reduced polarity, phenyl propane (P-C3) and phenyl pentane (P-C5) introduced steric hindrance and facilitated alkyl hydrophobic interactions with the nanosheets, primarily resulting in the modulation of eclipsed stacking for Zr-BTB-P-C3 (64.8%) and Zr-BTB-P-C5 (93.3%) nanosheets. The precise angle distributions of four Zr-BTB-P species were in agreement with theoretical calculations. The alkyl induction mechanism was confirmed by the sequential guest replacement and 2-D 13C-1H heteronuclear correlation (HETCOR). In addition, at the single-particle level, we first observed that rotational stacked pores exhibited similar desorption rates for xylene isomers, while eclipsed stacked pores showed significant discrepancy for xylenes. Moreover, the eclipsed nanosheets as stationary phases exhibited high resolution, selectivity, repeatability, and durability for isomer separation. The universality was proven by another series of bipolar acetate-alkanes. This bipolar molecular torque wrench strategy provides an opportunity to precisely control the stacking modes of porous nanosheets.

Greatly Intensified Guest Exchange Strategy for Highly-Efficient Activation of Metal-Organic Frameworks.

The preservation and accessibility of pores are prerequisites to the application of metal-organic frameworks (MOFs). Activation is a key step to eliciting rich features of pores, but it needs a repeated solvent-exchange process which is tedious and time/cost-consuming. Herein, a facile strategy for highly-efficient activation of MOFs utilizing rotating packed bed is proposed. With the tremendous enhancement of molecular mixing and mass transfer in high-gravity and strong-shearing surrounding, nine representative MOFs are completely activated within 2 h without structural change. Compared with conventional process, this activation displays surprising efficiency by accelerating the diffusion of solvents and redissolution of residual reactants in the pores. The complete activation time can be significantly shortened by over 90%. As a proof-of-concept, the methane storage of as-activated UiO-66 is five times that of as-synthesized UiO-66. This strategy provides a potential platform with industrial worth for the activation of MOF materials with ultra-high efficiency and versatility.

Construction of defective zirconium-based metal-organic frameworks for enhanced removal of toxic selenite: performance and mechanism studies.

The development of effective adsorbents for the adsorption and removal of toxic selenite (SeO32-) from wastewater is urgently required but challenging. Herein, formic acid (FA), a monocarboxylic acid, was used as a template to construct serial defective Zr-Fumarate (Fum) -FA based on a green and facile preparation method. Physicochemical characterization shows that the defect degree of Zr-Fum-FA can be flexibly controlled by regulating the amount of FA to be added. Owing to rich defect units, the diffusion and mass transfer of guest SeO32- into the channel can be boosted. Particularly, Zr-Fum-FA-6 with the most defects exhibits superior adsorption capacity (519.6 mg g-1) and rapid adsorption equilibrium (∼200 min). The adsorption isotherms and kinetics can be well described by the Langmuir and pseudo-second-order kinetic models. Moreover, this adsorbent possesses excellent resistance towards co-existing ions, high chemical stability and good applicability in a broad pH range of 3-10. Thus, our study provides a promising adsorbent for SeO32-, and more importantly, it proposes a strategy for rationally tailoring the adsorption behavior of adsorbents via defect construction.

Integrating Self-Partitioned Pore Space and Amine Functionality into an Aromatic-Rich Coordination Framework with Ph Stability for Effective Purification of C2 Hydrocarbons.

A great demand for high-purity C2 hydrocarbons calls for the development of chemically stable porous materials for the effective isolation of C2 hydrocarbons from CH4 and CO2. However, such separations are challenged by their similar physiochemical parameters and have not been systematically studied to date. In this work, we reported a cadmium-based rod-packing coordination framework compound ZJNU-140 of a new 5,6,7-c topology built up from a custom-designed tricarboxylate ligand. The metal-organic framework (MOF) features an aromatic-abundant pore surface, uncoordinated amine functionality, and self-partitioned pore space of suitable size. These structural characteristics act synergistically to provide the MOF with both selective recognition ability and the confinement effect toward C2 hydrocarbons. As a result, the MOF displays promising potential for adsorptive separation of C2-CH4 and C2-CO2 mixtures. The IAST-predicted C2/CH4 and C2/CO2 adsorption selectivities, respectively, fall in the ranges of 7.3-10.2 and 2.1-2.9 at 298 K and 109 kPa. The real separation performance was also confirmed by dynamic breakthrough experiments. In addition, the MOF can maintain skeleton intactness in aqueous solutions with a wide pH range of 3-11, as confirmed by powder X-ray diffraction (PXRD) and isotherm measurements, showing no loss of framework integrity and porosity. The excellent hydrostability, considerable uptake capacity, impressive adsorption selectivity, and mild regeneration make ZJNU-140 a promising adsorbent material applied for the separation and purification of C2 hydrocarbons.

Insights into the thermodynamic-kinetic synergistic separation of propyne/propylene in anion pillared cage MOFs with entropy-enthalpy balanced adsorption sites.

Propyne/propylene (C3H4/C3H6) separation is an important industrial process yet challenged by the trade-off of selectivity and capacity due to the molecular similarity. Herein, record C3H4/C3H6 separation performance is achieved by fine tuning the pore structure in anion pillared MOFs. SIFSIX-Cu-TPA (ZNU-2-Si) displays a benchmark C3H4 capacity (106/188 cm3 g-1 at 0.01/1 bar and 298 K), excellent C3H4/C3H6 IAST selectivity (14.6-19.3) and kinetic selectivity, and record high C3H4/C3H6 (10/90) separation potential (36.2 mol kg-1). The practical C3H4/C3H6 separation performance is fully demonstrated by breakthroughs under various conditions. 37.8 and 52.9 mol kg-1 of polymer grade C3H6 can be produced from 10/90 and 1/99 C3H4/C3H6 mixtures. 4.7 mol kg-1 of >99% purity C3H4 can be recovered by a stepped desorption process. Based on the in situ single crystal analysis and DFT calculation, an unprecedented entropy-enthalpy balanced adsorption pathway is discovered. MD simulation further confirmed the thermodynamic-kinetic synergistic separation of C3H4/C3H6 in ZNU-2-Si.

A Self-Evolutionary Methodology for Reverse Design of Novel MOFs.

A combination of high-throughput molecular simulation and machine learning (ML) algorithms has been widely adopted to seek promising metal-organic frameworks (MOFs) as energy gas carriers. However, the currently reported studies are mainly limited to extracting top performers from existing databases, not fully unleashing the ML capabilities for intelligently predicting novel structures with better performance. Herein, an efficient self-evolutionary methodology was proposed for searching high-performance MOFs that are unstructured in the origin database, in which a Tangent Adaptive Genetic Algorithm (TAGA) was newly put forward for structural evolution and the high-precision ML model of eXtreme Gradient Boosting (XGBoost) was employed as the fitness function. By taking CH4 storage in MOFs at room temperature as a showcase and using the database of 51,163 hMOFs, the TAGA-XGBoost coupling strategy rapidly suggested a certain number of possible combinations of the building blocks to form new structures with gravimetric storage capacity (35 bar) and volumetric working capacity (65-5.8 bar) higher than the best materials in the original database. The structures of some promising MOFs successfully used the finally optimized material genes for the two application conditions, and their performances were also confirmed by subsequent molecular simulations. The best materials can respectively reach a storage amount of 580 cm3(STP)/g at 35 bar and a working capacity of 218 cm3(STP)/cm3 between 65 and 5.8 bar. An analysis of the top 100 materials predicted from our method revealed that the choice of organic linkers has a systematic effect on the storage performance of MOFs. It might be believed that the proposed methodology offers an opportunity to expedite the discovery of unprecedented materials for other practical applications.

Isomer recognition by dynamic guest-adaptive ligand rotation in a metal-organic framework with local flexibility.

Local flexibility in a metal-organic framework is intriguing for reconstructing a microenvironment to distinguish different guest molecules by emphasizing their differences. Herein, guest-adaptive flexibility is observed in a metal-organic framework for efficiently discriminating aromatic isomers. Microcrystal electron diffraction directly reveals that the anthracene rings can rotate around the single bond with the adsorption of guest molecules. Disorder transformation of the ligand enables the preferential adsorption of ethylbenzene over other xylene isomers. Especially, a coated capillary column combining single/multi-component adsorption confirms a unique separation order of ethylbenzene > p-xylene > m-xylene > o-xylene with excellent selectivities, which has not been reported in other materials. Density functional theory calculations and the calculated Hirshfeld surface of guest molecules in the framework demonstrate that a guest-induced splint-like confinement structure makes the main contribution to such separation performance. This finding will provide a rational strategy for molecular recognition utilizing the local flexibility of metal-organic frameworks.

MIL-101-Cr/Fe/Fe-NH2 for Efficient Separation of CH4 and C3H8 from Simulated Natural Gas.

Adsorptive separation based on porous solid adsorbents has emerged as an excellent effective alternative to energy-intensive conventional separation methods in a low energy cost and high working capacity manner. However, there are few stable mesoporous metal-organic frameworks (MOFs) for efficient purification of methane from other light hydrocarbons in natural gas. Herein, we report a series of stable mesoporous MOFs, MIL-101-Cr/Fe/Fe-NH2, for efficient separation of CH4 and C3H8 from a ternary mixture CH4/C2H6/C3H8. Experimental results show that all three MOFs possess excellent thermal, acid/basic, and hydrothermal stability. Single-component adsorption suggested that they have high C3H8 adsorption capacity and commendable selectivity for C3H8 and C2H6 over CH4. Transient breakthrough experiments further certified the ability of direct separation of CH4 from simulated natural gas and indirect recovery of C3H8 from the packing column. Theoretical calculations illustrated that the van der Waals force proportional to the molecular weight is the key factor and that the structural integrity and defect can impact separation performances.

Highly Effective Removal of Ofloxacin from Water with Copper-Doped ZIF-8.

Residual antibiotics in wastewater have gained widespread attention because of their toxicity to humans and the environment. In this work, Cu-doped ZIF-8s (Cu-ZIF-8s) were successfully synthesized by the impregnation of Cu2+ in ZIF-8 and applied in the removal of ofloxacin (OFX) from water. Remarkably, excellent adsorption performance was obtained in Cu-ZIF-8s, especially for Cu-ZIF-8-1, in which the adsorption capacity (599.96 mg·g-1) was 4.2 times higher than that of ZIF-8 and superior to various adsorbents reported previously. The adsorption kinetics and adsorption isotherm follow the pseudo-second-order model and the Langmuir model, respectively. Furthermore, the removal efficiencies of OFX in Cu-ZIF-8-1 reached over 90% at low concentrations. It was revealed that electrostatic interaction and complexation play important roles in the adsorption process. In addition, the material can be regenerated by simple methods. Therefore, the obtained Cu-doped MOFs may have a promising application in the treatment of antibiotic-containing wastewater.

High-Throughput Computational Exploration of MOFs with Open Cu Sites for Adsorptive Separation of Hydrogen Isotopes.

Effective separation of hydrogen isotopes still remains one of the extremely challenging tasks in industry. Compared to the present methods that are energy- and cost-intensive, quantum sieving technology based on nanostructured materials offers a more efficient alternative approach, where metal-organic frameworks (MOFs) featuring open metal sites (OMS) can serve as an ideal platform. Herein, a combination of periodic density functional theory (DFT) with dispersive correction and high-throughput molecular simulation was employed from thermodynamic viewpoints to explore the D2/H2 separation properties of 929 experimental MOFs bearing a copper-paddlewheel unit. The DFT calculations showed that there is a negligible rotational energy barrier for the molecule adsorbed at the OMS, and the movement of the Cu atoms along the Cu-Cu axis vector almost has no influence on the interaction energy. On the basis of the DFT results, a new force field with a proposed cutoff scheme was developed to accurately describe the strong isotope-OMS interaction. Under practical conditions (40 K and 1.0 bar), large-scale computational material screening demonstrated that the OMS interaction plays a more important role in highly selective materials and ignoring such interactions can lead to completely wrong identification of the most promising materials. Using the adsorption selectivity and adsorbent performance score as evaluation metrics, this work demonstrated that the materials with sql topology notably outperform many benchmark adsorbents reported so far.

Enhancing CH4 Capture from Coalbed Methane through Tuning van der Waals Affinity within Isoreticular Al-Based Metal-Organic Frameworks.

Efficient separation of the CH4/N2 mixture is of great significance for coalbed methane purification. It is an effective strategy to separate this mixture by tuning the van der Waals interaction due to the nonpolar properties of CH4 and N2 molecules. Herein, we prepared several isoreticular Al-based metal-organic frameworks (MOFs) with different ligand sizes and polarities because of their high structural stability and low cost/toxicity feature of Al metal. Adsorption experiments indicated that the CH4 uptake, Qst of CH4, and CH4/N2 selectivity are in the order of Al-FUM-Me (27.19 cm3(STP) g-1, 24.06 kJ mol-1 and 8.6) > Al-FUM (20.44 cm3(STP) g-1, 20.60 kJ mol-1 and 5.1) > Al-BDC (15.98 cm3(STP) g-1, 18.81 kJ mol-1 and 3.4) > Al-NDC (10.86 cm3(STP) g-1, 14.89 kJ mol-1 and 3.1) > Al-BPDC (5.90 cm3(STP) g-1, 11.75 kJ mol-1 and 2.2), confirming the synergetic effects of pore sizes and pore surface polarities. Exhilaratingly, the ideal adsorbed solution theory selectivity of Al-FUM-Me is higher than those of all zeolites, carbon materials, and most water-stable MOF materials (except Al-CDC and Co3(C4O4)2(OH)2), which is comparable to MIL-160. Breakthrough results demonstrate its excellent separation performance for the CH4/N2 mixture with good regenerability. The separation mechanism of Al-FUM-Me for the CH4/N2 mixture was elucidated by theoretical calculations, showing that the stronger affinity of CH4 can be attributed to its relatively shorter interaction distance with adsorption binding sites. Therefore, this work not only offers a promising candidate for CH4/N2 separation but also provides valuable guidance for the design of high-performance adsorbents.

Homogeneously Mixing Different Metal-Organic Framework Structures in Single Nanocrystals through Forming Solid Solutions.

Pore engineering plays a significant role in the applications of porous materials, especially in the area of separation and catalysis. Here, we demonstrated a metal-organic framework (MOF) solid solution (MOSS) strategy to homogeneously and controllably mix NU-1000 and NU-901 structures inside single MOF nanocrystals. The key for the homogeneous mixing and forming of MOSS was the bidentate modulator, which was designed to have a slightly longer distance between two carboxylate groups than the original tetratopic ligand. All of the MOSS nanocrystals showed a uniform pore size distribution with a well-tuned ratio of mesopores to micropores. Because of the appropriate pore ratio, MOSS nanocrystals can balance the thermodynamic interactions and kinetic diffusion of the substrates, thus showing exceedingly higher separation abilities and a unique elution sequence. Our work proposes a rational strategy to design mixed-porous MOFs with controlled pore ratios and provides a new direction to design homogeneously mixed MOFs with a high separation ability and unique separation selectivity.

Temperature-Dependent Nitrous Oxide/Carbon Dioxide Preferential Adsorption in a Thiazolium-Functionalized NU-1000 Metal-Organic Framework.

Solvent-assisted ligand incorporation (SALI) of the ditopic linker 5-carboxy-3-(4-carboxybenzyl)thiazolium bromide [(H2PhTz)Br] into the zirconium metal-organic framework NU-1000 [Zr6O4(OH)8(H2O)4(TBAPy)2, where NU = Northwestern University and H4TBAPy = 1,3,6,8-tetrakis(p-benzoic-acid)pyrene], led to the SALIed NU-1000-PhTz material of minimal formula [Zr6O4(OH)6(H2O)2(TBAPy)2(PhTz)]Br. NU-1000-PhTz has been thoroughly characterized in the solid state. As confirmed by powder X-ray diffraction, this material keeps the same three-dimensional architecture of NU-1000 and the dicarboxylic extra linker bridges adjacent [Zr6] nodes ca. 8 Å far apart along the crystallographic c-axis. The functionalized MOF has a BET specific surface area of 1560 m2/g, and it is featured by a slightly higher thermal stability than its parent material (Tdec = 820 vs. 800 K, respectively). NU-1000-PhTz has been exploited for the capture and separation of two pollutant gases: carbon dioxide (CO2) and nitrous oxide (N2O). The high thermodynamic affinity for both gases [isosteric heat of adsorption (Qst) = 25 and 27 kJ mol-1 for CO2 and N2O, respectively] reasonably stems from the strong interactions between these (polar) "stick-like" molecules and the ionic framework. Intriguingly, NU-1000-PhTz shows an unprecedented temperature-dependent adsorption capacity, loading more N2O in the 298 K ≤ T ≤ 313 K range but more CO2 at temperatures falling out of this range. Grand canonical Monte Carlo simulations of the adsorption isotherms confirmed that the preferential adsorption sites of both gases are the triangular channels (micropores) in close proximity to the polar pillar. While CO2 interacts with the thiazolium ring in an "end-on" fashion through its O atoms, N2O adopts a "side-on" configuration through its three atoms simultaneously. These findings open new horizons in the discovery of functional materials that may discriminate between polluting gases through selective adsorption at different temperatures.

A Series of Functionalized Zirconium Metal-Organic Cages for Efficient CO2/N2 Separation.

Global warming associated with CO2 emission has led to frequent extreme weather events in recent years. Carbon capture using porous solid adsorbents is promising for addressing the greenhouse effect. Herein, we report a series of robust metal-organic cages (MOCs) featuring various functional groups, such as methyl and amine groups, for CO2/N2 separation. Significantly, the amine-group-functionalized MOC-QW-3-NH2 displays the best selective CO2 adsorption performance, as confirmed by single-component adsorption and transient breakthrough experiments. The distinct CO2 adsorption mechanism has been well studied via theoretical calculations, confirming that the amine groups play a vital role for efficiently selective CO2 adsorption resulting from hierarchical adsorbate-framework interaction.

A General Strategy for Instantaneous and Continuous Synthesis of Ultrasmall Metal-Organic Framework Nanoparticles.

Ultrasmall metal-organic frameworks (MOFs) may generate unique properties to expand the scope of applications. However, the synthesis is still a great challenge. Herein, we propose a strategy to synthesize ultrasmall MOFs by high gravity technology. With the aid of tremendous intensification of molecular mixing and mass transfer in high-gravity field, six typical MOFs were obtained instantaneously in a continuous way. These samples are monodispersed with sub-5 nm in size, smaller than the previously reported values and even close to the length of one crystal unit cell. As a proof-of-concept, catalytic activity for Knoevenagel reaction can be significantly enhanced using ultrasmall ZIF-8. Conversion time of benzaldehyde was decreased by 94 % or 75 % compared to those using conventional or hierarchically porous ZIF-8. More importantly, this approach is readily scalable with the highest space-time yield for nano-MOFs, which may promote the convenient synthesis and practical applications of ultrasmall MOFs in large-scale.

Fabrication of Ultrathin Membranes Using 2D-MOF Nanosheets for Tunable Gas Separation.

Two-dimensional (2D) metal-organic frameworks (MOF) nanosheets have emerged as novel membrane materials for gas separation. However, the development of ultrathin MOF membranes with tunable separation performances is still a challenge. Herein, we developed a facile GO-assisted restacking method to fabricate defect-free membranes with monolayer Zr-BTB nanosheets. Obtained ultrathin membranes ranging from 130 nm to 320 nm show tunable separation performances and exceed the 2008 Robeson upper bound by changing the amount of nanolayers in vertical stacking direction. Furthermore, a heating filtration method was used to change the restacking process of nanosheets in the horizontal direction. As a result, H2 /CO2 selectivity can be enhanced by two times with the same membrane thickness (130 nm) and H2 permeance is almost maintained to be 7.0×10-7  mol m-2 s-1 pa-1 . This method may provide a possible way to efficiently tune the gas separation performances of MOF membranes.

Controlling the Stacking Modes of Metal-Organic Framework Nanosheets through Host-Guest Noncovalent Interactions.

The tuning of metal-organic framework (MOF) nanosheet stacking modes from molecular level was rarely explored although it significantly affected the properties and applications of nanosheets. Here, the different stacking modes of Zr-1, 3, 5-(4-carboxylphenyl)-benzene framework nanosheets were synthesized through the induction of different host-guest noncovalent interactions. The solvents of methyl benzene and ethyl acetate induced twisted stacking of nanosheets with the specific rotation angles of 12°, 18°, 24° and 6°, 18°, 24°, 30°, respectively, which was in agreement with theoretical calculations. Meanwhile, the alkanes were likely to vertically enter the pores of Zr-BTB nanosheets because of steric hindrance and hydrophobic interactions, resulting in the untwisted stacking of nanosheets. The untwisted ordered nanopores showed the excellent gas chromatographic separations of benzene derivative isomers, which was better than twisted nanosheets stacking and commercial columns. This work uncovers a rational strategy to control the stacking of two-dimensional MOF nanosheets.

pH and H2S Dual-Responsive Magnetic Metal-Organic Frameworks for Controlling the Release of 5-Fluorouracil.

Along with the increasing cancer incidence, developing suitable drug delivery systems (DDSs) is becoming urgent to control drug release and further enhance therapeutic efficiency. Herein, a Fe-Zn bimetallic MOF-derived ferromagnetic nanomaterial was synthesized by a one-step method. The successful preparation of ferromagnetic Fe-ZIF-8 was verified by scanning electron microscopy, powder X-ray diffraction, Brunauer-Emmett-Teller, X-ray photoelectron spectroscopy, and physical property measurement system characterizations. Furthermore, the release behaviors of 5-FU from the ferromagnetic carrier were investigated in a simulative cancer microenvironment of PBS buffer solution (PBS = phosphate-buffered saline, pH = 5.8) and NaHS solution. The vehicle in PBS solution of pH = 5.8 and NaHS solution of 500 µM can rapidly release 5-FU with the cumulative release percentages of 68 and 36%, respectively, within two hundred minutes. The release mechanism in the weak acid environment can be mainly attributed to the decomposition of the Fe-ZIF-8. However, the strong interaction between Zn and Fe atoms in Fe-ZIF-8 and the S atom in H2S plays an important role in the release process in the simulated H2S cancer microenvironment. The investigation of release kinetic models indicates that the 5-FU release in the PBS solutions and NaHS solution of 500 µM can be accurately fitted by a second-degree polynomial model and first-order model, respectively. In addition, the decomposition products, zinc, iron, and 2-MeIM, are endogenous and show low toxicity values [LD50 (Zn) = 0.35 g·kg-1, LD50 (Fe) = 30 g·kg-1, and LD50 (2-MeIM) = 1.4 g·kg-1]. Therefore, the low-toxicity, pH and H2S dual-stimuli-responsive, and ferromagnetic nature make the obtained Fe-ZIF-8 an ideal candidate in the field of bioactive molecule delivery.