A Fine-Tuned Fluorinated MOF Addresses the Needs for Trace CO2 Removal and Air Capture Using Physisorption

Tailoring the separation properties of flexible metal-organic frameworks using mechanical pressure

Metal-organic frameworks are widely considered for the separation of chemical mixtures due to their adjustable physical and chemical properties. However, while much effort is currently devoted to developing new adsorbents for a given separation, an ideal scenario would involve a single adsorbent for multiple separations. Porous materials exhibiting framework flexibility offer unique opportunities to tune these properties since the pore size and shape can be controlled by the application of external stimuli. Here, we establish a proof-of-concept for the molecular sieving separation of species with similar sizes (CO₂/N₂ and CO₂/CH₄), via precise mechanical control of the pore size aperture in a flexible metal-organic framework. Besides its infinite selectivity for the considered gas mixtures, this material shows excellent regeneration capability when releasing the external mechanical constraint. This strategy, combining an external stimulus applied to a structurally compliant adsorbent, offers a promising avenue for addressing some of the most challenging gas separations.

Separation of gasses with similar physical and chemical properties can be energy demanding, and adsorption-based technologies may provide alternatives with lower energy consumption. Here, the authors show mechanical control of pore size aperture in flexible metal-organic frameworks for separation of gasses.

Halogenated Metal-Organic Framework Glasses and Liquids

The synthesis of four novel crystalline zeolitic imidazolate framework (ZIF) structures using a mixed-ligand approach is reported. The inclusion of both imidazolate and halogenated benzimidazolate-derived linkers leads to glass-forming behavior by all four structures. Melting temperatures are observed to depend on both electronic and steric effects. Solid-state NMR and terahertz (THz)/far-IR demonstrate the presence of a Zn-F bond for fluorinated ZIF glasses. In situ THz/far-IR spectroscopic techniques reveal the dynamic structural properties of crystal, glass, and liquid phases of the halogenated ZIFs, linking the melting behavior of ZIFs to the propensity of the ZnN₄ tetrahedra to undergo thermally induced deformation. The inclusion of halogenated ligands within metal-organic framework (MOF) glasses improves their gas-uptake properties.

Disappearing Polymorphs in Metal-Organic Framework Chemistry: Unexpected Stabilization of a Layered Polymorph over an Interpenetrated Three-Dimensional Structure in Mercury Imidazolate

The "disappearing polymorph" phenomenon is well established in organic solids, and has had a profound effect in pharmaceutical materials science. The first example of this effect in metal-containing systems in general, and in coordination-network solids in particular, is here reported. Specifically, attempts to mechanochemically synthesize a known interpenetrated diamondoid (dia) mercury(II) imidazolate metal-organic framework (MOF) yielded a novel, more stable polymorph based on square-grid (sql) layers. Simultaneously, the dia-form was found to be highly elusive, observed only as a short-lived intermediate in monitoring solvent-free synthesis and not at all from solution. The destabilization of a dense dia-framework relative to a lower dimensionality one is in contrast to the behavior of other imidazolate MOFs, with periodic density functional theory (DFT) calculations showing that it arises from weak interactions, including structure-stabilizing agostic C-H⋅⋅⋅Hg contacts. While providing a new link between MOFs and crystal engineering of organic solids, these findings highlight a possible role for agostic interactions in directing topology and stability of MOF polymorphs.

Curating Metal-Organic Frameworks To Compose Robust Gas Sensor Arrays in Dilute Conditions

Metal-organic frameworks (MOFs), tunable, nanoporous materials, are alluring recognition elements for gas sensing. Mimicking human olfaction, an array of cross-sensitive, MOF-based sensors could enable analyte detection in complex, variable gas mixtures containing confounding gas species. Herein, we address the question: given a set of MOF candidates and their adsorption properties, how do we select the optimal subset to compose a sensor array that accurately and robustly predicts the gas composition via monitoring the adsorbed mass in each MOF? We first mathematically formulate the MOF-based sensor array problem under dilute conditions. Instructively, the sensor array can be viewed as a linear map from gas composition space to sensor array response space defined by the matrix H of Henry coefficients of the gases in the MOFs. Characterizing this mapping, the singular value decomposition of H is a useful tool for evaluating MOF subsets for sensor arrays, as it determines the sensitivity of the predicted gas composition to measurement error, quantifies the magnitude of the response to changes in composition, and recovers which direction in gas composition space elicits the largest/smallest response. To illustrate, on the basis of experimental adsorption data, we curate MOFs for a sensor array with the objective of determining the concentration of CO₂ and SO₂ in the gas phase.

An overview on trace CO2 removal by advanced physisorbent materials

This review paper focuses on various gas processing technologies and materials that efficiently capture trace levels of carbon dioxide (CO₂). Fundamental separation mechanisms such as absorption, adsorption, and distillation technology are presented. Liquid amine-based carbon capture (C-capture) technologies have been in existence for over half a century, however, liquid amine capture relies upon chemical reactions and is energy-intensive. Liquid amines are thus not economically viable for broad deployment and offer little room for innovation. Innovative C-capture technologies must improve both the environmental footprint and cost-effectiveness. As a promising alternative, physisorbents have many advantages including considerably lower regeneration energy. Generally, existing classes of physisorbent materials, such as metal-organic frameworks (MOFs) and zeolites are selective toward C-capture. However, their selectivity is currently not high enough to remove trace levels (e.g., ~1%) of CO₂ from various natural gas process streams. This review summarizes the current advancements in physisorbent materials for CO₂ capture. Here, key performance parameters needed to select the most suitable candidate are highlighted. Furthermore, this review discusses the scope for the development of better performing CO₂ selective physisorbents from both environmental and economic perspectives. In addition, hybrid ultra microporous materials (HUMs), characterized mainly by ultra-micro pores (<0.7 nm), are discussed in reference to C-capture. Various characteristics of HUMs result in high selectivity and applicability in difficult separations such as the gas sweetening and C-capture from complex humid mixed gas streams.

Stable abnormal N-heterocyclic carbenes and their applications

Although N-heterocyclic carbenes (NHCs) have been known as ligands for organometallic complexes since the 1960s, these carbenes did not attract considerable attention until Arduengo et al. reported the isolation of a metal-free imidazol-2-ylidene in 1991. In 2001 Crabtree et al. reported a few complexes featuring an NHC isomer, namely an imidazol-5-ylidene, also termed abnormal NHC (aNHCs). In 2009, it was shown that providing to protect the C-2 position of an imidazolium salt, the deprotonation occurred at the C-5 position, affording imidazol-5-ylidenes that could be isolated. Over the last ten years, stable aNHCs have been used for designing a range of catalysts employing Pd(ii), Cu(i), Ni(ii), Fe(0), Zn(ii), Ag(i), and Au(i/iii) metal based precursors. These catalysts were utilized for different organic transformations such as the Suzuki-Miyaura cross-coupling reaction, C-H bond activation, dehydrogenative coupling, Huisgen 1,3-dipolar cycloaddition (click reaction), hydroheteroarylation, hydrosilylation reaction and migratory insertion of carbenes. Main-group metal complexes were also synthesized, including K(i), Al(iii), Zn(ii), Sn(ii), Ge(ii), and Si(ii/iv). Among them, K(i), Al(iii), and Zn(ii) complexes were used for the polymerization of caprolactone and rac-lactide at room temperature. In addition, based on the superior nucleophilicity of aNHCs, relative to that of their nNHCs isomers, they were used for small molecules activation, such as carbon dioxide (CO₂), nitrous oxide (N₂O), tetrahydrofuran (THF), tetrahydrothiophene and 9-borabicyclo[3.3.1]nonane (9BBN). aNHCs have also been shown to be efficient metal-free catalysts for ring opening polymerization of different cyclic esters at room temperature; they are among the most active metal-free catalysts for ε-caprolactone polymerization. Recently, aNHCs successfully accomplished the metal-free catalytic formylation of amides using CO₂ and the catalytic reduction of carbon dioxide, including atmospheric CO₂, into methanol, under ambient conditions. Although other transition metal complexes featuring aNHCs as ligand have been prepared and used in catalysis, this review article summarize the results obtained with the isolated aNHCs.

Solid Sorbents as a Retrofit Technology for CO2 Removal from Natural Gas Under High Pressure and Temperature Conditions

The capture of CO2 under high pressure and temperature is challenging and is required in a number for industrial applications including natural gas processing. In this work, we examine the use of benchmark hybrid ultraporous materials HUMs for their potential use in CO2 adsorption processes under high-pressure conditions, with three varying temperatures (283, 298 and 318 K). NbOFFOVE-1-Ni and SIFSIX-3-Ni were the selected HUMs given their established superior CO2 capacity under low pressure (0-1 bar). Both are microporous with highly ordered crystalline structures as compared to the mesoporous hexagonal silica (Santa Barbara Anhydrous-15 (SBA-15)). SBA-15 was previously tested for both low and high-pressure applications and can serve as a benchmark in this study. Sorbent characterization using XRD, SEM, FTIR and N2 adsorption were conducted to assure the purity and structure of the sorbents. TGA analysis were conducted to establish the thermal stability of the sorbents under various temperatures. High-pressure CO2 adsorption was conducted from 0-35 bar using magnetic suspension balance (Rubotherm). Although the SBA-15 had the highest surface (527 m3/g) are of the three adsorbents, the CO2 adsorption capacity (0.42 mmol/g) was an order of magnitude less than the studies HUMs with SIFSIX-3-Ni having 2.6 mmol/g, NbOFFIVE-1-Ni achieving 2.5 mmol/g at 298 K. Multistage adsorption isotherms were obtained at different pressures. In addition, results indicate that electrostatics in HUMs are most effective at improving isosteric heat of adsorption Qst and CO2 uptake. Higher temperatures had negative effect on adsorption capacity for the HUMs and SBA-15 at pressures between 7-9 bar. In SAB-15 the effect of temperature is reversed in what is known as a cross over phenomena.

Insights into the Gas Adsorption Mechanisms in Metal-Organic Frameworks from Classical Molecular Simulations

Classical molecular simulations can provide significant insights into the gas adsorption mechanisms and binding sites in various metal-organic frameworks (MOFs). These simulations involve assessing the interactions between the MOF and an adsorbate molecule by calculating the potential energy of the MOF-adsorbate system using a functional form that generally includes nonbonded interaction terms, such as the repulsion/dispersion and permanent electrostatic energies. Grand canonical Monte Carlo (GCMC) is the most widely used classical method that is carried out to simulate gas adsorption and separation in MOFs and identify the favorable adsorbate binding sites. In this review, we provide an overview of the GCMC methods that are normally utilized to perform these simulations. We also describe how a typical force field is developed for the MOF, which is required to compute the classical potential energy of the system. Furthermore, we highlight some of the common analysis techniques that have been used to determine the locations of the preferential binding sites in these materials. We also review some of the early classical molecular simulation studies that have contributed to our working understanding of the gas adsorption mechanisms in MOFs. Finally, we show that the implementation of classical polarization for simulations in MOFs can be necessary for the accurate modeling of an adsorbate in these materials, particularly those that contain open-metal sites. In general, molecular simulations can provide a great complement to experimental studies by helping to rationalize the favorable MOF-adsorbate interactions and the mechanism of gas adsorption.

Mechano-assisted synthesis of an ultramicroporous metal–organic framework for trace CO2 capture

Hybrid ultramicroporous ZU-36-Ni (GeFSIX-3-Ni) synthesized by a mechano-assisted thermal transformation method exhibits excellent trace CO₂ capture performance through strong host–guest interactions.

Trace CO2 capture by an ultramicroporous physisorbent with low water affinity

CO2 accumulation in confined spaces represents an increasing environmental and health problem. Trace CO2 capture remains an unmet challenge because human health risks can occur at 1000 parts per million (ppm), a level that challenges current generations of chemisorbents (high energy footprint and slow kinetics) and physisorbents (poor selectivity for CO2, especially versus water vapor, and/or poor hydrolytic stability). Here, dynamic breakthrough gas experiments conducted upon the ultramicroporous material SIFSIX-18-Ni-β reveal trace (1000 to 10,000 ppm) CO2 removal from humid air. We attribute the performance of SIFSIX-18-Ni-β to two factors that are usually mutually exclusive: a new type of strong CO2 binding site and hydrophobicity similar to ZIF-8. SIFSIX-18-Ni-β also offers fast sorption kinetics to enable selective capture of CO2 over both N2 (S CN) and H2O (S CW), making it prototypal for a previously unknown class of physisorbents that exhibit effective trace CO2 capture under both dry and humid conditions.

An Ultrastable Metal Azolate Framework with Binding Pockets for Optimal Carbon Dioxide Capture

In the evolution of metal-organic frameworks (MOFs) for carbon capture, a lasting challenge is to strike a balance between high uptake capacity/selectivity and low energy cost for regeneration. Meanwhile, these man-made materials have to survive from practical demands such as stability under harsh conditions and feasibility of scale-up synthesis. Reported here is a new MOF, Zn(imPim) (aka. MAF-stu-1), with an imidazole derivative ligand, featuring binding pockets that can accommodate CO₂ molecules in a fit-like-a-glove manner. Such a high degree of shape complementarity allows direct observation of the loaded CO₂ in the pockets, and warrants its optimal carbon capture performances exceeding the best-performing MOFs nowadays. Coupled with the record thermal (up to 680 °C) and chemical stability, as well as rapid large-scale production, both encoded in the material design, Zn(imPim) represents a most competitive candidate to tackle the immediate problems of carbon dioxide capture.

Negative emissions technologies: A complementary solution for climate change mitigation

Carbon dioxide (CO₂) is the main greenhouse gas (GHG) and its atmospheric concentration is currently 50% higher than pre-industrial levels. The continuous GHGs emissions may lead to severe and irreversible consequences in the climate system. The reduction of GHG emissions may be not enough to mitigate climate change. Consequently, besides carbon capture from large emission sources, atmospheric CO₂ capture may be also required. To meet the target defined for climate change mitigation, the removal of 10 Gt·yr^-1 of CO₂ globally by mid-century and 20 Gt·yr^-1 of CO₂ globally by the end of century. The technologies applied with this aim are known as negative emission technologies (NETs), as they lead to achieve a negative balance of carbon in atmosphere. This paper aims to present the recent research works regarding NETs, focusing the research findings achieved by academic groups and projects. Besides several advantages, NETs present high operational cost and its scale-up should be tested to know the real effect on climate change mitigation. With current knowledge, no single process should be seen as a solution. Research efforts should be performed to evaluate and reduce NETs costs and environmental impact.

Porous metal-organic frameworks for gas storage and separation: Status and challenges

Gases are widely used as energy resources for industry and our daily life. Developing energy cost efficient porous materials for gas storage and separation is of fundamentally and industrially important, and is one of the most important aspects of energy chemistry and materials. Metal-organic frameworks (MOFs), representing a novel class of porous materials, feature unique pore structure, such as exceptional porosity, tunable pore structures, ready functionalization, which not only enables high density energy storage of clean fuel gas in MOF adsorbents, but also facilitates distinct host-guest interactions and/or sieving effects to differentiate different molecules for energy-efficient separation economy. In this review, we summarize and highlight the recent advances in the arena of gas storage and separation using MOFs as adsorbents, including progresses in MOF-based membranes for gas separation, which could afford broader concepts to the current status and challenges in this field.

Oxidation-Resistant, Cost-Effective Epoxide-Modified Polyamine Adsorbents for CO2 Capture from Various Sources Including Air

CO₂ adsorbents based on the reaction of pentaethylenehexamine (PEHA) or tetraethylenepentamine (TEPA) with propylene oxide (PO) were easily prepared in "one pot" by impregnation on a silica support in water. The starting materials were readily available and inexpensive, facilitating the production of the adsorbents on a large scale. The prepared polyamine/epoxide adsorbents were efficient in capturing CO₂ and could be regenerated under mild conditions (50-85 °C). They displayed a much-improved stability compared with their unmodified amine counterparts, especially under oxidative conditions. Leaching of the active organic amine became minimal or nonexistent after treatment with the epoxide. The adsorption as well as desorption kinetics were also greatly improved. The polyamine/epoxide adsorbents were able to capture CO₂ from various sources including ambient air and indoor air with CO₂ concentrations of only 400-1000 ppm. The presence of water, far from being detrimental, increased the adsorption capacity. Their use for indoor air quality purposes was explored.

Fluorinated MOF platform for selective removal and sensing of SO2 from flue gas and air

Conventional SO₂ scrubbing agents, namely calcium oxide and zeolites, are often used to remove SO₂ using a strong or irreversible adsorption-based process. However, adsorbents capable of sensing and selectively capturing this toxic molecule in a reversible manner, with in-depth understanding of structure–property relationships, have been rarely explored. Here we report the selective removal and sensing of SO₂ using recently unveiled fluorinated metal–organic frameworks (MOFs). Mixed gas adsorption experiments were performed at low concentrations ranging from 250 p.p.m. to 7% of SO₂. Direct mixed gas column breakthrough and/or column desorption experiments revealed an unprecedented SO₂ affinity for KAUST-7 (NbOFFIVE-1-Ni) and KAUST-8 (AlFFIVE-1-Ni) MOFs. Furthermore, MOF-coated quartz crystal microbalance transducers were used to develop sensors with the ability to detect SO₂ at low concentrations ranging from 25 to 500 p.p.m.

Removal of SO₂ from flue gas is of prime importance to avoid its poisoning of CO₂-seperating agents. Here, the authors demonstrate that two fluorinated metal–organic frameworks selectively remove SO₂ from synthetic flue gas and can sense SO₂ with p.p.m.-level detection using quartz crystal microbalance transducers.

A new approach to enhancing the CO2 capture performance of defective UiO-66 via post-synthetic defect exchange

Zirconium-based metal-organic frameworks (Zr-MOFs) are a subclass of MOFs known for their remarkable stability, especially in the presence of water. This makes them extremely attractive for practical applications, including CO₂ capture from industrial emission sources; however, the CO₂ adsorption capacity of Zr-MOFs is moderate compared to that of the best performing MOFs reported to date. Functionalization of Zr-MOFs with amino groups has been demonstrated to increase their affinity for CO₂. In this work, we assessed the potential of post-synthetic defect exchange (PSDE) as an alternative approach to introduce amino functionalities at missing-cluster defective sites in formic acid modulated UiO-66. Both pyridine-containing (picolinic acid and nicotinic acid) and aniline-containing (3-aminobenzoic acid and anthranilic acid) monocarboxylates were integrated within defective UiO-66 with this method. Non-defective UiO-66 modified with linkers bearing the same amino groups (2,5-pyridinedicarboxylic acid and 2-aminoterephthalic acid) were prepared by classical post-synthetic ligand exchange (PSE), in order to compare the effect of introducing functionalities at defective sites versus installing them on the backbone. PSDE reduces the porosity of defective UiO-66, but improves both the CO₂ uptake and the CO₂/N₂ selectivity, whereas PSE has no effect on the porosity of non-defective UiO-66, improving the CO₂ uptake but leaving selectivity unchanged. Modification of defective UiO-66 with benzoic acid reveals that pore size reduction is the main factor responsible for the observed uptake improvement, whereas the presence of nitrogen atoms in the pores seems to be beneficial for increasing selectivity.

Ordered Porous Poly(ionic liquid) Crystallines: Spacing Confined Ionic Surface Enhancing Selective CO2 Capture and Fixation

Porous poly(ionic liquid)s (PPILs) combine the features of porous materials, polymers, and ionic liquids (ILs) or their derivatives, but they are normally of amorphous structure with disordered pores. Here, we report the facile synthesis of ordered porous poly(ionic liquid) crystallines (OPICs, specialized as a kind of PPIL analogues) with diverse and adjustable framework IL moieties through the Schiff base condensation of IL-derived ionic salts and neutral monomers. Ternary monomer mixtures are employed to artistically control the chemical composition and pore configurations. Compact atomic packing was achieved to give spacing confined ionic surface with strong CO₂ affinity. Through monomer control, OPICs exhibit high CO₂ uptakes with excellent CO₂/N₂(CH₄) selectivities and efficiently implement CO₂ fixation through catalyzing epoxides cycloaddition under down to ambient conditions.

Concurrent Sensing of CO2 and H2O from Air Using Ultramicroporous Fluorinated Metal-Organic Frameworks: Effect of Transduction Mechanism on the Sensing Performance

Conventional materials for gas/vapor sensing are limited to a single probe detection ability for specific analytes. However, materials capable of concurrent detection of two different probes in their respective harmful levels and using two types of sensing modes have yet to be explored. In particular, the concurrent detection of uncomfortable humidity levels and CO₂ concentration (400-5000 ppm) in confined spaces is of extreme importance in a great variety of fields, such as submarine technology, aerospace, mining, and rescue operations. Herein, we report the deliberate construction and performance assessment of extremely sensitive sensors using an interdigitated electrode (IDE)-based capacitor and a quartz crystal microbalance (QCM) as transducing substrates. The unveiled sensors are able to simultaneously detect CO₂ within the 400-5000 ppm range and relative humidity levels below 40 and above 60%, using two fluorinated metal-organic frameworks, namely, NbOFFIVE-1-Ni and AlFFIVE-1-Ni, fabricated as a thin film. Their subtle difference in a structure-adsorption relationship for H₂O and CO₂ was analyzed to unveil the corresponding structure-sensing property relationships using both QCM- and IDE-based sensing modes.

Two ultramicroporous metal-organic frameworks assembled from binuclear secondary building units for highly selective CO2/N2 separation

Two novel metal-organic frameworks [Ni₂(μ₂-Cl)(BTBA)₂·DMF]·Cl·3DMF (JLU-MOF56, BTBA = 3,5-bis(triazol-1-yl)benzoic acid) and [Co₂(μ₂-Cl)(BTBA)₂·DMF]·Cl·3DMF (JLU-MOF57) have been successfully synthesized under solvothermal conditions. Crystallographic analysis indicates that the two compounds with different metal ions are isoreticular and both are constructed from binuclear [M₂(μ₂-Cl)(COO)₂N₄] (M = Co, Ni) and a 3-connected hetero-N,O donor ligand. The overall framework possesses a (3,6)-connected dag topology. Furthermore, both of them feature ultramicroporous channels of 3.5 Å× 3.4 Å, which are suitable for adsorbing smaller carbon dioxide (CO₂) gas molecules but not larger nitrogen (N₂) gas molecules. Therefore, JLU-MOF56 and JLU-MOF57 exhibit good performance for CO₂/N₂ separation, and are promising materials for gas adsorption and purification.

A metal-organic framework with suitable pore size and dual functionalities for highly efficient post-combustion CO2 capture

Capturing carbon dioxide (CO2) from flue gases with porous materials has been considered as a viable alternative technology to replace traditional liquid amine adsorbents. A large number of microporous metal-organic frameworks (MOFs) have been developed as CO2-capturing materials. However, it is challenging to target materials with both extremely high CO2 capture capacity and gas selectivity (socalled trade-off) along with moderate regeneration energy. Herein, we developed a novel porous material, [Cu(dpt)2(SiF6)] n (termed as UTSA-120; dpt = 3,6-di(4-pyridyl)-1,2,4,5-tetrazine), which is isoreticular to the net of SIFSIX-2-Cu-i. This material exhibits simultaneously high CO2 capture capacity (3.56 mmol g-1 at 0.15 bar and 296 K) and CO2/N2 selectivity (~600), both of which are superior to those of SIFSIX-2-Cu-i and most other MOFs reported. Neutron powder diffraction experiments reveal that the exceptional CO2 capture capacity at the low-pressure region and the moderate heat of CO2 adsorption can be attributed to the suitable pore size and dual functionalities (SiF62- and tetrazine) which not only interact with CO2 molecules but also enable the dense packing of CO2 molecules within the framework. Simulated and actual breakthrough experiments demonstrate that UTSA-120a can efficiently capture CO2 gas from the CO2/N2 (15/85, v/v) and CO2/CH4 (50/50) gas mixtures under ambient conditions.

Exploration of porous metal-organic frameworks for gas separation and purification

As a new generation of porous materials, metal-organic frameworks (MOFs, also known as porous coordination polymers) have shown great promise for gas separation and purification because of their unique pore structures and surfaces for their differential recognition of small gas molecules. In this review article, we summarize our ongoing research endeavors to explore and discover microporous MOFs for gas separation and purification. We have developed several approaches to systematically tune the pores and to immobilize functional sites, including (1) the primitive cubic net of interpenetrated microporous MOFs from the self-assembly of the paddle-wheel clusters,M 2 ( C O 2 ) 4 ( M = C u 2 + , Z u 2 + … ) ), with two types of organic dicarboxylic acid and pillar bidentate linkers; (2) microporous mixed-metal-organic frameworks (M0 MOFs) through the metallo-ligands, and (3) microporous MOFs with dual functionalities. Such efforts have enabled us to make some breakthroughs on microporous MOFs for gas separation and purification, as demonstrated in the gas chromatographic separation of hexane isomers, kinetic D2/H2 separation, acetylene/ethylene separation, carbon dioxide capture, C2H2/CO2 and C3H4/C3H6 separation. Our group is one of the first groups who have envisioned the practical promise of microporous MOFs for the industrial gas separation and examined their separation capacities and efficiency using the fixed-bed adsorption and/or breakthrough experiments. Some of the very important and representative examples of these microporous MOFs for diverse gas separation and purification are highlighted in this review.

Uniform and directional growth of centimeter-sized single crystals of cyclodextrin-based metal organic frameworks

Although macroscopically-sized MOF crystals have proven of interest for efficient chromatographic separations, information processing, or optoelectronic devices, growing really large crystals has proven problematic. A growth-and-reseeding method can now produce MOF monocrystals ca. 1 cm³ in volume vs. at most ca. 0.025 cm³ by prior methods.

Enhanced Separation of Butane Isomers via Defect Control in a Fumarate/Zirconium-Based Metal Organic Framework

The discovery of appropriate synthetic reaction conditions for fabricating a stable zirconium-based molecular sieve (Zr-fum-fcu-MOF) with minimal defects and its utilization in the challenging separation of linear paraffins from branched paraffins is reported. The crystallinity and structural defects were modulated and adjusted at the molecular level by controlling the synthetic reaction conditions (i.e., amounts of modulators and ligands). The impact of molecular defects on the separation of n-butane from iso-butane was studied through the preparation, fine characterization, and performance evaluation of Zr-fum-fcu-MOFs with varying degrees of defects. Defect-rich Zr-fum-fcu-MOFs were found to have poor n-butane/iso-butane separation, mainly driven by thermodynamics, while Zr-fum-fcu-MOFs with fewer or minimal defects showed efficient separation, driven mainly by kinetics and full molecular exclusion mechanisms. The impact of intrinsic defects (i.e., missing organic or inorganic blocks) on the associated mechanisms involved in the separation of n-butane/iso-butane was evidenced through single-gas adsorption, mixed-gas column breakthrough experiments, and calorimetric studies. This investigation demonstrates, for the first time, the importance of controlling intrinsic defects to maintain the selective exclusion behavior of hydrocarbon isomers when using molecular sieves.

Transforming atmospheric CO2 into alternative fuels: a metal-free approach under ambient conditions

This work demonstrates the first-ever completely metal-free approach to the capture of CO₂ from air followed by reduction to methoxyborane (which produces methanol on hydrolysis) or sodium formate (which produces formic acid on hydrolysis) under ambient conditions. This was accomplished using an abnormal N-heterocyclic carbene (aNHC)-borane adduct. The intermediate involved in CO₂ capture (aNHC-H, HCOO, B(OH)₃) was structurally characterized by single-crystal X-ray diffraction. Interestingly, the captured CO₂ can be released by heating the intermediate, or by passing this compound through an ion-exchange resin. The capture of CO₂ from air can even proceed in the solid state via the formation of a bicarbonate complex (aNHC-H, HCO₃, B(OH)₃), which was also structurally characterized. A detailed mechanism for this process is proposed based on tandem density functional theory calculations and experiments.

Synergy between Isomorphous Acid and Basic Metal-Organic Frameworks for Anhydrous Proton Conduction of Low-Cost Hybrid Membranes at High Temperatures

Metal-organic frameworks (MOFs) embedded in polymer have showed efficiency in improving proton conduction of hybrid membranes under hydrated conditions. However, anhydrous proton conduction of such hybrid membranes over 100 °C remains great challenge. Here, proton conductive hybrid membranes combined acid group (-SO₃H)- and basic group (-NH₂)-modified isomorphous MOFs, namely UiO-66(SO₃H) (abbreviated as A, the initial of acid) and UiO-66(NH₂) (abbreviated as B, the initial of basic) and a low-cost polymer (chitosan, CS) were prepared. The proton conductivity of the optimum dual MOF-cofilled hybrid membranes (CS/A + B) reached 3.78 × 10^-3 S/cm at 120 °C and under anhydrous conditions, under which each component, that is MOF A, MOF B and CS, and single MOF-filled hybrid membranes (CS/A and CS/B) nearly lost proton conduction without exception, producing unprecedented results of one plus one more greater than two. The synergistic effects among UiO-66(SO₃H), UiO-66(NH₂), and CS on improving conductivity are also observed under hydrated conditions, the highest proton conductivity of CS/A + B reached 5.2 × 10^-2 S/cm, which is 1.86, compared to that of the pure CS membrane at 100 °C and 98% relative humidity. The anhydrous proton conductivity of CS/A + B over 100 °C is one of the highest for MOF-based hybrid membranes. MOFs and hybrid membranes were extensively characterized and the proton conductive mechanism was revealed. The achievements open a new avenue for MOF-based anhydrous proton-conducting membranes and would advance the exploration of future application of these MOFs in fuel cells.