Metal–organic frameworks (MOFs) are novel porous materials with application potential in multiple fields including gas storage and catalysis. However, most reported MOFs feature micropores smaller than 2 nm, and such a limited pore size will hinder mass transfer, a necessary process in industry. It is imperative to introduce hierarchical pores into MOFs, which can facilitate mass transfer due to mesoporosity while preserving the advantage of microporosity. Although nowadays lots of methods have been developed to generate hierarchically porous MOFs, it is still challenging to maintain the crystallinity and integrity of MOFs. Herein, controlled photolytic removal of linkers was developed to create mesopores within microporous MOFs at tens of milliseconds. Initially, a multicomponent MOF was synthesized based on robust and photolabile linkers. The ultraviolet laser has been applied to eliminate photolabile linkers to induce vacancies within the framework. Presumably, the generation of mesopores can be attributed to the missing-cluster defects, whose sizes can be controlled by varying the time of laser exposure and the ratio of robust/photolabile linkers. In addition, with assistance of computer programming, laser writing can also be utilized to fabricate complicated patterns with a high resolution at around 40 micrometers, paving the way towards MOF lithography, which has enormous potential in developing MOF-based devices for sensing and catalysis.

Understanding the factors that affectself-diffusion in isoreticular and multivariate (MTV) MOFs is key to their application indrug delivery, separations, and heterogeneous catalysis. Here, we measure the apparent self-diffusion of solvents saturated within the pores of large single crystals of MOF-5, IRMOF-3 (amino-functionalized MOF-5), and 17 MTV-MOF-5/IRMOF-3 materials at various mole fractions. We find that the apparent self-diffusion coefficientof N,N-dimethylformamide (DMF) may be tuned linearlybetween the diffusion coefficients of MOF-5 and IRMOF-3 as a function of the linkermole fraction. We compare a series of solvents at saturation in MOF-5 and IRMOF-3 to elucidatethe mechanism by which the linker amino groups tune molecular diffusion. The ratio of the self-diffusion coefficients for solventsin MOF-5 to those in IRMOF-3 is similar across all solvents tested, regardless of solvent polarity. We conclude that average pore aperture, not solvent-linker chemical interactions, is the primary factor responsible for the different diffusion dynamics upon introduction of anamino group to the linker.

Tapping into abundant, non-conventional water resources –such as seawater and municipal wastewater– will be critical for meeting rapidly increasing worldwide water needs. The capture of naturally occurring boric acid is of particular interest, as this contaminant is difficult to remove from water using traditional reverse osmosis membranes due to its neutral charge, causes detrimental health effects when at excess concentrations, and is ubiquitous in seawater (typically 4-6 ppm) and wastewater. Here, we report two new adsorbents, PAF-1-NMDG and P2-NMDG, in which boron-selective N-methyl-D-glucamine (NMDG) chelators are appended to the pore walls of high-surface-area porous aromatic frameworks (PAFs). Owing to their high porosities and NMDG loadings, these boron-selective PAFs demonstrate adsorption capacities up to 70% higher than those of commercial boron-selective resins and the fastest boron adsorption kinetic rate constants of any material reported to date. Remarkably, PAF-1-NMDG can also reduce the boron concentration in synthetic seawater from 2.91 to <0.5 ppm in less than 3 min. The frameworks can additionally be regenerated via mild acid/base treatment without loss in boron adsorption capacity over at least 10 regeneration cycles. These results highlight the numerous advantages of PAFs over traditional porous polymers in water purification applications.

Vapor phase ligand treatment (VPLT) of 2-aminobenzimidazole (2abIm) for 2-methylimidazole (2mIm) in ZIF-8 membranes prepared by two different methods (LIPS: ligand induced permselectivation and RTD: rapid thermal deposition) results in a notable shift of the molecular level cut-off to smaller molecules establishing selectivity improvements from ca. 1.8 to 5 for O₂/N₂; 2.2 to 32 for CO₂/CH₄; 2.4 to 24 for CO₂/N₂; 4.8 to 140 for H₂/CH₄ and 5.2 to 126 for H₂/N₂. Stable (based on a one-week test) oxygen-selective air separation performance at ambient temperature, 7 bar(a) feed, and 1 bar-(a) sweep-free permeate with a mixture separation factor of 4.5 and oxygen flux of 2.6 × 10^-3 mol/m²s¹ is established. LIPS and RTD membranes exhibit fast and gradual evolution upon a 2abIm-VPLT, respectively, reflecting differences in their thickness and microstructure. Functional reversibility is demonstrated by showing that the original permeation properties of the VPLT-LIPS membranes can be recovered upon 2mIm-VPLT.

The idea that complex systems have a hierarchical arrangement has been widely observed on various scales. We introduce the concept of modular programming, which emphasizes isolating the functionality of a system into independent modules, to tailor the hierarchy and diversity in these complex systems. Guided by modular programming, a system with multiple compatible components, including modules A, B, C, and so forth, can be constructed and subsequently modified into modules A′, B′, C′, and so forth independently. As a proof of concept, a series of multivariate hierarchical metal–organic frameworks (MOFs) with various compositions, ratios, and distributions were prepared as a compatible system. Sequential click reactions and acid treatments can be utilized to selectively modify a certain modular MOF into a polymer, while other modular MOFs either remain in their original state or dissolve upon treatment. As a result, a series of polymer/MOF composites that traditionally have been viewed as incompatible can be prepared with tailored properties and behaviors. This general synthesis approach guided by modular programming enables the investigation into hierarchy and its structured control flow, which is a critical design feature of future materials for their fast adaptivity and responses to variable environmental conditions.

The widespread deployment of carbon capture and sequestration as a climate change mitigation strategy could be facilitated by the development of more energy-efficient adsorbents. Diamine-appended metal–organic frameworks of the type diamine–M₂(dobpdc) (M = Mg, Mn, Fe, Co, Ni, Zn; dobpdc^4- = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) have shown promise for carbon capture applications, although questions remain regarding the molecular mechanisms of CO₂ uptake in these materials. Here, we leverage the crystallinity and tunability of this class of frameworks to perform a comprehensive study of CO₂ chemisorption. Using multinuclear nuclear magnetic resonance (NMR) spectroscopy experiments and calculations for thirteen diamine–M₂(dobpdc) variants, we demonstrate that the canonical CO₂ chemisorption products—ammonium carbamate chains and carbamic acid pairs—can be readily distinguished, and that ammonium carbamate chain formation dominates for diamine–Mg₂(dobpdc) materials. In addition, we elucidate a new chemisorption mechanism in the material dmpn–Mg₂(dobpdc) (dmpn = 2,2-dimethyl-1,3-diaminopropane), which involves formation of a 1:1 mixture of ammonium carbamate and carbamic acid and accounts for the unusual adsorption properties of this material. Finally, we show that the presence of water plays an important role in directing the mechanisms for CO₂ uptake in diamine–M₂(dobpdc) materials. Overall, our combined NMR and calculations approach gives a thorough understanding of CO₂ adsorption within diamine–M₂(dobpdc) compounds, which may aid similar studies in other amine-functionalized adsorbents in the future.

Over one million tons of CS₂ are produced annually, and emissions of this volatile and toxic liquid, known to generate acid rain, remain poorly controlled. As such, materials capable of reversibly capturing this commodity chemical in an energy-efficient manner are of interest. Recently, we reported diamine-appended metal-organic frameworks capable of selectively capturing CO₂ via a cooperative insertion mechanism that promotes efficient adsorption–desorption cycling. We therefore sought to explore the ability of these materials to capture CS₂ through a similar mechanism. Employing crystallography, spectroscopy, and gas adsorption analysis, we demonstrate that CS₂ is indeed cooperatively adsorbed in N,N-dimethylethylenediamine-appended framework M₂(dobpdc) (M = Mg, Mn, Zn; dobpdc⁴⁻ = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate), via the formation of electrostatically paired ammonium dithiocarbamate chains. In the weakly thiophilic Mg congener, chemisorption is cleanly reversible with mild thermal input. This work demonstrates that the cooperative insertion mechanism can be generalized to other high-impact target molecules.

Cooperative (sharp) adsorption of gases by porous frameworks, which permits more efficient uptake and removal than the more usual non-cooperative (Langmuir-type) adsorption, usually results from a phase transition of the framework. We show how cooperativity emerges in the class of metal-organic frameworks mmen-M₂(dobpdc) (mmen = N,N′-dimethylethylenediamine; dobpdc⁴⁻ = 4,4′-dioxido-3,3′-biphenyldicarboxylate; M = Mg, Mn, Fe, Co, Zn, and Ni) in the absence of a phase transition. Our study provides a microscopic understanding of the emergent features of cooperative binding, including the position, slope, and height of the isotherm step, and indicates how to optimize gas storage and separation in these materials.

Zeolitic imidazolate framework (ZIF) membranes are a promising energy-efficient separation technology. However, the existing fabrication methods of ZIF membranes rely on solvothermal synthesis that is difficult to scale up. The CGS team developed a novel all-vapor-phase processing method based on atomic layer deposition (ALD) and ligand-vapor treatment that holds promise for reliable and scalable manufacturing of high-performance molecular sieving membranes. This method starts with ALD, a well-established materials processing technology, to deposit ZnO inside a porous alumina support using diethyl zinc + water. After ALD, an impermeable and nonselective ZnO-alumina composite is obtained. The ALD treated membrane is then subjected to a treatment of vapors of ligands that can react with the oxide and transform it to ZIF. After ligand-vapor treatment (2-methylimidazole at 125 °C for 24 h), the ALD-deposited ZnO is partially converted to ZIF, resulting in a ZIF nanocomposite membrane that shows stable performance with high propylene flux and high propylene/propane selectivity. Because of the transformation from an impermeable to selective membrane, this process is named ligand-induced permselectivation (LIPS). The LIPS approach is based on solvent-free, seed-free, all-vapor processing with ALD, and is shown to be simple and highly reproducible. It establishes a robust approach for the fabrication of ZIF and possibly many other MOF membranes and nanocomposites.

Metal-organic frameworks (MOFs) that respond to external stimuli such as guest molecules, temperature, or redox conditions are highly desirable for their potential applications as smart absorbents for gas separation and storage. A representative example of framework flexibility is the breathing effect in which the framework experiences a reversible unit-cell dimensional change resulting from guest adsorption or desorption. We demonstrated that the flexibility and the associated breathing behavior of MOFs can be controlled by redox chemistry. Guided by topology, two flexible isomeric MOFs with a formula of In(Me₂NH₂)(TTFTB) were constructed via a combination of [In(COO)₄]^- metal nodes and tetratopic tetrathiafulvalene-based linkers (TTFTB). The breathing behaviors of two compounds upon N₂ sorption were studied by single-crystal X-ray diffractions and molecular simulations. More importantly, the rigidity of TTF-based linkers can be switched by reversible redox reaction, which in turn controls the flexibility of MOFs. The redox-controlled dynamic behavior of MOFs is reminiscent of sophisticated biological behavior such as redox regulation of enzymes. We believe that the redox-switchable flexible MOFs can potentially be applied to the design of smart absorbents with higher storage capacities and efficient gas release.

The separation of alkylaromatics o-xylene, m-xylene, p-xylene, and ethylbenzene is of enormous importance industrially, but very technically challenging due to the different isomers' similar physical properties. Metal–organic frameworks are promising as potential selective adsorbents, but separation of larger hydrocarbon mixtures remains a challenge. Here, we demonstrate the ability of Co₂(dobdc) (dobdc^4– = 2,5-dioxide-1,4-benzenedicarboxylate) and Co₂(m-dobdc) (m-dobdc^4– = 4,6-dioxido-1,3-benzenedicarboxylate) to differentiate these different isomers, with Co₂(dobdc) preferentially binding o-xylene > ethylebenzene > m-xylene > p-xylene. This was shown through single component vapor adsorption isotherms, multi-component vapor breakthrough measurements, and multi-component solution phase adsorption measurements. Structural analysis through single crystal and powder X-ray diffraction showed that these materials perform these separations through multiple metal site interactions, in which adjacent Co(II) centers bind to the same xylene molecule. Due to the different shapes of these isomers, the degree to which each xylene molecule can effectively bind to both Co(II) sites dictates the strength of interaction, providing an effective basis for separation. Finally, previously unobserved pore flexing in the hexagonal channels of Co₂(dobdc) allows for greater xylene uptake capacities.

Metal–organic frameworks are promising materials for energy-efficient gas separations, but little is known about the diffusion of adsorbates in materials featuring one-dimensional porosity at the nanoscale. An understanding of the interplay between framework structure and gas diffusion is important for the practical application of these materials as adsorbents or in mixed-matrix membranes. Here, we investigated the diffusion of CO₂ within the pores of Zn₂(dobpdc) (dobpdc^4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) using pulsed field gradient (PFG) nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations. Importantly, our PFG NMR method allowed measurement of self-diffusion in different crystallographic directions. In addition to observing CO₂ diffusion through the channels parallel to the crystallographic c axis we unexpectedly found that CO₂ is also able to diffuse between the hexagonal channels in the crystallographic ab plane, despite the walls of these channels appearing impermeable by single-crystal X-ray crystallography and flexible lattice MD simulations. Observation of such unexpected diffusion in the ab plane suggests the presence of defects that enable effective multidimensional CO₂ transport in a metal–organic framework with nominally one-dimensional porosity.

Metal-organic framework (MOF)-based membranes have gained increasing attention for their potential utility in the separation of CO₂ from natural gas and flue gas streams. Mixed matrix membranes (MMMs), which consist of MOF particles imbedded in a polymer matrix, are particularly appealing from a manufacturing standpoint. Compared to isotropic MOF crystals, the use of high aspect-ratio MOF nanosheets may enhance the separation performance of MMMs, although it has been a challenge to develop simple and scalable methods for nanosheet synthesis. In this work, we describe the facile preparation of Cu(BDC) (BDC^2– = 1,4-benzenedicarboxylate) nanosheets with aspect ratios as high as 100 via direct synthesis from a well-mixed solution of metal and ligand precursors. Notably, incorporation of the Cu(BDC) nanosheets into a Matrimid polymer matrix results in a MMM with a 70% increase in CO₂/CH₄ selectivity compared to neat Matrimid. Our approach is suitable for large-scale synthesis and can also be extended to the preparation of other varieties of MOF nanosheets. Analysis of gas permeation data for Cu(BDC) MMMs using a mathematical model also indicates that additional membrane performance improvements may be achieved by varying the choice of polymer used in the continuous phase.

How do molecules move when confined within small spaces? We placed three molecules (ortho-, meta-, and para-xylene) inside of a special material (metal-organic framework IRMOF-1) that has small pores, about the twice the size of these molecules. NMR spectroscopy revealed differences in how fast the molecules diffuse, as well as how the molecules rotate when confined to the MOF. Diffusion measurements come from a method known as NMR pulsed-field gradient diffusion and compare quite well with computer simulations of diffusion using sophisticated force fields created by quantum chemistry. The most cylindrically-shaped molecule, para-xylene, diffuses the fastest of the three because it has the weakest interactions with the MOF walls, as revealed by computer simulations. The rotation of these molecules was discerned via NMR relaxation times and, when combined with computer simulated probability maps, show that these molecules rotate like a Frisbee, and not like a flipping pancake, while in the MOF. Interestingly, the para-xylene molecules diffuse the fastest, yet have the highest energy barrier to rotate because these molecules don’t fit well in the MOF pores.