Linkers In Metal-organic Frameworks Research Articles

Construction of a Methoxy-Functionalized Pillar-Layer Metal-Organic framework for low-concentration SO2 uptake and adsorption separation

We report the synthesis and structural characterization of a novel methoxy (–OCH3) −functionalized pillar-layer metal–organic framework (MOF), denoted as DZU-17 (DZU=Dezhou University), which is isoreticular to Co6-MOF-3. DZU-17 is constructed by integrating a –OCH3 modified bipyridine ligand, 1,4-dipyridine-2,5-dimethoxybenzene (BPB-OCH3), into the framework of Co6-MOF-3. Single-crystal X-ray diffraction analysis confirms that the introduction of the –OCH3 groups effectively partitions the spacious channels of Co6-MOF-3 into more confined spaces within DZU-17. This structural modification results in a significant enhancement in SO2 uptake at low −pressure (P=0.1 bar, 112.7 cm3 g−1) and improved separation capacity for SO2/CO2/N2 mixture under ambient conditions. Our findings underscore the potential of functional group engineering on MOF linkers to modulate pore dimensions and chemical environments, thereby enhancing the frameworks’ performance in gas adsorption and separation processes.

Interconversion and functional composites of metal-organic frameworks and hydrogen-bonded organic frameworks.

Metal-organic frameworks (MOFs), an emerging class of highly ordered crystalline porous materials, possess structural tunability, high specific surface area, well-defined pores, and diverse pore environments and morphologies, making them suitable for various potential applications. Moreover, hydrogen-bonded organic frameworks (HOFs), constructed from organic molecules with complementary hydrogen-bonding patterns, are rapidly evolving into a novel category of porous materials due to their facile mild preparation conditions, solution processability, easy regeneration capability, and excellent biocompatibility. These distinctive advantages have garnered significant attention across diverse fields. Considering the inherent binding affinity between MOFs and HOFs along with the fact that many MOF linkers can serve as building blocks for constructing HOFs, their combination holds promise in creating functional materials with enhanced performance. This feature paper provides an introduction to the interconversion between MOFs and HOFs followed by highlighting the emerging applications of MOF-HOF composites. Finally, we briefly discuss the current challenges associated with future perspectives on MOF-HOF composites.

Engineering Frustrated Lewis Pair Active Sites in Porous Organic Scaffolds for Catalytic CO2 Hydrogenation.

Frustrated Lewis pairs (FLPs), featuring reactive combinations of Lewis acids and Lewis bases, have been utilized for myriad metal-free homogeneous catalytic processes. Immobilizing the active Lewis sites to a solid support, especially to porous scaffolds, has shown great potential to ameliorate FLP catalysis by circumventing some of its inherent drawbacks, such as poor product separation and catalyst recyclability. Nevertheless, designing immobilized Lewis pair active sites (LPASs) is challenging due to the requirement of placing the donor and acceptor centers in appropriate geometric arrangements while maintaining the necessary chemical environment to perform catalysis, and clear design rules have not yet been established. In this work, we formulate simple guidelines to build highly active LPASs for direct catalytic hydrogenation of CO2 through a large-scale screening of a diverse library of 25,000 immobilized FLPs. The library is built by introducing boron-containing acidic sites in the vicinity of the existing basic nitrogen sites of the organic linkers of metal-organic frameworks collected in a "top-down" fashion from the CoRE MOF 2019 database. The chemical and geometrical appropriateness of these LPASs for CO2 hydrogenation is determined by evaluating a series of simple descriptors representing the intrinsic strength (acidity and basicity) of the components and their spatial arrangement in the active sites. Analysis of the leading candidates enables the formulation of pragmatic and experimentally relevant design principles which constitute the starting point for further exploration of FLP-based catalysts for the reduction of CO2.

Data-Driven Discovery of Gas-Selective Organic Linkers in Metal-Organic Frameworks for the Separation of Ethylene and Ethane.

Metal-organic frameworks (MOFs) are potential candidates for gas-selective adsorbents for the separation of an ethylene/ethane mixture. To accelerate material discovery, high-throughput computational screening is a viable solution. However, classical force fields, which were widely employed in recent studies of MOF adsorbents, have been criticized for their failure to cover complicated interactions such as those involving π electrons. Herein, we demonstrate that machine learning force fields (MLFFs) trained on quantum-chemical reference data can overcome this difficulty. We have constructed a MLFF to accurately predict the adsorption energies of ethylene and ethane on the organic linkers of MOFs and discovered that the π electrons from both the ethylene molecule and the aromatic rings in the linkers could substantially influence the selectivity for gas adsorption. Four kinds of MOF linkers are identified as having promise for the separation of ethylene and ethane, and our results could also offer a new perspective on the design of MOF building blocks for diverse applications.

Binding of carbon dioxide and acetylene to free carboxylic acid sites in a metal-organic framework.

The functionalisation of organic linkers in metal-organic frameworks (MOFs) to improve gas uptake is well-documented. Although the positive role of free carboxylic acid sites in MOFs for binding gas molecules has been proposed in computational studies, relatively little experimental evidence has been reported in support of this. Primarily this is because of the inherent synthetic difficulty to prepare MOF materials bearing free, accessible -COOH moieties which would normally bind to metal ions within the framework structure. Here, we describe the direct binding of CO2 and C2H2 molecules to the free -COOH sites within the pores of MFM-303(Al). MFM-303(Al) exhibits highly selective adsorption of CO2 and C2H2 with a high selectivity for C2H2 over C2H4. In situ synchrotron X-ray diffraction and inelastic neutron scattering, coupled with modelling, highlight the cooperative interactions of adsorbed CO2 and C2H2 molecules with free -COOH and -OH sites within MFM-303(Al), thus rationalising the observed high selectivity for gas separation.

Exploring the effect of molecular size and framework functionalisation on transport in metal-organic frameworks using pulsed-field gradient nuclear magnetic resonance.

Molecular transport is an important aspect in metal-organic frameworks (MOFs) as it affects many of their applications, such as adsorption/separation, drug delivery and catalysis. Yet probing the fundamental diffusion mechanisms in MOFs is challenging, and the interplay between the MOF's features (such as the pore structure and linker dynamics) and molecular transport remains mostly unexplored. Here, the pulsed-field gradient nuclear magnetic resonance (PFG NMR) technique is used to probe the diffusion of several probe molecules, i.e., water, xylenes and 1,3,5-triisopropylbenzene (TIPB), within the UiO-66 MOF and its derivatives (UiO-66NH2 and UiO-66Br). Exploiting differences in the size of probe molecules we were able to probe the diffusion rate selectively in the different pore environments of the MOFs. In particular, when relatively small molecules, such as water and small hydrocarbons, were used as probes, the PFG NMR log attenuation plots were non-linear with two distinctive diffusion regions, suggesting faster diffusion in the inter-crystalline space and slower diffusion within crystal aggregates, the latter occurring mostly inside the framework of the MOFs. Conversely, experiments with a larger probe molecule, i.e., TIPB, with a kinetic diameter of 0.95 nm, which makes it unable to access the framework windows of the MOF crystals, showed linear PFG NMR log attenuation plots, which indicates diffusion occurring in a single environment, most likely in the inter-crystalline space. Analysis of the apparent tortuosity values of the systems under investigation highlights the role of linker functionalisation in influencing the molecular diffusion of the probe molecules, which affects both intra-molecular interactions and pore accessibility within the MOF crystals. The findings of this work demonstrate that the diffusion behaviour of probe molecules within MOFs is influenced by the pore size, structure, functionalisation of the MOF linker and molecular interactions. Our study contributes to further advance the understanding of mass transport in MOFs by PFG NMR and provides insights that can inform the design and optimisation of MOF-based materials for various applications.

Endogenous metal-ion dynamic nuclear polarization for NMR signal enhancement in metal organic frameworks.

Rational design of metal-organic framework (MOF)-based materials for catalysis, gas capture and storage, requires deep understanding of the host-guest interactions between the MOF and the adsorbed molecules. Solid-State NMR spectroscopy is an established tool for obtaining such structural information, however its low sensitivity limits its application. This limitation can be overcome with dynamic nuclear polarization (DNP) which is based on polarization transfer from unpaired electrons to the nuclei of interest and, as a result, enhancement of the NMR signal. Typically, DNP is achieved by impregnating or wetting the MOF material with a solution of nitroxide biradicals, which prevents or interferes with the study of host-guest interactions. Here we demonstrate how Gd(iii) ions doped into the MOF structure, LaBTB (BTB = 4,4',4''-benzene-1,3,5-triyl-trisbenzoate), can be employed as an efficient polarization agent, yielding up to 30-fold 13C signal enhancement for the MOF linkers, while leaving the pores empty for potential guests. Furthermore, we demonstrate that ethylene glycol, loaded into the MOF as a guest, can also be polarized using our approach. We identify specific challenges in DNP studies of MOFs, associated with residual oxygen trapped within the MOF pores and the dynamics of the framework and its guests, even at cryogenic temperatures. To address these, we describe optimal conditions for carrying out and maximizing the enhancement achieved in DNP-NMR experiments. The approach presented here can be expanded to other porous materials which are currently the state-of-the-art in energy and sustainability research.

Photoinduced Dynamic Ligation in Metal-Organic Frameworks.

Metal organic frameworks (MOFs), a class of porous crystalline materials consisting of metal-based nodes and organic linkers, have emerged as a promising platform for photocatalysis due to their ultrahigh functional surface area, customizable topologies, and tunable energetics. While interesting photochemistry has been reported, the related photoinduced structural dynamics of MOFs remains unclear. The consensus is that the coordination bonds between MOF nodes and linkers are considered static during photoexcitation, while the open-metal sites on the nodes are taken as the key active sites for catalysis. In this work, through a complementary time-resolved visible and infrared (IR) spectroscopic investigation, along with computational studies, we report for the first time light-induced structural bond dissociation (COO-M) and reformation in an iron-oxo framework, MIL-101(Fe). The probed excited state displayed ligand-to-metal charge transfer (LMCT) characteristics and exhibited a ca. 30 μs lifetime. The incredibly long excited-state lifetime led us to probe potential structural rearrangements that facilitated charge separation in MIL-101(Fe). By probing the vibrational fingerprints of the carboxylate linker upon LMCT photoexcitation, we observed the reversible transition of the carboxylate-Fe bond from a bidentate bridging mode to a monodentate mode, indicating the partial dissociation of the carboxylate ligand. Importantly, the bidentate configuration is recovered on the same time scale of the excited state lifetimes as probed via visible transient absorption spectroscopy. The elucidated photoinduced configurational dynamics provides a foundation for an in-depth understanding of MOF-based photocatalytic mechanisms.

Flattening of a Bent Sulfonated MOF Linker: Impact on Structures, Flexibility, Gas Adsorption, CO2/N2 Selectivity, and Proton Conduction.

Rational design of organic building blocks provides opportunities to control and tune various physicochemical properties of metal-organic frameworks (MOFs), including gas handling, proton conduction, and structural flexibility, the latter of which is responsible for new adsorption phenomena and often superior properties compared to rigid porous materials. In this work, we report synthesis, crystal structures, gas adsorption, and proton conduction for a flexible two-dimensional cadmium-based MOF (JUK-13-SO3H-SO2) containing a new sulfonated 4,4'-oxybis(benzoate) linker with a blocking SO2 bridge. This two-dimensional (2D) MOF is compared in detail with a previously reported three-dimensional Cd-MOF (JUK-13-SO3H), based on analogous, but nonflat, SO2-free sulfonated dicarboxylate. The comprehensive structure-property relationships and the detailed comparisons with insights into the networks flexibility are supported by five guest-dependent structures determined by single-crystal X-ray diffraction (XRD), and corroborated by spectroscopy (IR, 1H NMR), powder XRD, and elemental/thermogravimetric analyses, as well as by volumetric adsorption measurements (for N2, CO2, H2O), ideal adsorbed solution theory (IAST), density-functional theory (DFT+D) quantum chemical and grand-canonical Monte Carlo (GCMC) calculations, and electrochemical impedance spectroscopy (EIS) studies. Whereas both dynamic MOFs show moderate proton conductivity values, they exhibit excellent CO2/N2 selectivity related to the capture of CO2 from flue gases (IAST coefficients for 15:85 mixtures are equal to ca. 250 at 1 bar and 298 K). The presence of terminal sulfonate groups in both MOFs, introduced using a unique prechlorosulfonation strategy, is responsible for their hydrophilicity and water-assisted proton transport ability. The dynamic nature of the MOFs results in the appearance of breathing-type adsorption isotherms that exhibit large hysteresis loops (for CO2 and H2O) attributed to strong host-guest interactions. Theoretical modeling provides information about the adsorption mechanism and supports interpretation of experimental CO2 adsorption isotherms.

Metal-organic frameworks for efficient mephedrone detoxification or supervised withdrawal – synthesis, characterisation, and in vivo studies

In this study, we describe the innovative use of metal–organic frameworks (MOFs) as propranolol (PRO) carriers for mephedrone (4-MMC) overdose treatment or supervised withdrawal. The group of model Zr-based MOFs including UiO-66, UiO-67, MOF-808, and NU-1000, and PRO@MOF composites were synthesised and characterised. The PRO release was tested in water and simulated body fluid (SBF) environments. The prepared PRO@MOF differed in the amount and pore sizes, as well as in the surface properties, which affected the kinetics of PRO release. The inhibitory effect of SBF resulted in a more gradual, and controlled PRO release from PRO@MOF composites, further confirming the feasibility of this type of solution in real applications. The sorption mechanism in prepared PRO@MOF composites was studied by molecular dynamics (MD) and density functional theory (DFT) calculations. The molecular calculations (ring orientation, partial charges and bond orders) confirmed π–π stacking as the main contribution to the interaction between the electronic structures of PRO molecule and MOFs linker. The in vivo studies demonstrated the lack of cytotoxicity of MOFs and composites, and pre-evidence confirmed the effectiveness of using PRO-loaded MOFs as drug carriers in a 4-MMC overdose. The study provides a new potential detoxification system for addicts.

Imaging the dynamic influence of functional groups on metal-organic frameworks

Metal-organic frameworks (MOFs) with different functional groups have wide applications, while the understanding of functionalization influences remains insufficient. Previous researches focused on the static changes in electronic structure or chemical environment, while it is unclear in the aspect of dynamic influence, especially in the direct imaging of dynamic changes after functionalization. Here we use integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM) to directly ‘see’ the rotation properties of benzene rings in the linkers of UiO-66, and observe the high correlation between local rigidity and the functional groups on the organic linkers. The rigidity is then correlated to the macroscopic properties of CO2 uptake, indicating that functionalization can change the capability through not only static electronic effects, but also dynamic rotation properties. To the best of our knowledge this is the first example of a technique to directly image the rotation properties of linkers in MOFs, which provides an approach to study the local flexibility and paves the way for potential applications in capturing, separation and molecular machine.

Tunable Photocatalytic Singlet Oxygen Generation by Metal-Organic Frameworks via Functionalization of Pyrene-Containing Linkers.

Metal-organic frameworks (MOFs) are highly versatile materials that have shown great promise in chemical warfare agent (CWA) adsorption and decontamination. Sulfur mustard has been one of the most prominently used CWAs over the last century; therefore, the development of effective detoxification strategies is of utmost importance. However, typical routes of detoxification are slow and/or result in the production of harmful byproducts. NU-1000 has previously shown promise as a "soft" oxidizer that can readily detoxify sulfur mustard and its simulant 2-chloroethyl ethyl sulfide (2-CEES) through the generation of singlet oxygen in the presence of either UV (396 nm) or blue (465 nm) light. Several variants of NU-1000 were synthesized (MOF-R, R = -Cl, -NO2, -CH3) with functional groups positioned either ortho or meta to the carboxylic acid on the linker. NU-1000-o-(Cl)4 and NU-1000-m-(Cl)4 showed significant enhancement of photooxidation of 2-CEES due to spin-orbit coupling, enhancing the intersystem crossing into the MOF triplet (T1) state. Furthermore, substitution of MOF linkers led to pyrene-phenyl rotation. Linkers with substituents in the ortho-position were shown to have smaller pyrene-phenyl torsion angles, leading to enhanced conjugation between the rings and a subsequent red shift in the absorption spectra. This red shift leads to enhanced reactivity of NU-1000-o-(Cl)4 under blue light conditions and gives perspective on making materials with enhanced reactivity utilizing visible light.

MOF Linker Extension Strategy for Enhanced Atmospheric Water Harvesting.

A linker extension strategy for generating metal-organic frameworks (MOFs) with superior moisture-capturing properties is presented. Applying this design approach involving experiment and computation results in MOF-LA2-1 {[Al(OH)(PZVDC)], where PZVDC2- is (E)-5-(2-carboxylatovinyl)-1H-pyrazole-3-carboxylate}, which exhibits an approximately 50% water capacity increase compared to the state-of-the-art water-harvesting material MOF-303. The power of this approach is the increase in pore volume while retaining the ability of the MOF to harvest water in arid environments under long-term uptake and release cycling, as well as affording a reduction in regeneration heat and temperature. Density functional theory calculations and Monte Carlo simulations give detailed insight pertaining to framework structure, water interactions within its pores, and the resulting water sorption isotherm.

Structural Compromise Between Conflicted Spatial-Arrangements of Two Linkers in Metal-Organic Frameworks.

The structural control of metal-organic frameworks (MOFs) is essential for the development of superlative MOFs because the structural features of MOFs and their components play a critical role in determining their properties, and ultimately, their applications. The best components to endow the desired properties for MOFs are available via the appropriate choice from many existing chemicals or synthesizing new ones. However, to date, considerably less information exists regarding fine-tuning the MOF structures. Herein, a strategy for tuning MOF structures by merging two MOF structures into a single MOF, is demonstrated. Depending on the incorporated amounts and relative contributions of the two coexisting organic linkers, benzene-1,4-dicarboxylate (BDC2- ) and naphthalene-1,4-dicarboxylate (NDC2- ), which have conflicting spatial-arrangement preferences within an MOF structure, MOFs are rationally designed to have a Kagomé or rhombic lattice. In particular, MOFs with rhombic lattices are constructed to have specific lattice angles by compromising the optimal structural arrangements between the two mixed linkers. The relative contributions of the two linkers during MOF construction determine the final MOF structures, and the competitive influence between BDC2- and NDC2- is effectively regulated to produce specific MOF structures with controlled lattices.

Reactive Chlorine Capture by Dichlorination of Alkene Linkers in Metal-Organic Frameworks.

Chlorine (Cl2) is a toxic and corrosive gas that is both an essential reagent in industry and a potent chemical warfare agent. Materials that can strongly bind Cl2 at low pressures are essential for industrial and civilian personal protective equipment (PPE). Herein, we report the first examples of irreversible Cl2 capture via the dichlorination of alkene linkages in Zr-based metal-organic frameworks. Frameworks constructed from fumarate (Zr-fum) and stilbene (Zr-stilbene) linkers retain long-range order and accessible porosity after alkene dichlorination. In addition, energy-dispersive X-ray spectroscopy reveals an even distribution of Cl throughout both materials after Cl2 capture. Cl2 uptake experiments reveal high irreversible uptake of Cl2 (>10 wt %) at low partial pressures (<100 mbar), particularly in Zr-fum. In contrast, traditional porous carbons mostly display reversible Cl2 capture, representing a continued risk to users after exposure. Overall, our results support that alkene dichlorination represents a new pathway for reactive Cl2 capture, opening new opportunities for binding this gas irreversibly in PPE.

Reticular chemistry for the rational design of mechanically robust mesoporous merged-net metal-organic frameworks

Reticular chemistry for the rational design of mechanically robust mesoporous merged-net metal-organic frameworks

Light Harvesting Antenna Properties of Framework Solids

ConspectusEnergy transport within organic solids typically takes place via the movement of localized excitation or excitons, i.e., tightly bound electron–hole pairs. Exciton diffusion dynamics are crucial to applications like solar fuels production through photocatalysis and organic optoelectronics. Covalent organic frameworks (COFs) and metal–organic frameworks (MOFs) are classes of framework solids that integrate ordered assemblies of chromophores. These materials possess properties desirable for the aforementioned applications. Their absorption cross sections across the ultraviolet–visible spectrum can be tuned via linker design. They are often crystalline and provide control over the framework topology in terms of spatial separation and relative orientation of chromophores. These properties allow for detailed understanding and tuning of structure–activity relationships.In this Account, we highlight our recent studies on photophysics and photonics of MOFs and COFs. Three materials are discussed with the focus on structural factors that govern the exciton properties in terms of diffusion, delocalization, and transport. The first is COF-5, a layered, two-dimensional COF with boronate ester linkages. We show that the molecular excitons in COF-5 are delocalized over multiple constituent building blocks with optical signatures of mixed (e.g., H- and J-type) excitonic interactions. The exciton of the S1 state decays and evolves into an excimer trap state in a few picoseconds. The excitons in COF-5 possess a diffusivity of about 0.02 cm2/s and a diffusion radius of about 5 nm based on exciton–exciton annihilation studies. To investigate the range of exciton diffusion, parallel exciton dynamics measurements were performed on COF-5 colloids with varying crystallite size, revealing that exciton diffusion is limited by crystallite size. The second system studied is an imine-linked two-dimensional COF with a hexagonal structure, the TAPB-DiOMe COF. To understand the photophysics of this material, we extended the study toward isomeric materials with a lower dimensionality: cofacially stacked nanotubes and free macrocycles assembled from the same primary building blocks. In-plane exciton delocalization and movement cannot occur in these lower dimensional species. We find that the exciton lifetime is prolonged significantly in the π-stacked nanotubes where the macrocycles undergo planarization that restricts atomic movements of the imine bonds in the peripheral groups, which may reduce the rate of internal conversion. In addition, orientation-dependent anisotropic energy transfer through the nanotubes was detected via optical transient absorption anisotropy. The third system discussed is the MOF NDC-NU-1000 decorated with single-atom metal-oxy (i.e., mix of oxo, hydroxo, and/or aqua) TiIV, CoIII, and NiII species mimicking single-atom catalysts (SACs). These composite materials display photoinduced charge separation. We show the formation of charge-separated states in these M-oxy decorated MOF samples upon photoexcitation with an electron transferred from the MOF linker to the M-oxy moiety. We compare and rationalize the differences in photoinduced charge separation kinetics among these M-oxy decorated MOFs and hope to identify energetic and structural factors that make the material an effective light harvesting platform and facilitate formation of the charge-separated state. Finally, we review our progress in context of the research published previously in the field and provide our perspective on the future of crystalline porous assemblies serving as light-harvesting antennae.

Data-mining based assembly of promising metal-organic frameworks on Xe/Kr separation

The adsorptive separation of xenon (Xe) and krypton (Kr) becomes increasingly important for the treatment of used nuclear fuel (UNF), and thus the novel high-performing metal–organic frameworks (MOFs) on Xe/Kr adsorption separation are urgently needed. In this work, the 200 MOFs formally used for ethane/ethylene separation were adapted to construct structure-adsorption property relationships (SAPR) for Xe/Kr mixture (20/80 v/v) at 298 K and 1 bar, to screen for MOFs with large Xe/Kr selectivities and Xe uptakes in the CoRE MOF, G-MOFs and hMOFs databases with more than 320,000 structures. Then based on the screened 1499 MOFs, the important metal nodes and organic linkers of MOFs (genes) governing the Xe uptake were identified by data-mining of feature engineering, which were assembled crossly into three novel promising MOFs according to material genomics strategy. After considering of the regenerabilities, it is found that Xe uptake (4.2857 mmol/g) and Xe/Kr selectivity (19.70) of the assembled Al2O6-fum_B-hmof8_No1 are larger than those of the most of experimentally synthesized frameworks including Al-Fum-Me, overcoming the “trade-off” problem between adsorption selectivity and capacity. From the multiscale calculations at GCMC and DFT levels, it is found that the large 1D pore size that can accommodate a double-atom chain and large electrostatic potential gradient (EPG) should be responsible for the high Xe uptake and Xe/Kr selectivity of Al2O6-fum_B-hmof8_No1, respectively. Note that the present work first report double-atom chain of rare gas adsorbed in MOF materials. The present data mining and cross assembly strategies are expected to assist the discovery of novel high-performing MOF absorbents for the separation of Xe-Kr even light hydrocarbon in the future.

Mechanism of Methanol Dehydration Catalyzed by Al8O12 Nodes Assisted by Linker Amine Groups of the Metal–Organic Framework CAU-1

Elucidation of catalytic sites and mechanisms of reactions on metal oxides is hindered by the complexity and heterogeneity of the surfaces. In contrast, the metal oxide cluster nodes of metal–organic frameworks (MOFs) have well-defined, essentially molecular structures that provide excellent platforms for understanding catalytic reactions in depth. We report an experimental and density functional theory (DFT) investigation of methanol dehydration on the Al8O12 nodes of the MOF CAU-1, presenting catalyst performance data obtained with vapor-phase methanol contacting MOF particles in a flow reactor at temperatures in the range of 180–250 °C. These data and infrared spectra of the MOF characterizing its reactivity with methanol and water show that the reactivities of node μ2-OH groups, influenced by the coordination environments of the node aluminum atoms, react with methanol to form catalytic sites incorporating methoxy ligands that bridge paired aluminum atoms, with the dehydration mechanism engaging nearby MOF linker amine groups that bond to methanol. DFT investigations of the mechanism suggest that the SN2-type methanol dehydration reaction is energetically favored to occur at MOF linker amine groups, which allow for a near-linear transition state (TS) geometry; in contrast, the central cavity of the Al8O12 node adsorbs methanol strongly but, because of geometric constraints, renders the TS for methanol dehydration energetically unfavorable.

Separation of Aromatic Hydrocarbons in Porous Materials.

Industrial-scale thermal separation processes have contributed greatly to the rise in carbon dioxide emissions. Porous materials, such as metal-organic frameworks (MOFs), can potentially reduce these emissions by achieving nonthermal chemical separations through the physical adsorption of targeted species with high selectivity. Here, we report the synthesis of the channel-based MOFs NU-2000 and NU-2001, which are constructed from three-dimensional (3D) linkers, to separate the industrially relevant xylene isomers under ambient conditions by leveraging sub-Ångstrom differences in the sizes of each isomer. While the rotation of two-dimensional (2D) linkers in MOFs often affords changes in pore apertures and pore sizes that are substantial enough to hinder separation efficiency, increasing the linker dimensionality from 2D to three-dimensional (3D) enables precise control of the MOF pore size and aperture regardless of the linker orientation, establishing this design principle as a broadly applicable strategy.