Amides Do Not Always Work: Observation of Guest Binding in an Amide-Functionalized Porous Metal-Organic Framework

Analysis and Refinement of Host-Guest Interactions in Metal-Organic Frameworks

ConspectusMetal-organic frameworks (MOFs) are a class of hybrid porous materials characterized by their periodic assembly using metal ions and organic ligands through coordination bonds. Their high crystallinity, extensive surface area, and adjustable pore sizes make them promising candidates for a wide array of applications. These include gas adsorption and separation, substrate binding, and catalysis, of relevance to tackling pressing global issues such as climate change, energy challenges, and pollution. In comparison to traditional porous materials such as zeolites and activated carbons, the design flexibility of organic ligands in MOFs, coupled with their orderly arrangement with associated metal centers, allows for the precise engineering of uniform pore environments. This unique feature enables a rich variety of interactions between the MOF host and adsorbed gas molecules, which are fundamental to understanding the observed uptake capacity and selectivity for target gas molecules and thus the overall performance of the material.In this Account, a data set for three-dimensional MOFs has been constructed based upon the structural analysis of host-guest interactions using the largest experimental database, the Cambridge Structural Database (CSD). A full screening was performed on structures with guest molecules of H2, C2H2, CO2, and SO2, and the relationship between the primary binding site, the isosteric heats of adsorption (Qst), and the adsorption uptake was extracted and established. We review the methodologies to refine host-guest interactions based primarily on our studies on the host-guest chemistry of MOFs. The methods include ligand functionalization, variation of metal centers, formation of defects, addition of single atom sites, and control of pore size and structure. In situ structural and dynamic investigations using diffraction and spectroscopic techniques are powerful tools to visualize the details of host-guest interactions upon the above modifications, affording key insights into functional performance at a molecular level. Finally, we give an outlook of future research priorities in the study of host-guest chemistry in MOF materials. We hope this Account will encourage the rational development and improvement of future MOF-based sorbents for applications in challenging gas adsorption, separations, and catalysis.

Correlation between the Metal and Organic Components, Structure Property, and Gas-Adsorption Capacity of Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are materials in which metals and organic compounds form crystalline and porous structures. Previous studies have investigated the relationships between the structure properties and physical properties of MOFs through molecular simulations, but the overall relationships in MOFs, including the relationships between the metals and organic components and the experimentally measured physical properties, have not been clarified. In this study, we developed two regression models between three elements in MOFs: the components, structure properties, and gas-adsorption capacities as physical properties. Using a nonlinear regression analysis method, we succeeded in predicting the structure properties from the components and the physical properties from the structure properties.

MOFs in the time domain

Many of the proposed applications of metal-organic framework (MOF) materials may fail to materialize if the community does not fully address the difficult fundamental work needed to map out the 'time gap' in the literature - that is, the lack of investigation into the time-dependent behaviours of MOFs as opposed to equilibrium or steady-state properties. Although there are a range of excellent investigations into MOF dynamics and time-dependent phenomena, these works represent only a tiny fraction of the vast number of MOF studies. This Review provides an overview of current research into the temporal evolution of MOF structures and properties by analysing the time-resolved experimental techniques that can be used to monitor such behaviours. We focus on innovative techniques, while also discussing older methods often used in other chemical systems. Four areas are examined: MOF formation, guest motion, electron motion and framework motion. In each area, we highlight the disparity between the relatively small amount of (published) research on key time-dependent phenomena and the enormous scope for acquiring the wider and deeper understanding that is essential for the future of the field.

Deciphering the Weak CO2···Framework Interactions in Microporous MOFs Functionalized with Strong Adsorption Sites-A Ubiquitous Observation

Carbon capture from industrial effluents such as flue gas or natural gas mixture (cf. landfill gas), the primary sources of CO₂ emission, greatly aids in balancing the environmental carbon cycle. In this context, the most energy-efficient physisorptive CO₂ separation process can benefit immensely from improved porous sorbents. Metal organic frameworks (MOFs), especially the ultramicroporous MOFs, built from readily available small and rigid ligands, are highly promising because of their high selectivity (CO₂/N₂) and easy scalability. Here, we report two new ultramicroporous Co-adeninato isophthalate MOFs. They concomitantly carry basic functional groups (-NH₂) and Lewis acidic sites (coordinatively unsaturated Co centers). They show good CO₂ capacity (3.3 mmol/g at 303 K and 1 bar) along with high CO₂/N₂ (∼600 at 313 K and 1 bar and ∼340 at 303 K and 1 bar) selectivity, working capacity, and smooth diffusion kinetics (D_c = 7.5 × 10^-9 m² s^-1). The MOFs exhibit good CO₂/N₂ kinetic separation under both dry and wet conditions with a smooth breakthrough profile. Despite their well-defined CO₂ adsorption sites, these MOFs exhibit only a moderately strong interaction with CO₂ as evidenced from their HOA values. This counterintuitive observation is ubiquitous among many MOFs adorned with strong CO₂ adsorption sites. To gain insights, we have identified the binding sites for CO₂ using simulation and MD studies. The radial distribution function analysis reveals that despite the amine and bare-metal sites, the pore size and the pore structure determine the positions for the CO₂ molecules. The most favorable sites become the confined spaces lined by aromatic rings. A plausible explanation for the lack of strong adsorption in these MOFs is premised from these collective studies, which could aid in the future design of superior CO₂ sorbents.

Metal-Organic Framework Materials for Light Hydrocarbon Separation

In practical industrial applications, the separation of light hydrocarbon mixtures is a very important technology. In recent years, some progress has been made in metal-organic framework materials for light hydrocarbon separation, but further research is still needed. This Minireivew presents a systematic discussion on the latest developments and separation mechanisms of metal-organic framework materials for C₂ and C₃ mixtures, discusses the problems faced by metal-organic framework materials in the study of light hydrocarbon adsorption and separation, and provides a reference for the design, preparation and process development of low-carbon hydrocarbon adsorption and separation materials in the future.

A mechanistic insight into the rapid and selective removal of Congo Red by an amide functionalised Zn(ii) coordination polymer

New CPs [Zn(μ-1κOO':2κN-L)(H₂O)(BDC)_0.5]_n·n(DMF) (1), [Cd(μ-1κO:2κN-L)₂(H₂O)₂]_n (2), and [Pb(μ-1κOO':2κO'-L)(μ-1κO:2κO':3κN-L)]_n (3) [L = 4-(pyridin-3-ylcarbamoyl)benzoate; BDC = benzene-1,4-dicarboxylate] were synthesized and characterized by elemental, FT-IR, powder, and single-crystal X-ray diffraction analyses. Single crystal X-ray diffraction analysis discloses 1D polymeric architectures for 1 and 2 and a 2D one for 3. The topological analysis exemplifies a 2,2,3-connected 3-nodal net with the point symbol {8²·12}₂{8}₃ for 1, a 2,4-connected bimodal net for 2, and a 3,4,7-connected trinodal net for 3. CP 1 shows a selective removal of the Congo Red (CR) dye amongst various dyes. It can be recycled and reused without any significant loss of its dye removal efficiency. An insight into the selective removal of the Congo dye is provided by in silico studies, being accounted for by anion-π, cation-π, and π-π stacking interactions, involving the Zn(ii) ion, phenyl rings, and O_carboxylate of L, and the phenyl rings, naphthalene rings, and O_sulfonate of CR.

Observation of binding of carbon dioxide to nitro-decorated metal–organic frameworks

Synergistic effects between –NO₂ groups and open metal sites lead to optimal binding of CO₂ molecules within MFM-102-NO₂via hydrogen bonding to C–H groups.

Molecular Insights into Carbon Dioxide Sorption in Hydrazone-Based Covalent Organic Frameworks with Tertiary Amine Moieties

Tailorable sorption properties at the molecular level are key for efficient carbon capture and storage and a hallmark of covalent organic frameworks (COFs). Although amine functional groups are known to facilitate CO₂ uptake, atomistic insights into CO₂ sorption by COFs modified with amine-bearing functional groups are scarce. Herein, we present a detailed study of the interactions of carbon dioxide and water with two isostructural hydrazone-linked COFs with different polarities based on the 2,5-diethoxyterephthalohydrazide linker. Varying amounts of tertiary amines were introduced in the COF backbones by means of a copolymerization approach using 2,5-bis(2-(dimethylamino)ethoxy)terephthalohydrazide in different amounts ranging from 25 to 100% substitution of the original DETH linker. The interactions of the frameworks with CO₂ and H₂O were comprehensively studied by means of sorption analysis, solid-state NMR spectroscopy, and quantum-chemical calculations. We show that the addition of the tertiary amine linker increases the overall CO₂ sorption capacity normalized by the surface area and of the heat of adsorption, whereas surface areas and pore size diameters decrease. The formation of ammonium bicarbonate species in the COF pores is shown to occur, revealing the contributing role of water for CO₂ uptake by amine-modified porous frameworks.

2D covalent organic frameworks with built-in amide active sites for efficient heterogeneous catalysis

Two new amide functionalized covalent organic frameworks (COFs) were synthesized via a bottom-up strategy and used as heterogeneous catalysts toward Knoevenagel condensation with excellent performance.

Host-guest selectivity in a series of isoreticular metal-organic frameworks: observation of acetylene-to-alkyne and carbon dioxide-to-amide interactions

We report a series of six isoreticular metal–organic frameworks (MOFs) for selective gas adsorption, specifically for selective adsorption of CO₂ and C₂H₂.

In order to develop new porous materials for applications in gas separations such as natural gas upgrading, landfill gas processing and acetylene purification it is vital to gain understanding of host–substrate interactions at a molecular level. Herein we report a series of six isoreticular metal–organic frameworks (MOFs) for selective gas adsorption. These materials do not incorporate open metal sites and thus provide an excellent platform to investigate the effect of the incorporation of ligand functionality via amide and alkyne groups on substrate binding. By reducing the length of the linker in our previously reported MFM-136, we report much improved CO₂/CH₄ (50 : 50) and CO₂/N₂ (15 : 85) selectivity values of 20.2 and 65.4, respectively (1 bar and 273 K), in the new amide-decorated MOF, MFM-126. The CO₂ separation performance of MFM-126 has been confirmed by dynamic breakthrough experiments. In situ inelastic neutron scattering and synchrotron FT-IR microspectroscopy were employed to elucidate dynamic interactions of adsorbed CO₂ molecules within MFM-126. Upon changing the functionality to an alkyne group in MFM-127, the CO₂ uptake decreases but the C₂H₂ uptake increases by 68%, leading to excellent C₂H₂/CO₂ and C₂H₂/CH₄ selectivities of 3.7 and 21.2, respectively. Neutron powder diffraction enabled the direct observation of the preferred binding domains in MFM-126 and MFM-127, and, to the best of our knowledge, we report the first example of acetylene binding to an alkyne moiety in a porous material, with over 50% of the acetylene observed within MFM-127 displaying interactions (<4 Å) with the alkyne functionality of the framework.

Nanospace within metal-organic frameworks for gas storage and separation

Porous metal-organic frameworks (MOFs), also known as porous coordination polymers, represent a new class of porous materials, and one of their striking features lies in their tunable, designable, and functionalizable nanospace. This nanospace within MOFs provides virtually plenty of room for imagination, allowing designed incorporation of different size, shape, and functionalities for targeted gas storage and separation applications. Furthermore, the features of high porosities, tunable framework structures and pore sizes, and immobilized functional sites enable MOF materials to fully make use of their nanopore space for gas storage, to optimize their sieving effects, and to differentiate their interactions with gas molecules for gas separation. In this review article, we highlight some recent significant advances in developing microporous MOFs for some of the most important gas storage and separation applications.

An unprecedented water stable acylamide-functionalized metal–organic framework for highly efficient CH4/CO2 gas storage/separation and acid–base cooperative catalytic activity

HNUST-8 exhibits water stable, efficient CH₄/CO₂ storage and separation, acid–base cooperative catalytic activity in a tandem deacetalization Knoevenagel densation.

Organic amines as templates: pore imprints with exactly matching sizes in a series of metal–organic frameworks

Three metal–organic frameworks have been synthesized with an amine template strategy whose pore sizes exactly matched those of templates.

An Amino-Coordinated Metal-Organic Framework for Selective Gas Adsorption

A novel 3D porous metal organic framework, JUC-141, constructed by 5-aminoisophthalic acid and Cu(NO₃)₂, has been synthesized successfully. The carboxyl groups in the ligand coordinate to Cu²⁺ to form the classic Cu₂(COO)₄ paddle wheel SBU, and the assembly of the SBUs with the isophthalic acid moieties leads to a kagome lattice. Interestingly, the amino groups in the ligand also take part in the coordination and link to the dipole of the paddle wheel as pillars, thus forming a 3D porous framework with eea topology. The sizes of the channels are 5.2 Å in the direction of [111] and 10.9 Å in the direction of [001]. Gas sorption tests show that the CO₂ adsorption capacities of JUC-141 are 79.94 and 51.39 cm³ g^-1 at 273 and 298 K under 1 atm pressure, respectively. However, the N₂ adsorption capacities of JUC-141 are 13.90 and 6.76 cm³ g^-1 at 273 and 298 K under 1 atm pressure, respectively. IAST calculations indicate that the selectivity values of CO₂/N₂ are 21.62 at 273 K and 27.60 at 298 K under 101 kPa, respectively. Good selective adsorption of CO₂ over N₂ makes JUC-141 possible for CO₂ storage and separation.

A new set of Cd(ii)-coordination polymers with mixed ligands of dicarboxylate and pyridyl substituted diaminotriazine: selective sorption towards CO2and cationic dyes

Pyridyl substituted diaminotriazine based three new Cd(ii)-CPs have been synthesized. CP3showed its potential for CO₂/N₂and CO₂/CH₄separation selectivity and its ability for selective adsorption to a cationic dye.