Reversible structural change of Cu-MOF on exposure to water and its CO2 adsorptivity

Enzyme Immobilization on Metal Organic Frameworks: the Effect of Buffer on the Stability of the Support

Metal organic frameworks (MOFs) have been used to encapsulate an array of enzymes in a rapid and facile manner; however, the stability of MOFs as supports for enzymes has not been examined in detail. This study examines the stability of MOFs with different compositions (Fe-BTC, Co-TMA, Ni-TMA, Cu-TMA, and ZIF-zni) in buffered solutions commonly used in enzyme immobilization and biocatalysis. Stability was assessed via quantification of the release of metals by inductively coupled plasma optical emission spectroscopy. The buffers used had varied effects on different MOF supports, with incubation of all MOFs in buffers resulting in the release of metal ions to varying extents. Fe-BTC was completely dissolved in citrate, a buffer that has a profound destabilizing effect on all MOFs analyzed, precluding its use with MOFs. MOFs were more stable in acetate, potassium phosphate, and Tris HCl buffers. The results obtained provide a guide for the selection of an appropriate buffer with a particular MOF as a support for the immobilization of an enzyme. In addition, these results identify the requirement to develop methods of improving the stability of MOFs in aqueous solutions. The use of polymer coatings was evaluated with polyacrylic acid (PAA) providing an improved level of stability. Lipase was immobilized in Fe-BTC with PAA coating, resulting in a stable biocatalyst with retention of activity in comparison to the free enzyme.

Sustainable Ketalization of Glycerol with Ethyl Levulinate Catalyzed by the Iron(III)-Based Metal-Organic Framework MIL-88A

The catalytic properties of a simple iron-containing MOF based on fumaric acid, MIL-88A, were investigated in the ketalization of ethyl levulinate with glycerol. The corresponding product is a component of current interest as a renewable building block for many uses. Under the following conditions (solventless, 120 °C, stoichiometric ratio, 1% cat.), the reaction proceeds with good yields (85%), and the catalyst can be recovered and recycled without loss of activity, despite some changes in the crystalline lattice and morphology. Moreover, the residual iron content in the product is in the order of units of ppm (≤2), which demonstrates the robustness of the MOF under the reaction conditions.

In Situ Growth of a Stable Metal-Organic Framework (MOF) on Flexible Fabric via a Layer-by-Layer Strategy for Versatile Applications

Fabrics have been used broadly in daily life for an enormous variety of applications due to their intrinsic advantages, such as flexibility, renewability, and good processability. Integrating natural fabrics with metal-organic frameworks (MOFs) is an effective strategy to improve the added value of textiles with special functionalities. Here, a facile, low-cost, and scalable technology is reported for the in situ growth of MOFs on cotton fabrics. A uniform and dense coating of regular octahedral Cu-1,3,5-benzenetricarboxylic acid (CuBTC) crystals was formed on the fiber surface, followed by treatment with 1H,1H,2H,2H-perfluorooctyltriethoxysilane and triethoxyoctylsilane to create a superhydrophobic CuBTC@cotton fabric (SMCF), which greatly improved its water stability and extended superhydrophobic CuBTC's potential applications. The as-prepared MCF has a specific surface area of 229 m²/g, which is 11 times that of pristine fabrics (21 m²/g). This high porosity further endows the fabric with enhanced loading capacity of essential oils to enable excellent antibacterial ability. Moreover, the SMCF also exhibits excellent self-cleaning, UV shielding, and anti-icing performances. In addition, we performed COMSOL simulations to investigate the dynamic freezing process of water on the surface of samples, which agrees well with our experimental observations. By combining the merits of both fabrics and MOFs, the MCF is expected to extend the applications of traditional textiles in antifouling, safety, the fragrance industry, and healthcare for the next-generation multifunctional fabrics.

Recent advances of the core-shell MOFs in tumour therapy

Coordination chemistry has always been vital to explore the material prominence of metal-organic systems. The metal-organic chemistry plays a fundamental role in decisive structural features, which are accountable for tuning the properties of materials. Tumour therapy has become an important research field of medical treatment in the world. Metal-organic frameworks (MOFs) have attracted extensive interest in medical science research due to their large effective surface area, clear pore network, and critical catalytic performance. Compared with traditional MOF materials, MOF materials with core-shell structures have a higher loading rate and better stability, which can overcome a single function. They have been successfully used in tumour medical research and have excellent prospects for diagnosing and treating various tumours. The current review article thoroughly describes the various synthetic approaches for engineering core-shell MOF materials, the structural types, and the potential functional applications. We also discussed core-shell MOF materials for the various treatment of tumours, such as tumour chemotherapy, tumour phototherapy and tumour microenvironment anti-hypoxia therapy. In this paper, the synthesized procedures of core-shell MOFs and their applications for tumour treatment have been discussed, and their future research has prospected. The current improved strategies, challenges, and prospects are also presented because of the metal-organic chemistry governing the structural modification of core-shell MOFs for tumour therapy applications. Therefore, the present review article opens a new door for medicinal chemists to tune the structural features of the core-shell MOF materials to modulate tumour therapy with simple, low-cost materials for better human lives.

High Working Capacity Acetylene Storage at Ambient Temperature Enabled by a Switching Adsorbent Layered Material

Unlike most gases, acetylene storage is a challenge because of its inherent pressure sensitivity. Herein, a square lattice (sql) coordination network [Cu(4,4'-bipyridine)₂(BF₄)₂]_n (sql-1-Cu-BF₄) is investigated with respect to its C₂H₂ sorption behavior from 189 to 298 K. The C₂H₂ sorption studies revealed that sql-1-Cu-BF₄ exhibits multistep isotherms that are temperature-dependent and consistent with the transformation from "closed" (nonporous) to four "open" (porous) phases induced by the C₂H₂ uptake. The Clausius-Clapeyron equation was used to calculate the performance of sql-1-Cu-BF₄ for C₂H₂ storage at pressures >1 bar, which revealed that its volumetric working capacity at 288 K is slightly superior to acetone (174 vs 170 cm³ cm^-3) over a safer pressure range (1-3.5 vs 1-15 bar). Molecular simulations provided insights into the observed switching phenomena, revealing that the layer expansion of sql-1-Cu-BF₄ occurs via intercalation and inclusion of C₂H₂. These results indicate that switching adsorbent layered materials offer promise for utility in the context of C₂H₂ storage and delivery.

Thermogravimetric Analysis and Mass Spectrometry Allow for Determination of Chemisorbed Reaction Products on Metal Organic Frameworks

Thermogravimetric analysis (TGA) is a technique which can probe chemisorption of substrates onto metal organic frameworks. A TGA method was developed to examine the catalytic oxidation of S-nitrosoglutathione (GSNO) by the MOF H₃[(Cu₄Cl)₃(BTTri)₈] (abbr. Cu-BTTri; H₃BTTri = 1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene), yielding glutathione disulfide (GSSG) and nitric oxide (NO). Thermal analysis of reduced glutathione (GSH), GSSG, GSNO, and Cu-BTTri revealed thermal resolution of all four analytes through different thermal onset temperatures and weight percent changes. Two reaction systems were probed: an aerobic column flow reaction and an anaerobic solution batch reaction with gas agitation. In both systems, Cu-BTTri was reacted with a 1 mM GSH, GSSG, or GSNO solution, copiously rinsed with distilled-deionized water (dd-H₂O), dried (25 °C, < 1 Torr), and assessed by TGA. Additionally, stock, effluent or supernatant, and rinse solutions for each glutathione derivative within each reaction system were assessed by mass spectrometry (MS) to inform on chemical transformations promoted by Cu-BTTri as well as relative analyte concentrations. Both reaction systems exhibited chemisorption of glutathione derivatives to the MOF by TGA. Mass spectrometry analyses revealed that in both systems, GSH was oxidized to GSSG, which chemisorbed to the MOF whereas GSSG remained unchanged during chemisorption. For GSNO, chemisorption to the MOF without reaction was observed in the aerobic column setup, whereas conversion to GSSG and subsequent chemisorption was observed in the anaerobic batch setup. These findings suggest that within this reaction system, GSSG is the primary adsorbent of concern with regards to strong binding to Cu-BTTri. Development of similar thermal methods could allow for the probing of MOF reactivity for a wide range of systems, informing on important considerations such as reduced catalytic efficiency from poisoning, recyclability, and loading capacities of contaminants or toxins with MOFs.

Soft Matter Technology at KIT: Chemical Perspective from Nanoarchitectures to Microstructures

Bioinspiration has emerged as an important design principle in the rapidly growing field of materials science and especially its subarea, soft matter science. For example, biological cells form hierarchically organized tissues that not only are optimized and designed for durability, but also have to adapt to their external environment, undergo self-repair, and perform many highly complex functions. Being able to create artificial soft materials that mimic those highly complex functions will enable future materials applications. Herein, soft matter technologies that are used to realize bioinspired material structures are described, and potential pathways to integrate these into a comprehensive soft matter research environment are addressed. Solutions become available because soft matter technologies are benefitting from the synergies between organic synthesis, polymer chemistry, and materials science.

Activated carbons from common nettle as potential adsorbents for CO2 capture

Activated carbons (ACs) prepared from common nettle (Urtica Dioica L.) were studied in terms of carbon dioxide adsorption. ACs were prepared by KOH chemical activation in a nitrogen atmosphere at temperatures (ranging from 500 to 850°C). The pore structure and the surface characterization of the ACs were specified based on adsorption-desorption isotherms of nitrogen measured at –196°C and carbon dioxide at 0°C. The specific surface area was calculated according to the BET equation. The pore volume was estimated using the DFT method. The highest values of the specific surface area (SSA) showed activated carbons produced at higher carbonization temperatures. All samples revealed presence of micropores and mesopores with a diameter range of 0.3–10 nm. The highest value of the CO2 adsorption, 4.22 mmol/g, was found for the material activated at 700°C.

Elucidating the Formation and Transformation Mechanisms of the Switchable Metal-Organic Framework ELM-11 by Powder and Single-Crystal EPR Study

The effect of the synthesis conditions on the structure and guest-responsive properties of a "gate pressure" metal-organic framework (MOF) with composition [Cu(4,4'-bipy)₂(BF₄)₂] _n (4,4'-bipy = 4,4'-bipyridine), also known as ELM-11 (ELM = elastic layer material) was investigated. Two different batches of ELM-11, synthesized from water-methanol and water-acetonitrile solutions, have been entirely characterized by PXRD, nitrogen (77 K) and carbon dioxide (195 K) physisorption, elemental analysis, DRIFT, TG, and SEM. Both ELM-11 samples were studied by electron paramagnetic resonance (EPR) spectroscopy in order to follow the change in the local structure of the copper ion during the activation and resolvation. Continuous wave X-band EPR measurements on powder samples provided an elongated octahedral coordination symmetry of the cupric ions and revealed different axial ligands in the as-synthesized and activated forms in both bulk samples of ELM-11. One of the procedures was amended in order to slow down the crystallization that allows isolation of single crystals of two polymorphic modifications of Cu-4,4'-bipyridine coordination polymers, namely [Cu(4,4'-bipy)₂(CH₃CN)₂](BF₄)₂ and [Cu₂O(4,4'-bipy)₃(CH₃CN)₄](BF₄)₂, one of which shows a crystal structure similar to that of ELM-11. Further single-crystal EPR experiments on the as-synthesized material [Cu(4,4'-bipy)₂(CH₃CN)₂](BF₄)₂ revealed the orientation of the g tensor of the cupric ions and proved that layers of acetonitrile-synthesized ELM-11 are arranged perpendicularly to the crystallographic c axis.

Explicit treatment of hydrogen bonds in the universal force field: Validation and application for metal-organic frameworks, hydrates, and host-guest complexes

A straightforward means to include explicit hydrogen bonds within the Universal Force Field (UFF) is presented. Instead of treating hydrogen bonds as non-bonded interaction subjected to electrostatic and Lennard-Jones potentials, we introduce an explicit bond with a negligible bond order, thus maintaining the structural integrity of the H-bonded complexes and avoiding the necessity to assign arbitrary charges to the system. The explicit hydrogen bond changes the coordination number of the acceptor site and the approach is thus most suitable for systems with under-coordinated atoms, such as many metal-organic frameworks; however, it also shows an excellent performance for other systems involving a hydrogen-bonded framework. In particular, it is an excellent means for creating starting structures for molecular dynamics and for investigations employing more sophisticated methods. The approach is validated for the hydrogen bonded complexes in the S22 dataset and then employed for a set of metal-organic frameworks from the Computation-Ready Experimental database and several hydrogen bonded crystals including water ice and clathrates. We show that the direct inclusion of hydrogen bonds reduces the maximum error in predicted cell parameters from 66% to only 14%, and the mean unsigned error is similarly reduced from 14% to only 4%. We posit that with the inclusion of hydrogen bonding, the solvent-mediated breathing of frameworks such as MIL-53 is now accessible to rapid UFF calculations, which will further the aim of rapid computational scanning of metal-organic frameworks while providing better starting points for electronic structure calculations.

Catalytic properties of a cobalt metal-organic framework with a zwitterionic ligand synthesized in situ

Herein we describe the high yield synthesis of a highly crystalline cobalt(ii) MOF with a novel zwitterionic ligand made up of 3,3',4,4'-BPTC and 1,4-cyclohexanediamine, obtained in situ during the hydrothermal synthesis. The compound with the molecular formula C₄₆H₃₈N₄O₁₄Co·2H₂O has a molecular mass of 965.7783 g mol^-1, a triclinic crystalline system (a = 5.86 Å, b = 9.28 Å, c = 19.92 Å, α = 83.93°, β = 88.01°, γ = 78.59), it is thermally stable up to 300 °C and presents structural stability before and after removing the solvent molecules from its pores. This novel material showed catalytic properties in an electrophilic substitution reaction of indoles and aldehydes allowing the syntheses of bis(indolyl)methanes in high yields under mild reaction conditions and can be reused at least once with the same catalytic activity.

Recyclable switching between nonporous and porous phases of a square lattice (sql) topology coordination network

A 2D switching material holds great potential for exceptional working capacity of gas storage.

Liquid/vapor-induced reversible dynamic structural transformation of a three-dimensional Cu-based MOF to a one-dimensional MOF showing gate adsorption

A new 3D metal–organic framework (MOF) is reversibly transformed to a 1D chain MOF showing selective adsorption properties.

Nitrogen-doped 3D porous carbons with iron carbide nanoparticles encapsulated in graphitic layers derived from functionalized MOF as an efficient noble-metal-free oxygen reduction electrocatalysts in both acidic and alkaline media

Functionalized 3D porous carbons comprising encased iron carbide species, derived from a MOF, display outstanding ORR performance in both acidic and alkaline solutions.

Can a highly flexible copper(i) cluster-containing 1D and 2D coordination polymers exhibit MOF-like properties?

The p-TolS(CH₂)₈STol-p and p-tBuC₆H₄S(CH₂)₈SC₆H₄-tBu-p ligands react with CuI respectively in MeCN and EtCN and in EtCN form the 2D and 1D polymers [Cu₈I₈(p-TolS(CH₂)₈STol-p)₃(solvent)₂]_n (solvent = MeCN, EtCN) and [Cu₄I₄(p-tBuC₆H₄S(CH₂)₈SC₆H₄-tBu-p)₂(EtCN)]_n susceptible to exchange solvent molecules.

A green strategy to prepare metal oxide superstructure from metal-organic frameworks

Metal or metal oxides with diverse superstructures have become one of the most promising functional materials in sensor, catalysis, energy conversion, etc. In this work, a novel metal-organic frameworks (MOFs)-directed method to prepare metal or metal oxide superstructure was proposed. In this strategy, nodes (metal ions) in MOFs as precursors to form ordered building blocks which are spatially separated by organic linkers were transformed into metal oxide micro/nanostructure by a green method. Two kinds of Cu-MOFs which could reciprocally transform by changing solvent were prepared as a model to test the method. Two kinds of novel CuO with three-dimensional (3D) urchin-like and 3D rods-like superstructures composed of nanoparticles, nanowires and nanosheets were both obtained by immersing the corresponding Cu-MOFs into a NaOH solution. Based on the as-formed CuO superstructures, a novel and sensitive nonenzymatic glucose sensor was developed. The small size, hierarchical superstructures and large surface area of the resulted CuO superstructures eventually contribute to good electrocatalytic activity of the prepared sensor towards the oxidation of glucose. The proposed method of hierarchical superstructures preparation is simple, efficient, cheap and easy to mass production, which is obviously superior to pyrolysis. It might open up a new way for hierarchical superstructures preparation.

A two-dimensional flexible porous coordination polymer based on Co(ii) and terpyridyl phosphine oxide

A 2D framework with –P(O)Ph₂ phenyl groups on the layer surface has been obtained, which shows remarkable dynamic sorption behaviours.

Tuning of gate adsorption: modification of a flexible metal-organic framework by secondary organic ligands

For realizing selective adsorption of targeted molecules, a flexible metal-organic framework (MOF) was modified with monodentate secondary ligands. Although the modified MOF retains CO2 adsorptivities with a vertical adsorption uptake, the material also shows gate adsorptivities of a specific gas molecule that the pristine MOF does not adsorb.

The biocompatibility of metal-organic framework coatings: an investigation on the stability of SURMOFs with regard to water and selected cell culture media

Highly porous thin films based on a [Cu(bdc)(2)](n) (bdc = benzene-1,4-dicarboxylic acid) metal-organic framework, MOF, grown using liquid-phase epitaxy (LPE) show remarkable stability in pure water as well as in artificial seawater. This opens the possibility to use these highly porous coatings for environmental and life science applications. Here we characterize in detail the stability of these SURMOF 2 thin films under aqueous and cell culture conditions. We find that the material degrades only very slowly in water and artificial seawater (ASW) whereas in typical cell culture media (PBS and DMEM) a rapid dissolution is observed. The release of Cu(2+) ions resulting from the dissolution of the SURMOF 2 in the liquids exhibits no adverse effect on the adhesion of fibroblasts, prototype eukaryotic cells, to the substrate and their subsequent proliferation, thus demonstrating the biocompatibility of SURMOF 2 surface coatings. Thus, the results are an important step toward application of these porous materials as a slow release matrix, for example, for pharmaceuticals and growth factors.

Tuning of gate opening of an elastic layered structure MOF in CO2 sorption with a trace of alcohol molecules

It is important to tune the sorption behavior of metal-organic framework (MOF) materials. Ethanol treatment on the hydrated form of [Cu(bpy)(2)(BF(4))(2)], which is a representative flexible MOF showing the fascinating gate phenomenon on CO(2) sorption, induces an easier dehydration and a significant decrease in the CO(2) gate pressure. The results of IR, X-ray diffraction (XRD), and X-ray absorption fine structure (XAFS) measurements indicated that water molecules in the lattice of the hydrated form can be removed even at room temperature after the ethanol treatment and the basic 2D layered structure remains with a slight interlayer expansion. The results of thermogravimetric (TG) and gas chromatograph/mass spectrometry (GC/MS) analyses and of CO(2) sorptions indicated that the change of the gate phenomenon was caused by a trace of residual ethanol molecules included in the structure. Similar phenomena were observed on alcohols with different polarity and molecular size.

Reactive adsorption of ammonia on Cu-based MOF/graphene composites

New composites based on HKUST-1 and graphene layers are tested for ammonia adsorption at room temperature in both dry and moist conditions. The materials are characterized by X-ray diffraction, FT-IR spectroscopy, adsorption of nitrogen, and thermal analyses. Unlike other MOF/GO composites reported in previous studies, these materials are water-stable. Ammonia adsorption capacities on the composites are higher than the ones calculated for the physical mixture of components, suggesting the presence of a synergetic effect between the MOF and graphene layers. The increased porosity and dispersive forces being the consequence of the presence of graphene layers are responsible for the enhanced adsorption. In addition to its retention via physical forces, ammonia is also adsorbed via binding to the copper sites in HKUST-1 and then, progressively, via reaction with the MOF component. This reactive adsorption is visible through two successive changes of the adsorbents' color during the breakthrough tests. More ammonia is adsorbed in moist conditions than in dry conditions owing to its dissolution in a water film present in the pore system.

Sorbents for CO(2) capture from flue gas--aspects from materials and theoretical chemistry

Predictions of future climate change have triggered a search for ways to reduce the release of greenhouse gases into the atmosphere. Carbon capture and storage (CCS) assists this goal by reducing carbon dioxide emissions, and CO(2) adsorbents in particular can reduce the costs of CO(2) capture. Here, we review the nanoscale sorbent materials that have been developed and the theoretical basis for their function in CO(2) separation, particularly from N(2)-rich flue gases.

Flexible two-dimensional square-grid coordination polymers: structures and functions

Coordination polymers (CPs) or metal-organic frameworks (MOFs) have attracted considerable attention because of the tunable diversity of structures and functions. A 4,4'-bipyridine molecule, which is a simple, linear, exobidentate, and rigid ligand molecule, can construct two-dimensional (2D) square grid type CPs. Only the 2D-CPs with appropriate metal cations and counter anions exhibit flexibility and adsorb gas with a gate mechanism and these 2D-CPs are called elastic layer-structured metal-organic frameworks (ELMs). Such a unique property can make it possible to overcome the dilemma of strong adsorption and easy desorption, which is one of the ideal properties for practical adsorbents.