Pore opening and breathing transitions in metal-organic frameworks: Coupling adsorption and deformation

Synthesis and Characterization of MOF-Derived Structures: Recent Advances and Future Perspectives

Due to their facile tunability, metal-organic frameworks (MOFs) are employed as precursors and templates to construct advanced functional materials with unique and desired chemical, physical, mechanical, and morphological properties. By tuning MOF precursor composition and manipulating conversion processes, various MOF-derived materials commonly known as MOF derivatives can be constructed. The possibility of controlled and predictable properties makes MOF derivatives a preferred choice for numerous advanced technological applications. The innovative synthetic designs besides the plethora of interdisciplinary characterization approaches applicable to MOF derivatives provide the opportunity to perform a myriad of experiments to explore the performance and offer key insight to develop the next generation of advanced materials. Though there are many published works of literature describing various synthesis and characterization techniques of MOF derivatives, it is still not clear how the synthesis mechanism works and what are the best techniques to characterize these materials to probe their properties accurately. In this review, the recent development in synthesis techniques and mechanisms for a variety of MOF derivates such as MOF-derived metal oxides, porous carbon, composites/hybrids, and sulfides is summarized. Furthermore, the details of characterization techniques and fundamental working principles are summarized to probe the structural, mechanical, physiochemical, electrochemical, and electronic properties of MOF and MOF derivatives. The future trends and some remaining challenges in the synthesis and characterization of MOF derivatives are also discussed.

Upcycling of post-consumer polyethylene terephthalate bottles into aluminum-based metal-organic framework adsorbents for efficient orthophosphate removal

2-Phosphonobutane-1,2,4,-tricarboxylic acid (PBTC) is an orthophosphate compound widely used as an antiscalant chemical and corrosion inhibitor in manufacturing. However, PBTC poses persistent environmental concerns due to its stability and resistance to conventional water treatment. In addressing the issues of PBTC in aquatic systems, Al-based metal-organic frameworks (MOFs) have been developed and applied as sustainable adsorbents. The materials are synthesized from terephthalic acid (TPA) linkers derived from upcycling products of post-consumer polyethylene terephthalate (PET) bottles. The PET-derived linker was prepared using alkaline hydrolysis followed by acidification and employed in forming MIL-53 (Al), with a comparative assessment against the corresponding MOFs made from commercial-grade TPA. The structures and properties of the materials were characterized with microscopic and spectroscopic methods. The synthesized adsorbents achieved a phosphate adsorption capacity of 826 mg/g at pH 5, with kinetics fitting a pseudo-second-order model and isotherm patterns aligning with Langmuir, Freundlich, and Sips models, indicative of diverse adsorption on heterogeneous surfaces. The results highlight the role of electrostatic interactions and hydrogen bonding mechanisms in PBTC adsorption. The eco-friendly materials with high adsorption performance offer an innovative route for sustainable waste management and water purification.

Co-Adsorption of Alcohols and Water in JUK-8 Studied Using Quasi-Equilibrated Thermodesorption

JUK-8 ([Zn(oba)(pip)]n, oba2- = 4,4'-oxybis(benzenedicarboxylate), pip = 4-pyridyl-functionalized benzene-1,3-dicarbohydrazide) is a hydrolytically stable flexible metal-organic framework. Owing to its unusual adsorptive properties, JUK-8 can be considered as a promising sensing material for construction of detectors of volatile organic compounds (VOCs) in air. Quasi-equilibrated temperature-programmed desorption and adsorption (QE-TPDA) is a versatile method dedicated to characterization of porous materials. In this work, QE-TPDA was employed to study co-adsorption of water and selected alcohols in JUK-8. For the first time an infrared detector sensitive to organic compounds was used in the QE-TPDA measurements, allowing the study of the influence of water vapor on sorption of VOCs. The QE-TPDA profiles of the studied alcohols, exhibiting two desorption maxima and two adsorption minima, are consistent with the standard sorption isotherms, revealing a two-step adsorption-desorption mechanism. The profiles recorded in the presence of water are noticeably changed in different ways for different alcohols. While at low relative humidity (RH) (ca. 20%) the low temperature adsorption states of ethanol and 1-propanol were only slightly destabilized, for 2-propanol almost complete suppression of adsorption was observed. The results found for moderate RH levels (ca. 50%) indicated that the opening of the JUK-8 structure, responsible for its breathing behavior, was followed by the filling of the just generated pores with a water-alcohol mixture.

Modulation of the Dynamics of a Two-Dimensional Interweaving Metal-Organic Framework through Induced Hydrogen Bonding

Inducing, understanding, and controlling the flexibility in metal-organic frameworks (MOFs) are of utmost interest due to the potential applications of dynamic materials in gas-related technologies. Herein, we report the synthesis of two isostructural two-dimensional (2D) interweaving zinc(II) MOFs, TMU-27 [Zn(bpipa)(bdc)] and TMU-27-NH2 [Zn(bpipa)(NH2-bdc)], based on N,N'-bis-4-pyridyl-isophthalamide (bpipa) and 1,4-benzenedicarboxylate (bdc) or 2-amino-1,4-benzenedicarboxylate (NH2-bdc), respectively. These frameworks differ only by the substitution at the meta-position of their respective bdc groups: an H atom in TMU-27 vs an NH2 group in TMU-27-NH2. This difference strongly influences their respective responses to external stimuli, since we observed that the structure of TMU-27 changed due to desolvation and adsorption, whereas TMU-27-NH2 remained rigid. Using single-crystal X-ray diffraction and CO2-sorption measurements, we discovered that upon CO2 sorption, TMU-27 undergoes a transition from a closed-pore phase to an open-pore phase. In contrast, we attributed the rigidification in TMU-27-NH2 to intermolecular hydrogen bonding between interweaving layers, namely, between the H atoms from the bdc-amino groups and the O atoms from the bpipa-amide groups within these layers. Additionally, by using scanning electron microscopy to monitor the CO2 adsorption and desorption in TMU-27, we were able to establish a correlation between the crystal size of this MOF and its transformation pressure.

Tuning microscopic structure of La-MOFs via ligand engineering effect towards enhancing phosphate adsorption

The selection of different organic ligands when synthesizing metal organic framework (MOFs) can change their effects on the adsorption performance. Here, four La-MOFs adsorbents (La-SA, La-FA, La-TA and La-OA) with different organic ligands and structures were synthesized by solvothermal method for phosphate adsorption, and the relationship between their adsorption properties and structures was established. Among four La-MOFs, their phosphate adsorption capacities and adsorption rates followed La-SA > La-FA > La-TA > La-OA. The results indicated that average pore diameter played a key role in phosphate adsorption and there was a positive correlation between average pore diameter and adsorption capacity (R2 = 0.86). Coexisting ion experiments showed that phosphate adsorptions on three La-MOFs (La-SA, La-FA and La-TA) were inhibited in the presence of CO32- and HCO3-. The inhibition of CO32- was the most pronounced and the results of redundancy analysis pointed out that it was mainly due to the change of pH value. In contrast, La-OA showed enhanced phosphate adsorption in the presence of CO32- and HCO3-, and the combination of pH experiments showed that phosphate adsorption by La-OA was increased under alkaline conditions. Further combined with FT-IR, XRD, high resolution energy spectra of XPS (La 3d, P 2p and O 1s) and XANES, the adsorption mechanisms were derived electrostatic attraction, chemical precipitation and inner sphere complexation, and the last two were identified as the main mechanisms. Moreover, it can be identified from XPS 2p that the phosphate adsorption on La-FA and La-OA were mainly in the LaPO4 state, while La-SA and La-TA mainly existed in the form of LaPO4·xH2O crystals and inner sphere complexes. From the perspective of material morphology, this work provides a thought for the rational design of MOFs with adjustable properties for phosphate adsorption.

A highly stable terbium(III) metal-organic framework MOF-76(Tb) for hydrogen storage and humidity sensing

The present study described the synthesis and characterization of MOF-76(Tb) for hydrogen storage and humidity sensing applications. The structure and morphology of as-synthesized material were studied using powder X-ray diffraction, scanning, and transmission electron microscopy. The crystal structure of MOF-76(Tb) consists of terbium(III) and benzene-1,3,5-tricarboxylate(-III) ions, one coordinated aqua ligand and one crystallization N,N´-dimethylformamide molecule. The polymeric framework of MOF-76(Tb) contains 1D sinusoidally shaped channels with sizes of 6.6 × 6.6 Å propagating along c crystallographic axis. The thermogravimetric analysis of the prepared material exhibited thermal stability up to 600 °C. At 77 K and pressure up to 20 bar; 0.6 wt.% hydrogen storage capacity for MOF-76(Tb) was observed. Finally, the humidity sensing measurements (water adsorption experiments) were performed, and the results indicate that MOF-76(Tb) is not a suitable material for moisture sensing applications.

Theoretical isotherm equation for adsorption-induced structural transition on flexible metal-organic frameworks

Flexible metal-organic frameworks (MOFs) exhibit an adsorption-induced structural transition known as "gate opening" or "breathing," resulting in an S-shaped adsorption isotherm. This unique feature of flexible MOFs offers significant advantages, such as a large working capacity, high selectivity, and intrinsic thermal management capability, positioning them as crucial candidates for revolutionizing adsorption separation processes. Therefore, the interest in the industrial applications of flexible MOFs is increasing, and the adsorption engineering for flexible MOFs is becoming important. However, despite the establishment of the theoretical background for adsorption-induced structural transitions, no theoretical equation is available to describe S-shaped adsorption isotherms of flexible MOFs. Researchers rely on various empirical equations for process simulations that can lead to unreliable outcomes or may overlook insights into improving material performance owing to parameters without physical meaning. In this study, we derive a theoretical equation based on statistical mechanics that could be a standard for the structural transition type adsorption isotherms, as the Langmuir equation represents type I isotherms. The versatility of the derived equation is shown through four examples of flexible MOFs that exhibit gate opening and breathing. The consistency of the formula with existing theories, including the osmotic free energy analysis and intrinsic thermal management capabilities, is also discussed. The developed theoretical equation may lead to more reliable and insightful outcomes in adsorption separation processes, further advancing the direction of industrial applications of flexible MOFs.

Kinetics of Guest-Induced Structural Transitions in Metal-Organic-Framework MIL-53(Al)-NH2 Probed by High-Pressure Nuclear Magnetic Resonance

A nuclear magnetic resonance (NMR) study of a pore opening in amino-functionalized metal-organic framework (MOF) MIL-53(Al) in response to methane pressure variation is presented. Variations of both NMR signal intensities and transversal relaxation rates for methane are found to reveal hysteretic structural transitions in the MOF material, which are smeared out over broad pressure ranges. Experiments with pressure reversals upon an incomplete adsorption/desorption gave deeper insight into the microscopic transition mechanisms. These experiments have unequivocally proven that the non-stepwise pore opening/closing transitions observed in the experiments are governed by a distribution of the opening/closing pressures over different MOF crystallites, for example, due to a distribution of the crystal sizes or shapes. The slow kinetics of the structural transitions measured in the hysteresis regime revealed a complex free energy landscape for the phase transition process.

Spatiotemporal Design of the Metal-Organic Framework DUT-8(M)

Switchable metal-organic frameworks (MOFs) change their structure in time and selectively open their pores adsorbing guest molecules, leading to highly selective separation, pressure amplification, sensing, and actuation applications. The 3D engineering of MOFs has reached a high level of maturity, but spatiotemporal evolution opens a new perspective toward engineering materials in the 4th dimension (time) by t-axis design, in essence exploiting the deliberate tuning of activation barriers. This work demonstrates the first example in which an explicit temporal engineering of a switchable MOF (DUT-8, [M₁ M₂ (2,6-ndc)₂ dabco]_n , 2,6-ndc = 2,6-naphthalene dicarboxylate, dabco = 1,4diazabicyclo[2.2.2]octane, M₁  = Ni, M₂  = Co) is presented. The temporal response is deliberately tuned by variations in cobalt content. A spectrum of advanced analytical methods is presented for analyzing the switching kinetics stimulated by vapor adsorption using in situ time-resolved techniques ranging from ensemble adsorption and advanced synchrotron X-ray diffraction experiments to individual crystal analysis. A novel analysis technique based on microscopic observation of individual crystals in a microfluidic channel reveals the lowest limit for adsorption switching reported so far. Differences in the spatiotemporal response of crystal ensembles originate from an induction time that varies statistically and widens characteristically with increasing cobalt content reflecting increasing activation barriers.

Tröger's Base Network Polymers of Intrinsic Microporosity (TB-PIMs) with Tunable Pore Size for Heterogeneous Catalysis

Heterogeneous catalysis plays a pivotal role in the preparation of value-added chemicals, and it works more efficiently when combined with porous materials and supports. Because of that, a detailed assessment of porosity and pore size is essential when evaluating the performance of new heterogeneous catalysts. Herein, we report the synthesis and characterization of a series of novel microporous Tröger’s base polymers and copolymers (TB-PIMs) with tunable pore size. The basicity of TB sites is exploited to catalyze the Knoevenagel condensation of benzaldehydes and malononitrile, and the dimension of the pores can be systematically adjusted with an appropriate selection of monomers and comonomers. The tunability of the pore size provides the enhanced accessibility of the catalytic sites for substrates, which leads to a great improvement in conversions, with the best results achieving completion in only 20 min. In addition, it enables the use of large benzaldehydes, which is prevented when using polymers with very small pores, typical of conventional PIMs. The catalytic reaction is more efficient than the corresponding homogeneous counterpart and is ultimately optimized with the addition of a small amount of a solvent, which facilitates the swelling of the pores and leads to a further improvement in the performance and to a better carbon economy. Molecular dynamic modeling of the copolymers’ structures is employed to describe the swellability of flexible chains, helping the understanding of the improved performance and demonstrating the great potential of these novel materials.

A Comprehensive Review on the Use of Metal–Organic Frameworks (MOFs) Coupled with Enzymes as Biosensors

Several studies have shown the development of electrochemical biosensors based on enzymes immobilized in metal–organic frameworks (MOFs). Although enzymes have unique properties, such as efficiency, selectivity, and environmental sustainability, when immobilized, these properties are improved, presenting significant potential for several biotechnological applications. Using MOFs as matrices for enzyme immobilization has been considered a promising strategy due to their many advantages compared to other supporting materials, such as larger surface areas, higher porosity rates, and better stability. Biosensors are analytical tools that use a bioactive element and a transducer for the detection/quantification of biochemical substances in the most varied applications and areas, in particular, food, agriculture, pharmaceutical, and medical. This review will present novel insights on the construction of biosensors with materials based on MOFs. Herein, we have been highlighted the use of MOF for biosensing for biomedical, food safety, and environmental monitoring areas. Additionally, different methods by which immobilizations are performed in MOFs and their main advantages and disadvantages are presented.

Coupling external and internal pressure for the structural transition of MIL-53(Cr)

Flexible metal-organic framework (MOF) materials have the ability to perform stimulated sudden volume contractions, and thus attract increasing attention for use in potential applications such as: actuators or sensors. Here, the structural transition of MIL-53(Cr) loaded with a high concentration of CH₃OH (CH₃OH) guest molecules, which cause internal pressure due to guest-guest interactions, was investigated. The pressure triggering the structural transition can be enhanced by high guest molecule loadings (1 CH₃OH per unit cell (UC): 5 MPa, empty: 53 MPa, 7 CH₃OH per UC: 90 MPa, and 8 CH₃OH per UC: 280 MPa). The asymmetrical and small distortion of the organic-inorganic connections are the main microscopic characteristic of the structural transition of MIL-53(Cr) with a high CH₃OH loading. The external pressure and the internal pressure, instead of the adsorption of the guest molecules, became dominant in the structural transition of MIL-53(Cr). Current studies showed that the high-pressure response of the flexible MOF structure may broaden the acceptable pressure range in future actuator or sensor applications.

Inlaying metal-organic framework derived pancake-like TiO2 into three-dimensional BiOI for visible-light-driven generation of vanillin from sodium lignosulfonate

Pancake-like TiO₂ (M-TiO₂) derived from the metal-organic framework was inlaid into three-dimensional flower-like BiOI through a facile solvothermal method. M-TiO₂ supplies large surface area and mesoporous structure for attachment and transfer of the substrates and products, while BiOI acts as a photosensitizer to absorb visible light and generates electrons and holes. The distinct structure of M-TiO₂/BiOI gives a favorable contact between the two monomers, and promotes the transfer of charge carriers. In conjunction with the proper band positions of M-TiO₂ and BiOI, the efficient separation of electron-hole pairs is attained. Benefiting from the above cooperative effects of M-TiO₂ and BiOI, the performance for the vanillin generation from sodium lignosulfonate (SLS) over M-TiO₂/BiOI composites has a prominent improvement under visible light. Specifically, the yield over optimal M-TiO₂/BiOI sample is about 5.8 mg/g_SLS, obviously superior to that over pristine M-TiO₂ (~1 mg/g_SLS) and BiOI (~1.1 mg/g_SLS). It is found that h⁺ and O₂^- play the key role for vanillin generation from sodium lignosulfonate, and the low vanillin generation under UV-vis light sheds light on that OH is an adverse factor. We hoped that this work could inspire the studies on the photocatalytic valorization of biomass using noble metal-free catalysts.

Elucidating pore chemistry within metal–organic frameworks via single crystal X-ray diffraction; from fundamental understanding to application

The application of metal–organic frameworks (MOFs) to diverse chemical sectors is aided by their crystallinity, which permits the use of X-ray crystallography to characterise their pore chemistry and provides invaluable insight into their properties.