In Situ Single-crystal X-ray Diffraction Studies of Physisorption and Chemisorption of SO2 within a Metal-Organic Framework and Its Competitive Adsorption with Water.

Living on an increasingly polluted planet, the removal of toxic pollutants such as sulfur dioxide (SO2) from the troposphere and power station flue gas is becoming more and more important. The CPO-27/MOF-74 family of metal-organic frameworks (MOFs) with their high densities of open metal sites is well suited for the selective adsorption of gases that, like SO2, bind well to metals and have been extensively researched both practically and through computer simulations. However, until now, focus has centered upon the binding of SO2 to the open metal sites in this MOF (called chemisorption, where the adsorbent-adsorbate interaction is through a chemical bond). The possibility of physisorption (where the adsorbent-adsorbate interaction is only through weak intermolecular forces) has not been identified experimentally. This work presents an in situ single-crystal X-ray diffraction (scXRD) study that identifies discrete adsorption sites within Ni-MOF-74/Ni-CPO-27, where SO2 is both chemisorbed and physisorbed while also probing competitive adsorption of SO2 of these sites when water is present. Further features of this site have been confirmed by variable SO2 pressure scXRD studies, DFT calculations, and IR studies.

Correction: Toxicity of metal-organic framework nanoparticles: from essential analyses to potential applications.

Correction for 'Toxicity of metal-organic framework nanoparticles: from essential analyses to potential applications' by Romy Ettlinger et al., Chem. Soc. Rev., 2022, 51, 464-484, https://doi.org/10.1039/D1CS00918D.

Exploring cation distribution in ion-exchanged Al,Ga-containing metal-organic frameworks using 17O NMR spectroscopy.

A mixed-metal metal-organic framework, (Al,Ga)-MIL-53, synthesised by post-synthetic ion exchange has been investigated using solid-state nuclear magnetic resonance (NMR) spectroscopy, microscopy and energy dispersive X-ray (EDX) spectroscopy. 17O enrichment during the ion-exchange process enables site specific information on the metal distribution to be obtained. Within this work two ion-exchange processes have been explored. In the first approach (exchange of metals in the framework with dissolved metal salts), 17O NMR spectroscopy reveals the formation of crystallites with a core-shell structure, where the cation exchange takes place on the surface of these materials forming a shell with a roughly equal ratio of Al3+ and Ga3+. For the second approach (exchange of metals between two frameworks), no core-shell structure is observed, and instead crystallites containing a majority of Al3+ are obtained with lower levels of Ga3+. Noticeably, these particles show little variation in the metal cation distribution between crystallites, a result not previously observed for bulk (Al,Ga)-MIL-53 materials. In all cases where ion exchange has taken place NMR spectroscopy reveals a slight preference for clustering of like cations.

Computational NMR investigation of mixed-metal (Al,Sc)-MIL-53 and its phase transitions.

Compositionally complex metal-organic frameworks (MOFs) have properties that depend on local structure that is often difficult to characterise. In this paper a density functional theory (DFT) computational study of mixed-metal (Al,Sc)-MIL-53, a flexible MOF with several different forms, was used to calculate the relative energetics of these forms and to predict NMR parameters that can be used to evaluate whether solid-state NMR spectroscopy can be used to differentiate, identify and characterise the forms adopted by mixed-metal MOFs of different composition. The NMR parameters can also be correlated with structural features in the different forms, giving fundamental insight into the nature and origin of the interactions that affect nuclear spins. Given the complexity of advanced NMR experiments required, and the potential need for expensive and difficult isotopic enrichment, the computational work is invaluable in predicting which experiments and approaches are likely to give the most information on the disorder, local structure and pore forms of these mixed-metal MOFs.

Toxicity of metal-organic framework nanoparticles: from essential analyses to potential applications.

In the last two decades, the field of metal-organic frameworks (MOFs) has exploded, and MOF nanoparticles in particular are being investigated with increasing interest for various applications, including gas storage and separation, water harvesting, catalysis, energy conversion and storage, sensing, diagnosis, therapy, and theranostics. To further pave their way into real-world applications, and to push the synthesis of MOF nanoparticles that are 'safe-and-sustainable-by-design', this tutorial review aims to shed light on the importance of a systematic toxicity assessment. After clarifying and working out the most important terms and aspects from the field of nanotoxicity, the current state-of-the-art of in vitro and in vivo toxicity studies of MOF nanoparticles is evaluated. Moreover, the key aspects affecting the toxicity of MOF nanoparticles such as their chemical composition, their physico-chemical properties, including their colloidal and chemical stability, are discussed. We highlight the need of more targeted synthesis of MOF nanoparticles that are 'safe-and-sustainable-by-design', and their tailored hazard assessment in the context of their potential applications in order to tap the full potential of this versatile material class in the future.

Synthesis of FeAPO-34 Molecular Sieve under Ionothermal Condition.

FeAPO-34 with a chabazite (CHA) topology structure was successfully synthesized under ionothermal conditions using 1-ethyl-3-methylimidazoliumchloride (EMIMCl) ionic liquid in the presence of ethylenediamine (EDA). The material was characterized using powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), elemental analyses, and solid-state NMR spectroscopy. Incorporation of iron within the covalent framework of the material was confirmed by the presence of broad signals between 1000 and 14,000 ppm in the 31P NMR spectrum, corresponding to the P(OFe)x(OAl)4-x species.

A structural investigation of organic battery anode materials by NMR crystallography.

Conjugated alkali metal dicarboxylates have recently received attention for applications as organic anode materials in lithium- and sodium-ion batteries. In order to understand and optimise these materials, it is important to be able to characterise both the long-range and local aspects of the crystal structure, which may change during battery cycling. Furthermore, some materials can display polymorphism or hydration behaviour. NMR crystallography, which combines long-range crystallographic information from diffraction with local information from solid-state NMR via interpretation aided by DFT calculations, is one such approach, but this has not yet been widely applied to conjugated dicarboxylates. In this work, we evaluate the application of NMR crystallography for a set of model lithium and sodium dicarboxylate salts. We investigate the effect of different DFT geometry optimisation strategies and find that the calculated NMR parameters are not systematically affected by the choice of optimisation method, although the inclusion of dispersion correction schemes is important to accurately reproduce the experimental unit cell parameters. We also observe hydration behaviour for two of the sodium salts and provide insight into the structure of an as-yet uncharacterised structure of sodium naphthalenedicarboxylate. This highlights the importance of sample preparation and characterisation for organic sodium-ion battery anode materials in particular.

Controlled Synthesis of Large Single Crystals of Metal-Organic Framework CPO-27-Ni Prepared by a Modulation Approach: In situ Single-Crystal X-ray Diffraction Studies.

The size of single crystals of the metal-organic framework CPO-27-Ni was incrementally increased through a series of modulated syntheses. A novel linker modulated synthesis using 2,5-dihydroxyterephthalic acid and the isomeric ligand 4,6-dihydroxyisophthalic acid yielded large single crystals of CPO-27-Ni (∼70 µm). All materials were shown to have high crystallinity and phase purity through powder X-ray diffraction, electron microscopy methods, thermogravimetry, and compositional analysis. For the first time single-crystal structure analyses were carried out on CPO-27-Ni. High BET surface areas and nitric oxide (NO) release efficiencies were recorded for all materials. Large single crystals of CPO-27-Ni showed a prolonged NO release and proved suitable for in situ single-crystal diffraction experiments to follow the NO adsorption. An efficient activation protocol was developed, leading to a dehydrated structure after just 4 h, which subsequently was NO-loaded, leading to a first NO loaded single-crystal structural model of CPO-27-Ni.

Solvothermal Synthesis of a Novel Calcium Metal-Organic Framework: High Temperature and Electrochemical Behaviour.

The rapid growth in the field of metal-organic frameworks (MOFs) over recent years has highlighted their high potential in a variety of applications. For biological and environmental applications MOFs with low toxicity are vitally important to avoid any harmful effects. For this reason, Ca-based MOFs are highly desirable owing to their low cost and high biocompatibility. Useful Ca MOFs are still rare owing to the ionic character and large size of the Ca2+ ion tending to produce dense phases. Presented here is a novel Ca-based MOF containing 2,3-dihyrdoxyterephthalate (2,3-dhtp) linkers Ca(2,3-dhtp)(H2O) (SIMOF-4). The material undergoes a phase transformation on heating, which can be followed by variable temperature powder X-ray diffraction. The structure of the high temperature form was obtained using single-crystal X-ray diffraction. The electrochemical properties of SIMOF-4 were also investigated for use in a Na ion battery.

Following the unusual breathing behaviour of 17O-enriched mixed-metal (Al,Ga)-MIL-53 using NMR crystallography.

The breathing behaviour of 17O-enriched (Al,Ga)-MIL-53, a terephthalate-based metal-organic framework, has been investigated using a combination of solid-state nuclear magnetic resonance (NMR) spectroscopy, powder X-ray diffraction (PXRD) and first-principles calculations. These reveal that the behaviour observed for as-made, calcined, hydrated and subsequently dehydrated mixed-metal MIL-53 materials differs with composition, but cannot be described as the compositionally weighted average of the breathing behaviour seen for the two end members. Although the form of MIL-53 adopted by the as-made material is independent of metal composition, upon calcination, materials with higher levels of Al adopt an open pore (OP) form, as found for the Al end member, but substitution of Ga results in mixed pore materials, with OP and narrow pore (NP) forms co-existing. Although the Ga end member is prone to decomposition under the calcination conditions used, a low level of Al in the starting synthesis (5%) leads to an OP mixed-metal MOF that is stable to calcination. Upon hydration, all materials almost exclusively adopt a closed pore (CP) structure, with strong hydrogen bonding interactions with water leading to two distinct resonances from the carboxylate oxygens in 17O NMR spectra. When dehydrated, different framework structures are found for the two end members, OP for Al-MIL-53 and NP for Ga-MIL-53, with the proportion of NP MOF seen to increase systematically with the Ga content in mixed-metal materials, in contrast to the forms seen upon initial calcination. 17O NMR spectra of mixed-metal MIL-53 materials show an increased preference for clustering of like cations as the Ga content increases. This is not a result of the small-scale dry gel conversion reactions used for enrichment, as a similar cation distribution and clustering is also observed for (Al0.5,Ga0.5)-MIL-53 synthesised hydrothermally and enriched with 17O via post-synthetic steaming.

Kinetics and Mechanism of the Hydrolysis and Rearrangement Processes within the Assembly-Disassembly-Organization-Reassembly Synthesis of Zeolites.

The hydrolysis (disassembly, D) and rearrangement (organization, O) steps of the assembly-disassembly-organization-reassembly (ADOR) process for the synthesis of zeolites have been studied. Germanium-rich UTL was subjected to hydrolysis conditions in water to understand the effects of temperature (100, 92, 85, 81, 77, and 70 °C). Samples were taken periodically over an 8-37 h period, and each sample was analyzed by powder X-ray diffraction. The results show that the hydrolysis step is solely dependent on the presence of liquid water, whereas the rearrangement is dependent on the temperature of the system. The kinetics have been investigated using the Avrami-Erofeev model. With increasing temperature, an increase in the rate of reaction for the rearrangement step was observed, and the Arrhenius equation was used to ascertain an apparent activation energy for the rearrangement from the kinetic product of the disassembly (IPC-1P) to the thermodynamic product of the rearrangement (IPC-2P). From this information, a mechanism for this transformation can be postulated.

13C pNMR of "crumple zone" Cu(II) isophthalate metal-organic frameworks.

NMR spectroscopy of paramagnetic materials (pNMR) has the potential to provide great structural insight, but many challenges remain in interpreting the spectra in detail. This work presents a study of a series of structurally analogous metal-organic frameworks (MOFs) based on 5-substituted isophthalate linkers and Cu(II) paddlewheel dimers, of interest owing to their "crumple zone" structural rearrangement on dehydration/rehydration. 13C MAS NMR spectra reveal a wide variation in the observed resonance position for chemically similar C species in the different MOFs but, despite this, resonances are overlapped in several cases. However, by considering a combination of the integration of quantitative spectra, the resonance position as a function of temperature and T1 relaxation measurements, the spectra can be fully assigned. It is also demonstrated that the prototypical MOF in this series, STAM-1, displays a crumple zone rearrangement on dehydration, similar to the well-characterised 5-ethoxyisophthalate MOF (STAM-17-OEt) although, while the materials have similar local C environments, dehydrated STAM-1 exhibits less long-range order.

Metal-Organic Framework-Activated Carbon Composite Materials for the Removal of Ammonia from Contaminated Airstreams.

Metal-organic frameworks (MOFs) are a class of porous materials that show promise in the removal of toxic industrial chemicals (TICs) from contaminated airstreams, though their development for this application has so far been hindered by issues of water stability and the wide availability and low cost of traditionally used activated carbons. Here a series of three MOF-activated carbon composite materials with different MOF to carbon ratios are prepared by growing STAM-17-OEt crystals inside the commercially available BPL activated carbon. The composite materials display excellent water stability and increased uptake of ammonia gas when compared to unimpregnated carbon. Such properties make these composites very promising in the fields of air purification and personal protective equipment.

Coordination change, lability and hemilability in metal-organic frameworks.

Metal-organic frameworks (MOFs) are some of the most exciting materials in current science. Their utility and diversity of applications depends on a combination of their chemistry, their framework topology and the spatial dimensions of their pores. In this review we concentrate on the chemistry of MOFs. Specifically we bring together many aspects of MOFs that underpin their stability, reactivity and dynamic behaviour within a common theme related to (changes in) metal-ligand bonding. In each area we provide examples to illustrate the behaviour and discuss it in the context of metal lability and coordination changes. Starting with flexible behaviour in which metal-linker bonds undergo deformation rather than cleavage, we then consider coordination changes that lead to open metal sites, changes in framework topology, framework dimensionality or degree of network interpenetration. We show how these changes are linked to development of new properties, including changes in magnetic behaviour, gas adsorption characteristics, construction of composite MOFs and amorphous MOFs, as well as providing new synthetic routes for MOF preparation. We discuss how the lability of the species that make up the MOFs can affect aspects from their synthesis to the possibility of metal and linker exchange reactions that may lead to defects and disorder. The final section reviews hemilability in MOFs, where regions of different chemical behaviour within MOFs can lead to unusual properties, such as self-accelerating and ultraselective adsorption.

Synthesis, Isotopic Enrichment, and Solid-State NMR Characterization of Zeolites Derived from the Assembly, Disassembly, Organization, Reassembly Process.

The great utility and importance of zeolites in fields as diverse as industrial catalysis and medicine has driven considerable interest in the ability to target new framework types with novel properties and applications. The recently introduced and unconventional assembly, disassembly, organization, reassembly (ADOR) method represents one exciting new approach to obtain solids with targeted structures by selectively disassembling preprepared hydrolytically unstable frameworks and then reassembling the resulting products to form materials with new topologies. However, the hydrolytic mechanisms underlying such a powerful synthetic method are not understood in detail, requiring further investigation of the kinetic behavior and the outcome of reactions under differing conditions. In this work, we report the optimized ADOR synthesis, and subsequent solid-state characterization, of 17O- and doubly 17O- and 29Si-enriched UTL-derived zeolites, by synthesis of 29Si-enriched starting Ge-UTL frameworks and incorporation of 17O from 17O-enriched water during hydrolysis. 17O and 29Si NMR experiments are able to demonstrate that the hydrolysis and rearrangement process occurs over a much longer time scale than seen by diffraction. The observation of unexpectedly high levels of 17O in the bulk zeolitic layers, rather than being confined only to the interlayer spacing, reveals a much more extensive hydrolytic rearrangement than previously thought. This work sheds new light on the role played by water in the ADOR process and provides insight into the detailed mechanism of the structural changes involved.

Expansion of the ADOR Strategy for the Synthesis of Zeolites: The Synthesis of IPC-12 from Zeolite UOV.

The assembly-disassembly-organization-reassembly (ADOR) process has been used to disassemble a parent zeolite with the UOV structure type and then reassemble the resulting layers into a novel structure, IPC-12. The structure of the material has previously been predicted computationally and confirmed in our experiments using X-ray diffraction and atomic resolution STEM-HAADF electron microscopy. This is the first successful application of the ADOR process to a material with porous layers.

Post-Synthesis Stabilization of Germanosilicate Zeolites ITH, IWW, and UTL by Substitution of Ge for Al.

Germanosilicate zeolites often suffer from low hydrothermal stability due to the high content of Ge. Herein, we investigated the post-synthesis introduction of Al accompanied by stabilization of selected germanosilicates by degermanation/alumination treatments. The influence of chemical composition and topology of parent germanosilicate zeolites (ITH, IWW, and UTL) on the post-synthesis incorporation of Al was studied. Alumination of ITH (Si/Ge=2-13) and IWW (Si/Ge=3-7) zeolites resulted in the partial substitution of Ge for Al (up to 80 %), which was enhanced with a decrease of Ge content in the parent zeolite. In contrast, in extra-large pore zeolite UTL (Si/Ge=4-6) the hydrolysis of the interlayer Ge-O bonds dominated over substitution. The stabilization of zeolite UTL was achieved using a novel two-step degermanation/alumination procedure by the partial post-synthesis substitution of Ge for Si followed by alumination. This new method of stabilization and incorporation of strong acid sites may extend the utilization of germanosilicate zeolites, which has been until now been limited.

Coordination polymers of 5-substituted isophthalic acid.

The synthesis and characterisation of five coordination polymers - Ni2(mip)2(H2O)8·2H2O (1), Zn6(mip)5(OH)2(H2O)4·7.4H2O (2), Zn6(mip)5(OH)2(H2O)2·4H2O (3), Mn(HMeOip)2 (4), and Mn3(tbip)2(Htbip)2(EtOH)2 (5) - are reported. Preliminary nitric oxide release data on compounds 2 and 3 are also given.

Crystal structure resolution of two different chlorhexidine salts.

Two salts of the chlorhexidine di-cation (H2CHx2+) - (H2CHx)(SO4)·3H2O and (H2CHx)(CO3)·4H2O - have been synthesised and characterised crystallographically.

The ADOR mechanism for the synthesis of new zeolites.

A novel methodology, called ADOR (assembly-disassembly-organisation-reassembly), for the synthesis of zeolites is reviewed here in detail. The ADOR mechanism stems from the fact that certain chemical weakness against a stimulus may be present in a zeolite framework, which can then be utilized for the preparation of new solids through successive manipulation of the material. In this review, we discuss the critical factors of germanosilicate zeolites required for application of the ADOR protocol and describe the mechanism of hydrolysis, organisation and condensation to form new zeolites starting from zeolite UTL. Last but not least, we discuss the potential of this methodology to form other zeolites and the prospects for future investigations.