NMR crystallography to probe the breathing effect of the MIL-53(Al) metal–organic framework using solid-state NMR measurements of 13C–27Al distances

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.

Terahertz Spectroscopy Unambiguously Determines the Orientation of Guest Water Molecules in a Structurally Elusive Metal-Organic Framework

Porous materials, particularly metal-organic frameworks (MOFs), hold great promise for advanced applications. MIL-53(Al) is an exceptionally well-studied MOF that exhibits a phase transition upon guest capture─in this case, water─resulting in a dramatic change in the pore volume. Despite extensive studies, the structure of the water-loaded narrow-pore phase, MIL-53(Al)-np, remains controversial, particularly with respect to the positions of the adsorbed water molecules. We use terahertz spectroscopy, coupled with powder X-ray diffraction and density functional theory simulations, to unambiguously resolve this controversy. We show that the low-frequency (<100 cm-1) vibrational spectrum depends on weak long-range forces that are extremely sensitive to the orientation of the adsorbed water molecules. This enables definitively determining the correct structure of MIL-53(Al)-np while highlighting the extreme sensitivity of terahertz spectroscopy to bulk structure, suggesting its potential as a robust complement to X-ray diffraction for precise characterization of host-guest complexes.

Computational Protocol for the Spectral Assignment of NMR Resonances in Covalent Organic Frameworks

Solid-state nuclear magnetic resonance spectroscopy is routinely used in the field of covalent organic frameworks to elucidate or confirm the structure of the synthesized samples and to understand dynamic phenomena. Typically this involves the interpretation and simulation of the spectra through the assumption of symmetry elements of the building units, hinging on the correct assignment of each line shape. To avoid misinterpretation resulting from library-based assignment without a theoretical basis incorporating the impact of the framework, this work proposes a first-principles computational protocol for the assignment of experimental spectra, which exploits the symmetry of the underlying building blocks for computational feasibility. In this way, this protocol accommodates the validation of previous experimental assignments and can serve to complement new NMR measurements.

Surface Hydrophobicity and Guest Permeability in Polydimethylsiloxane-Coated MIL-53 as Studied by Solid-State Nuclear Magnetic Resonance Spectroscopy

Experimental characterization of the hydrophobic porous materials at the atomic and molecular levels is of great significance, but exploring their hydrophobicity characteristics and interactions with guest molecules with distinct polarity is still challenging. In this work, solid-state NMR is employed to characterize the surface hydrophobicity and explore the guest solvent permeability in polydimethylsiloxane (PDMS)-coated MIL-53. It was found that the PDMS-coated MIL-53 was hydrophobic to water and infiltrated to methanol, acetone, benzene, toluene, and ethylbenzene solvents. In addition, two types of guest solvents (methanol, acetone, benzene, toluene, and ethylbenzene), inside the pore and outside the pore of PDMS-coated MIL-53, were clearly identified using two-dimensional 1H-1H homo-nuclear correlation NMR experiments. Moreover, the membrane thickness of the PDMS-coated MIL-53 could be determined from the analysis of the 1H-1H spin diffusion buildup curves. Furthermore, the permeability of benzene, toluene, and ethylbenzene at different PDMS coating levels was extracted from 1H MAS NMR. The increase of the hydrophobic PDMS layer resulted in a decrease of the penetration of aromatic guests to the internal pore of MIL-53. This work provides deep insights into the understanding of guest solvent permeability of hydrophobic layer-coated MOFs in the application fields of catalysis and separation.

A critical review on the synthesis of NH2-MIL-53(Al) based materials for detection and removal of hazardous pollutants

Nowadays, emerging hazardous pollutants have caused many harmful effects on the environment and human health, calling for the state of the art methods for detection, qualification, and treatment. Metal-organic frameworks are porous, flexible, and versatile materials with unique structural properties, which can solve such problems. In this work, we reviewed the synthesis, activation, and characterization, and potential applications of NH₂-MIL-53(Al). This material exhibited intriguing breathing effects, and obtained very high surface areas (182.3-1934 m²/g) with diverse morphologies. More importantly, NH₂-MIL-53(Al) based materials could be used for the detection and removal of various toxic pollutants such as organic dyes, pharmaceuticals, herbicides, insecticides, phenols, heavy metals, and fluorides. We shed light on plausible adsorption mechanisms such as hydrogen bonds, π-π stacking interactions, and electrostatic interactions onto NH₂-MIL-53(Al) adsorbents. Interestingly, NH₂-MIL-53(Al) based adsorbents could be recycled for many cycles with high stability. This review also recommended that NH₂-MIL-53(Al) based materials can be a good platform for the environmental remediation fields.

Application of solid-state NMR techniques for structural characterization of metal-organic frameworks

Solid-state NMR can afford the structural information about the chemical composition, local environment, and spatial coordination at the atomic level, which has been extensively applied to characterize the detailed structure and host-guest interactions in metal-organic frameworks (MOFs). In this review, recent advances for the structural characterizations of MOFs using versatile solid-state NMR techniques were briefly introduced. High-field sensitivity-enhanced solid-state NMR method enabled the direct observation of metal centers in MOFs containing low-γ nuclei. Two-dimensional (2D) homo- and hetero-nuclear correlation MAS NMR experiments provided the spatial proximity among linkers, metal clusters and the introduced guest molecules. Moreover, quantitative measurement of inter-nuclear distances using solid-state NMR provided valuable structural information about the connectivity geometry as well as the host-guest interactions within MOFs. Furthermore, solid-state NMR has exhibited great potential for unraveling the structure property of MOFs containing paramagnetic metal centers.

Crystal Engineering of Supramolecular 1,4-Benzene Bisamides by Side-Chain Modification - Towards Tuneable Anisotropic Morphologies and Surfaces

Benzene bisamides are promising building blocks for supramolecular nano‐objects. Their functionality depends on morphology and surface properties. However, a direct link between surface properties and molecular structure itself is missing for this material class. Here, we investigate this interplay for two series of 1,4‐benzene bisamides with symmetric and asymmetric peripheral substitution. We elucidated the crystal structures, determined the nano‐object morphologies and derived the wetting behaviour of the preferentially exposed surfaces. The crystal structures were solved by combining single‐crystal and powder X‐ray diffraction, solid‐state NMR spectroscopy and computational modelling. Bulky side groups, here t‐butyl groups, serve as a structure‐directing motif into a packing pattern, which favours the formation of thin platelets. The use of slim peripheral groups on both sides, in our case linear perfluorinated, alkyl chains, self‐assemble the benzene bisamides into a second packing pattern which leads to ribbon‐like nano‐objects. For both packing types, the preferentially exposed surfaces consist of the ends of the peripheral groups. Asymmetric substitution with bulky and slim groups leads to an ordered alternating arrangement of the groups exposed to the surface. This allows the hydrophobicity of the surfaces to be gradually altered. We thus identified two leitmotifs for molecular packings of benzene bisamides providing the missing link between the molecular structure, the anisotropic morphologies and adjustable surface properties of the supramolecular nano‐objects.

17 O solid-state NMR at ultrahigh magnetic field of 35.2 T: Resolution of inequivalent oxygen sites in different phases of MOF MIL-53(Al)

MIL-53(Al) is a member of the most extensively studied metal-organic framework (MOF) families owing to its "flexible" framework and superior stability. 17 O solid-state NMR (SSNMR) spectroscopy is an ideal site-specific characterization tool as it probes local oxygen environments. Because oxygen local structure is often altered during phase change, 17 O SSNMR can be used to follow phase transitions. However, 17 O is a challenging nucleus to study via SSNMR due to its low sensitivity and resolution arising from the very low natural abundance of 17 O isotope and its quadrupolar nature. In this work, we describe that by using 17 O isotopic enrichment and performing 17 O SSNMR experiments at an ultrahigh magnetic field of 35.2 T, all chemically and crystallographically inequivalent oxygen sites in two representative MIL-53(Al) (as-made and water adsorbed) phases can be completely resolved. The number of signals in each phase is consistent with that predicted from the space group refined from powder X-ray diffraction data. The 17 O 1D magic-angle spinning (MAS) and 2D triple-quantum MAS (3QMAS) spectra at 35.2 T furnish fine information about the host-guest interactions and the structural changes associated with phase transition. The ability to completely resolve multiple chemically and crystallographically inequivalent oxygen sites in MOFs at very high magnetic field, as illustrated in this work, significantly enhances the potential for using the NMR crystallography approach to determine crystal structures of new MOFs and verify the structures of existing MOFs obtained from refining powder X-ray diffraction data.

Solid-state NMR spectroscopy as a powerful tool to investigate the location of fluorinated lipids in highly porous hybrid organic-inorganic nanoparticles

Nanosized metal-organic frameworks (nanoMOFs) have emerged as a new class of biodegradable and nontoxic nanomaterials of high interest for biomedical applications thanks to the possibility to load large amounts of a wide variety of therapeutic molecules in their porous structure. The surface of the highly porous nanoMOFs is usually engineered to increase their colloidal stability, to tune their interactions with the biological environment, and to allow targeting specific cells or organs. However, the atomic-scale analysis of these complex core-shell materials is highly challenging. In this study, we report the investigation of aluminum-based nanoMOFs containing two fluorinated lipids by solid-state NMR spectroscopy, including ²⁷ Al, ¹ H and ¹⁹ F MAS NMR. The ensemble of NMR data provides a better understanding of the localization and conformation of the fluorinated lipids inside the pores or on the nanoMOF surface.

Natural abundance oxygen-17 solid-state NMR of metal organic frameworks enhanced by dynamic nuclear polarization

The 17O resonances of zirconium-oxo clusters that can be found in porous Zr carboxylate metal-organic frameworks (MOFs) have been investigated by magic-angle spinning (MAS) NMR spectroscopy enhanced by dynamic nuclear polarization (DNP). High-resolution 17O spectra at 0.037% natural abundance could be obtained in 48 hours, thanks to DNP enhancement of the 1H polarization by factors ε(1H) = Swith/Swithout = 28, followed by 1H → 17O cross-polarization, allowing a saving in experimental time by a factor of ca. 800. The distinct 17O sites from the oxo-clusters can be resolved at 18.8 T. Their assignment is supported by density functional theory (DFT) calculations of chemical shifts and quadrupolar parameters. Protonation of 17O sites seems to be leading to large characteristic shifts. Hence, natural abundance 17O NMR spectra of diamagnetic MOFs can thus be used to probe and characterize the local environment of different 17O sites on an atomic scale.

Gallium Species Incorporated into MOF Structure: Insight into the Formation of a 3D Polycrystalline Gallium-Imidazole Framework

The formation of a polycrystalline 3D gallium-imidazole framework (MOF) was closely studied in three steps using ssNMR, XRPD, and TGA. In all steps, the reaction products show relatively high temperature stability up to 500 °C. The final product was examined by structural analysis using NMR crystallography combined with TG and BET analyses, which enabled a detailed characterization of the polycrystalline MOF system on the atomic-resolution level. ⁷¹Ga ssNMR spectra provided valuable structural information on the coexistence of several distinct gallium species, including a tunable liquid phase. Moreover, using an NMR crystallography approach, two structurally asymmetric units of Ga(Im₆)^6- incorporated into the thermally stable polycrystalline 3D matrix were identified. Prepared polycrystalline MOF material with polymorphic gallium species is promising for use in catalytic processes.

Molecular Mechanism of Crystalline-to-Amorphous Conversion of Pharmaceutical Solids from 19F Magic Angle Spinning NMR

Crystalline and amorphous materials usually possess distinct physicochemical properties due to major variations in long-range and local molecular packings. Enhanced fundamental knowledge of the molecular details of crystalline-to-amorphous interconversions is necessary to correlate the intermolecular structure to material properties and functions. While crystal structures can be readily obtained by X-ray crystallography, the microstructure of amorphous materials has rarely been explored due to a lack of high-resolution techniques capable of probing local molecular structures. Moreover, there is increasing interest in understanding the molecular nature of amorphous solids in pharmaceutical sciences due to the widespread utilization of amorphous active pharmaceutical ingredients (APIs) in pharmaceutical development for solubility and bioavailability enhancement. In this study, we explore multidimensional ¹³C and ¹⁹F magic angle spinning (MAS) NMR spectroscopy to study the molecular packing of amorphous posaconazole (POSA) in conjunction with the crystalline counterpart. Utilizing methods integrating homonuclear and heteronuclear ¹H, ¹³C, and ¹⁹F correlation spectroscopy and atomic ¹⁹F-to-¹³C distance measurements, we identified the major differences in molecular packing between crystalline and amorphous POSA. The intermolecular "head-to-head" interaction along the molecule's major axis, as well as the "head-to-tail" molecular packing perpendicular to the major axis in POSA crystals, was recapitulated by MAS NMR. Furthermore, critical intermolecular distances in the crystal lattice were determined. Most importantly, the head-to-tail contact of two neighboring molecules was found to be preserved in amorphous POSA, suggesting localized molecular order, whereas crucial interactions for head-to-head packing are absent in the amorphous form resulting in long-range disorder. Our study, likely one of the first documented examples, provides molecular-level structural details to understand the molecular mechanism of crystalline-to-amorphous conversion of fluorine-containing drug substances occurring in drug processing and development and establish a high-resolution experimental protocol for investigating amorphous materials.

New frontiers for solid-state NMR across the periodic table: a snapshot of modern techniques and instrumentation

Selected highlights of the recent literature on solid-state NMR of some of the lesser studied nuclei are provided. The roles of ultrahigh magnetic fields, radiofrequency pulse sequences, dynamic nuclear polarization, isotopic enrichment, and nuclear quadrupole resonance in opening up the periodic table to in-depth study are discussed.

Facile synthesis of Fe3O4@MOF-100(Fe) magnetic microspheres for the adsorption of diclofenac sodium in aqueous solution

In this research, the adsorptive removal of diclofenac sodium, one of the representative pharmaceuticals and personal care products, from aqueous solution using Fe₃O₄@MOF-100(Fe) magnetic microspheres was studied for the first time. The Fe₃O₄@MOF-100(Fe) microspheres exhibit strong magnetism and stability, which were observed as a core-shell structure. The maximum adsorption capacity of Fe₃O₄@MOF-100(Fe) for diclofenac sodium can reach 377.36 mg L^-1, which was higher than most of the adsorbents reported. The adsorption kinetics follows the pseudo-second-order kinetic equation. And the adsorption equilibrium of DCF can be described with Langmuir isotherm. In the cycle experiment, Fe₃O₄@MOF-100(Fe) material performed high adsorption efficiency for low-concentration diclofenac sodium solution, and the removal rate can still reach 80% after 5 cycles of adsorption without desorption. The mechanisms including electrostatic interaction, H-bond interaction, and π-π interaction that coexisted in the adsorption processes would be of benefit to enhance the adsorption capacity. The Fe₃O₄@MOF-100(Fe) magnetic microspheres offer exciting opportunities for further application.

Observation of proximities between spin-1/2 and quadrupolar nuclei in solids: Improved robustness to chemical shielding using adiabatic symmetry-based recoupling

We introduce a novel heteronuclear dipolar recoupling based on the R2₁^-1 symmetry, which uses the tanh/tan (tt) shaped pulse as a basic inversion element and is denoted R2₁^-1(tt). Using first-order average Hamiltonian theory, we show that this sequence is non-γ-encoded and that it reintroduces the |m| = 1 spatial component of the Chemical Shift Anisotropy (CSA) of the irradiated isotope and its heteronuclear dipolar interactions. Using numerical simulations and one-dimensional (1D) ²⁷Al-{³¹P} through-space D-HMQC (Dipolar Heteronuclear Multiple-Quantum Correlation) experiments on VPI-5, we compare the performances of this recoupling to those of other non-γ-encoded |m| = 1 heteronuclear recoupling schemes: REDOR (Rotational-Echo DOuble Resonance), SFAM (Simultaneous Frequency and Amplitude Modulation) and R4₂^-1(tt). Such comparison indicates that the R2₁^-1(tt) scheme is more robust to CSA, offset and radiofrequency field inhomogeneities than the other schemes. We take advantage of the high robustness of R2₁^-1(tt) to CSA and offset to demonstrate the possibility to correlate the signals of ²⁰⁷Pb isotope with those of neighboring half-integer spin quadrupolar nuclei. Such approach is demonstrated experimentally by acquiring ¹¹B-{²⁰⁷Pb} D-HMQC 2D spectra of Pb₄O(BO₃)₂ crystalline powder.

Host-guest interaction of styrene and ethylbenzene in MIL-53 studied by solid-state NMR

Solid-state NMR was utilized to explore the host-guest interaction between adsorbate and adsorbent at atomic level to understand the separation mechanism of styrene (St) and ethylbenzene (EB) in MIL-53(Al). ¹³C-²⁷Al double-resonance NMR experiments revealed that the host-guest interaction between St and MIL-53 was much stronger than that of EB adsorption. In addition, ¹³C DIPSHIFT experiments suggested that the adsorbed St was less mobile than EB confined inside the MIL-53 pore. Furthermore, the host-guest interaction model between St, EB and MIL-53 was established on the basis of the spatial proximities information extracted from 2D ¹H-¹H homo-nuclear correlation NMR experiments. According to the experimental observation from solid-state NMR, it was found that the presence of π-π interaction between St and MIL-53 resulted in the stronger host-guest interaction and less mobility of St. This work provides direct experimental evidence for understanding the separation mechanism of St and EB using MIL-53 as an adsorbent.

Structural aspects of displacive transformations: what can optical microscopy contribute? Dehydration of Sm2(C2O4)3·10H2O as a case study

For martensitic transformations the macroscopic crystal strain is directly related to the corresponding structural rearrangement at the microscopic level. In situ optical microscopy observations of the interface migration and the change in crystal shape during a displacive single crystal to single crystal transformation can contribute significantly to understanding the mechanism of the process at the atomic scale. This is illustrated for the dehydration of samarium oxalate decahydrate in a study combining optical microscopy and single-crystal X-ray diffraction.