Solvent Exchange Dynamics in M2(dobdc): An Interplay among Binding Strength, Exchange Kinetics, and Cooperativity.

Solvent exchange is a crucial step in ensuring the complete activation of metal-organic frameworks (MOFs); however, the conditions for solvent exchange vary among MOFs, even within the isostructural variants. This study examines the factors contributing to solvent exchange by investigating the isostructural M2(dobdc) (M═Mg, Co, Zn) series. Common solvents N,N-dimethylformamide (DMF), ethanol (EtOH), and methanol (MeOH) are employed to assess the solvent exchange at coordinatively unsaturated sites (CUS) of M2(dobdc). By monitoring both solvents released from the MOF during solvent exchange and the coordination environment of metals within the MOF, a picture is constructed of exchange rates during early stages of solvent exchange as well as expulsion of the last traces of bound solvents. This differentiation is achieved by a combination of bulk monitoring of solvent phase composition and microscopic application of Raman spectroscopy on the single-crystal level. The kinetics of solvent replacement is revealed to have a substantial contribution from cooperativity; this phenomenon is observed in both the forward and reverse directions. Thermogravimetric analysis coupled with IR spectroscopy and density functional theory (DFT) calculations are employed to elucidate the relationship between solvent exchange rates and solvent binding energy. The solvent exchange rates are determined by the kinetic barriers of solvent exchange that do not follow the order of the solvent binding affinity. This work contributes to understanding the solvent exchange of MOFs by examining the interplay among the binding strength, exchange kinetics, and cooperativity. It further provides valuable insights for scrutinizing MOF activation protocols.

An evaluation of resolution, accuracy, and precision in FT-IR spectroscopy.

Infrared spectroscopy is a foundational technique for the elucidation of chemical structures. The advancements in interferometric spectroscopy, and specifically the development of Fourier transform infrared (FT-IR) spectroscopy, are responsible for the widespread usage of IR spectrometers ranging from teaching labs to pharmaceutical quality control. FT-IR affords an excellent signal-to-noise ratio that permits sensitive sampling with quantitative accuracy and high wavenumber precision based on well documented advantages (Jacquinot, Fellgett, Connes). However, the effect of resolution and instrument-to-instrument variation on wavenumber accuracy is not well understood, with previous work grossly overestimating error. Here, a recommendation of wavenumber accuracy as a function of spectral resolution, accounting for instrument variation among leading manufacturers, is given based on an experimental study of polystyrene and acetaminophen. For peaks that are well resolved and not saturated, the position can be known within 1.1 cm-1 at a spectral resolution of 4 cm-1 or higher, and within 2.2 cm-1 at 8 cm-1 resolution. Other sources of variation are also discussed (e.g., poorly resolved peaks, peak saturation, water interference, spectral noise) to give general recommendations on when IR peak positions can be considered significantly different. Such guidelines are critical for interpreting subtle positional variations, as are often present in different crystal forms of pharmaceuticals.

Kidney stone growth through the lens of Raman mapping.

Bulk composition of kidney stones, often analyzed with infrared spectroscopy, plays an essential role in determining the course of treatment for kidney stone disease. Though bulk analysis of kidney stones can hint at the general causes of stone formation, it is necessary to understand kidney stone microstructure to further advance potential treatments that rely on in vivo dissolution of stones rather than surgery. The utility of Raman microscopy is demonstrated for the purpose of studying kidney stone microstructure with chemical maps at ≤ 1 µm scales collected for calcium oxalate, calcium phosphate, uric acid, and struvite stones. Observed microstructures are discussed with respect to kidney stone growth and dissolution with emphasis placed on < 5 µm features that would be difficult to identify using alternative techniques including micro computed tomography. These features include thin concentric rings of calcium oxalate monohydrate within uric acid stones and increased frequency of calcium oxalate crystals within regions of elongated crystal growth in a brushite stone. We relate these observations to potential concerns of clinical significance including dissolution of uric acid by raising urine pH and the higher rates of brushite stone recurrence compared to other non-infectious kidney stones.

Adsorption of Natural Gas in Metal-Organic Frameworks: Selectivity, Cyclability, and Comparison to Methane Adsorption.

Evaluation of metal-organic frameworks (MOFs) for adsorbed natural gas (ANG) technology employs pure methane as a surrogate for natural gas (NG). This approximation is problematic, as it ignores the impact of other heavier hydrocarbons present in NG, such as ethane and propane, which generally have more favorable adsorption interactions with MOFs compared to methane. Herein, using quantitative Raman spectroscopic analysis and Monte Carlo calculations, we demonstrate the adsorption selectivity of high-performing MOFs, such as MOF-5, MOF-177, and SNU-70, for a methane and ethane mixture (95:5) that mimics the composition of NG. The impact of selectivity on the storage and deliverable capacities of these adsorbents during successive cycles of adsorption and desorption, simulating the filling and emptying of an ANG tank, is also demonstrated. The study reveals a gradual reduction in the storage performance of MOFs, particularly with smaller pore volumes, due to ethane accumulation over long-term cycling, until a steady state is reached with substantially degraded storage performance.

Sensitizing Explosives Through Molecular Doping.

Cocrystallization assembles multicomponent crystals in defined ratios that are held together by intermolecular interactions. While cocrystals have seen extensive use in the pharmaceutical industry for solving issues with stability and solubility, extension to the field of energetic materials for improved properties has proven difficult. Predicting successful coformers remains a challenge for systems lacking well-understood synthons that promote reliable intermolecular assembly. Herein, an alternative method is investigated for altering energetic properties that operates in the absence of well-defined interactions by molecular doping. An impact sensitive primary explosive, cyanuric triazide (CTA), was selected as the dopant to test if less impact sensitive secondary explosives could gain increased sensitization to impact when CTA is inserted into their crystal lattices. Molecular doping was successful in sensitizing three melt-castable energetics: 2,4,6-trinitrotoluene (TNT), 2,4-dinitroanisole (DNAN), and 1,3,3-trinitroazetidine (TNAZ). CTA could also be incorporated as a stabilized inclusion to sensitize DNAN further. These results demonstrate how the judicious choice of dopant can lead to specific property improvements, providing a method for creating energetic materials with new properties to access metal-free primary explosives and physical hot spot models for explosive ignition.

Perchlorate-Free Energetic Oxidizers Enabled by Ionic Cocrystallization.

The search for a suitable replacement for the ubiquitous oxidizer ammonium perchlorate (AP) is a top priority to enable more sustainable solid rocket motors. The oxidizing salts ammonium nitrate (AN) and ammonium dinitramide (ADN) are regarded as potential green replacements for AP, but suffer from a plethora of handling and processing issues including poor stability and a needle-like crystal morphology which inhibits dense packing; these prevent their widespread use. In the present work, ionic cocrystallization is leveraged to produce the first cocrystals of these oxidizing salts with an energetic coformer and the first such cocrystals to maintain a positive oxygen balance. The azole-based energetic molecule 5,5'-dinitro-2H,2H'-3,3″-bi-1,2,4-triazole (DNBT) is successfully cocrystallized with AN to yield the cocrystal 2AN:DNBT. Differential scanning calorimetry data confirms that AN, which in its pure form suffers from a problematic solid-state phase transition, is stabilized in the cocrystal. Application of this cocrystallization strategy to ADN produces 2ADN:DNBT, which has the highest oxygen balance of any organic cocrystal.

Performance of brushite plaster as kidney stone phantoms for laser lithotripsy.

Artificial phantoms used in photothermal near-infrared laser lithotripsy research generally fail to mimic both the chemical and the physical properties of human stones. Though high-energy, 1 J pulses are capable of fracturing hard human stones into several large fragments along natural boundaries, similar behavior has not been observed in commonly used gypsum plasters like BegoStone. We developed a new brushite-based plaster formulation composed of ≈90% brushite that undergoes rapid fracture in the manner of human stones under fragmentation pulse regimes. Single-pulse (1 J) ablation crater volumes for phantoms were not significantly different from those of pure brushite stones. Control over crater volumes was demonstrated by varying phosphorous acid concentration in the plaster formulation. Fragmentation of cylindrical brushite phantoms was filmed using a high-speed camera which demonstrated rapid fragmentation in < 100 µs during the bubble expansion phase of a short pulse from a high-powered Ho:YAG laser (Lumenis Pulse 120 H). The rapid nature of observed fracture suggests increasing laser pulse energy by increasing laser pulse duration will not improve fragmentation performance of laser lithotripters. Brushite plaster phantoms are a superior alternative to gypsum plasters for laser lithotripsy research due to their better mimicry of stone composition, controllable single-pulse crater volumes, and fragmentation behavior.

In Situ Observation of Solvent Exchange Kinetics in a MOF with Coordinatively Unsaturated Sites.

Solvent exchange of synthesis solvent within metal-organic frameworks (MOFs) is an essential process for the activation of coordinatively unsaturated sites (CUS) to achieve an optimal surface area; activation of the CUS is required to exploit the versatile applications of MOFs. However, it is challenging to replace CUS-bound synthesis solvent prior to MOF activation, which can lead to a structural collapse and reduced surface area post-evacuation. Herein, we quantify the exchange behavior of a copper paddlewheel-based CUS-MOF (HKUST-1) in the presence of three different solvents: ethanol (EtOH), dichloromethane (DCM), and N,N-dimethylformamide (DMF). The DMF release profiles are monitored via in situ observation of the exchange solvent composition via 1H NMR and Raman spectroscopy at the macroscopic scale. Furthermore, the change in solvent within a single crystal is measured to directly elucidate the exchange behavior. We demonstrate the DMF release profile from HKUST-1 exhibits different rate laws depending on whether the solvent exchange occurs at the CUS or is purely diffusive through the pores. This contribution represents the first characterization of release from a CUS-MOF as a function exchange solvent and reveals that solvent exchange in a CUS-MOF is not diffusion-limited, but rather is limited by the solvent exchange kinetics at the metal center. Insights from this study can be generalized to the variety of copper-paddlewheel-based MOFs, informing best practices for solvent exchange.

Progress in Predicting Ionic Cocrystal Formation: The Case of Ammonium Nitrate.

In contrast to the mature predictive frameworks applied to neutral cocrystals, ionic cocrystals, those including an ion pair, are difficult to design. Furthermore, they are generally excluded categorically from studies which correlate specific molecular properties to cocrystal formation, leaving the prospective ionic cocrystal engineer with few clear avenues to success. Herein ammonium nitrate, an energetic oxidizing salt, is targeted for cocrystallization in a potential coformer group selected based on likely interactions with the nitrate ion as revealed in the Cambridge Structural Database; six novel ionic cocrystals were discovered. Molecular descriptors previously identified as being related to neutral cocrystal formation were examined across the screening group but showed no relationship with ionic cocrystal formation. High packing coefficient is shown to be a constant among the successful coformers in the set and is utilized to directly target two more successful coformers, bypassing the need for a large screening group.

Structural Modifications of Polyethylenimine to Control Drug Loading and Release Characteristics of Amorphous Solid Dispersions.

Crystalline drugs with low solubility have the potential to benefit from delivery in the amorphous form. The polymers used in amorphous solid dispersions (ASDs) influence their maximum drug loading, solubility, dissolution rate, and physical stability. Herein, the influence of hydrophobicity of crosslinked polyethylenimine (PEI) is investigated for the delivery of the BCS class II nonsteroidal anti-inflammatory drug flufenamic acid (ffa). Several synthetic variables for crosslinking PEI with terephthaloyl chloride were manipulated: solvent, crosslinking density, reactant concentration, solution viscosity, reaction temperature, and molecular weight of the hyperbranched polymer. Benzoyl chloride was employed to cap amine groups to increase the hydrophobicity of the crosslinked materials. Amorphous deprotonated ffa was present in all ASDs; however, the increased hydrophobicity and reduced basicity from benzoyl functionalization led to a combination of amorphous deprotonated ffa and amorphous neutral ffa in the materials at high drug loadings (50 and 60 wt %). All ASDs demonstrated enhanced drug delivery in acidic media compared to crystalline ffa. Physical stability testing showed no evidence of crystallization after 29 weeks under various relative humidity conditions. These findings motivate the broadening of polymer classes employed in ASD formation to include polymers with very high functional group concentrations to enable loadings not readily achieved with existing polymers.

Superior Metal-Organic Framework Activation with Dimethyl Ether.

Metal-organic frameworks (MOFs) are demonstrated to be readily activated by treatment with the low surface tension, low boiling point solvent dimethyl ether (DME). The mildness of the method enables access to high surface areas by avoiding structural changes in the framework that often plague thermal activation methods. A distinction from previous methods is that DME activation succeeds for materials with coordinatively unsaturated sites (CUS) and non-CUS MOFs as well. DME displaces solvent molecules occupying the pores of the MOF as well as those coordinated to metal centers; reducing evacuation temperature by using a coordinating, yet highly volatile guest enables low temperature activation with structural retention as demonstrated surface area measurements that match or exceed existing activation protocols.

Discovery strategy leads to the first melt-castable cocrystal based on an energetic oxidizing salt.

Cocrystallization is a synthetic method employed across fields to improve functional materials while preserving properties inherent to the molecules/ions involved. However, there is no guarantee that cocrystals will demonstrate improved properties relative to the constituent materials. Oxygen balance, which is closely correlated to the performance of energetic materials, is an exception in that this attribute may be targetted with certainty. The combination of energetic oxidizing salts with small molecules presents a seemingly straightforward path to energetic materials with desirable performance properties. Unfortunately no general approach for the cocrystallization of salts and small molecules (salt cocrystallization) has yet emerged. Presented here is such an approach, focussing on ammonium salts, and applied to the energetic oxidizing salt ammonium dinitramide to achieve a melt-castable energetic material. Though focused on ammonium salts, this salt cocrystallization paradigm is a general approach that may be extended to other ions.

Microscale Determination of Binary Gas Adsorption Isotherms in MOFs.

The experimental determination of mixed gas isotherms is challenging and thus rarely performed. Nevertheless, characterizing the performance of adsorbents toward mixtures of gases is critical in most adsorptive separations. Here, the utility of Raman spectroscopy in determining binary gas adsorption isotherms on the microscale with metal-organic framework (MOF) single crystals is demonstrated for quantifying C2H6/CH4 selectivity. The influence of pore size on sorption selectivity is determined experimentally. The technique also allows determination of kinetics of methane adsorption in MOFs, which is critical for refueling times in adsorbed natural gas storage.

Peroxosolvate discovery method leads to first cocrystal with three energetic components.

In spite of their exceptional performance, energetic peroxosolvates are rare. Research in this area is slowed by the poor availibility of concentrated hydrogen peroxide solutions. Presented here is an efficient peroxosolvate discovery method that is applied in the discovery of the first ternary cocrystal comprising only energetic components.

Laser Heating of Fluid With and Without Stone Ablation: In Vitro Assessment.

Introduction: Laser lithotripsy can cause excessive heating of fluid within the collecting system and lead to tissue damage. To better understand this effect, it is important to determine the percentage of applied laser energy that is converted to heat and the percentage used for stone ablation. Our objective was to calculate the percentage of laser energy used for stone ablation based on the difference in fluid temperature measured in an in vitro model when the laser was activated without and with stone ablation. Methods: Flat BegoStone disks (15:5) were submerged in 10 mL of deionized water at the bottom of a vacuum evacuated double-walled glass Dewar. A Moses 200 D/F/L laser fiber was positioned above the surface of the stone at a distance of 3.5 mm for control (no stone ablation) or 0.5 mm for experimental (ablation) trials. The laser was activated and scanned at 3 mm/second across the stone in a preprogrammed pattern for 30 seconds at 2.5 W (0.5 J × 5 Hz) for both short-pulse (SP) and Moses distance (MD) modes. Temperature of the fluid was recorded using two thermocouples once per second. Results: Control trials produced no stone ablation, while experimental trials produced a staccato groove in the stone surface, simulating efficient lithotripsy. The mean temperature increase for SP was 1.08°C ± 0.04°C for control trials and 0.98°C ± 0.03°C for experimental trials, yielding a mean temperature difference of 0.10°C ± 0.06°C (p = 0.0005). With MD, the mean temperature increase for control trials was 1.03°C ± 0.01°C and for experimental trials 0.99°C ± 0.06°C, yielding a smaller mean temperature difference of 0.04°C ± 0.06°C (p = 0.09). Conclusions: Even under conditions of energy-efficient stone ablation, the majority of applied laser energy (91%-96%) was converted to heat.

Factors influencing hydrogen peroxide versus water inclusion in molecular crystals.

Hydrate formation is often unavoidable during crystallization, leading to performance degradation of pharmaceuticals and energetics. In some cases, water molecules trapped within crystal lattices can be substituted for hydrogen peroxide, improving the solubility of drugs and detonation performance of explosives. The present work compares hydrates and hydrogen peroxide solvates in two ways: (1) analyzing structural motifs present in crystal structures accessed from the Cambridge Structural Database and (2) developing potential energy surfaces for water and hydrogen peroxide interacting with functional groups of interest at geometries relevant to the solid state. By elucidating fundamental differences in local interactions that can be formed with molecules of hydrogen peroxide and/or water, the analyses presented here provide a foundation for the design and selection of candidate molecules for the formation of hydrogen peroxide solvates.

Computational Identification and Experimental Demonstration of High-Performance Methane Sorbents.

Remarkable methane uptake is demonstrated experimentally in three metal-organic frameworks (MOFs) identified by computational screening: UTSA-76, UMCM-152 and DUT-23-Cu. These MOFs outperform the benchmark sorbent, HKUST-1, both volumetrically and gravimetrically, under a pressure swing of 80 to 5 bar at 298 K. Although high uptake at elevated pressure is critical for achieving this performance, a low density of high-affinity sites (coordinatively unsaturated metal centers) also contributes to a more complete release of stored gas at low pressure. The identification of these MOFs facilitates the efficient storage of natural gas via adsorption and provides further evidence of the utility of computational screening in identifying overlooked sorbents.

Ethylene oxide functionalization enhances the ionic conductivity of a MOF.

Varying the degree of ethylene oxide (EO) functionalization of the zirconium MOF UiO-68 affords two novel MOFs; UiO-68-EO and UiO-68-2EO exhibit solvent-free ionic conductivity upon loading LiTFSI in their pores. Incorporating EO chains provides a pathway for lithium ion migration between the coordinated sites and results in an ionic conductivity of 3.8 × 10-7 S cm-1 and 3.9 × 10-4 S cm-1 at 90 °C for UiO-68-EO/LiTFSI and UiO-68-2EO/LiTFSI respectively.

Linker Deprotonation and Structural Evolution on the Pathway to MOF-74.

The synthesis of MOF-74 (MOF = metal-organic framework) proceeds first through the generation of chemically and topologically distinct materials, referred to as phases, displaying exclusively carboxylate coordination, followed by further deprotonation to enable oxo coordination and MOF-74 formation. The synthesis of Mg-MOF-74 at high concentrations of linker and metal enables the stabilization and characterization of the previously unobserved, exclusively carboxylate coordinating phases. Ex situ and in situ approaches are leveraged to provide the time-resolved observation of Mg-MOF-74 synthesis and the formation of phases that precede Mg-MOF-74 formation as well as metastable phase dissolution. These data support dissolution and redeposition as the mechanism of MOF-74 formation and provide insight into the formation mechanism of MOFs with multiple linker coordination types.

Metal-Organic Framework (MOF) Morphology Control by Design.

Exerting morphological control over metal-organic frameworks (MOFs) is critical for determining their catalytic performance and to optimize their packing behavior in areas from separations to fuel gas storage. A mechanism-based approach to tailor the morphology of MOFs is introduced and experimentally demonstrated for five cubic Zn4 O-based MOFs. This methodology provides three key features: 1) computational screening for selection of appropriate additives to change crystal morphology based on knowledge of the crystal structure alone; 2) use of additive to metal cluster geometric relationships to achieve morphologies expressing desired crystallographic facets; 3) potential for suppression of interpenetration for certain phases.