Exploring mechanochemistry of pharmaceutical cocrystals: effect of incident angle on molecular mixing during simulated indentations of two organic solids

The solid-state reaction of the active pharmaceutical ingredient theophylline with citric acid is a well-established example of a mechanochemical reaction, leading to a model pharmaceutical cocrystal. Here, classical force field molecular dynamics was employed to investigate the molecular mixing and structural distortion that take place on the mechanically driven indentation of a citric acid nanoparticle on a slab of crystalline theophylline. Through non-equilibrium molecular dynamics simulations, a 6 nm diameter nanoparticle of citric acid was introduced onto an open (001) surface of a theophylline crystal, varying both the angle of incidence of the nanoparticle between 15° and 90° and the indentation speed between 1 m s-1 and 16 m s-1. This theoretical study enabled the evaluation of how these two parameters promote molecular mixing and overall structural deformation upon the mechanical contraction of theophylline and citric acid, both of which are important parameters underlying mechanochemical cocrystallisation. The results show that the angle of incidence plays a key role in the molecular transfer ability between the two species and in the structural disruption of the initially spherical nanoparticles. Changing the indentation speed, however, did not lead to a discernible trend in molecular mixing, highlighting the importance of the incident angle in mechanochemical events in the context of supramolecular chemistry, such as the disruption of the crystal structure and molecular transfer between molecular crystals.

Effects of Fluorinated Functionalization of Linker on Quercetin Encapsulation, Release and Hela Cell Cytotoxicity of Cu-Based MOFs as Smart pH-Stimuli Nanocarriers

Controlled delivery of target molecules is required in many medical and chemical applications. For such purposes, metal-organic frameworks (MOFs), which possess desirable features such as high porosity, large surface area, and adjustable functionalities, hold great potential as drug carriers. Herein, Quercetin (QU), as an anticancer drug, was loaded on Cu2 (BDC)2 (DABCO) and Cu2 (F4 BDC)2 )DABCO) MOFs (BDC=1,4-benzenedicarboxylate and DABCO=1,4-diazabicyclo[2.2.2]octane). As these Cu-MOFs have a high surface area, an appropriate pore size, and biocompatible ingredients, they can be utilized to deliver QU. The loading efficiency of QU in these MOFs was 49.5 % and 41.3 %, respectively. The drug-loaded compounds displayed sustained drug release over 15 days, remarkably high drug loading capacities and pH-controlled release behavior. The prepared nanostructures were characterized by different characterization technics including FT-IR, PXRD, ZP, TEM, FE-SEM, UV-vis, and BET. In addition, MTT assays were carried out on the HEK-293 and HeLa cell lines to investigate cytotoxicity. Cellular apoptosis analysis was performed to investigate the cell death mechanisms. Grand Canonical Monte Carlo simulations were conducted to analyze the interactions between MOFs and QU. Moreover, the stability of MOFs was also investigated during and after the drug release process. Ultimately, kinetic models of drug release were evaluated.

Vapor-assisted synthesis of the MOF-74 metal-organic framework family from zinc, cobalt, and magnesium oxides

In this work, we investigate the vapor-assisted synthesis of the metal-organic framework MOF-74 starting from three metal oxides (ZnO, CoO, and MgO). Depending on the nature of the added vapor (H2O, DMF, DMSO), the metal oxide, and the temperature, the outcome of the reaction can be directed towards the desired porous phase. Ex situ and in situ XRD measurements reveal the formation of an intermediate phase during the reaction of MgO with H4dobdc, while the MOF-74 phase forms directly for ZnO and CoO. The reduced CO2 uptake of the resulting materials compared to solvothermally prepared MOFs might be offset by the convenience of the presented route and the promise of a high space time yield.

Effect of Surface Functionality on the Rheological and Self-Assembly Properties of Chitin and Chitosan Nanocrystals and Use in Biopolymer Films

Chitin nanocrystals (ChNCs) are unique to all other bio-derived nanomaterials in one aspect: the inherent presence of a nitrogen moiety. By tuning the chemical functionality of this nanomaterial, and thus its charge and hydrogen bonding capacity, one can heavily impact its macroscopic properties such as its rheological and self-assembly characteristics. In this study, two types of ChNCs are made using acid hydrolysis (AH-ChNCs) and oxidative (OX-ChNCs) pathways, unto which deacetylation using a solvent-free procedure is utilized to create chitosan nanocrystals (ChsNCs) of varying degree of deacetylation (DDA). These nanocrystals were then studied for their rheological behavior and liquid crystalline ordering. It was found that with both deacetylation and carboxylation of ChNCs, viscosity continually increased with increasing concentrations from 2 to 8 wt %, contrary to AH-ChNC dispersions in the same range. Interestingly, increasing the amine content of ChNCs was not proportional to the storage modulus, where a peak saturation of amines provided the most stiffness. Conversely, while the introduction of carboxylation increased the elastic modulus of OX-ChNCs by an order of magnitude from that of AH-ChNCs, it was decreased by increasing DDA. Deacetylation and carboxylation both inhibited the formation of a chiral nematic phase. Finally, these series of nanocrystals were incorporated into biodegradable pectin-alginate films as a physical reinforcement, which showed increased tensile strength and Young's modulus values for the films incorporated with ChsNCs. Overall, this study is the first to investigate how surface functionalization of chitin-derived nanocrystals can affect their rheological and liquid crystalline properties and how it augments pectin/alginate films as a physical reinforcement nanofiller.

Free-Standing Metal-Organic Framework Membranes Made by Solvent-Free Space-Confined Conversion for Efficient H2/CO2 Separation

Metal-organic frameworks (MOFs) are promising candidates for the advanced membrane materials based on their diverse structures, modifiable pore environment, precise pore sizes, etc. Nevertheless, the use of supports and large amounts of solvents in traditional solvothermal synthesis of MOF membranes is considered inefficient, costly, and environmentally problematic, coupled with challenges in their scalable manufacturing. In this work, we report a solvent-free space-confined conversion (SFSC) approach for the fabrication of a series of free-standing MOF (ZIF-8, Zn(EtIm)₂, and Zn₂(BIm)₄) membranes. This approach excludes the employment of solvents and supports that require tedious pretreatment and, thus, makes the process more environment-friendly and highly efficient. The free-standing membranes feature a robust and unique architecture, which comprise dense surface layers and highly porous interlayer with large amounts of irregular-shaped micron-scale pore cavities, inducing satisfactory H₂/CO₂ selectivities and exceptional H₂ permeances. The ZIF-8 membrane affords a considerable H₂ permeance of 2653.7 GPU with a competitive H₂/CO₂ selectivity of 17.1, and the Zn(EtIm)₂ membrane exhibits a high H₂/CO₂ selectivity of 22.1 with an excellent H₂ permeance (6268.7 GPU). The SFSC approach potentially provides a new pathway for preparing free-standing MOF membranes under solvent-free conditions, rendering it feasible for scale-up production of membrane materials for gas separation.

ZIF-8@Rhodamine B as a Self-Reporting Material for Pollutant Extraction Applications

Herein, we have evaluated the potential of dye-encapsulation as a simple mechanism to self-report the stability of MOFs for pollutant extraction applications. This enabled the visual detection of material stability issues during the selected applications. As proof-of-concept, the zeolitic imidazolate framework (ZIF-8) material was prepared in aqueous medium and at room temperature in the presence of the dye rhodamine B. The total amount of loaded rhodamine B was determined using UV-vis spectrophotometry. The prepared dye-encapsulated ZIF-8 showed a comparable extraction performance with bare ZIF-8 for the removal of hydrophobic endocrine-disrupting phenols, such as 4-tert-octylphenol and 4-nonylphenol, and improved the extraction performance of more hydrophilic endocrine disruptors, such as bisphenol A and 4-tert-butylphenol.

The "η-sweet-spot" (ηmax) in liquid-assisted mechanochemistry: polymorph control and the role of a liquid additive as either a catalyst or an inhibitor in resonant acoustic mixing (RAM)

Resonant acoustic mixing (RAM) offers a simple, efficient route for mechanochemical synthesis in the absence of milling media or bulk solvents. Here, we show the use of RAM to conduct the copper-catalysed coupling of sulfonamides and carbodiimides. This coupling was previously reported to take place only by mechanochemical ball milling, while in conventional solution environments it is not efficient, or does not take place at all. The results demonstrate RAM as a suitable methodology to conduct reactions previously accessed only by ball milling and provide a detailed, systematic overview of how the amount of liquid additive, measured by the ratio of liquid volume to weight of reactants (η, in μL mg^-1), can affect the course of a mechanochemical reaction and the polymorphic composition of its product. Switching from ball milling to RAM allowed for the discovery of a new polymorph of the model sulfonylguanidine obtained by catalytic coupling of di(cyclohexyl)carbodiimide (DCC) and p-toluenesulfonamide, and the ability to control reaction temperature in RAM enabled in situ control of the polymorphic behaviour of this nascent product. We show that the reaction conversion for a given reaction time does not change monotonically but, instead, achieves a maximum for a well-defined η-value. This "η-sweet-spot" of conversion is herein designated η_max. The herein explored reactions demonstrate sensitivity to η on the order of 0.01 μL mg^-1, which corresponds to an amount of liquid additive below 5 mol% compared to the reactants, and is at least one to two orders of magnitude lower than the η-value typically considered in the design of liquid-assisted ball milling mechanochemical reactions. Such sensitivity suggests that strategies to optimise liquid-assisted mechanochemical reactions should systematically evaluate η-values at increments of 0.01 μL mg^-1, or even finer. At η-values other than η_max the reaction conversion drops off, demonstrating that the same liquid additive can act either as a catalyst or an inhibitor of a mechanochemical reaction, depending on the amount.

Spontaneous Synthesis of [FeII (Atrz)3 ]SO4 and its Analogues Through Accelerated Ageing: New Insights from Small-Scale Reactions

Accelerated ageing reactions that take place between two solid materials on contact in the absence of added solvent have been used to synthesize two spin-crossover-active 1D coordination polymers and one of their Cu(II) analogues. The hygroscopy of the ligands and the relative humidity of the reaction chamber have been shown to be particularly important factors in the rate of reaction. Small-scale reactions between a few individual crystals have allowed observation of deliquescence of the 4-aminotriazole ligand at high humidity. The metal salt does not dissolve, and the ligand diffuses into the crystal of the metal salt during the reaction. In the case of the Cu analogue, the formation of the product causes the crystal of the metal salt to deform with the formation of pseudocrystals, which have a fibrous structure.

The crystal structure of imidazolium nitrate, C3H5O3N3

C3H5O3N3, tetragonal, P43212 (no. 96), a = 7.2479(2) Å, c = 21.6138(8) Å, V = 1135.42(8) Å3, Z = 8, Rgt (F) = 0.0344, wRref (F 2) = 0.0903, T = 100 K.

Mechanochemical Enhancement of the Structural Stability of Pseudorotaxane Intermediates in the Synthesis of Rotaxanes

Mechanochemical syntheses of rotaxanes have attracted considerable attention of late because of the superior reaction rates and higher yields associated with their production compared with analogous reactions carried out in solution. Previous investigators, however, have focused on the demonstration of the mechanochemical syntheses of rotaxanes per se, rather than on studying the solid-phase host-guest molecular interplay related to their rapid formation and high yields. In this investigation, we attribute the lower yields of rotaxanes prepared in solution to the limited concentration and a desolvation energy penalty that must be compensated for by host-guest interactions during complexation that precedes the templation leading to rotaxane formation. It follows that, if the desolvation energy can be removed and higher concentrations can be attained, even weak host-guest interactions can drive the complexation of host and guest molecules efficiently. In order to test this hypothesis, we chose two host-guest pairs of permethylated pillar[5]arene/1,6-diaminohexane and permethylated pillar[5]arene/2,2'-(ethylenedioxy)bis(ethylamine) for the simple reason that they exhibit extremely low binding constants (2.7 ± 0.4 M^-1 when 1,6-diaminohexane is the guest and <0.1 M^-1 when 2,2'-(ethylenedioxy)bis(ethylamine) is the guest in CDCl₃; i.e., ostensibly no pseudorotaxane formation is observed). We argue that the amount of pseudorotaxanes formed in the solid state is responsive to mechanical treatments or otherwise and changes in temperature during stoppering reactions. Compared to the amount of pseudorotaxanes that can be obtained in solution, large quantities of pseudorotaxanes are formed in the solid state because of concentration and desolvation effects. This mechanochemical enhancement of pseudorotaxane formation is referred to as a self-correction in the current investigation. Rotaxanes based on permethylated pillar[5]arene/1,6-diaminohexane and permethylated pillar[5]arene/2,2'-(ethylenedioxy)bis(ethylamine) have been synthesized in much higher yields compared to those obtained in solution, aided and abetted by self-correction effects during mechanical treatments and heating at a mild temperature of 50 °C.

Challenges, Opportunities and Future Directions of Membrane Technology for Natural Gas Purification: A Critical Review

Natural gas is an important and fast-growing energy resource in the world and its purification is important in order to reduce environmental hazards and to meet the required quality standards set down by notable pipeline transmission, as well as distribution companies. Therefore, membrane technology has received great attention as it is considered an attractive option for the purification of natural gas in order to remove impurities such as carbon dioxide (CO₂) and hydrogen sulphide (H₂S) to meet the usage and transportation requirements. It is also recognized as an appealing alternative to other natural gas purification technologies such as adsorption and cryogenic processes due to its low cost, low energy requirement, easy membrane fabrication process and less requirement for supervision. During the past few decades, membrane-based gas separation technology employing hollow fibers (HF) has emerged as a leading technology and underwent rapid growth. Moreover, hollow fiber (HF) membranes have many advantages including high specific surface area, fewer requirements for maintenance and pre-treatment. However, applications of hollow fiber membranes are sometimes restricted by problems related to their low tensile strength as they are likely to get damaged in high-pressure applications. In this context, braid reinforced hollow fiber membranes offer a solution to this problem and can enhance the mechanical strength and lifespan of hollow fiber membranes. The present review includes a discussion about different materials used to fabricate gas separation membranes such as inorganic, organic and mixed matrix membranes (MMM). This review also includes a discussion about braid reinforced hollow fiber (BRHF) membranes and their ability to be used in natural gas purification as they can tackle high feed pressure and aggressive feeds without getting damaged or broken. A BRHF membrane possesses high tensile strength as compared to a self-supported membrane and if there is good interfacial bonding between the braid and the separation layer, high tensile strength, i.e., upto 170Mpa can be achieved, and due to these factors, it is expected that BRHF membranes could give promising results when used for the purification of natural gas.

Mechanochemical halogenation of unsymmetrically substituted azobenzenes

The direct and selective mechanochemical halogenation of C–H bonds in unsymmetrically substituted azobenzenes using N-halosuccinimides as the halogen source under neat grinding or liquid-assisted grinding conditions in a ball mill has been described. Depending on the azobenzene substrate used, halogenation of the C–H bonds occurs in the absence or only in the presence of Pd^II catalysts. Insight into the reaction dynamics and characterization of the products was achieved by in situ Raman and ex situ NMR spectroscopy and PXRD analysis. A strong influence of the different 4,4’-substituents of azobenzene on the halogenation time and mechanism was found.

Time-Resolved In Situ Monitoring of Mechanochemical Reactions

Mechanochemical transformations offer environmentally benign synthesis routes, whilst enhancing both the speed and selectivity of reactions. In this regard, mechanochemistry promises to transform the way in which chemistry is done in both academia and industry but is greatly hindered by a current lack of mechanistic understanding. The continued development and use of time-resolved in situ (TRIS) approaches to monitor mechanochemical reactions provides a new dimension to elucidate these fascinating transformations. We here discuss recent trends in method development that have pushed the boundaries of mechanochemical research. New features of mechanochemical reactions obtained by TRIS techniques are subsequently discussed, which sheds light on how different TRIS approaches have been used. Emphasis is placed on the strength of combining complementary techniques. Finally, we outline our views on the potential of TRIS methods in mechanochemical research, towards establishing a new, environmentally benign paradigm in the chemical sciences.

Mechanoenzymatic Reactions Involving Polymeric Substrates or Products

Mechanoenzymology is an emerging field in which mechanical mixing is used to sustain enzymatic reactions in low-solvent or solvent-free mixtures. Many enzymes have been reported that thrive under such conditions. Considering the central role of biopolymers and synthetic polymers in our society, this minireview highlights the use of mechanoenzymology for the synthesis or depolymerization of oligomeric or polymeric materials. In contrast to traditional in-solution reactions, solvent-free mechanoenzymology has the advantages of avoiding solubility issues, proceeding in a minimal volume, and reducing solvent waste while potentially improving the reaction efficiency and accessing new reactivity. It is expected that this strategy will continue to gain popularity and find more applications.

Multi-Component Crystals Containing Urea: Mechanochemical Synthesis and Characterization by 35Cl Solid-State NMR Spectroscopy and DFT Calculations

Mechanochemical synthesis provides new pathways for the rational design of multi-component crystals (MCCs) involving anionic or cationic components, which offer molecular-level architectures unavailable to MCCs comprised of strictly neutral components....

Mechanochemically Assisted Synthesis of Hexaazatriphenylenehexacarbonitrile

1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HAT CN) was synthesized mechanochemically at room temperature. The coupling of hexaketocyclohexane and diaminomaleonitrile was conducted in 10 min by vibratory ball milling. The effects of milling parameters, acids, dehydrating agents, and liquid-assisted grinding were rationalized. With 67%, the yield of this mechanochemical approach exceeds that of state-of-the-art wet-chemical syntheses while being superior with respect to time-, resource-, and energy-efficiency as quantified via green metrics.

A Case Study on the Desired Selectivity in Solid-State Mechano- and Slow-Chemistry, Melt, and Solution Methodologies

Solution-based syntheses are omnipresent in chemistry but are often associated with obvious disadvantages, and the search for new mild and green synthetic methods continues to be a hot topic. Here, comparative studies in four different reaction media were conducted, that is, the solid-state mechano- and slow-chemistry synthesis, melted phase, and solution protocols, and the impact of the employed solvent-free solid-state versus liquid-phase synthetic approaches was highlighted on a pool of products. A moderately exothermic model reaction system was chosen based on bis(pentafluorophenyl)zinc, (C₆ F₅ )₂ Zn, and 2,2,6,6-tetramethylpiperidinyl oxide (TEMPO) as a stable nitroxyl radical, anticipating that these reagents may offer a unique landscape for addressing kinetic and thermodynamic aspects of wet and solvent-free solid-state processes. In a toluene solution two distinct paramagnetic Lewis acid-base adducts (C₆ F₅ )₂ Zn(η¹ -TEMPO) (1) and (C₆ F₅ )₂ Zn(η¹ -TEMPO)₂ (2) equilibrated, but only 2 was affordable by crystallization. In turn, crystallization from the melt was the only method yielding single crystals of 1. Moreover, the solid-state approaches were stoichiometry sensitive and allowed for the selective synthesis of both adducts by simple stoichiometric control over the substrates. Density functional theory (DFT) calculations were carried out to examine selected structural and thermodynamic features of the adducts 1 and 2. Compound 2 is a unique non-redox active metal complex supported by two nitroxide radicals, and the magnetic studies revealed weak-to-moderate intramolecular antiferromagnetic interactions between the two coordinated TEMPO molecules.

Enzymatic depolymerization of highly crystalline polyethylene terephthalate enabled in moist-solid reaction mixtures

Less than 9% of the plastic produced is recycled after use, contributing to the global plastic pollution problem. While polyethylene terephthalate (PET) is one of the most common plastics, its thermomechanical recycling generates a material of lesser quality. Enzymes are highly selective, renewable catalysts active at mild temperatures; however, they lack activity toward the more crystalline forms of PET commonly found in consumer plastics, requiring the energy-expensive melt-amorphization step of PET before enzymatic depolymerization. We report here that, when used in moist-solid reaction mixtures instead of the typical dilute aqueous solutions or slurries, the cutinase from Humicola insolens can directly depolymerize amorphous and crystalline regions of PET equally, without any pretreatment, with a 13-fold higher space-time yield and a 15-fold higher enzyme efficiency than reported in prior studies with high-crystallinity material. Further, this process shows a 26-fold selectivity for terephthalic acid over other hydrolysis products.

Raman spectroscopy for real-time and in situ monitoring of mechanochemical milling reactions

Solid-state milling has emerged as an alternative, sustainable approach for preparing virtually all classes of compounds and materials. In situ reaction monitoring is essential to understanding the kinetics and mechanisms of these reactions, but it has proved difficult to use standard analytical techniques to analyze the contents of the closed, rapidly moving reaction chamber (jar). Monitoring by Raman spectroscopy is an attractive choice, because it allows uninterrupted data collection from the outside of a translucent milling jar. It complements the already established in situ monitoring based on powder X-ray diffraction, which has limited accessibility to the wider research community, because it requires a synchrotron X-ray source. The Raman spectroscopy monitoring setup used in this protocol consists of an affordable, small portable spectrometer, a laser source and a Raman probe. Translucent reaction jars, most commonly made from a plastic material, enable interaction of the laser beam with the solid sample residing inside the closed reaction jar and collection of Raman-scattered photons while the ball mill is in operation. Acquired Raman spectra are analyzed using commercial or open-source software for data analysis (e.g., MATLAB, Octave, Python, R). Plotting the Raman spectra versus time enables qualitative analysis of reaction paths. This is demonstrated for an example reaction: the formation in the solid state of a cocrystal between nicotinamide and salicylic acid. A more rigorous data analysis can be achieved using multivariate analysis.

A Showcase of Green Chemistry: Sustainable Synthetic Approach of Zirconium-Based MOF Materials

Zirconium-based metal-organic framework materials (Zr-MOFs) have more practical usage over most conventional benchmark porous materials and even many other MOFs due to the excellent structural stability, rich coordination forms, and various active sites. However, their mass-production and application are restricted by the high-cost raw materials, complex synthesis procedures, harsh reaction conditions, and unexpected environmental impact. Based on the principles of "Green Chemistry", considerable efforts have been done for breaking through the limitations, and significant progress has been made in the sustainable synthesis of Zr-MOFs over the past decade. In this review, the advancements of green raw materials and green synthesis methods in the synthesis of Zr-MOFs are reviewed, along with the corresponding drawbacks. The challenges and prospects are discussed and outlooked, expecting to provide guidance for the acceleration of the industrialization and commercialization of Zr-MOFs.

Template-Mediated Control over Polymorphism in the Vapor-Assisted Formation of Zeolitic Imidazolate Framework Powders and Films

The landscape of possible polymorphs for some metal-organic frameworks (MOFs) can pose a challenge for controlling the outcome of their syntheses. Demonstrated here is the use of a template to control in the vapor-assisted formation of zeolitic imidazolate framework (ZIF) powders and thin films. Introducing a small amount of either ethanol or dimethylformamide vapor during the reaction between ZnO and 4,5-dichloroimidazole vapor results in the formation of the porous ZIF-71 phase, whereas other conditions lead to the formation of the dense ZIF-72 phase or amorphous materials. Time-resolved in situ small-angle X-ray scattering reveals that the porous phase is metastable and can be transformed into its dense polymorph. This transformation is avoided through the introduction of template vapor. The porosity of the resulting ZIF powders and films was studied by N₂ and Kr physisorption, as well as positron annihilation lifetime spectroscopy. The templating principle was demonstrated for other members of the ZIF family as well, including the ZIF-7 series, ZIF-8_Cl, and ZIF-8_Br.

Comment on "The solvent and zinc source dual-induced synthesis of a two dimensional zeolitic imidazolate framework with a farfalle-shape and its crystal transformation to zeolitic imidazolate framework-8" by C.-X. Jin et al., 2020, 49, 2437

In a recently published article Jin et al. described a new farfalle-shaped zeolitic imidazolate framework (ZIF-F) and its transformation into the well-known ZIF-8 in methanol. However, the shown powder diffraction patterns of ZIF-F are similar to another ZIF polymorph called ZIF-8dia, which has previously been synthesized from synthesis mixtures of different compositions.

Mechanochemical methods for the transfer of electrons and exchange of ions: inorganic reactivity from nanoparticles to organometallics

For inorganic metathesis and reduction reactivity, mechanochemistry is demonstrating great promise towards both nanoparticles and organometallics syntheses.

Crystal engineering strategies towards halogen-bonded metal–organic multi-component solids: salts, cocrystals and salt cocrystals

This highlight presents an overview of the current advances in the preparation of halogen bonded metal–organic multi-component solids, including salts and cocrystals comprising neutral and ionic constituents.

Mechanochemical Metathesis between AgNO3 and NaX (X = Cl, Br, I) and Ag2XNO3 Double-Salt Formation

Here we describe real-time, in situ monitoring of mechanochemical solid-state metathesis between silver nitrate and the entire series of sodium halides, on the basis of tandem powder X-ray diffraction and Raman spectroscopy monitoring. The mechanistic monitoring reveals that reactions of AgNO₃ with NaX (X = Cl, Br, I) differ in reaction paths, with only the reaction with NaBr providing the NaNO₃ and AgX products directly. The reaction with NaI revealed the presence of a novel, short-lived intermediate phase, while the reaction with NaCl progressed the slowest through the well-defined Ag₂ClNO₃ intermediate double salt. While the corresponding iodide and bromide double salts were not observed as intermediates, all three are readily prepared as pure compounds by milling equimolar mixtures of AgX and AgNO₃. The in situ observation of reactive intermediates in these simple metathesis reactions reveals a surprising resemblance of reactions involving purely ionic components to those of molecular organic solids and cocrystals. This study demonstrates the potential of in situ reaction monitoring for mechanochemical reactions of ionic compounds as well as completes the application of these techniques to all major compound classes.

Advances in metal-organic framework coatings: versatile synthesis and broad applications

Metal-organic frameworks (MOFs) as a new kind of porous crystalline materials have attracted much interest in many applications due to their high porosity, diverse structures, and controllable chemical structures. However, the specific geometrical morphologies, limited functions and unsatisfactory performances of pure MOFs hinder their further applications. In recent years, an efficient approach to synthesize new composites to overcome the above issues has been achieved, by integrating MOF coatings with other functional materials, which have synergistic advantages in many potential applications, including batteries, supercapacitors, catalysis, gas storage and separation, sensors, drug delivery/cytoprotection and so on. Nevertheless, the systemic synthesis strategies and the relationships between their structures and application performances have not been reviewed comprehensively yet. This review emphasizes the recent advances in versatile synthesis strategies and broad applications of MOF coatings. A comprehensive discussion of the fundamental chemistry, classifications and functions of MOF coatings is provided first. Next, by modulating the different states (e.g. solid, liquid, and gas) of metal ion sources and organic ligands, the synthesis methods for MOF coatings on functional materials are systematically summarized. Then, many potential applications of MOF coatings are highlighted and their structure-property correlations are discussed. Finally, the opportunities and challenges for the future research of MOF coatings are proposed. This review on the deep understanding of MOF coatings will bring better directions into the rational design of high-performance MOF-based materials and open up new opportunities for MOF applications.

Exploring the Scope of Macrocyclic "Shoe-last" Templates in the Mechanochemical Synthesis of RHO Topology Zeolitic Imidazolate Frameworks (ZIFs)

The macrocyclic cavitand MeMeCH2 is used as a template for the mechanochemical synthesis of 0.2MeMeCH2@RHO-Zn16(Cl2Im)32 (0.2MeMeCH2@ZIF-71) and RHO-ZnBIm2 (ZIF-11) zeolitic imidazolate frameworks (ZIFs). It is shown that MeMeCH2 significantly accelerates the mechanochemical synthesis, providing high porosity products (BET surface areas of 1140 m2/g and 869 m2/g, respectively). Templation of RHO-topology ZIF frameworks constructed of linkers larger than benzimidazole (HBIm) was unsuccessful. It is also shown that cavitands other than MeMeCH2-namely MeHCH2, MeiBuCH2, HPhCH2, MePhCH2, BrPhCH2, BrC5CH2-can serve as effective templates for the synthesis of x(cavitand)@RHO-ZnIm2 products. The limitations on cavitand size and shape are explored in terms of their effectiveness as templates.

Towards Controlling the Reactivity of Enzymes in Mechanochemistry: Inert Surfaces Protect β-Glucosidase Activity During Ball Milling

The activity of β-glucosidases-the enzymes responsible for the final step in the enzymatic conversion of cellulose to glucose-can be maintained and manipulated under mechanochemical conditions in the absence of bulk solvent, either through an unexpected stabilization effect of inert surfaces, or by altering the enzymatic equilibrium. The reported results illustrate unique aspects of mechanoenzymatic reactions that are not observable in conventional aqueous solutions. They also represent the first reported strategies to enhance activity and favor either direction of the reaction under mechanochemical conditions.

Tomislav Friščić

"My motto is 'work hard, play hard'. My favorite quote is 'research not published is equivalent to research not done' …" Find out more about Tomislav Friščić in his Author Profile.

Mechanochemistry: an efficient and versatile toolbox for synthesis, transformation, and functionalization of porous metal–organic frameworks

Multiple ways in which the synergy of mechanochemistry and MOFs advances the field of materials chemistry are presented here.

Controlling the morphology of metal–organic frameworks and porous carbon materials: metal oxides as primary architecture-directing agents

We give a comprehensive overview of how the morphology control is an effective and versatile way to control the physicochemical properties of metal oxides that can be transferred to metal–organic frameworks and porous carbon materials.

Efficient Enzymatic Hydrolysis of Biomass Hemicellulose in the Absence of Bulk Water

Current enzymatic methods for hemicellulosic biomass depolymerization are solution-based, typically require a harsh chemical pre-treatment of the material and large volumes of water, yet lack in efficiency. In our study, xylanase (E.C. 3.2.1.8) from Thermomyces lanuginosus is used to hydrolyze xylans from different sources. We report an innovative enzymatic process which avoids the use of bulk aqueous, organic or inorganic solvent, and enables hydrolysis of hemicellulose directly from chemically untreated biomass, to low-weight, soluble oligoxylosaccharides in >70% yields.

Mechanochemical and slow-chemistry radical transformations: a case of diorganozinc compounds and TEMPO

From the green chemistry perspective, molecular solid-state transformations conducted under mild conditions are of great interest and desirability. However, research in this area lacked popularity in the previous century, and thus progressed slowly. In particular, the application of radical reactions in solid-state chemistry has been hampered by several long-standing challenges that are intrinsically associated with the apparent unpredictable nature of radical chemistry. We present a comparative study of model mechanochemical, slow-chemistry and solution radical reactions between TEMPO and homoleptic organozinc compounds (i.e., di-tert-butylzinc and diphenylzinc). In the case of the tBu2Zn/TEMPO reaction system only a dimeric diamagnetic complex [tBuZn(μ-TEMPO*)]2 is obtained in yields slightly varying with the method chosen. In contrast, when TEMPO is mixed with diphenylzinc in a 2 : 1 molar ratio a novel paramagnetic Lewis acid-base adduct [[Ph2Zn(η1-TEMPO)]·TEMPO] is isolated in high yields regardless of the applied methodology. This adduct is also formed in the slow-chemistry process when TEMPO is gently mixed with Ph2Zn in a 1 : 1 molar ratio and left for two weeks at ambient temperature. Within the next week the reaction mixture gives in high yield a diamagnetic dinuclear compound [PhZn(μ-TEMPO*)][PhZn(μ2-η1:η1-TEMPO*)] and biphenyl. The analogous reaction conducted in toluene results in a much lower conversion rate. The reported results open up a new horizon in molecular solid-state radical transformations.

Mechanoenzymatic Breakdown of Chitinous Material to N-Acetylglucosamine: The Benefits of a Solventless Environment

Chitin is not only the most abundant nitrogen-containing biopolymer on the planet, but also a renewable feedstock that is often treated as a waste. Current chemical methods to break down chitin typically employ harsh conditions, large volumes of solvent, and generate a mixture of products. Although enzymatic methods have been reported, they require a harsh chemical pretreatment of the chitinous substrate and rely on dilute solution conditions that are remote from the natural environment of microbial chitinase enzymes, which typically consists of surfaces exposed to air and moisture. We report an innovative and efficient mechanoenzymatic method to hydrolyze chitin to the N-acetylglucosamine monomer by using chitinases under the recently developed reactive aging (RAging) methodology, based on repeating cycles of brief ball-milling followed by aging, in the absence of bulk solvent. Our results demonstrate that the activity of chitinases increases several times by switching from traditional solution-based conditions of enzymatic catalysis to solventless RAging, which operates on moist solid substrates. Importantly, RAging is also highly efficient for the production of N-acetylglucosamine directly from shrimp and crab shell biomass without any other processing except for a gentle wash with aqueous acetic acid.

Geomimetic approaches in the design and synthesis of metal-organic frameworks

The recent discovery of minerals with metal-organic framework (MOF) structures has challenged the view of MOFs as purely synthetic materials. At the same time, the application of geo-inspired synthetic methodologies, such as accelerated ageing and pseudomorphic replication, has enabled a cleaner, more environmentally friendly synthesis of MOFs from mineral-like feedstocks, as well as the assembly of materials with structure controlled at both micro- and meso-scales. These almost concomitant developments have highlighted the previously unknown relationships between geology and MOF chemistry. Here, we outline examples of MOF structures found in minerals, and note geologically inspired approaches to MOF synthesis, as a means to highlight how the emergent geomimetic concepts in MOF chemistry can lead to advances in the design and synthesis of MOFs. This article is part of the theme issue 'Mineralomimesis: natural and synthetic frameworks in science and technology'.