Real-Time and In Situ Monitoring of Mechanochemical Reactions: A New Playground for All Chemists

Direct in situ measurement of polymorphic transition temperatures under thermo-mechanochemical conditions

For the first time, we directly measured the onset and completion temperatures of polymorphic transitions under thermo-mechanochemical conditions by simultaneous in situ synchrotron powder X-ray diffraction and temperature monitoring. We determined the thermo-mechanochemical polymorphic transition temperature in 1-adamantyl-1-diamantyl ether to be 31 °C lower than the transition temperature determined by DSC. Our findings highlight the uniqueness of thermo-mechanochemical conditions, with potential applications in polymorph screening.

In situ monitoring of mechanochemical MOF formation by NMR relaxation time correlation

In this paper, we present a new approach to monitoring mechanochemical transformations, based on a magnetic resonance (MR) method in which relaxation time correlation maps are used to track the formation of the popular metal-organic framework (MOF) materials Zn-MOF-74 and ZIF-8. The two-dimensional (2D) relaxation correlation measurement employed yields a spectrum which visually and analytically identifies different 1H environments in the sample of interest. The measurement is well-suited to analyzing solid mixtures, and liquids, in complex systems. Application in this work to monitoring MOF formation shows changes in signal amplitudes, and their MR lifetime coordinates, within the 2D plots as the reaction progresses, confirming reaction completion. This new measurement provides a simple way to analyse solid-state reactions without dissolution, and there is a logical pathway to benchtop measurement with a new generation of permanent magnet-based MR instruments. The methodology described permits measurement in an MR compatible milling container, which may be directly transferred from the shaker assembly to the MR magnet for in situ measurement of the entire reaction mixture.

Mechanistic model for quantifying the effect of impact force on mechanochemical reactivity

Conventional mechanochemical synthetic tools, such as ball mills, offer no methodology to quantitatively link macroscale reaction parameters, such as shaking frequency or milling ball radius, to fundamental drivers of reactivity, namely the force vectors applied to the reactive molecules. As a result, although mechanochemistry has proven to be a valuable method to make a wide variety of products, the results are seldom reproduceable between reactors, difficult to rationally optimize, and hard to ascribe to a specific reaction pathway. Here we have developed a controlled force reactor, which is a mechanochemical ball mill reactor with integrated force measurement and control during each impact. We relate two macroscale reactor parameters-impact force and impact time-to thermodynamic and kinetic transition state theories of mechanochemistry utilizing continuum contact mechanics principles. We demonstrate force controlled particle fracture of NaCl to characterize particle size evolution during reactions, and force controlled reaction between anhydrous copper(II) chloride and (1, 10) phenanthroline. During the fracture of NaCl, we monitor the evolution of particle size as a function of impact force and find that particles quickly reach a particle size of ∼100 μm largely independent of impact force, and reach steady state 10-100× faster than reaction kinetics of typical mechanochemical reactions. We monitor the copper(II) chloride reactivity by measuring color change during reaction. Applying our transition state theory developed here to the reaction curves of copper(II) chloride and (1, 10) phenanthroline at multiple impact forces results in an activation energy barrier of 0.61 ± 0.07 eV, distinctly higher than barriers for hydrated metal salts and organic ligands and distinctly lower than the direct cleavage of the CuCl bond, indicating that the reaction may be mediated by the higher affinity of Fe in the stainless steel vessel to Cl. We further show that the results in the controlled force reactor match rudimentary estimations of impact force within a commercial ball mill reactor Retsch MM400. These results demonstrate the ability to quantitatively link macroscale reactor parameters to reaction properties, motivating further work to make mechanochemical synthesis quantitative, predictable, and fundamentally insightful.

Mechanochemical Synthesis of Corannulene: Scalable and Efficient Preparation of A Curved Polycyclic Aromatic Hydrocarbon under Ball Milling Conditions

Corannulene, a curved polycyclic aromatic hydrocarbon, is prepared in a multigram scale through mechanochemical synthesis. Initially, a mixer mill approach is examined and found to be suitable for a gram scale synthesis. For larger scales, planetary mills are used. For instance, 15 g of corannulene could be obtained in a single milling cycle with an isolated yield of 90 %. The yields are lower when the jar rotation rate is lower or higher than 400 revolutions per minute (rpm). Cumulatively, 98 g of corannulene is produced through the ball milling-based grinding techniques. These results indicate the future potential of mechanochemistry in the rational chemical synthesis of highly curved nanocarbons such as fullerenes and carbon nanotubes.

The Role of Crystalline Intermediates in Mechanochemical Cyclorhodation Reactions Elucidated by in-Situ X-ray Powder Diffraction and Computation

The occurrence of crystalline intermediates in mechanochemical reactions might be more widespread than previously assumed. For example, a recent study involving the acetate-assisted C-H activation of N-Heterocycles with [Cp*RhCl2 ]2 by ball milling revealed the formation of transient cocrystals between the reagents prior to the C-H activation step. However, such crystalline intermediates were only observed through stepwise intervallic ex-situ analysis, and their exact role in the C-H activation process remained unclear. In this study, we monitored the formation of discrete, stoichiometric cocrystals between benzo[h]quinoline and [Cp*RhCl2 ]2 by ball milling using in-situ synchrotron X-ray powder diffraction. This continuous analysis revealed an initial cocrystal that transformed into a second crystalline form. Computational studies showed that differences in noncovalent interactions made the [Cp*RhCl2 ]2 unit in the later-appearing cocrystal more reactive towards NaOAc. This demonstrated the advantage of cocrystal formation before the acetate-assisted metalation-deprotonation step, and how the net cooperative action of weak interactions between the reagents in mechanochemical experiments can lead to stable supramolecular assemblies, which can enhance substrate activation under ball-milling conditions. This could explain the superiority of some mechanochemical reactions, such as acetate-assisted C-H activation, compared to their solution-based counterparts.

Hydrogenation of different carbon substrates into light hydrocarbons by ball milling

The conversion of carbon-based solids, like non-recyclable plastics, biomass, and coal, into small molecules appears attractive from different points of view. However, the strong carbon-carbon bonds in these substances pose a severe obstacle, and thus-if such reactions are possible at all-high temperatures are required1-5. The Bergius process for coal conversion to hydrocarbons requires temperatures above 450 °C6, pyrolysis of different polymers to pyrolysis oil is also typically carried out at similar temperatures7,8. We have now discovered that efficient hydrogenation of different solid substrates with the carbon-based backbone to light hydrocarbons can be achieved at room temperature by ball milling. This mechanocatalytic method is surprisingly effective for a broad range of different carbon substrates, including even diamond. The reaction is found to proceed via a radical mechanism, as demonstrated by reactions in the presence of radical scavengers. This finding also adds to the currently limited knowledge in understanding mechanisms of reactions induced by ball milling. The results, guided by the insight into the mechanism, could induce more extended exploration to broaden the application scope and help to address the problem of plastic waste by a mechanocatalytic approach.

Twice-milled magnetic biochar: A recyclable material for efficient removal of methylene blue from wastewater

Although magnetic modification has potential for preparing recyclable biochar, the traditional preparation methods of loading magnetic materials on biochar will probably lead to pore blockage and consequently remarkable adsorption recession. Herein, a preparation method was developed in which ball milled biochar was loaded with ultrafine magnetite and then milled for a second time, thus generating a magnetic, recyclable biochar with minimal pore blockage. The deposits of magnetite did not significantly wrap the biochar, although a decreased sorption performance was still detectable. Benefitting from the extra milling step, surface functional groups and specific surface areas of the adsorbents were largely restored, thus leading to a 93.8 % recovery adsorption of 84.6 ± 2.5 mg/L on methylene blue. Meanwhile, the recyclability of the material was not affected. The adsorption was driven by multiple interactions. These twice-milled magnetic biochar is quite outstanding for sustainable removal of aqueous contaminants with its recyclability and high sorption efficiency.

Disentangling the Effect of Pressure and Mixing on a Mechanochemical Bromination Reaction by Solid-State NMR Spectroscopy

Mechanical forces, including compressive stresses, have a significant impact on chemical reactions. Besides the preparative opportunities, mechanochemical conditions benefit from the absence of any organic solvent, the possibility of a significant synthetic acceleration and unique reaction pathways. Together with an accurate characterization of ball-milling products, the development of a deeper mechanistic understanding of the occurring transformations at a molecular level is critical for fully grasping the potential of organic mechanosynthesis. We herein studied a bromination of a cyclic sulfoximine in a mixer mill and used solid-state nuclear magnetic resonance (NMR) spectroscopy for structural characterization of the reaction products. Magic-angle spinning (MAS) was applied for elucidating the product mixtures taken from the milling jar without introducing any further post-processing on the sample. Ex situ ¹³ C-detected NMR spectra of ball-milling products showed the formation of a crystalline solid phase with the regioselective bromination of the S-aryl group of the heterocycle in position 4. Completion is reached in less than 30 minutes as deduced from the NMR spectra. The bromination can also be achieved by magnetic stirring, but then, a longer reaction time is required. Mixing the solid educts in the NMR rotor allows to get in situ insights into the reaction and enables the detection of a reaction intermediate. The pressure alone induced in the rotor by MAS is not sufficient to lead to full conversion and the reaction occurs on slower time scales than in the ball mill, which is crucial for analysing mixtures taken from the milling jar by solid-state NMR. Our data suggest that on top of centrifugal forces, an efficient mixing of the starting materials is required for reaching a complete reaction.

Shaking Things from the Ground-Up: A Systematic Overview of the Mechanochemistry of Hard and High-Melting Inorganic Materials

We provide a systematic overview of the mechanochemical reactions of inorganic solids, notably simple binary compounds, such as oxides, nitrides, carbides, sulphides, phosphides, hydrides, borides, borane derivatives, and related systems. Whereas the solid state has been traditionally considered to be of little synthetic value by the broader community of synthetic chemists, the solid-state community, and in particular researchers focusing on the reactions of inorganic materials, have thrived in building a rich and dynamic research field based on mechanically-driven transformations of inorganic substances typically seen as inert and high-melting. This review provides an insight into the chemical richness of such mechanochemical reactions and, at the same time, offers their tentative categorisation based on transformation type, resulting in seven distinct groupings: (i) the formation of adducts, (ii) the reactions of dehydration; (iii) oxidation-reduction (redox) reactions; (iv) metathesis (or exchange) reactions; (v) doping and structural rearrangements, including reactions involving the reaction vessel (the milling jar); (vi) acid-base reactions, and (vii) other, mixed type reactions. At the same time, we offer a parallel description of inorganic mechanochemical reactions depending on the reaction conditions, as those that: (i) take place under mild conditions (e.g., manual grinding using a mortar and a pestle); (ii) proceed gradually under mechanical milling; (iii) are self-sustained and initiated by mechanical milling, i.e., mechanically induced self-propagating reactions (MSRs); and (iv) proceed only via harsh grinding and are a result of chemical reactivity under strongly non-equilibrium conditions. By elaborating on typical examples and general principles in the mechanochemistry of hard and high-melting substances, this review provides a suitable complement to the existing literature, focusing on the properties and mechanochemical reactions of inorganic solids, such as nanomaterials and catalysts.

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.

Mechanochemistry-driven engineering of 0D/3D heterostructure for designing highly luminescent Cs–Pb–Br perovskites

Embedding metal-halide perovskite particles within an insulating host matrix has proven to be an effective strategy for revealing the outstanding luminescence properties of perovskites as an emerging class of light emitters. Particularly, unexpected bright green emission observed in a nominally pure zero-dimensional cesium–lead–bromide perovskite (Cs₄PbBr₆) has triggered intensive research in better understanding the serendipitous incorporation of emissive guest species within the Cs₄PbBr₆ host. However, a limited controllability over such heterostructural configurations in conventional solution-based synthesis methods has limited the degree of freedom in designing synthesis routes for accessing different structural and compositional configurations of these host–guest species. In this study, we provide means of enhancing the luminescence properties in the nominal Cs₄PbBr₆ powder through a guided heterostructural configuration engineering enabled by solid-state mechanochemical synthesis. Realized by an in-depth study on time-dependent evaluation of optical and structural properties during the synthesis of Cs₄PbBr₆, our target-designed synthesis protocol to promote the endotaxial formation of Cs₄PbBr₆/CsPbBr₃ heterostructures provides key insights for understanding and designing kinetics-guided syntheses of highly luminescent perovskite emitters for light-emitting applications.

While emission and stability of metal–halide perovskites can be enhanced through heterostructural encapsulation, a controlled synthesis route to such structures is not trivial to realize. Here, the authors design a mechanochemistry-driven protocol for synthesizing highly luminescent CsPbBr₃/Cs₄PbBr₆ heterostructures.

Advancing mechanochemical synthesis by combining milling with different energy sources

Owing to its efficiency and unique reactivity, mechanochemical processing of bulk solids has developed into a powerful tool for the synthesis and transformation of various classes of materials. Nevertheless, mechanochemistry is primarily based on simple techniques, such as milling in comminution devices. Recently, mechanochemical reactivity has started being combined with other energy sources commonly used in solution-based chemistry. Milling under controlled temperature, light irradiation, sound agitation or electrical impulses in newly developed experimental setups has led to reactions not achievable by conventional mechanochemical processing. This Perspective describes these unique reactivities and the advances in equipment tailored to synthetic mechanochemistry. These techniques - thermo-mechanochemistry, sono-mechanochemistry, electro-mechanochemistry and photo-mechanochemistry - represent a notable advance in modern mechanochemistry and herald a new level of solid-state reactivity: mechanochemistry 2.0.

A Review on Mechanochemistry: Approaching Advanced Energy Materials with Greener Force

Mechanochemistry with solvent-free and environmentally friendly characteristics is one of the most promising alternatives to traditional liquid-phase-based reactions, demonstrating epoch-making significance in the realization of different types of chemistry. Mechanochemistry utilizes mechanical energy to promote physical and chemical transformations to design complex molecules and nanostructured materials, encourage dispersion and recombination of multiphase components, and accelerate reaction rates and efficiencies via highly reactive surfaces. In particular, mechanochemistry deserves special attention because it is capable of endowing energy materials with unique characteristics and properties. Herein, the latest advances and progress in mechanochemistry for the preparation and modification of energy materials are reviewed. An outline of the basic knowledge, methods, and characteristics of different mechanochemical strategies is presented, distinguishing this review from most mechanochemistry reviews that only focus on ball-milling. Next, this outline is followed by a detailed and insightful discussion of mechanochemistry-involved energy conversion and storage applications. The discussion comprehensively covers aspects of energy transformations from mechanical/optical/chemical energy to electrical energy. Finally, next-generation advanced energy materials are proposed. This review is intended to bring mechanochemistry to the frontline and guide this burgeoning field of interdisciplinary research for developing advanced energy materials with greener mechanical force.

Direct Dry Synthesis of Supported Bimetallic Catalysts: A Study on Comminution and Alloying of Metal Nanoparticles

Ball milling is growing increasingly important as an alternative synthetic tool to prepare catalytic materials. It was recently observed that supported metal catalysts could be directly obtained upon ball milling from the coarse powders of metal and oxide support. Moreover, when two compatible metal sources are simultaneously subjected to the mechanochemical treatment, bimetallic nanoparticles are obtained. A systematic investigation was extended to different metals and supports to understand better the mechanisms involved in the comminution and alloying of metal nanoparticles. Based on this, a model describing the role of metal-support interactions in the synthesis was developed. The findings will be helpful for the future rational design of supported metal catalysts via dry ball milling.

The Mechanochemical Synthesis and Activation of Carbon-Rich π-Conjugated Materials

Mechanochemistry uses mechanical force to break, form, and manipulate chemical bonds to achieve functional transformations and syntheses. Over the last years, many innovative applications of mechanochemistry have been developed. Specifically for the synthesis and activation of carbon-rich π-conjugated materials, mechanochemistry offers reaction pathways that either are inaccessible with other stimuli, such as light and heat, or improve reaction yields, energy consumption, and substrate scope. Therefore, this review summarizes the recent advances in this research field combining the viewpoints of polymer and trituration mechanochemistry. The highlighted mechanochemical transformations include π-conjugated materials as optical force probes, the force-induced release of small dye molecules, and the mechanochemical synthesis of polyacetylene, carbon allotropes, and other π-conjugated materials.

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.

Toward Mechanistic Understanding of Mechanochemical Reactions Using Real-Time In Situ Monitoring

The past two decades have witnessed a rapid emergence of interest in mechanochemistry-chemical and materials reactivity achieved or sustained by the action of mechanical force-which has led to application of mechanochemistry to almost all areas of modern chemical and materials synthesis: from organic, inorganic, and organometallic chemistry to enzymatic reactions, formation of metal-organic frameworks, hybrid perovskites, and nanoparticle-based materials. The recent success of mechanochemistry by ball milling has also raised questions about the underlying mechanisms and has led to the realization that the rational development and effective harnessing of mechanochemical reactivity for cleaner and more efficient chemical manufacturing will critically depend on establishing a mechanistic understanding of these reactions. Despite their long history, the development of such a knowledge framework for mechanochemical reactions is still incomplete. This is in part due to the, until recently, unsurmountable challenge of directly observing transformations taking place in a rapidly oscillating or rotating milling vessel, with the sample being under the continuous impact of milling media. A transformative change in mechanistic studies of milling reactions was recently introduced through the first two methodologies for real-time in situ monitoring based on synchrotron powder X-ray diffraction and Raman spectroscopy. Introduced in 2013 and 2014, the two new techniques have inspired a period of tremendous method development, resulting also in new techniques for mechanistic mechanochemical studies that are based on temperature and/or pressure monitoring, extended X-ray fine structure (EXAFS), and, latest, nuclear magnetic resonance (NMR) spectroscopy. The new technologies available for real-time monitoring have now inspired the development of experimental strategies and advanced data analysis approaches for the identification and quantification of short-lived reaction intermediates, the development of new mechanistic models, as well as the emergence of more complex monitoring methodologies based on two or three simultaneous monitoring approaches. The use of these new opportunities has, in less than a decade, enabled the first real-time observations of mechanochemical reaction kinetics and the first studies of how the presence of additives, or other means of modifying the mechanochemical reaction, influence reaction rates and pathways. These studies have revealed multistep reaction mechanisms, enabled the identification of autocatalysis, as well as identified molecules and materials that have previously not been known or have even been considered not possible to synthesize through conventional approaches. Mechanistic studies through in situ powder X-ray diffraction (PXRD) and Raman spectroscopy have highlighted the formation of supramolecular complexes (for example, cocrystals) as critical intermediates in organic and metal-organic synthesis and have also been combined with isotope labeling strategies to provide a deeper insight into mechanochemical reaction mechanisms and atomic and molecular dynamics under milling conditions. This Account provides an overview of this exciting, rapidly evolving field by presenting the development and concepts behind the new methodologies for real-time in situ monitoring of mechanochemical reactions, outlining key advances in mechanistic understanding of mechanochemistry, and presenting selected studies important for pushing forward the boundaries of measurement techniques, data analysis, and mapping of reaction mechanisms.

Mechanochemically synthesised dicyanamide hybrid organic-inorganic perovskites, and their melt-quenched glasses

Here we present efficient and scalable mechanochemical formation of hybrid organic-inorganic perovskites of the form [TPrA][M(dca)₃] (M = Mn²⁺, Co²⁺) and the subsequent formation of their bulk melt-quenched glasses. In situ X-ray diffraction reveals direct, facile, and almost instantaneouos formation of both crystalline materials, while slow cooling limits recrystallisation in glasses. The glasses show good stability to acidic and basic aqueous solutions and display higher carbon dioxide uptakes than their crystalline precursors.

From Operando Raman Mechanochemistry to "NMR Crystallography": Understanding the Structures and Interconversion of Zn-Terephthalate Networks Using Selective 17O-Labeling

The description of the formation, structure, and reactivity of coordination networks and metal-organic frameworks (MOFs) remains a real challenge in a number of cases. This is notably true for compounds composed of Zn²⁺ ions and terephthalate ligands (benzene-1,4-dicarboxylate, BDC) because of the difficulties in isolating them as pure phases and/or because of the presence of structural defects. Here, using mechanochemistry in combination with operando Raman spectroscopy, the observation of the formation of various zinc terephthalate compounds was rendered possible, allowing the distinction and isolation of three intermediates during the ball-milling synthesis of Zn₃(OH)₄(BDC). An "NMR crystallography" approach was then used, combining solid-state NMR (¹H, ¹³C, and ¹⁷O) and density functional theory (DFT) calculations to refine the poorly described crystallographic structures of these phases. Particularly noteworthy are the high-resolution ¹⁷O NMR analyses, which were made possible in a highly efficient and cost-effective way, thanks to the selective ¹⁷O-enrichment of either hydroxyl or terephthalate groups by ball-milling. This allowed the presence of defect sites to be identified for the first time in one of the phases, and the nature of the H-bonding network of the hydroxyls to be established in another. Lastly, the possibility of using deuterated precursors (e.g., D₂O and d ₄-BDC) during ball-milling is also introduced as a means for observing specific transformations during operando Raman spectroscopy studies, which would not have been possible with hydrogenated equivalents. Overall, the synthetic and spectroscopic approaches developed herein are expected to push forward the understanding of the structure and reactivity of other complex coordination networks and MOFs.

Mechanistic Insights on the Mechanosynthesis of Phenytoin, a WHO Essential Medicine

In recent years, mechanochemistry has enriched the toolbox of synthetic chemists, enabling faster and more sustainable access to new materials and existing products, including active pharmaceutical ingredients (APIs). However, molecular‐level understanding of most mechanochemical reactions remains limited, delaying the implementation of mechanochemistry in industrial applications. Herein, we have applied in situ monitoring by Raman spectroscopy to the mechanosynthesis of phenytoin, a World Health Organization (WHO) Essential Medicine, enabling the observation, isolation, and characterization of key molecular‐migration intermediates involved in the single‐step transformation of benzil, urea, and KOH into phenytoin. This work contributes to the elucidation of a reaction mechanism that has been subjected to a number of interpretations over time and paints a clear picture of how mechanosynthesis can be applied and optimized for the preparation of added‐value molecules.

A review of recent developments for the in situ/operando characterization of nanoporous materials

This is a review on up-to-date in situ/operando methods for a comprehensive characterization of nanoporous materials. The group of nanoporous materials is constantly growing, and with it, the variety of...

Infrared spectroscopic monitoring of solid-state processes

We put a spotlight on IR spectroscopic investigations in materials science by providing a critical insight into the state of the art, covering both fundamental aspects, examples of its utilisation, and current challenges and perspectives focusing on the solid state.

Pharmaceutical nanococrystal synthesis: a novel grinding approach

Nanococrystals – a new green in situ surfactant-assisted mechanochemical synthesis.

In situ synchrotron x-ray diffraction studies monitoring mechanochemical reactions of hard materials: Challenges and limitations

In situ monitoring of mechanochemical reactions of soft matter is feasible by synchrotron diffraction experiments. However, so far, reactions of hard materials in existing polymer milling vessels failed due to insufficient energy input. In this study, we present the development of a suitable setup for in situ diffraction experiments at a synchrotron facility. The mechanochemical transformation of boehmite, γ-AlOOH, to corundum, α-Al₂O₃, was chosen as a model system. The modifications of the mill's clamping system and the vessels themselves were investigated separately. Starting from a commercially available Retsch MM 400 shaker mill, the influence of the geometrical adaptation of the setup on the milling process was investigated. Simply extending the specimen holder proved to be not sufficient because changes in mechanical forces need to be accounted for in the construction of optimized extensions. Milling vessels that are suitable for diffraction experiments and also guarantee the required energy input as well as mechanical stability were developed. The vessels consist of a steel body and modular polymer/steel rings as x-ray transparent windows. In addition, the vessels are equipped with a gas inlet and outlet system that is connectable to a gas analytics setup. Based on the respective modifications, the transformation of boehmite to corundum could be observed in an optimized setup.

Mechanochemical transformation of planar polyarenes to curved fused-ring systems

The transformation of planar aromatic molecules into π-extended non-planar structures is a challenging task and has not been realized by mechanochemistry before. Here we report that mechanochemical forces can successfully transform a planar polyarene into a curved geometry by creating new C-C bonds along the rim of the molecular structure. In doing so, mechanochemistry does not require inert conditions or organic solvents and provide better yields within shorter reaction times. This is illustrated in a 20-minute synthesis of corannulene, a fragment of fullerene C60, in 66% yield through ball milling of planar tetrabromomethylfluoranthene precursor under ambient conditions. Traditional solution and gas-phase synthetic pathways do not compete with the practicality and efficiency offered by the mechanochemical synthesis, which now opens up a new reaction space for inducing curvature at a molecular level.

Paving the Way to Establish Protocols: Modeling and Predicting Mechanochemical Reactions

Parametrization of mechanochemical reactions, or relating the evolution of the reaction progress to the supplied input power, is required both to establish protocols and to gain insight into mechanochemical reactions. Thus, results could be compared, replicated, or scaled up even under different milling conditions, enlarging the domains of application of mechanochemistry. Here, we propose a procedure that allows the parametrization of mechanochemical reactions as a function of the supplied input power from the direct analysis of the milling experiments in a model-free approach, where neither the kinetic model function nor the rate constant equation are previously assumed. This procedure has been successfully tested with the mechanochemical reaction of CH₃NH₃PbCl₃, enabling the possibility to make predictions regardless of the milling device as well as gaining insight into the reaction dynamic. This methodology can work for any other mechanical reaction and definitely paves the way to establish mechanochemistry as a standard synthetic procedure.

Mechanochemistry: A Green Approach in the Preparation of Pharmaceutical Cocrystals

Mechanochemistry is considered an alternative attractive greener approach to prepare diverse molecular compounds and has become an important synthetic tool in different fields (e.g., physics, chemistry, and material science) since is considered an ecofriendly procedure that can be carried out under solvent free conditions or in the presence of minimal quantities of solvent (catalytic amounts). Being able to substitute, in many cases, classical solution reactions often requiring significant amounts of solvents. These sustainable methods have had an enormous impact on a great variety of chemistry fields, including catalysis, organic synthesis, metal complexes formation, preparation of multicomponent pharmaceutical solid forms, etc. In this sense, we are interested in highlighting the advantages of mechanochemical methods on the obtaining of pharmaceutical cocrystals. Hence, in this review, we describe and discuss the relevance of mechanochemical procedures in the formation of multicomponent solid forms focusing on pharmaceutical cocrystals. Additionally, at the end of this paper, we collect a chronological survey of the most representative scientific papers reporting the mechanochemical synthesis of cocrystals.

Sustainability Assessment of Mechanochemistry by Using the Twelve Principles of Green Chemistry

In recent years, mechanochemistry has been growing into a widely accepted alternative for chemical synthesis. In addition to their efficiency and practicality, mechanochemical reactions are also recognized for their sustainability. The association between mechanochemistry and Green Chemistry often originates from the solvent-free nature of most mechanochemical protocols, which can reduce waste production. However, mechanochemistry satisfies more than one of the Principles of Green Chemistry. In this Review we will present a series of examples that will clearly illustrate how mechanochemistry can significantly contribute to the fulfillment of Green Chemistry in a more holistic manner.

Mechanochemical Synthesis of Catalytic Materials

The mechanochemical synthesis of nanomaterials for catalytic applications is a growing research field due to its simplicity, scalability, and eco-friendliness. Besides, it provides materials with distinct features, such as nanocrystallinity, high defect concentration, and close interaction of the components in a system, which are, in most cases, unattainable by conventional routes. Consequently, this research field has recently become highly popular, particularly for the preparation of catalytic materials for various applications, ranging from chemical production over energy conversion catalysis to environmental protection. In this Review, recent studies on mechanochemistry for the synthesis of catalytic materials are discussed. Emphasis is placed on the straightforwardness of the mechanochemical route-in contrast to more conventional synthesis-in fabricating the materials, which otherwise often require harsh conditions. Distinct material properties achieved by mechanochemistry are related to their improved catalytic performance.

Metastability and Seeding Effects in the Mechanochemical Hybrid Lead(II) Iodide Formation

The mechanism for the mechanochemical synthesis of (C(NH₂ )₃ )₃ PbI₅ 3 and (C(NH₂ )₃ )₄ PbI₆ 4 and their conversion into each other is presented. We investigated the synthesis of 3 at different frequencies and energies using in situ powder X-ray diffraction. By splitting the reaction into single parts we could prove that the formation of 3 is simply dependent on the energy and mixing speed. The nucleation of 4 instead is slightly negative dependent on the energy but dependent on the mixing speed, while its growth is mostly independent of any influence. We were able to influence the reaction pathways by seeding the mixture with a small amount of powdery 4. The formation of 4 is very likely an auto-catalytic process. 3 instead is metastable. It can be stabilized by energy, which beside mechanochemistry can also be achieved by temperature. The results showcases the complex nature of mechanochemical reactions.

Highly Efficient Solid-State Hydrolysis of Waste Polyethylene Terephthalate by Mechanochemical Milling and Vapor-Assisted Aging

Despite significant methodological and technological advancements in chemical recycling of synthetic polymers, an efficient and quantitative conversion of post-consumer polyethylene terephthalate (PET) into terephthalic acid (TPA) under ambient conditions of temperature and pressure still remains a challenge. In this respect, the application of mechanochemistry and multiple advantages offered by solid-state ball milling and vapor-assisted aging have remained insufficiently explored. To further expand their potential, the implementation of organic solvent-free milling as a superior methodology for successful alkaline depolymerization of waste PET (e. g., bottles and textile) into TPA monomer in near-quantitative yields was reported herein. The solid-state alkaline PET hydrolysis was also shown to proceed in excellent yields under aging conditions in humid environment or in the presence of alcohol vapors. Moreover, the performance of mechanochemical ball milling and aging in the gram-scale depolymerization of PET into TPA was demonstrated.

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.

Synthesis of Metal Organic Frameworks by Ball-Milling

Metal-organic frameworks (MOFs) have been used in adsorption, separation, catalysis, sensing, photo/electro/magnetics, and biomedical fields because of their unique periodic pore structure and excellent properties and have become a hot research topic in recent years. Ball milling is a method of small pollution, short time-consumption, and large-scale synthesis of MOFs. In recent years, many important advances have been made. In this paper, the influencing factors of MOFs synthesized by grinding were reviewed systematically from four aspects: auxiliary additives, metal sources, organic linkers, and reaction specific conditions (such as frequency, reaction time, and mass ratio of ball and raw materials). The prospect for the future development of the synthesis of MOFs by grinding was proposed.

Mechanochemistry of Metal-Organic Frameworks under Pressure and Shock

ConspectusMetal-organic framework solids (MOFs) are synthetic nanoporous materials that have drawn intense efforts in synthesis and characterization of chemical properties, most notably for their ability to adsorb liquids and gases. They are constructed as "node-spacer" nanostructured materials: metal centers (ions or clusters) connected by organic linkers (commonly containing carboxylate or imidazolate groups) to form crystalline, extended, often highly nanoporous structures. MOFs exhibit a variety of advantages over conventional porous materials: rationally designed synthesis of desired crystal structures and crystal engineering become feasible; great synthetic versatility and ease of incorporating different chemical functionalities are realized; and the use of lightweight organic linkers allows for ultrahigh surface area and porosity previously not accessible to conventional materials (i.e., zeolites and porous carbon). As a consequence, MOFs show great promise for a rapidly expanding collection of applications such as gas storage, separations, catalysis, sensing, and drug delivery.The mechanochemistry of MOFs and their response to shock waves, which we discuss in this Account, have been only partially explored. Mechanochemistry, the connection between the mechanical and the chemical worlds, has ancient origins. Rubbing sticks together to start a fire is mechanochemistry. Only in the past decade or so, however, has mechanochemistry gained a notable focus in the chemical community. In the following discussion, we present a general introduction to the complex mechanochemical behavior of MOFs both under quasi-static compression and under shock loading created by high-speed impact. During elastic deformation, MOFs undergo reversible structural or phase transitions. Plastic deformation of MOFs can result in mechanochemistry and can permanently modify the crystal structure, the pore dimensions and configuration, and the chemical bonding. The large energies required to induce bond rearrangement during plastic deformation suggest an interesting potential of MOFs for shock wave mitigation applications.MOFs are promising materials for shock energy dissipation because of the high density of nanopores which can absorb shock energy as they collapse. We have recently developed a platform to assess shock wave energy attenuation by MOFs and other powdered materials. It uses a tabletop laser-driven flyer plate to impact MOF samples at velocities of up to 2.0 km/s. The pressure of the shock waves that break out from the MOF sample can be measured by photon Doppler velocimetry. By measuring the shock profiles of MOF layers with different thicknesses, we can determine the shock pressure attenuation by the MOF layer. We have identified the two-wave structure of shocks in MOFs caused by nanopore collapse. Electron micrographs of recovered shocked MOFs show distinct zones in the shocked material corresponding to shock powder compaction, nanopore collapse, and chemical bond destruction.

Direct Mechanocatalysis: Using Milling Balls as Catalysts

Direct mechanocatalysis describes catalytic reactions under the involvement of mechanical energy with the distinct feature of milling equipment itself being the catalyst. This novel type of catalysis features no solubility challenges of the catalysts nor the substrate and on top offering most facile way of separation.

Ball milling as a mechanochemical technology for fabrication of novel biochar nanomaterials

Mechanochemical synthesis of nano-biochar by ball-milling technology is gaining attention for the sake of its low-cost and eco-friendly nature. Ball milling as a non-/less-solvent technology can propel environmental sustainability and waste valorization into engineered biochar for advanced applications. Scalable production of biochar nanomaterials with superior properties (e.g., 400-500 m² g^-1 surface area and 0.5-1000 nm pore sizes) enables diverse applications in the field of energy and environment. This review critically evaluates the synthesis, characterization, and application of ball-milled biochar nanomaterials based on the latest findings. Limitations such as feedstock selection, process optimization, product homogeneity and reusability, environmental risks, and sustainability assessment remain challenging for further studies. This work highlights the recent advances on mechanochemical biochar technology and offer insights into opportunities and future prospects related to sustainable and facile synthesis of biochar-based novel materials for achieving sustainable development goals.

Tribochemistry as an Alternative Synthesis Pathway

While reactions driven by mechanical force or stress can be labeled mechanochemical, those specifically occurring at a sliding interface inherit the name tribochemical, which stems from the study of friction and wear: tribology. Increased perception of tribochemical reactions has been gained through technological advancement, and the development of new applications remains on-going. This surprising physico-kinetic process offers great potential in novel reaction pathways for synthesis techniques and nanoparticle interactions, and it could prove to be a powerful cross-disciplinary research area among chemists, engineers, and physicists. In this review article, a survey of the history and recent usage of tribochemical reaction pathways is presented, with a focus on forging new compounds and materials with this sustainable synthesis methodology. In addition, an overview of tribochemistry’s current utility as a synthesis pathway is given and compared to that of traditional mechanochemistry.

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.

Tetrathiafulvalene: A Gate to the Mechanochemical Mechanisms of Electron Transfer Reactions

This report describes aspects of our previous studies of the mechanochemical synthesis of charge transfer complexes of the electron donor tetrathiafulvalene, which are relevant to the use of laboratory X-ray powder diffraction for ex situ monitoring of mechanochemical reactions toward investigating their mechanisms. In particular, the reaction of tetrathiafulvalene and chloranil was studied under neat mechanochemical conditions and liquid-assisted grinding with diethyl ether (1 μL/mg). The product in both cases is the green tetrathiafulvalene chloranil polymorph and the mechanism of the redox reaction is presumably the same. However, while the kinetic profile of the neat mechanochemical synthesis was fitted with a second-order rate law, that of the overall faster liquid-assisted grinding reaction was fitted with the Ginstling-Brounshtein 3D diffusion-controlled model. Hence, the diffusional processes and mass transfer bringing the reactants together and separating them from products must be different. Diffraction measurements sensitive to crystalline phases and amorphous material, combined with in situ monitoring by spectroscopic techniques, will ultimately afford a better understanding of mechanochemical reaction mechanisms, a hot topic in mechanochemistry.