CaO assisted mechanochemical remediation of lindane-contaminated soil

In this study, the planetary ball milling with CaO addition was used to remediate lindane-contaminated soil. Based on Hertzian theory, a mathematical model was proposed to simulate the trajectory of grinding ball and the local energy transfer during a planetary operation at the disk rotation velocities of 150-250 rpm. Besides, the influence of different parameters on lindane removal in soil was investigated, whose results showed that disk rotation velocity and reagent-to-soil ratio had a positive effect, while soil moisture, initial concentration of lindane, and mass of polluted soil demonstrated a negative influence. The mechanochemical method exhibited a higher degradation performance at 3 wt% CaO addition, and a disk rotation velocity of 250 rpm. Active species generated by ball collisions in the presence of CaO, especially superoxide (·O2-) demonstrated a significant role in participating in the lindane conversion. In combination with GCMS and XPS analysis, the proposed model provides insight into mechanochemical remediation process from physical and chemical perspectives, which mainly includes four main steps: mixing, inducing, chemical reaction, and structure destruction.

Kinetics of the mechanically induced ibuprofen-nicotinamide co-crystal formation by in situ X-ray diffraction

Mechanochemistry is drawing attention from the pharmaceutical industry given its potential for sustainable material synthesis and manufacture. Scaling mechanochemical processes to industrial level remains a challenge due to an incomplete understanding of their underlying mechanisms. We here show how time-resolved in situ powder X-ray diffraction data, coupled with analytical kinetic modelling, provides a powerful approach to gain mechanistic insight into mechanochemical reactions. By using the ibuprofen-nicotinamide co-crystal mechanosynthesis as a benchmark system, we investigate the behaviour of the solids involved and identify the factors that promote the reaction. As mechanochemical mechanisms become increasingly clear, it promises to become a breakthrough in the industrial preparation of advanced pharmaceuticals.

Modeling Mechanochemical Depolymerization of PET in Ball-Mill Reactors Using DEM Simulations

Developing efficient and sustainable chemical recycling pathways for consumer plastics is critical for mitigating the negative environmental implications associated with their end-of-life management. Mechanochemical depolymerization reactions have recently garnered great attention, as they are recognized as a promising solution for solvent-free transformation of polymers to monomers in the solid state. To this end, physics-based models that accurately describe the phenomena within ball mills are necessary to facilitate the exploration of operating conditions that would lead to optimal performance. Motivated by this, in this paper we develop a mathematical model that couples results from discrete element method (DEM) simulations and experiments to study mechanically-induced depolymerization. The DEM model was calibrated and validated via video experimental data and computer vision algorithms. A systematic study on the influence of the ball-mill operating parameters revealed a direct relationship between the operating conditions of the vibrating milling vessel and the total energy supplied to the system. Moreover, we propose a linear correlation between the high-fidelity DEM simulation results and experimental monomer yield data for poly(ethylene terephthalate) depolymerization, linking mechanical and energetic variables. Finally, we train a reduced-order model to address the high computational cost associated with DEM simulations. The predicted working variables are used as inputs to the proposed mathematical expression which allows for the fast estimation of monomer yields.

Mechanochemical ignition of self-propagating reactions in equimolar Al-Ni powder mixtures and multilayers

This work addresses a long standing question in the field of mechanochemistry, namely the role of mesostructure in the initiation of self-propagating high-temperature reactions in exothermic chemical systems, commonly referred to as ignition. In an attempt to find robust evidence in this regard, we compare the ignition behaviour of equimolar Al-Ni powder mixtures and equimolar Al-Ni multilayers. To achieve the best possible control of experimental conditions and allowing high reproducibility, we used elemental powders sieved in the range between 20 μm and 44 μm, and multilayers with bi-layer thickness between 10 nm and 800 nm. We carried out systematic ball milling experiments involving pristine powder mixtures and multilayers as well as a mix of pristine material and material prone to ignition suitably prepared. Experimental findings suggest that pristine powder mixtures and multilayers with bi-layer thickness of 240 nm have analogous ignition behaviour. Along the same lines, data suggest that pristine powder mixtures undergo ignition when they attain a mesostructure similar to that of multilayers with bi-layer thickness of 10 nm.

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.

The first solid-state route to luminescent Au(I)-glutathionate and its pH-controlled transformation into ultrasmall oligomeric Au10-12(SG)10-12 nanoclusters for application in cancer radiotheraphy

There is still a need for synthetic approaches that are much faster, easier to scale up, more robust and efficient for generating gold(I)-thiolates that can be easily converted into gold-thiolate nanoclusters. Mechanochemical methods can offer significantly reduced reaction times, increased yields and straightforward recovery of the product, compared to the solution-based reactions. For the first time, a new simple, rapid and efficient mechanochemical redox method in a ball-mill was developed to produce the highly luminescent, pH-responsive Au(I)-glutathionate, [Au(SG)]_n. The efficient productivity of the mechanochemical redox reaction afforded orange luminescent [Au(SG)]_n in isolable amounts (mg scale), usually not achieved by more conventional methods in solution. Then, ultrasmall oligomeric Au_10-12(SG)_10-12 nanoclusters were prepared by pH-triggered dissociation of [Au(SG)]_n. The pH-stimulated dissociation of the Au(I)-glutathionate complex provides a time-efficient synthesis of oligomeric Au_10-12(SG)_10-12 nanoclusters, it avoids high-temperature heating or the addition of harmful reducing agent (e.g., carbon monoxide). Therefore, we present herein a new and eco-friendly methodology to access oligomeric glutathione-based gold nanoclusters, already finding applications in biomedical field as efficient radiosensitizers in cancer radiotherapy.

Role of Crystal Disorder and Mechanoactivation in Solid-State Stability of Pharmaceuticals

Common energy-intensive processes applied in oral solid dosage development, such as milling, sieving, blending, compaction, etc. generate particles with surface and bulk crystal disorder. An intriguing aspect of the generated crystal disorder is its evolution and repercussion on the physical- and chemical stabilities of drugs. In this review, we firstly examine the existing literature on crystal disorder and its implications on solid-state stability of pharmaceuticals. Secondly, we discuss the key aspects related to the generation and evolution of crystal disorder, dynamics of the disordered/amorphous phase, analytical techniques to measure/quantify them, and approaches to model the disordering propensity from first principles. The main objective of this compilation is to provide special impetus to predict or model the chemical degradation(s) resulting from processing-induced manifestation in bulk solid manufacturing. Finally, a generic workflow is proposed that can be useful to investigate the relevance of crystal disorder on the degradation of pharmaceuticals during stability studies. The present review will cater to the requirements for developing physically- and chemically stable drugs, thereby enabling early and rational decision-making during candidate screening and in assessing degradation risks associated with formulations and processing.

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.

Spiers Memorial Lecture: Mechanochemistry, tribochemistry, mechanical alloying - retrospect, achievements and challenges

The paper presents a view on the achievements, challenges and prospects of mechanochemistry. The extensive reference list can serve as a good entry point to a plethora of mechanochemical literature.

Mechanochemical reaction kinetics scales linearly with impact energy

Inelastic collisions of the milling media in ball milling provide energy to the reaction mixture required for chemical transformations. However, movement of the milling media also results in physical mixing of reactants, which may enable a chemical reaction too. Separating the two contributions is challenging and gaining a direct insight into the purely mechanochemically driven reactivity is accordingly hindered. Here, we have applied in situ reaction monitoring by Raman spectroscopy to a suitable, purely mechanically activated, chemical reaction and combined kinetic analysis with numerical simulations to access experimentally unattainable milling parameters. The breadth of milling conditions allows us to establish a linear relationship between the reaction rate and the energy dose received by the sample. Consequently, different kinetic profiles in time scale to the same profile when plotted against the energy dose, which increases with the ball mass, the average ball velocity and the frequency of impacts, but decreases with the hardness of the milling media due to more elastic collisions. The fundamental relationship between kinetics and energy input provides the basis for planning and optimisation of mechanochemical reactions and is essential for transferability of mechanochemical reactions across different milling platforms.

Mechanochemistry: New Tools to Navigate the Uncharted Territory of "Impossible" Reactions

Mechanochemical transformations have made chemists enter unknown territories, forcing a different chemistry perspective. While questioning or revisiting familiar concepts belonging to solution chemistry, mechanochemistry has broken new ground, especially in the panorama of organic synthesis. Not only does it foster new "thinking outside the box", but it also has opened new reaction paths, allowing to overcome the weaknesses of traditional chemistry exactly where the use of well-established solution-based methodologies rules out progress. In this Review, the reader is introduced to an intriguing research subject not yet fully explored and waiting for improved understanding. Indeed, the study is mainly focused on organic transformations that, although impossible in solution, become possible under mechanochemical processing conditions, simultaneously entailing innovation and expanding the chemical space.

Controlling Nonlinear Dynamics of Milling Bodies in Mechanochemical Devices Driven by Pendular Forcing

Understanding the dynamics of milling bodies is key to optimize the mixing and the transfer of mechanical energy in mechanochemical processing. In this work, we present a comparative study of mechanochemical reactors driven by harmonic pendular forcing and characterized by different geometries of the lateral borders. We show that the shape of the reactor bases, either flat or curved, along with the size of the milling body and the elasticity of the collisions, represents relevant parameters that govern the dynamical regimes within the system and can control the transition from periodic to chaotic behaviors. We single out possible criteria to preserve target dynamical scenarios when the size of the milling body is changed, by adapting the relative extent of the spatial domain. This allows us to modulate the average energy of the collisions while maintaining the same dynamics and paves the way for a unifying framework to control the dynamical response in different experimental conditions. We finally explore the dynamical and energetic impact of an increasingly asymmetric mechanical force.

Mechanochemical Conversion of Aromatic Amines to Aryl Trifluoromethyl Ethers

Increased interest in the trifluoromethoxy group in organic synthesis and medicinal chemistry has induced a demand for new, selective, general, and faster methods applicable to natural products and highly functionalized compounds at a later stage of hit-to-lead campaigns. Applying pyrylium tetrafluoroborate, we have developed a mechanochemical protocol to selectively substitute the aromatic amino group with the OCF₃ functionality. The scope of our method includes 31 examples of ring-substituted anilines, including amides and sulfonamides. Expected S_NAr products were obtained in excellent yields. The presented concise method opens a pathway to new chemical spaces for the pharmaceutical industry.

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.

Improved fractal kinetic model to predict mechanochemical destruction rate of organic pollutants

Mechanochemical destruction of organic pollutants by high energy milling with inorganic reagents is considered a promising non-thermal technology to detoxify hazardous waste. However, due to complex nature of the physicochemical phenomena involved, pollutant destruction kinetics heavily depends on the used reagents and operating parameters, thus varying case by case. In the present work, a fractal model was validated as flexible tool to interpolate pollutant mechanochemical destruction data satisfactorily. In addition, such model was expanded to estimate the contributions of the inorganic reagent and the pollutant to the overall reaction rate. Specifically, the kinetic constant associated to mechanical activation of the co-milling reagent and that related to pollutant destruction reaction were calculated. Their values resulted to depend only on the specific compound, hence, the tabulated data could be used to predict the pollutant mechanochemical degradation rate for any kind of mixture.

Using Desmotropes, Cocrystals, and Salts to Manipulate Reactivity in Mechanochemical Organic Reactions

Performing reactions in the solid state offers the largely unexplored possibility of influencing reactivity by manipulating the solid form of the starting reactants. In this work, we explore the use of various solid forms of barbituric acid and its effect on reaction paths and kinetics in a Knoevenagel condensation reaction with vanillin. Modifications of barbituric acid included the use of its desmotrope, a cocrystal, and a salt as the starting reactant. Comparing these reactions with the reaction starting from the commercial keto tautomer of barbituric acid, we find that the reaction kinetics could be accelerated or decelerated, together with a change in the reaction mechanism. Exploring solid forms of reactants can be used as general methodology for manipulating mechanochemical reactivity, further highlighting the benefits of conducting reactions in the solid state, because many of the modifications of solids become unavailable upon dissolution.

Mechanochemical Rearrangements

Molecular rearrangements are a powerful tool for constructing complex structures in an atom- and step-economic manner, translating multistep transformations into an intrinsically more sustainable process. Mechanochemical molecular rearrangements become an even more appealing eco-friendly synthetic approach, especially for preparing active pharmaceutical ingredients (APIs) and natural products. Still in their infancy, rearrangements promoted by mechanochemistry represent a promising approach for chemists to merge molecular diversity and green chemistry perspectives toward more selective and efficient syntheses with a reduced environmental footprint.

Solvent-free Reactions for the Synthesis of Indolenine-based Squaraines and Croconaines: Comparison of Thermal Heating, Mechanochemical Milling, and IR Irradiation

Squaraines and croconaines are organic dyes characterized by intense absorption in the visible or near-infrared spectral regions with applications ranging from biology to material sciences. They are commonly synthesized by condensation reactions of oxocarbonic acids (squaric or croconic acid, respectively) with electron-rich aromatic compounds in high-boiling organic solvents. Here, a simple, cost-effective, and environmentally benign process was developed for the synthesis of indolenine-based squaraines and croconaines under solvent-free conditions. Protocols based on conventional thermal heating, mechanochemical milling, and IR-light activation were compared.

Coupling of mechanical deformation and reaction in mechanochemical transformations

Co-deformation and forced mixing contribute to the rate of chemical reaction between molecular solids subjected to mechanical processing.

A phenomenological kinetic equation for mechanochemical reactions involving highly deformable molecular solids

With its ability to enable solvent-free chemical reactions, mechanochemistry promises to open new and greener synthetic routes to chemical products of industrial interest. Its practical exploitation requires understanding the relationships between processing variables, powders' mechanical behaviour, and chemical reactivity. To this aim, rationalizing experimental kinetics is of paramount importance. In this work, we propose a phenomenological kinetic model that could help experimentalists to disentangle the mechanical, chemical, and statistical factors underlying mechanochemical reactions. The model takes into account the statistical nature of ball milling and relates the global kinetic curve that can be obtained experimentally to the deformation and chemical processes that occur on the mesoscopic and microscopic scales during individual impacts. We show that our model equations can satisfactorily best fit experimental datasets, providing information on the underlying mechanochemistry.

Mechanochemical Preparation of Active Pharmaceutical Ingredients Monitored by In Situ Raman Spectroscopy

The mechanochemical preparation of silver sulfadiazine and dantrolene, two marketed active pharmaceutical ingredients, was investigated by in situ Raman spectroscopy. For the first time, the mechanochemical transformations involving highly fluorescent compounds could be studied in situ with a high-resolution Raman system combined with a unique suitable Raman probe. Moreover, the kinetic features of the mechanochemical process were examined by a mathematical model allowing to describe the chemical changes under mechanical stress. This approach is promising both to broaden the scope of Raman in situ investigations that would otherwise be impossible and for process optimization at any scale.