Rafael Luque, Professor, Departamento de Quimica Organica, Universidad de Cordoba, Spain
Mechanochemistry deals with chemical transformations induced by mechanical means such as compression, shear or friction. In mechanochemical processes, the energy required for the activation of chemical reactions is usually provided by mechanical force as similar to thermochemistry, photochemistry or electrochemistry where energy is provided by heat, light or electrical potential, respectively.
Importantly, the solvent often plays a key role in energy dispersion, dissolution/solvation and transportation of chemicals in conventional chemical synthesis. Mass and energy transport may also be hampered in solventless reactions. The efficient mixing process under ball milling or grinding can offer an effective way out of this problem, enabling the reactions between solids/powders or solidified reagents in solvent-free conditions.
Solventless, “dry milling” mechanochemical approaches, highly advantageous for certain applications, can also be replaced by “liquid-assisted grinding” (LAG) as bridging alternative to minimize the use of solvents in mechanochemical syntheses. In contrast to “dry milling”, LAG may offer advantages such as greater time efficiency, enhancing molecular mobility and can result in the discovery of new or improved reactivity and (nano)materials.
Mechanochemical processes have a number of relevant advantages as compared to conventional syntheses including 1) an inherently “greener” approach to conduct chemical/materials syntheses (solvent-free or solvent-limited); 2) improved energy efficiency and solvent use (up to 1000-fold reduction/improvement); 3) swiftness and remarkably faster than solution synthesis (allowing a rapid screening of synthesis conditions for materials and/or chemical reactions); 4) wider choice of starting materials and possibilities (e.g. cheaper and more environmentally friendly reactants); 5) high yielding and facilitating/avoiding purification/isolation steps.
In view of these relevant advantages, the potential of mechanochemistry is significant, not only in the design of advanced and new (nano)materials for applications in multiple fields (adsorption, catalysis, energy storage, sensing, etc.) but also in the promotion of chemical reactions (mechanocatalysis). Some relevant examples are given in the following sections.
Design of advanced (nano)materials
Mechanochemistry has already paved the way to the design of advanced and new functional (nano)materials which includes (but not limited to) perovskites, spinels, metal-organic frameworks (MOFs), supported nanoparticles on porous materials, bionanoconjugates an electrodes/biosensors (i.e. laccase@TiO2@C) and many more. The possibilities are enormous and mechanochemistry was found to provide access to new structures, enhanced properties and improved activities in certain applications (e.g. catalysis).
Organic-inorganic hybrid perovskites are materials that have attracted significant attention due to their extraordinary optoelectronic properties with applications in the fields of solar energy, lighting, photodetectors, and lasers. The rational design of these hybrid materials is a key factor in the optimization of their performance in perovskite-based devices. These could be successfully synthesized using a highly efficient, simple, and reproducible solventless mechanochemical approach. Materials could be synthesized 1) in large amounts (multi-gram scale), 2) as polycrystalline powders with high purity, and 3) in a very short synthesis time (typically 10-30 mins). Three-dimensional (3D) (e.g. MAPbI3 and FAPbI3), bidimensional (2D) (e.g. Gua2PbI4) and one-dimensional (1D) perovskites (e.g. GuaPbI3) were reported, indicating also a unique flexibility of the mechanochemical step to provide access to different types of structures (Figure 1).
Figure 1: Source http://onlinelibrary.wiley.com/doi/10.1002/anie.201607397/full
Mixed spinel inorganic materials (e.g. MgFe2O4 and MgAl2O4 can be also synthesized in high yields, purity and short times of syntheses (15-30 mins) under solventless mechanochemical conditions. The mechanochemical approach provides a simple and efficient alternative to conventional methods to prepare spinels which typically employ large quantities of solvents (sol-gel, hydrothermal methods) or extremely high temperatures >1200ºC (combustion methods), illustrating the potential of this methodology. In addition to the green credentials of mechanochemistry, spinel materials obtained by this method were reported to be highly crystalline, homogeneous in shape and particle size and could be again obtained in large quantities (multi-gram scale) within a short processing time.
Similarly, metal-organic frameworks (MOFs) comprising organic molecules linking transition metals to form a porous material network have been also synthesized using solventless mechanochemical methods, providing access to new structures (e.g. pillared MOFS from their metal oxides, new porous MOFs or quasi-MOFs, etc.). Apart from the different new structures that can be potentially designed by means of the mechanochemical approach, the green chemistry advantages of the mechanochemical methodology are also clear: a 30 min grinding with limited quantities of solvents (via LAG) at room temperature could replace a 24-48 h solvothermal synthesis (100-160ºC) using large quantities of solvents and 10,000 times more energy consuming (http://onlinelibrary.wiley.com/wol1/doi/10.1002/anie.200906583/abstract)
Last, but not least importantly, bio(nano)conjugates have been recently developed using mechanochemical syntheses comprising redox proteins (e.g. horse hemoglobin) and magnetic nanoparticles for various relevant applications including the synthesis of carbon-based fluorescent polymers at room temperature (see Figure 2), electrochemistry and energy storage (http://pubs.rsc.org/en/content/articlelanding/2018/nr/c8nr00512e/unauth#!divAbstract). In some cases, the utilized magnetic (and other non-magnetic systems) can also be effectively synthesized in a ball mill under mild reaction conditions (room temperature, solid-state reactions, solvent-free, typically in minutes).
In addition to the mechanochemical syntheses, the possibility to conduct chemical reactions using mechanochemistry (mechanocatalysis) also recently emerged as a promising alternative to promote a number of chemistries under mild and environmentally friendly reaction conditions. Stemming from the aforementioned advantages, oxidations, C-C coupling reactions, acid-based catalyzed processes (e.g. esterifications) and related others have been already reported to take place under mechanochemical conditions.
Interestingly, biomass conversion was also successfully accomplished using mechanocatalysis, with examples of cellulose depolymerization to sugars and lignin deconstruction to valuable aromatics. This has a significant potential for further studies and its combination with a rational understanding of catalyst/process design will undoubtedly lead to important scientific advances in biomass conversion in future years.
From the beginning, this contribution has been aimed to provide an overview of the relevance and inherent advantages of mechanochemistry for multiple applications (materials design, catalysis, organic syntheses, biomass deconstruction, etc.). Reported results to date clearly illustrate the present and future potential and possibilities of mechanochemistry despite the relatively poor understanding of the phenomenon as such. Further studies are needed to be able to fully understand chemical, physical and structural changes taking place in mechanochemical syntheses (in-situ methodologies) to rationally design processes and methodologies based on such fundamental understanding. These studies will in any case complement nicely the burgeoning possibilities of mechanochemistry in various fields based on its inherent green credentials.
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