The past decade has seen a reawakening of solid-state approaches to chemical synthesis, driven by the search for new, cleaner synthetic methodologies. Mechanochemistry, i.e., chemical transformations initiated or sustained by mechanical force, has been advancing particularly rapidly, from a laboratory curiosity to a widely applicable technique that not only enables a cleaner route to chemical transformations but offers completely new opportunities in making and screening for molecules and materials. This Outlook provides a brief overview of the recent achievements and opportunities created by mechanochemistry, including access to materials, molecular targets, and synthetic strategies that are hard or even impossible to access by conventional means.

http://pubs.acs.org/doi/pdf/10.1021/acscentsci.6b00277

Mechanochemistry: A Force of Synthesis

Mechanochemistry is becoming more widespread as a technique for molecular synthesis with new mechanochemical reactions being discovered at increasing frequency. Whilst mechanochemical methods are solvent free and can therefore lead to improved sustainability metrics, it is more likely that the significant differences between reaction outcomes, reaction selectivities and reduced reaction times will make it a technique of interest to synthetic chemists. Herein, we provide an overview of mechanochemistry reaction examples, with ‘direct’ comparators to solvent based reactions, which collectively seemingly show that solid state grinding can lead to reduced reaction times, different reaction outcomes in product selectivity and in some instances different reaction products, including products not accessible in solution.

/pdf/mechanochemistry-as-an-emerging-tool-for-molecular-synthesis-1fikz7tsl4.pdf

Mechanochemistry as an emerging tool for molecular synthesis: what can it offer?

Mechanochemical activation achieved by grinding, shearing, pulling, or milling opens unique opportunities in synthetic organic chemistry. Common features are that mechanochemistry facilitates reactions with insoluble reactants, enables high-yielding solvent-free synthetic procedures, and shortens reaction times, among others. However, mechanochemical techniques can also alter chemical reactivity and selectivity compared to the analogous solution-based protocols. As a consequence, solvent-free milling can lead to different product compositions or equilibration mixtures than in solution. Reactions by milling have also allowed the trapping and characterization of elusive intermediates and materials. In this Perspective we highlight a few selected examples that illustrate the value of mechanochemistry in uncovering interesting chemical reactivity, which is often masked in typical liquid-phase synthesis.

Altering Product Selectivity by Mechanochemistry

Research on covalent organic frameworks (COFs) has recently gathered significant momentum by the virtue of their predictive design, controllable porosity, and long-range ordering. However, the lack of solvent-free and easy-to-perform synthesis processes appears to be the bottleneck toward their greener fabrication, thereby limiting their possible potential applications. To alleviate such shortcomings, we demonstrate a simple route toward the rapid synthesis of highly crystalline and ultraporous COFs in seconds using a novel salt-mediated crystallization approach. A high degree of synthetic control in interlayer stacking and layer planarity renders an ordered network with a surface area as high as 3000 m2 g-1. Further, this approach has been extrapolated for the continuous synthesis of COFs by means of a twin screw extruder and in situ processes of COFs into different shapes mimicking the ancient terracotta process. Finally, the regular COF beads are shown to outperform the leading zeolites in water sorption performance, with notably facile regeneration ability and structural integrity.

Constructing Ultraporous Covalent Organic Frameworks in Seconds via an Organic Terracotta Process

Mechanochemistry and organic synthesis: from mystical to practical

Grinding solid reagents under solvent-free or low-solvent conditions (mechanochemistry) is emerging as a general synthetic technique which is an alternative to conventional solvent-intensive methods. However, it is essential to find ways to scale-up this type of synthesis if its promise of cleaner manufacturing is to be realised. Here, we demonstrate the use of twin screw and single screw extruders for the continuous synthesis of various metal complexes, including Ni(salen), Ni(NCS)2(PPh3)2 as well as the commercially important metal organic frameworks (MOFs) Cu3(BTC)2 (HKUST-1), Zn(2-methylimidazolate)2 (ZIF-8, MAF-4) and Al(fumarate)(OH). Notably, Al(fumarate)(OH) has not previously been synthesised mechanochemically. Quantitative conversions occur to give products at kg h−1 rates which, after activation, exhibit surface areas and pore volumes equivalent to those of materials produced by conventional solvent-based methods. Some reactions can be performed either under completely solvent-free conditions whereas others require the addition of small amounts of solvent (typically 3–4 mol equivalents). Continuous neat melt phase synthesis is also successfully demonstrated by both twin screw and single screw extrusion for ZIF-8. The latter technique provided ZIF-8 at 4 kg h−1. The space time yields (STYs) for these methods of up to 144 × 103 kg per m3 per day are orders of magnitude greater than STYs for other methods of making MOFs. Extrusion methods clearly enable scaling of mechanochemical and melt phase synthesis under solvent-free or low-solvent conditions, and may also be applied in synthesis more generally.

/pdf/synthesis-by-extrusion-continuous-large-scale-preparation-of-4ajypj1rd1.pdf

Synthesis by extrusion: continuous, large-scale preparation of MOFs using little or no solvent

A process for the preparation of a metal-organic compound, said metal- organic compound comprising at least one metal ion and at least one organic ligand, wherein said organic ligand is capable of associating with said metal ion, comprising at least the steps of; providing a first reactant comprising at least one metal in ionic form; providing a second reactant comprising at least one organic ligand capable of associating with said metal in ionic form; and admixing said first and second reactants under conditions of prolonged and sustained pressure and shear sufficient to synthesise said metal-organic compound.

Process for the preparation of a metal-organic compound

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Synthesis by extrusion: continuous, large-scale preparation of MOFs using little or no solvent

Process for the preparation of a metal-organic compound