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Solventless synthesis of zeolites.

In chemistry, the age-old question “Which solvent should I use?” is gradually being replaced with “Do I actually need a solvent at all?” It is of course very timely to develop solventfree approaches to synthesis because of the great scope for improving upon the environmental impact, safety, energy consumption, and economic cost associated with traditional solvent-intensive chemical processes. Of these, the cost associated with recycling waste solvent provides a very strong driver for increasing the product to solvent ratio, with the eventual goal of removing the need for solvents altogether. Of course, this driver is strongest when the solvent itself is expensive or toxic (as is the case with most organic solvents) but even when the solvent is water the treatment and recycling costs can be significant. For these reasons we are currently witnessing the accelerated development of solventfree synthesis. Of course solventless synthesis is not new. It has long been applied to the synthesis of inherently insoluble inorganic materials, and the synthesis of many ceramic oxides, hydrides, and nitrides is carried out in the solid state routinely. However, these synthetic procedures often require very high temperatures and repeat firings to ensure that the bondmaking/-breaking and organization processes have enough energy for the formation of crystalline phases. An alternative method of adding energy into the system is through grinding, in what is commonly called mechanochemical synthesis. This involves simply grinding, either by hand using a pestle and mortar or nonmanually using a ball mill or other apparatus. Although still counterintuitive to many chemists, this simple method can often provide quantitative reactions within a few minutes. We aren t (yet) taught this in university courses! Strikingly, in addition to generating less waste, there are often practical benefits of ease, simplicity, and rate compared to traditional solvent-based methods. Although this method has traditionally been applied to inherently insoluble inorganic materials such as alloys, oxides, nitrides, and hydrides, it is now being extended very effectively into the molecular domain. It has been found to be highly efficient for a wide range of organic C C and C X bond-forming reactions, for the formation of pharmaceutical cocrystals, and for the synthesis of metal complexes including porous metal organic frameworks (MOFs). Encouragingly, we are also seeing increased patent activity and industrial application of “molecular mechanochemistry”, even performed at scales of several tens of kilograms, such as in the synthesis of certain drug–carrier composites. Further unconventional but effective approaches to solventless molecular synthesis include the use of melt phases and the exploitation of reactions between solids and gases. The relationships between the various types of synthetic techniques are shown in Figure 1.