Solid-state reactions can be scaled up for industrial chemical production in a waste-free and environmentally benign manner, claims a German chemist.

Gerd Kaupp, from the University of Oldenburg, performed a selection of the thousand-plus known solid-state reactions at kilogram scale. He found that the reactions were easy to perform, proceeded with a 100% yield, and did not require a purifying workup.

For solid-state reactions to be successful, molecules must be able to move within the crystal, the product phase must form quickly, and the crystal must break down to allow formation of the new product surface. This three-step solid state mechanism is very efficient in reactions between solids and gases. However, in reactions between two solids, new contacts between the particles have to be continuously generated by milling the solids.

Kaupp realised that the key to the mechanism is to keep the reactants solid. Molecules in solution are less reactive due to solvation, and the reactions less specific because the molecules are not aligned as they are in a crystal, he said. In the solid state, the reactivity remains high, catalysts are unnecessary, and the reactions are cheaper than solution or melt reactions.

'This technique is so efficient that new and delicate products can now be obtained, products that could not be made by any other techniques,' explained Kaupp.

Dario Braga, an expert in crystal engineering from the University of Bologna, Italy, recognises the significance of these results: 'The notion that solid-solid and solid-gas reactions can be used to make new chemicals and new crystals is important for crystal engineers. Crystal makers should always try to make their materials without solvent first!'

Kaupp hopes that his work will help the scientific community understand that waste-free solid-state chemistry is 'much more than an academic lab curiosity.' He intends for his research to form the guidelines for industrial chemists scaling-up reactions in the future.

Joanne Thomson