Molecular design of macrocyclic extractants for extraction and separation of alkali and alkaline-earth metals

The distribution coefficients of the alkaline and alkali-earth metal nitrates are determined for the processes of extraction from nitric acid solutions with crown ethers, which differ in sizes of their cycles (from 18 to 22 atoms) and in side phenyl and cyclohexyl substituents, but have unchanged number of the O atoms in a cycle. The phenyl and cyclohexyl derivatives of 18-crown-6 are found to be more efficient extractants of alkali metals as compared with unsubstituted crown ethers. However, the selectivity of separation of a pair Cs-Na remains low. The 18-crown-6 derivatives with one and two cyclohexyl fragments selectively extract Sr, which can be used to separate it from the remaining alkali and alkaline-earth metals. An increase in the size of a macrocycle from 18 to 20 and 22 atoms leads to a reduction in the extraction efficiency for all alkali and alkaline-earth metals, insignificantly improves separation of a pair Cs-Na, but noticeably deteriorates Sr-Na separation.