Recent Advances in Molten Salts as Reaction Media

The factors which distinguish molten salts from other non-aqueous solvents are described and the range of systems available outlined. Some illustrative examples of the various classes of reactions carried out in molten salts are then given. These include the exchange of halides and pseudohalides in organic and inorganic compounds, organic chlorinations, eliminations and rearrangements, a consideration of acid—base phenomena in tetrachloroaluminate and in nitrate melts and the problems of distinguishing the acidic and basic species in the latter, the coordination geometries adopted by nickel(ii) in a series of halide melts, the formation of lower oxidation state cations including some novel clusters, the stabilization of high oxidation states and the kinetics of some oxidations in molten nitrates and, fmally, two industrially important processes in molten sulphates, the corrosion of steel surfaces and the catalytic oxidation of sulphur dioxide. Molten salts differ from conventional non-aqueous solvents in two major respects. Firstly, their melting points are considerably higher, and secondly, they are much more ionic in character. The first difference is not necessarily a great disadvantage, especially if the lower melting eutectics are used (see Table 1), and indeed the higher temperatures available can be of positive advantage in certain reactions. The second difference enables a range of Table 1. Melting points and compositions of some molten salt eutectics mole m.pt LiCl/KC1 (59:41) 352° KC1/ZnC12 (48:52) 228° NaCl/AlCl3 (50:50) 175° LiF/NaF/KF (46.5:11.5:42) 454° LiBr/KBr (60:40) 348° Lil/KI (63:37) 260° KCN 634° KSCN 177° LiNO3/KNO3 (43:57) 132° NaNO2/KNO2 (65:35) 220° Li2SO4/Na2SO4/K2S04 (78:13.5:8.5) 512° LiClO4/KC1O4 (76:24) 207°