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(n) 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 I. Melting points and compositions of some molten salt eutectics LiCljKCI KCI/ZnCI 2 NaCI/AICI3 LiF/NaF/KF LiBrjKBr Lii/KI KCN KSCN LiN03/KN03 NaN02/KN02 Li2SO 4 /Na2SO JK2 SO 4 LiCIOJKC104 mole ~~ (59:41) (48:52) (50:50) (46.5 :11.5 :42) (60:40) (63:37) (43:57) (65:35) (78: 13.5 :8.5) (76:24) 355 m.pt 352° 228° 175° 454° 348° 260° 634b 177° 132° 220° 512° 207° D.H.KERRIDGE chemical reactions tobe approached more readily, the ionic nature conferring high electrical and thermal conductivities and promoting the solution of ionic solutes. Solubility in molten salts is partly physical in that solutes can enter the 'holes' in the ionic structure created by the quite large increase in molar volume caused by melting (e.g. (LH~/V.) x 100 for LiCI is 26.2; for NaBr, 22.4; for KI, 15.9; for LiN03, 21.4; for KN03, 3.3; for KSCN, 10). Such solutions are typified by the rare gases in molten nitrates where the solubilities increase with temperature (as well as with pressure). For example, the Henry's Law Constant for helium in sodium nitrate is 1.86 and 2.80 and for argon 0.64 and 1.04 mol ml- 1 bar- 1 at 332 and 441 oc, respectively1• However, many other solutions are the result of 'chemical' interactions, as, for example, hydrogen fluoride dissolving in molten fluorides (cf. HF in NaF/ZnF4 (60:40mole %), K8 = 1.53, 1.03, 0.81 molml- 1 bar- 1 at 600, 700 and 800°C)2 , while the solution oftransition metal cations in chloride and nitrate melts Ieads to coordination, as is clearly shown by their spectra. It is perhaps less expected that metals are generally soluble in their molten halides 900 800 650 o 4 eCooling curves x Equilibration and sampling KI-K 658.5" ro w ~ ~ w oo m ~ ~ mol "IoK Figure 1. Temperature-composition phase diagrams of the potassium metal-potassium halide (ref. 3) 356 REACTIONS IN MOLTEN SALTS and that they are usually completely miscible at temperatures not far above the melting points of the molten salt (see Figure 1). Such solutions have frequently been interpreted in terms of the formation of lower oxidation states3• The study of reactions in molten salts is often facilitated not only by such solubilities and the higher temperatures but also by the fact that the solvent itself can act as a reactant and that it is at the highest possible concentration (e.g. for molten LiOH molality is 42 m). Earlier chemists (e.g. Scheele) took advantage of this to some extent, e.g. Mn02 + 0 2 in fused KOH- K2Mn04 (1) Fe in fused KN03 (2) and the use of melts in prepara tive reactions has never completely ceased: Cr 2 0 3 in fused Li2C03 - Li3Cr04 (ref. 4) (3) 6()()0 M2Fe04 + MOH---+M3Fe04 (ref. 5) (4) Finally, the point may be made that a really vast choice ofpotential solvents exists, since Iiterally any salt which is thermally stable can be melted to provide a solvent with potentially interesting properties. The chemical characteristics vary widely from the alkali metal halides to more covalent halides, including transition metal halides, pseudo-halides, oxoanion salts such as nitrates, nitrites, sulphates, carbonates, to the higher melting complex polymeric borates and silicates. Some account of recent advances in the use of such non-aqueous solvents as reaction mediawill now be given. For convenience they aresetout under five headings: 1. Exchange reactions; 2. Reactions of organic compounds; 3. Acid-base phenomena; 4. Coordination equilibria; 5. Oxidation-reduction reactions.