SPECIATION OF THE PROTON IN AMBIENT-TEMPERATURE MOLTEN-SALTS

Ionic equilibria associated with acidic protons present in HCl:ImCl (Im+=-1-ethyl-3-methyl-H-1-imidazolium)ionic liquids (0.38 < HCI mole fraction, X(HCl) < 0.67) have been investigated by H-1-NMR spectroscopy (200 MHz, 297 K). We conclude that proton speciation in these liquids is governed largely by the reaction H2Cl3- + Cl- half arrow right over half arrow left 2HCl2-, for which the stoichiometric equilibrium constant has been evaluated as 218 +/- 25; the concentration of molecular HCl is small or negligible relative to that of the anionic species. Virtually identical behavior was observed in an AlCl3:ImCl (AlCl3 mole fraction = 0.35) ionic liquid. As in the case of the HCl:ImCl system, it was found that, maximally, 2 equiv of HCl (relative to available chloride ion) could be introduced into the AlCl3:ImCl liquid under ambient conditions. On the basis of these observations, and those of other workers, we have constructed an approximate phase diagram for the ternary system HCl:ImCl:AlCl3 near ambient temperature and with HCI pressure less than 1 atm. Depending on the activity of Al2Cl7-, the behavior of protic species in this system may range from that of a weak Lewis acid to that of a Bronsted superacid.