Fundamental Studies On NH₃ Capture Using Ionic Liquids

Fixation and conversion of NH3 using ILs has been raised due to the advantages of ionic liquids (ILs). Two key problems of fixation and conversion of NH3 are the equilibrium of NH3+ILs and the conversion of NH3 using ILs, in which the equilibrium of NH3+ILs is of most importance. In this work, the equilibrium of NH3+conventional imidazolium ionic liquids have been systematically investigated, which can provide valuable fundamental data for the industrial applications of NH3 fixation and conversion.(1) In this work, Solubilities of ammonia in four conventional imidazolium ionic liquids:[Cnmim][BF4] (n=2,4,6,8) have been measured. Isothermally fixed temperatures are 293.15,303.15,313.15,323.15 and 333.15 K; the pressure is from 0 to 1.0 MPa. High solubilities of ammonia are found; the effects of temperature and pressure on the solubilities of NH3 in ILs were studied:with the increasing of the temperature the solubilities decreased, and with the increasing of pressure the solubilities increased; and it is also found that the solubilities of ammonia increase when the length of cations'alkyl increases (the ILs have the same anion), that is: [C8mim]+> [C6mim]+> [C4mim]+> [C2mim]+(2) The solubility data have been correlated by the Krichevisky-Kasarnovsky equation, and then Henry's constants and partial molar volumes of NH3 at infinite dilution are obtained. The thermodynamic properties such as solution enthalpy (AsolH), solution Gibbs free energy (AsolG), solution entropy (AsolS), and solution heat capacity (AsolCp) of these systems are obtained.(3) The effect of water content on the solubility of ammonia in the ionic liquid [C8MIM][BF4] has been studied. The water mass fractions are 0.93%,21.28%,28.67%, and the solubility data were measured at 313.15K and less than 1MPa. In this work, solubility of ammonia in pure [C8MIM][BF4] is higher than in those with water; solubility of ammonia in [C8MIM][BF4] with smaller amount of water is higher than in those with greater amount of water. The reason is that, between ammonia and [C8MIM][BF4], there is no chemical reactions occur, the process is chemical absorption. During the process, ammonia molecular occupied the interspaces among the structure of ILs. When some water entered, water molecular would occupy some interspaces which ammonia molecular should occupy if there was no water. (4) Because ammonia is alkalescence and ILs can be designed, in this study, two acidic ILs:[HSO3MIM][BF4] and [HSO3MIM][HSO4] has been synthesized. Solubility of ammonia in those two ILs has been studied. The result is that, chemical reactions occurred between ammonia and the two ILs. The products are the white solid zwitterion which is the material used to synthesize the acidic ILs, and the relevant ammonium salts. Further, the solubility data of ammonia in the two ILs have been studied. The solubility of ammonia in [HSO3MIM][BF4] is 0.8578, and the solubility of ammonia in [HSO3MIM][HSO4] is 0.9024.