Crystallization thermodynamics of 2,4,6-trinitrophenol and 2,4,6-trinitroresorcinol∙2/3H2O in different pure solvents

The by-products in nitration of benzene were analyzed by HPLC, X-ray single crystal diffraction and so on, and the main components were: 2,4,6-trinitroresorcinol∙2/3H2O (TNR∙2/3H2O, 76.7% by HPLC) and 2,4,6-trinitrophenol (TNP, 14.6% by HPLC). The solubility of TNP and TNR∙2/3H2O in six pure solvents (dichloromethane, chloroform, 1,2-dichloroethane, toluene, xylene and anisole) were determined by laser dynamic method under atmospheric pressure at T = 298.15 K to 333.15 K. When the temperature and solvent are the same, the solubility of TNR∙2/3H2O is significantly lower than that of TNP. The solubility of the two substances is the highest in anisole, the smallest in chloroform, similar in toluene, xylene, dichloromethane and 1,2-dichloroethane at the same temperature and increases at different rates with the increase of temperature in the same solvent. The solubility data of the two substances was correlated by the modified Apelblat equation, Van't Hoff equation, polynomial empirical equation and λh equation. All of these models gave a satisfactory result and the polynomial empirical equation and the modified Apelblat equation are better. The apparent thermodynamic parameters such as the standard dissolution enthalpy (ΔH°dis), standard dissolution entropy (ΔS°dis) and the standard Gibbs energy (ΔG°dis) were calculated by the Van't Hoff method, which indicates the dissolution behavior of TNP and TNR∙2/3H2O is considered to be an “endothermic, non-spontaneous and entropy-driven” process in selected solvents. According to the solubility data, toluene and xylene are more suitable for the purification of TNR∙2/3H2O from the by-products of benzene nitration by cooling crystallization. Cooling crystallization experiments were carried out on the by-products of benzene nitration, and the pure TNR∙2/3H2O can be obtained after twice crystallizations by toluene.