THE PROCESS OF ISOLATION BY CRYSTALLIZATION OF CIS- AND TRANS-ISOMERS OF PERFLUORODECALINES FROM INDUSTRIAL MIXTURE OF ELECTROCHEMICAL FLUORINATION OF NAPTHALINE

The process of separation by crystallization of an industrial mixture of perfluorinated cycloalkanes, products of electrochemical fluorination of naphthalene, is considered. The content of the target products – cis- and trans-isomers of perfluorodecalin (PFD) in all initial fractions of the investigated samples of the reaction mixture was more than 70 wt %, the mixture itself was taken directly from industrial plant. In this work the distribution of components (target products, identified and unidentified impurities; the nomenclature of the latter is unknown because of the difficulties during their purifying that is required to identify their structure and, accordingly, the lack of relevant scientific studies) is shown by composition in solid and liquid phases. Based on experimental data the dependences of the crystallization (partition) coefficients between the solid and liquid phases (D^s/l) and recovery ratio on proportion between trans-PFD and cis-PFD contents, presence of low- and high-melting impurities and crystallization temperature in the range -50÷-10 ºС were obtained. According to the experimental data, the D^s/l value significantly depends on the presence of trans-PFD in the initial solution. Thus, with an increase in the ratio of trans-PFD/cis-PFD: the D^s/l value for the low-melting identified as perfluoro(butylcyclohexane) (BCH) impurity tends to 0 – BCH is completely absent in the solid phase; the majority of unidentified impurities are concentrated in the mother liquor (D^s/l< 1); the D^s/l_trans-PFD value has a linear dependence with the point of inversion at the ratio of trans-PFD/cis-PFD = 1; D^s/l_cis-PFD≥ 1 for almost all trans-PFD/cis-PFD ratios and D^s/l_cis-PFD ≫ 1 at trans-PFD/cis-PFD < 1. The latter is very interesting given that the melting point of cis-PFD is lower than that of trans-PFD. Thus, according to the experimental data the crystallization of the fractions with high trans-PFD content is less effective than that with high cis-PFD content.