Purification of molecular mixtures below the eutectic by emulsion crystallization

Abstract

MELT crystallization¹ is used increasingly by the chemical industry to purify organic compounds, in part because of the availability of new processing techniques² and in part because of the commercial and legislative pressures demanding that such compounds be supplied with precisely defined compositions. At present, purification is generally achieved by a succesion of steps involving freezing on a cooled surface followed by partial melting³. As an alternative, we have been examining the process of emulsion crystallization⁴, in which a melt is crystallized within emulsified droplets so that homogeneous nucleation occurs at a lower rate than in a bulk melt. This approach requires no special equipment or solvents, and so offers the possibility of purification at low capital and operating costs. Here we report the use of this method to purify mixtures of meta and para chloronitrobenzene (m- and P-CNB) below their eutectic temperature at compositions where bulk crystallization would yield a mixture of pure m- or P-CNB and a m/p combination of the eutectic composition. Adding seed crystals of m-CNB to various m/p mixtures below the eutectic composition results in the formation of crystals highly enriched in m-CNB, while the p-CNB remains primarily in the emulsion phase. Thus emulsion crystallization has the potential to 'break' the eutectic, which otherwise presents a barrier to the separation of eutectic-forming mixtures.