Focusing Impurities during Inert Gassing Crystallization of Complex Mixtures

Selective crystallization of a pure product from complex mixtures is challenging due to the variety of impurities which can potentially affect the crystallization process and hence dramatically increase the complexity of the separation task. Focusing on specific mechanisms of product contamination, we demonstrate that suitable operating parameters for efficient purification can be derived in experiments with simplified crystallization systems and transferred to crystallization from complex mixtures. Systematic investigations were carried out for the selective crystallization of linear 1,12-dimethyl dodecanedioate (l-C₁₂-DME) from reaction mixtures as the model system. In the reference binary system l-C₁₂-DME/methanol, we observed the preferential formation of inclusions in platelet-shaped crystals at a low cooling rate κ = 0.1 K·min^–1, whereas pure crystals were obtained at faster cooling with κ = 0.5 K·min^–1. Furthermore, we verified that the structurally similar reaction substrate strongly promotes agglomeration, indicating that the isolation of a pure product from the reaction mixture requires sufficient conversion in the preceding reaction step. Finally, we demonstrate that in crystallization from complex mixtures, the introduction of gas bubbles enables controlled nucleation, improving product purity and reproducibility compared to simple cooling crystallization with uncontrolled nucleation. Using suitable operating parameters derived from simplified crystallization systems, by means of gassing crystallization with inert argon, l-C₁₂-DME can be purified from a complex reaction mixture with multiple impurities at a target purity of >99.9% and simultaneously high yield.