Focusing Impurities during Inert Gassing Crystallization of Complex Mixtures

Abstract

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.

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“…Gassing crystallization was carried out analogously to our preceding investigations with simplified crystallization systems [22,23]. While stirring with n = 350 rpm, after holding the initial temperature ϑ start = 20 • C for 5 min, linear cooling with a cooling rate of κ = 0.5 K min −1 was applied until reaching the final temperature ϑ end = 0 • C, followed by a second holding time of 5 min.…”

“…Based on preceding investigations revealing immediate degradation of supersaturation in cooling crystallization of l-C 12 -DME [23], we assume that during the holding time of 5 min, a near-equilibrium state was reached with c* (ϑ end = 0 • C) in the mother liquor. According to our previous study, the solubility c* of l-C 12 -DME strongly depends on the concentration of remaining substrate and branched by-products [20].…”

“…In gassing crystallization, upon cooling below the solubility, gas bubbles are introduced into the supersaturated solution, thus reducing the energy for the initial formation of crystal nuclei on the bubble surface, inducing primary heterogeneous nucleation [32]. For the crystallization of l-C 12 -DME, we could demonstrate that compared to simple cooling crystallization with spontaneous nucleation from the solution, by gassing crystallization using inert argon, fewer nuclei are formed which grow to larger, more uniform crystals with a reduced agglomeration level, achieving the target purity of >99.9% [22,23]. However, in our preceding investigations, a significant impact of substrate and isomers, as well as branched by-products, on l-C 12 -DME crystallization was identified.…”

“…Building on our work [20], we have recently focused on improving the steps of crystallization, filtration, and washing in systematic investigations, which revealed a high potential for further process optimization regarding purity [22,23]. In the present study, we aim to implement the optimized procedures in the integrated catalyst recycling process, focusing on a constant high product purity over consecutive recycling runs.…”

“…A controlled crystallization is crucial to produce suitable product crystals for an efficient separation. Here, we have recently presented the concept of inert gassing crystallization for enhanced purification of l-C 12 -DME [22,23]. In gassing crystallization, upon cooling below the solubility, gas bubbles are introduced into the supersaturated solution, thus reducing the energy for the initial formation of crystal nuclei on the bubble surface, inducing primary heterogeneous nucleation [32].…”