A deracemization technique using periodic temperature fluctuations on a conglomerate forming system undergoing a swift racemization in solution is demonstrated. The method uses heating and cooling periods of the suspension in order to create cycles of partial dissolution of the crystal phase followed by crystal regrowth: this enables symmetry breaking in the solid phase. The technique is an effective, simple, and cheap operation, and can promote understanding of the effects of dissolution and recrystallization on chiral symmetry breaking in the solid phase. The heating period leads to the decrease of the size of crystals and the destruction of small crystals; the surviving crystals can then grow during the cooling period. A succession of such cycles allows the autocatalytic transformation from a racemic suspension into pure enantiomer, with an enantiomeric excess (ee) > 99% within a few days. The results demonstrate a possible mechanism for the emergence of homochirality of molecules of biological significance on Earth.

An improved process for the deracemization of a racemic conglomerate suspension of enantiomorphs has been created based on principles developed in an earlier method using temperature fluctuations. The method consists of circulating the suspension between two vessels, each controlled at a specific temperature in order to make the process more effective and faster to achieve a homochiral solid state. Crystals in the suspension were partially dissolved in the hot vessel, and the remaining crystals were regrown in the cold vessel. The crystals in the cold vessel have a longer residence time than those in the hot vessel to allow more time for the crystal growth process. The results show that complete deracemization can be achieved via this process far more rapidly than by using the previous temperature cycling (one-vessel) process. Moreover, the new process could easily be scaled up to an industrial scale. The current process can be an effective alternative to currently used enantiopurification methods, with simple processing implementation and low cost.

The current research has developed a potential route toward process optimization for the deracemization of a racemizable conglomerate forming system. The use of damped temperature cycleswhere the magnitude of the temperature cycles is reduced as the enantiomeric excess (ee) in the solid phase increasesis a promising methodology for optimizing the temperature cycle induced deracemization process. This process requires significantly less time and energy to reach an enantiopure state compared with the use of constant amplitude temperature cycles. There was evidence of some crystal breakage occurring in the system; however, the limited amount of breakage did not produce any change in the enantiomeric excess in the solid phase in the absence of temperature cycles in the system. Hence, the primary mechanism in this process is neither Viedma ripening nor Ostwald ripening. Scanning electron micrographs of the crystals taken during the series of temperature cycles show that the amplification of the ee in the solid phase is caused by dissolution and growth phenomena induced by the temperature cycles. The promotion of the larger or faster growing crystals at the expense of the smaller or slower growing crystals during the cycles and entrainment from a mother liquor having ee = 0 are both responsible for this deracemization.

Two simple models are proposed and tested for the mechanism of ripening of a conglomerate suspension to a single enantiomorph by temperature cycles. In both models, the initial crystal size distributions and masses of the two enantiomorphs are equal, but either the crystal growth rate or the growth rate distribution is varied. The difference in the crystal growth kinetics of the two enantiomorphs may be caused by the intrinsic thermodynamic stability of the crystals occurring in the initial suspension. The initial nucleation of one of the two enantiomers will occur earlier than that of the counter-enantiomer. This results in the formation of two populations occurring under different conditions, leading to different internal crystalline perfection and therefore different thermodynamic stability.

A racemic suspension of a conglomerate-forming system can be converted to a homochiral suspension where the solute in the solution phase undergoes fast racemization through temperature fluctuations, i.e., temperature cycle-induced deracemization. Previously, a mathematical model of chiral symmetry breaking due to differences in distributions of crystal growth rate activities between the two enantiomorphs was proposed. This model was a simplification since it used a very low number of crystals. Herein, the deracemization process was simulated using continuous distributions for the population of crystals and the growth rate of crystals which results in far more accurate simulations. The mechanism produces results similar to that achieved in experiments. The results from the gCrystal program demonstrated that complete chiral conversion to the enantiomorph with the wider dispersion in growth rate activity occurred in the solid phase.