Ultrasound-enhanced grinding is a more practical alternative to glass bead-enhanced grinding for performing attrition-enhanced deracemization at large scale or in continuous flow. In this work, both ultrasound-enhanced grinding (41.2 kHz) and glass bead-enhanced grinding were applied to induce Viedma deracemization of sodium chlorate (NaClO3) crystals in isothermal conditions. The results demonstrate that high intensity, low frequency ultrasound can achieve efficient grinding of enantiomorphous NaClO3 crystals, producing small crystal size and narrow size distribution, both being highly desirable final product properties. Monitoring the width of the crystal size distribution reveals its crucial role and offers further insight into the underlying phenomena in the deracemization process. Compared to glass bead-enhanced grinding, ultrasound-enhanced grinding resulted in faster crystal size reduction and rapid initial deracemization. However, a further increase in the enantiomeric excess was hindered after prolonged times of ultrasonication. This ensues probably due to the absence of crystal size-induced solubility gradients, owing to the existence of close to monodispersed sized crystals after the initial stage in the ultrasound-enhanced grinding process. We show that this can be overcome by combining (a) ultrasound with glass beads or (b) ultrasound with seeding, both of which led to enantiopurity.
展开▼
