Particle breakage kinetics and mechanisms in attrition-enhanced deracemization

In this study, we report on experiments designed to deconvolute the particle breakage kinetics and mechanism from the parallel phenomena (growth-dissolution, agglomeration) in attrition-enhanced deracemization processes. Through such experiments, we derived the specific breakage rates and cumulative breakage distribution functions for three grinding methods typically used in deracemization experiments: (a) bead grinding, (b) ultrasound grinding, and (c) the combination of bead and ultrasound grinding. Subsequently, we tested these methods on their ability to induce deracemization. We show that in the conventional bead grinding process, breakage occurs mostly by fracture. This results in slow deracemization rates due to the delayed formation of submicron particles that are essential to the process. Conversely, ultrasound grinding very efficiently breaks particles by abrasion. This leads to fast generation of an abundance of submicron fragments resulting in fast deracemization. However, using ultrasound, large crystals fracture rates are an order of magnitude lower than those using bead grinding, which results in an insufficient size decrease of the large counter enantiomer crystals and eventually to incomplete deracemization. Remarkably, the simultaneous application of bead and ultrasound grinding leads, due to synergistic effects of both fracture and abrasion, to 2-fold higher total deracemization rates compared to bead grinding alone. The present work offers new insights into the key role of particle breakage in attrition-enhanced deracemization, together with a basis for decoupling the individual phenomena involved in the process.