Sonocrystallisation of ZIF-8 in water with high excess of ligand: Effects of frequency, power and sonication time.

A systematic study on the sonocrystallisation of ZIF-8 (zeolitic imidazolate framework-8) in a water-based system was investigated under different mixing speeds, ultrasound frequencies, calorimetric powers and sonication time. Regardless of the synthesis technique, pure crystals of ZIF-8 with high BET (Brunauer, Emmett and Teller) specific surface area (SSA) can be obtained in water after only 5 s. Furthermore, 5 s sonication produced even smaller crystals (~0.08 µm). The type of technique applied for producing the ZIF-8 crystals did not have any significant impact on crystallinity, purity and yield. Crystal morphology and size were affected by the use of ultrasound and mixing, obtaining nanoparticles with a more spherical shape than in silent condition (no ultrasound and mixing). However, no specific trends were observed with varying frequency, calorimetric power and mixing speed. Ultrasound and mixing may have an effect on the nucleation step, causing the fast production of nucleation centres. Furthermore, the BET SSA increased with increasing mixing speed. With ultrasound, the BET SSA is between the values obtained under silent condition and with mixing. A competition between micromixing and shockwaves has been proposed when sonication is used for ZIF-8 production. The former increases the BET SSA, while the latter could be responsible for porosity damage, causing a decrease of the surface area.

A review on possible mechanisms of sonocrystallisation in solution.

Sonocrystallisation is the application of ultrasound to the crystallisation process. The benefits obtained by sonication have been widely studied since the beginning of the 20th century and so far it is clear that ultrasound can be a very useful tool for enhancing crystallisation and controlling the properties of the final product. Crystal size, polymorphs, purity, process repeatability and lower induction time are only some of the advantages of sonocrystallisation. Even though the effects of sonication on crystallisation are quite clear, the physical explanation of the phenomena involved is still lacking. Is the presence of cavitation necessary for the process? Or is only the bubbles surface responsible for enhancing crystallisation? Are the strong local increases in pressure and temperature induced by cavitation the main cause of all the observed effects? Or is it the strong turbulence induced in the system instead? Many questions still remain and can only be appreciated with an understanding of the complexity behind the individual processes of crystallisation and acoustic cavitation. Therefore, this review will first summarise the theories behind crystallisation and acoustic cavitation, followed by a description of all the current proposed sonocrystallisation mechanisms, and conclude with an overview on future prospects of sonocrystallisation applications.

Texture analysis as a tool to study the kinetics of wet agglomeration processes.

In this work wet granulation experiments were carried out in a planetary mixer with the aim to develop a novel analytical tool based on surface texture analysis. The evolution of a simple formulation (300g of microcrystalline cellulose with a solid binders pre-dispersed in water) was monitored from the very beginning up to the end point and information on the kinetics of granulation as well as on the effect of liquid binder amount were collected. Agreement between texture analysis and granules particle size distribution obtained by sieving analysis was always found. The method proved to be robust enough to easily monitor the process and its use for more refined analyses on the different rate processes occurring during granulation is also suggested.