No abstract

A universal multistage cascade CSTR has been developed that is suitable for a wide range of continuous-flow processes. Coined by our group the “Freactor” (free-to-access reactor), the new reactor integrates the efficiency of pipe-flow processing with the advanced mixing of a CSTR, delivering a general “plug-and-play” reactor platform which is well-suited to multiphasic continuous-flow chemistry. Importantly, the reactor geometry is easily customized to accommodate reactions requiring long residence times (≥3 h tested).

The ability to control crystallization reactions is required in a vast range of processes including the production of functional inorganic materials and pharmaceuticals and the prevention of scale. However, it is currently limited by a lack of understanding of the mechanisms underlying crystal nucleation and growth. To address this challenge, it is necessary to carry out crystallization reactions in well-defined environments, and ideally to perform in situ measurements. Here, a versatile microfluidic synchrotron-based technique is presented to meet these demands. Droplet microfluidic-coupled X-ray diffraction (DMC-XRD) enables the collection of time-resolved, serial diffraction patterns from a stream of flowing droplets containing growing crystals. The droplets offer reproducible reaction environments, and radiation damage is effectively eliminated by the short residence time of each droplet in the beam. DMC-XRD is then used to identify effective particulate nucleating agents for calcium carbonate and to study their influence on the crystallization pathway. Bioactive glasses and a model material for mineral dust are shown to significantly lower the induction time, highlighting the importance of both surface chemistry and topography on the nucleating efficiency of a surface. This technology is also extremely versatile, and could be used to study dynamic reactions with a wide range of synchrotron-based techniques.

The reaction of amino acid derived N-carboxyanhydrides (NCAs) with unprotected amino acids under carefully controlled aqueous continuous flow conditions realized the formation of range of di-and tripeptide products in 60-85% conversion at productivities of up to 535 g.L-1 h-1. This required a fundamental understanding of the physicochemical aspects of the reaction resulting in the design of a bespoke continuous stirred tank reactor (CSTR) with continuous solids addition, high shear mixing, automated pH control to avoid the use of buffer, and efficient heat removal to control the reaction at 1±1 °C.

Characterizing the pathways by which crystals form remains a significant challenge, particularly when multiple pathways operate simultaneously.Here, an imaging-based strategy is introduced that exploits confinement effects to track the evolution of a population of crystals in 3D and to characterize crystallization pathways. Focusing on calcium sulfate formation in aqueous solution at room temperature, precipitation is carried out within nanoporous media, which ensures that the crystals are fixed in position and develop slowly. The evolution of their size, shape, and polymorph can then be tracked in situ using synchrotron X-ray computed tomography and diffraction computed tomography without isolating and potentially altering the crystals. The study shows that bassanite (CaSO 4 0.5H 2 O) forms via an amorphous precursor phase and that it exhibits long-term stability in these nanoscale pores. Further, the thermodynamically stable phase gypsum (CaSO 4 2H 2 O) can precipitate by different pathways according to the local physical environment. Insight into crystallization in nanoconfinement is also gained, and the crystals are seen to grow throughout the nanoporous network without causing structural damage. This work therefore offers a novel strategy for studying crystallization pathways and demonstrates the significant impact of confinement on calcium sulfate precipitation, which is relevant to its formation in many real-world environments.