Molecular Cages for Automated Chemical Separation in Continuous Flow System

Chemical separation is essential for several industrial processes, waste water purification, air cleaning, drug synthesis and synthetic research in laboratory. The challenges we are facing now such as global warming and limited oil supplies mean that we must use resources more wisely. To solve these problems, new strategies are required that are more efficient, more environmentally friendly and less energetically costly than those currently employed, as well as economically worth for applying to industry. Metal-organic coordination cages provide new opportunities to address the need for improved separation technologies. Recent work in this field has demonstrated that these containers can encapsulate specific guest species and act as vehicles for targeted molecular transport and separation.<br/><br/>We synthesised Fe^II₄L₄ water-soluble cage via ‘subcomponent self-assembly’ technique, based on a diamino terphenylene subcomponent, bearing chiral glyceryl groups. The aqueous solution of this cage was used as a liquid membrane in a triphasic U-shaped tube system, separating two organic layers, the feedstock and receiving phases. The cage is able to encapsulate specific guest compounds from a mixture in the feedstock phase and release them in the receiving phase. We could separate naphthalene from larger aromatic compounds such as triisopropylbenzene, as well as ferrocene from its larger derivatives. Within the U-tube system, the transport of compounds was monitored by NMR, GCMS and UV-Vis spectroscopy. To understand the system better, we proposed a kinetic model that can be used to explain the guest transport process, and studied the effect of competing guests on the rate of transport of the selected compounds.<br/><br/>We demonstrated that there is need to improve the bulk liquid membrane separation system to facilitate faster transport, in order to allow it to be used for extraction in industries. It took over 50 days in the U-shaped tube setup for the transport of ferrocene through the cage solution to reach equilibrium. Hence, we designed and optimised a slug flow system to obtain the conditions that offer best extraction efficiency. The continuous flow system gave significantly greater contact areas between phases with better mixing caused by the chaotic internal circulation within the flow, resulting in faster transport (within hours), less amount of solvents needed and less energy required for extraction. The cage could be automatically recycled, and the extraction progress could be monitored in real time by UV-Vis spectroscopy. Further work is being done to precipitate out ferrocene after the guest release, shifting the equilibrium toward the completion of the extraction process in continuous flow system.<br/><br/>This finding is highly industrially relevant, and the main goal is to open up new opportunity for industries requiring novel chemical separation technique.