CBE PhD Dissertation Defense | “Bijels for Continuous Reactive Separation”

Abstract: 

Several chemical processes involve the use of mutually immiscible molecules, for example a water-soluble catalyst is used in combination with oil-soluble reagents to produce water soluble products. There are two possible strategies to run these processes. A mutual solvent can be used to promote mixing of the otherwise immiscible molecules; however, such a system would make the separation of the product challenging. Biphasic systems comprising two immiscible phases provide a very powerful platform because their ability to host both polar and apolar species and at the same time allowing separation of molecules of different polarity by simple phase partitioning, without relying on complex and often energy intensive separation processes. Such a strategy is employed in several industrially relevant scenarios like fatty acid and pharmaceutical manufacturing for which the desired product is made from an oil-soluble precursor, while the desired products and the catalyst are water-soluble. Two forms of biphasic reactive separation systems have been widely studied: emulsion-based and membrane-based systems. There are several examples of successful implementation of simultaneous reactive separation based on emulsions and membranes; however, the efficiency and effectiveness of these systems can be further enhanced. Significantly, all emulsion based systems comprise a continuous phase and a droplet phase; the discrete nature of the droplets makes the supply or reagent or retrieval of product challenging. Although continuous processing is possible, membrane reactors have limitations in their interfacial area.

A recently developed class of soft materials, bicontinuous interfacially jammed emulsion gels or bijels provide opportunity to overcome the shortcomings of emulsions and membrane-based biphasic reactors. Bijels have a bicontinuous architecture which allows intimate contact between the two fluid phases; moreover, bijels provide a significantly higher water/oil interfacial area than membranes. Traditionally bijels are made by triggering the spinodal decomposition of a binary mixture of fluids either by temperature change or co-solvent removal; the three dimensional bicontinuous structure arising from spinodal decomposition has several advantages since both the water and the oil phases are co-continuous and therefore allow for the continuous transport of chemicals throughout its domains. Despite these promising features, several aspects of bijels must be further addressed to enable their application as reactive media. For examples, bijels are fragile; they lose their microscopic and macroscopic structure under mechanical (e.g. flow, agitation) and chemical stresses (e.g. pH change). Moreover, most bijel formulations are unsuitable for system involving biological molecules such as enzymes, as they require surfactants to control nanoparticle wetting properties. This study addresses these shortcomings and develops simple and inexpensive methods to engineer bijels for reactive separation. Reactive separation is successfully demonstrated in these bijels in batch mode. Lastly, for the first time, bijels are used for continuous reactive separation; bijel-based reactor conversion is compared to membrane based reactors and simple batch stirred tank reactors. Future studies will include the further development of bijels microreactor design and its development as a potentially disruptive technology in multiphase catalysis.