Process Intensification in a Liquid Biphasic Reaction System by Application of an External Electric Field

Abstract

Due to the increasing concerns on global warming and depletion of fossil fuels, sustainable chemistry and engineering has been a research focus in the 21st century. Tremendous efforts have been devoted to designing inherently safe chemicals and processes with minimal waste emission and energy consumption. Among the many ongoing technologies, conducting chemistry in liquid biphasic systems is a promising approach and has found many successful applications in organic synthesis, biocatalysis, biomass pretreatment, etc. The development of green solvents, such as ionic liquids, further extends the scope of liquid biphasic systems for investigation of sustainable chemistry. While liquid biphasic systems feature many advantages, reactions in those systems usually suffer from mass transfer limitation. To facilitate mass transfer across the interfacial boundary, conventional methods, such as vigorous mechanical agitation or organic additions (e.g., phase transfer catalysts, or surfactants) are being extensively used. These methods, however, require significant energy input, increase the complexity of the reaction systems, and add to environmental concerns. To address this challenge, an intensification method achieved by application of external electric fields is proposed in this thesis. The electrostatic intensification in reaction performance in a liquid biphasic system was studied in two aspects: 1) controlling the migration of reactive species in a batch system, 2) increasing interfacial areas and reaction rates by application of an electrospray in a continuous system. This approach is easy to apply with relatively low energy consumption, thus showing great potential for a wide range of applications. Chapter I is an introduction of the research background. It discussed the importance and challenges of liquid biphasic systems in the development of sustainable chemistry and engineering. Three typical applications of liquid biphasic reaction systems, phase transfer catalysis, aqueous biphasic systems, and biomass processing, were discussed in terms of their abilities to achieve sustainability in chemistry and engineering. Recent advances in intensification methods in liquid biphasic systems were discussed with continuous flow processes, including microfluidic systems, and application of external electric fields as representatives. Afterwards, the model reaction system was discussed, followed by the proposals of research motivations and goals. In Chapter II, the roles of external electric fields in the batch reaction system were examined. It was demonstrated that water participated in the transfer hydrogenation of acetophenone when aqueous sodium formate was employed as hydrogen source. The external electric field was found to act as promoter or inhibitor for the phase transfer hydrogenation depending on the orientation of the electric field, which suggests the great potential of external electric fields in controlling reaction rates. In Chapter III, the ability of external electric fields to control reaction rates was further explored. It was found that the reaction performance was not linearly dependent on the applied voltages when the reaction time increased. The application of a negative voltage may result in the decomposition of the catalyst, which led to the decreased product enantioselectivity. It was also demonstrated that the reaction could be externally controlled by simply switching the applied electric potential over the course of the reaction. In Chapter IV, a continuous reaction system based on electrospray was established to further intensify mass transfer and reaction rates. Unlike the observations in Chapter II and III, the orientation of external electric fields was found to show no effects on the reaction performance. The induced electric current in the reaction system due to the increased conductivity of the continuous phase was proposed correlated to the improved conversion and yields. In this electrospray system, 71% conversion could be achieved in 1 h with a 13-fold increase in throughput compared to that in the batch system studied in Chapter II and III. In Chapter V, the importance of this work in understanding the roles of external electric fields in organic synthesis and catalysis was summarized. Further research directions and some new ideas were also proposed as complement to this thesis.