(516j) An Oscillatory Flow Reactor for High-Throughput Studies of CO2-Mediated Switchable Hydrophilicity Solvents

Experimental Process development and optimization are key aspects of adopting a newly developed chemical process into a real-world process. In order to examine the feasibility for the industrial adoption of a chemical process, a wide range of process parameters have to be explored thoroughly, resulting in a significant amount of chemical consumption in addition to the required time and labor cost. Currently, most of the process parameter optimization efforts are relying on conventional batch techniques due to its ease of assembly. However, time-, material- and labor-intensive batch (i.e., flask-based) processes suffer from inherent mass transfer limitation and lack of in situ information availability. The aforementioned challenges of batch processes thereby hinder further development and industrial adoption of many exciting chemical reactions. In particular, multi-phase chemical reactions face the greatest difficulties during the process development in batch reactors mainly due to poorly defined interfaces as well as irreproducible and uncontrollable mass and heat transfer rates.

Recently, switchable hydrophilicity solvents (SHSs) have emerged as a promising green candidate for energy-efficient solvent recovery utilizing carbon dioxide (CO₂) as the hydrophilicity switching trigger. SHSs are nitrogenous bases that offer a facile reversible change in hydrophilicity/hydrophobicity in the presence/absence of CO₂.¹ This intriguing characteristic of SHSs has been demonstrated in various fields ranging from the extraction of bitumen and bio-algal mass to the recycling of plastics and homogenous catalysis. Despite the wide range of successful demonstrations of SHSs over the last decade, the above-mentioned challenges associated with conventional multiphase batch processes have hindered their adoption by chemical industries. The gas-liquid mass transfer limitation in batch reactors results in prolonged SHS extraction times up to 10 h per experiment with 10-1000 mL of chemical consumption and waste generation. Such challenges have resulted in limited fundamental understanding and available chemical database of SHSs, demanding further exploration and development of optimized process conditions for highly efficient and accelerated CO₂-mediated SHS recovery.

Over the past decade, continuous multi-phase flow chemistry strategies have successfully been utilized for fundamental and applied studies of gas-liquid processes ranging from physical properties (e.g., solubility and diffusivity) measurements to reaction kinetic studies. Despite the well-defined interfaces and small volumes compared to batch systems, continuous multi-phase flow reactors possess limitations in the accessible reaction (residence) time and the inter-related reaction and mixing times. To overcome the aforementioned challenges associated with continuous multi-phase flow strategies, an oscillatory flow strategy utilizing a single droplet has recently been developed for studies of multi-phase physical/chemical processes.² In a single-droplet oscillatory flow reactor, a few microliters of the reactive phase is introduced into the fluidic system by an inert phase (e.g., nitrogen or perfluorinated oil). The single droplet is then moved back-and-forth within a small heated volume of the flow reactor which can be integrated with in situ diagnostic probes enabling real-time process characterization.

Building on the recently developed single-droplet flow chemistry strategy in our group,³ in this work, we developed and utilized a fully automated time- and material-efficient microfluidic approach for gas-liquid-liquid reactive extraction of SHSs triggered by CO₂.⁴ Utilizing a highly CO₂-permeable tubular membrane (Teflon AF 2400) in a tube-in-tube configuration, the flow chemistry platform offers dramatically increased gas-liquid mass transfer rates with interfacial areas exceeding 4900 m²/m³_, resulting in 4 min SHS extraction time per experimental condition while consuming only 4-8 μL of SHS. Utilizing the developed flow chemistry platform integrated with an intensified flow reactor, we systematically studied a broad range of SHS process parameters including reaction time, SHS concentration, CO₂ pressure, flow velocity, and SHS chemical structure. Next, the identified optimized process conditions were transferred to a continuous flow reactor for continuous accelerated SHS extraction under similar mass transport characteristics. Such flow chemistry platforms can be extended for high-throughput studies of other switchable solvents and further accelerate their adoption by chemical industries.

References

(1) Jessop, P. G.; Phan, L.; Carrier, A.; Robinson, S.; Dürr, C. J.; Harjani, J. R. A Solvent Having Switchable Hydrophilicity. Green Chem. 2010, 12 (5), 809–814. https://doi.org/10.1039/B926885E.

(2) Abolhasani, M.; F. Jensen, K. Oscillatory Multiphase Flow Strategy for Chemistry and Biology. Lab on a Chip 2016, 16 (15), 2775–2784. https://doi.org/10.1039/C6LC00728G.

(3) Zhu, C.; Raghuvanshi, K.; Coley, C. W.; Mason, D.; Rodgers, J.; Janka, M. E.; Abolhasani, M. Flow Chemistry-Enabled Studies of Rhodium-Catalyzed Hydroformylation Reactions. Chem. Commun. 2018, 54 (62), 8567–8570. https://doi.org/10.1039/C8CC04650F.

(4) Han, S.; Raghuvanshi, K.; Abolhasani, M. Accelerated Material-Efficient Investigation of Switchable Hydrophilicity Solvents for Energy-Efficient Solvent Recovery. ACS Sustainable Chem. Eng. 2020. https://doi.org/10.1021/acssuschemeng.9b07304.