(711f) Flow Chemistry-Enabled Investigations of Switchable Hydrophilicity Solvents

The dramatic environmental impacts on society have created the demands for fast development of energy-efficient and environmental-friendly processes towards green and sustainable industrial processes. Currently, most manufacturing processes in chemical and pharmaceutical industries are highly dependent on energy-intensive distillation processes with harmful volatile solvents due to their ease of separation. Recently, switchable solvents have emerged as a promising candidate for liquid-liquid extraction (LLE) processes with a facile reversible tunability of their physiochemical properties triggered by carbon dioxide (CO₂) under a mild condition (i.e., room temperature and atmospheric pressure). In the presence of water and CO₂, switchable solvents react with the formed carbonic acid (H₂CO₃), producing protonated SHS and bicarbonate salts. This reaction can be easily reversed with an inert gas (e.g., nitrogen) or moderate heating (e.g., ~60 °C), thereby providing an ideal characteristic for the next-generation sustainable LLE-based solvent recovery.¹ In particular, Switchable Hydrophilicity Solvents (SHSs) offer a dramatic solubility change in hydrophilic/hydrophobic media upon their protonation/de-protonation, opening a new frontier for energy-efficient chemical processes in various fields from the extraction of bitumen and bio-algal mass and homogeneous catalysis to the recycling of packaging materials and plastics.

Despite the groundbreaking advancements in the discovery and characterization of SHSs over the last decade, the time-, material-, and labor-intensive nature of conventional batch (flask-based) reactors have hindered further exploration of the broad parameter space of SHSs. Conventional batch reactors have significant mass transfer limitations for conducting gas-liquid reactions due to their poorly defined interfacial area (e.g., bubble column provides 50-600 m²/m³). Thus, screening of an SHS-based LLE process using a batch reactor might take up to10 h/experiment, further resulting in a limited available chemical database for the design of next-generation SHSs. Furthermore, batch screening methods require 10-1000 mL of chemical consumption for each SHS characterization experiment, which leads to significant waste generation and high cost for parameter space exploration of SHSs. Thus, lack of comprehensive understanding of thermodynamic characteristics of CO₂-triggered hydrophilicity switching of SHSs, mainly due to the above-mentioned drawbacks of conventional batch processes, has negatively impacted the industrial adoption of this exciting class of green solvents.

In this work, we developed and utilized an automated microscale flow chemistry platform utilizing a membrane-based flow reactor for accelerated studies of both continuous (e.g., CO₂ pressure, reaction time, SHS concentration, and flow velocity) and discrete (e.g., the chemical structure of SHS) process parameters associated with CO₂-mediated SHS recovery process. Utilizing a highly CO₂-permeable membrane in a tube-in-tube configuration, offering a significantly higher interfacial area of 4900 m²/m³ than conventional batch reactors, we systematically studied CO₂-mediated SHS extraction efficiency and kinetics, in situ, as fast as 4 min/experiment (~100 times faster than batch reactor), while utilizing only 4-8 μL of SHS mixture (~2500 times less chemical consumption than batch reactor) for each experimental condition. The single-droplet flow reactor provided a time- and material-efficient approach for comprehensive fundamental and applied studies of the extraction efficiency and kinetics of SHS-based LLE processes, providing accelerated access to the optimized process conditions. Next, the tube-in-tube flow reactor was reconfigured from the single-droplet screening mode to the continuous flow configuration to continuously recover the SHS under the optimized conditions identified by the single-droplet process screening flow reactor.² Since the single-droplet and continuous flow reactor configurations share the same intensified mass transfer characteristics, the optimized conditions identified by the screening flow reactor are directly transferrable to the continuous flow reactor. The proposed research will provide a promising solvent utilization technique with valuable insight and robust guidelines for green and sustainable chemical processing.

Reference.

(1) Jessop, P. G.; Mercer, S. M.; Heldebrant, D. J. CO2-Triggered Switchable Solvents, Surfactants, and Other Materials. Energy Environ. Sci. 2012, 5 (6), 7240–7253. https://doi.org/10.1039/C2EE02912J.

(2) Han, S.; Raghuvanshi, K.; Abolhasani, M. Accelerated Material-Efficient Investigation of Switchable Hydrophilicity Solvents for Energy-Efficient Solvent Recovery. ACS Sustainable Chem. Eng. 2020, 8 (8), 3347–3356. https://doi.org/10.1021/acssuschemeng.9b07304.