Roadmap for integrating deep eutectic solvents into adsorption processes: A critical review & design blueprint

The adoption of green chemistry and sustainable engineering approaches into various processes became a trending, proactive practice. On this front, mature and well-developed processes, such as adsorption, have been complemented with different nuances in green chemistry and neoteric solvents, such as Deep Eutectic Solvents (DESs). This review provides a detailed reading of the adsorption studies that incorporated DESs for the development of adsorption materials, referred here as DES-based Adsorbents (DES-ADS). The first part of this review summarizes the different DES-ADS in the literature under specific themes, namely, (i) applications, (ii) adsorbent materials, and (iii) DES components. The majority of DES-ADS are investigated for application in water/wastewater treatment (55.1%), followed by applications in protein isolation (12.7%), food (12.7%), biomass (7.6%), medical (3.4%) and other application (8.5%). The adsorbents were prepared from different base materials, and two or more base materials are often used as hybrids. As for the DES constituents, hydrogen bond acceptors were mainly chosen as choline chloride or other quaternary ammonium salts, while hydrogen bond donors include ethylene glycol, glycerol, urea in addition to other organic acids, polymerizable monomers, and sugars. The second part of the review traces the different methodologies used within DES-ADS field through homogenization of different terms in the literature and categorizing each methodology based on the role of the DES within the different DES-ADS development schemes. Accordingly, three main synthesis routes were identified, namely, (i) Mixing, (ii) Dispersion, and (iii) Solvothermal methods. The discussion includes a critique about certain generalizations, assumptions, and shortcomings with regard to the DES's nature and intrinsic properties during the development of such DES-ADS. This includes using improper reaction environments and dismissing the basic thermal properties of DES during the synthesis/functionalization of DES-ADS. Lastly, an alternative bottom-up framework is proposed for developing functional and task-specific DES-ADS. This work provides a detailed mapping of trends and trajectories in the field of DES-based adsorbents and a critical discussion on the different methodologies used within the field. The framework devised in the light of such meticulous reading is a methodical, bottom-up approach that considers the principles from both fields (Adsorption and DES). Ultimately, this framework allows researchers to take the necessary steps towards answering the research question(s) imposed by the need for DES-ADS development. The framework can be also extrapolated into other fields in order to develop various DES-based adsorbents, membranes or other functional materials.