CO2 Capture Using Slurries of Immobilized Deep Eutectic Solvents: From Synthetic Gas Studies to Flue Gas Implementation

The mitigation of anthropogenic CO2 emissions requires the development of efficient, stable, and economically viable capture technologies beyond conventional amine-based systems. Deep eutectic solvents (DESs) have emerged as promising alternatives due to their tunable properties and high CO2 affinity; however, their practical application is often limited by high viscosity and mass-transfer constraints. This study investigates the design, optimization, and validation of DES-based systems for CO2 capture, with particular emphasis on combining cosolvent addition and immobilization strategies to enhance overall performance. In the first part, [MEACl][EDA]-based DESs with varying molar ratios were synthesized and evaluated as aqueous CO2 absorbents. An aqueous 40 wt.% [MEACl][EDA] (1:5) system was identified as optimal, exhibiting higher CO2 uptake (22.09 wt.% at 22 ºC and 1 atm), faster absorption kinetics (1.24 mol CO2/(kg sorbent·min) after 2 min at 22 ºC), and comparable viscosity (4.401 mPa·s before and 13.330 mPa·s after CO2 capture) relative to benchmark 30 wt.% aqueous monoethanolamine (MEA) (15.74 wt.% CO2 capture capacity, viscosity of 3.318 mPa·s before and 8.413 mPa·s after CO2 capture), along with good thermal stability and recyclability (~88% regeneration). To further improve performance, a novel hybrid approach was developed by immobilizing 5 wt.% DES within mesoporous silica and dispersing 3 wt.% of the composite in 40 wt.% aqueous DES to form slurries. These hybrid systems demonstrated enhanced CO2 capacity (up to 24.93 wt.% at 22 ºC and 1 atm), improved sorption rates (1.4 mol CO2/(kg sorbent·min) after 2 min at 22 ºC), and acceptable viscosity (7.32 and 21.82 mPa·s before and after CO2 capture) and cyclic stability (~91% recovery). To study the desorption performance, the strategy was extended to non-aqueous systems by immobilizing 5 wt.% DES within mesoporous silica and dispersing 3 wt.% of the composite in 20 wt.% DES in ethylene glycol as a cosolvent, resulting in slurries with improved desorption kinetics (0.38 mol CO2/(kg sorbent·min) after 2 min at 110 ºC), superior thermal stability, minimal solvent loss, and promising regeneration performance (96.4% recovery). Finally, the practical applicability of the developed systems was validated using real biomass combustion flue gas and synthetic gas mixtures (CO2/N2 and CO2/CH4) over a wide pressure range. The aqueous DES-based systems achieved CO2 removal efficiencies >97%, whereas the non-aqueous systems reached 75-85% at 25 ºC and 1 atm for real flue gas. Pressure-dependent studies using CO2/N2 and CO2/CH4 mixtures showed enhanced CO2 uptake with increasing pressure for all systems, with aqueous slurries consistently outperforming their non-aqueous counterparts, while the latter exhibited additional CH4 uptake due to physical sorption in the ethylene glycol phase.