The existing CO2 absorption by deep eutectic solvents is limited by the unavoidable water absorption problem during use. In this study, we prepared three deep eutectic solvents with different alcohol aminations and added different water contents to discuss the effect of water content on the absorption of carbon dioxide by deep eutectic solvents. All deep eutectic solvents have a low melting point at room temperature as a liquid and have high thermal stability, where the choline chloride-diethanolamine deep eutectic solvents have a high viscosity. Anhydrous choline chloride-monoethanolamine deep eutectic solvents have the largest CO2 absorption, reaching 0.2715 g/g, and the absorption of CO2 by anhydrous choline chloride-N-methyldiethanolamine deep eutectic solvents is only 0.0611 g/g. Water content inhibited the absorption of CO2 in primary amine and secondary amine systems, whereas it enhanced the absorption of CO2 in tertiary amine systems, which was related to the reaction process of deep eutectic solvent and CO2.
Deep eutectic solvents (DESs) are a new class of green
solvents
that exhibit unique properties in various process applications. In
this regard, this study evaluated imidazole-type DESs as solvents
for carbon dioxide (CO2) capture. A series of imidazole-type
DESs with different ratios was prepared through one-step synthesis.
The absorption capacity of CO2 in imidazole-type DESs was
measured through weighing, and the effects of temperature, hydrogen
bond acceptors, hydrogen bond donors, and water content were discussed.
DESs absorbed the effects of CO2. Im-MEA (1:2) was selected
to linearly fit lnη and 1/T using the Arrhenius
equation under variable temperature conditions, and a good linear
relationship was found. The results show the best absorption effect
for Im-MEA (1:4). At 303.15 K and 0.1 MPa, the absorption capacity
of Im-MEA (1:4) was as high as 0.323 g CO2/g DES; through
five times of absorption–desorption after the cycle, the absorption
capacity of DES was almost unchanged. Finally, the mechanism of CO2 absorption was studied using Fourier transform infrared and
nuclear magnetic resonance spectroscopy. The absorption mechanism
of imidazole-type DESs synthesized using imidazole salt and an amine-based
solution was chemical absorption, and the reaction formed carbamate
(−NHCOO) to absorb CO2.
In this study, we used tetraethylammonium chloride (TEAC), diethanolamine (DEA), and N-methyldiethanolamine (MDEA) to prepare ternary DES and binary DES to absorb CO2. We found that their formation was due to the hydrogen bond interaction between hydrogen bond acceptor and hydrogen bond donor (HBD). Surprisingly, TEAC/MDEA/DEA can react with CO2, but TEAC/MDEA cannot react with CO2. Unexpectedly, after adding DEA to TEAC/MDEA, the ternary TEAC/MDEA/DEA DES can react with CO2. Nuclear magnetic resonance spectroscopy and Fourier infrared spectroscopy results showed that the accidental CO2 absorption behavior mainly depended on the HBD DEA, because the imine group in DEA reacted with CO2 to form carbamate, thereby absorbing CO2, while the hydroxyl group on MDEA and the hydroxyl group of DEA did not interact with CO2. Through thermal stability analysis, TEAC/MDEA/DEA system with the molar ratio of 1:3:4 is more stable. We further studied the influence of molar ratio, temperature, water content, and other factors on the absorption of CO2 by ternary DES. In addition, TEAC/MDEA/DEA (1:3:4) was regenerated at 80°C, and the absorption capacity of DES was almost unchanged after five absorption–desorption cycles.
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