CO₂ Absorption Performance By MDEA-based Deep Eutectic Solvents And Its Molecular Simulation

In recent years,CO₂ emissions have been increasing rapidly,becoming a major contributor for climate change,further influencing the ecological environment and human life.CO₂ capture is one of the effective ways to solve global warming.Herein,a series of deep eutectic solvents(DES)based on N-methyldiethanolamine(MDEA)as hydrogen bond donor with primary,secondary,and tertiary amine hydrochlorides as hydrogen acceptors were developed.Their CO₂ absorption performance was investigated.the hydrogen bond network composition of DES and the interaction mechanism between DES and CO₂ was exploited using molecular simulations.The main research results and conclusions are as follows:Three different DESs were prepared:[MEAHCl][MDEA](DES1),[DEAHCl][MDEA](DES2)and[MDEAHCl][MDEA](DES3).The CO₂ uptake capacity after 6 h of absorption at room temperature is 0.1150,0.1082 and 0.0594 g CO₂/g DES,respectively.The NMR and FTIR results indicate that the CO₂ absorption of DES1 and DES2 is mainly via the amino group in MEAHCl to form carbamate,and through the hydroxyl group in MDEA to generate carbonate;while DES3 reacts with CO₂ through the hydroxyl group in MDEA to produce CO₃^2-.The desorption activation energy was determined using According to the Flynn-Wall-Ozawa method.DES1 can be regenerated at 80?without solvent loss Moreover,DES1 is able to maintain its absorption capacity after 5 cycles of absorption-desorption,indicating that the DES1can be recycled and reused with low energy consumption.Quantum chemistry and molecular dynamics simulation methods were used to analyze the interaction between DES and CO₂ and the structural features of DES before and after CO₂ absorption.The Density Function Theory(DFT)calculation was performed based on the B3LYP/6-31G(d,p)basis set combined with D3 dispersion.The charge distribution was obtained through the structural optimization.It is found that both the-NH₂ group in MEAH⁺and the-OH groups in MDEA of DES1 exhibit strong interaction with CO₂.GAFF force field and RESP charge were adopted to calculate the radial distribution function and spatial distribution function of DES before and after CO₂ absorption using molecular dynamics simulations.The simulation results revel that in DES1 and DES2 systems,hydrogen bonds may exist between the-NH₂,Cl^-groups of HBA and-OH group of MDEA,and the-OH groups between MDEA and MDEA.In DES3,hydrogen bonds may be formed between HBA and MDEA,and-OH group between MDEA and MDEA.The-NH₂ group in DES1 and DES2 and the hydroxyl group in MDEA have an interaction with CO₂ while in DES3 the hydroxyl group in HBA and MDEA interacts with CO₂.Furthermore,the CO₂ absorption promotion factor of ethanolamine(MEA)was introduced to prepare the ternary DES of[MEAHCl][MDEA][MEA]to improve the absorption capacity of CO₂.The CO₂ uptake capacity was examined at different molar ratio of 1:2:0.5,1:3:0.3,and 1:3:0.5.The absorptive capacity of CO₂ is dependent on the amount of MEA.However,high MEA content eventually lead to the difficulty in regeneration.At 1:3:0.5,the CO₂ absorption capacity is significantly higher than[MEAHCl][MDEA],reaching to 0.1383 g CO₂/g DES.The TGA results indicate that the system is renewable.The NMR and FTIR results indicate that CO₂ mainly reacts with the amino groups on MEAHCl and MEA to form carbamate,and reacts with the hydroxyl groups on MDEA to form carbonate.The desorption activation energy is slightly higher than[MEAHCl][MDEA]due to the formation of carbamate between MEA and CO₂,but it can be successfully desorbed at 80?.No loss in the CO₂absorption was observed after 5 cycles of absorption-desorption,revealing that the system can be regenerated with low energy consumption.