| The most critical method to achieve CO2 emission reduction and promote China moving to low-carbon energy economy is developing efficient and low-energy technologies for capturing CO2. Amine-based absorption process has some fatal defects, involving solvent volatilization and degradation, equipment corrosion and high energy consumption of regeneration, in order to overcome these defects, a series of absorbents of amine-functionalized ionic liquids(ILs) were designed and the mechanisms of capturing CO2 were systematically investigated by employing B3LYP and MP2 quantum chemistry methods at 6-311++G(d, p) basis sets level. Through optimization calculation of geometry structure, energy calculation, natural bond orbital(NBO) calculation and topological calculation of electron density, the absorption mechanisms of ionic liquids absorbing CO2 were revealed and all the calculation works were done by using GAUSSIAN03 programs, GTA2000 and AIM2000 programs. A theoretical method of screening ionic liquids was presented based on quantum chemisty theory for guiding the development of novel ionic liquids, screening efficient CO2 absorbents and prividing theoretical reliable basis and guidance for applications of CO2 capture technologies based on ionic liquids.Selecting (1-(4-aminobutyl)-3-methyl ([Abmim]+) cation and PF6-, BF4-, Cl-, Br-anions, thirty five geometry structures of functionalized ILs were designed and ten stable geometry structures were obtained through optimization calculations, based on these stable structures, thirty initial structures of products were designed when CO2 is introduced into ILs and ten stable geometry structures of products were obtained by optimization calculation. The interaction energy between B-structure ionic liquid including BF4- (BIL(BF4)) and CO2 is about 32kJ/mol, which is the greatest and enough to capture CO2 efficiently. Generally, it is a main physical absorption when the absorption energy is less than 80kJ/mol, it is therefore concluded that the energy consumption of regeneration is lower. When another cation of (1-(4-butyalminoethyl)-3-ethyl ([Abeim]+) interact with the above four anions respectively, it shows the same the interaction types with those of [Abmim]+ and the absoption energies of the main geometry sturctures capturing CO2 are about 16kJ/mol, 19kJ/mol,14kJ/mol and 16kJ/mol, respectively. Obviously, BF4-may well be the suitable anon.When the cations with substituent on N1 atom of [Abmim]+ remains the same, the substituents on N3 atom were designed as methyl, ethyl and propyl, interact with BF4 and form ionic liquids respectively, the investigation results about the influence on absorbing CO2 of the substituents on N3 showen that the interaction energy between IL and CO2 decreases with increasing of the carbon chain length, which led to less efficiency for capturing CO2. Thus, methyl is the optimum substituent on N3.When the cation with a mehtyl on N3 atom, the substituents on N1 atom are dsigned as aminopropy, aminobutyl and aminoamyl, and form ionic liquids with BF4- repectively. Exploring researches results indicates that the absorption energy for capturing CO2 are about 16kJ/mol for the ILs containing aminopropy and aminoalkyl, which is poor efficiency for capturing CO2, therefore aminobutyl is the best substituent on N1 atom. Synthesizes the above analysis, it is obviously that [Abmim]BF4 is the optimum absorbent for capturing CO2.The AIM2000 calculation results implied that the mechanism of amine-functionalized ionic liquids absorbing CO2 mainly display as C…N weak chemical bond between CO2 and amine, as well as an hydrogen bond between CO2 and buthyl. Based on the geometry structures and combine the calculation results of the energy and the topological properties of electronic density, it can be inferred that this kind of ionic liquid absorbing CO2 may well be a physical process. The calulation results obtained from natural bond orbital(NBO) and atom in molecular theories indicated that the interaction forces of ILs absorbing CO2 are mainly from discrete electrosatctic forces between polarized CO2 by charge distribution and cation, the electrostatic forces between bonding atoms and the orbital interaction forces.The influences on [Abmim]BF4 absorbing CO2 of the coexistence gases (SO2, NO, O2 and N2) were studied and the results show that no stable structures were optimized when NO, O2 and N2 were introduced into [Abmim]BF4, which means a poor absorption efficiency for NO and O2. But two stable geometry structures of products were obtained from [Abmim]BF4 absorbing SO2 and both the interaction energy are about 167 kJ/mol. That is to say, [Abmim]BF4can absorb CO2 and SO2 simultaneously, and it can be inferred from the difference of absorbing energy that CO2 and SO2 can be seperated through grading regeneration. Results obtained from NBO investigation and AIM analysis suggested that [Abmim]BF4 absorbing SO2 is also a physical process. |
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