DFT Calculations on the Role of Base in the Reaction between CO 2 and Monoethanolamine Jae-Goo Shim, † Jun-Han Kim, † Young H. Jhon,* ,‡ Jaheon Kim,* ,‡ and Kwang-Hwi Cho* ,§ Global EnVironment Research Group, EnVironment & Structure Laboratory, Korea Electric Power Research Institute, 103-16 Munji-Dong, Yuseong-Gu, Daejeon 305-380, Korea, and Departments of Chemistry and Bioinformatics and Life Science and CAMDRC, Soongsil UniVersity, 1-1 Sangdo-Dong, Dongjak-Gu, Seoul 156-743, Korea DFT calculations using a PCM solvation model were carried out to examine the reaction pathways for the formation of carbamate from CO 2 and monoethanolamine (MEA) in the presence of an additional proton acceptor, MEA or H 2 O. In both cases, the zwitterionic species [HO(CH 2 ) 2 NH 2 + sCOO - ] has a lower energy than the reactants. Proton transfer from the zwitterionic species to the base (MEA) produced a carbamate product with an accompanying gradual decrease in energy. However, a similar process for the water case resulted in an implausible increase in energy, which suggests that MEA is suitable as a base but H 2 O is not. Introduction Chemical absorption using an aqueous alkanolamine solution is an important industrial method for capturing CO 2 from the flue gas of power plants. 1 Among the alkanolamines available, monoethanolamine [MEA, HO(CH 2 ) 2 NH 2 ] has been studied extensively because it absorbs CO 2 quickly in the form of a stable carbamate. 2 Although extensive studies have been conducted on absorption reactions involving MEA, the reaction mechanism(s) is (are) unclear. 3 The main controversy is the identity of the reaction intermediate, a zwitterionic species formed from CO 2 and MEA. Currently, a zwitterion mechanism is the prevailing mecha- nism for processes using primary and secondary alkanolamines as absorbing agents. In this process, a zwitterionic carbamate is an intermediate and transfers a proton to a base, B, to produce a carbamate and a conjugate acid, BH + , as products. 4 According to the mechanism, the reaction consists of the following two unit reactions CO 2 + RNH 2 a RNH 2 + COO - (1) RNH 2 + COO - + B f RNHCOO - + BH + (2) In the above equations, R is HOCH 2 CH 2 s for MEA, and B is either a base, such as OH - , HCO 3 - , CO 3 2- , or H 2 O, or another MEA molecule in the reaction medium. Caplow originally proposed this two-step mechanism to explain the biotin car- bamate formation reaction, where hydrogen transfer is very fast and the first step is considered the rate-limiting step. 5 On the other hand, if carbamate formation and proton transfer take place simultaneously, the overall reaction becomes a single-step, third- order reaction. This termolecular mechanism proposed by Crooks and Donnellan does not involve a zwitterion. 6 CO 2 + RNH 2 + B a RNHCOO - + BH + (3) In addition to kinetic studies, theoretical investigations have been used to examine the reaction mechanism. da Silva and Svendsen examined the reaction pathways between CO 2 and alkanolamines, such as MEA and diethanolamine (DEA) by Hartree-Fock ab initio calculations. 7 They used the coordinate driving method to perform a series of calculations for the reacting species in which the key parameters were chosen and kept fixed at various values while all other geometric parameters were optimized. The calculated reaction energy profile did not involve any intermediates and supported the one-step termo- lecular mechanism. Recently, Arstad et al. used density func- tional theory (DFT) and ab initio calculations to examine the reactions between CO 2 and various amines. 8 They proposed a two-step reaction mechanism, but one that was different from the zwiiterionic mechanism. In this case, the intermediate was not a zwitterion but a carbamic acid, as shown in eqs 4 and 5. Their calculations showed that carbamic acid was produced by a proton-catalyzed pathway where additional amine or water molecules, although not incorporated in eq 4, relayed the proton from the nitrogen atom to a terminal CO 2 group in the zwitterionic encounter complex RNH 2 + CO 2 - . CO 2 + RNH 2 a RNHCOOH (4) RNHCOOH + B f RNHCOO - + BH + (5) However, in their report, only the first reaction (eq 4) was examined without involving the second step. Instead, direct proton transfer from a zwitterionic complex to another MEA base was calculated. This study examined in more detail the absorption reaction paths between CO 2 and MEA in the presence of additional MEA or H 2 O using DFT calculations with a polarizable continuum model (PCM) solvation environment. This approach is similar to that used by da Silva and Svendsen. 7 However, a higher level of calculation in a different solvation model was used in the present case. This study compared the proton-transfer processes from a zwitterionic species formed from CO 2 and MEA to a base molecule, H 2 O or MEA. The results obtained were used to describe the role of the proton acceptors, MEA and H 2 O, as the base during the overall MEA-CO 2 absorption reaction. Computational Details All calculations were carried out using the Gaussian 03 9 suite of programs utilizing density functional theory (DFT) 10 with * To whom correspondence should be addressed. Tel.: +82-2-820- 0459. Fax: +82-2-824-4383. E-mail: youngjhon@gmail.com (Y.H.J.), jaheon@ssu.ac.kr (J.K.), chokh@ssu.ac.kr (K.-H.C.). † Korea Electric Power Research Institute. ‡ Department of Chemistry, Soongsil University. § Department of Bioinformatics and Life Science and CAMDRC, Soongsil University. Ind. Eng. Chem. Res. 2009, 48, 2172–2178 2172 10.1021/ie800684a CCC: $40.75  2009 American Chemical Society Published on Web 01/05/2009