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Nucleophilicity and Accessibility Calculations of Alkanolamines: Applications to Carbon Dioxide Absorption Reactions Young H. Jhon,† Jae-Goo Shim,‡ Jun-Han Kim,‡ Ji Hyun Lee,‡ Kyung-Ryong Jang,‡ and Jaheon Kim*,† Department of Chemistry, Soongsil UniVersity, 511 Sangdo-Dong, Dongjak-Gu, Seoul 156-743, Korea, and Global EnVironment Research Group, EnVironment & Structure Laboratory, Korea Electric Power Research Institute, 103-16 Munji-Dong, Yuseong-Gu, Daejeon 305-380, Korea ReceiVed: June 26, 2010; ReVised Manuscript ReceiVed: October 12, 2010
Both nucleophilicities and accessibilities of three alkanolamines [monoethanolamine (MEA), (2-(methylamino)ethanol (MAE), and 2-amino-2-methyl-1-propanol (AMP)] were calculated to predict their reactivities with CO2. After DFT geometry-optimization calculations, the global, group, and atomic nucleophilicities of each amine were obtained using MP2 quantum mechanical calculations. Only global nucleophilicity matched an experimental pKa order (MAE > AMP > MEA). However, it failed to predict the slow rate of the sterically hindered AMP and the order of rate constants, MAE > MEA > AMP. The accessibilities of amines to CO2 have been calculated by MD simulations by monitoring collisions at the reaction centers: N atoms in amines and C in CO2. The accessibility results indicate that global nucleophilicity needs quantitative correction for steric effects to predict better reactivities of amines with CO2. I. Introduction Carbon capturing is hoped to help moderate global warming. It can be applied to a major source of CO2:1 power plants that burn fossil fuels. A postcombustion process using aqueous amines, such as MEA, can easily be applied to currently operating power plants.2 Motivated by cost reduction, more efficient amines are sought that can absorb CO2 more quickly and in larger amounts and that can also be regenerated at lower temperatures than MEA.3 Computational chemistry can predict physical properties of various amines to understand their reactions with CO2.4 Study at a molecular level5 shows that the electronic properties of amines correlate with the amines’ capacity to absorb CO2.6a However, there are scarce reports6b about calculations of amines’ reactivity in relation to CO2 absorption. Herein both the electronic reactivity and the accessibility of three amines (MEA, AMP, and MAE) with CO2 are investigated and compared with known experimental results.
using group philicity,8c and (iv) the use of global electrophilicity descriptor for explaining the rate of a chemical reaction.8d In CO2 absorption reactions, amines act as nucleophiles, and therefore, their global, group, and atomic nucleophilcities are calculated and assessed for electronic reactivity. These electronic descriptors largely focus on the energies and populations of frontier orbitals of the reactants, requiring calculations on chemical potential9 and Fukui functions,10 in addition to the frontier molecular orbitals (FMO).11 Effective absorption of CO2 is unlikely even in an amine of high electronic reactivity if the reaction center (N atom in amines) is sterically congested. Therefore, collisions of reaction centers were also investigated to complement electronic reactivity findings that express the reaction only in terms of the electronic structures and energies of the reactants. Molecular dynamics (MD) simulations find likely collision rates, considering the dynamic accessibility of an amine. The effective collisions of amines have been calculated by considerations of both the kinetic energies of the reactants and reactions’ activation energies. II. Reaction Properties of MEA, AMP, and MAE
Various philicities have been used to explain basicities and chemical reactivities.7,8 For instance, the relationships between quantum mechanical descriptors and experimental results have been recently discussed by many research groups: (i) between both electrophilicity and nucleophilicity with the Mayr-Patz experimental free-energy equation for diaryl carbenium ions,8a (ii) between basicity and nucleophilicity,8b (iii) pKa prediction * Corresponding author. E-mail:
[email protected]. † Soongsil University. Fax: 82 2 824 4383. Tel: 82 2 820 0459. ‡ Korea Electric Power Research Institute. Fax: 82 042 865 5725. Tel: 82 042 865 5239.
There are two proposed meachanisms for the absorption of CO2 by primary or secondary amines. The prevailing mechanism involves a zwitterionic carbamate intermediate which transfers a proton to a base, B, to produce a carbamate and a conjugate acid, BH+.12 It consists of the following two unit reactions:
RNH2 + CO2 a RNH2+ + COO-
(1)
RNH2+COO- + B f RNHCOO- + BH+
(2)
The other, termolecular, mechanism proposed by Crooks and Donnellan, does not involve zwitterions because it assumes simultaneous carbamate formation and proton transfer.13
10.1021/jp105914c 2010 American Chemical Society Published on Web 11/16/2010
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TABLE 1: Experimentally Determined Properties of Some Aqueous Aminesa MEA 15a
second-order rate constant (m3 mol-1s-1) at 298 K
3.63 3.6315b 4.0915c 5.54515d activation energy, Ea (kcal/mol) 4.2816a 10.2815d reaction enthalpy, ∆H (kcal/mol) -19.617a -18.417b capacity (mol CO2/mol Amine) 0.566a 12.514 carbamate stability constant, KC at 40 °C
MAE
AMP
15a
µ ) ∂E/∂N
(7)
η ) ∂2E/∂N2
(8)
15a
7.99 7.9415b 5.0115c
0.42 0.52015d 1.18515e
9.216b 9.9715d not found -16.117c 0.636a not found
where µ is chemical potential and η is chemical hardness:20
where E is the total energy and N is the number of electrons. Both µ and η are estimated using IP (ionization potential) and EA (electron affinity):10b
0.726a AMP (0.1161) > MEA (0.1114), implying that MAE is the most reactive (Table 3). This trend is in accordance with experimental pKa values, although the global nucleophilicty reflects the kinetic aspect. As an amine exhibiting a higher pKa is a stronger base, MAE is more likely to form favorably a Lewis acid-base complex with CO2 than the other amines. The enhanced electronic reactivity of MAE is attributed to the electrondonating methyl group bonded to the donor N atom. The carbamate structures were examined to determine the effect of the electron-donating groups (Table S5, Supporting Information). In general, the sp3 N atoms became sp2 with planar geometry and partial double bond character in the N-C bonds.44 The charge densities in the N atoms of the carbamates increased by 0.112e (MAE), 0.128e (AMP), and 0.144e (MEA), on the basis of NPA population analysis. Therefore, the presence of electron-donating groups facilitated electron delocalization and stronger C-N bonds. A similar discussion can be found in a previous report on ionic liquid absorbents for CO2 capture by Yu et al.45 The group and atomic nucleophilicities were calculated using various population analyses (Tables 4 and 5). Only the group
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TABLE 2: Atomic Electrophilicity Indices of the Carbon Atom of CO2 Using Various Population Analyses population analysis method atomic electrophilicity
NPA
MKS
CHelpG
HPA-I
FPA
MPA
LPA
AIM
HPA
1.114
1.904
1.760
1.142
0.809
0.946
0.673
1.676
3.659
nucleophilicity from FPA resulted in the same philicity order as experimental pKa values. The other population analyses failed to produce philicity trends similar to that for pKa. These results show that global and local (group and atomic) nucleophilicities do not always produce the same trend.46 Accessibility. Sterically hindered amines have lower reaction rates than nonhindered amines. However, this expectation did not match the calculated electronic reactivities because AMP had a similar but larger global nucleophilicity than MEA. The orgin of this inaccuracy of the calculated reactivities of amines is a lack of consideration of steric effects. This led to calculations of mutual accessibility of the reaction centers using MD simulations. The exposed accessible area of each amine’s N atom is dependent on both the orientation and the path of the incoming CO2. Water molecules also affect access of the reactants. This complex situation was simulated in an explicit solvent environment, as shown in Figure 4. As expected, the number of collisions between N atoms in amines and C atoms in CO2 molecules showed a decreasing
TABLE 6: Average Number of Collisions over 10 Configurations between the Reactive Centers of Amines and CO2 (Tables S6 and S7, Supporting Information)a compound
no. of collisions
no. of effective collisions
activation energies (kcal/mol)
MEA MAE AMP
1210 (195) 637 (193)a 1065 (160)
294 (49) 125 (30) 226 (40)
4.2816a 9.216b 10.2815d
a Effective collisions are those having larger available kinetic energies than activation energies. Parentheses show standard deviations.
Figure 4. MD image captured 50 fs after the simulation’s start. The MEA molecules are shown as stick models and CO2 is shown with a CPK model. Green lines represent water molecules.
Figure 3. ESP-mapped electron densities for (a) CO2, (b) MEA, (c) MAE, and (d) AMP are displayed with an isoelectron density of 4 × 10-4 e/Bohr.3 Color codes: C, gray; H, white; N, blue; O, red.
TABLE 3: Calculated ω- (Global Nucleophilicity) Indices of Three Amines and a Comparison of Their Respective Empirical pKa Values compound
IP (eV)
EA (eV)
ω-
pKa47 (25 °C, in water)
MEA MAE AMP
6.268 6.220 5.637
0.469 0.470 0.460
0.1114 0.1316 0.1161
9.5 9.88 9.694
trend: MEA (1210) > AMP (1065) > MAE (637) (Table 6). Accessibility is also dependent on viscosity. Therefore, the large accessibility of MEA may be partly due to its lower viscosity (Table S8, Supporting Information). However, the results indicated that any additional substituent in MEA would decrease the accessibility. Interestingly, the order of the collision frequency (MEA > AMP > MAE) is different from that of the global nucleophilicities (MAE > AMP > MEA). The streically hindered AMP has a better nucleophilic character than MEA. However, as AMP has a lower chance of interacting with CO2 in solution, its absorption reaction would show a lower rate than MEA. Effective collisions were those having energies larger than the activation energy. The proportions of effective collisions of MEA, MAE, and AMP are 24.3, 21.2, and 19.6%, respectively, in accordance with the trend of the activation energies.
TABLE 4: Calculated ωg- (Group Nucleophilicity) Values of the Nitrogen Atoms of Three Amines Using Various Population Analyses compound
NPA
MKS
CHelpG
HPA-I
FPA
MPA
LPA
AIM
HPA
MEA MAE AMP
0.0713 0.0812 0.1014
0.0439 0.0397 0.0411
0.0403 0.0248 0.0416
0.0617 0.0469 0.0867
0.0690 0.0790 0.0747
0.0683 0.0497 0.3884
0.1141 0.0861 0.0784
0.0503 0.0409 0.0588
0.1358 0.1149 0.0485
TABLE 5: Calculated ωk- (Atomic Nucleophilicity) of the Nitrogen Atoms of Three Amines Using Various Population Analyses compound
NPA
MKS
CHelpG
HPA-I
FPA
MPA
LPA
AIM
HPA
MEA MAE AMP
0.0613 0.0827 0.0380
0.1027 0.1505 0.0931
0.1009 0.1522 0.0846
0.0633 0.1007 0.0315
0.0435 0.0520 0.0354
0.0494 0.0450 -0.0078
0.1123 0.0344 0.0254
0.0206 0.0358 -0.0132
0.1189 0.0247 0.0209
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TABLE 7: Summary of Magnitude Trends of Reaction Properties for Three Aminesa properties 15
rate constant pKa17 global nucleophilicity accessibility effective collision a
order
methods
MAE > MEA > AMP MAE > AMP > MEA MAE > (AMP > MEA) (MEA > AMP) > MAE MEA > AMP > MAE
experiments experiments calculations calculations calculations
The amines enclosed by parentheses are of similar magnitude.
It can be deduced that the introduction of methyl group(s) acts to decrease the reaction rate. VI. Conclusions Various nucleophilicities of three amines have been calculated to evaluate their electronic reactivity with CO2. While group and atomic nucleophilicities were too dependent on the population analysis method, the global nucleophilicity trend followed the pKa values of the amines (Table 7). There was a discrepancy in trends between global nucleophilcities and experimental rates of AMP and MEA, imputable to global nucleophilicty’s neglect of steric effects without consideration of explicit intermolecular interactions. The dynamic accessibility between the reactive centers of amines and CO2 in water were represented by the number of collisions. Both MAE and AMP were considered largely sterically hindered to incoming CO2. However, the current calculations on accessibility do not explain clearly the behavior of MAE, which shows a low accessibility but has a larger rate constant than MEA. A possible argument is that the large nucleophilicty of MAE can overcome its low accessibility. This is not applicable to AMP because its steric hindrance is too large to be countered by its moderate nulceophilcity. This work shows that the global nucleophilicity calculation of amines requires some correction for steric effects to explain and predict amines’ reactivity with CO2. A proper combination of steric effects with global nucleophilicities is something that further research will hopefully provide. Acknowledgment. We thank the Korea Institute of Energy Technology Evaluation and Planing (KETEP) for financial support through the Energy Efficiency & Resources Program. We are grateful to Dr. Bultinck for calculating FPA population analysis and also the reviewers for their valuable comments. Supporting Information Available: MEA conformer calculations, MD simulation results, charge distribution for amines and their carbamates, viscosity data, and Fukui functions of amines. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Johnson, J. Chem. Eng. News 2007, (October 29), 25–28. (b) Kintisch, E. Science 2007, 317, 184–186 (in News Focus). (2) Rochelle, G. T. Science 2009, 325, 1652–1654 (in Perspecitve). (3) (a) Goto, K.; Okabe, H.; Shimizu, S.; Onoda, M.; Fujioka, Y. Energy Procedia 1 2009, 1083–1089. (b) Singh, P.; Versteeg, G. F. Process. Saf. EnViron. 2008, 86, 347–359. (c) Singh, P.; Niederer, J. P. M.; Versteeg, G. F. Chem. Eng. Res. Des. 2009, 87, 135–144. (4) (a) da Silva, E. F. Computational Chemistry Study of Solvents for Carbon Dioxide. Doctorial Thesis, Norwegian University of Science and Technology, 2005. (b) da Silva, E. F.; Svendsen, H. F. Int. J. Greenh. Gas. Con. 2007, 1, 151–157. (5) (a) Ismael, M.; Sahnoun, R.; Suzuki, A.; Koyama, M.; Tsuboi, H.; Hatakeyama, N.; Endou, A.; Takaba, H.; Kubo, M.; Shimizu, S.; Del Carpio, C. A.; Miyamoto, A. Int. J. Greenhouse Gas Control 2009, 3, 612–616.
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