Notes Bull. Korean Chem. Soc. 2009, Vol. 30, No. 12 3131 DOI 10.5012/bkcs.2009.30.12.3131
Computational Studies on the Reaction between Monoethanolamine and Nitrogen Oxides
Jae-Goo Shim, Jun-Han Kim, Young H. Jhonj* and Jaheon Kim%*
GlobalEnvironment Research Group, Environment &StructureLaboratory, Korea ElectricPowerResearchInstitute, Daejeon 305-380, Korea
'Department ofChemistry, Soongsil University, Seoul156-743, Korea :E-mail: YoungJhon@gmail. com (YHJ); jaheon@ssu. ac. (JK) Received September 4, 2009,Accepted October 28, 2009
KeyWords:Nitrogen oxides, Monoethanolamine, Reaction, Transitionstate,DFT
The flue gasemittedbypower plants is composedof mainly N2 andCO2 withasmallamountof the harmful gasesSO2 and NO2. ThegreenhousegasCO2 is removed byabsorptionpro cessusing alkanolamines such asmonoethanolamine(MEA).1 SO2 canalso becapturedbyforming a Lewisacid-basecom plex with amines.2 While both CO2 and SO2 can form stable adductswithMEA,itshould be expected that NO2 would be
have differently sinceit is aparamagnetic gas, and in equi libriumwithN2O4 atambienttemperature. It is possiblefor MEA to react withN2O4, too. There are patent reports on the removal ofNO2 by amines.3 However,noresearchhasyetbeen reported onthe mechanism oftheNO2 elimination reaction by amines, which motivated us to investigate the reaction be tweenNO2 andMEA by quantummechanicalcalculations.
As MEAcanreact withboth NO2 andN2O4, possiblereac tionsare classifiedas shown inScheme 1: (1)MEA- NO2 or MEA-N2O4 complex formation, and(2)dissociation of N2O4
byMEAtoHO(CH2)2N-NO2 and nitrous acid. N2O4, aplanar structurewith D2h symmetry,hasa long andweakN-Nbondin which d(N-N) is 1.75 A. Its dissociation energy is only 57 kJ/mol(13.6kcal/mol).4 N2O4 iseasily hydrolyzed to produce HNO3 and HNO2. As itis, it is expectedthat MEA could also break theN-N bond by an SN2 typereaction.
In this work, all calculations were carriedout using theGau ssian 03 suite of programs.5 Geometry optimizationand energy calculations were performed onthecomputational level of Har- tree-Fock,anddensity functional theory(DFT)6 with theB3LYP functional, respectively. For the energycalculation, we used theaqueoussolvation model, PCM,developed byTomasi et al.
Theadopted basis set was 6-31+G(d,p),andthe cavitiesof the solvation model weregiven Bondiradii.8
Optimizationand energy calculations for the MEA-NO2
complex yieldedapositive Gibbs energy, +7.6kcal/mol. Al
though there are some stable compounds having anN-NO2
bond,9-11 the MEA-NO2 complex is notlikely toform spon taneously. Similarly, the MEA-N2O4 complex was veryunsta ble, and each species was separated, anddidnotform a che
mical bondduring calculations.
Asboth NO2 and N2O4 cannot form stablecomplexes with MEA, a nucleophilic reaction between MEA andN2O4 was investigated. The reactionpathway for a reaction between MEA and N2O4 is monitoredby a “coordinate driving” me
thod,12,13withakey parameter being d(N-N), a distance between
Scheme 1. Reactions considered in this work are displayed with formal charges and electron rearrangement. R- is HO(CH，2- for MEA
-0 u」- ro0
〉-)
>5」
쯔」山
Reaction Coordinate d(N-N) (A)
Figure 1. Energy vs. reaction coordinate 刁(N-N) for the reaction between MEA and N2O4, which produces HNO3 + HO(CH2)2 NH- NO2. There is a discontinuous change between the points (i), corre
sponding to pseudo-TS, and (ii), a product.
the nitrogen atom in MEA and a nitrogen atom in N2O4, as shown in Figure 1. The value ofd(N-N) wasfixed to 3.07 A, and all other geometric parameters were optimized. In this manner,the energy profile was obtained byreducingd(N-N)
3132 Bull.KoreanChem. Soc. 2009, Vol. 30, No. 12 Notes
Table 1. Selected inter-atomic distances of the N2O4-MEA com
plexes
distance (A) reactants Pseudo-TS TS products
d(N-N) 2.99 1.75 1.60 1.33
d(N-N)' 1.57 1.62 1.81 3.70
d(O-H) 3.09 2.39 1.98 0.95
(a)
-( OL U- ro°
〉-)
>5」
으
」山
Reaction Coordinate d(N-N) (A)
( O-L U- ro°
〉-)
>5」
으」山
Reaction Coordinate d(N-N) (A)
Figure 2. (a) IRC plot around TS, and (b) energies for the reactants, TS, and products for the reaction between MEA and N2O4.
to 1.2 A.The resulting relativeenergies areplottedagainst the reactioncoordinate. The energy of theinitial optimized chemi cal structure with thefixedkeyparameter is defined aszero.
The reaction wasinvolved with oneenergy barrier at 1.75 A.
Whend(N-N) was reduced further abovethe pseudo-TS (tran
sition state), and thechemicalconnectivity of thesystemwas changed greatlyat 1.55 〜1.6 A, theenergydropped from 13.3 kcal/mol to-17.7kcal/mol. This unusualresultindicated that thereaction coordinate andthe application of the “coordinate driving” method tothis workwas not pertinent, or neededother factorsto make it so.14
Instead of monitoring the reaction profile, the transition state (TS)wasdetermined using thepseudo-TS structure ob tained by the coordinate-driving method. The obtained TS struc ture (Figure 2a) had only one negative eigenvalue, corres ponding to an imaginary vibration of i292.4 cm-1 along the
line connectingtworeacting N atoms. Intrinsic reactioncoordi nate(IRC)calculations performed attenpoints aroundthe TS confirmedthatthe energy valuesatthese points were lower than that of the TS (Figure 2a).5 The eneigy vs.reactioncoordi nateis plotted in Figure 2b; here,structures for reactants,TS, and products, are displayed. The activation energy was 9.7 kcal/mol. Optimization and energycalculationswerealso car
ried outfor both reactants and products. Thecalculatedreac tion energy was -22.5 kcal/mol, showingthat the proposed reaction is spontaneous. Vibrational frequency analyses showed that allthe stationary points of the reaction models had no imaginary frequencies, which confirmed that the obtained geo
metrieswereat their energy minima.
The important interatomic distance changes of d(N-N), d(N-N)', and d(O-H) (Figure 2a), thedistances of thebonds which are formed or broken duringthe reaction, are listedin Table 1. It is seen thatN2O4 maintainsits structurewith d(N-N)' alittle elongated at TS, while MEA andN2O4 are partially bonded,withd(N-N) = 1.33A. It is also foundthat the Hatom in MEA transferred tothe oxygen atom in N2O4 toproducea nitrousacid, HNO2.
Inshort, NO2 seemstobe eliminated in a fundamentally different manner from CO2 and SO2. While thesetwo gases form stable adducts, NO2 canbe removedindirectlyby anN2O4
eliminationpathway. Itispossiblethat NO2 isalsoeliminated by H2Oto give nitiric acid,15and N2O5, like N2O4,can react with MEA. For the lattercase, theonlydifference isthat nitric acid ratherthan nitrous acidis produced; in all,thecomputed reaction energywas -43.8 kcal/mol.
Acknowledgments.This work was supported by the KEMCO (Korea Energy Management Corporation).
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15. As a CO2 absorption process using alkanolamine operates at 40 〜 60 oC, the reaction between NO2 and H2O was calculated at 50 oC and 25 oC, respectively. The energies were almost the same, with -0.46 kcal/mol at 50 oC, and -0.43 kcal/mol at 25 oC. In a real CO2-absorption process, amines are used as an aqueous solution.
As it is, NO2 elimination may take place competitively by amines and water.
