Ab Intio Calculations on Reaction Process of Piperazine Absorbing CO 2

Abstract

The increasing amount of CO2 emitted to the atmosphere has a deep impact on the environment. It is urgent to take effective measures to reduce CO2 emissions from its intensive emission source. Chemical absorption is an effective way to cover the large reduction of CO2, and piperazine (PZ) is one of the amines that have good performance on CO2 capture. To make a deep understanding of the chemical mechanism of PZ absorbing CO2, computational studies on PZ chemically absorbing CO2 are discussed in this work using Density functional theory (DFT).
Possible reaction pathways of PZ absorbing CO2 are analyzed. Due to the special chemical structure of PZ, the reaction processes can be divided into two parts, the process of (PZ+CO2)+PZ and (PZ+2CO2)+PZ. To make a good understanding of the reaction process, a comparison of that between PZ and MEA are discussed. The results show that PZ has lower forward and backward energy barriers than MEA in the zwitterion formation, but in the intermolecular hydrogen transfer process, PZ has higher forward and backward barriers than MEA. For the reaction process of PZ+2CO2, it can be stated as a two-step zwitterion formation with two transition states. For the following intermolecular hydrogen transfer of PZ(COO)2+PZ, the atom movements are similar to that of (PZCOO+PZ). All the discussions provide theoretical information of PZ chemical absorbing CO2.

Full text

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of GHGT-13.
doi: 10.1016/j.egypro.2017.03.1346
Energy Procedia 114 ( 2017 ) 2121 – 2127
ScienceDirect
13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18
November 2016, Lausanne, Switzerland
Ab Intio Calculations on Reaction Process of Piperazine Absorbing
CO2
Tingting Zhang 1,2, Yunsong Yu 1, Guoxiong Wang 3, Zaoxiao Zhang 1, 2,
*
1. School of Chemical Engineering and Technology, Xi’an Jiaotong University
No.28 Xianning West Road, Xi’an 710049, P.R. China
2. State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University
No.28 Xianning West Road, Xi’an 710049, P.R. China
3. School of Chemical Engineering, University of Queensland, Brisbane, Qld. 4072, Australia
Abstract
The increasing amount of CO2 emitted to the atmosphere has a deep impact on the environment. It is urgent to take effective
measures to reduce CO2 emissions from its intensive emission source. Chemical absorption is an effective way to cover the large
reduction of CO2, and piperazine (PZ) is one of the amines that have good performance on CO2 capture. To make a deep
understanding of the chemical mechanism of PZ absorbing CO2, computational studies on PZ chemically absorbing CO2 are
discussed in this work using Density functional theory (DFT).
Possible reaction pathways of PZ absorbing CO2 are analyzed. Due to the special chemical structure of PZ, the reaction processes
can be divided into two parts, the process of (PZ+CO2)+PZ and (PZ+2CO2)+PZ. To make a good understanding of the reaction
process, a comparison of that between PZ and MEA are discussed. The results show that PZ has lower forward and backward
energy barriers than MEA in the zwitterion formation, but in the intermolecular hydrogen transfer process, PZ has higher forward
and backward barriers than MEA. For the reaction process of PZ+2CO2, it can be stated as a two-step zwitterion formation with
two transition states. For the following intermolecular hydrogen transfer of PZ(COO)2+PZ, the atom movements are similar to
that of (PZCOO+PZ). All the discussions provide theoretical information of PZ chemical absorbing CO2.
© 2017 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the organizing committee of GHGT-13.
Keywords: CO2 absorption, Piperazine, Reaction mechanism, DFT, Energy barriers
* Corresponding author. Tel.: +86-29-82660689; fax: +86-29-82660689.
E-mail address: zhangzx@mail.xjtu.edu.cn
Available online at www.sciencedirect.com
© 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of GHGT-13.

2122 Tingting Zhang et al. / Energy Procedia 114 ( 2017 ) 2121 – 2127
1. Introduction
The large amount of greenhouse gases emitted to the atmosphere has aroused many worldwide environmental
problems. Carbon dioxide (CO2) is recognized as a typical greenhouse gas. To alleviate these problem, intensive
researches on CO2 capture with energy-efficient method are of essential[1]. Chemical absorption is the commonly
used method, and amines are the typical absorbents, such as monoethanolamine(MEA), diethanolamine(DEA), N-
methyldiethanolamine (MDEA), and 2-amino-2-methyl-1-propanol (AMP)[2, 3]. Generally, primary and secondary
amines have fast reaction rate and high regeneration energy consumption, ternary and hinder amines have lower
regeneration energy consumption but slow reaction rate. Based on this, mixed solvents are discussed to take
advantage of the fast reactivity of primary or secondary amines and low solvent regeneration energy consumption of
tertiary or sterically hindered amines [4-6].
Among the discussed amines, piperazine (C4H10N2, PZ) was found to perform well with CO2 absorption in
mixing with the commonly used amines. The using of piperazine to activate aqueous MDEA solutions was patented
by BASF for its good application in the bulk removal of CO2 in ammonia plants. And at the temperature range of 40
to 60 °C, the mass transfer rate of CO2 into concentrated PZ is more than 1.5-2.0 times faster than MEA. And in the
presence of stainless steel materials, PZ has lower oxidative degradation than that of MEA[7, 8]. There are many
experimental works on the PZ performance at CO2 absorption, but limited works had been found about the detailed
reactions between CO2 and aqueous piperazine. Hence, to make a better understanding of the mechanism and
working principles of the absorption process, simulations discussions are discussed in this work.
The reaction mechanism of primary and secondary alkanolamines with CO2 can be described using the zwitterion
mechanism, as originally proposed by Caplow (1968)[9] and later reintroduced by Danckwerts (1979)[10]. The
chemical structure and amino group of piperazine is different from that of the typical alkanolamines, but the
chemical functional groups are same and they are all R-NH. The main chemical interaction between amino group
and CO2 are universal. It is assumed that this mechanism is applicable to PZ, though it is a diamine with cyclic
structure. However, the different chemical structures that connect with the amino functionality have influence on the
reaction configuration and reaction energy, of which has influence on the performance of CO2 absorption capacity
and energy consumption.
Combining the reaction processes of MEA absorbing CO2, it is assumed that piperazine first reacts with CO2 to
form a zwitterion. Then the zwitterion will be deprotonated by a base (here, PZ is the discussed base and all the
discussions in this work do not involved the H2O molecules) present in the liquid. As piperazine is a heterocycle and
has two amine groups, it has the potential to absorb two CO2 molecules at one time. Thus, the main reactions
discussed in this paper contain two parts, PZ+CO2 and PZ+2CO2. The reaction processes are shown in equations (1-
4),
2
PZ+CO PZCOOo
(1)
-+
PZCOO+PZ PZCOO +PZHo
(2)
222
CO +PZ+CO CO -PZCOO (COO)PZ(COO)oo
(3)
-+
22
PZ(COO) + PZ PZ(CO )COO +PZHo
(4)
The reaction processes of equation (1) and (2) are similar to that of MEA. The reaction processes of equation (3)
and (4) are special ones for PZ. The reaction process of equation (2) is compared with that of equation (4) to find out
the effect of the extra CO2 molecule on the reaction process. Based on those analyses, we will finally find out the
reaction properties of PZ chemically absorbing CO2.

Tingting Zhang et al. / Energy Procedia 114 ( 2017 ) 2121 – 2127 2123
2. Computational details
The simulation works were performed with Gaussian 09 software using density functional theory (DFT). The
chemical interactions of PZ absorbing CO2 were studied in aqueous solvation effect with pauling atomic radii. The
optimized geometries and frequencies of the reactants, transition states and products were obtained at the B3LYP/6-
311G++(d, p)//B3LYP/6-311G level. The transition states have only one imaginary frequency and the vibrational
mode is consistent with the reactant and product. The IRC (Intrinsic reaction coordinate) and Gibbs free energy are
calculated to analyze the reaction. The reaction energies is defined as: ΔEreaction = Eproducts-Ereactants, where ΔEreaction is
the energy difference between products and reactants for the corresponding reaction process, Eproducts and Ereactants are
the energy of products and reactants, respectively[11].
3. Result and discussions
In the reaction process of PZ absorbing CO2, zwitterion formation and the followed hydrogen transfer as shown
in eqs (1-4) are the main reaction mechanism. The atom movements (ie. N-C bond) were discussed. Fig. 1(a) gives a
comparison of structure between MEA and PZ, and it clearly demonstrates the typical geometry difference between
MEA (an industry standard alkanolamine) and PZ (circle structure).
3.1. PZ+CO2 and the following hydrogen transfer
Fig. 1(b) shows the main atom movements of PZ absorbing CO2 in zwitterion formation. The C-N bond lengths
are marked in the figure. As the zwitterion formed, the following reaction step is the hydrogen transfer. In the
process of zwitterion PZCO2 reacting with another PZ molecule, hydrogen atom moves from zwitterion to the
second PZ, and finally produce PZCOO- and PZH+, and this is an intermolecular hydrogen transfer process.
To make a good understanding of the reaction process and energy changes of this process, a comparison between
PZ and MEA is made as the process of MEA absorbing CO2 were also simulated at the same calculation level. The
energy changes are shown in Fig. 2. The results show that in the zwitterion formation, the IRC and Gibbs free
energy barriers of PZ are lower than those of MEA no matter in the forward and backward processes. The forward
Gibbs free energy barriers are much higher than the backward ones, and the energy gaps are 24kJ/mol both for PZ
and MEA. For the IRC, the backward energy barriers are higher than those forward ones, the energy gaps are more
than 3kJ/mol both for PZ and MEA. For the followed hydrogen transfer process, seen from Fig. 2, the backward
energy barriers are much higher than the forward processes for PZ and MEA. Compared the results of PZ and MEA,
the IRC and Gibbs free energy barriers of PZ are higher than those of MEA for both forward and backward
processes.
Combining the zwitterion formation and the following intermolecular hydrogen transfer, the reaction energy
barriers show that in PZ absorbing CO2 process, the zwitterion formation is the dominant reaction unit during the
reaction pattern of (PZ+CO2)+PZ. Thus, small reaction energy barrier advantage is observed for PZ over MEA in in
the reaction pattern of (RNH+CO2)+RNH. However, for the desorption process, the hydrogen transfer process is the
dominant unit and MEA has lower energy barriers than PZ.
3.2. CO2+PZ+CO2 series and the following hydrogen transfer
As a diamine, one PZ has the potential to absorb two CO2 at the same time. The complex reaction process of
PZ+2CO2 can be summarized as a two-step zwitterion formation with two transition states, as shown in Fig. 1(c). It
includes the reaction unit of series1 to series 3 through series/TS 2 and series 3 to series 5 through series/TS 4. The
reaction energy changes are shown in Fig. 3. The IRC energy change of the five states is regular, that is the
transition state has higher relative energy over the corresponding reactant and product. For the reaction process of
the first zwitterion, the Gibbs energy of series/TS 4 is higher than that of series 1 and series 3. However, for the
second zwitterion formation of series 3 to series 5 through series/TS 4, the Gibbs free energy increases from series 3

2124 Tingting Zhang et al. / Energy Procedia 114 ( 2017 ) 2121 – 2127
to series 5 as shown in Fig. 3. Hence, for the two-step zwitterion formation of PZ+2CO2, series 3 is a rational
reaction product while the series 5 may not easily exist in the solution at the discussed temperature (298.15K).
Comparing the zwitterion formation of series 1 to series 3 in (CO2+PZ+CO2) with that in (PZ+CO2) process, the
forward Gibbs free energy is higher in the process of PZ+2CO2 than that in PZ+CO2. And for the IRC energy
barriers and the backward Gibbs free energy barrier, there is no significant different between these two reaction
processes. Thus, the zwitterion formation process of series 1 to series 3 is similar to that of zwitterion formation of
(PZ+CO2) for the similar atom movements and reaction energy barriers, and the extra CO2 atom has no significant
effect on the zwitterion formation.
4.31
4.28
4.31 2.22
4.31 1.64
1.64
2.15
1.66
1.66
Series 1
Series/TS 2
Series 3
Series/TS 4
Series 5
4.32
2.22
1.63
(b) Zwitterion formation
PZ MEA
(c) CO2+PZ+CO2reaction series
(a) PZ and MEA molecular structure
Fig. 1. The typical structure for MEA and PZ

Tingting Zhang et al. / Energy Procedia 114 ( 2017 ) 2121 – 2127 2125
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
-30
-20
-10
0
10
20
30
40
Energy change ( kJ/mol)
Reaction Coordinate
PZ,IRC
PZ,Gibbs
MEA,IRC
MEA,Gibbs
15.10
17.81
31.33
38.77
0.00
-4.01
-3.70
24.39
29.37
0.00
-27.04
-19.11
-23.23
-17.81
0.84
3.78
4.05
6.93
Zwitterion formation Intermolecular hydrogen transfer
Fig. 2. Comparison of energy barrier on the reaction process of zwitterion and hydrogen transfer process
Series 1 Series/TS 2 Series 3 Series/TS 4 Series 5
0
10
20
30
40
50
60
70
1.11
72.08
13.05
67.18
-4.29
32.45
14.83
39.68
PZ+2CO2,IRC
PZ+2CO2,Gibbs
Reaction Energy Barrier (kJ/mol)
Reaction Coordinate
0.00
Fig. 3. The reaction energy changes for the PZ reacting with two CO2 process

2126 Tingting Zhang et al. / Energy Procedia 114 ( 2017 ) 2121 – 2127
20.399
1.384
1.245
1.462
1.185
3.389
B3LYP/6-311G B3LYP/6-311G++(d,p)
Fig. 4. The transition states geometry under different base set (the unit of the bond is Å)
In the above discussed series reaction process, the formed zwitterion can react with another PZ molecule to
produce the PZ(COO)COO- anion and PZH+ cation. The series 3 is a rational product and then could react with the
second PZ molecule. And during the simulation process, it was observed that with different calculation base set, the
optimized geometry of the transition state is different which is shown in Fig. 4. The typical bond lengths are marked.
The relative situation of H atom is a little different in these two geometry, and the significant different structure is
the bond length of N-C between PZ and the extra CO2 molecule, which is 3.389 Å at the base set of B3LYP/6-311G
and 20.399 Å at the base set of B3LYP/6-311G++(d, p).
The reaction energies at the calculating base set of B3LYP/6-311G++(d, p) are used to analyze the energy
barriers. The forward IRC energy barrier is 6.67kJ/mol and the backward is 34.17kJ/mol. The results are very
similar to that of the (PZCOO+PZ) hydrogen transfer process, which are 6.93kJ/mol for the forward and
33.97kJ/mol for the backward process. For the Gibbs free energy, the forward barrier is 6.17kJ/mol and the
backward is 28.64kJ/mol, while they are 4.05 kJ/mol and 23.16kJ/mol, respectively in the (PZCOO+PZ) hydrogen
transfer process. Hence, in those two intermolecular hydrogen transfer processes, the extra CO2 molecule has little
impact on the atom movements of the reaction but the forward and backward Gibbs free energy barriers become
higher.
4. Conclusions
The main reactions and possible pathways of PZ absorbing CO2 are analyzed in aqueous solutions and H2O
molecules are not involved in the chemical reactions. The corresponding reaction energies are calculated using the
ab initio simulations. Zwitterion formation and the followed intermolecular hydrogen transfer processes are the main
reaction paths. Due to the special chemical structure of PZ, the reaction processes can be divided into two parts, the
process of (PZ+CO2)+PZ and (PZ+2CO2)+PZ. In the discussion of (PZ+CO2)+PZ, to make a good understanding of
the reaction process, a comparison of that between PZ and MEA are discussed. The results show that PZ has lower
forward and backward energy barriers than MEA in the zwitterion formation, but in the intermolecular hydrogen
transfer process, PZ has higher forward and backward barriers than MEA. As the energy barriers of zwitterion
formation are much higher than those of the intermolecular hydrogen transfer in the forward reaction process, it can
be concluded that in the absorption process, PZ can absorb CO2 with lower energy consumption than MEA
theoretically. For the reaction process of PZ+2CO2, higher forward and backward Gibbs free energy barriers are
observed in the first zwitterion formation (series 1 to series 3 through series/TS 2) than that in PZ+CO2. The second
zwitterion formation of series3 to series 5 is turns out to be a rational IRC energy change but the Gibbs free energy
is sustained increasing. For the following intermolecular hydrogen transfer of (PZ(COO)2+PZ), the atom movements
are similar to that of (PZCOO+PZ), but the forward and backward energy barriers are a bit higher than that of
(PZCOO+PZ) both for IRC and Gibbs free energy barrier. All those discussions provide theoretical information and
better understanding of PZ chemical absorbing CO2. And further works about the effect of H2O molecules on the
reactions are needed.

Tingting Zhang et al. / Energy Procedia 114 ( 2017 ) 2121 – 2127 2127
Acknowledgements
This work is financially supported by the National Natural Science Foundation of China (No. 51276141).
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