Supramolecular binding of amines with functional magnesium tetraphenylporphyrin for CO2 capture

Article abstract of DOI:10.1016/j.cclet.2013.01.043

In this work, magnesium tetraphenylporphyrin (MgTPP) was used as a new supramolecular amine-fixing agent. Once introduced, CO₂ easily competes with MgTPP for amines, leading to the release of MgTPP. The processes can be explained by the fact that the association constant (K_assoc) values of MgTPP with amines were in the range of 0.6 (ethanolamine) to 3.9 (ethylenediamine), which are lower than the K_assoc values of CO ₂ with these amines. MgTPP interacted with aniline, ethanolamine, pyrrolidine, or ethylenediamine to form 1:1 adducts. Ethylenediamine presents a stronger K_assoc value for MgTPP, so it was considered an optimal agent for CO₂ capture.

Full text of DOI:10.1016/j.cclet.2013.01.043

Chinese Chemical Letters 24 (2013) 249–252
Contents lists available at SciVerse ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Original article
Supramolecular binding of amines with functional magnesium
tetraphenylporphyrin for CO2 capture
a
a
b,
Fei Gao a, Jian-Bin Zhang a, , Chun-Ping Li , Tian-Rui Huo , Xiong-Hui Wei
*
*
a College of Chemical Engineering, Inner Mongolia University of Technology, Huhhot 010051, China
b Department of Applied Chemistry, College of Chemistry & Molecular Engineering, Peking University, Beijing 100871, China
A R T I C L E I N F O
A B S T R A C T
Article history:
In this work, magnesium tetraphenylporphyrin (MgTPP) was used as a new supramolecular amine-fixing
agent. Once introduced, CO2 easily competes with MgTPP for amines, leading to the release of MgTPP.
The processes can be explained by the fact that the association constant (Kassoc) values of MgTPP with
amines were in the range of 0.6 (ethanolamine) to 3.9 (ethylenediamine), which are lower than the Kassoc
values of CO2 with these amines. MgTPP interacted with aniline, ethanolamine, pyrrolidine, or
ethylenediamine to form 1:1 adducts. Ethylenediamine presents a stronger Kassoc value for MgTPP, so it
was considered an optimal agent for CO2 capture.
Received 8 November 2012
Received in revised form 2 January 2013
Accepted 15 January 2013
Keywords:
Supramolecular binding
Fixation
ß 2013 Jian-Bin Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights
reserved.
Amine
Magnesium tetraphenylporphyrin
CO2 capture
1. Introduction
reduce amine losses, but these changes have decreased the CO2
loading capacity and solvent regeneration [13–15].
The concentrations of CO2 in the atmosphere have increased
because of human activities, such as the burning of fuels (oil, coal,
petroleum and gas), deforestation, and hydrogen production from
hydrocarbons (steam conversion and partial oxidation) [1–3].
Research has focused on ways to slow or stop this trend [4–6].
Selective removal of CO2 from various gases is desirable for
operational, economical, and environmental reasons. Chemical
absorption of CO2 using amine-based solvents, such as mono-
ethanolamine, diethanolamine, and methyldiethanolamine [7–
10], is a commercially mature technology and the preferred option
for CO2 removal [11,12]. In amine scrubbing, CO2 reacts with an
amine solvent via an exothermic and reversible reaction in a gas/
liquid reactor. However, the subsequent mechanical breakdown,
entrainment, foaming, and chemical degradation processes could
lead to amine losses in the gas treatment plant. In the final step,
CO2 is removed from the solvent in a regenerator at low pressure
and/or high temperature resulting in considerable vaporization
and amine losses, which reduce plant performance and increase
operating costs. Recently, attention has been focused on alterna-
tive amine solvents and controlling the lean amine temperature to
In this work, we present data for the use of magnesium
tetraphenylporphyrin (MgTPP) as an amine-fixing agent for CO2
capture. The amines initially form adducts with MgTPP. When CO2
is introduced, it competes with MgTPP for the amines, which
eventually leads to the release of MgTPP (Figs. 1 and 2). The amines
can form adducts with the regenerated MgTPP when separated
from CO2, which may reduce amine losses. The results showed that
several amines formed adducts with MgTPP and MgTPP were
replaced by CO2 during CO2 capture (see supporting information
(Fig. S1)). In the present work, only the liposoluble porphyrin
complex was used. We have attempted to develop new aqueous
porphyrin complexes as amine-fixing agents for CO2 capture in our
recent work.
2. Experimental
Fluorescence spectra were acquired using an F-4500 fluores-
cence spectrophotometer. UV–vis spectra were recorded on a
Varian CARY 1E UV–vis spectrometer. All solid reagents were
weighed using a Sartorius BS224S electric balance.
MgTPP was prepared using meso-tetraphenylporphyrin (H2TPP,
>98%) purchased from Acros Organics (New Jersey, USA). All other
reagents and solvents were reagent grade and used as received.
* Corresponding authors.
A solution of 1.5 mL MgTPP in CH2Cl2 (65
mmol/L) and a series
E-mail addresses: tadzhang@pku.edu.cn (J.-B. Zhang),
xhwei@pku.edu.cn (X.-H. Wei).
of amine solutions were kept at four temperatures for 20 min by a
1001-8417/$ – see front matter ß 2013 Jian-Bin Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2013.01.043

250
F. Gao et al. / Chinese Chemical Letters 24 (2013) 249–252
Fig. 1. An amine-fixing agent for CO2 capture. MgTPP is initially coordinated to amines and then displaced by CO2.
constant temperature water SPY-III bath apparatus (accuracy:
ꢀ0.01 K).
The reaction mechanism of MgTPP with several amines (see
supporting information (Figs. S2–S4)), including aniline, ethanol-
amine, pyrrolidine, ethylenediamine, diethylamine, diphenylamine,
and triethylamine, was investigated by a fluorescence titration
experiment at four temperatures. The spectral changes were
3. Results and discussion
monitored by fluorescence spectroscopy (
When aniline (0.000–0.691 mol/L) was added to a CH2Cl2
solution of MgTPP (19.51 mol/L), the emission peak of MgTPP
was quenched with the sequential additions of aniline at
temperatures of 298.15, 303.15, 308.15, and 313.15 K, respectively.
This could be attributed to the electron transfer in the complexes
between MgTPP and the electron donating amine molecules,
because the amine nitrogen with lone pair electrons is coordinated
to the metal center of MgTPP [19].
The binding constants of MgTPP with various amines were
calculated from the fluorescence spectral data. The binding
constant (K) and the binding number (n) are calculated by the
following equation,
lex = 550 nm) (Fig. 3).
Before the CO2 capturing processes, we first determined the
effects of CH2Cl2 on the binding of amines with MgTPP and CO2.
The results showed that CH2Cl2 can be considered an optimal
solvent and does not affect the binding of MgTPP with CO2 or the
amines.
Several amines were tested in this work and the process can be
monitored by the color changes of the reaction mixtures. For
example, when aniline was added to the CH2Cl2 solution of MgTPP,
thecolor of theMgTPPsolution changed frompurpletoblue (Fig. 2A).
This could be attributed to the interaction of MgTPP with aniline.
When CO2 gas was bubbled through the solution, the color changed
fromblue tovioletbecause CO2 competes withMgTPP foranilineand
MgTPP is released. The ultraviolet–visible (UV–vis) and fluorescence
spectra were collected to confirm these interpretations.
m
M þ nQ ! MQn
(1)
In a typical experiment, aniline was added to the MgTPP
solution, and the absorbance changes were recorded on an UV–vis
where Q denotes the quencher (the various amines), M denotes
MgTPP, and MQn denotes their bound complex, and
spectrometer (Fig. 2B). The absorption spectra showed a typical
*
Soret band at 424 nm, which can be attributed to the a1u
(p
)–eg (
p
)
½MQnꢁ
transition of MgTPP [16]. Similar Soret absorption peaks (B-bands)
can be observed for most porphyrinic compounds [17,18]. The
color of the MgTPP solution changed from purple to blue, and a
bathochromic shift of 3 nm was observed for the Soret band. This
suggests that MgTPP interacted with aniline to form the
corresponding MgTPP–aniline adduct. When CO2 gas was bubbled
through the solution, the color changed from blue to violet, and the
Soret band returned to its position for free MgTPP (Fig. 2B). This
result confirmed that CO2 competed with MgTPP for aniline, which
eventually led to the dissociation of MgTPP–aniline complex and
the formation of the CO2–aniline adduct. The regenerated MgTPP
was able to form amine adducts again when CO2 was removed.
MgTPP acted as a recyclable amine-fixing agent in amine scrubbing
for CO2 capture, and reduced amine losses. Under the same
conditions, similar observations were made for ethanolamine,
pyrrolidine, and ethylenediamine.
K ¼
(2)
n
½Mꢁ½Qꢁ
where K is the intrinsic binding constant, and n is the binding
number of amines to MgTPP. [M] is the concentration of free
MgTPP, and [MQn] is the concentration of bound complex. If [M]0 is
the total concentration of MgTPP,
½Mꢁ0 ¼ ½Mꢁ þ ½MQnꢁ
(3)
and hence Eq. (2) can be replaced by
½Mꢁ0 ꢂ ½Mꢁ ½MQnꢁ
n
¼
¼ K½Qꢁ
(4)
½Mꢁ
½Mꢁ
In static quenching,
½Mꢁ0 F0
¼
(5)
½Mꢁ
F
Fig. 2. (A) Photograph of MgTPP (left), MgTPP + aniline (middle), and MgTPP + aniline + CO2 (right). (B) Changes in the absorbance spectra of MgTPP and MgTPP–aniline in
CH2Cl2 at room temperature ([MgTPP]: 1.95 ꢃ 10ꢂ5 mol/L, [aniline]: 0.69 mol/L).

F. Gao et al. / Chinese Chemical Letters 24 (2013) 249–252
251
350
300
250
200
150
100
50
calculated binding entropy using the standard relationship,
G ¼ H ꢂ T
608 nm
D
D
D
S
(7)
The fluorescence titration results suggested that MgTPP
interacted with aniline (Figs. 4 and 5), pyrrolidine, ethylenedia-
mine, or ethanolamine to form 1:1 adducts.
According to previous methods [20,21], we calculated the
association constants (Kassoc) and molar Gibbs energy of reaction
665 nm
(
D
G) (Table 1). Using a van’t Hoff plot, the molar enthalpy (
entropy ( S) of the reactions between MgTPP and amines were
calculated (Table 2).
DH) and
D
The thermodynamic data showed that interactions of MgTPP
with aniline or ethylenediamine were spontaneous and entropy-
driven coordination reactions. The interactions of MgTPP with
ethanolamine and pyrrolidine were enthalpy-driven coordination
reactions, and the latter interactions may spontaneously occur at
0
600
625
650
675
700
low temperature. The
DH values indicated that these coordination
Wavelength
reactions are endothermic processes except for the reaction with
pyrrolidine. However, the UV–vis and fluorescence spectra showed
that the interactions between MgTPP and diethylamine were weak,
and MgTPP hardly interacted with diphenylamine or triethylamine
(see supporting information (Figs. S5–S7)). Steric hindrance could
have contributed to these results [22].
Fig. 3. Fluorescence spectral changes of MgTPP–aniline with addition of aniline in
CH2Cl2 at 298.15 K.
where [M]0 denotes the total concentration of MgTPP, [M] denotes
the concentration of free MgTPP, and F0 and F are the fluorescence
intensities in the absence and presence of the amines, respectively.
The fluorescence measurements of MgTPP–amine binding were
performed at four temperatures. The resulting data were analyzed
using Origin software to estimate the binding affinity (K) and the
Rudkevich reported that the Kassoc values for Zn-tetraphenyl-
porphyrin (ZnTPP) with amines were between 7.0 ꢃ 102 (diethy-
lamine) and 7.0 ꢃ 104 (quinuclidine) [23]. However, the binding
constants between CO2 and amines are about 10 [24,25]. In the
present study, the Kassoc values of MgTPP with amines were
between 0.6 (ethanolamine) and 3.9 (ethylenediamine). The
binding constants between MgTPP and amines are smaller than
or comparable to those between CO2 and amines, and it is easier for
CO2 gas to compete with MgTPP for amines than with ZnTPP.
Thermodynamic data for the binding of MgTPP with amines
were calculated. The results suggested that central magnesium ion
of MgTPP interacted with aniline, pyrrolidine, ethanolamine, and
ethylenediamine to form 1:1 adducts. The aniline, pyrrolidine, and
ethanolamine only present an N atom, so one MgTPP molecule only
binds one amine molecule. On the other hand, the ethylenediamine
enthalpy of the binding process (DH). Thermodynamic parameters
were estimated by an analysis of the plots of ln K vs. 1/T (van’t Hoff
plot) obtained using the experimental data at the above tempera-
tures[20,21]. The gradient ofthestraightlineofa plot ofln Kvs. 1/T is
equal to ꢂ
D
H/R, which indicates the value of
DH and the
corresponding free energy (
D
G) were calculated from Eq. (6),
D
G ¼ ꢂRT ln K
(6)
The binding free energy coupled with the binding enthalpy
derived from the fluorescence data allowed the calculation of the
entropic contribution to the binding (TDS), where DS is the
2.4
2.2
B
A
2.2
2.0
1.8
1.6
1.4
1.2
1.0
2.0
1.8
1.6
1.4
1.2
1.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
[Q]/(mol/L)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
[Q]/(mol/L)
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
2.4
C
D
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
0.0
0.1
0.2
0.3
0.4
0.5
0.6
[Q]/(mol/L)
[Q]/(mol/L)
Fig. 4. Plots of F0/F vs. [Q] for MgTPP with aniline at 298.15 K (A, R2 = 0.981, Kassoc = 1.585 ꢀ 0.069), 303.15 K (B, R2 = 0.976, Kassoc = 1.600 ꢀ 0.075), 308.15 K (C, R2 = 0.969,
Kassoc = 1.613 ꢀ 0.055), and 313.15 K (D, R2 = 0.989).

252
F. Gao et al. / Chinese Chemical Letters 24 (2013) 249–252
Table 1
The association constants (Kassoc) and
D
G of binding of MgTPP with amines (a: ethanolamine; b: aniline; c: ethylenediamine; and d: pyrrolidine).
T (K)
Kassoc
DG (kJ/mol)
a
b
c
d
a
b
c
d
298.15
303.15
308.15
313.15
0.584 ꢀ 0.024
0.646 ꢀ 0.048
0.686 ꢀ 0.029
0.784 ꢀ 0.030
1.585 ꢀ 0.069
1.600 ꢀ 0.075
1.613 ꢀ 0.055
1.638 ꢀ 0.055
3.901 ꢀ 0.040
4.002 ꢀ 0.167
4.113 ꢀ 0.124
3.965 ꢀ 0.071
1.190 ꢀ 0.044
1.091 ꢀ 0.017
0.993 ꢀ 0.021
0.860 ꢀ 0.031
1.333
1.101
0.966
0.644
ꢂ1.141
ꢂ1.186
ꢂ1.225
ꢂ1.285
ꢂ3.374
ꢂ3.495
ꢂ3.622
ꢂ3.586
ꢂ421.075
ꢂ214.444
17.588
383.894
Autonomous Region’s Educational Commission (No. NJZZ11068),
the Inner Mongolia Talented People Development Fund, Program
for Young Talents of Science and Technology in Universities of
Inner Mongolia Autonomous Region (No. NJYT-12-B13), Program
for New Century Excellent Talents in University (NCET-12-1017),
and Yongfeng Boyuan Industry Co., Ltd. (Jiangxi Province, China).
1.64
1.63
1.62
1.61
1.60
1.59
1.58
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.cclet.2013.01.043.
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0.00320
0.00325
0.00330
0.00335
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MgTPP could be used as a renewable amine-fixing agent to
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This work was supported by the National Natural Science
Foundation of China (No. 21166017), the Research Fund for the
Doctoral Program of Higher Education of China (No.
20111514120002), the Natural Science Foundation of Inner
Mongolia Autonomous Region (No. 2011BS0601), Inner Mongolia