Extra Unsaturated Metal Centers of Zirconium-Based MOFs: a Facile Approach towards Increasing CO2 Uptake Capacity at Low Pressure

Article abstract of DOI:10.1002/ejic.201701296

Developing adsorption materials to capture CO₂ at low pressure has attracted great attention due to the low CO₂ partial pressure in fuel gases. Herein, zirconium-based metal–organic frameworks (PCN-X) are successfully synthesized and open metal sites (Fe³⁺ and Al³⁺) are incorporated into the PCN-X MOFs. The results indicate that the CO₂ adsorption capability is enhanced through the strong interaction between extra open metal centers and CO₂ molecules at low pressure. Meanwhile, the CO₂ adsorption capacity of PCN-X with incorporated Al³⁺ is higher than with incorporated Fe³⁺, which can be attributed to the higher isosteric heat of CO₂ adsorption. The adsorption kinetics further indicate the faster CO₂ transmission on PCN-X with extra Al³⁺ than with extra Fe³⁺. Besides, modified PCN-X, treated by some inorganic acids, exhibits favorable chemical stability, which is suitable for CO₂ capture from acidic fuel gases.

Full text of DOI:10.1002/ejic.201701296

A Journal of
Accepted Article
Title: Extra unsaturated metal centers of Zirconium-based MOFs: a
facile approach towards increasing CO2 uptake capacity at low
pressure
Authors: Zhong Li, Xue Li, Chong Chen, Lijin Zhou, Qirui Guo, Dashui
Yuan, Hui Wan, Jing Ding, and Guan guofeng
This manuscript has been accepted after peer review and appears as an
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using the Digital Object Identifier (DOI) given below. The VoR will be
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201701296
Link to VoR: http://dx.doi.org/10.1002/ejic.201701296

10.1002/ejic.201701296
European Journal of Inorganic Chemistry
FULL PAPER
Extra unsaturated metal centers of Zirconium-based MOFs: a
facile approach towards increasing CO2 uptake capacity at low
pressure
Zhong Li,[a] Xue Li,[a] Chong Chen,[a] Lijin Zhou,[b] Qirui Guo,[a] Dashui Yuan,[a] Hui Wan,[a] Jing Ding,*[a]
Guofeng Guan*[a]
Abstract: Developing adsorption materials to capture CO2 at low applications in gas storage,[5] separations,[6] drug delivery,[7]
pressure has attracted great attention due to the low CO2 partial catalysis[8] and so on.[9] MOFs are formed through the bridge
pressure in fuel gases. Herein, zirconium-based metal organic
between metal-based nodes and organic linkers which are a
one-, two-, or three-dimensional coordination network.[10]
frameworks (PCN-X) were successfully synthesized and open metal
sites (Fe3+ and Al3+) were incorporated into the PCN-X MOFs. The Compared to traditional porous adsorption materials,[11] the
results indicated that the CO2 adsorption capability was enhanced outstanding surface areas,[12] adjustable pore properties[13] and
through the strong interaction between extra open metal centers and multi-functionalities[14] of MOFs make them as a potential and
CO2 molecule at low pressure. Meanwhile, the CO2 adsorption suitable CO2 scavenger. Particularly, Zirconium-based MOFs, as
capacity on PCN-X with incorporated Al3+ was higher than with a member of MOFs family, exhibiting their advantages of
incorporated Fe3+, which could be attributed to the higher isosteric abundant structure types, excellent stability, adjustable and
heat of CO2 adsorption. The adsorption kinetics further indicated the functional properties, have been regarded as a kind of potential
faster CO2 transmission on PCN-X with extra Al3+ than with extra
MOFs.[15] Actually, Zr-MOFs have been under inchoate exploring,
Fe3+. Besides, modified PCN-X was treated by some inorganic acids due to the symmetry and connectivity of Zr6 cluster which makes
and exhibited favorable chemical stability, which was suitable for Zr ions coordinate with various organic ligands to form MOFs.[14]
CO2 capture from acidic fuel gases.
In recent years, Porphyrin derivatives as new organic linkers
with adjustable functionalities have been used in the synthesis of
Zr-MOFs. Xu et al.[16] utilized 5, 10, 15, 20-Tetra-(4-
carboxyphenyl) -porphyrin (TCPP) as ligand and successfully
obtained PCN-222. Compared to the porphyrin ligand itself, the
PCN-222 as a catalyst is the same with porphyrin ligand for
converting CO2 to formate anion under visible-light irradiation
and the CO2 conversion efficiency of PCN-222 significantly
improved. However, PCN-222 as a CO2 scavenger was only 18
cm3 (STP)/cm3 at 1 bar and 298 K.[17] It was a relatively low CO2
uptake capacity among many MOFs. To enhance the CO2
uptake capacity on PCN-222, Zhang and co-workers[17] used N-
rich tetrazolate to replace benzene in TCPP. At the same
temperature and pressure, the result showed that the CO2
adsorption amount on PCN-222 almost increased 120%
(cm3/cm3) by N-rich functionalization. However, in this way,
owing to almost Zr-MOFs including PCN-222 obtained via de
novo synthesis which was unfavorable to incorporate
functionalities into Zr-MOFs,[14] the process of N-rich
functionalization was very complex and difficult. Therefore,
developing a facile route to enhance the CO2 adsorption amount
on Zr-TCPP MOFs currently is the main research direction.
Since CO2 is mainly derived from fossil fuels and the partial
pressure of CO2 in fuel gases is 0.01 MPa~0.02 MPa, it is
valuable to capture CO2 from post-combustion under a lower-
pressure and room temperature.[18] Monoethanolamine (MEA) is
the main CO2 sorbent in industry which can capture more than
90% CO2 from fuel gases. However, 0.0032 tonnes of MEA may
be discharged into the atmosphere when every tonne of CO2 is
captured via MEA.[19] It means that a great deal of solvent loss
will occur, resulting in the environmental pollution.[20] Meanwhile,
based on physical adsorption property, activated carbon has
been applied to capture CO2. Although the adsorption behavior
of CO2 on activated carbon is reversible, the performance of
Introduction
CO2 emissions leading to a rise of CO2 concentration in
atmosphere and consequent plenty of environment problems is
extensively brought to attention of all sectors of society.[1]
Reducing carbon emission has become an imperative task to
meet the demand of sustainable development. Currently, carbon
capture, storage, and utilization scheme (CCSU) was admitted
as an effective approach to decarbonize the exhaust gases in
energy, steel and chemical industry.[2] Particularly, in the
frontline process, carbon capture involving in the gas selective
concentration plays
a dominant role in CCSU. Vigorous
exploration is incited to seek a favorable material for carbon
capture.[3]
MOFs (Metal Organic Frameworks)[4] as novel porous materials
are under research in recent years on account of their potential
[a]
Z. Li, X Li, C. Chen, Q. Guo, D. Yuan, Prof. H. Wan, Dr. J. Ding,
Prof. G. Guan
State Key Laboratory of Materials-Oriented Chemical Engineering
College of Chemical Engineering
Nanjing Tech University (former Nanjing University of Technology)
Nanjing, Jiangsu 210009, P. R. China
E-mail: jding@njtech.edu.cn (J. Ding)
guangf@njtech.edu.cn (G. Guan)
[b]
L. Zhou
Yangzi Petrochemical Company Ltd., Sinopec
Nanjing, Jiangsu 210048, P. R. China
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/ejic.201701296
European Journal of Inorganic Chemistry
FULL PAPER
CO2 uptake is not remarkable at low pressure (˂1.72 bar) in
comparison with that at higher pressure (˃1.72 bar).[21] As a
result, the designability of MOFs allow them to be a kind of
advanced materials for CO2 capture. Indeed, at low pressure,
large specific surface area (BET) isn’t a main factor for CO2
adsorption. For instance, MOF-177 has a large BET surface
area up to 4500 m2/g, but its CO2 adsorption capacity is only 3.4
wt.% at 1 bar and 298 K.[22] On the contrary, functionalization
such as grafting polar functional groups onto the inner or outer
surface could be a well way to enhance CO2 adsorption amount
at room conditions, although the incorporated groups may cause
a decrease in the specific surface area. It is attributed to the
strongly interaction between CO2 molecule and pore surface
through these functional groups.[23] For example, Yaghi and his
co-workers[24] reported that CO2 adsorption capacity of -NH3
modified IRMOF-3 was 9% (wt.%) higher than pristine IRMOFs.
Yaghi[25] further studied ZIF-70 modified with -NO2 groups (ZIF-
78) and -CN groups (ZIF-82). Meanwhile, the CO2 adsorption
capacities of ZIF-78 and ZIF-82 were obviously enhanced due to
the strong interaction between CO2 molecule and strong polar
groups (-NO2 and -CN). However, the incorporation of these
functional groups might accompany with complex chemical
reaction. Instead, generating unsaturated metal centers or open
metal sites is more facile in some cases as a method of
functionalization. M-MOF-74[22,26] is a kind of MOFs with open
metal sites (M represents different metals). Compared to Ni-
MOF-74, Co-MOF-74 and Zn-MOF-74, Mg-MOF-74 has highest
CO2 uptake capacity (35.2 wt.%) at 1 atm and 298 K since Mg2+
ions have interaction with lone pair electrons in CO2 which
increases the CO2 adsorption heat. HKUST-1 shows excellent
CO2 adsorption capacity at 0.1 MPa and 298 K, it also owes to
Cu2+ which has unsaturated metal centers.[27]
Herein, a facile method was presented to successfully fabricate
porphyrinic Zr-MOF named PCN-X. Considering that the Zr4+
ions tended to be coordinated to the oxygen donor of the
carboxylate groups in TCPP which was attribute to the oxo-philic
nature of the early transition elements, Fe3+ ions were
incorporated into these Zr-MOFs by the virtue of the strong
interaction between Fe3+ ions and nitrogen atoms of porphyrin
ring center to enhance CO2 adsorption ability on PCN-X.
Obviously, with the increasing addition of Fe3+ ions into PCN-X,
CO2 uptake capacity on PCN-X was increased gradually. When
Fe3+ ions were replaced by Al3+ ions, CO2 adsorption capacity
was further increased which was attributed to the higher
isosteric heat of CO2 adsorption caused by the higher ratio of
valence to diameter of Al3+ than that of Fe3+. The adsorption
kinetics was utilized to confirm the faster CO2 transmission on
PCN-X with Al3+ ions than that with Fe3+ ions. In addition, these
Zr-MOFs exhibited favorable stability under acidic environment,
allowing them as good candidates for CO2 capture from fuel
gases.
Results and Discussion
Physical properties
The scanning electron microscopy (SEM) images of PCN-X,
PCN-X-100%Fe and PCN-X-100%Al prepared in this work were
showed in Fig 1. PCN-X appeared two types of morphology
which were ellipsoidal crystals and cubic crystals.[28] The size of
ellipsoidal crystals ranged from 1 to 6 um and the size of cubic
crystals was from 1 to 2 um. When Fe3+ was incorporated into
PCN-X, ellipsoidal crystals were remained without any cubic
crystals and the size range for ellipsoidal crystals of PCN-X-
100%Fe was the same with PCN-X (Fig 1b). This indicated that
incorporated Fe3+ in PCN-X guided the crystal morphology
growth directionally which finally generated onefold ellipsoidal
crystal. When the incorporated metal ion was changed from Fe3+
to Al3+, the crystal morphology was converted. Different from
Fe3+, the incorporated Al3+ made structure of PCN-X change
from ellipsoidal crystals and cubic crystals to onefold lamellar
crystal. This phenomenon revealed that incorporated metal ions
played a guiding role of the growth of crystals.
N2 adsorption-desorption isotherms were measured to obtain
data of the Brunauer-Emmett-Teller (BET) surface areas, pore
diameter and pore volume (Table 1). The N2 isotherm of PCN-X
was a reversible typical type-I with a rapid growth of N2
adsorption amount at low pressure zone, which resulted from
micropore filling process (Fig S1). The BET surface area of
PCN-X was 855 m2 g-1, pore volume was 0.72 cm3 g-1 and
average pore diameter was 3.36 nm. It was interesting to note
that Fe-TCPP and Al-TCPP was not simple metal complex
compound. They formed certain pore structures, and the BET
surface areas of Fe-TCPP and Al-TCPP respectively were 421
m2 g-1 and 874 m2 g-1. After the coordination between Zr4+ and
Fe-TCPP (Al-TCPP) to form PCN-X MOFs, the BET surface
areas did not have a great change between PCN-X-100%Fe and
Fe-TCPP or PCN-X-100%Al and Al-TCPP (Table 1). It revealed
that the process of coordination was carried out on the surface
of Fe-TCPP, when Zr4+ was coordinated with Fe-TCPP, leading
to the specific surface area of PCN-X-100%Fe was smaller than
PCN-X, larger than Fe-TCPP. The pore diameter of PCN-X-
100%Fe laid between the PCN-X and Fe-TCPP. With the
Figure 1. Scanning electron microscopy (SEM) images of (a) PCN-X; (b) PCN-X-100%Fe; (c) PCN-X-100%Al
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10.1002/ejic.201701296
European Journal of Inorganic Chemistry
FULL PAPER
Table 1. Surface area, average pore diameter and pore volume for as-
synthesised PCN-X MOFs
[b]
[c]
Sample
Surface area[a]
(m2 g-1)
da
Vtotal
(nm)
(cm3 g-1)
Fe-TCPP
421
874
855
457
450
441
774
5.26
8.50
3.36
4.23
3.18
2.51
7.05
0.55
1.86
0.72
0.48
0.36
0.28
1.36
Al-TCPP
PCN-X
PCN-X-25%Fe
PCN-X-50%Fe
PCN-X-100%Fe
PCN-X-100%Al
Scheme 1. Schematic illustration of the preparation of PCN-X MOFs
[a] Area evaluated by following the BET method. [b] Average pore diameter.
[c] Total pore volume estimated from the amount of N2 adsorbed at p/p0= 0.99.
increase of Fe3+, the specific surface area, the pore volume and
pore diameter all had a decrease which attributed to the porosity
clogging. Meanwhile, the similar phenomenon happened on
PCN-X-100%Al sample. Since PCN-X was grown on the
surfaces of Fe-TCPP and Al-TCPP, the obtained BET results of
PCN-X-100%Fe (PCN-X-100%Al) should lay between PCN-X
and Fe-TCPP (Al-TCPP), and should get access to Fe-TCPP
(Al-TCPP). Due to the specific surface area of Fe-TCPP was
close to one-half of PCN-X, and the specific surface area of Al-
TCPP was similar to PCN-X, the phenomenon that the specific
surface area for the Al-material was similar to the pristine while
for the Fe-material it got twice smaller happened. Meanwhile,
the reason for the specific surface area was also suitable for the
average pore size which depended on Fe-TCPP (Al-TCPP). The
decrease of the specific surface area and average pore size of
PCN-X-100%Fe (PCN-X-100%Al) could be attributed to the
porosity clogging caused by incorporated metal ions. All above
were attributed to the structure-directing effect reciprocally
proved with SEM images which PCN-X with Fe-TCPP as ligand
only formed ellipsoidal crystal and with Al-TCPP as ligand
formed lamellar crystal. However, large BET surface area was
not the determinant of CO2 adsorption on MOFs at low pressure.
Instead, extra open metal sites engendered strong interaction
with CO2 to enhance the CO2 adsorption capacity which was in
accordance with the CO2 adsorption capacity on PCN-X-100%Al
112.2% higher than PCN-X and the adsorption capacity on
PCN-X-100%Fe 22.1% higher than PCN-X.
element with smaller diameter. Al-TCPP was same with PCN-X-
100%Al which matched our thought about the guiding role of
incorporated Al3+ ions. It revealed that incorporated Fe3+ and Al3+
played a guiding role for PCN-X MOFs during the crystal growth
process.
MOF-525
PCN-223
*




(d)
(c)
(b)


*
*
(a)


4
8
12
16
20
2-theta / º
(g)
(f)
(d)
(e)
4
8
12
16
20
2-theta / º
The Powder XRD patterns were shown in Fig 2 and simulated
Powder XRD patterns were shown in Fig S5. The XRD of PCN-
X revealed that the typical peaks of PCN-223 and MOF-525
were appeared simultaneously,[28] which was coincident with
SEM image. With the addition of Fe-TCPP, the peaks
transformed to the characteristic peaks of PCN-223, it revealed
a transformation from mixed phases to single phase with the
increase of incorporated Fe3+. Meanwhile, peak position of PCN-
X-100%Al was shifted to higher 2θ compared with PCN-X, it was
attributed to the decrease of lattice constant caused by Al
Figure 2. Powder XRD patterns of (a) PCN-X, (b) PCN-X-25%Fe, (c) PCN-X-
50%Fe, (d) PCN-X-100%Fe, (e) Fe-TCPP, (f) Al-TCPP, (g) PCN-X-100%Al
The thermogravimetry (TG) curves were measured to test
thermal stability of PCN-X, PCN-X-100%Fe and PCN-X-100%Al.
The TGA was carried out under a N2 atmosphere and from
30 °C to 800 °C at a rate of 10 °C min-1 (Fig 3). 10% weight loss
of three samples from 30 °C to 100 °C was considered as guest
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10.1002/ejic.201701296
European Journal of Inorganic Chemistry
FULL PAPER
solvent within the pores and the sublimation of MOFs. The initial
decomposition temperature was around 400 °C for PCN-X and
PCN-X-100%Fe and around 420 °C for PCN-X-100%Al. The
slight weight loss between 100 °C and decomposition
temperature was attributed to the continuous sublimation of
MOFs. Even up to 800 °C, the weight loss of PCN-X was less
than 40% and the weight loss of PCN-X-100%Fe and PCN-X-
100%Al was less than 50%. It revealed that all of the three
samples owned well thermal stability. To measure acid
resistance, three samples were treated in different acids without
attenuation for one day. The results were shown in Fig S6, PCN-
X-100%Fe and PCN-X-100%Al remained the stability compared
to PCN-X, which did not dissolve in hydrochloric acid and slightly
dissolve in nitric acid. In addition, PCN-X slightly dissolved in
nitric acid while PCN-X with Fe3+ and Al3+ did not dissolved in
nitric acid. However, they all dissolved in pure concentrated
sulfuric acid. When pure sulfuric acid was replaced by 0.1 M
sulfuric acid, they all slightly dissolved, and this property
remained in 1 M sulfuric acid. The results revealed that the
products owned well chemical stability which could be taken
advantage for CO2 capture from fuel gases.
coupling and mass of metal ions which caused slight change on
electronic structure of ligand TCPP.
(d)
(c)
1005
(b)
(a)
1025
969
2000
1500
1000
500
-1
Wavenumber (cm
)
(g)
(f)
(d)
(e)
969
781
2000
1500
1000
500
-1
Wavenumber (cm
)
100
90
Figure 4. FT-IR spectra of (a) PCN-X, (b) PCN-X-25%Fe, (c) PCN-X-50%Fe,
(d) PCN-X-100%Fe, (e) Fe-TCPP, (f) Al-TCPP, (g) PCN-X-100%Al.
80
Adsorption equilibrium
70
Adsorption equilibrium isotherms of CO2 were obtained at 298 K
and up to 1 bar to investigate the CO2 adsorption ability on PCN-
X MOFs. As shown in Fig 5, at 298 K, when the pressure was up
to 1 bar, the adsorption of CO2 on PCN-X was 18.42 cm3 g-1
which was according to the previous work.[17] To enhance the
CO2 adsorption capacity, Fe3+ was incorporated into PCN-X to
generate open metal sites. With the increasing amount of Fe3+,
CO2 adsorption capacity on PCN-X increased continuously due
to the increasing amount of open metal sites. When the mass
ratio of Zr4+/Fe3+ was up to 1:2 (Table S1) which TCPP was
completely replaced by Fe-TCPP as a ligand, the growth of CO2
capacity reached maximum (up to 22.50 cm3 g-1), which was up
22.1%. It was attributed to the interaction between CO2 and
central Fe3+ with Lewis acid property to form endpoint adduct
O=C=O...Fe3+ (Fig 9).[30]
As a comparison, Fe3+ was changed to Al3+, when Al-TCPP
acted as a ligand without any TCPP, the mass ratio of Zr4+/Al3+
was up to 1:2.1 (Table S1). The adsorption capacity of CO2 had
a significant promotion. At 298 K and 1 bar, compared to PCN-X,
the adsorption capacity of CO2 on PCN-X-100%Al was up to
39.10 cm3 g-1, up 112.2%. In order to prove that incorporated
Al3+ was better than Fe3+ in CO2 capture, isosteric heat of CO2
adsorption on PCN-X-100%Fe and PCN-X-100%Al calculated in
the further study were showed in the next section.
PCN-X
60
PCN-X-100% Fe
PCN-X-100% Al
50
0
200
400
600
800
Temperature (oC)
Figure 3. TGA plot of as-synthesised PCN-X, PCN-X-100%Fe and PCN-X-
100%Al at N2 atmosphere
The FT-IR spectra of ligand TCPP was shown in Fig S7. The
bond around 1655 cm-1 was attributed to the stretching vibration
of C=N in pyrrole. The bond around 1702 cm-1 represented the
vibrational peak of C=O. The weak adsorption peaks at 3240
cm-1 and 969 cm-1 were respectively attributed to the stretching
vibration and formation vibration of N-H in pyrrole. The medium
strong band at 800 cm-1 was caused by the vibration of the
pyrrole ring. The adsorption peaks around 1605, 1564, 1480,
1450 cm-1 were attributed to the skeletal vibration of benzene
ring outside pyrrole ring. The broad and strong absorption band
around 3200~3650 cm-1 was the adsorption peak of -OH in
carboxyl. The characteristic peak at 872 cm-1 represented that
carboxyl located in the para-position of benzene ring. All above
could confirm TCPP was prepared successfully.[29] The FT-IR
spectrums of PCN-X MOFs were shown in Fig 4. When Fe3+
ions coordinated with N-H to form Fe-TCPP, the bending
vibration of N-H at 969 cm-1 was weakened with the increasing
addition of Fe3+, the peak disappeared until TCPP was replaced
by Fe-TCPP absolutely. However, the peak at 969 cm-1 of Al-
TCPP was weakened without disappearance. It was owing to
Al3+ did not coordinate with TCPP completely. The peak at 1025
cm-1 migrated to 1005 cm-1 and the peak at 800 cm-1 migrated to
781 cm-1. It was attributed to the inductive effects, vibronic
Isosteric heat of adsorption
The adsorption of CO2 on MOFs material is physical adsorption
which depends on Van Der Waals’ force between guest
molecule CO2 and MOFs material. Therefore, the adsorption
force is relatively weak, and it is remarkable that the adsorption
capacity of CO2 on MOFs is related to the heat of CO2
adsorption at low pressure.
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10.1002/ejic.201701296
European Journal of Inorganic Chemistry
FULL PAPER
The isosteric heat of adsorption is an important parameter to
characterize surface heterogeneity, which can provide
information about adsorbent and adsorption capacity. It
represents the intensity of the interactions between adsorbent
lattice atoms and absorbate molecules which is also very helpful
to test the performance of adsorbents in an adsorption process.
To measure the isosteric heat, isothermal adsorption lines of the
same sample for the same gas at different temperatures are
necessary. According to these isothermal adsorption lines, the
isosteric heat can be calculated via the Clausius-Clapeyron
equation[31] as
the isosteric heat of CO2 adsorption might not have such a big
difference. It confirmed to rule that cations were preferential
adsorption sites for CO2 adsorption and the adsorption capacity
increased with the increasing values of valence of cations
versus diameter of cations at low pressure.[32] It was proved that
the heat of CO2 adsorption capacity increased due to the higher
values of valence of cations versus diameter of cations.
Because of the higher adsorption heat given rise through higher
ratio of valence to diameter of Al3+ than that of Fe3+, CO2
adsorption capacity on PCN-X-100%Al was higher than PCN-X-
100%Fe.
InP
 T


Qst = RT2
(1)



n
Adsorption kinetics
a
Where Qst is the isosteric heat of adsorption (kJ mol-1), T is
analytical temperature (K), na is CO2 adsorption amount, R is
gas constant (0.00831 kJ mol-1 K-1), P is absolute equilibrium
pressure (kPa). Integration of the equation 1 obtains
Qst
Adsorption kinetics curves of CO2 on PCN-X-100%Fe and PCN-
X-100%Al were surveyed from Fig 8. It was obvious that CO2
uptake capacity on PCN-X-100%Al was reached equilibration in
a short time similar with PCN-X-100%Fe. However, the CO2
uptake capacity on PCN-X-100%Al was higher than PCN-X-
100%Fe which suggested a higher CO2 adsorption rate on PCN-
X-100%Al. To confirm this conclusion, the kinetic rate constants
of CO2 adsorption on PCN-X-100%Fe and PCN-X-100%Al were
calculated based on experimental data via the linear driving
force (LDF) model[33] as
InP = -
+ C
(2)
RT
Due to C is a constant, InP is linear with 1/T and the isosteric
heat can be determined by the slope.
In this work, two values of temperature (313 K and 328 K) were
chosen to get adsorption isotherms and utilized for calculation of
isosteric heat (Fig 6). At an appointed adsorption amount, values
of InP versus 1/T were obtained from two adsorption isotherms.
Qst was then calculated via equation 2. At the same adsorption
amount, Qst of PCN-X-100%Al was higher than that of PCN-X-
100%Fe (Fig 7). For example, when CO2 adsorption amount
was 10 cm3 g-1, Qst of PCN-X-100%Al was 28.60 kJ mol-1 while
Qst of PCN-X-100%Fe was 21.81 kJ mol-1. On the contrary, if
only physical adsorption based on BET surface areas remained,
q
t
 k(q * q)
(3)
Conversion of the equation 3 obtains
q
In(1
)   kt
(4)
q*
Where q is CO2 adsorption amount (mmol g-1), q* is CO2
equilibrium adsorption amount at 25 °C and 1 bar (mmol g-1), k is
the kinetic rate constant (s-1). In(1-q/q*) is linear with t and the
25
42
(a)
(b)
35
20
28
21
14
15
10
PCN-X
PCN-X-25% Fe
PCN-X-50% Fe
PCN-X-100% Fe
5
PCN-X
PCN-X-100% Fe
PCN-X-100% Al
7
0
0
0.0
0.2
0.4
0.6 0.8 1.0
0.0
0.2
0.4
0.6
0.8
1.0
P / P
P / P
0
0
Figure 5. CO2 adsorption curve: (a) PCN-X with different amount of Fe3+ at 298 K; (b) PCN-X with different metal ions at 298 K
25
20
15
10
5
42
35
28
21
14
7
(a)
(b)
298 K
313 K
328 K
298 K
313 K
328 K
0
0
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
P / P
P / P
0
0
Figure 6. CO2 adsorption isotherms: (a) PCN-X-100%Fe; (b) PCN-X-100%Al
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10.1002/ejic.201701296
European Journal of Inorganic Chemistry
FULL PAPER
40
PCN-X-100% Fe
PCN-X-100% Al
Where q is CO2 adsorption amount (mmol g-1), q* is CO2
equilibrium adsorption amount at 25 °C and 1 bar (mmol g-1), k is
the kinetic rate constant (s-1), z is the constant about adsorption
mechanism. The Avrami model was used for kinetics of CO2
adsorption on mesoporous silica with amine functional groups.[34]
In this work, the Avrami model was suitable for PCN-X-100%Fe
and PCN-X-100%Al, and the fitting coefficients R2 were 0.98498
and 0.98473 (Fig S10 and S11). The results were shown in table
2, which k of PCN-X-100%Al was a bit larger than that of PCN-
X-100%Fe. It revealed that the larger pore diameter of PCN-X-
100%Al was in favor of enhancing the mass transfer of CO2.[31]
30
20
10
0
5
10
15
20
25
30
3
-1
CO adsorbed (cm
g
)
2
Figure 7. Isosteric heat of CO2 adsorption on PCN-X-100%Fe and PCN-X-
100%Al
Table 2. Kinetic Rate Constants k of CO2 on PCN-X-100%Fe and PCN-X-
100%Al
Sample
k (s-1)[a]
z[b]
2.0
1.5
1.0
PCN-X-100%Fe
PCN-X-100%Al
9.22×10-4
9.38×10-4
1.54703
1.48348
[a] Kinetic rate constants k evaluated by following the Avrami method. [b] z is
the constant about adsorption mechanism.
0.5
PCN-X-100% Fe
PCN-X-100% Al
0.0
0
1000
2000
3000
4000
5000
Conclusions
Time (s)
Figure 8. Adsorption kinetic curves of CO2 on PCN-X-100%Fe and PCN-X-
100%Al
In summary, several PCN-X MOFs were prepared by a facile
one-pot method. With the addition of Fe-TCPP and Al-TCPP,
PCN-X was generated via directed growth with open metal sites
and decreased BET surface area in varying degrees. The CO2
uptake capacity increased continuously due to the increase of
open metal sites at 25 °C and 1 bar. When ligand was full Fe-
TCPP and Al-TCPP, the mass ratios of Zr4+/Fe3+ and Zr4+/Al3+
were respective 1:2 and 1:2.1, and the CO2 adsorption
capacities of PCN-X-100%Fe and PCN-X-100%Al were
respective up 22.1% and 112.2 in comparison to original PCN-X.
Moreover, the incorporated Al3+ was with better performance of
CO2 uptake capacity due to the higher adsorption heat brought
about through higher ratio of valence to diameter of Al3+ than
that of Fe3+, and better CO2 transmission speed according to the
kinetic rate constant can be determined by the slope.
However, the calculation error made LDF model not suitable for
this CO2 adsorption process owing to the physical adsorption
along with chemical adsorption. It could prove that the open
metal sites played a role in chemical adsorption. The interaction
between open metal sites and CO2 was a chemical process
which made significant deviating from the experimental data
after the beginning of the adsorption process according to LDF
model. The In(1-q/q*) was obvious nonlinear with t (Fig S12 and
S13). Herein, the Avrami model based on particle nucleation
was utilized
q = q * - q * / e (k t)
z
(5 )
Figure 9. Possible mechanism of CO2 adsorption on PCN-X with open metal sites (Fe3+ and Al3+
)
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10.1002/ejic.201701296
European Journal of Inorganic Chemistry
FULL PAPER
lined autoclave. The two mixtures were heated in 150 °C oven for 16 h.
After cooling down to room temperature, the products were washed with
DMF for 3 times and then dissolved in acetone. The solutions were
standing for 24 h. After centrifugation, the solids were dried in vacuum
temperature, the product was washed by DMF for 3 times. To activate
the sample, the solid was suspended in a solution of 4 M HCl (2.5 mL) in
100 mL DMF and stirred at 120 °C for 12 h. Subsequently, the products
were dissolved in acetone and standing for 24 h. After centrifugation, the
solid was washed by methanol for several times and dried in vacuum.
The obtained product was denoted as PCN-X.
higher kinetic rate constant. However, the BET surface areas of
force between metal ions and CO2 molecules. Taking into
account the acid resistance of PCN-X with incorporated metal
ions, these materials could be promising candidates for CO2
capture from fuel gases which accompanied with the existence
of acid gases (SO2, SO3). Moreover, integrating the facile
functionalization and stability, we lay our account with such
strategy which facilitated the fabrication of PCN-X MOFs with
open metal sites to provide a convenient method to design
multifunctional materials for a variety of applications.
To incorporate Fe3+ into PCN-X, partial TCPP was replaced by Fe-TCPP.
0.3 g ZrCl4, TCPP and Fe-TCPP mixtures (sample a: 0.3 g Fe-TCPP, b.
0.15 g Fe-TCPP and 0.15 g TCPP, c. 0.075 g Fe-TCPP and 0.225 g
TCPP) and 5.4 g benzoic acid were dissolved in 40 mL DMF. The rest of
steps were followed by the synthesis of PCN-X. Then, several solids
(sample a: PCN-X-100%Fe, b. PCN-X-50%Fe, c. PCN-X-25%Fe) were
obtained.
Experimental Section
Materials
For the synthesis of PCN-X MOFs, zirconium chloride (ZrCl4), Benzoic
acid 4-Formylbenzoic acid and N,N-Dimethylformamide (DMF) were
purchased from Aladdin Industrial Inc, Propanoic acid, Hydrochloric acid,
Acetone, Sulfuric acid were obtained from Shanghai Lingfeng Chemical
Co., Ltd, Methanol was received from Wuxi City Yasheng Chemical Co.,
Ltd, Pyrrole was acquired from Macklin Biochemical Technology Co., Ltd.
Aluminum chloride hexahydrate was gained from Sinopharm Chemical
Reagent Co., Ltd, Ferrous chloride was got from Richjoint Chemical Co.,
Ltd. Hydrofluoric acid was received from Shanghai Shenbo Chemical Co.,
Ltd. Nitric acid was purchased from Yangzhou Hubao Chemical Reagent
Co., Ltd. All of the chemicals were used without further purification. 4 M
hydrochloric acid was self-prepared from pure hydrochloric acid, 0.1 M
and 1 M sulfuric acids were self-prepared from pure sulfuric acid.
To incorporate Al3+ into PCN-X, TCPP was replaced by Al-TCPP. 0.3 g
ZrCl4, 0.3 g Al-TCPP and 5.4 g benzoic acid were dissolved in 40 mL
DMF. The rest of steps were followed by the synthesis of PCN-X. Then
PCN-X-100%Al was obtained.
Characterization
A SmartLab diffractometer (Rigaku, Japan) with Cu Kα (λ=1.5406 Å)
radiation was used to recorded powder x-ray diffraction (XRD) patterns.
A step increment of 0.02° in 2θ was used to collect the data from 2° to
20°. The Brunauer-Emmett-Teller (BET) surface areas, pore volumes,
and pore size distributions of the samples received from N2 adsorption-
desorption isotherms which were measured on a Micromeritics ASAP
2000 instrument at 77 K, and the samples were pretreated at 150 °C for
8 h under vacuum. A Hitachi S-4800 field-emission scanning electron
microscope was used to observe scanning electron microscopy (SEM)
images to ascertain the crystal morphology of the samples. A Thermo
Nicolet 6700 spectrometer (United States) was used with samples fully
dried and mixed with anhydrous potassium bromide to get the fourier
transform infrared (FT-IR) spectra in the wavenumber range of 4000-500
cm-1. A STA 409PC thermogravimetric analyzer was used to test the
thermal stability, the thermogravimetry (TG) curves were from 30 to
800 °C and under a continuous N2 flow with a heating increment of 10 °C
min-1. An inductively coupled plasma-mass spectrometer (ICP-MS, ICP
OPTMA20000V) was used to ascertain the mass ratios of Zr4+/Fe3+ and
Preparation of TCPP
To prepare TCPP, 18 g 4-formylbenzoic acid were added into 300 mL
propionic acid in a 500 mL three necked flask, 8.4 mL pyrrole were then
slowly added into the mixed solution, the solution was stirred and
refluxed for 12 h and cooled to room temperature naturally. Black solids
were obtained after centrifugation. the products were washed with
acetone and methanol in alternation until black solids turned to dark
purple solids and then dried in vacuum.
COOH
Zr4+/Al3+
.
NH
N
N
propionic acid
12 h reflux
CHO +
HOOC
4
4
HOOC
COOH
N
HN
H
CO2 adsorption experiments and kinetics measurements
The CO2 adsorption equilibrium data were obtained from a BElSORP-
max instrument (MicrotracBEL, Japan) through
a static volumetric
COOH
Scheme 2. Synthesis of porphyrin ligand TCPP
method under low pressure (0-1.0 bar). Temperatures were set as three
fixed values (298 K, 313 K and 328 K). About 0.1 g of adsorbent samples
were used for tests with a pretreatment, samples were placed in vacuum
at 150 °C for 8 h. CO2 with purity 99.999% from Nanjing Special Gases
Company was used as received.
COOH
COOH
N
N
N
M
N
NH
N
DMF
HOOC
COOH
MCl ·nH O
HOOC
COOH
x
2
+
The CO2 adsorption kinetic data were received from an intelligent
gravimetric analyzer (IGA-100, Hiden) at 298 K and 1 bar. About 0.02 g
of adsorbent samples which was pretreated at 150 °C for 4 h was used
for pure CO2 adsorption. The weight increase of samples overtime was
recorded to calculate the kinetic rate constant until reaching adsorption
equilibrium.
16 h 150 ℃
N HN
M = Fe , Al
COOH
COOH
Scheme 3. Synthesis of Fe-TCPP and Al-TCPP
Preparation of Fe-TCPP/Al-TCPP
Acknowledgements
To prepare Fe-TCPP/Al-TCPP, 1 g TCPP and 1 g FeCl2·4H2O were
added into 60 mL DMF in a 100 mL Teflon-lined autoclave, 1 g TCPP
and 1.2 g AlCl3·6H2O were added into 60 mL DMF in a 100 mL Teflon-
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10.1002/ejic.201701296
European Journal of Inorganic Chemistry
FULL PAPER
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