(653f) A One-Step Ion Exchange Method to Functionalized Zif-67 for Enhanced CO2 Capture | AIChE

(653f) A One-Step Ion Exchange Method to Functionalized Zif-67 for Enhanced CO2 Capture

A one-step ion exchange method to functionalized ZiF-67 for enhanced CO2 capture

Fujiao
Song, Qin Zhong*

School
of Chemical Engineering, Nanjing University of Science and Technology, Nanjing
210094, PR China. E-mail: Song_fj2006@126.com (Fujiao Song), zq304@mail.njust.edu.cn (Qin Zhong).

ZiFs have shown great potential applications
in gas separation [1]. Ion exchange could improve the surface
area, pore volume as well as surface charge of the adsorbents, further
enhancing the CO2 adsorption capacities. However, it is notable that the standard ion-exchange
process needs to be repeated for several times, which brings a series of
disadvantages, such as complicated in synthesis, time consuming and solvent
consuming [2, 3].

In this work, we develop a new one-step ion exchange method
(donated as ion as-exchange technique) to overcome the disadvantages mentioned
above. Since the cations for exchange (Li+ and Na+)
was added before the hydrothermal preparation of ZiF-67, time consuming and solvent
consuming in the standard ion-exchange procedure was eliminated. The product was
designated as M-as, where ¡°M¡± represents the alkali metal ion (Li and
Na). By comparison, the standard
ion-exchange procedure was also carried out to synthesize the adsorbents, and the product was
designated as M-post. The structure, surface
charge and CO2 adsorption properties of the adsorbents prepared via
the two methods were investigated and compared.

Fig. 1(A) shows the XRD patterns of ZiF-67 exchanged with Li+ and Na+.
all the ion exchanged ZiF-67 maintains crystallinity of the original phase [4].
And almost no changes in peak intensity and position could be observed. Fig. 1(B)
presents the particle size distributions of as-synthesized and ion exchanged
ZiF-67 samples, which shows the Gaussian type distribution with different distribution
ranges.

The CO2 adsorption isotherms of parent
and these ion-exchanged ZiFs at 0 ¡ãC are shown in Fig. 1(C).
The CO2 uptake of
parent ZiF-67 at 1 bar is 35.4 cm3/g. Li/Na-post exhibited decreased CO2
adsorption capacity (30.4 and 31.2 cm3/g) compared
to the parent, while Li/Na-as showed improved CO2 uptake values (48.7 and 45.1 cm3/g). The highest CO2 capacity is achieved by Li-as, which is 37.6 % higher than that of the parent ZiF-67. An interesting phenomenon is found that all of the isotherms
show linear patterns with stable adsorption rate from 0 bar to 1 bar, which
reveals the materials could get excellent adsorption performance at higher
pressure range (> 1 bar).

Fig. 1. (A) XRD patterns of parent ZiF-67
and ion exchanged with Li+ and Na+, (B) Particle size
distribution, (C) CO2/N2 adsorption isotherms and (D) CO2
adsorption-desorption isotherms

The CO2 adsorption mechanism of the adsorbents prepared via the two methods was
investigated. The surface area
and pore volume (Table 1) of Li+/Na+
as-exchanged ZiF-67 increase significantly
because the radius of Li+
and Na+ is smaller than that of Co2+. However, the surface area of post-exchanged materials decreases contrarily due to the agglomerate and pore block. PZC characterization
(Table 2) indicates that parent ZiF-67 has a neutral character, whereas ion-exchanged ZiFs has
basic surfaces. And pHPZC values of Li+ and Na+
as-exchanged ZiFs is higher
than that of post-exchanged
samples, which could improve CO2 adsorption due to the coordination
interaction. All of the CO2 adsorption isotherms show linear patterns with stable
adsorption rate from 0 bar to 1 bar, which reveals the materials could get
excellent adsorption performance at higher pressure range (> 1 bar). Van der
Waals interaction determined by the surface area and coordination interaction resulting
from electrostatic interaction [5] work in synergy to enhance CO2 adsorption performance of Li+/Na+ as-exchanged
ZiF-67 adsorbents.

Table 1. Physical properties of parent ZiF-67 and ion exchanged ZiF-67.

ZiFs

BET

(m2/g)

Pore volume (m3/g)

SF Median pore width (nm)

parent

1424.59

0.84

0.868

Li-as

1450.71

0.87

0.875

Na-as

1435.15

0.86

0.871

Li-post

1366.14

0.76

0.851

Na-post

1359.50

0.77

0.864

Table 2. The pHPZC
values of
the parent ZiF-67
and ion exchanged ZiF-67
before and after CO2 adsorption

Sample

pHPZC values

before CO2 adsorption

after CO2 adsorption

parent

6.92

5.36

Li-post

7.79

5.10

Na-post

7.51

5.63

Li-as

9.26

5.27

Na-as

8.83

5.49

References

[1] J. An, N.L. Rosi, J. Am. Chem. Soc. 132
(2010) 5578-5579.

[2] M. Eddaoudi, J. Eubank, Y. Liu, V.Ch. Kravtsov,
R. Larsen, J. Brant, Stud. Surf. Sci. Catal. 170 (2007) 2021-2029.

[3] F. Nouar, J. Eckert, J. Eubank, P. Forster,
M. Eddaoudi, J. Am. Chem. Soc. 131 (2009) 2864- 2870.

[4] J. Qian, F. Sun, L. Qin, Mater. Lett. 82
(2012) 220-223.

3601-3606.

[5] H. Zhuo, Q. Li,
W. Li, J. Cheng, New J. Chem. 39 (2015) 2067-2074.




*Corresponding author. Tel.:+86 025-84315517; fax: +86
025-84315517.

E-mail: zq304@mail.njust.edu.cn (Qin Zhong).