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 pH_PZC 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 pH_PZC 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.

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

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

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