(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 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.

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

References

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W. Li, J. Cheng, New J. Chem. 39 (2015) 2067-2074.