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Article

The Reversible Removal of SO2 by Amino Functionalized ZIF8 with 5-Aminotetrazole via Post-Synthesis Modification

by
Chiran Wang
,
Helin Xu
,
Pengbing Huang
,
Xiaoqing Xu
,
Hao Wang
,
Yangrui Zhang
and
Renpan Deng
*
School of Chemistry and Chemical Engineering/Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan, Shihezi University, Shihezi 832003, China
*
Author to whom correspondence should be addressed.
Atmosphere 2022, 13(3), 462; https://doi.org/10.3390/atmos13030462
Submission received: 6 February 2022 / Revised: 3 March 2022 / Accepted: 8 March 2022 / Published: 12 March 2022
(This article belongs to the Special Issue Greener Energy, Air Quality, and Carbon Neutrality)
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Versions Notes

Abstract

:
The post-synthesis modification is a highly efficient method for the modification of Metal-organic framework (MOF) materials, which has been used to synthesize MOF materials purposefully that cannot be prepared by direct synthesis and impregnation method. In this work, amino modified ZIF8 with 5-aminotetrazole was prepared by the post synthesis modification method and was employed to reversibly remove SO2 from flue gas. Based on the characterization and analysis of X-ray diffraction (XRD), Scanning Electron Microscope (SEM), and Brunner Emmet Teller (BET), it was found that the functionalized ZIF8 (Zn(5-ATZ)1.5) was a microporous material with a two-dimensional nano-layered structure. According to the SO2 adsorption experiments, the adsorption capacity of SO2 at the concentration of 1.6% vol can reach to 122 mg/g under the optimal conditions (25 °C, 2865 h−1). Five successive adsorption-desorption experiments exhibited that Zn(5-ATZ)1.5 had excellent regeneration performance. The characterization results of Raman Spectra (Raman) and X-ray photoelectron spectroscopy (XPS) as well as the DFT simulation calculations revealed that SO2 mainly interacted with Zn(5-ATZ)1.5 by hydrogen bonds between O of SO2 and amino H in the Zn(5-ATZ)1.5, and the interaction of SO2 with amino N and 5-aminotetrazole N by forming a non-covalent charge transfer complex. This work suggested that Zn(5-ATZ)1.5 is an excellent potential sorbent for SO2 removal.

1. Introduction

In recent years, more and more metal–organic frameworks (MOFs) are widely used in the research of gas separation [1,2,3] and purification [4,5] because of their high metal center density, large specific surface area, porous structure and easy modification [1,6,7]. As a subclass of MOF, zeolite–imidazole framework (ZIF), an emerging molecular sieve material with superior stability, has a tetrahedral three-dimensional mesh structure similar to that of zeolite, using zinc or cobalt to replace the silicon in zeolite, and replacing the oxygen bridge in zeolite with the ligand of imidazole [8,9]. ZIF-8 and ZIF-11 are stable at 550 °C [8] and in boiling alkaline aqueous solutions as well as organic solvents [10], which are available for many applications.
ZIFs are easily to be modified into efficient catalysts [11], adsorbents [12], and membrane materials [13] as needed. Normally, there are three methods to modify ZIFs, that is, physical impregnation method [14], direct synthesis method [15,16] and post-synthetic modification (PSM) method [17]. The impregnation method is a simple way with no chemical reactions and only the combination between the loadings and the microporous ZIFs. This method needs appropriate solvents for loadings, and the modification process requires a long period of heating and stirring, however, the yield is low [14,18]. In addition, the loss of the loadings due to the weak physical combination with ZIF during prolonged use is a challenging problem [19]. The direct synthesis method needs appropriate organic reagents containing the target modified group to self-assemble with metal ions after they completely dispersed in organic solvents. In some cases, the yield of direct synthesis may be low and it has some limitations [20]. The PSM is an emerging method to functionalized ZIFs by introduce other groups and ions into the original ZIFs via the metathesis of linkers [17,21,22]. Up to today, PSM has been used to improve ZIFs properties, enhance ZIFs stability and synthesize new ZIFs materials, which were unavailable for impregnation and directly synthesis [21,22]. The modified ZIFs by PSM are usually more stable than that by physical impregnation, because the ligands interact with the center metals via chemical bonds after the metathesis of organic linkers [17,21,22].
SO2 released during the utilization of fossil fuels accounts for more than 87% of global SO2 emissions and has caused lots of serious environmental problems [23]. SO2 can easily cause heart disease, cancer, and respiratory tract damage due to the formation of fog and haze [24]. Besides, SO2 could cause soil and water pollution, which threatens the health of human beings as well as disrupt the ecological balance [25,26,27]. ZIFs materials are widely used for the separation of CO2, N2, and alkanes [10,28], however, there are less studies on ZIFs materials for flue gas desulfurization due to the high reactivity and corrosiveness of SO2 [4,29]. Simon H. et al. reported the SO2 adsorption performance of ZIF8 at different exposure conditions and found that SO2 would corrode ZIF8 in moisture conditions due to the formation of sulfite and sulfate groups [30]. Ding et al. stimulated adsorption of SO2 by ZIF10, ZIF69, and ZIF71 under different SO2 concentration and found that SO2 can be interacted with the open-metal sites of the adsorbents [31]. By molecule simulations, ZIF-71 and ZIF-69 were found to be ideal adsorbents to remove SO2 [32]. Ma and his colleague reported the Ethylene diamine tetraacetic acid (EDTA) modified MOF-808 (EDTA-MOF-808) exhibited high SO2 adsorption capacity at 273 K and 0.2 bar. It was found that SO2 interacted with EDTA-MOF-808 through moderate dipole–dipole interaction and hydrogen bonds [33]. In the process of SO2 adsorption, the SO2 concentration and the adsorption temperature have a great influence on the adsorption. The increase in SO2 concentration leads to a greater concentration difference between the gas phase and the adsorbent surface, increasing the driving force for the SO2 adsorption [34]. The adsorption of SO2 is exothermic and will lead to a decrease in the free energy and enthalpy of adsorption of the system, so increasing the adsorption temperature is usually detrimental to the adsorption process [35].
In our previous research, we found that the amino functionalized ZIF8 with 3-amino-1,2,4 triazole could be used to adsorb SO2, and it has been confirmed that the introduction of amino groups could enhance the SO2 adsorption capacity due to the formation of hydrogen bond between SO2 and the amino [36]. According to the work reported by Panda and Wang [37,38], imidazolium salts contain N1 and N4 which can be coordinated with metal centers. The ligand 2-methylimidazole in ZIF8 has only two N and no amino group, while 5-aminotetrazole has four N and an additional amino group, therefore, 5-aminotetrazole was employed to modify ZIF8 aiming to enhance the SO2 adsorption in this work. In fact, there are lots of reports about amino functionalizing MOFs with 5-aminotetrazole [39,40,41]. Herein, we reported the SO2 adsorption performance of a MOF material with nano-layered structure. Specifically, 5-aminotetrazole modified ZIF8 (i.e., Zn(5-ATZ)1.5) was efficiently prepared by PSM in this work. We focused on the effects of the SO2 concentration and adsorption temperature on the removal of SO2 as well as the mechanism of interaction between SO2 and Zn(5-ATZ)1.5. The results of this work hope to provide a theoretical basis for the potential application of Zn(5-ATZ)1.5.

2. Materials and Methods

2.1. Materials and Reagents

Firstly, 2-methylimidazole (98%, MIM), 5-aminotetrazole (5-ATZ), methanol (99.9%) and zinc nitrate hexahydrate (99%, Zn (NO3)2·6H2O) were procured from Adams Reagent Co., Ltd. (Shanghai, China). The simulated flue gas (vol %: 1.6% SO2, 98.4% N2) and pure N2 (99.99%) were procured from Shanghai Weichuang Standard Gas Analysis Technology Co., Ltd. (Shanghai, China).

2.2. Synthesis of ZIF8 and Zn(5-ATZ)1.5

The preparation of ZIF8 was referred to the method reported by Cho [17]. Then, 5.9498 g of Zn (NO3)2·6H2O and 6.5683 g of MIM were dissolved in 200 mL of methanol, respectively. The methanol solution of MIM was then added into the methanol of Zn (NO3)2·6H2O, and the mixture was placed in an ultrasound for 4 h at the temperature of 22~25 °C. The white precipitate of ZIF8 was obtained after the mixture incubated for 24 h. ZIF8 was then centrifuged and washed three times with methanol. The activation of ZIF8 was in a vacuum drying oven at 80 °C for 6 h. A unit of ZIF8 is composed of four coordination bonds formed between Zn in ZIF8 and N in MIM [42], shown as Figure 1.
A total of 0.6 g of ZIF8 was evenly dispersed in 300 mL of methanol. After ultrasonic at 50 °C for 30 min, 1.7865 g of 5-aminotetrazole was added and stirred at the speed of 400 rpm for 8 h. The precipitate of modified ZIF8 was then incubation for 20 min. The centrifugation and wash of the modified ZIF8 was the same with that of the ZIF8. The activation of the modified ZIF8 was placed in a vacuum drying oven at 120 °C for 12 h.

2.3. The Removal of SO2

The adsorption and desorption experiments were carried out in a quartz tubular reactor with the inner diameter of 11 mm and the length of 550 mm. A sand core was arranged inside the reactor to hold adsorbents. The adsorption and desorption temperature were controlled by a heating furnace. All the experiments were carried out at the pressure of 1.0 bar.
The schematic diagram of adsorption and desorption experiments are shown in Figure 2. 0.1 g of the adsorbent was placed in the reactor. The simulated flue gas was flowed through the reactor when the volume flow and adsorption temperature was steady. The SO2 content in the tail gas at the outlet was detected and analyzed by the iodine titration method (HJ/T 56-2000, a standard method of State Environmental Protection Administration of China). The SO2 capacity of the adsorbent was expressed by mg/g, namely the amount of SO2 (mg) contained in per gram of the adsorbent. After SO2 saturation, the simulated flue gas was switched to N2 with the volume flow of 15 mL/min for desorption. The desorption experiment was at the temperature of 120 °C with a heating rate of 10 °C/min. The determination of SO2 in the tail gas is the same with that of the adsorption experiment. The desorption rate was defined as the ratio of SO2 desorption capacity to saturation capacity, for evaluating the desorption effect.

2.4. Specific Brunauer−Emmett−Teller (BET) Surface Areas

The specific areas of ZIF8 and Zn(5-ATZ)1.5 were calculated on Micromeritics Model ASAP 2000 (Atlanta, GA, USA) recorded at the temperature of liquid nitrogen from nitrogen adsorption isotherms by using the BET equation.

2.5. X-ray Photoelectron Spectroscopy (XPS) Analysis

In order to analyze the S species on the adsorbents, electronic binding energies for S 2p peaks of Zn(5-ATZ)1.5 after adsorbing SO2, Zn(5-ATZ)1.5 after desorbing SO2 at 120 °C and Zn(5-ATZ)1.5 after five continuous adsorption–desorption cycles were measured by Thermo ESCALAB 250XI, respectively. The binding energy was calibrated by C 1s as the reference energy (C 1s = 284.8 EV).

2.6. Sanning Electron Microscope (SEM)

The SEM samples were prepared at room temperature by depositing uniformly dispersed ethanol solution droplets with adsorbent on a single-sided polished silicon wafer with a width and length of no more than 25 mm, which can be clearly observed on Hitachi SU8010 scanning electron microscope with a resolution of 1.0 nm. The sample was pre-treated by spray-gold to minimize charging effects before observation using SEM. The acceleration voltage of the microscope used in SEM analyses is 15 kV.

2.7. X-ray Diffractometer (XRD)

RIGAKU ultimate IV X-ray diffractometer of Japanese science is adopted, with wavelength of 1.5418 Å, working voltage of 40 kV, working current of 40 Ma, scanning speed of 10°/min and scanning angle of 5~90°.

2.8. Raman Spectrometer (Raman)

The Raman spectra of Zn(5-ATZ)1.5 before and after SO2 adsorption were measured by Bruker Raman spectrometer at room temperature, and excited by Ar+ laser at 514.5 nm.

2.9. Theoretical Calculation

In the calculation and simulation part, the adsorption energies between adsorbent and SO2 at different adsorption sites were calculated by density functional theory (DFT). Using the DMOL3 module in the materials studio program and all different adsorption site structures were determined under the DFT method. The exchange and correlation terms are determined using the generalized gradient approximation (GGA) in the form proposed by Perdew, Burke, and Ernzerhof (PBE) [43]. Then, Hirshfeld charge analysis was carried out for the optimized structure.

3. Result and Discussion

3.1. Characterization of Zn(5-ATZ)1.5

Figure 3a and b gives the SEM patterns of ZIF8 and Zn(5-ATZ)1.5, respectively. After modification, the nanoparticle of ZIF8 with octahedron morphology structure turned into a two-dimensional nano-layered structure, which was in line with the work conducted by Zhang [44]. The XRD patter of Zn(5-ATZ)1.5 as shown in Figure 3c, which was quite different with that of ZIF8 (see Figure S1 in the Supplementary Materials) due to the change of the structure. The observed diffraction peaks were consistent with that of the sample synthesized by direct synthesis [44]. This clearly suggested that Zn(5-ATZ)1.5 could also be prepared by PSM, which was proved to greatly reduce the preparation time and had a higher yield compared with the reported methods [44]. The structural diagram of Zn(5-ATZ)1.5 was exhibited in Figure 3d. Compared with ZIF8, three coordination bonds are formed between Zn in Zn(5-ATZ)1.5 and N in 5-ATZ, which led to its structure into two-dimensional and nano-layered, instead of octahedron morphology.
To investigate the microstructure of Zn(5-ATZ)1.5, the N2 adsorption–desorption isotherms and pore size distribution of Zn(5-ATZ)1.5 were tested and the results were shown in Figure S2, which exhibited microporous structure for Zn(5-ATZ)1.5. As shown in Table 1, the specific surface area, average pore diameter, and average pore volume of ZIF8 were 1243 m2/g, 29.5 nm and 0.65 m3/g, respectively. After modified by 5-ATZ, the specific surface area and average pore diameter decreased to 386 m2/g and 4.5 nm, respectively. Besides, the average pore volume of Zn(5-ATZ)1.5 decreased by 33.8% compared to ZIF8. Clearly, the pore structure was significant changed after modified due to the change of crystal structures.

3.2. Desulfurization Performance

The SO2 adsorption activity of Zn(5-ATZ)1.5 and ZIF8 is shown in Figure 4. It can be seen that under the same conditions, the SO2 saturation capacity of Zn(5-ATZ)1.5 is 122 mg/g, which is about 1.8 times higher than that of ZIF8 (68.8 mg/g). Considering the fact that the specific surface area of ZIF8 decreased significantly after modified while its saturation capacity on SO2 increased notably, it can be concluded that the introduction of amino group greatly promoted the adsorption of SO2. In addition, from the SO2 desorption ratio of Zn(5-ATZ)1.5, SO2 is considered to be desorbed completely, indicating that the adsorbent has good regeneration performance. The SO2 adsorption capacities of some adsorbents mentioned previously were summarized in Table 2. Obviously, the saturation capacity of Zn(5-ATZ)1.5 was higher than those of HKUST-1, KAUST-7, KAUST-8, FMOF-2, and ZnCo, some of which were even adsorbed in pure SO2 atmosphere.

3.3. The Effect of SO2 Concentration on SO2 Removal

Figure 5 displayed the influence of SO2 concentration on SO2 removal with Zn(5-ATZ)1.5. Clearly, the breakthrough time of Zn(5-ATZ)1.5 on SO2 adsorption decreased significantly as the SO2 concentration increased, and the SO2 saturation capacity increased with the increasing of SO2 concentration. This is mainly due to the stronger diffusion driving force under the higher gas concentration, which promoted the adsorption of SO2. It is worth mentioning that when the SO2 concentration reduced by 31.4 times (from 2.2% to 0.07%), the SO2 saturation capacity of Zn(5-ATZ)1.5 still remains at about 74 mg/g (about 37% of that at the SO2 concentration of 2.2%), indicating that the adsorbent can be used for SO2 removal even at low SO2 concentration.

3.4. Effect of Adsorption Temperature on the Removal of SO2

Adsorption temperature is an important factor affecting the removal of SO2. Figure 6 shows the effect of temperature on SO2 adsorption by Zn(5-ATZ)1.5 in the range of 25 to 65 °C. The breakthrough time and the saturation time decreased as the adsorption temperature increasing. The SO2 saturation capacity is 122 mg/g at 25 °C, however it decreased to 70 mg/g when the temperature rose up to 65 °C. This clearly shows that high temperature is not conducive to the removal of SO2 with Zn (5-ATZ)1.5, suggesting that the interaction between the adsorbent and SO2 is physical and a weak interaction rather than strong chemical interaction. This would be discussed in detail later.

3.5. Regeneration Performance of Adsorbent

For any potential applications, the regeneration performance of adsorbents determines whether they can be recycled. Five continues adsorption–desorption experiments of Zn(5-ATZ)1.5 on SO2 removal were shown in Figure 7. The SO2 saturation capacity of Zn(5-ATZ)1.5 remains at about 122 mg/g without obvious decline. Obviously, the adsorbent of Zn(5-ATZ)1.5 possesses an excellent regeneration performance, which is beneficial for its potential application. Besides, the excellent regeneration performance also implied the interaction between the Zn(5-ATZ)1.5 and SO2 is completely reversible under the given experiment conditions.

3.6. Adsorption Mechanism

3.6.1. Raman Analysis

In order to explore the interaction mechanism between Zn(5-ATZ)1.5 and SO2, the Raman spectra of the fresh Zn(5-ATZ)1.5 (1), Zn(5-ATZ)1.5 after saturated with SO2 (2), Zn(5-ATZ)1.5 after SO2 desorption (3) and Zn(5-ATZ)1.5 after five continuous adsorption–desorption cycles (4) were characterized, respectively, as shown in Figure 8. Obviously, a new band appeared at 1151.5 cm−1 after SO2 adsorption, which belongs to gaseous SO2 [48,49]. This indicates that there is physical adsorption between SO2 and Zn(5-ATZ)1.5. The band (at 1151.5 cm−1) of Zn(5-ATZ)1.5 after SO2 desorption (3) and five continuous adsorption–desorption cycles (4) disappeared, which also shows that the SO2 adsorbed by Zn(5-ATZ)1.5 can be completely desorbed. This result is consistent with the good regeneration performance mentioned above.

3.6.2. XPS Analysis

The interaction mechanism between Zn(5-ATZ)1.5 and SO2 was further studied by the characterization and analysis of XPS. The XPS spectra of the fresh Zn(5-ATZ)1.5 (1), Zn(5-ATZ)1.5 after being saturated with SO2 (2), Zn(5-ATZ)1.5 after SO2 desorption (3), and Zn(5-ATZ)1.5 after five continuous adsorption–desorption cycles (4) are shown in Figure 9, respectively. Clearly, the fresh Zn(5-ATZ)1.5 only contains zinc, carbon, and nitrogen (Zn 2p, C 1s, and N 1s, see Figure 9a), however, a new peak assigned to S 2p was observed after Zn(5-ATZ)1.5 saturated with SO2, confirming that SO2 was adsorbed on Zn(5-ATZ)1.5. According to the fitting of the S 2p spectrum (Figure 9b), there are three peaks that can be observed which may be assigned to the interaction of SO2 with H of amino (161.0 eV), the N of amino (167.4 eV) and N of 5-ATZ ring (163.7 eV), respectively. As shown in Figure 9c, the spectrum of N 1s slightly shifts from 397.0 to 396.9 eV after Zn(5-ATZ)1.5 adsorbed with SO2 owing to the averaging of the N and S electron density regions. This may suggest that the amino H of Zn(5-ATZ)1.5 interacted with O of SO2 to form hydrogen bonds [50] or the N of amino and the N on the 5-aminotetrazole ring of Zn(5-ATZ)1.5 interacted with S of SO2 to form non-covalent charge transfer complexes [51]. After SO2 desorption at high temperature (120 °C), the peak of N 1s completely restored to its original state [52], which is in line with the excellent regeneration performance. In addition, as shown in Figure 9d, there is no change in the Zn 2p spectra of Zn(5-ATZ)1.5 before and after adsorbing SO2, indicating that SO2 did not interact with Zn due to the coordination saturation between Zn and 5-ATZ. The conclusion is different from our previous research on ZIF8-A [29].

3.6.3. Theoretical Calculation

In this study, three possible adsorption sites of Zn(5-ATZ)1.5 were attempted to adsorb SO2. As shown in Figure 10, the three possible interactions include: SO2 with the H of -NH2 in Zn (5-ATZ)1.5 by hydrogen bond (Figure 10a), SO2 with the N of -NH2 in Zn(5-ATZ)1.5 by non-covalent charge transfer complexes (Figure 10b), and SO2 with the N of the 5-aminotetrazole ring in Zn(5-ATZ)1.5 by non-covalent charge transfer complexes (Figure 10c). Considering that the interaction between SO2 and coordination unsaturated metals in MOFs is unreversible [53], and the fact that the regeneration performance of Zn(5-ATZ)1.5 was excellent, the calculation of adsorption energy of the interaction between SO2 with Zn in Zn(5-ATZ)1.5 can be excluded.
The adsorption energy of the SO2 molecule in the Zn(5-ATZ)1.5 framework was calculated by the following equation (Equation (1)) [54]:
E a d s = E f r a m e w o r k − g a s − ( E f r a m e w o r k + E g a s )
where Eframework and Egas represent the energies of Zn(5-ATZ)1.5 and the single SO2 molecule, respectively, and Eframework-gas is the total energy of adsorbed complex (kJ/mol).
The adsorption energies of SO2 molecule with different adsorption sites in Zn(5-ATZ)1.5 are summarized in Table 3. The adsorption energy of SO2 with the H of -NH2 in Zn(5-ATZ)1.5 was the highest (−19.44 kJ/mol), followed by the adsorption energy of SO2 with the N of -NH2 in Zn(5-ATZ)1.5 (−14.45 kJ/mol), and the adsorption energy of SO2 with the N of the 5-aminotetrazole ring in Zn(5-ATZ)1.5 was the lowest (−9.25 kJ/mol). According to Equation (1), a negative value of Eads indicates that the process is an exothermic reaction. The higher negative values mean stronger interactions, resulting in more heat release and more stable product formation. The results showed SO2 preferentially interacts with the H of -NH2 in Zn(5-ATZ)1.5, followed by the N of -NH2 in Zn(5-ATZ)1.5, and the N of 5-aminotetrazole ring in Zn(5-ATZ)1.5. This is consistent with the regeneration performance data and XPS analysis. According to the characterization analysis and calculation simulation results, it can be concluded that the interaction between SO2 and Zn(5-ATZ)1.5 is mainly through four ways: physical adsorption, the O in SO2 interacts with the H of -NH2 in Zn(5-ATZ)1.5 by hydrogen bond, the S in SO2 interacts with the N of -NH2 in Zn(5-ATZ)1.5, and the N of 5-aminotetrazole ring in Zn(5-ATZ)1.5 by non-covalent charge transfer complex. The four reversible interactions are the reasons why Zn(5-ATZ)1.5 has good adsorption and recycling performance.

4. Conclusions

In this work, a desulfurization adsorbent (Zn(5-ATZ)1.5) was prepared by the modification of ZIF8 with 5-aminotetrazole via post-synthesis modification. The acquired adsorbent Zn(5-ATZ)1.5 was substantiated to be a porous material with two-dimensional nano-layered structure. The SO2 adsorption capacity increased with the increasing of SO2 concentration and decreased as the increased adsorption temperature. Under the optimum conditions (adsorption temperature: 25 °C; desorption temperature: 120 °C; air speed: 2865 h−1), the SO2 adsorption capacity of Zn(5-ATZ)1.5 reached to 122 mg/g (SO2 concentration: 1.6%), which was 77.3% higher than that of ZIF8. Five adsorption–desorption cycles indicated that Zn(5-ATZ)1.5 had excellent regeneration performance. Based on the characterization and analysis of Raman, XPS, and DFT calculation, it is concluded that the interaction between SO2 and Zn(5-ATZ)1.5 is mainly through four reversible ways: physical adsorption, the O of SO2 interacts with the H of -NH2 in Zn(5-ATZ)1.5 by hydrogen bond, the S of SO2 interacts with the N of -NH2 in Zn(5-ATZ)1.5, and the N of 5-aminotetrazole ring in Zn(5-ATZ)1.5 by non-covalent charge transfer complex. The adsorbent of Zn(5-ATZ)1.5 can be potentially applied in SO2 adsorption and separation due to the high adsorption capacity and excellent regeneration performance.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/atmos13030462/s1, Figure S1: The XRD patterns of ZIF8, Figure S2: N2 adsorption–desorption isotherm (a) and pore size distribution of Zn(5-ATZ)1.5 (b).

Author Contributions

Conceptualization, R.D.; Methodology, C.W., P.H., H.X. and X.X.; Data curation: C.W., H.X. and X.X.; Formal analysis: P.H. and Y.Z.; Software: H.W. and P.H.; Writing—original draft preparation: C.W.; Writing—review and editing: C.W. and R.D.; Supervision: R.D.; Project administration: R.D.; Funding acquisition, R.D. All authors have read and agreed to the published version of the manuscript.

Funding

This project was supported by the Shihezi University Young Innovative Talents Program Project (Grant No. CXBJ201902), the National Natural Science Foundation of China (Grant No. 31800828), the “Double First-Class” General Science and Technology Project of Shihezi University (Grant No. SHYL-YB201902), and the “Open Project” of Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan (Grant No. KF201705).

Acknowledgments

The authors highly acknowledge the resources and opportunity provided by Shihezi Unniversity and the Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan.

Conflicts of Interest

The authors declare no conflict of interest.

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Atmosphere 13 00462 g001 550
Figure 1. The schematic of ZIF8 unit structure (white: H; blue: N; dark gray: C; light gray: Zn). (Created by authors).
Figure 1. The schematic of ZIF8 unit structure (white: H; blue: N; dark gray: C; light gray: Zn). (Created by authors).
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Figure 2. Schematic of SO2 adsorption and desorption experiments (MFC: mass flow controller; 1–4: check valve; 5: temperature sensor; 6: heating furnace; 7: sand core; 8: the adsorption tube; 9: gas absorption flask for tail gas). (Created by authors).
Figure 2. Schematic of SO2 adsorption and desorption experiments (MFC: mass flow controller; 1–4: check valve; 5: temperature sensor; 6: heating furnace; 7: sand core; 8: the adsorption tube; 9: gas absorption flask for tail gas). (Created by authors).
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Figure 3. The SEM patterns of ZIF8 (a) and Zn(5-ATZ)1.5 (b), the XRD pattern of Zn(5-ATZ)1.5 (c), and the structural diagram of Zn(5-ATZ)1.5 (d) (blue: N; green: Zn; black: C; white: H). (Created by authors).
Figure 3. The SEM patterns of ZIF8 (a) and Zn(5-ATZ)1.5 (b), the XRD pattern of Zn(5-ATZ)1.5 (c), and the structural diagram of Zn(5-ATZ)1.5 (d) (blue: N; green: Zn; black: C; white: H). (Created by authors).
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Figure 4. The activity of Zn(5-ATZ)1.5 and ZIF8 on the removal of SO2 (adsorption temperature: 25 °C, desorption temperature: 120 °C, gas composition (vol %): 1.6% SO2 and 98.4% N2, volume airspeed: 2865 h−1).
Figure 4. The activity of Zn(5-ATZ)1.5 and ZIF8 on the removal of SO2 (adsorption temperature: 25 °C, desorption temperature: 120 °C, gas composition (vol %): 1.6% SO2 and 98.4% N2, volume airspeed: 2865 h−1).
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Figure 5. The effect of SO2 concentration on the removal of SO2 by Zn(5-ATZ)1.5 ((a)—breakthrough curve; (b)—saturation capacity; adsorption temperature: 25 °C, desorption temperature: 120 °C, volume airspeed: 2865 h−1).
Figure 5. The effect of SO2 concentration on the removal of SO2 by Zn(5-ATZ)1.5 ((a)—breakthrough curve; (b)—saturation capacity; adsorption temperature: 25 °C, desorption temperature: 120 °C, volume airspeed: 2865 h−1).
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Figure 6. The effect of temperature on the removal of SO2 by Zn(5-ATZ)1.5 (a)—breakthrough curve; (b)—saturation capacity; adsorption temperature: 25 °C, desorption temperature: 120 °C, gas composition (vol %): 1.6% SO2 and 98.4% N2, volume airspeed: 2865 h−1.
Figure 6. The effect of temperature on the removal of SO2 by Zn(5-ATZ)1.5 (a)—breakthrough curve; (b)—saturation capacity; adsorption temperature: 25 °C, desorption temperature: 120 °C, gas composition (vol %): 1.6% SO2 and 98.4% N2, volume airspeed: 2865 h−1.
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Figure 7. Regeneration performance of Zn(5-ATZ)1.5 on SO2 removal. (Adsorption temperature: 25 °C, desorption temperature: 120 °C, gas composition (vol %): 1.6% SO2 and 98.4% N2, volume airspeed: 2865 h−1).
Figure 7. Regeneration performance of Zn(5-ATZ)1.5 on SO2 removal. (Adsorption temperature: 25 °C, desorption temperature: 120 °C, gas composition (vol %): 1.6% SO2 and 98.4% N2, volume airspeed: 2865 h−1).
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Figure 8. The Raman spectra of the fresh Zn(5−ATZ)1.5 (1), Zn(5−ATZ)1.5 saturated with SO2 (2), Zn(5−ATZ)1.5 after SO2 desorption (3) and Zn(5−ATZ)1.5 after five continuous adsorption–desorption cycles (4).
Figure 8. The Raman spectra of the fresh Zn(5−ATZ)1.5 (1), Zn(5−ATZ)1.5 saturated with SO2 (2), Zn(5−ATZ)1.5 after SO2 desorption (3) and Zn(5−ATZ)1.5 after five continuous adsorption–desorption cycles (4).
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Figure 9. The XPS spectra of the Zn(5-ATZ)1.5 before and after SO2 adsorption ((a)—survey spectra; (b)—S 2p; (c)—N 1s; (d)—Zn 2p; 1—fresh Zn(5-ATZ)1.5; 2—Zn(5-ATZ)1.5 after saturation with SO2; 3—Zn(5-ATZ)1.5 after SO2 desorption; 4—Zn(5-ATZ)1.5 after five continuous adsorption–desorption cycles).
Figure 9. The XPS spectra of the Zn(5-ATZ)1.5 before and after SO2 adsorption ((a)—survey spectra; (b)—S 2p; (c)—N 1s; (d)—Zn 2p; 1—fresh Zn(5-ATZ)1.5; 2—Zn(5-ATZ)1.5 after saturation with SO2; 3—Zn(5-ATZ)1.5 after SO2 desorption; 4—Zn(5-ATZ)1.5 after five continuous adsorption–desorption cycles).
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Figure 10. Possible interaction of SO2 with the H of -NH2 in Zn (5-ATZ)1.5 (a); SO2 with the N of -NH2 in Zn(5-ATZ)1.5 (b); and SO2 with the N of the 5-aminotetrazole ring in Zn(5-ATZ)1.5 (c); blue: N; green: Zn; black: C; white: H; red: O; yellow: S. (Created by authors).
Figure 10. Possible interaction of SO2 with the H of -NH2 in Zn (5-ATZ)1.5 (a); SO2 with the N of -NH2 in Zn(5-ATZ)1.5 (b); and SO2 with the N of the 5-aminotetrazole ring in Zn(5-ATZ)1.5 (c); blue: N; green: Zn; black: C; white: H; red: O; yellow: S. (Created by authors).
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Table
Table 1. The specific surface area, average aperture, and average pore volume of the ZIF8 and Zn(5-ATZ)1.5.
Table 1. The specific surface area, average aperture, and average pore volume of the ZIF8 and Zn(5-ATZ)1.5.
EntrySpecific Surface AreaAverage ApertureAverage Pore Volume
ZIF81243 m2/g29.5 nm0.65 m3/g
Zn(5-ATZ)1.5386 m2/g4.5 nm0.43 m3/g
Table
Table 2. The SO2 uptake of different adsorbents.
Table 2. The SO2 uptake of different adsorbents.
EntryPressure/BarTemperature (°C)SO2 Concentration (vol %)SO2 Uptake (mg/g)Reference
Zn(5-ATZ)1.51.0252.2200.1This work
HKUST-11.025 32[6]
ZIF81.0251.668.8This work
KAUST-71.025Pure SO2160.1[45]
FMOF-21.025Pure SO2115.2[46]
ZnCo1.025Pure SO2115.2[47]
KAUST-81.025Pure SO2184[45]
Table
Table 3. The adsorption energies of three kinds of initial adsorption structures (kJ/mol).
Table 3. The adsorption energies of three kinds of initial adsorption structures (kJ/mol).
EntryEads
(H (NH2) Site)		Eads
(N (NH2) Site)		Eads
(N (5-ATZ) Site)
Zn(5-ATZ)1.5−19.44−14.45−9.25
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Wang, C.; Xu, H.; Huang, P.; Xu, X.; Wang, H.; Zhang, Y.; Deng, R. The Reversible Removal of SO2 by Amino Functionalized ZIF8 with 5-Aminotetrazole via Post-Synthesis Modification. Atmosphere 2022, 13, 462. https://doi.org/10.3390/atmos13030462

AMA Style

Wang C, Xu H, Huang P, Xu X, Wang H, Zhang Y, Deng R. The Reversible Removal of SO2 by Amino Functionalized ZIF8 with 5-Aminotetrazole via Post-Synthesis Modification. Atmosphere. 2022; 13(3):462. https://doi.org/10.3390/atmos13030462

Chicago/Turabian Style

Wang, Chiran, Helin Xu, Pengbing Huang, Xiaoqing Xu, Hao Wang, Yangrui Zhang, and Renpan Deng. 2022. "The Reversible Removal of SO2 by Amino Functionalized ZIF8 with 5-Aminotetrazole via Post-Synthesis Modification" Atmosphere 13, no. 3: 462. https://doi.org/10.3390/atmos13030462

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