N2 by a Water

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Separations

Selective Adsorptive Separation of CO2/CH4 and CO2/N2 by a Water Resistant Zirconium–Porphyrin Metal–Organic Framework Daofei Lv, Renfeng Shi, Yongwei Chen, Yang Chen, Houxiao Wu, Xin Zhou, Hongxia Xi, Zhong Li, and Qibin Xia Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02596 • Publication Date (Web): 23 Aug 2018 Downloaded from http://pubs.acs.org on August 23, 2018

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Industrial & Engineering Chemistry Research

Selective Adsorptive Separation of CO2/CH4 and CO2/N2 by a Water

Resistant

Zirconium–Porphyrin

Metal–Organic

Framework Daofei Lv1, Renfeng Shi1, Yongwei Chen1, Yang Chen1, Houxiao Wu1, Xin Zhou1, Hongxia Xi1,2*, Zhong Li1, and Qibin Xia1,3* 1 School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, PR China 2 Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, South China University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou 510006, PR China 3 Guangdong Provincial Key Lab of Green Chemical Product Technology, South China University of Technology, Guangzhou 510640, PR China

ABSTRACT: A water resistant zirconium–porphyrin metal–organic framework PCN-222 was synthesized for selectively separating CO2/CH4 and CO2/N2. Isotherms of CO2, CH4 and N2 on PCN-222 at low and high pressure were respectively measured. At 298 K, CO2 uptakes at 100 kPa and 3000 kPa were 1.16 and 13.67 mmol/g, respectively. At 298 K and 100 kPa, CO2/CH4 (50:50, v/v) and CO2/N2 (50:50, v/v) adsorption selectivities separately reached up to 4.3 and 73.7. At 298 K and 3000 kPa, CO2/CH4 (50:50, v/v) and CO2/N2 (50:50, v/v) adsorption selectivities were 4.7 and 32.8, respectively. Importantly, breakthrough experiments confirmed that PCN-222 could achieve the effective separation of CO2/CH4 (both 50:50 and 10:90) and CO2/N2 (both 50:50 and 15:85) gas mixtures. The dynamic regeneration experiments showed that PCN-222 maintained its initial CO2 adsorption capacity after five cycles of CO2 adsorption-desorption. This work suggested that PCN-222 was a potential candidate for selectively separating CO2/N2 and CO2/CH4. 1. Introduction Nowadays, the greenhouse effect, which occurs mainly due to the emission of lots *

Corresponding authors.

E-mail addresses: [email protected] (Qibin Xia), [email protected] (Hongxia Xi).

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of CO2 into the atmosphere, has become an intractable issue for humankind.1 Compared with the atmospheric CO2 concentration in the beginning of the industrial revolution (280 ppm), the CO2 concentration has risen to 403 ppm in 2015.2 To mitigate the greenhouse effect, it is desired to develop effective CO2 capture and storage methods to reduce CO2 emissions.3 According to the statistics, the power plant contributes about 30% of total CO2 emissions, due to the burning of a large amount of fossil fuels.4 Therefore, it is necessary to separate CO2 from power plant flue gas, which contains ~85% N2 and ~15% CO2, before it is emitted to the atmosphere.5 On the other hand, landfill gas and biogas, mainly consisting of CO2 and CH4, are known as promising alternative cleaner fuels to replace oil and coal.6 However, undesired impurity of CO2 in landfill gas and biogas will lower their heating values and corrode pipelines and equipments during transportation.7-9 Thus, it is significant importance to remove CO2 in landfill gas and biogas before their widespread utilization.10,11 To date, numerous advanced technologies have been developed to separate CO2 from flue gas or landfill gas and biogas, such as chemical absorption,12-14 cryogenic distillation,15-17 membrane separation18,19 and physical adsorption.20-23 Among these different technologies, the adsorption-based CO2 capture technology is considered as a very promising technology because it needs low energy consumption and operation cost.16,24 Adsorbents are the key to the adsorption-based CO2 capture technologies.6,25 A variety of traditional adsorbents including molecule sieves and zeolites have been reported for separating CO2/N2 and CO2/CH4.26-28 However, most of these traditional adsorbents have the problems of low CO2 adsorption uptakes and poor CO2/N2 and CO2/CH4 adsorption selectivities. Therefore, it is desirable to explore novel adsorbents with high CO2 adsorption uptakes and superior CO2/N2 and CO2/CH4 selectivities. In comparison with traditional adsorbents such as molecule sieves, zeolites and porous carbon materials, metal-organic frameworks (MOFs) exhibit outstanding potential for selectively separating CO2/N2 and CO2/CH4 mixtures owing to their extra-large pore volumes and surface areas, designable pore structures, flexible surface functionality and many unsaturated metal centers.21,29-33 Numerous MOFs


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have been reported as adsorption materials for CO2 capture at low and high pressure. For CO2 capture operated at low pressure, a good example is Mg-MOF-74, which could adsorb 8.78 mmol/g of CO2 at 298 K and 100 kPa.34 Moreover, Nugent and co-workers21 measured the isotherms of CO2 on SIFSIX-3-Zn and SIFSIX-2-Cu-I up to an atmospheric pressure. They found that CO2/CH4 selectivity of SIFSIX-3-Zn and CO2/N2 selectivity of SIFSIX-2-Cu-i reached up to 231 and 1818 at 298 K and 100 kPa, respectively. For CO2 capture operated at high pressure, Yaghi et al.35 found that CO2 adsorption uptake on Cu-BTC at 298 K and 3000 kPa was 10.83 mmol/g. Peng et al.36 prepared a zinc based UMCM-2, and at 298 K and 3000 kPa, UMCM-2 could adsorb 37.50 mmol/g of CO2. However, most MOFs such as Mg-MOF-74,34 MOF-537 and Cu-BTC38 exhibited instabilities after exposure to water. Thermally, many MOFs including TKL-101-10739, bio-MOF-140 and UTSA-1641 suffered from poor thermal stability. The poor water or thermal stability of MOFs will largely lower the adsorption properties of CO2 and impede their practical application. Thus, it is significant to develop novel adsorbents which not only have excellent water stability and high thermal stability but also high CO2 uptakes and superior CO2/N2 and CO2/CH4 selectivities. Recently, zirconium–porphyrin MOFs attracted extensive attention owing to their robust structures, ultrahigh surface areas and extraordinary water and thermal stability.42,43

PCN-222,

composed

of

zirconium

(IV)

and

tetrakis(4-carboxyphenyl)porphyrin (H2TCPP), was first reported by Feng et al.44 In this work, PCN-222 was selected as an adsorbent for separating CO2/CH4 and CO2/N2 due to its ultrahigh surface area, extraordinary water and thermal stability. We systematically study the adsorption behaviors of CO2, CH4 and N2 on PCN-222, and results showed that PCN-222 had excellent separation performance towards CO2/CH4 and CO2/N2 mixtures at low and high pressures. More importantly, PCN-222 exhibited extraordinary water and thermal stability as well as superior regeneration performance. 2. Experimental section



2.1. Chemicals Zirconium (IV) chloride (ZrCl4, >99.95%) was purchased from Strem Chemicals. The organic ligand tetrakis(4-carboxyphenyl)porphyrin (H2TCPP, >97%) was bought from J&K Scientific Co., Ltd. Benzoic acid (99.5%, AR), N,N-dimethylformamide (DMF, ≥99.5%, AR) and reagent grade 37% hydrochloric acid (HCl) were obtained from Tianjin Chemical Plant. N,N-diethylformamide (DEF, 99%, AR) was purchased from Aladdin. The ultrahigh purity grade gases used in adsorption experiments were CO2 (≥99.999%, Guangzhou Shengying), CH4 (>99.99%, Foshan Huate) and N2 (≥99.999%, Guangzhou Shengying). Mixture gases of CO2/CH4 (10:90 or 50:50, v/v) and CO2/N2 (15:85 or 50:50, v/v) were obtained from Guangzhou Shengying. 2.2. Synthesis of PCN-222 PCN-222 (MOF-545) was prepared according to the reported procedures with slight modification.44 Briefly, ZrCl4 (75 mg), H2TCPP (50 mg) and benzoic acid (2.7 g) were ultrasonically dissolved in a 20 mL disposable scintillation vial containing 8 mL of DEF. The mixture was heated at 393 K for 48 h. The resultant purple rod-shaped crystals (PCN-222) were harvested by filtration. After that, we carried out the activation procedures of PCN-222 crystals before sorption experiment. The as-synthesized PCN-222 (60 mg) sample was soaked in 40 mL DMF with an additional 1.5 mL of 8 M HCl to remove trapped benzoic acid, unreacted inorganic metal salts and organic ligands at 393 K for 12 h. Thereafter, extract was decanted and obtained PCN-222 samples were immersed in 80 mL DMF for 36 h. Finally, activated PCN-222 samples were dried at 393 K for 6 h under vacuum. 2.3. Characterization of PCN-222 Powder X-ray diffraction (PXRD) patterns were recorded on a Philips XPERT Powder Diffractometer, using monochromatic Cu‐Kα radiation in the scanning range of 2-30°. Scanning electron microscope (SEM) images were obtained on a Hitachi SU-70 instrument. Thermogravimetric analysis (TGA) was performed using a Netzsch STA 449F3 instrument in a nitrogen atmosphere in the temperature range of


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302-1000 K. N2 isotherms at 77 K were recorded with a Micromeritics ASAP 2460 analyzer. Brunauer-Emmett-Teller (BET) equation was employed to calculate the specific surface area, according to N2 adsorption isotherms data at relative pressure (P/P0) range of 0.08-0.28. N2 adsorption uptake at P/P0 = 0.95 was employed to calculate the total pore volume. The density functional theory method was applied to determine the pore size distribution. 2.4. Water Stability Experiment Water stability experiment was carried out by placing 60 mg samples in 20 mL vial containing 10 mL deionized water with pH value of 6.1 for 24 h at ambient temperature. Thereafter, the samples were harvested by filtration followed by drying at 393 K for 6 h under vacuum. The water stability was characterized by PXRD and N2 adsorption experiments. 2.5. CO2, CH4 and N2 Adsorption Isotherms Measurements Low-pressure CO2, CH4 and N2 isotherms in the pressure range from 0.002 to 100 kPa at 288, 298 and 308 K were determined with the 3Flex Surface Characterization Analyzer. Adsorption temperatures were strictly controlled by immersing sample cell into a thermostat. Before each run, ~80 mg of sample was outgased at 393 K for 6 h under vacuum. High-pressure CO2, CH4 and N2 isotherms for experimental pressures up to 3000 kPa at 288, 298 and 308 K were recorded on a magnetic-suspension microbalance (Rubotherm GmbH, Germany). Before each run, ~80 mg of sample was outgased at 393 K for 6 h under vacuum. 2.6. Breakthrough Experiments A dynamic self assembly breakthrough setup was employed to separate CO2/N2 and CO2/CH4 mixtures (Figure S1). Breakthrough curves were measured at ambient temperature and pressure with a gas chromatography, which was equipped with a Porapak Q column (3.15 m long and 3 mm diameter). Prior to each breakthrough experiment, 150 mg of pre-activated sample (393 K under high vacuum) was packed in a stainless steel adsorption column (10 cm long and 6 mm diameter). The length of



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the packed sample was 4.2 cm. The flow rate (0.5 mL/min) of CO2/N2 or CO2/CH4 mixture was precisely controlled using the mass flow controller. Hydrogen (23 mL/min) was used as carrier gas. The concentrations of CO2, CH4 and N2 were detected by a thermal conductivity detector (TCD). 3. Results and discussions 3.1. Characterization Figure 1 presents the PXRD patterns of PCN-222. Experimental PCN-222 samples exhibited characteristic peaks at 2.4, 4.7, 6.5, 6.9, 8.2, and 9.5°, which were agreement with those simulated in the literature,45 confirming that PCN-222 samples with high purity were successfully synthesized via a solvothermal method.

Figure 1. PXRD patterns of PCN-222.

SEM images were collected to observe the crystal morphologies of PCN-222 samples, as shown in Figure 2. It demonstrated that PCN-222 crystals had comparatively

regular

rod-shaped

morphologies.

These

regular

rod-shaped

morphologies indicated that PCN-222 crystals had high degree of crystallization. It was observed from Figure 2a that the lengths of rod-shaped PCN-222 crystals were in the range of 20-80 um. Figure S2 exhibits N2 isotherms at 77 K and the pore size distribution of PCN-222. The N2 isotherms displayed a typical type-Ⅱ profile with a steep increase in N2 uptake at low pressure region and a H2 type of hysteresis loop in the light of the IUPAC classifications,46 suggesting that PCN-222 was a porous material with micropores and


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mecropores. The BET surface area was 1371.4 m2/g (Figure S3), and total pore volume was 1.07 cm3/g. The pore size distribution on the basis of DFT calculation showed that PCN-222 mainly had two types of channels (13 and 32 Å).

Figure 2. SEM images of PCN-222 at (a) low resolution and (b) high resolution.

To study the thermal stability of PCN-222, TGA measurement was conducted in the temperature range of 302-1000 K. As shown in Figure S4, this material showed three steps of weight loss. The first step between 303 and 373 K was induced by the removal of water adsorbed on the surfaces. The second step was occurred at 373-637 K, due to the release of solvent molecules trapped in the channels. The framework of PCN-222 began to collapse at temperatures beyond 637 K, due to the decomposition of H2TCPP ligand. Thus, the TGA curve manifested that PCN-222 could keep thermally stable up to 637 K. The excellent thermostability could be ascribed to the strong Zr-O coordination bonds in PCN-222. 3.2. Water Stability of PCN-222 Figure 3 shows PXRD patterns of PCN-222 after being soaked in water at room temperature for 24 h. It was noticed that the PXRD characteristic peaks of PCN-222 before and after being soaked in water for 24 h were similar, indicating that the frameworks of PCN-222 samples remained intact after being immersed in water for 24 h. The superior water stability was related to the very strong Zr-O coordination bonds between Zr4+ and -COO- and the rigid structure of PCN-222. To further verify the water stability of PCN-222, we measured the N2 isotherms of PCN-222 after water treatment for 24 h, as shown in Figure S5. Interestingly,



PCN-222 after being immersed in water for 24 h exhibited a higher N2 uptake compared to original PCN-222. The reason may be that the H3O+ in deionized water (pH = 6.1) contributes to decompose the coordination compounds which are stuck in PCN-222 pores and makes them can be removed by water or DMF. The similar phenomenon had also been seen in other porphyrin based MOFs, like PCN-222 (Fe)44 and PCN-600 (Fe)47. Hence, PCN-222 had good water stability like UiO-66 (Figures S6 and S7).

Figure 3. PXRD patterns of PCN-222 after being immersed in water at room temperature for 24 h.

3.3. Adsorption Isotherms of CO2, CH4, and N2 on PCN-222 Figure 4 presents low-pressure CO2, CH4 and N2 isotherms on PCN-222 at three various temperatures. It showed that the adsorption capacities of these three gases, under all test temperatures and pressures, followed the order CO2 > CH4 > N2. Furthermore, it was noticed that these three gases adsorption uptakes increased as adsorption temperature decreased, suggesting that CO2, CH4 and N2 adsorbed on PCN-222 mainly belonged to physical adsorption. The CO2, CH4 and N2 adsorption uptakes, at 298 K and 100 kPa, separately were 1.16, 0.35 and 0.03 mmol/g. Accordingly, PCN-222 exhibited a moderate CO2 uptake at 298 K and 100 kPa, compared to other adsorbents listed in Table S1.


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Figure 4. Low-pressure CO2, CH4 and N2 isotherms on PCN-222 at (a) 288 K, (b) 298 K and (c) 308 K.

Figure 5 exhibits high-pressure CO2, CH4 and N2 isotherms on PCN-222 at three various temperatures. It was observed that the adsorption capacities of CO2, CH4 and N2 still followed the order CO2 > CH4 > N2 for pressures up to 3000 kPa. In addition, the adsorption capacity of CO2 at 298 K and 3000 kPa reached up to 13.67 mmol/g. CO2 uptake of PCN-222 at 298 K and 3000 kPa was superior to many other reported porous materials, including Zeolite 13X,48,49 AC Norit R1,50 LTA zeolite,51 Ni-MOF-74,52 MOF-2,35,53 Cu-BTC,35 ZIF-854 and Al-BDC55, as listed in Table 1. The relatively high CO2 adsorption uptake was ascribed to the relatively high pore volume (1.07 cm3/g) of PCN-222 and nitrogen functionalities in the host framework which provide the basic adsorption sites. The order of CO2 > CH4 > N2 can be explained as follows: (i) the quadrupole moment of CO2 is far greater than that of CH4 or N2, resulting in that CO2 has stronger interactions with the framework;56 (ii) relative to N2 (17.4×1025 cm3), the polarizability of CH4 (25.9×1025 cm3) is higher,57 causing a stronger electrostatic interaction with the framework; (iii) The channels of PCN-222 are uniformly decorated with Lewis basic sites due to the presence of -N= and -NH-,



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which can have strong interactions with incoming CO2 adsorbate molecules. The similar adsorption behavior can also be seen from other reported adsorbents with many Lewis basic sites in their frameworks, such as Glc-C-4,4 Qc-5-Cu-dia10 and JUC-100038. The large difference of adsorption uptake among the three gases implied that PCN-222 would effectively separate CO2/CH4 and CO2/N2 with high adsorption selectivities.

Figure 5. High-pressure CO2, CH4 and N2 isotherms on PCN-222 at (a) 288 K, (b) 298 K and (c) 308 K. Table 1 CO2 adsorption performance on the selected adsorbents at 3000 kPa Adsorbents

BET area (m2/g)

Temperature (K)

Vpf (cm3/g)

CO2 uptake (mmol/g)

Zeolite 13X AC Norit R1 PPY-4700 Glc-C-4 LTA zeolite MOF-205 MOF-210 UMCM-1

164 3568 3153 140 4460 6240 4100

298 298 298 298 298 298 298 298

0.21 2.46 2.07 0.35 2.16 3.60 2.21

6.06g 10.58 29.6 22.40 5.30 34.09 32.12 30.75

Selectivity CO2/CH4 CO2/N2 (50:50,v/v) (50:50,v/v) 1.5 4.5 3.1


2.2 20 5.2

Qst(CO2) (kJ/mol)

Ref.

21.8 19.4

48,49 50 58 4 51 59 59 36,60

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UMCM-2 Ni-MOF-74 MOF-2 Cu-BTC ZIF-8 PCN-16 Al-BDC DUT-49 Cu-BTTri Co(BDP) PCN-222

5200 1218 345 1781 1980 2273 1100 5476 1750 2030 1371.4

298 298 298 298 300 300 298 298 313 313 298

2.20 0.47 0.09 0.70 0.65 1.06 0.35h 2.91 0.71 0.93 1.07

37.50 11.63 3.04 10.83 8.75 16.59 8.29 42.73 15.36 15.89 13.67

4.4 4.5

7.2 2.21 32.5

24.0 42.0 27.4 31.2 19.5 18.3

36 52 35,53 35 53 53 55 61 62 62

This work

f

Vp is the total pore volume. gThe pressure is 2900 kPa. hMicropore volume

3.4. The Adsorption Selectivities of CO2/CH4 and CO2/N2 The low-pressure and high-pressure CO2, CH4 and N2 isotherms at three different temperatures were fitted utlizing DSLF model.63 The DSLF model was defined as Eq. (1): 𝑞=𝑞

(1)

+𝑞

where q represents the equilibrium adsorbed amount (mmol/g) at gas pressure p (kPa); q1 and q2 are the saturation capacities of sites 1 and 2, respectively (mmol/g); k1 and k2 represents the affinity coefficients of sites 1 and 2 (1/kPa), respectively; 1/m and 1/n present the ideal homogeneous surface deviations. Table S2 and S3 list the fitting parameters. It was observed that the regression coefficients R2 were > 0.9980, suggesting that the DSLF model could fit well CO2, CH4 and N2 isotherms on PCN-222 at low and high pressure. To further investigate the separation performance of PCN-222 towards CO2/CH4 and CO2/N2, we combined IAST calculation24,64 with DSLF model to calculate CO2/CH4 and CO2/N2 adsorption selectivities. Figure 6 exhibits IAST-predicted selectivities for CO2/CH4 mixtures (10:90 or 50:50, v/v) and CO2/N2 mixtures (15:85 or 50:50, v/v) on PCN-222 at 298 K under low pressure. It was seen that the CO2/CH4 and CO2/N2 adsorption selectivities decreased sharply with the increase of pressure in the low pressure region followed by



no notable change as the pressure kept increasing. It was also noted that the change in the proportion of CO2/CH4 and CO2/N2 had negligible influence on CO2/CH4 and CO2/N2 adsorption selectivities. The CO2/CH4 adsorption selectivity was in the range of 4.3-8.7. The CO2/CH4 adsorption selectivity of PCN-222 at 298 K and 100 kPa is comparable to those of Al(OH)(NDC) (4.4),65 ZJU-8a (5.4),66 and MOF-508b (3).67 Furthermore, the adsorption selectivity of CO2/N2 mixture (50:50, v/v) for PCN-222 at 298 K and 100 kPa was up to 73.7, which exceeded many other reported porous materials such as Zeolite Na-4A (17.8),27 Zeolite 13X (17.2),49 Al(OH)(NDC) (13),65 PCN-88 (5.3),68 and MOF-508b (19),67 as shown in Table S1.

Figure 6. IAST-predicted selectivities for (a) CO2/CH4 mixtures (10:90 or 50:50, v/v) and (b) CO2/N2 mixtures (15:85 or 50:50, v/v) on PCN-222 at 298 K under low pressure.

Figure 7 presents IAST-predicted selectivities for CO2/CH4 mixtures (10:90 or 50:50, v/v) and CO2/N2 mixtures (15:85 or 50:50, v/v) on PCN-222 at 298 K under high pressure. It showed that the CO2/CH4 and CO2/N2 adsorption selectivities increased with the increase of pressure. The CO2/CH4 adsorption selectivity for CO2/CH4 mixtures (10:90) and CO2/CH4 mixtures (50:50) at 3000 kPa separately were 3.9 and 4.7. In addition, for binary CO2/N2 mixture, the adsorption selectivity at 3000 kPa was 13.0 for CO2/N2 mixtures (15:85) and 32.8 for CO2/N2 mixtures (50:50). In comparison with other reported porous materials, as listed in Table 1, the CO2/CH4 and CO2/N2 adsorption selectivities of PCN-222 were at relatively high level. This was because nitrogen-donor porphyrin ligands (H2TCPP) in PCN-222 presented many basic adsorption sites which could selectively combine with CO2.69


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Figure 7. IAST-predicted selectivities for (a) CO2/CH4 mixtures (10:90 or 50:50, v/v) and (b) CO2/N2 mixtures (15:85 or 50:50, v/v) on PCN-222 at 298 K under high pressure.

3.5. Isosteric Heat of Adsorption To evaluate the interactions between the framework of PCN-222 and these gas molecules (CO2, CH4 and N2), the isosteric heat of adsorption Qst was calculated. In detail, Qst was obtained by fitting high-pressure adsorption isotherms of CO2, CH4 and N2 at 288, 298 and 308 K with the Virial equation16 Eq. (2). Then, the obtained fitting parameters were employed to calculate Qst by using Eq. (3). ln(𝑝) = ln(𝑁) + 𝑄 = −𝑅 ∑

∑

𝑎 𝑁 +∑

𝑏 𝑁

(2) (3)

𝑎 𝑁

where p and N represent the pressure (Torr) and the quantity adsorbed (mg/g), T refers to the temperature (K), ai and bj represent empirical parameters, m and n represent the number of coefficients required to give a good fit to the isotherms, R the is ideal gas constant (J·K-1·mol-1). Figures S8-S10 present the experimental and Virial fitting curves of CO2, CH4, N2 isotherms of PCN-222 at three different temperatures as well as fitting parameters. The correlation coefficients R2 were up to 0.98, indicating that the Qst values calculated by the Virial method were reliable. Figure 8 shows the Qst of CO2, CH4 and N2 adsorption for PCN-222. The Qst values of CO2, CH4 and N2 followed the order CO2 > CH4 > N2. It implied that the interactions between the framework of PCN-222 and these gas molecules followed



the order: CO2 > CH4 > N2.70 In addition, it was noticed that the Qst of CO2 adsorption at different CO2 loading were about 18.3 kJ/mol. The relative low Qst value suggested that the regeneration process of this adsorbent would consume less energy,71 compared with most other reported porous materials, as shown in Table 1 and S1. It was also noted that the Qst values of CO2 remained approximately constant as the CO2 loading increased, indicating that adsorption sites for CO2 on PCN-222 were uniform distribution. However, Qst of CH4 and N2 adsorption decreased slightly with increasing adsorbed amounts. This was because the binding sites of the host framework for CH4 and N2 adsorption was energetically heterogeneity.4

Figure 8. The isosteric heats of adsorption of CO2, CH4 and N2 over PCN-222

3.6. Breakthrough Curves of CO2, CH4 and N2 Gas Mixtures on PCN-222 To investigate the feasibility of applying PCN-222 to dynamically separate CO2/CH4 and CO2/N2 gas mixtures, the breakthrough experiments of binary CO2/CH4 (50:50 and 10:90) and CO2/N2 (50:50 and 15:85) gas mixtures were carried out at 298 K. Figure 9 shows that PCN-222 could effectively separate CO2/CH4 (both 50:50 and 10:90) and CO2/N2 (both 50:50 and 15:85) gas mixtures. It was visible that the breakthrough time of CH4 and N2 was shorter than that of CO2, suggesting that CO2 was preferentially adsorbed. Comparing Figure 9a and 9b, we could observe that the breakthrough time of CO2 for CO2/CH4 (50:50) was shorter than that of CO2 for CO2/CH4 (10:90). This was because the higher CO2 concentration in CO2/CH4 (50:50) meant that CO2 adsorption needed less time to reach saturation, resulting in a shorter


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breakthrough time of CO2 for CO2/CH4 (50:50). Similarly, Figure 9c and 9d showed that the breakthrough time of CO2 for CO2/N2 (50:50) was shorter than that of CO2 for CO2/N2 (15:85). Furthermore, small roll-ups were noticed from the breakthrough curves of CH4 and N2. These roll-ups were due to the competitive adsorption between CO2 and CH4 or N2, resulting in initially adsorbed CH4 or N2 partially replaced by CO2.

Figure 9. Breakthrough curves of binary mixtures (a) CO2/CH4 (50:50, v/v), (b) CO2/CH4 (10:90, v/v), (c) CO2/N2 (50:50, v/v), and (d) CO2/N2 (15:85, v/v).

3.7. Regeneration of Multi-Cycle Adsorption-Desorption on PCN-222 In regard to practical application, the adsorbent is required to maintain its CO2 adsorption capacity after multiple cycles of adsorption-desorption. Hence, we conducted the dynamic regeneration experiments using CO2/N2 (50:50, v/v) mixtures. For each adsorption process, CO2/N2 (50:50, v/v) mixtures flowed through a packed column of PCN-222 with a flow of 0.5 mL/min at 298 K. For each desorption process, the adsorbed sample was purged with a helium flow of 40 mL/min until no signal of



Page 16 of 34

CO2 and N2 was detected at the outlet of the packed column. It was worth mentioning that the desorption process could be completed within 10 min at 298 K. Such regeneration condition makes it more feasible that PCN-222 is used for the industrial separation

of

CO2/CH4

and

CO2/N2.

Figure

10

shows

multi-cycle

adsorption–desorption curves for PCN-222 adsorbed with CO2/N2 (50:50, v/v) mixtures. It was noted that the breakthrough curve nearly maintained unchanged after five cycles, implying an excellent regeneration performance of PCN-222.

Figure 10. Multi-cycle adsorption–desorption curves for PCN-222 adsorbed with CO2/N2 (50:50, v/v) mixtures.

4. Conclusions In summary, zirconium–porphyrin based PCN-222 exhibited good thermal and water stability. In detail, PCN-222 maintains stability up to 637 K, even being immersed in water for 24 h. Adsorption isotherms of CO2 at low and high pressure showed that PCN-222 had moderated CO2 adsorption uptake at low pressure and relatively high CO2 adsorption uptake at high pressure. Notably, this material had high CO2/CH4 and CO2/N2 adsorption selectivities at low and high pressure. Breakthrough experiments verified that PCN-222 could effectively separate CO2/CH4 and CO2/N2 mixtures. Besides, the relative low isosteric heat of adsorption (18.3 kJ/mol) of CO2 adsorption for PCN-222 suggested that the regeneration process of this adsorbent would consume less energy, compared to many other reported adsorbents. Multi-cycle adsorption–desorption experiments showed that the CO2 adsorbed by the framework could be completely desorbed within 10 min at 298 K under a helium flow of 40


Page 17 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


mL/min.

More importantly,

PCN-222 exhibited

an excellent

regeneration

performance. These results suggested that PCN-222 had great potential of adsorption-based CO2/CH4 and CO2/N2 industrial separation. ASSOCIATED CONTENT Supporting Information The self assembly setup for breakthrough experiments; N2 isotherms at 77 K; plot of the linear region on the N2 isotherm for the BET equation; TG curve; N2 isotherms of PCN-222 after being immersed in water; PXRD patterns of UiO-66 after being immersed in water; N2 isotherms of UiO-66 after being immersed in water; adsorption performance on the selected adsorbents at 100 kPa; fitting parameters of DSLF model for CO2, N2, and CH4 adsorption at low and high pressures; Virial fitting of CO2 isotherms at three different temperatures. AUTHOR INFORMATION Corresponding Authors *

E-mail: [email protected](Q.X.).

*

E-mail: [email protected] (H.X.).

ORCID Xin Zhou: 0000-0002-4317-7354 Qibin Xia: 0000-0002-8563-6715 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS We gratefully acknowledge the research grants provided by the Guangdong Natural Science Foundation (No. 2017A030313052), and the National Natural Science Foundation of China (Nos. 21576092, 21576094 and 21436005). REFERENCES (1) Zhang, Z.; Yao, Z.-Z.; Xiang, S.; Chen, B. Perspective of microporous metal–organic frameworks for CO2 capture and separation. Energy Environ. Sci. 2014, 7, 2868-2899. (2) Snoeckx, R.; Bogaerts, A. Plasma technology - a novel solution for CO2 conversion? Chem. Soc. Rev. 2017, 46, 5805-5863.



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Abstract Graphic



Graphic Abstract 38x17mm (300 x 300 DPI)


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Figure 1 63x50mm (300 x 300 DPI)





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Figure 4 100x76mm (300 x 300 DPI)


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Figure 5 111x81mm (300 x 300 DPI)





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Figure 9 115x89mm (300 x 300 DPI)



Figure 10 60x45mm (300 x 300 DPI)


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N2 by a Water

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