CH4 Selectivity with

Article Cite This: J. Chem. Eng. Data 2018, 63, 1657−1662

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Improving CO2 Adsorption Capacity and CO2/CH4 Selectivity with Amine Functionalization of MIL-100 and MIL-101 Majideh Babaei,† Samira Salehi,‡ Mansoor Anbia,*,‡ and Maryam Kazemipour† †

Department of Chemistry, Kerman Branch, Islamic Azad University, Kerman 7635131167, Iran Research Laboratory of Nanoporous Materials, Faculty of Chemistry, Iran University of Science and Technology, Narmak, Tehran 16846-13114, Iran

‡

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S Supporting Information *

ABSTRACT: In the present study, we have compared the adsorption behavior of as-prepared and amine-functionalized MIL-100 and MIL-101 in the selective adsorption process. The synthesized adsorbents by the hydrothermal method were investigated by the different methods of analysis and identification. The volumetric method was used to investigate CO2 and CH4 adsorption at the different temperatures and P < 10 bar. Also, the selectivity of CO2 to CH4 was studied. The surface area and pore volume of MIL-101 were higher than those of MIL-100; consequently, the CO2 adsorption capacity of MIL-101 was higher than that of MIL-100. Although the surface area and pore volume of MIL-100 and MIL-101 significantly decreased after amine functionalization, the adsorbents showed the enhanced CO2 adsorption capacity at ambient temperature. The amine-modified adsorbents displayed ultrahigh selectivity for CO2 over CH4. Also, an increase in the temperature decreased the adsorption capacity of CO2. According to the results, modified MIL-101 can be a promising adsorbent for the purification and separation of gases. in comparison to conventional materials.28 MIL-101 and MIL100 contain both mesopores and micropores and display excellent thermal stability, high porosity, chemical stability, and high resistance to moisture.29−34 They are the promising adsorbents among the many MOFs that have been investigated to date. Llewellyn et al.28 have studied the adsorption capacity of CO2 for MIL-101 and MIL-100. These MOFs showed high CO2 capacities of separately 40 and 18 mmol/g at 303 K and 50 bar. Xian et al.35 studied the CO2 adsorption on MIL-100(Fe) and MIL-101(Cr) and also the effect of the water vapor on the adsorption capacity. The results showed that the CO 2 adsorption capacity and CO2/CH4 selectivity of MIL-100(Fe) were enhanced in the presence of water vapor. Munusamy et al.36 examined the separation properties of MIL-101(Cr). The adsorption capacity of pure gases CO2, N2, CH4, and CO on MIL-101 (both powdery and granular forms) was measured at different temperatures and up to 850 mmHg. The selectivity of CO2 over N2 (12.6) at 303 K was more than that for CO2 over CH4 (5.69) and CO (2.90). The adsorption capacity of CO2 and CH4 on HKUST-1 and MIL-101 using GCMC simulation was studied by H. W. B. Teo.37 The simulation results were compared with experimental results, and it was found that the simulation results were consistent with experimental results (5−10% error range). Ye et al.38 described the comparative

1. INTRODUCTION Carbon dioxide as a result of fossil fuels combustion is the main greenhouse gas. The removal of CO2 from the flue gases is critical for reduction of its effect on global warming and climate change. Also, in recent years, for reduction of CO2 emissions, use of natural gas as an energy source and fuel has reached its highest level. The main component of natural gas is methane with variable amounts of impurities such as water, CO2, N2, and other impurities.1 In order to prevent pipeline and equipment corrosion and increase the heat value of natural gas, removal of CO2 from methane is important. Adsorption based techniques play an important role in the separation and purification of gas mixtures.2,3 The diverse porous materials such as activated carbons, alumina, and zeolites have been used as adsorbents in the adsorption processes.4−9 In the last two decades, research and investigation to synthesize and develop the metal organic frameworks (MOFs) as one new class of porous materials have been carried out.10−12 MOFs due to their unique properties, such as extremely high surface areas, ultrahigh porosity, tunable pore size, and the possibility of functionality, have been investigated for the application in gas separation and storage.13−20 The surface modification of MOFs improves the selective adsorption performance. The coordinative unsaturated metal sites in MOFs interact with gases. These sites can graft with organic groups such as amines.21−27 The two kinds of Material Institute Lavoisier (MIL) series, MIL-101 and MIL-100, exhibited high gas adsorption capacities © 2018 American Chemical Society

Received: January 5, 2018 Accepted: March 30, 2018 Published: April 6, 2018 1657

DOI: 10.1021/acs.jced.8b00014 J. Chem. Eng. Data 2018, 63, 1657−1662

Journal of Chemical & Engineering Data

Article

powder was filtrated, washed with ethanol, and dried at 343 K in a vacuum for 24 h. 2.3. Amine Functionalization of Adsorbents. The amine-functionalized MIL-100 and MIL-101 were prepared according to the following process. First, the PPD solution with a concentration of 10 wt % was prepared in ethanol. A total of 1 g of the adsorbents was added to the PPD solution and then refluxed at 373 K for 12 h. Eventually, the resulting samples were filtrated, washed by ethanol, and then dried at room temperature. The products were denoted as MIL-100/PPD and MIL-101/PPD. 2.4. Characterization. Nitrogen adsorption−desorption isotherms of the samples were measured at 77 K using a Micromeritics ASAP 2010 analyzer. The specific surface areas and the pore size were calculated by the Brunauer−Emmett− Teller (BET) and Barrett−Joyner−Halenda (BJH) methods, respectively. The XRD patterns of the adsorbents were obtained with a powder X-ray diffractometer (Philips PW 1830 X-ray diffraction) with a Cu Kα radiation source. The FTIR spectra of the adsorbents were obtained at ambient temperature by a DIGILAB FTS 7000 spectrometer. The morphology of the adsorbents was examined by SEM (KYKYEM3200). 2.5. Gas Adsorption Measurement. To investigate the gas adsorption capacity of the adsorbents, a laboratory setup based on the volumetric method was used (Figure 1). In the

investigation of CO2 capture by using HKUST-1 and MIL101(Cr). The results reveal that at high pressure and low temperature the CO2 adsorption capacity of these adsorbents improves. The CO2 adsorption capacities for HKUST-1 and MIL-101(Cr) at 30 °C and 10 bar were 7.19 and 8.07 mmol/g, respectively. Li et al.39 compared CO2, CH4, and N2 adsorption capacities for the M/DOBDC frameworks with activated carbon and MIL-100(Cr). The M/DOBDC materials showed a higher capacity and selectivity for CO2 and CH4; therefore, they are suitable for gas separation. Lin et al. reported a CO2 adsorption capacity of 15 mmol/g at 16 °C on aminefunctionalized MIL-101.22 The polyethylenimine (PEI) functionalized MIL-101 have been utilized by Lin et al. for selective CO2 capture. The PEI/MIL-101 at low pressures exhibited enhanced CO2 adsorption capacity.24 Yan investigated the adsorption behavior of CO2 and CH4 on amine functionalized MIL-101. The CO2 adsorption capacity of MIL-101(Cr) was enhanced after amine modification.26 In order to improve the CO2 selective adsorption capacity from the CO2/N2 mixture, Lin et al. functionalized MIL-101 with polyamine.40 As far as we know, no direct comparison of the adsorption behavior of MIL-100 and MIL-101 and amine functionalized MIL-100 and MIL-101 adsorbents in the selective adsorption process has been reported. In this work, the adsorption capacity of CO2 and CH4 on amine-functionalized MIL-100 and MIL101 adsorbents comparatively was investigated. MIL-100 and MIL-101 were synthesized via hydrothermal reaction. Then, pphenylenediamine (PPD) was grafted to these MILs and the resulting samples were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) analysis, and Brunauer− Emmett−Teller (BET). At final, the volumetric method was used for measurement of gas adsorption capacities at different temperatures. Also, the effect of amine functionalization on the improvement of the separation of the CO2/CH4 mixture and the adsorption/desorption behavior of CO2 on MIL-101/PPD was studied.

2. EXPERIMENTAL METHODS 2.1. Materials. All materials such as chromium nitrate nonahydrate (Cr(NO3)3·9H2O), 1,4-benzenedicarboxylic acid (terephthalic acid), fluorhydric acid 38−40% (HF), benzene1,3,5-tricarboxylate (H3BTC), chromium trioxide (CrO3), ethanol, and PPD were obtained from E. Merck (Germany). These materials without purification were utilized. N,NDimethylformamide (DMF, 99%) was purchased from Fluka Co. 2.2. Synthesis of MIL-100 and MIL-101. The hydrothermal method was used for synthesis of MIL-100, as described previously.41 Briefly, CrO3 (2.5 mmol, 250 mg), H3BTC (2.5 mmol, 525 mg), HF (2.5 mmol, 100 mL), and H2O (666.7 mmol, 12 mL) were mixed and stirred at ambient temperature for a few minutes. Then, the prepared mixture was poured into a Teflon-lined autoclave and then placed in an oven at 220 °C for 4 days. The resulting green solid was washed by acetone and deionized water and then dried at ambient temperature under an air atmosphere. MIL-101 was synthesized by hydrothermal reaction between Cr(NO3)3·9H2O (3.76 g) with terephthalic acid (6 g), HF (0.75 mL), and H2O (75 mL) at 493 K for 8 h.42 The resulting green powder was filtered and dried at 343 K in a vacuum for 12 h. This powder was dissolved in the DMF solution to remove the unreacted 1,4-benzenedicarboxylic acid. MIL-101

Figure 1. Setup for the adsorption capacity test.

beginning, some of the adsorbent was poured into the adsorption reactor and attached to the system. For degassing the system, the valves 6, 7, 8, and 9 were opened and the rest of the valves were closed, and then, the system was vacuumed by the vacuum pump at 120 °C for 1.5 h. The temperature of the adsorption system was decreased to the desired temperature after degassing. The adsorption test was carried out by opening valves 1, 3, 5, 6, 7, and 8 and closing the rest of the valves. The pressure decreased during the adsorption process due to the gas adsorption and some dead volumes in the reactor. The pressure reduction relevant to the gas adsorption can be calculated by measuring the dead volumes via helium test. The CO2, CH4, and N2 with 99.99% purity were used for the experiment.

3. RESULTS AND DISCUSSION 3.1. Adsorbent Characterization. Figures S1 and S2 show the powder X-ray diffraction patterns of samples. As is clear from these figures, the synthesized adsorbents are well crystalline and the peak positions are the same as those of the MIL-10043 and MIL-10144 reported previously. XRD patterns of MIL-100 and MIL-101 did not change after amine modification, indicating that the crystalline structure of 1658



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The N2 adsorption/desorption isotherms of MIL-100 and MIL-101 at 77 K before and after loading PPD are the type-IV isotherm, indicating that MIL-100 and MIL-101 are the mesoporous compounds (Figures S5 and S6). As seen from these figures, the N2 uptakes of the adsorbents after loading PPD are decreased because their pores were occupied by amine. The pore characteristics of adsorbents are listed in Table 1. The results show that the surface area and pore volume of

these adsorbents was maintained after incorporation of PPD. However, the peak intensity slightly decreased after incorporation of amine. The filling of the adsorbent pores by amines decreases the peak intensity.42,45,46 Figures S3 and S4 present the FT-IR spectra of the asprepared and amine-functionalized MIL-100 and MIL-101, respectively. The peaks at 1450−1600 cm−1 can be assigned to the aromatic rings (Figure S3). Also, the peaks at 523 and 568 cm−1 can be attributed to the Cr−O bonds of the MOF compound. The bands at 3423, 1386, and 1450 cm−1 are assigned to tricarboxylate coordination within the MIL-100 framework. The broad peak at 3200−3400 cm−1 and the peak at 2950 cm−1 confirm incorporation of the amine group into the MIL-100 structure. As shown in Figure S4, a peak at 576 cm−1 appears which is associated with CrO vibrations and demonstrates the formation of the MOF. In the spectrum of MIL-101, peaks in the range 1400−1600 cm−1 are attributed to the stretching vibrations of the CC bond of the benzene ring of the organic ligand. The peak at 1560−1650 cm−1 is due to the vibration of the CO group. The incorporation of PPD into the MIL-101 framework is confirmed by the broad peak at 3200−3400 cm−1 that is due to the NH stretching vibrations of the NH group. The peaks between 2900 and 3400 cm−1 that are due to the ν(NH) and ν(CH) stretching vibrations confirm the incorporation of PPD into the MIL-101 framework.24 The morphologies of MIL-100 and MIL-101 before and after loading PPD were obtained by SEM. Figures 2 and 3 present SEM images of pristine and amine-functionalized adsorbents. Results show that the morphologies of the adsorbents after loading the PPD remained undamaged.

Table 1. Structural Properties of the Adsorbents at 77 K average pore size (nm)

VP (cm3/g)

SBET (m2/g)

adsorbent

3.5 2.8 2.9 2.3

1.72 1.10 1.14 0.59

1420 1007 1170 820

MIL-101 MIL-101/PPD MIL-100 MIL-100/PPD

amine-modified adsorbents also decrease, indicating the occupation of the channels in MIL-100 and MIL-101 by PPD. Therefore, the N2 uptakes, the BET surface areas, and the pore volume of MIL-101 are higher compared to those of MIL100. Figures S7 and S8 show the pore size distribution of MIL100 and MIL-101 and amine-modified adsorbents, respectively. 3.2. Adsorption Capacity and Selectivity of Adsorbents. Figure 4 shows the adsorption behaviors of CO2 and CH4

Figure 4. Adsorption isotherms of CO2 and CH4 on as-prepared and amine-functionalized MIL-100 and MIL-101 at 298 K.

on the as-prepared and amine-functionalized of MIL-100 and MIL-101 at 298 K. As expected, the CO2 adsorption capacity of MIL-101 was higher than that of MIL-100 because of the higher pore volume and surface area of MIL-101 compared to MIL-100. However, the amine-functionalized adsorbents display higher CO2 adsorption capacities than the as-prepared adsorbents that could be for the reaction between the CO2 molecules with the surface amine groups, whereas the CH4 adsorption capacities are decreased after amine functionalization. The reaction between the amine-modified adsorbents and the CO2 gas is formation of the carbamate, reported in many literatures:47−49

Figure 2. SEM images of (a) MIL-100 and (b) MIL-100/PPD.

CO2 + 2RNH 2 ↔ RNH3+ + RNHCOO−

(1)

CO2 + 2R 2NH ↔ R 2NH 2+ + R 2NCOO−

(2)

The CO2 adsorption capacities of MIL-100, MIL-101, MIL100/PPD, and MIL-101/PPD at 298 K and 10 bar are 3.5, 6.1, 4.6, and 7.3 mmol/g, respectively. In order to study the effect of temperature on the adsorption capacity of adsorbents, the adsorption process was carried out at three temperatures and their results were shown in Table 2. It can be clearly seen that the gas adsorption capacity decreased with the increase in temperature, which shows the exothermic nature of the adsorption process and the decrease in chemical

Figure 3. SEM images of (a) MIL-101 and (b) MIL-101/PPD.

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CH4 adsorption mechanism on the amine-modified adsorbents is physisorption, whereas the mechanism of CO2 adsorption on amine-modified adsorbents is a hybrid mechanism of chemisorption and physisorption. The higher adsorption capacity of CO2 than that of CH4 in the amine-modified adsorbents results in a higher selectivity of CO2/CH4. Also, the results show higher selectivity at lower pressures; therefore, amine-functionalized MIL-100 and MIL-101 can be the promising candidates for separation and purification of CO2 from CO2/CH4 mixtures at atmospheric pressure and ambient temperature. 3.3. Cyclic CO2 Adsorption. Absorbents in addition to high absorption capacity and selectivity should have high stability at various cycles of adsorption and desorption. The cyclical CO2 adsorption/desorption on MIL-101/PPD at 298 K and 1 bar was carried out. The desorption temperature of 373 K was selected in the cyclic CO2 adsorption. The CO2 adsorption capacities at various cycles of adsorption and desorption for MIL-101/PPD were shown in Figure 7. It is

Table 2. Effect of Temperature on CO2 Adsorption in AsPrepared and Amine-Functionalized MIL-100 and MIL-101 CO2 adsorption capacity (mmolCO2/gadsorbent) adsorbent

temperature (K)

1 (bar)

5 (bar)

10 (bar)

MIL-100/PPD

298 323 348 298 323 348

1.3 0.9 0.6 1.7 1.1 0.8

3.5 2.4 1.8 5.5 4.2 3.1

4.6 3.4 2.6 7.3 5.8 4.7

MIL-101/PPD

interactions between CO2 molecules and amino groups of the adsorbent surface.50−52 In adsorption and separation processes, the high adsorption capacity and selectivity of adsorbent are requirements. The gas adsorption isotherms were used to calculate the pure component selectivity of the gases by the following equation53 q α1/2 = 1 q2 (TP)

(3)

where α1/2 is the adsorption selectivity of gas 1 over gas 2 and q is the adsorption capacity of gases in mmol/g at a certain pressure (P) and temperature (T). Figures 5 and 6 show the adsorption selectivity of CO2/CH4 on as-prepared and amine-functionalized MIL-100 and MIL-

Figure 7. CO2 adsorption capacity as a function of adsorption cycle on MIL-101/PPD.

seen that the CO2 adsorption capacity decreased from 1.7 to 1.56 mmol/g after the first cycle of operation and then was constant during the eight cycles of adsorption and desorption. The results indicate that amine-modified MIL-101 can be employed in cyclic CO2 adsorption operation.

Figure 5. Adsorption selectivity of CO2/CH4 on MIL-100 and MIL100/PPD at different pressures and 298 K.

4. CONCLUSION The synthesized MIL-100 and MIL-101 adsorbents have been modified with PPD and characterized. The pore volume and surface area of adsorbents after amine modification decrease. The CO2 adsorption capacity of MIL-101 was higher than that of MIL-100 because of the higher surface area and pore volume of MIL-101 compared to those of MIL-100. CO2 adsorption capacities of MIL-100 and MIL-101 at 10 bar and 298 K are 3.5 and 6.1 mmol/g, respectively. The amine modification of adsorbents improved significantly the CO2 adsorption capacity, whereas CH4 adsorption capacity increased slightly, consequently adsorption selectivity CO2/CH4 enhanced. The effect of temperature on the adsorption capacity of adsorbents was studied, and the results showed that the gas adsorption capacity decreased with the increase in temperature, which shows the adsorption is exothermic. According to the results of this study, amine-modified MIL-101 can be a promising adsorbent for the purification and separation of gases in the industrial applications.

Figure 6. Adsorption selectivities of CO2/CH4 on MIL-101 and MIL101/PPD at different pressures and 298 K.

101 at different pressures and 298 K. The results indicate that the adsorption selectivity of the MIL-101 for CO2/CH4 is higher than that of MIL-100. As seen in both figures, the adsorption selectivity of CO2/CH4 on the two samples has significantly improved after amine modification. The dominant 1660



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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00014. The characterization figures, including XRD patterns of MIL-100 and MIL-100/PPD, XRD patterns of MIL-101 and MIL-101/PPD, FTIR spectra of MIL-100 and MIL100/PPD, FTIR spectra of MIL-101 and MIL-101/PPD, adsorption−desorption isotherms of nitrogen at 77 K on MIL-100 and MIL-100/PPD samples, adsorption− desorption isotherms of nitrogen at 77 K on MIL-101 and MIL-101/PPD samples, PSD of the MIL-100 and MIL-100/PPD, and PSD of the MIL-101 and MIL-101/ PPD (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +98 21 77240516. Fax: +98 21 77491204. ORCID

Mansoor Anbia: 0000-0002-0180-5244 Funding

The authors are grateful to the Research Council of Iran University of Science and Technology (Tehran) and Islamic Azad University, Kerman Branch for the financial support of this project. Notes

The authors declare no competing financial interest.

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