Ethylenediamine grafting to functionalized NH2-UiO-66 using green

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Ethylenediamine grafting to functionalized NH2-UiO-66 using green azaMichael addition reaction for improving CO2/CH4 adsorption selectivity Hossein Molavi, Farhad Ahmadi Joukani, and Akbar Shojaei Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00372 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 27, 2018

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Industrial & Engineering Chemistry Research

Ethylenediamine grafting to functionalized NH2-UiO-66 using green azaMichael addition reaction to improve CO2/CH4 adsorption selectivity Hossein Molavi1, Farhad Ahmadi Joukani2, Akbar Shojaei*1,2 1

Institute for Nanoscience and Nanotechnology (INST), Sharif University of Technology, P.O. Box 11155-8639, Tehran, Iran.

2

Department of Chemical and Petroleum Engineering, Sharif University of Technology, PO Box 11155-9465, Tehran, Iran. Corresponding author: Akbar Shojaei, Email: [email protected]

Abstract Three versions of zirconium-based metal organic framework, NH2-UiO-66, GMA-UiO-66 and EDA-UiO-66, were synthesized and employed as adsorbent for CO2/CH4 separation. GMA-UiO-66 was synthesized via ring opening reaction between the amine species in the framework and epoxy groups in glycidyl methacrylate (GMA), while the green aza-Michael addition reaction was used for the first time to functionalize GMA-UiO-66 with ethylenediamine (EDA). The products were characterized by BET, XRD, TGA, FESEM, ICP-OES, 1H NMR, mass spectroscopy and FTIR-ATR methods to monitor their textural properties before and after functionalization. The results indicated that GMA was

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successfully grafted to the NH2-UiO-66 framework, and most of the alkene groups were grafted with EDA, which indicated that the Michael addition reaction was very effective. The experimental gas adsorption results indicated that the EDA-grafted UiO-66 possessed the highest CO2 adsorption capacity as well as CO2/CH4 selectivity due to the introduction of alkaline nitrogen groups on the surface, which could form strong quadrupole-dipole interactions as well as the chemical reaction with CO2. EDA-grafted UiO-66 exhibited a 53% increase in CO2 adsorption capacity and a 95% increase in CO2/CH4 adsorption selectivity at 298 K and 5 bar relative to the untreated NH2-UiO-66. Moreover, EDA-UiO66 exhibited excellent stability in the cyclic CO2 adsorption-desorption runs, which indicated that no EDA leaching had occurred during this process.

Keywords: Metal organic framework; Gas adsorption; NH2-UiO-66; Post synthetic modification; Aza-Michael addition.

1.

Introduction

Metal-organic frameworks (MOFs), also known as metal-organic porous materials (MOPMs), porous coordination polymer (PCPs), metal organic materials (MOMs), or porous coordination networks (PCNs), are rather a new class of highly crystalline and nanoporous materials which are constructed from metal ions or metal clusters nodes linked

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to each other by organic ligands to build –one, -two or –three dimensional structures1-4. Owing to their high specific surface area, low density, adjustable pore structure, high pore volume, well-controlled pore size distribution, tunable surface chemistry and high affinity for CO2 molecules, MOFs are promising materials for adsorption and separation of CO2 from industrial flue gas and landfill gas5-8. CO2 is the main anthropogenic greenhouse gas in the atmosphere and its average concentration in the atmosphere is currently around 400 ppm, and it is expected to reach 550 ppm by 2050 even if CO2 emission be stable for the next three decades6,9,10. Therefore, adsorption of CO2 and CH4 are two of the main important applications of MOFs mainly due to the high global warning effect of these two gases and the applications in natural gas purification11,12. In spite of many advantages, most of the reported MOFs have low hydrothermal stability, low mechanical stability, and vulnerability to organic solvents, water, oxygen, and other chemical compounds13-15. Therefore, to address the above issues, UiO-66-derived MOFs are considered to be most suitable candidate for CO2 adsorption due to their extraordinary water stability (more than one year), high thermal stability (up to 500°C), good chemical resistance toward polar solvents like HCl (pH = 1), aqueous NaOH (pH = 14), several alcohols, and organic solvents including acetone, benzene, dimethylformamide, and chloroform14,16-18, lower regeneration cost, and affinity toward CO2, have been investigated as sorbents9. Although, thermal and chemical stability of these MOFs are high, the adsorption capacity of them for CO2 is low. Functionalization of this MOF with polar groups such as NO2, NH2, OH, SO3H, COOH, 3



and OCH3 is one of the most promising techniques to increase CO2 adsorption capacity5,9,1921

. For example, Walton et al.19 reported adsorption properties of UiO-66 and four

functionalized variants of UiO-66 containing –NH2, -NO2, -OMe, and -1,4-Naphtyl. The results indicated that polar functional groups improve CO2/CH4 selectivity, with the –OMe functionalized derivative showing the best performance in room temperature while the amine functionalized MOF showing the best performance at 303 k. Recently, mixed functionalized groups based on UiO-66 have been studied for selective adsorption of CO2 against CH4. For instance, Rada et al.22 synthesized mixed functionalized UiO-66 for CO2 and CH4 adsorption. The results revealed that the CO2 adsorption capacity on UiO-6-(OH)2-NO2 and UiO-66-NO2-(OH)2 are 4.65 and 7.35 mmol/g, respectively, while the UiO-66-(OH)2 show the highest CO2/CH4 separation factor. The similar results was also reported by Kronast et al.23 when functionalized UiO-66 with various polar groups. The results exhibited

that

CO2 adsorption capacity of aminoalcohol

functionalized one is nearly four times higher than that of the pristine compound. Introduction of uncoordinated nitrogen atoms could increase adsorption capacity of CO2 due to the strong quadrupole-dipole interaction between them. Therefore, grafting of amine groups onto the surface of MOFs is another strategy to enhance CO2 adsorption capacity as well as selectivity24-27. Herein, we report an alternative approach to attach ethylenediamine covalently to a water stable MOF structure. For this goal, NH2-UiO-66 (Zr), was chosen as a parent MOF for post-synthetic modification with glycidyl methacrylate (GMA). GMA functionalized MOF 4


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(GMA-UiO-66) contains various functional groups, i.e. hydroxyl, ester, and alkene, and is able to functionalize with amine groups via aza-Michael addition reaction. Polyethylene glycol (PEG) as a non-toxic, inexpensive, simple and widely available polymer was chosen as recycle able solvent and catalyst for this green functionalization28. The steps of reactions and chemical structures of the final products were also shown in Scheme 1.

Scheme 1. Post-synthetic modification of NH2-UiO-66 with GMA (1) and EDA (2).

2.

Experimental

2.1. Materials All chemicals including DMF (Dimethylformamide 99%), 2-ATA (2-aminoterephthalic 5



acid 99%), EDA (ethylenediamine 98%), chloroform (99%), PEG (polyethylene glycol 400 g/mol), THF (tetrahydrofuran 99.5%), GMA (glycidyl methacrylate 97%), and ZrCl4 were supplied by Sigma–Aldrich and Merck and were used without further purification. 2.2. Synthesis of MOFs NH2-UiO-66 and GMA-UiO-66 were prepared according to the procedure reported in previous studies (see also supporting information)16,29-31. 2.3. Functionalization of GMA-UiO-66 GMA-UiO-66 nanoparticles (60 mg) were suspended in PEG (3 g) through sonication for 10 min, and then EDA (3 mmol) was added to the solution. The mixture was heated at 60°C for 24 h. After the completion of reaction, the solvent was decanted and the powder washed three times with chloroform under sonication for 10 min to remove unreacted amines. Fresh chloroform (10 mL) was added once a day for three days to rinse the amine grafted GMA-UiO-66 (EDA-UiO-66) crystals free of any excess amines. Finally, the yellow nanoparticles were dried at 50°C for 24 h under vacuum. 2.4. Characterization Fourier transform infrared (FTIR) spectra (Spectrum 100-FT-IR Spectrometer, PerkinElmer) was obtained to evaluate functionalization of NH2-UiO-66 with GMA and EDA. The spectrum was scanned from 500 to 4000 cm-1 with a wavenumber resolution of 4 cm-1. 1

H NMR spectroscopy was performed on a Bruker model AVANCE DPX 500 MHz. In a

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typical experiment approximately 10 mg of NH2-UiO-66, GMA-UiO-66 and EDA-UiO-66 were dried under vacuum at 100°C and digested by sonication in 600 µL of DMSO-d6 and 20 µL of HF (40% aqueous solution). Mass spectroscopy was also used to confirm the functionalization by using a mass spectrometer (Agilent 5975C VL MSD with Triple-Axis Detector). Typically, approximately 10 mg of MOFs were digested by sonication in 0.5 mL of CH3CN and 10 µL of HF (40% aqueous solution). X-ray diffraction (XRD) patterns were obtained at room temperature using a X-pert Philips, pw 3040/60 diffractometer with accelerating voltage and current of 40 kV and 40 mA, respectively. Thermogravimetric analyses (TGA) were performed using a Perkin-Elmer Pyris thermogravimetric analyzer from ambient temperature to 800°C at a heating rate of 10°C/min under nitrogen atmosphere. Field emission scanning electron microscopy (FESEM – MIRA3 TESCAN) was used to determine the morphologies of the crystalline MOFs. Nitrogen physisorption measurements were carried out using a BELSORP-miniII at 70 K after a pretreatment at 150°C under vacuum for 8 h. The specific surface area, pore volume, and the pore size distribution were calculated according to the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods. Inductively coupled plasma-optical emission spectrometry (ICP-OES, Spectro Arcos) and TruSpecR CHN were used for determination of zirconium and CHN amount, respectively, due to their high sensitivity and high selectivity.

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2.5. Gas adsorption measurements The gas adsorption capacity of the MOFs was evaluated using volumetric method by an experimental apparatus setup in our laboratory. A schematic representation of the gas adsorption apparatus is given in Figure S1 (see the supporting information). The samples with approximate weights of 0.5g were used for the measurements. Before starting the adsorption measurements; the samples were activated under high vacuum at 100°C for 12 h. Then, the temperature increased to 200°C and held at this temperature for 2 h. Gas adsorption was carried out using ultra-high-purity gas in a pressure range of 0–20 bar at 273 and 298 K. The ideal adsorption selectivity for CO2 over CH4 was calculated from their adsorption isotherms according to the following relation16,32:

S=

(1)

where S is the ideal adsorption selectivity, qCO2 and qN2 are the adsorption capacities of CO2 and N2, respectively at any given pressure P and temperature T.

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3.

Results and discussion

3.1. Characterization of MOFs Figure 1 shows ATR-FTIR spectra of NH2-UiO-66, GMA-UiO-66 and EDA-UiO-66 activated by chloroform. The bands observed around 1400-1700 cm-1 are related to asymmetric and symmetric stretching vibration of carboxyl functional groups (-COO)16,17,22. The absorption bands of 3376 cm-1 and 3457 cm-1 are referred to the –NH2 stretching vibration of primary amines in the 2-aminoterephthalic acid. The small bands appeared around 2850-3050 cm-1 are assigned to the C-H stretching vibration of saturated and unsaturated carbons16,29,30. The amine absorption bands of NH2-UiO-66 at 3376 cm-1 and 3457 cm-1 disappear in GMA-UiO-66 and instead, absorption peaks are appeared around 3200-3600 cm-1, which could be due to the formation of hydrogen bond between amine and hydroxyl groups of GMA-UiO-66. The absorption band at 1732 cm-1 is related to the ester carbonyl stretching of GMA-UiO-66 which indicates that GMA is successfully attached to the NH2-UiO-66. As shown in Figure 1, some differences are observed between the spectra of GMA-UiO-66 and EDA-UiO-66. It can be seen that there are two sharp absorption bands around 33503500 cm-1, which are assigned to the secondary and primary amine groups of attached EDA. The strong bands observed at 2850-2950 cm-1 are assigned to the C-H stretching vibration of attached GMA and EDA. It can also be seen that the intensity of these peaks in the spectrum of EDA-UiO-66 is more in comparison with the spectra of NH2-UiO-66 and

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GMA-UiO-66, which indicates that EDA is successfully attached to the GMA-UiO-66.

Figure 1. ATR-FTIR spectra of all MOFs.

To confirm the chemical structure of modified MOFs nuclear magnetic resonance (NMR) spectroscopy was used and the results are depicted in Figure 2 and Figure S2. The 1H NMR spectrum of NH2-UiO-66 displays three resonance signals at 7.00, 7.37 and 7.75 ppm. These three resonance signals which are attributed to the 2-aminoterephthalate ligand are also observed in the spectra of modified MOFs. As seen in these spectra, some impurities are also observed in the aromatic region, which are associated with the 2aminoterephthalate ligand32,33. As can be inferred in this figure, some new signals are observed in the spectra of modified MOFs. Thus, in order to better recognize, the spectra of these modified samples are expanded and shown in Figure 2.

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After GMA are attached to the NH2-UiO-66 via ring-opening reaction, two special signals at 5.65 and 6.05 ppm for =CH2 of alkene groups are observed on the spectrum of GMAUiO-66. All the other resonance signals of GMA-UiO-66 are related to the corresponding hydrogen according to the chemical structure drawn in Figure 2. Appearance of these new peaks indicating that the NH2-UiO-66 is successfully connected to the epoxy ring of GMA16,31. The chemical shifts for the =CH2 groups of GMA-UiO-66, at 5.65 and 6.05 ppm, are disappeared after addition EDA to the α, β-unsaturated ester via Michael addition reaction. In addition, effective grafting of EDA onto the GMA-UiO-66 is confirmed by the peak of secondary and primary amines at 1.25 and -CH2- at 2.52 ppm of EDA, as shown on the spectrum of EDA-UiO-6634,35. All the other resonance signals of EDA-UiO-66 are related to the corresponding hydrogen according to the chemical structure drawn in Figure 2. These results indicate that all of the alkene groups of GMA-UiO-66 participate in Michael addition reaction and EDA is completely connected to the GMA-UiO-66 nanoparticles16,31. The samples are further investigated by Mass spectroscopy in order to investigate the molecular weight of modified samples, and the results are depicted in Figure S3.

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Figure 2. Expanded 1H NMR spectra of GMA-UiO-66 and EDA-UiO-66 in DMSO.

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The structural stability or changes in the crystallinity was examined by comparing XRD patterns of pristine NH2-UiO-66 and modified MOFs. As shown in Figure 3, all the characteristic diffraction peaks of the NH2-UiO-66 (2θ= 7.42, 8.62 and 25.82°) are well correspondent with the previously reported diffraction peaks in literature16,29,30. This result confirms that synthesis of NH2-UiO-66 is complete and there is not any impurity. As inferred from Figure 3, presence of GMA and EDA groups in the structure of NH2-UiO-66 does nearly not change the crystallinity of modified MOFs. In GMA modified samples the peak positions remain the same indicating no loss of crystallinity, while the peak positions of EDA-UiO-66 shift to lower degrees and two sharp peaks overlap with each other, but the overall crystallinity does not change. This observation might be due to lower crystallinity as well as the formation of an amorphous layer of EDA in the surface of GMA-UiO-66 nanoparticles. The other reason for this observation might be due to diffraction patterns of trapped solvent and unreacted reactants in the pores of modified MOF.

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Figure 3. XRD patterns of NH2-UiO-66, GMA-UiO-66 and EDA-UiO-66.

To examine the thermal stability of the MOFs, thermogravimetric analysis was used and the TGA and derivative TG (DTG) of pristine NH2-UiO-66 and modified samples are given in Figure 4. The TGA and DTG curves of the NH2-UiO-66 and GMA-UiO-66 show a threestep weight loss, which are in agreement with literature16,36,37, while the TGA and DTG curves of the EDA-UiO-66 show a four-step weight loss. The initial losses observed around 100°C are most likely owing to the removal of the physisorbed water molecules on the surface of MOF nanoparticles. It can be inferred from Figure 4a that the amount of the adsorbed water is reduced by functionalization with organic compounds. The second weight loss observed around 200ºC can be due to the loss or residual solvent (DMF) molecules. The third weight loss for NH2-UiO-66 is attributed to the decomposition of MOF to CO, CO2 and ZrO2, while there is a nearly continuous weight loss around 350600ºC with no specific steps for GMA-UiO-66 nanoparticles. This continuous weight loss probably corresponds to the removal of grafted GMA groups on the surface of NH2-UiO-66 and also decomposition of MOF frameworks16. The third weight loss for EDA-UiO-66 around 390ºC represents the removal of organic compounds (GMA and EDA) from the structure of MOF, and the final loss can be attributed to the decomposition/collapse of the framework, which is slightly overlapped with the third weight loss. Therefore, the decomposition temperature of this MOF is slightly shifted to the lower temperatures. Comparing TGA curves of pristine NH2-UiO-66 14


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with modified samples indicates that the final residue ash value of NH2-UiO-66 at temperatures higher than 600°C is higher than that of modified samples, mainly because of the presence of organic compounds in the structure of modified MOFs. Therefore, the difference between the residue ashes of these samples would be attributed to the extent of grafted organic compounds.

Figure 4. (a) TGA and (b) derivative TG (DTG) for all MOFs. The nitrogen isotherms and pore size distributions of NH2-UiO-66, GMA-UiO-66 and EDA-UiO-66 are presented in Figure S4. The shape of the N2 adsorption isotherms indicates the microporous structure of these MOFs. As shown in Table 1, the surface areas and pore volumes of modified MOFs are lower than those of the pristine NH2-UiO-66, which might be due to the partially blocking the pores as a consequence of introduction the bulky GMA and EDA groups on the NH2-UiO-66 nanoparticles. The lower surface areas 15



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and pore volumes of EDA-UiO-66 than those of pristine NH2-UiO-66 are the results of the large size of the organic compounds (GMA + EDA) used for functionalization. As given in Table 1, the Zr content determined by ICP-OES decreases while the nitrogen content first decreases and then increases due to the introduction of GMA and EDA on NH2-UiO-66 nanoparticles, respectively. Table 1. Physical properties and elemental analysis of MOFs.

Elemental analysis of MOFs (%)

Total pore volume MOFs

2

SBET (m /g)

(cm3/g)

C

H

N

Zra

NH2-UiO-66

1258

0.51

31.8

4.11

5.62

55.68

GMA-UiO-66

965

0.43

36.5

4.61

2.71

46.33

EDA-UiO-66

612

0.31

32.2

4.43

7.78

27.56

a

Zr contents were analyzed by ICP-OES.

In order to investigate the particle morphology of the samples, the FESEM analysis was further used and the images of the parent and functionalized samples are presented in Figure 5. In the case of NH2-UiO-66, the triangular base pyramid shape of the MOF nanoparticles is seen and the size distribution of them is quite uniform, and the particle size is about 100 nm. The FESEM image of GMA-UiO-66 nanoparticles show no change in the 16


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nanoparticle morphology, which confirms that NH2-UiO-66 is stable after functionalization with GMA. In the case of EDA-UiO-66 the tetrahedral and octahedral morphology of the crystals are visible in the FESEM images. The images of NH2-UiO-66 and GMA-UiO-66 exhibit the same surface roughness, while the edges of the EDA-UiO-66 crystals are less sharp. This change is due to the functionalization of GMA-UiO-66 with EDA and formed a thin shell around GMA-UiO-66 nanoparticles. Nevertheless, the overall morphology of the modified crystals is preserved. As a consequence of formation of a thin shell around the GMA-UiO-66 nanoparticles, limited agglomeration of EDA-UiO-66 nanoparticles is observed in the FESEM image.

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Figure 5. FESEM images of pristine NH2-UiO-66 and modified MOFs.

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3.2. Adsorption performance of MOFs Figure 6 displays CO2 and CH4 adsorption isotherms on NH2-UiO-66, GMA-UiO-66 and EDA-UiO-66 at 273 and 298 K. It is obvious that the adsorption capacity of both gases decreases by increasing the temperature due to the exothermic character of sorption22,38. The modified samples present a milder decrease in CO2 adsorption with temperature than pristine NH2-UiO-66, which could be due to the stronger interactions between this gas and the basic amine groups on the surface of modified samples. This trend is more sensitive in the case of EDA-UiO-66 due to the highest nitrogen content (see Table 1).

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Figure 6. CO2 and CH4 adsorption isotherms and CO2/CH4 adsorption selectivity on UiO66-derived MOFs at 273 and 298 K. 20


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The heat of adsorption for CO2 and CH4 were calculated using the Clausius-Clapeyron equation and the results are shown in Figure S522,39.

= ∆

(2)

where P and T are the pressure and temperature, respectively and R (8.314 J/mol k) is the universal gas constant. The heat of CO2 adsorption in all MOFs is higher than that of CH4 because of presence the amine groups, which could create an electric filed by changing the charge distribution inside the pores against more polarizable gases. Other possible results for this observation could be due to the chemical reaction between these amine groups and CO2 molecules. As can be inferred from Figure S5, the adsorption enthalpy for CH4 slightly increases when the adsorbent was modified with the bulkier compounds and results to smaller pore volume. Therefore, the adsorption enthalpy for CH4 in the functionalized adsorbents is governed by the pore dimensions, while the adsorption enthalpy for CO2 not only depends on the pore dimensions and accessible pore space but also on the polarity of the grafted functional groups. The similar trend was also reported by other researchers20,22. As shown in Figure 6, the CO2 adsorption capacity of both modified samples are found to be higher than that of pristine NH2-UiO-66, while the CH4 adsorption capacity increases and then decreases after functionalization the NH2-UiO-66 nanoparticles with GMA and EDA, respectively. This adsorption capacity trend is also in agreement with the previous report16. Additionally, it can be inferred that all the three samples show much higher CO2 adsorption capacity compared with CH4, which could be due to its high quadrupole

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moment, small molecular size, more polarizable, and the strong interaction with the polar groups in porous structures. In this case the polar functional groups increase the polarity of the organic ligands due to changing the charge distribution, which could indirectly enhance the CO2 adsorption capacity40-42. So, the adsorption capacity as well as the adsorption selectivity of CO2 over CH4 in the presence of NH2 groups increases due to the strong dipole-quadrupole interaction between them. In the case of GMA-UiO-66, accelerating the reversible chemical reaction between CO2 molecules and this amine group in presence of hydroxyl groups of the functionalized MOF nanoparticles. In this case, one mole of amino group in presence of hydroxyl groups reacts with one mole of CO2 molecule, while without the hydroxyl groups, two moles of amino groups react with one mole of CO2 molecule as recently reported16 (see Scheme 2). Moreover, as CO2 has a high quadrupole moment and polarizable property, it interacts through dispersion forces more significantly with the polarizable π- electron system in alkene groups16. Contrarily with CO2, CH4 as a nonreactive and nonquadrupole moment gas is adsorbed solely by physisorption mechanism. However, the adsorption capacity of CH4 increases after functionalization of the MOF with GMA, which might be due to the increase of the Vander Waals interaction at potential pocket state43,44. Therefore, the adsorption selectivity of CO2 against CH4 decreases in GMA-UiO-66 in comparison to NH2-UiO-66. Although the surface area of EDA-UiO-66 is the lowest, the highest CO2 adsorption capacity is observed for this sample due to the highest nitrogen content, which could 22


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increase the dipole-quadrupole interaction with CO2 and increase the amount of chemisorption CO245-47. The similar chemical reaction occurs in presence of EDA-UiO-66 as adsorbent, in which one mole of amino group in presence of hydroxyl groups reacts with one mole of CO2 molecule. Additionally, in the presence of EDA-UiO-66, the reaction between CO2 molecule and this modified MOF is expected to be dominated with the formation of an intramolecular carbamat. As shown in Scheme 2, in this case each grafted EDA can react with one mole of CO2 molecule. The similar observation was also reported by Feng Zheng et al. using EDA-SBA-15 as CO2 adsorbent45. Even though, the adsorption enthalpy for CH4 in EDA-UiO-66 is the highest value, the adsorption capacity of this adsorbent is lower than the other modified sample, which could be due to the partially blocking the pores leading to the lower accessible surface area, as corroborated by BET analysis. Thus, in this case the adsorption selectivity of CO2 over CH4 increases significantly.

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Scheme 2. Reaction of CO2 with amine groups in different UiO-66-dreived MOFs.

3.3. Adsorbent regeneration Although high CO2 adsorption capacity as well as high adsorption selectivity is essential for the commercial application of the adsorbent, regeneration is also a critical factor which must be investigated48. In order to study the circular use of adsorbents, the adsorption capacity of all adsorbent for CO2 were determined using Temperature Swing Adsorption (TSA) and Vacuum Swing Adsorption (VSA) after four consecutive adsorption-desorption cycles and the results are shown in Figure 7a and b, respectively. In TSA method, the adsorbed CO2 is desorbed by rising the temperature to 400 K, then the setup is cooled down 24


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to room temperature and the second CO2 adsorption cycle is measured again. As shown in Figure 7a, the EDA-UiO-66 shows stable and very reversible adsorption-desorption performance during four consecutive runs, which indicates that no EDA leaching had occurred during the four repeated CO2 adsorption-desorption cycles, and very well reusability of this adsorbent. The similar trend is also observed for the other adsorbents24,49. As can be inferred from Figure 7b, the adsorption capacity of these adsorbents for CO2 is slightly smaller than that of fresh adsorbents, which could be due to the contribution of chemisorption which hinders regeneration of these adsorbents in VSA method38,46. The fact that the pristine NH2-UiO-66 presents the lowest change in the adsorption capacity of CO2 after four consecutive adsorption-desorption cycles is due to the higher contribution of physisorption in this adsorbent because of its highest surface area. In the case of modified adsorbents, the physisorption is limited due to the decrease of the surface areas by partially blocking the pore with organic compounds, while the chemisoption increases due to the increase of nitrogen content.

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Figure 7. Effect of recycle times on the adsorption capacity of CO2 onto UiO-66-dreived MOFs in TSA (a) and VSA (b) methods.

4.

Conclusions

Two functionalized version of NH2-UiO-66 were synthesized to increase the CO2 adsorption capacity as well as the CO2/CH4 adsorption selectivity. The prepared samples were characterized by BET, XRD, TGA, FESEM, ICP-OES, 1H NMR, Mass spectroscopy and FTIR-ATR methods before and after functionalization. The BET results indicated that the chemical functionalization of NH2-UiO-66 with EDA via Michael addition reaction results in a significant decrease of the surface area of the EDA-UiO-66, possibly due to pore blockage. The presence of multiple polar functional groups such as hydroxyl, ester, and alkene together with the amine groups significantly enhances the affinity of the 26


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framework respect to CO2, resulting in the increase of the CO2/CH4 adsorption selectivity. This effect was more acute in the case of EDA-UiO-66 due to the highest nitrogen content. Using VSA method for regeneration, the CO2 adsorption capacity of adsorbents decreased over consecutive adsorption cycles, because the CO2 adsorbed in the form of carbamate was stable, and must be thermally regenerated. This result indicated that these materials are suitable for use as an adsorbent in a temperature swing adsorption process for CO2 capture. Supporting Information Procedure for synthesis NH2-UiO-66 and GMA-UiO-66, Procedure for gas adsorption, the image of experimental setup used for the adsorption capacity tests, 1H NMR spectrum of all MOFs, mass spectroscopy of all MOFs, N2 adsorption/desorption and pore size distribution of all MOFs, Isosteric heat of adsorption for CO2 and CH4 at low coverage. Acknowledgment The authors acknowledge Institute of Water and Energy of Sharif University of Technology particularly Dr. Ali A. Alamolhoda for the instrumental supports.

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Highlights EDA-UiO-66 demonstrated great potential for CO2 adsorption and CO2/CH4 separation Using polyethylene glycol as a non-toxic, inexpensive, simple and widely available polymer Novel UiO-66-drived MOFs prepared by post-synthetic modification

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Graphical abstract

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Ethylenediamine grafting to functionalized NH2-UiO-66 using green

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