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. 2024 Feb 14;9(8):9516–9525. doi: 10.1021/acsomega.3c09243

Improving the Separation of CO2/N2 Using Impregnation of a Deep Eutectic Solvent on a Porous MOF

Narmin Noorani 1, Abbas Mehrdad 1,*
PMCID: PMC10905700  PMID: 38434863

Abstract

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As the partial pressure of CO2 in flue gas is 0.1–0.2 bar, CO2 capture at a low pressure needs more attention. Under low pressure conditions, the functional metal–organic framework (MOF) is powerful for CO2 capture. One of the effective methods to increase the absorption capacity of the MOF is impregnation with deep eutectic solvents. In this research, NH2-MIL101(Cr) is impregnated with a deep eutectic solvent of choline chloride:urea (DES ChCl:urea) to enhance the adsorption capacity. The CO2 and N2 adsorption capacity of NH2-MIL101(Cr) and DES/NH2-MIL101(Cr) was investigated at temperatures of 288.15–303.15 K and pressures up to 1 bar. The obtained results indicate that the adsorption capacity of the MOF increases by 1.7 and 3 times with the impregnated DES for CO2 and N2, respectively. Nevertheless, the pore volume of the MOF decreased after impregnation, but the adsorption capacity of the MOF increased due to the interaction of the adsorbate with the confined DES in pores. The contribution of the impregnated DES to adsorption capacity is explained according to Henry’s law. Also, high heats of adsorption are attributed to the strong interaction between modified NH2-MIL101(Cr) and CO2. Also, the sample was refined at 298 K and vacuum and was reused without considerable reduction of the CO2 capture capacity after 6 times. Moreover, the impregnation of ChCl:urea into NH2-MIL101(Cr) nanostructures was studied using density functional theory-based approaches.

Introduction

One of the most important ecological concerns at present is related to the increase of the CO2 concentration in the atmosphere, which arises due to fossil fuels. The CO2 partial pressure in the flue gas varies between 0.1 and 0.2 bar with other components, especially some acidic compounds and N2. This requires that the substance be able to continuously uptake CO2 from postcombustion at this lower pressure.1 Environmental problems such as ocean acidification and global warming arise due to increases of atmospheric CO2. The majority of carbon capture technologies improved to date focus on capturing CO2 from these considerable sources.2 However, in order to widely use these methods, their energy use must be reduced, and their cost-effectiveness must be raised. To reduce CO2 emissions in the energy sector, many studies have been carried out on the development of CO2 capture and storage (CCS).3−5 Numerous efforts have been made to develop novel CO2 capture technologies/processes, particularly absorption based on using nanoporous solids like metal–organic frameworks, carbon materials, silica gel, carbon nanotubes, and zeolites.6−9 These materials’ special pore structures are designed to selectively inhibit CO2 absorption and CO2 movement in the surface of the material.10,11

Recently, metal–organic frameworks (MOFs) have shown good potential for CO2 capture. MOFs have been noted as a novel category of nanoporous material. The high interior surface area, flexible porosity, tunable multifunctional pores, and good chemical and thermal stabilities support plenty of MOFs for diverse applications in gas separation.12,13 Among the many MOFs studied so far, one of the most topical solids is the porous MIL101(Cr).14 The MIL101(Cr) is a 3D framework due to the high pore volume, BET surface area, and high density of open metal sites. Both the open space within the porous framework and the Cr open metal sites can be impregnated with different amine species for CO2 adsorption applications. As the CO2 partial pressure in flue gases is 0.1–0.2 bar, CO2 capture at this pressure range needs more attention.15 Also, at low pressure, the BET surface area is not a significant factor for CO2 capture. Nevertheless, the CO2 adsorption capacity in MOFs is low, and therefore, an improvement is needed. The modification of MOFs using deep eutectic solvents (DESs) and ionic liquids can be performed to improve the CO2 adsorption capacity.16,17

DESs have achieved growing attention as “green” alternatives to the highly expensive ionic liquid, owing to unique properties including high solvation capacity, relatively low cost, higher biodegradability, and nontoxicity making them environmentally and technologically superior.18 Moreover, DESs due to intriguing benefits such as low volatility, low vapor pressure, and biocompatibility are favorable in many green technologies like CO2 capture.19 DESs are most commonly obtained by mixing a hydrogen bond donor with hydrogen bond acceptor molecules at a certain ratio. This decrease in the melting point has been related to the wide hydrogen bond network formed between different moieties in the mixture.20 For the purpose of CO2 capture, numerous DESs have been synthesized. The most common chemical combinations utilized in the design of DESs are mixtures of urea and choline chloride. The DES of ChCl:urea is a nontoxic, biodegradable, biocompatible, readily available, and affordable substance.21 Li et al.22 reported that ChCl:urea (mole ratio of 1:2) indicates the highest CO2 solubility (0.309 molCO2/molDES) at 313 K and 12.5 MPa. Also, Leron et al.23 found that the CO2 solubility in ChCl:urea was higher than those of ChCl:ethylene glycol and ChCl:glycerol. However, the high viscosity of ChCl:urea needs a high pressure environment for increasing CO2 absorption, which leads to an increase in operating costs. Furthermore, DESs and other solvents in a liquid form at ambient temperature are problematic to control.

Recently, the immobilization of DESs on porous MOFs has been attracting much attention due to the overcoming of this drawback that may lead to increased CO2 adsorption capacity. Ariyanto et al.19 studied DES-impregnated porous carbon for the CH4/CO2 separation. DESs of choline chloride:alcohols impregnated on the porous carbon were derived from the palm kernel shell (C-PKS). Lin et al.24 also investigated CO2 capture in a DES (ChCl:ethylene glycol) confined into graphene oxide (GO) with different molar ratios of HBA/HBD by molecular dynamics simulations, i.e., GO makes available confined space for the DES, consequently the special spatial configuration, and also the DES-GO interaction. Also, the interaction between DES molecules weakened, which increases the free volume and results in the diffusion of gas. Ghazali et al.25 found that the modification of mesoporous silica gel (SG) with impregnation of ChCl:urea can be considered as a promising adsorbent for CO2 capture at atmospheric pressure and ambient temperature. However, the data of the isotherm of the NH2-MIL101(Cr) impregnated with the DES for the separation process are scarce. In this research, a choline chloride:urea (1:2) DES was prepared and impregnated to NH2-MIL101(Cr) for CO2 adsorption. The CO2 adsorption in the DES/NH2-MIL101(Cr) was determined by a quartz crystal microbalance (QCM) at temperatures of 288.15 and 303.15 K. The adsorption isotherm to study the potential of the material for separation purposes was investigated. A new hybrid model has been proposed to correlate the CO2 adsorption isotherm. In addition, the CO2/N2 selectivity was carried out to examine the practical efficacy of the prepared adsorbents.

2. Experimental Section

2.1. Materials

Chromium nitrate nonahydrate (Cr(NO3)3·9H2O) (>99% purity), 2-aminoterephthalic acid (NH2-H2BDC, ≥99% purity), and choline chloride (>99% purity) were purchased from Sigma-Aldrich. N,N-Dimethylformamide (DMF) (>99% purity), urea (>99% purity), sodium hydroxide (NaOH), and ethanol (>99% purity) were supplied by Merck. CO2 gas (>99.9% purity) was used in gas absorption tests.

2.2. Synthesis of NH2-MIL101(Cr)

NH2-MIL101(Cr) was synthesized by using the hydrothermal technique. To synthesize NH2-MIL101(Cr), 1.6 g of chromium nitrate nonahydrate (Cr(NO3)3·9H2O), 0.72 g of 2-aminoterephthalic acid (NH2-H2BDC), 0.4 g of NaOH, and 30 mL of deionized water were combined in a Teflon-lined autoclave. The homogeneous solution was heated in an autoclave in a particular Teflon container for 8 h at 423 K. Then, the slurry was centrifuged. The green product obtained was washed with DMF, deionized water, and ethanol to eliminate the unreacted chemicals in the pores. Then, the synthesized NH2-MIL101(Cr) became dry at 373 K for about 12 h under vacuum.

2.3. Synthesis of the DES

In this work, a ChCl:urea (1:2 mol ratio) DES was obtained by mixing choline chloride and urea under stirring at 360 K for about 2 h. Then, the obtained homogeneous solution was cooled to ambient temperature and is liquid, homogeneous, viscous, and colorless at ambient temperature, and its melting point was below room temperature. The melting point of ChCl/urea is 285 K, which is in good agreement with the literature.26

2.4. Synthesis of DES Confinement in NH2-MIL101(Cr)

NH2-MIL101(Cr) was impregnated by ChCl/urea (1:2) using a vacuum impregnation method. The product was dried at 378 K for 20 h.

2.5. Characterization of the MOF

The morphology of NH2-MIL101(Cr) was studied by providing FESEM images (EDX, map and line). To distinguish the Cr of the MOF, EDX spectra were recorded in the same microscope working at 10 keV. The samples were degassed in vacuum at 120 °C for 10 h before measurement. The BET surface area of the NH2-MIL101(Cr) and DES/NH2-MIL101(Cr) was also obtained by measuring the nitrogen adsorption at 77 K using a BELSORP MINI II instrument. Also, the FTIR spectra of NH2-MIL101(Cr) and DES/NH2-MIL101(Cr) were recorded using a spectrometer (Bruker, Tensor 27).

2.6. Gas Adsorption Apparatus

For the purpose of measuring gas adsorption, a QCM sensor was applied. The adsorption apparatus performance has been discussed in the previous works by authors in detail.27−31 The adsorbent adsorption capacity, Qe (Inline graphic) was calculated as follows:

2.6. 1

where ΔFC is the difference in frequencies between uncoated and coated crystals with the adsorbent. ΔFS is the difference in frequency between DES/NH2-MIL101(Cr)-coated crystals under vacuum and after gas adsorption.

2.7. Thermodynamic Model

The three-parameter Redlich–Peterson (R–P) model was used to fit the experimental isotherms in the MOF as follows:32

2.7. 2

where p indicates gas pressure in equilibrium conditions, n is the dimensionless parameter that was assumed as n = 1 in this work, and c and qm are R–P model’s parameters. A hybrid R–P and Henry’s law model was used to correlate the experimental adsorption data in DES/NH2-MIL101(Cr) as follows:

2.7. 3

where H is Henry’s law constant. The adsorption selectivity for CO2/N2 was calculated as follows:

2.7. 4

where QCO2 and QN2 are the absolute adsorbed values of CO2 and N2, respectively.

3. Results and Discussion

3.1. Characterization

3.1.1. FTIR Spectra

The FTIR spectra of the choline chloride-urea DES, NH2-MIL101(Cr), and DES-impregnated NH2-MIL101(Cr) are represented in Figure 1. As seen from Figure 1a, the spectra of the DES indicate a broad band at ∼3400 cm–1, which corresponds to the hydrogen-bonded hydroxyl functional group between urea and choline.33,34 The existence of the carboxylate linker in NH2-MIL101(Cr) was shown by the presence of bands in 1300–1750 cm–1, which are related to the COO symmetric and asymmetric stretching vibrations and the stretching vibrations of C–C. The weak signal at 960 cm–1 and the sharp signal at 760 cm–1 were attributed to C–H of the aromatic ring out-of-plane and in-plane bending vibration, respectively (Figure 1b).35 Also, analysis of FTIR peaks in Figure 1c reveals the bands at 3450 and 3500 cm–1, which are related to the symmetric and asymmetric stretching vibration of N–H in the amino groups, respectively.36 Also, the obvious bands in the region of 1660–1380 cm–1 are attributed to the COO– symmetric vibration, C–C stretching vibration, and asymmetric stretching vibration, indicating the NH2-MIL101(Cr) carboxylate linker. The peaks at 1660 and 1582 cm–1 are attributed to the asymmetric CO2 stretching mode of carboxylic groups. In addition, analysis of FTIR peaks in Figure 1c reveals that the bands at 3370 and 3430 cm–1 are related to the asymmetric and symmetric stretching of primary amines, which shows the presence of amino groups; moreover, another band at 596 cm–1 agrees with previous observations for NH2-MIL101(Cr).

Figure 1.

Figure 1

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FTIR spectra of the synthesized (a) DES, (b) NH2-MIL101(Cr), and (c) the DES/NH2-MIL101(Cr).

3.1.2. EDX Spectroscopy Pattern

EDX spectroscopy was used to evaluate the elemental distribution in the NH2-MIL101(Cr). Figure 2 shows the pattern that corresponds to the distinctive components of NH2-MIL101(Cr). EDX confirmed elements of chrome, carbon, nitrogen, and oxygen in the structure of NH2-MIL101(Cr). Based on the results of the characteristic elements, the mass fractions of C and N are 47.68 and 8.21%, respectively. Therefore, molar ratio of C to N is equal to 5.81, which is close to the molar ratio of C to N in the NH2-H2BDC molecule. These outcomes demonstrate the purity of synthesized NH2-MIL101(Cr).

Figure 2.

Figure 2

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EDX pattern of synthesized NH2-MIL101(Cr).

3.1.3. Scanning Electron Microscopy

The morphology of the samples’ structures is investigated using SEM. Scanning electron microscopy (SEM) images of the NH2-MIL101(Cr) are depicted in Figure 3. The morphology and topology of the NH2-MIL101(Cr) were confirmed to be a quasi-cubic structure.

Figure 3.

Figure 3

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SEM images of synthesized NH2-MIL101(Cr).

3.1.4. Textural Properties of NH2-MIL101(Cr)

The textural characteristics of the DES/NH2-MIL101(Cr) such as the specific surface area (ABET), total pore volume (VP), and mean pore diameter (DMP) were measured using the N2 adsorption/desorption isotherms. As depicted in Figure 4, as typical of microporous crystalline materials, type IV isotherms were obtained.37 The surface areas of the samples were obtained by the BET method based on N2 adsorption/desorption isotherms. The total pore volumes were determined from the N2 adsorption volume at p/p0 = 0.991. Observation of hysteresis loops for NH2-MIL101(Cr) implies that NH2-MIL101(Cr) is mesoporous. The textural properties of NH2-MIL101(Cr) and DES/NH2-MIL101(Cr) are reported in Table 1, which are consistent with the values reported in the literature.38,39 Textural properties show that the impregnation procedure reduces the specific surface area, total pore volume, and micropore volume compared to the original NH2-MIL101(Cr) sample. The obvious decrease in the pore volume as well as the surface area indicated that the NH2-MIL101(Cr) pores were occupied by the bulky DES. The adsorbed nitrogen over NH2-MIL101(Cr) impregnated with DESs was comparably low, probably because of partial filling or blocking of the pores of NH2-MIL101(Cr) with an excess of DES species. This behavior may be due to the DES located near the pore opening of NH2-MIL101(Cr) or inside the porous framework.

Figure 4.

Figure 4

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Nitrogen desorption/adsorption isotherms at 77 K: (A) NH2-MIL101(Cr) and (B) DES/NH2-MIL101(Cr).

Table 1. Textural Characteristics of the Samples.
adsorbent ABET (m2·g–1) VP (cm3·g–1) DMP (nm)
NH2-MIL101(Cr) 1073.1 0.882 3.29
DES/NH2-MIL101(Cr) 115.47 0.284 9.83
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3.2. Adsorption Isotherms

CO2 and N2 gas adsorption in NH2-MIL101(Cr) and DES/NH2-MIL101(Cr) was measured at a pressure of up to 1 bar and temperatures of 288.15–303.15 K. The CO2 and N2 gas adsorption data are listed in Tables 2 and 3. The gas adsorption capacities of NH2-MIL101(Cr) and DES/NH2-MIL101(Cr) are fitted using the P–R and hybrid model, respectively. The calculated parameters of these models for CO2 and N2 adsorption in NH2-MIL101(Cr) and DES/NH2-MIL101(Cr) along with the absolute average relative deviation (AARD) are reported in Tables 4 and 5. The reported AARD indicates that the capability of the proposed model was suitable. As seen from Tables 2 and 3, the impregnation of the DES into NH2-MIL101(Cr) can make novel stronger adsorption sites and improves the CO2 adsorption capacity. Compared to this pure DES,40 the DES/NH2-MIL101(Cr) owned higher CO2 capture, which is attributed to both chemical and physical adsorption. Also, the increase of CO2 capture on DES/NH2-MIL101(Cr) compared to NH2-MIL101(Cr) at 298 K and 0.15 bar, which corresponds to the pressure of CO2 in flue gases, was 16.8%. This behavior can be attributed to the interaction between guest CO2 molecules and amine of the DES. The −NH2 groups and CO2 can be chemically combined with a molar fraction of 2:1 (−NH2:CO2) due to forming carbamate. Subsequently, sites of the −NH2 adsorption were entirely occupied, and CO2 can interact more strongly with the −OH group of choline chloride. During the adsorption process, sites of the open metal would also be advantageous for the CO2 uptake.

Table 2. CO2 Adsorption Capacity Qe (Inline graphic) of NH2-MIL101(Cr) and DES/NH2-MIL101(Cr) at the 288.15–303.15 K Temperature Range and Pressures up to 1 bara.

T = 288.15 K
 T = 293.15 K
 T = 298.15 K
 T = 303.15 K
p/bar Qe/Inline graphic p/bar Qe/Inline graphic p/bar Qe/Inline graphic p/bar Qe/Inline graphic
NH2-MIL101(Cr)
0.151 0.576 0.157 0.521 0.15 0.434 0.153 0.421
0.283 1.040 0.258 0.788 0.263 0.698 0.275 0.707
0.346 1.234 0.310 0.882 0.337 0.892 0.313 0.809
0.487 1.587 0.487 1.345 0.412 1.054 0.387 0.942
0.51 1.664 0.534 1.478 0.487 1.217 0.496 1.164
0.624 1.900 0.607 1.614 0.528 1.351 0.538 1.234
0.703 2.084 0.723 1.8261 0.64 1.555 0.678 1.425
0.787 2.236 0.787 1.945 0.787 1.821 0.739 1.537
0.823 2.34 0.856 2.106 0.844 1.925 0.862 1.682
0.921 2.490 0.923 2.204 0.921 2.080 0.937 1.765
1.011 2.620 1.003 2.320 1.002 2.189 1.006 1.854
DES/NH2-MIL101(Cr)
0.153 0.851 0.151 0.783 0.158 0.729 0.157 0.617
0.229 1.242 0.256 1.242 0.223 0.943 0.215 0.773
0.345 1.772 0.348 1.615 0.345 1.511 0.392 1.253
0.425 2.089 0.431 1.915 0.433 1.799 0.487 1.511
0.565 2.642 0.542 2.318 0.523 2.093 0.579 1.760
0.618 2.851 0.613 2.575 0.601 2.349 0.678 2.029
0.717 3.242 0.787 3.208 0.796 2.985 0.784 2.238
0.816 3.618 0.854 3.450 0.884 3.275 0.887 2.509
0.966 4.225 0.923 3.701 0.978 3.583 0.981 2.736
1.004 4.376 1.008 3.978 1.010 3.688 1.014 2.782
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a

Standard uncertainties are u(Qe) = 0.001, u(T) = 0.05 K, and u(p) = 0.001.

Table 3. N2 Adsorption Capacity Qe (Inline graphic) of NH2-MIL101(Cr) and DES/NH2-MIL101(Cr) at the 288.15–303.15 K Temperature Range and Pressures up to 1 bara.

T = 288.15 K
 T = 293.15 K
 T = 298.15 K
 T = 303.15 K
p/bar Qe/Inline graphic p/bar Qe/Inline graphic p/bar Qe/Inline graphic p/bar Qe/Inline graphic
NH2-MIL101(Cr)
0.152 0.0667 0.157 0.0598 0.156 0.0489 0.159 0.0423
0.220 0.094 0.275 0.092 0.221 0.067 0.283 0.076
0.367 0.148 0.313 0.113 0.359 0.110 0.321 0.084
0.450 0.180 0.427 0.155 0.421 0.132 0.42 0.111
0.574 0.228 0.516 0.188 0.523 0.162 0.590 0.154
0.628 0.245 0.615 0.220 0.615 0.195 0.623 0.163
0.705 0.272 0.703 0.247 0.716 0.219 0.695 0.181
0.804 0.302 0.787 0.275 0.805 0.249 0.769 0.200
0.887 0.325 0.897 0.310 0.876 0.272 0.818 0.210
0.978 0.350 0.978 0.334 0.945 0.292 0.904 0.234
1.002 0.357 1.005 0.342 1.001 0.310 1.004 0.252
DES/NH2-MIL101(Cr)
0.151 0.264 0.153 0.244 0.158 0.244 0.151 0.176
0.237 0.367 0.238 0.333 0.256 0.335 0.229 0.241
0.337 0.484 0.329 0.438 0.347 0.440 0.384 0.371
0.415 0.569 0.477 0.593 0.433 0.520 0.475 0.446
0.516 0.677 0.564 0.684 0.547 0.627 0.583 0.536
0.609 0.791 0.628 0.751 0.635 0.709 0.678 0.616
0.782 0.974 0.739 0.867 0.737 0.804 0.768 0.691
0.877 1.066 0.811 0.942 0.846 0.905 0.845 0.755
0.976 1.173 0.900 1.036 0.963 1.014 0.920 0.817
1.005 1.204 1.005 1.145 1.010 1.058 1.004 0.887
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a

Standard uncertainties are u(Qe) = 0.001, u(T) = 0.05 K, and u(p) = 0.001.

Table 4. Henry’s Law Constant (H), qm, and c That Are Also Parameters of the Redlich–Peterson Isotherm Model, the Correlation Coefficient (R2), and Absolute Average Relative Deviation (AARD) for CO2 Adsorption on NH2-MIL101(Cr) and DES/NH2-MIL101(Cr) at Different Temperatures (T)a.

adsorbent T/Kaa H/bar qm/Inline graphic c/bar–1 bAARD%
NH2-MIL101(Cr) 288.15   6.601 0.656 0.67
293.15   8.400 0.458 1.09
298.15   8.396 0.354 0.78
303.15   4.540 0.684 0.75
DES/NH2-MIL101(Cr) 288.15 0.269 0.781 4.210 1.50
293.15 0.294 0.678 4.577 0.27
298.15 0.363 1.459 1.563 0.86
303.15 0.442 0.627 4.111 0.72
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a

The standard uncertainty is u(T) = 0.05 K.

b

Inline graphic.

Table 5. Henry’s Law Constant (H), qm, and c That Are Also Parameters of the Redlich–Peterson Isotherm Model, the Correlation Coefficient (R2), and Absolute Average Relative Deviation (AARD) for N2 Adsorption on NH2-MIL101(Cr) and DES/NH2-MIL101(Cr) at Different Temperatures (T)a.

adsorbent T/Ka H/bar qm/Inline graphic c/bar–1 bAARD%
NH2-MIL101(Cr) 288.15   1.685 0.270 0.61
293.15   3.759 0.100 1.27
298.15   3.768 0.060 0.72
303.15   3.515 0.070 0.59
DES/NH2-MIL101(Cr) 288.15 0.957 0.169 10.561 0.19
293.15 0.962 0.104 26.294 0.33
298.15 1.094 0.145 12.460 0.43
303.15 1.237 0.090 5.445 0.57
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a

The standard uncertainty is u(T) = 0.05 K.

b

Inline graphic.

The BET surface area and pore volume of the DES/NH2-MIL101(Cr) decreased with a further increase of the incorporated DES into NH2-MIL101(Cr). This behavior implies a further decrease in the BET surface area through porosity blockage resulting from the added DES. The isotherm curves of the CO2 and N2 gas adsorption of the considered systems at different temperatures and pressures are illustrated in Figure 5. As mentioned, NH2-MIL101(Cr) impregnated with the DES not only reduced the surface area and pore volume but also modified the surface chemistry owing to the presence of numerous −NH2 groups. Hence, the rise of the CO2 adsorption capacity in DES/NH2-MIL101(Cr) is attributed to the production of more adsorptive sites on the adsorbent. The improved CO2 adsorption performance on NH2-MIL101(Cr) is attributed to the incorporation of the DES into NH2-MIL101(Cr), which leads to CO2 approaching to extra active sites on the pore surface, interaction between −NH2 and −OH groups, clogging of porosity, and reduction of the BET surface area. According to Figure 5, NH2-MIL101(Cr) has a CO2 adsorption capacity of 2.189 Inline graphic, while DES/NH2-MIL101(Cr) has an adsorption capacity of 3.688 Inline graphic at a temperature of 298.15 K and a pressure of 1 bar. The reported CO2 adsorption capacity value in the literature for nonamine-functionalized MIL101(Cr) at 298.15 K and a pressure of 1 bar is 1.6 Inline graphic.41 Therefore, the amine-functionalized MIL101(Cr) has a higher adsorption capacity than nonamine-functionalized MIL101(Cr). Also, Lawson et al.41 have achieved adsorption capacities of 2 and 3.6Inline graphic by impregnation of polyethylenimine and tetraethylenepentamine into MIL101(Cr) at 298.15 K and a pressure of 1 bar. This behavior implies the presence of amine groups in NH2-MIL101(Cr) owing to the fact that the Lewis basic amine groups can interact strongly with the CO2 molecules. Moreover, the quadrupole moment of CO2 (−14 × 10–40 C m2) is much higher than that of N2 (−4.6 × 10–40 C m2) that leads to the higher capture of CO2 than that of N2. Also, in the DES-impregnated MOF, gas adsorption can be due to two factors; the first factor is confinement of the DES in pores of the MOF, and the second factor is immobilization of the DES on the pore surface. At a low pressure, gas adsorption occurred on the immobilized DES on the pore surface of NH2-MIL101(Cr), while at a high pressure, gas adsorption happened on the confined DES in MIL101(Cr) pores. In addition, the chemical reaction between the amine group and CO2 molecules affects the CO2 adsorption in NH2-MIL101(Cr). The several active sites in NH2-MIL101(Cr) such as the NH2 functional group and carboxylate oxygen atoms can increase preferential interactions between CO2 and the MOF.

Figure 5.

Figure 5

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CO2 adsorption in (A) NH2-MIL101(Cr) and (B) DES/NH2-MIL101(Cr) and N2 adsorption in (C) NH2-MIL101(Cr) and (D) DES/NH2-MIL101(Cr) at different temperatures (diamonds) 288.15K; (triangles) 293.15 K; (square) 298.15 K; (circles) 303.15 K; (lines) fitting results by eq 3.

The selectivity of CO2/N2 for NH2-MIL101(Cr) and DES/NH2-MIL101(Cr) was evaluated at a temperature of 288.15 K and different pressures. The selectivities of CO2/N2 for NH2-MIL101(Cr) and DES/NH2-MIL101(Cr) are illustrated in Figure 6. The CO2 adsorption in the bare amine-functionalized MOF often is chemisorption, but N2 adsorption in the bare amine-functionalized MOF is physisorption. However, in the DES/MOF, moreover, surface adsorption on one portion of adsorption is related to the physical absorption in the confined DES in the pores of the MOF, which is governed by Henry’s law. Therefore, the selectivity of the bare amine-functionalized MOF is higher than that of the DES/MOF. The value of CO2/N2 selectivity decreases with an increase in the pressure and tends to plateau, which is in good agreement with the literature.42

Figure 6.

Figure 6

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Selectivity of CO2/N2 based on the ratio of adsorption uptake at 288.15 K for (filled circles) NH2-MIL101(Cr) and (open circles) DES/NH2-MIL101(Cr).

3.3. Enthalpy of Adsorption

The molar enthalpy of adsorption is a measure of the strength of interaction between the adsorbate molecules and the adsorbent surface, which can be calculated by measuring gas adsorption at various temperatures.43,44 The isotropic heat adsorption at a constant adsorption amount, q, was calculated as follows:45,46

3.3. 5

where R is the universal gas constant and T is temperature. In order to calculate the heat of absorption, absorption experiments were performed at temperatures of 303.15–288.15 K. The influence of temperature on the absorption of CO2 by DES/NH2-MIL101(Cr) adsorbents is shown in Figure 5. According to Figure 5, the adsorption isotherm is temperature-dependent. The isosteric heat values for NH2-MIL101(Cr) and DES/NH2MIL101(Cr) are obtained from plotting ln(P) vs 1/T. The obtained isosteric heat versus adsorption capacity is illustrated in Figure 7. According to Figure 7, isosteric heat of adsorption is decreased with increasing adsorption. Gas absorption in the confined DES in the pores of the MOF is physical sorption, which is governed by Henry’s law. The heat of physical adsorption is lower than that of chemical sorption; therefore, a higher adsorption heat in the amine-functionalized MOF than in the DES/MOF is expected.

Figure 7.

Figure 7

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Isosteric heat of CO2 adsorption on (filled circles) NH2-MIL101(Cr) and (open circles) DES/NH2-MIL101(Cr).

3.4. Regeneration Efficiency

To evaluate the reusability of the adsorbent, an adsorption/desorption process was performed for up to seven cycles. For regeneration tests, CO2 absorption was tested at 298.15 K and 1 bar, and vacuum and desorption processes were done at vacuum and 298.15 K up to 90 min for CO2 elimination. The regeneration capability of the DES/NH2-MIL101(Cr) adsorbent in seven cycles is shown in Figure 8. As shown in Figure 8, the amount of adsorbed CO2 was slightly reduced after seven cycles. The amounts of CO2 adsorption in DES/NH2-MIL101(Cr) are obtained to be 2.817, 2.817, 2.817, 2.817, 2.809, 2.809, and 2.809 in seven repeated cycles of adsorption/desorption, and the regeneration performance of DES/NH2-MIL101(Cr) is 99.7% after seven consecutive cycles of adsorption/desorption.

Figure 8.

Figure 8

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CO2 absorption capacity of DES/NH2-MIL101(Cr) at p = 1.000 bar and T = 298.15 K in seven adsorption/desorption cycles.

3.5. DFT Calculations

The study of the CO2 adsorption mechanism in the ChCl:urea-incorporated NH2-MIL101(Cr) complex has been carried out at the molecular level. In order to find the structural properties and stability of ChCl/urea that was incorporated into NH2-MIL101(Cr) pores, the different structures of the pure ChCl:urea DES, NH2-MIL101(Cr) structure, and ChCl:urea-incorporated NH2-MIL101(Cr) complex were optimized at the B3LYP-D3/6-311G*(d,p) level of DFT theory as realized in the Gaussian 03 computational package.47−49 The most stable configurations were used to study CO2 adsorption. The adsorption energy (Eads) was obtained using the following equation:50

3.5. 6

To obtain the most probable sites of the interactions between the ChCl/urea and ChCl/urea confinement in NH2-MIL101(Cr), different several structures of ChCl/urea and NH2-MIL101(Cr) were considered based on the partial charge distribution and the full optimization of structures, and their energies were calculated. The interaction energy of ChCl/urea-impregnated NH2-MIL101(Cr) is −112 kcal·mol–1. The geometries of the selected ChCl/urea, NH2-MIL101(Cr) structure, and ChCl/urea-impregnated NH2-MIL101(Cr) complex are shown in Figure 9. This approach enables an obvious picture of the microscopic interfacial interaction between ChCl/urea and NH2-MIL101(Cr). In addition, the degree of stability was based on the nature of surface charge transfer in the complex. The optimized geometries along with the shortest interaction distances between ChCl/urea and the NH2-MIL101(Cr) framework are shown in Figure 9. ChCl/urea geometries of the DES are not altered even in the confined environment and stabilized by the electrostatic interactions and H-bonds. The calculated interacting distance of ChCl/urea–surface was varied from 2.13 to 2.67 Å. This phenomenon implies that Cl strongly interacts with the surface that notably increases the calculated adsorption energies and causes the increase of the volume of the NH2-MIL101(Cr) structure.

Figure 9.

Figure 9

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Structures of (A) DES with CO2 (B) NH2-MIL101(Cr) and (C) DES/NH2-MIL101(Cr) with CO2.

4. Conclusions

The potential of DES-impregnated NH2-MIL101(Cr) was assessed for the separation of CO2/N2. Choline chloride was used in conjunction with urea, which was used to modify amino-functionalized NH2-MIL101(Cr). BET, SEM-EDX, and FTIR analysis confirmed that the DES is impregnated into the porous MOF. CO2/N2 adsorption isotherms revealed that DES/NH2-MIL101(Cr) exhibited a better performance. The results show that in addition to physical adsorption, the chemical adsorption of CO2 by the functional group of NH2 in the structure of the adsorbent also has a significant effect on the adsorption mechanism. The DES/NH2-MIL101(Cr) can be employed repeatedly without losing adsorption performance and could increase the CO2 uptake capacity, which introduces a new class of extremely porous adsorbents for effective absorption.

Acknowledgments

The authors are thankful for the postdoctoral grant (no. SAD/3938-1400 1225) from the University of Tabriz.

The authors declare no competing financial interest.

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