A metal–organic framework MIL-101 doped with metal nanoparticles (Ni & Cu) and its effect on CO₂ adsorption properties

Abstract

In this work, MIL-101-Cu and MIL-101-Ni were successfully synthesized via a microwave irradiation technique to enhance the adsorption capacity and adsorbent cyclability. The prepared adsorbents were characterized by various techniques such as XRD, FE-SEM, EDS, ICP, TEM and BET. TEM images clearly demonstrated that Cu and Ni NPs of 3–7 nm and 2–4 nm, respectively, were incorporated within the pores of the MIL-101 adsorbent. The CO₂ adsorption capacity was measured by a volumetric method. The equilibrium CO₂ adsorption capacities were measured as 9.7, 10.6, 11.8 and 12.4 mmol g⁻¹ for the parent MIL-101, activated MIL-101, MIL-101-Cu and MIL-101-Ni adsorbents, respectively at 7.1 bar and 298.2 K. The initial isosteric heats of CO₂ adsorption on the above mentioned adsorbents were estimated to be 22, 27, 31 and 38 kJ mol⁻¹, respectively. Successive adsorption–desorption cycles were conducted to explore the cyclability of the adsorbents. The results confirmed that the adsorption capacity remained constant after 100 cycles. The equilibrium experimental data were well-fitted with a Freundlich isotherm model.

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1. Introduction

The increase in atmospheric CO₂ concentration, mainly emitted from fossil fuel combustion, is a major issue of the current century, because it greatly contributes to global warming.¹ Currently, 85% of the total world energy consumption is supplied by fossil fuels.^2,3 Carbon capture and storage (CCS) is widely regarded as an effective approach to reduce the total CO₂ emission in the atmosphere.⁴ The main technologies for CCS include absorption, membranes and adsorption.^5,6 Absorption currently proves to be a commercial process for CO₂ capture. This process suffers from an energy penalty problem due to the energy-intensive stage of liquid solvent regeneration.^6,7 Adsorption technology is a promising alternative since desorption from solid sorbents requires less energy than that from liquid absorbents.^7–9 Efficient industrial CO₂ capturing by adsorption requires solid sorbents with high CO₂ adsorption capacity, high CO₂ selectivity, favorable adsorption–desorption kinetics at mild operating conditions, high thermal and mechanical stability, high cyclability, and low cost.^7,10–13 A diversity of sorbents including carbonaceous materials, zeolite molecular sieves, amine supported mesoporous materials, hydrotalcite-like compounds, limestone, lithium zirconate, lithium silicate, metal oxides materials and metal–organic framework materials (MOFs) have been introduced through literature so far.^{2,7,10,14–16}

MOFs are emerging crystalline porous materials composed of metal centers connected by organic linkers.^17,18 These components have high surface area, low density, high porosity, high thermal and chemical stability as well as adjustable chemical functionality.^18–23 The pore size and chemical functionality of MOFs can be tailored for different purposes by modifying their metal centers and organic linkers.²¹ Because of the above mentioned significant properties, MOFs are favorable candidates in gas storage and separation,^24–30 drug delivery,^24,31,32 luminescence,^24,33,34 and catalysis.^24,35–39 The chromium(III)terephthalate, known as MIL-101 (ref. 19) is standing out for adsorption studies, because in addition to the above mentioned properties, its pore size is 2.9–3.4 nm which makes it favorable for CCS and gas separation.^20,25,28

Yang et al. reported the CO₂ and SO₂ adsorption on a non-amine-containing porous material (NOTT-300).⁴⁰ In situ INS and PXRD studies were investigated to understand the detailed binding mechanism. Their results showed high adsorption capacity for CO₂ adsorption at low pressure, a maximum value of 7.0 mmol g⁻¹ at 273 K and 1.0 bar.

Llewellyn et al. studied the adsorption of pure CH₄ and CO₂ on MIL-101, MIL-101b (ethanol treated), MIL-101c (ethanol + NH₄F-treated) and MIL-100.²⁵ They reported CO₂ adsorption capacities for parent MIL-101 and MIL-100 as 28 and 18 mmol g⁻¹ at 300 K and 50 bar, respectively. They showed the activation treatment was able to enhance this capacity at the same conditions, respectively to 34 and 40 mmol g⁻¹ for MIL-101b and MIL-101c. Chowdhury et al. investigated adsorption isotherms for Ar, CO₂, CH₄, C₃H₈, and SF₆ on MIL-101.²⁶ The adsorption capacities of C₃H₈, SF₆, CO₂, CH₄ and Ar gases were reported to be 13.4, 9.1, 8, 2.2 and 0.9 mmol g⁻¹, respectively at 5.3 bar and 283 K. In another research Chowdhury and coworkers presented a comparative adsorption study of industrially relevant gases such as CH₄, CO₂ and CO on Cu-BTC and MIL-101 frameworks.²⁷ CO₂ adsorption capacity on Cu-BTC and MIL-101 were obtained 15.5 and 21.3 mmol g⁻¹ at 295 K and 36 bar, respectively. Anbia et al. carried out CO₂ adsorption study on MIL-101 and PEHA-MIL-101.²⁸ The CO₂ adsorption capacities of MIL-101 and PEHA-MIL-101 were reported to be 0.85 and 1.3 mmol g⁻¹ at 10 bar and 298 K, respectively. Zhang et al. used MIL-101 to investigate equilibrium and kinetic of CO₂ adsorption.²⁹ They reported that the maximum uptake of CO₂ on MIL-101 was 22.9 mmol g⁻¹ at 298 K and 30 bar. Munusamy et al. investigated the equilibrium adsorption capacities of CO₂, N₂, CH₄, and CO on both powder and granules of MIL-101.³⁰ They found the equilibrium adsorption capacity of CO₂ in MIL-101 was much higher than those of other gases. Also the powder form of MIL-101 showed higher adsorption capacity than the granules.

Although, literature reviews demonstrate that generally MOFs show large CO₂ adsorption capacities at high pressure, improving their CO₂ uptake at relatively low partial pressure is a scientific challenge.⁴¹ Therefore, a number of modification techniques have been developed to overcome this disadvantage of MOFs, such as metal doping,⁴² chemical functionalization with alkyl amine group^43,44 and making empty unsaturated metal coordination sites.^25,26,45,46 However, from an engineering point of view, there must be a need to consider other important properties include cyclability and mild regeneration conditions. Metal nanoparticles (MNPs) doping in MOFs, which is widely used for catalytic applications,³⁶ seems to be an encouraging method of the modification for enhancing adsorption capacity in gas storage.^43,47–49 To our knowledge, there exit no report for MNP-doped MIL-101 application for CO₂ adsorption. There are several techniques for MNP doping including chemical vapor deposition (CVD), solution infiltration, solid grinding, and microwave irradiation (MWI) for introducing MNPs in MOFs.⁵⁰ Botas et al. reported cobalt incorporation into Zn based MOF-5 (IRMOF-1) framework for gas adsorption process.⁴² Their results showed the important role of Co for gas adsorption. Qin et al. synthesized Pd-doped MIL-101 as adsorbent for adsorption of toluene and hydrogen.⁵¹ Their results indicated that adsorption capacities of toluene and hydrogen on Pd-doped MIL-101 were enhanced comparing with that of the parent MIL-101. Zhao et al. synthesized the bimetallic NiIr NPs immobilized on MIL-101 for dehydrogenation of hydrazine monohydrate.⁵² Their results exhibited that NiIr@MIL-101 is a potential catalyst for chemical hydrogen storage. Hermannsdorfer et al. introduced PdNi@MIL-101 catalyst in hydrogenation reactions.⁵³ High catalytic efficiency was observed for bimetallic Ni/Pd NP catalysts. Yingya et al. prepared the Ni/MIL-101 material for hydrogen adsorption process.⁵⁴ Their results showed that Ni/MIL-101 had higher hydrogen capacity than original MIL-101.

In the following section MWI method as a facile and efficient technique will be introduced for incorporating MNPs in MIL-101. The capability of this method is investigated by CO₂ adsorption experiments at three different temperatures of 298.2, 310.2 and 320.2 K and pressure range of 0 and 7.1 bar. In the present work, the effects of MNPs on CO₂ adsorption capacity of MIL-101 will be investigated. Also adsorbent cyclability, which is an important parameter being used to evaluate the adsorption performance, will be evaluated for all synthesized adsorbents.

2. Experimental

2.1. Materials

All the chemicals used for adsorbents synthesis in this work include terephthalic acid (H₂bdc, 98% purity), chromium nitrate nonahydrate (Cr(NO₃)₃·9H₂O, 98%), hydrofluoric acid (HF, 38–40%), N,N-dimethyl formamide (DMF, 99.8%), ethanol (EtOH, 99.9%), copper(II) nitrate trihydrate (Cu(NO₃)₂·3H₂O, 99.5%) and nickel(II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O, 99–102%) were purchased from E. Merck Inc. The CO₂ gas used in this study was purchased from Farafan Gas Company with high purity grade, 99.995%.

2.2. Synthesis

2.2.1. Synthesis of the parent MIL-101. MIL-101 was synthesized by hydrothermal method according to the well-documented procedures,^55,56 and briefly described here. A mixture of Cr(NO₃)₃·9H₂O (1.2 g, 3 mmol), terephthalic acid (H₂bdc, 500 mg, 3 mmol), and HF (5 M, 0.6 mL, 3 mmol) in H₂O (15 mL) was heated in a teflon-lined stainless steel autoclave at 493.2 K for 8 h. Then, the mixture was left exposed to atmosphere to be gradually cooled down to room temperature. The solid green product was filtered to eliminate unreacted crystalline H₂bdc and then filtered on a thick glass filter. After drying at 393.2 K overnight, the powder was washed with hot DMF (373.2 K, 8 h, twice) and hot EtOH (353.2 K, 8 h, twice). The washed solid was dried at 423.2 K in air overnight followed drying under vacuum for 4 h at same temperature to obtain the final fine green powder of MIL-101. To evaluate the reproducibility, the synthesis of MIL-101 was repeated three times under identical synthesis conditions.

2.2.2. Synthesis of MIL-101-Cu and MIL-101-Ni. Prior to metal loading, activation of synthesized MIL-101 was performed as following. A mixture of MIL-101 (400 mg), water (20 ml) and sodium borohydride (50 mg) in methanol as a reducing agent was combined in a beaker and the mixture was stirred for 10 min at room temperature. Then, the solution was sonicated using an HD2200 Bandelin Sonopuls sonicator. Next, the solution was placed in a microwave oven (Samsung M9G45, 900 W, 2450 MHz) at 700 W for 1 min. Finally, the solution was filtered with a thick glass filter. The obtained solid was washed with water, dried overnight at 423.2 K, and named as activated-MIL-101. For the synthesis of the MNPs-doped MIL-101, an aqueous dispersion of the activated-MIL-101 (100 mg in 6 ml H₂O) was prepared. An appropriate amount of nitrate salt (19 mg of Cu(NO₃)₂·3H₂O or 25 mg of Ni(NO₃)₂·6H₂O) was added to this solution. The mixture was vigorously stirred for 10 min at room temperature. Then, a prepared solution of sodium borohydride (12 mg) in methanol was added to the mixture under vigorous stirring. The color of the mixture immediately turned from pale green to dark green, indicating the formation of MNPs-doped MIL-101. The solution was subjected to ultrasonication for 10 min at room temperature and then placed in the microwave set at 700 W for 2 min. Microwave irradiation made the color of the solution go back to the initial pale green-colored materials. After cooling down, the solution was filtered, washed with deionized water, dried overnight at 423.2 K, and named MIL-101-Cu or MIL-101-Ni, depending to used nitrate salt. This procedure was repeated once again to evaluate reproducibility.

2.3. Characterization

The X-ray powder diffraction (XRD) patterns of samples were recorded using a D₈ ADVANCE XRD instrument (Bruker Corporation) in the 2θ range of 1–30 degree using CoKα radiation with a wavelength of 1.7890 angstrom (Å). A Zeiss Sigma VP field-emission electron microscope was used to obtain field emission scanning electron microscopy (FE-SEM) images. The microscope was coupled with an Oxford Instruments energy dispersive X-ray spectrometer (EDS) for qualitatively determining the elemental composition of nanoparticles. Inductively coupled plasma (ICP) was used to determine the actual metal content in MIL-101 samples. The measurements were performed using a Perkin-Elmer Optima 7300 DV ICP-OES instrument. N₂ adsorption–desorption isotherms of the MIL-101 were measured at 77 K using a Nova instrument (Quantachrome NovaWin2). The specific surface area was obtained employing BET method. The material structure and the presence of MNPs in MIL-101 samples were examined under transmission electron microscopy (TEM) using a Zeiss-EM10C-100 kV transmission electron microscope. TGA test was performed using a Bahr STA-503 instrument at a heating rate of 10 K min⁻¹ from 298.2 K to 853.2 K under argon atmosphere.

2.4. CO₂ adsorption measurements

To investigate the CO₂ adsorption capacities of MIL-101, MIL-101-Cu and MIL-101-Ni samples, an experimental setup based on volumetric method was used as shown in Fig. 1. The CO₂ isotherms were measured at three different temperatures of 298.2, 310.2, and 320.2 K and pressures ranging from 0 to 7.1 bar. At first, the samples were degassed at the heating temperature of 423.2 K overnight. Then, 100 mg of each degassed adsorbent was loaded into the adsorption cell. The equilibrium amount of the adsorbed CO₂ was calculated by using the difference of initial and final pressures in adsorption cell for different consecutive cycles. Before starting a new adsorption cycle test, the sample was regenerated in an oven at 423.2 K for 4 h. The accuracy in measuring pressure and temperature were evaluated as much as 1000 Pa and 0.1 K, respectively.

Fig. 1 Experimental setup for adsorption measurements.

3. Results and discussion

3.1. Characterization of the synthesized MIL-101s

The XRD patterns for the simulated and synthesized MIL-101, activated-MIL-101 and MIL-101-Cu and MIL-101-Ni are shown in Fig. 2. Results show that the diffraction peak positions are consistent with the simulated XRD pattern of MIL-101 reported in the literature.⁵⁶ As mentioned earlier, the synthesis of MIL-101 was performed three times using the same conditions to evaluate the reproducibility of the MIL-101. After each batch synthesis, XRD analysis was carried out. These XRD patterns are consistent with each other as shown in Fig. S1 in the ESI.†

Fig. 2 X-ray diffraction patterns of simulated MIL-101, synthesized MIL-101, activated-MIL-101, MIL-101-Ni, and MIL-101-Cu.

It is clearly seen from Fig. 2 that the structure of crystalline MIL-101 was retained regardless of the metal content. According to the monolayer dispersion theory,⁵⁷ no characteristic peaks of MNPs would be detected in XRD patterns if the nanoparticles were dispersed as a monolayer or submonolayer.^50,58 As shown in this figure, XRD patterns of the adsorbents were intact, suggesting a better metal dispersion.

Table 1 shows the Cu and Ni content in metal-doped adsorbents, measured by ICP analysis.

Table 1 Cu and Ni content in metal-doped adsorbents

	Copper (wt%)	Nickel (wt%)
MIL-101-Ni	0	5.67
MIL-101-Cu	6.00	0

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FE-SEM images of MIL-101-Ni and MIL-101-Cu are shown in Fig. 3. These images reveal that the morphologies of the crystals of both synthesized adsorbents remained the same during MNP doping lending support to the fact that process of Ni/Cu insertion does not change the morphology of MIL-101 adsorbent.

Fig. 3 FE-SEM images of (a) MIL-101-Ni, and (b) MIL-101-Cu.

The EDS results obtained from FE-SEM analysis of the both metal doped MIL-101 clearly confirm the presence of Ni and Cu nanoparticles in the MIL-101 framework. To investigate the uniformity of the Ni(II) and Cu(II) distribution, an elemental mapping analysis of the metal-doped MIL-101 were conducted with EDS, as depicted in Fig. 4. Also, the corresponding EDS plots of MIL-101-Ni and MIL-101-Cu, indicate the presence of Ni and Cu elements in the MIL-101 (Fig. S2 in the ESI†).

Fig. 4 (a) EDS mapping of MIL-101-Cu. (b) EDS mapping of MIL-101-Ni. SEM image (left) and the corresponding elemental distributions of Cr (middle) and doped metal nanoparticles (Cu, Ni) (right).

TEM images, given by Fig. 5, show MNPs as dark-gray areas, which are easily recognized from the surrounding light gray regions of MIL-101. It can be seen that the Ni nanoparticles are well dispersed throughout the MIL-101 crystal and no obvious aggregation is observed. The Cu nanoparticles are mostly found to be accumulated at the edge and surface of MIL-101. More TEM images with different scale up are presented in ESI (Fig. S3 and S4†). Also, the histograms of the nanoparticles size distribution, estimated by TEM images, are shown in Fig. 5. The mean diameter of Cu and Ni particles are estimated to be 5.35 ± 0.4 nm and 2.85 ± 0.5 nm for MIL-101-Cu and MIL-101-Ni, respectively.

Fig. 5 TEM images of MIL-101-Cu (left), MIL-101-Ni (right), and their corresponding size distribution of metal nanoparticles.

Nitrogen adsorption–desorption isotherms on all the synthesized MIL-101 at 77 K are given by Fig. S5 (ESI†) and Table 2. The BET surface area and pore volume of the parent MIL-101 were 2730 m² g⁻¹ and 1.36 cm³ g⁻¹, respectively. After the activation, BET surface area and pore volume of MIL-101 increased to 2931 m² g⁻¹ and 1.43 cm³ g⁻¹, respectively, that clearly indicates that unreacted materials trapped in the pores have removed. Following doping metals in MIL-101, the BET surface areas of MIL-101-Cu and MIL-101-Ni samples decreased to 1850, 2270 m² g⁻¹ and the corresponding pore volumes changed to 1.13 and 1.20 cm³ g⁻¹, respectively. The BET surface area and pore volume of MNPs-doped MIL-101 are less than those of MIL-101. This can be attributed to the pore blocking caused by the doped MNPs. The BET surface area and pore volume of MIL-101-Ni are higher than those of MIL-101-Cu; it can be contributed to better dispersion of Ni nanoparticles inside the adsorbent pores.

Table 2 Surface areas and pore volumes of MIL-101 adsorbents

Adsorbents	BET surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)
MIL-101	2730	1.36
Activated-MIL-101	2931	1.43
MIL-101-Cu	1850	1.13
MIL-101-Ni	2270	1.20

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Thermogravimetric analysis (TGA) curves of MIL-101, MIL-101-Cu and MIL-101-Ni are displayed in Fig. S6 (ESI†). For MIL-101, three distinct weight loss steps are observed. The first step falling within the range of 298.2–423.2 K corresponds to the loss of guest water molecules from the large pores. The second weight loss step which is situated within the range of 423.2–573.2 K can be attributed to the loss of guest water molecules from the medium-sized pores. The third weight loss step (573.2–823.2 K) is consistent with the elimination of OH/F groups, which is followed by the decomposition of the frameworks.^19,26,29 For MIL-101-Ni again, three stages are observed in the TG curve similar to that obtained for MIL-101. However, MIL-101-Cu has only two steps of weight loss. The first step can be attributed to loss of water molecules from large pores and the second step is due to the decomposition of organic frameworks of MIL-101. For MIL-101-Cu, the weight loss step, corresponded to the loss of guest water molecules from the medium-sized pores, is not observed because the doped MNPs block the pores of the adsorbent and also reduce surface area of MIL-101.

3.2. CO₂ adsorption study

CO₂ adsorption isotherms of MIL-101 for each synthesis time are presented in Fig. S7–S9 in the ESI.† The results show reasonable agreement of experimental isotherm data for each batch synthesis. Therefore, MIL-101 was synthesized with good reproducibility. The CO₂ adsorption capacity obtained for MIL-101 in this study is compared with those previously reported in the literature in Table S1 (ESI†).

CO₂ adsorption isotherms for MIL-101, activated-MIL-101, MIL-101-Cu and MIL-101-Ni are plotted in Fig. 6, at three different temperatures of 298.2, 310.2 and 320.2 K under CO₂ pressure up to 7.1 bar. After repeating the steps of metal doping, CO₂ adsorption isotherms were replicated for both new synthesized MIL-101-Cu and MIL-101-Ni adsorbents and showed good reproducibility. The results are shown in Fig. S10 and S11 (ESI†).

Fig. 6 The CO₂ adsorption isotherms for (a) MIL-101, (b) activated-MIL-101, (c) MIL-101-Ni, and (d) MIL-101-Cu adsorbents at different temperatures, 298.2, 310.2 and 320.2 K.

Fig. 7 compares the CO₂ adsorption capacity of studied adsorbents at 298.2 K. As shown in this figure, MIL-101 and activated-MIL-101 have CO₂ adsorption capacities of 9.7 and 10.6 mmol g⁻¹, respectively, at 298.2 K and 7.1 bar. By metal doping in MIL-101, the CO₂ uptakes on MIL-101-Ni and MIL-101-Cu have been enhanced to 12.4 and 11.8 mmol g⁻¹, respectively. To our knowledge, there is no report for MNP-loaded MIL-101 for CO₂ adsorption in the relevant literature.

Fig. 7 CO₂ adsorption capacity of MIL-101 adsorbents at 298.2 K.

According to Fig. 6 and 7, CO₂ adsorption isotherms on all the synthesized adsorbents are nearly linear and no apparent saturation was observed within studied range of pressure. It means that the adsorbents can adsorb more CO₂ by further increasing of pressure.

The higher adsorption capacity of activated-MIL-101 than MIL-101, shown by Fig. 7, can be ascribed to higher surface area achieved as a result of the activation of MIL-101 using microwave. The unreacted chromium nitrate salt and terephthalic acid trapped inside the pores may be reduced in the presence of the reducing agent (sodium borohydride) during the activation process. It can be clearly seen that the amounts of CO₂ adsorbed in both metal-doped MIL-101 adsorbents are higher than those of non-doped MIL-101.

Although metal doping reduces surface area of adsorbents, according to Table 2, an increase in CO₂ uptake for metal-doped MIL-101 adsorbents is revealed by Fig. 7. It can be attributed to the creation of strong affinity between MNPs and CO₂ molecules. Also according to Fig. 7, the CO₂ adsorption capacities for different adsorbents follow the order of MIL-101-Ni > MIL-101-Cu > activated-MIL-101 > MIL-101. The maximum CO₂ uptake reached 12.4 mmol g⁻¹ at 7.1 bar and 298.2 K on MIL-101-Ni adsorbent. It is noticeable that the adsorption capacity of MIL-101-Ni is clearly higher than that of MIL-101-Cu, which can be attributed to the well dispersion of Ni nanoparticles in the adsorbent. This finding has been approved by TEM images, (given by Fig. 5).

It is well worth mentioning that at low pressures, CO₂ adsorption capacity of MIL-101-Cu is higher than that of MIL-101-Ni. It is because the interaction between CO₂ molecules and Cu nanoparticle is stronger than that between CO₂ molecules and Ni nanoparticle. However, at higher pressures, the MIL-101-Ni adsorption capacity surpasses that of MIL-101-Cu.

The CO₂ uptake for synthesized adsorbents was also examined by fitting the Freundlich isotherm equation. Table 3 presents the fitted values of Freundlich model parameters and their respective regression coefficients. The results demonstrate a good agreement between experimental data and model prediction.

Table 3 Freundlich isotherm model (q = Kp_i^1/n) constants for CO₂ adsorption on MIL-101 adsorbents

Temperature (K)		298.2	310.2	320.2	Temperature (K)		298.2	310.2	320.2
MIL-101 adsorbent	n	0.903	1.068	0.829	MIL-101-Cu adsorbent	n	1.260	1.050	0.986
	K	1.148	1.199	0.641		K	2.360	1.620	1.080
	R^2	0.999	0.999	0.988		R^2	0.989	0.987	0.993
Activated-MIL-101 adsorbent	n	0.978	0.815	0.797	MIL-101-Ni adsorbent	n	1.104	0.927	0.858
	K	1.408	0.887	0.642		K	2.060	1.375	0.898
	R^2	0.999	0.998	0.999		R^2	0.998	0.999	0.999

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Since the isosteric heat of adsorption is a measure of the interaction energy between CO₂ molecules and MOF adsorbents, it was evaluated by applying the Van't Hoff's equation on adsorption isotherms data, as follows:

where Q_st is the isosteric heat of adsorption (kJ mol⁻¹), P is the pressure (bar), T is the absolute temperature (K), R is the universal gas constant and q is the amount adsorbed (mmol g⁻¹).

According to eqn (1), the isosteric heat of adsorption can be obtained as a function of surface loading. The results are given by Fig. 8 that shows the isosteric heats of adsorption decrease with the adsorption amount for all adsorbents. It may be related to heterogeneous surface, which is composed of adsorption sites having different energy of adsorption.⁵⁹ The low-loading limit (q → 0) of isosteric heats for CO₂ adsorption on the MIL-101, activated-MIL-101, MIL-101-Cu and MIL-101-Ni were calculated as much as 22, 27, 31 and 38 kJ mol⁻¹ respectively. The higher attained isosteric heat for MIL-101-Ni is in line with the greater CO₂ adsorption capacity for this type of adsorbent, both showing stronger interaction between Ni NPs and CO₂ molecules.

Fig. 8 Isosteric heat of CO₂ adsorption on synthesized MIL-101 adsorbents.

The stability and regeneration ability of adsorbent, which can be called as cyclability, during consecutive adsorption–desorption cycles is crucial for any type of adsorbents which is going to be employed in a commercial adsorption plant. TSA method was employed to conduct more than 100 successive cycles of CO₂ adsorption–desorption on the adsorbents synthesized in this work.

The CO₂ adsorption capacities as a function of number of cycles for all synthesized adsorbents are presented in Fig. S12† at 310.2 K. To increase the clarity of displaying the adsorption capacity during adsorption–desorption cycles, the obtained results are plotted as ln(adsorption capacity) versus ln(number of cycles) in Fig. 9 at 310.2 K. As shown in Fig. S12,† adsorption capacity does not change significantly during these cycles, and remains nearly constant at 8.1, 9.8, 11.3, 11.0 mmol g⁻¹ up to 100 repeated cycles for MIL-101, activated-MIL-101, MIL-101-Ni and MIL-101-Cu, respectively. Although CO₂ adsorption capacity of MIL-101 increases during the first 9 cycles, from 6.4 to 8.1 mmol g⁻¹, it remains constant at 8.1 mmol g⁻¹ for up to 100 repeated cycles. As mentioned, MIL-101 is the only adsorbent that does not go through the activation process. During this process, the adsorbents lose the residual materials left over after synthesis stage, and hence adsorption capacity and surface area increase. For MIL-101, the regenerations occurring in the first cycles act as an activation process, and hence these stages enhance the adsorption capacity. As another important finding, the doping of MIL-101 with MNP does not show any adverse effect on cyclability. It means that the whole cycle of CO₂ adsorption can be performed by employing TSA method.

Fig. 9 Plot of ln(adsorption capacity) versus ln(number of cycles) at 310.2 K for (a) MIL-101, (b) activated-MIL-101, (c) MIL-101-Ni, and (d) MIL-101-Cu.

4. Conclusion

In this work, MIL-101 was synthesized by hydrothermal method reproducibly and then activated by microwave irradiation. Microwave irradiation method was recognized as a facile, flexible and effective method for incorporating of MNPs (Ni and Cu) within activated-MIL-101. All adsorbents were characterized by using XRD, SEM, EDS, ICP and TEM analyses. The adsorption capacity and cyclability were considered as the primary parameters influencing CO₂ capturing performance of the synthesized adsorbents. These parameters were investigated through a set of volumetric adsorption experiments performed at temperatures 298.2, 310.2 and 320.2 K, and pressures up to 7.1 bar.

The maximum uptake amounts of CO₂ on MIL-101, activated-MIL-101, MIL-101-Cu and MIL-101-Ni were 9.7, 10.6, 11.8 and 12.4 mmol g⁻¹, respectively at 298.2 K and 7.1 bar. The metal doped MIL-101 has higher adsorption capacities for CO₂ adsorption that it's metal-free MIL-101. It may be related to attractive interaction of the transition metal and the carbon dioxide. Uniform distribution of Ni NPs on MIL-101 without aggregation was confirmed by TEM results that explained its higher adsorption capacity than other adsorbents. The low-loading isosteric adsorption heats of CO₂ in the synthesized adsorbents were estimated. Retaining the capacity after a large number of successive adsorption-regeneration cycles demonstrated the good cyclability of all adsorbents synthesized in this research.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra22450k

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