Environ Eng Res > Volume 28(1); 2023 > Article
Lin, Zeng, Liu, Zhong, Su, Gong, and Liu: Amino-modified Mg-MOF-74: Synthesis, characterization and CO2 adsorption performance

Abstract

Based on the solvothermal method to synthesize Mg-MOF-74, the amino-functionalized Mg-MOF-nNH2 was prepared through addition of amino-containing ligands. Physicochemical properties of Mg-MOF-nNH2 materials were well characterized by Powder X-ray diffraction (PXRD), Scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT-IR), N2 adsorption/desorption isotherms (BET). The adsorption properties of the resulting materials were studied for CO2. In all the samples, the micropore volume of Mg-MOF-1/8NH2 was 0.46 cm3 g−1 and the specific surface area was 924.19m2 g−1, the highest CO2 saturated adsorption capacity was 3.9 mmol g−1, and the dynamic adsorption capacity was 1.27 mmol g−1. The adsorption processes agreed well with the Langmuir isotherm and Avrami model.

1. Introduction

Global warming has become increasingly serious and environmental problems such as rising sea levels, floods and frequent ocean storms caused by climate change have become increasingly prominent [1]. Burning of fossil fuels such as coal and petroleum, oil refining, steel smelting and other chemical industry are the main activity that emits CO2 to the atmosphere [2]. In order to minimize CO2 emissions, the most effective method available for control the increasing concentration of carbon dioxide in the atmosphere is carbon capture and sequestration (CCS) [3]. Metal-organic frameworks (MOFs) can be used as an adsorbent in the adsorption of carbon dioxide and have shown better performance compared to traditional adsorbents such as zeolite, mesoporous silica and activated carbon. Currently, several MOFs materials have been used in the adsorption of carbon dioxide research, such as Mg-MOF-74 [4], MIL-101(Cr) [5], Cu-BTC [6], MIL-53(Al) [7], UiO-66(Zr) [8], and so on. Of these, Mg-MOF-74 has a broad prospect of adsorption of carbon dioxide application due to their unique structural characteristics, large surface areas and enriched unsaturated metal sites [9]. However, the adsorption interaction between the Mg-MOF-74 and the carbon dioxide molecules is a weak physical action, and it is suitable for applicable to low temperature and low pressures environment, which has largely restricted their practical applications. Therefore, it is of great significance to enhance the force between the Mg-MOF-74 and the carbon dioxide molecules in order to increase the adsorption capacity of the material at a higher temperature [10].
It is found that the presence of nitrogen-containing functional groups can enhance the CO2 adsorption capacity of MOFs. The Lewis basic site provided by the nitrogen-containing group has a strong affinity with CO2 as a conjugated base. Presently, most of the modifications of basic groups are reported about post-synthetic modification. For example, loading nitrogen-containing groups in MOFs pore canals by impregnating with various alkaline nitrogen-containing substances [11], grafting nitrogen-containing groups after the material’s synthesis [12]. Although these post-synthetic modification methods can increase the affinity of the materials to carbon dioxide molecules, the introduction of nitrogen-containing groups will block the original microporous structure of the materials and reduce the specific surface area of the materials, resulting in a decrease in physical adsorption capacity. The direct synthesis of functionalized MOFs materials from the connectors containing amino or nitrogen heterocycles can not only maintain the original topological structure of the materials, have a large specific surface area, but also enhance CO2 affinity, so it is an excellent alternative [13].
To summarize, in this work, functionalized Mg-MOF-74 was synthesized by adding amino-containing ligands. the 2,5-dihydroxyterephthalic acid and 2-amino terephthalic acid were used as the ligands material, Mg (NO3)2·6H2O was used as a metal center ion donor, the mixed solution of N, N-dimethylformamide (DMF), distilled water and ethanol were used as the solvent, to synthesize Mg-MOF-nNH2 and the carbon dioxide adsorption capacity of synthetic materials was measured, and look forward to its application prospect.

2.1. Materials

Magnesium nitrate hexahydrate (Mg (NO3)2·6H2O) (≥ 98%, Sinopharm), N, N–dimethylformamide (DMF) (≥ 99%, Sinopharm), 2,5-dihydroxyterephthalic acid (C8H6O6) (≥ 99%, Aladdin), 2-aminoterephthalic acid(C8H7NO4) (≥ 99%, Aladdin), methanol (CH4O) (≥ 99.5%, Sinopharm), anhydrous ethanol(C2H6O) (≥ 99.5%, Sinopharm), carbon dioxide(CO2) (≥ 99.999%, Fuzhou Xinhang Gas Co., Ltd.), nitrogen(N2) (≥ 99.999%, Fuzhou Xinhang Gas Co., Ltd.).

2.2. Preparation of Materials

Synthesis of Mg-MOF-74: The typical synthetic process was performed as follows: Mg (NO3)2·6H2O (1.95 mmol, 0.712 g) and 2,5-dihydroxyterephthalic acid (0.83 mmol, 0.167 g) were dissolved in the mixed solution of N, N-Dimethylformamide (DMF), 67.5 mL deionized water 4.5 mL, ethanol 4.5 mL (V(DMF):V(H2O): V(CH3CH2OH) = 15:1:1). After ultrasound was used for 30 min, the solution was transferred into a Teflon-lined autoclave (100 mL) and kept at 398K for 24 h. Next, the sample was cooled naturally to room temperature, the precipitate was filtered and the product was immersed in methanol for 3 d, the supernatant solution was changed every 12 h, then the solid product was separated by centrifugation. Finally, the product was placed in a vacuum drying box and dried for 12 h at 373K, dark yellow solid was obtained. The samples were designated as Mg-MOF-74.
Synthesis of Mg-MOF-nNH2: The Mg (NO3)2·6H2O (1.95 mmol, 0.712 g), 2,5-dihydroxyterephthalic acid (x mmol), 2-aminoterephthalic acid (y mmol) were weighed (where x: y = 10:1, 8:1, 6:1, 4:1), dissolved in the mixed solution of N, N-dimethylformamide (DMF), 67.5mL deionized water 4.5 mL, ethanol 4.5 mL (V(DMF): V(H2O):V(CH3CH2OH) = 15:1:1). The next steps were the same as for the synthesis of Mg-MOF-74. the samples are designated as Mg-MOF-nNH2. (n = y/x)

2.3. Characterization

Structural analysis was carried out by powder Xray diffraction (PXRD, Ultima IV, Buker, Germany). The Fourier infrared analysis was carried out by Fourier transforming infrared spectrometry (FTIR, SP2000, Thermo Nicolet Corporation, USA). The surface morphology of the samples was observed using a scanning electron microscope (SEM, Hitachi SU8010, Hitachi High Technologies Corp, Japan). The thermal stability of the samples was determined by Thermogravimetric Analysis (TG, TG209F3, Netzsch, Germany) The specific surface area and the pore size of samples were determined by an automatic surface area and pore size analyzer (AutosorbiQ2, Quantachrome Instruments Co., USA).

The static adsorption-desorption experiment of CO2 was carried out by using a thermogravimetric analyzer (TG209-F3, Netzsch, Germany). The sample was heated and degassed to constant weight in high purity N2 flow (40 cm3/min), and introduction of high purity CO2/N2 mixture for 1 h after the system temperature dropped to test temperature, recorded sample weight. Next, the gas in the system was converted into a high purity N2 flow, and the temperature was raised to 373k and kept for 1 h. After desorption was completed, the temperature was set to the next adsorption temperature, and the next adsorption process continued. The adsorption capacity qa is calculated using Eq. (1).
(1)
$qa=1000(me-m0)44m0$
Where qa (mmol/g) is the adsorption capacity; me (mg) is the weight at the end of adsorption; m0 (mg) is the initial sample weight; 44 is the molecular weight of CO2.

Lagergren pseudo first-order kinetic model [18], Ho pseudo second-order kinetic model [19] and Avrami fractional order dynamics model [20] were used to explore the adsorption mechanism. The expression of the Lagergren pseudo first-order adsorption kinetic model is as follows Eq. (2):
(2)
$dqtdt=k1(qe-qt)$
where qt (mmol/g) is the adsorption capacity at the time t; qe (mmol/g) is saturated adsorption capacity; k1 (mmol·g−1·s−1) is the quasi-first-order adsorption rate constant.
The integral form of Eq. (2) is
(3)
$qt=qe(1-e-k1t)$
The Ho pseudo-second-order adsorption kinetic model can be written as below in Eq. (4)
(4)
$dqtdt=k2(qe-qt)2$
where k1 (mmol·g−1·s−1) is the quasi-second-order adsorption rate constant.
The integral form of Eq. (5) is
(5)
$qt=t1k2qe2+1qe$
The Avrami fractional-order dynamics expressions such as Eq. (6)
(6)
$dqtdt=kantn-1(qe-qt)$
In the equations above, ka (min−1) is the Avrami model kinetic constant; n is the Avrami index, dimensionless constant; qe (mmol/g) is the saturated adsorption capacity; qt (mmol/g) is the adsorption capacity at the time t.
The integral form of Eq. (7) is
(7)
$qt=qe [1-e-(kat)n]$

Langmuir and Freundlich models [21, 22] were used to analyze and fit the sorption isotherm data, the linear form of the Langmuir is as follows in Eq. (8):
(8)
$qe=qmKLP1+KLP$
where qm (mmol/g) is the maximum adsorption capacity of the Langmuir model; KL is the Langmuir model constant; p is the CO2 partial pressure; qe (mmol/g) is the adsorption capacity at equilibrium.
The linear form of the Freundlich is as follows in Eq. (9):
(9)
$qe=Kfp1/n$
where Kf is the Freundlich model constant; n is a nonuniformity factor; p is the CO2 partial pressure; qe (mmol/g) is the adsorption capacity at equilibrium.

Thermodynamic parameters were calculated using the following equations [23]:
(10)
$ln KL=(ΔS0R)-(ΔH0RT)$
(11)
$ΔG0=ΔH0-TΔS0$
where ΔS0 is the entropy change; ΔH0 is the enthalpy change; ΔG0 is the Gibbs free energy change.
Isosteric heats of adsorption were estimated by the Clausius–Clapeyron equation [24], the expression is as follows:
(12)
$ln (p)=-QstRT+C$
where ΔH is the equivalent adsorption heat; T (K) is the adsorption temperature.

2.4.5. Breakthrough performance test

CO2 dynamic adsorption experiments were carried out at atmospheric pressure and different temperatures (303K, 313K, 323K). The experimental setup is shown in Fig. S1. The size of the adsorption column is D × L = 13 × 100 mm, the loading capacity of the adsorbent was 1,000 mg, the outlet CO2 concentration was detected by gas chromatography (GC9790, Zhejiang Fuli analysis instrument Co., Ltd., China). The working conditions of the instrument included the injector temperature was maintained at 423K, detection temperature was 423K and the temperature of the chromatographic column was 423K. Helium was used as the carrier gas. The continuous injection method was used to detect the gas concentration at the outlet once per 1.5 min.
The adsorption breakthrough curve is the variation curve of outlet gas concentration with time and the position where the concentration of gas at the outlet rises sharply is the breakthrough point, corresponding time t1 is the adsorption penetration time [25]. In this paper, the saturated adsorption capacity of adsorbent for CO2 is calculated based on the adsorption breakthrough curve and the formula is as follows:
(13)
$q=1M×[∫0tQ×C0-C1-Cdt]×T0T×1Vm$
where q (mmol/g) is the adsorption capacity; Q (cm3/min−1) is the gas flow rate; C0 (%) is the inlet concentration of CO2; C (%) is the outlet concentration of CO2; T (K) is the adsorption temperature; T0 is 273K; M (mg) is the mass of adsorbent; Vm is the molar volume of gas, 22.4 cm3/min.
According to the model of gas-solid reaction proposed by Yasyeli et al. [26] and the isothermal mass conservation equation of CO2 concentration in the adsorption column is shown in the following Eq. (14):
(14)
$-QdCdM-k0Cα=0$
where k0(m3·kg−1·min−1)is the initial adsorption rate constant; α is the rate of change of the solid reactants activity; α can be expressed by the following Eq. (15):
(15)
$-dαdt=kdCjαm$
The unit of kd is min−1; j indicates the independence of gas adsorption; m means gas adsorption and put-in activity; j = 1, m = 1, the modified form of the breakthrough curve model is as follows:
(16)
$CC0=exp[[1-(exp(k0MQ(1-exp(-kdt))))]1-exp(-kdt)exp(-kdt)]$
Avrami fractional-order dynamics can be used to simulate the adsorption process, the formula is as follows:
(17)
$y=1-e-(kat)n$
where y (%) is desorption ratio; ka (min−1) is desorption kinetic constant; n is Avrami desorption index, dimensionless constants.

3.1. Characterization Results

3.1.1. XRD characterization

The XRD patterns of the Mg-MOF-74 and Mg-MOF-nNH2 are shown in Fig. 1(a). From the figure, the XRD spectra of all samples exhibit two obvious diffraction peaks appeared at 6.9° and 11.9°. The diffraction peak intensity at the 2θ = 11.9º of Mg-MOF-nNH2 is weaker than Mg-MOF-74, and this indicates that the addition of amino ligands affects the crystallinity of materials [27, 28] A new sharp diffraction peak appears at 2θ = 9.7º, it shows that the addition of amino ligand changes the original crystal structure. When adding a small amount of aminoligands, a small amount of -NH2 can promote the ionization of -COOH and the probability of coordination between magnesium ion and carboxyl group was increased, this is the reason that the diffraction peak intensity of Mg-MOF-1/10NH2 and Mg-MOF-1/8NH2 are sharper than other samples. However, excessive content of -NH2 causes steric hindrance effect which hinders the coordination of magnesium ions with ligands.

3.1.2. FT-IR analysis

The FT-IR spectrums of Mg-MOF-74 and Mg-MOF-nNH2 are presented in Fig. 1(b). Peaks at 3,500cm−1 and 3,200cm−1 are the O-H stretching vibration. The peak of Mg-MOF-nNH2 here is slightly sharper than that of Mg-MOF-74. It is attributed to the superposition of vibration of -NH2 which from newly added ligands and stretching vibration of O-H [29].

3.1.3. TG analysis

Thermogravimetric analysis (TGA) weight loss curves are presented in Fig. 2. The slight weight loss at 423K is due to the removal of moisture and residual solvents inside the channel. There is an obvious weight loss in 673K~773K, and the material structure collapses gradually. When the temperature reaches about 903K, with a weight loss of about 50 wt.%. At this time, the coordination bond has been completely broken, the organic ligand is volatilized, and the structure collapses [30]. From Mg-MOF-1/10NH2 to Mg-MOF-1/4NH2, the equilibrium point gradually shifts to the left, and the thermal stability decreases.

N2 adsorption/desorption isotherms and pore size distributions of the samples are depicted in Fig. 3. Typical Type-IV isotherms are found in the Mg-MOF-nNH2, demonstrating that the material obvious mesoporous structure. With the increase of the amount of 2-aminoterephthalic acid, the specific surface area of Mg-MOF-nNH2 decreased gradually. It may be due to the competitive coordination between 2-aminoterephthalic acid ligand and 2.5 dihydroxyterephthalic acid, and excessive -NH2 makes the crystal produce defects and increase pore size [31].
Table S1 shows the parameters of pore structure. Compared with Mg-MOF-74, the specific surface area of Mg-MOF-1/8NH2 decreased 198m2/g, but the proportion of micropore volume increased, and the specific surface areas of Mg-MOF-1/6NH2 and Mg-MOF-1/4NH2 are reduced by about 430m2/g and 607m2/g, respectively. The specific surface area has great impact on the adsorption properties of adsorbent material. Therefore, Mg-MOF-1/8NH2 was selected to further study its CO2 adsorption properties.

3.1.5. SEM analysis

Fig. 4 shows SEM images and EDS spectrums of Mg-MOF-74 and Mg-MOF-1/8NH2. The crystal of Mg-MOF-74 is mainly composed of two kinds of morphology, one is cone-shaped needle shape, the other is cauliflower-like morphology bonded by short columnar grains, and the crystal surface is smooth. The result is consistent with reports in the literature [32]. Compared with Mg-MOF-74, Mg-MOF-1/8NH2 crystal has a certain thickness, single crystal is the hexagonal pyramid and exhibites a rough and uneven surface, indicating that the addition of new ligands changes the morphology and structure of the materials. The content of the N element in Mg-MOF-1/8NH2 was 0.25% determined by EDS, indicating that the amino ligand successfully entered the crystal structure.

Fig. 5(b) shows the results of the isothermal model simulation. With the increase of CO2 concentration, the adsorption capacity increases, and the CO2 adsorption properties of the materials are better in the lower temperature range. As the results in Table S3, the R2 obtained by fitting the data with the Langmuir equation is larger than that of the Freundlich equation, and the adsorption process is more fitted the Langmuir model. 1/n < 1 indicates that adsorption is easy to proceed.
Table S4 shows the comparison of the saturated adsorption capacity of CO2 between Mg-MOF-74 and Mg-MOF-1/8NH2 at different temperatures. The binding force between Mg-MOF-74 and adsorbate is weak, and the adsorption capacity decreases with the increase of temperature, due to the CO2 molecules desorption from adsorbents at high temperatures. The adsorption capacity of Mg-MOF-1/8NH2 which modified by 2-aminoterephthalic acid increased by 9, 18 and 44% at 303K, 313K and 323K, respectively. It is possible due to the chemical adsorption is increased after amino functionalization, and the binding force between CO2 molecules and adsorbents is stronger.

Fig. 6. shows the Van’ Hoff plot of 1/T and ln(P) plot of 1/T at three temperatures. Table S6 shows the results of thermodynamic parameters, and the value of Qst is shown in Table S5. The ΔH value is −8.64 kJ/mol, which shows heat release during the adsorption process, ΔS is 5.2 J/mol suggests a more random contact at the solid/gas, and the affinity of the material to CO2 is high. The average isovolumetric heat of adsorption is 41.54 kJ/mol, which is slightly higher than the enthalpy of physical adsorption (< 40 kJ/mol). During adsorption, there is both chemical adsorption and physical adsorption, mainly chemical adsorption.

3.2.4. Results of dynamic a Cyclic adsorption performance

The relationship between the adsorption of the CO2 and the number of cycles after five cycles can be seen in the Fig. 7. After five adsorption and desorption cycles, the fifth adsorption capacity of Mg-MOF-1/8NH2 still retains 77% of that of the first adsorption, indicating that Mg-MOF-1/8NH2 has a good regeneration ability.

Fig. 8 (c), (d) shows the breakthrough curves of different CO2 concentrations at 303K and 40 mL/min flow rate of Mg-MOF-74 and Mg-MOF-1/8NH2. According to the figures, the saturation point gradually delays with the decrease of initial concentration, and the breakthrough curve becomes smooth at the same time. Table S9 and Table S10 show the parameters of the deactivation model obtained by fitting the experimental results of Mg-MOF-74 and Mg-MOF-1/8NH2. According to the data of tables, the correlation coefficient R2 is more than 0.9, and the adsorbent activity decreases significantly with the increase of the adsorption degree. The change of the adsorption capacity of adsorbent may correlate with the change of pore structure, specific surface area and active site distribution of the adsorbent changes. The inactivation rate kd increases with the increase of initial concentration, indicating that the high CO2 partial pressure promotes the diffusion of CO2 to the reaction site. The adsorption capacity q increases with the increase of concentration, indicating that high pressure CO2 promotes the reaction with the adsorbent

4. Application Prospect

The carbon dioxide adsorption capacity of several common adsorbents are listed in Table S11, and it shows that the material prepared in this work has a high carbon dioxide adsorption capacity at room temperature and normal pressure, which suggests that it is potentially an good sorbent for CO2. The characterization results confirmed that the high temperature has a certain effect on the structure of the material, and then affects the CO2 adsorption capacity of materials. So, we suggest that it can be applied to CO2 capture in confined spaces under room temperature and normal pressure, such as submarines, space station and other environments.

Acknowledgments

This work was supported by Natural Science Foundation of Fujian Province of China (2020Y0055), Project in Fuzhou Science and Technology Bureau (2020-PT-144) and The Startup Fund for Scientific Research (GY-Z19168), Fujian University of Technology.

Notes

Conflict-of-Interest

The authors declare that they have no conflict of interest.

Author Contributions

Dr. X.L. (Professor) and Dr. M.L. (Assistant Professor) jointly drafted the manuscript. Ms. Q.Z. (M.Sc. Student) and Mr. W.Z. (M.Sc. Student) have done all the experiments. Ms. T.S (M.Sc. Student) did literature survey, revised the manuscript. Dr. Y.L. (Professor) and Dr. L.G. (Professor) reviewed the whole work and provided suggestions for the improvement of the paper. All authors have read and agreed to the published version of the manuscript.

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Fig. 1
(a) XRD patterns of Mg-MOF-74 and Mg-MOF-nNH2, (b) FT-IR spectrum of Mg-MOF-74 and Mg-MOF-nNH2.
Fig. 2
TGA curve of Mg-MOF-74 and Mg-MOF-nNH2.
Fig. 3
Nitrogen adsorption/desorption isotherm and pore size distribution of Mg-MOF-74 and Mg-MOF-nNH2:(a) Nitrogen adsorption/desorption isotherm of Mg-MOF-74 and Mg-MOF-nNH2; (b) Pore size distribution of Mg-MOF-74 and Mg-MOF-nNH2.
Fig. 4
SEM images and EDS spectrum of Mg-MOF-74 and Mg-MOF-1/8NH2 :(a) SEM image of Mg-MOF-74; (b), (c) SEM image of Mg-MOF-1/8NH2; (d) EDS spectrum of Mg-MOF-1/8NH2.
Fig. 5
(a) The curves of adsorption kinetic, (b) The curves of adsorption isotherms.
Fig. 6
(a) Van’t Hoff plots for estimation of thermodynamic parameters for CO2 adsorption (b) ln(P) versus 1/T and regression lines at different amounts of adsorbed CO2.
Fig. 8
Breakthrough curves at different temperatures of (a) Mg-MOF-1/8NH2 (b) Mg-MOF-74, Breakthrough curve of different concentrations of (c) Mg-MOF-1/8NH2 (d) Mg-MOF-74.
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