Synthesis, characterization and adsorption ability of UiO-66-NH₂

The emission of CO₂ leads to adverse impacts on the environment such as global warming, rising of sea-level as well ocean acidity. The CO₂ and CH₄ are currently the major source of emissions leading to greenhouse effect [1]. Therefore, there are many researches focused on removal of these gases from the environment. Among methods used for this purpose, adsorption and separation by porous materials is shown to have high yield and to be economically competitive. A good adsorbent should have large specific surface, high porosity, flexible frame structure and periodic arrangement for the reversible adsorption and desorption process [2]. However, some traditional adsorbents such as zeolites, activated carbons, calcium oxides, hydrotalcites amino etc, are very difficult to regenerate at low temperature, then adsorption capacity of adsorbents will decrease from time to time, resulting in high cost of treatment [3, 4]. Recently, metal organic framework (MOFs) have become high potential materials for storage and separation of gases thanks to their high specific surface area, large pore volume, molecular cell and homogeneous structure, adjustable chemical functionality [5–9]. The adsorption of CO₂ in MOF-2 was first studied by Yaghi et al [10]. Then, the CO₂ storage capacity in nine MOFs at room temperature and 42 bar was also tested [11]. In other research, the result showed that adsorption capacity of CO₂ on MOF-177 at 35 bar was reached 33.5 mmol g⁻¹, nine times higher than the amount of CO₂ in a container without adsorbent [11]. The adsorption capacity of CH₄ on MOFs was also studied and it reached 155 cm³ g⁻¹ on IRMOF-6 at 35 bar [12]. Mueller et al have synthesized MOF-210 with high specific surface area which reached 10 400 m² g⁻¹ with CH₄ storage capacity of 666.82 cm³ g⁻¹ CH₄, that was higher than other porous materials [13]. Although the adsorption ability of CO₂ and CH₄ on MOFs is confirmed, the important feature of MOFs is their thermal and water stability. Previous investigations showed that although adsorption capacity of UiO-66 is not high compared to other MOFs, the advantage of UiO-66 consists in its structural stability in water and high thermal stability [14].

Adding functional groups in structures is one of the methods to increase the adsorption capacity of MOFs. Therefore, UiO-66-NH₂ was synthesized and its physical and chemical properties are also studied. The effect of preparation parameters such as solvents volume, reaction times, exchange solvents and activated times on the propertiy of MOF were also investigated. The adsorption capacity of CO₂ and CH₄ on UiO-66-NH₂ are obtained and compared to UiO-66.

UiO-66 and UiO-66-NH₂ were prepared following the procedure described by Luu Cam Loc et al [15] and Kandiah et al [16]. In order to determine the optimal condition of UiO-66-NH₂ preparation, in this study the volume of solvent was varied from 10 ml to 30 ml, reaction time was varied from 12 h to 48 h and activation time was varied in the range of 3 h to 9 h at a temperature of 473 K.

The morphologies of the synthesized product were obtained using a scanning electron microscope (SEM, HITACHI S−4800). Powder x-ray diffraction (PXRD) data were recorded on a Bruker D8 Advance diffractometer. Thermogravimetric analysis (TGA, Q500 V20.10) was also performed on samples to study the thermostability of the materials. Brunauer–Emmett–Teller (BET) surface areas and total pore volumes of the samples were determined from N₂ adsorption isotherms at 77 K on Quantachrome NovaWin machine. Fourier transform infrared (FT-IR) spectra (4000 − 400 cm⁻¹) were obtained from KBr pellets using a Bruker Tensor 22 instrument.

The gas (CO₂, CH₄ and mixture 50%CO₂/50%CH₄) adsorption capacity on UiO-66 and UiO-66-NH₂ was investigated using a high pressure volumetric analyzer (Micromeritics HPVA-100) at temperature 303 K and pressure in range from 1 to 30 bar.

3.1. UiO-66-NH₂ synthesis

3.1.1. Effect of reaction solvent volume

It can be observed from XRD patterns (figure 1(a)) that for the UiO-66-NH₂ synthesized in different solvent volumes, the highest intensity of characteristic peaks is obtained in the case of 16 ml solvent. The reason may be that the volume of solvent is too little (10 ml), being not enough to dissolve the reactants in the process of UiO-66-NH₂ creation. In contrast, with a large volume of solvent (≥22 ml), the crystallization of UiO-66-NH₂ decreased due to the low concentration of reactants.

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3.1.2. Effect of reaction times

The XRD pattern of UiO-66-NH₂, synthesized in different reaction times (figure 1(b)), contained characteristic peaks of UiO-66 at 2θ = 7.34°, 8.48° [14–17] with different intensity dependences on the reaction times. The variation of peak intensity has extreme character. The intensity of characteristic peaks increased with increasing reaction times from 12 to 36 h, reached maximum at 36 h, then, when reaction time was increased up to 48 h, the characteristic peaks of UiO-66-NH₂ decreased. This might be due to the short time (24 h) in which the reaction has not reached equilibrium, leading to low intensity peaks. When reaction time is 36 h, the reaction reach equilibrium, the intensity of peak became strongest.

3.1.3. Effect of exchange solvents

The experimental results showed that for the UiO-66-NH₂ prepared in optimal conditions (16 ml of reaction solvent, temperature and time of reaction: 393 K in 36 h) and using CH₂Cl₂ as exchanged solvent, the highest specific surface area was obtained (S_BET = 754.2 m² g⁻¹). Using C₂H₅OH or CH₃OH as an exchanged solvent instead of CH₂Cl₂, the specific surface area of the sample decreased, reached S_BET = 122.9 m² g⁻¹ and S_BET = 576.4 m² g⁻¹, respectively. The boiling point solvent of CH₂Cl₂ (boiling point = 313 K) is lower than that of CH₃OH (boiling point = 337.7 K) and C₂H₅OH (boiling point = 351.4 K), allowing the activation to be performed at lower temperature. Besides, CH₂Cl₂ has low acidity so its bonding with NH₂ group is also weaker than that of CH₃OH and C₂H₅OH, therefore, it is easier to separate during activated process.

3.1.4. Effect of activation time.

Generally, basing on results showed in figure 2, the specific surface area of product increased with activation time, and then decreased after reaching its maximum value. At the temperature 473 K, the activation times increased from 3 h to 5 h and 7 h, the specific surface area increased from 434.12 m² g⁻¹ up to 753.95 m² g⁻¹ and 873.07 m² g⁻¹, respectively. With further increasing the activated time to 9 h, the specific surface area decreased to 571.28 m² g⁻¹.

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The results of the above study allowed proposing the method for the preparation of UiO-66- NH₂ as follows: the mixture including ZrCl₄ (324 mg) and 2-aminoterephthalic axit (252 mg) was dissolved in 16 ml N,N-dimethylformamide (DMF). The solution was filled in a tightly capped vial, placed in a hot air oven and heated to 393 K overnight (36 h). The solid product then was removed by decanting with mother liquor and washed with DMF. After that solvent exchange was carried out with CH₂Cl₂. Finally, the solid product was activated under vacuum pressure at 473 K for 7 h. The physico-chemical characteristics of UiO-66-NH₂ are introduced below.

The XRD pattern in figure 3(a) allowed us to confirmed that UiO-66-NH₂ was synthesized successfully with characteristic peaks of UiO-66 at 2θ = 7.34°, 8.48°. Figure 3(a) displayed narrow lines and high crystallinity of the sample and showed good agreement with that reported in the literature [14–17]. The morphologies of the sample are shown in figure 3(b). As is seen, the crystals of UiO-66-NH₂ formed ball shape and the particle size was about 200 nm. The thermal stability of UiO-66-NH₂ was investigated by thermogravimetric analysis (TGA) as shown in figure 3(c). Three weight loss steps were observed. The first weight loss of 4.3 wt% occurred between 303 K and 403 K due to vaporization of water. The second step of weight loss was 5.8 wt% at 403–673 K due to dehydroxylation of OH⁻ at 523 K and was completed at 573 K [18]. The third step of weight loss was 22 wt% above 673 K due to decomposition of material. The result indicated that UiO-66-NH₂ was stable up to 673 K.

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The IR spectrum of UiO-66-NH₂ is shown in figure 3(d). Overall it looked similar to the spectrum reported by Abid et al [17]. The appearance of absorption band at 1570.35 cm⁻¹ indicated the existence of the reaction of –COOH with Zr⁴⁺. The band of 1506.02 cm⁻¹ is referred to C = C from aromatic. The peak appearing at 3420.52 cm⁻¹ is NH₂ on the organic linker.

Synthesized UiO-66-NH₂ showed high surface area up to 873 m² g⁻¹, micropore volume of 0.379 cm³ g⁻¹, pore diameter of 18.96 Å and stability up to 673 K. Meanwhile, UiO-66 prepared by us in a previous study [15] obtained BET surface area 1041 m² g⁻¹, micropore volume 0.913 cm³ g⁻¹, pore diameter of 30 Å and thermal stability up to 696 K [15]. From these data it follows that the addition of NH₂ to UiO-66 led to a decrease in surface area, pore volume, and pore diameter.

3.2. CO₂ adsorption on UiO-66-NH₂

Figure 4 describes the CO₂ adsorption and desorption isotherms of UiO-66 and UiO-66-NH₂ at pressure range from 0 to 30 bar and temperature 303 K. At low pressure (≤15 bar), CO₂ adsorption on UiO-66-NH₂ is higher than that on UiO-66, although the surface area of UiO-66-NH₂ is lower than that of UiO-66 (873 m² g⁻¹ compared to 1041 m² g⁻¹). This could be attributed to the presence of NH₂ group in porous structure. At 1 bar and 303 K, CO₂ adsorption on UiO-66-NH₂ could reach 52.15 cm³ g⁻¹, which is twice higher than that on UiO-66 (28.43 cm³ g⁻¹).

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In contrast, at high pressure (15 to 30 bar), CO₂ adsorption on UiO-66-NH₂ is always lower than that on UiO-66. At 30 bar and 303 K, CO₂ adsorption on UiO-66-NH₂ is lower than that on UiO-66, reaching 145 cm³ g⁻¹ và 167.7 cm³ g⁻¹, respectively. The obtained result indicates that the presence of NH₂ functional group increased the adsorption only at low pressure. The explanation for this result is that at low pressure, the adsorption capacity of CO₂ of UiO-66-NH₂ is high because of the effect of Lewis base center NH₂, while the effect of specific area is still not clear yet. But at high pressure specific surface becomes the deciding factor for CO₂ adsorption. Thus, in this condition, UiO-66-NH₂ with lower specific surface area compared to UiO-66, expressed the lower adsorption capacity. Besides, the isothermal adsorption curve of CO₂ on UiO-66-NH₂ reaches saturated state at 30 bar. Meanwhile, the isothermal adsorption curve of CO₂ on UiO-66 is continuously increasing at 30 bar. The explanation for this is that the adsorption process is limited by NH₂ groups on the surface of UiO-66-NH₂ at high pressure.

3.3. CH₄ adsorption on UiO-66-NH₂

The adsorption and desorption isotherms of CH₄ on UiO-66 and UiO-66-NH₂ at pressure 0 to 30 bar and 303 K are shown in figure 5. As seen, at range 0–30 bar and 303 K, CH₄ adsorption on UiO-66-NH₂ is always higher than that on UiO-66. At 1 bar and 303 K, the volumes of CH₄ adsorption of 11.1 cm³ g⁻¹ and 6.68 cm³ g⁻¹ on UiO-66-NH₂ and UiO-66, respectively, were obtained. At 30 bar and 303 K, CH₄ adsorption on UiO-66-NH₂ is also higher than that on UiO-66, reaching 57 cm³ g⁻¹ and 49.1 cm³ g⁻¹, respectively. The result indicated that the presence of NH₂ groups is the more important factor affecting the adsorption ability of CH₄ in comparison with specific surface area or pore volume. A similar result was reported by Wu et al [19].

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3.4. The adsorption of mixture 50%CO₂/50%CH₄

Figure 6 shows the increase of adsorption capacity of mixture CO₂ and CH₄ with the ratio 1:1 (v/v) on UiO-66-NH₂ when pressure increases. At 30 bar and 303 K, on UiO-66-NH₂ the CO₂ adsorption is twice those of CH₄, which were 145 cm³ g⁻¹ and 75.38 cm³ g⁻¹, respectively. It can be explained by the strong interaction between CO₂ and adsorbent and high dipole moment of CO₂ (13.4 × 10⁻⁴⁰ C m²) while CH₄ is nonpolar molecular [1]. The adsorption ability of the mixture of 50% CO₂/50% CH₄ is lower than that of pure CO₂ but higher than that of pure CH₄ on UiO-66-NH₂, as shown in figure 6 also.

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Adsorption characteristics of the mixture of 50%CO₂/50%CH₄ (figure 7) on UiO-66-NH₂ and UiO-66 similar to CO₂ adsorption (figure 5), i.e. at pressure lower 15 bar, the adsorption capacity of 50%CO₂/50%CH₄ mixture on UiO-66-NH₂ was higher than that on UiO-66, but in the region of pressure higher than 15 bar, in contrast, the adsorption capacity of UiO-66 was higher than that of UiO-66-NH₂. It can be related to the fact that CO₂ storage capacity of the UiO-66 and UiO-66-NH₂ is three times higher than that of CH₄.

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3.5. Study on adsorption stability of UiO-66-NH₂

The reusability of UiO-66-NH₂ in adsorption process is also shown in table 1.

Table 1.  Volume uptake of CO₂ and CH₄ on UiO-66-NH₂ in ten adsorption-desorption cycles (from 1 to 10) under different pressures (P).

		Cycles of adsorption (cm3 g−1)
Gas	P(bar)	1	2	3	4	5	6	7	8	9	10	Standard error (%)
	1	52	51	55	50	42	53	52	53	50	52	1.1
CO2	14	133	136	132	132	138	134	135	137	137	132	0.78
	30	145	148	142	144	155	145	147	149	150	143	1.25
	1	11.1	11.1	11.3	11.4	11.4	14	10.9	10.9	11.1	12.1	0.29
CH4	14	46.1	53.1	53.8	52.5	52.2	53.7	54.5	54.8	53.9	54.8	0.8
	30	58	70	71.2	68.2	67.8	65.6	73.1	73.7	71.5	70.8	1.45

From the data of table 1 it follows that the adsorption capacity of CO₂ and CH₄ on UiO-66-NH₂ are nearly unchanged after 10 cycles of adsorption–desorption. Besides, as was shown from the XRD spectrum (figure 8), the crystal structure of UiO-66-NH₂ is nearly not changed after 10 adsorption–desorption cycles. These data confirmed that UiO-66-NH₂ is a potential material for storage of gases even at high pressure. Besides, it could be easily regenerated by decreasing pressure at low temperature of desorption.

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UiO-66-NH₂ was successfully synthesized; the optimal conditions for preparation were determined. The synthesized UiO-66 and UiO-66-NH₂ should be considered as an effective adsorbent for CO₂, CH₄, and CO₂/CH₄ mixture. Unchanged structure of UiO-66-NH₂ under high pressure adsorption process and reversible adsorption of CO₂ and CH₄ allow one to reuse UiO-66-NH₂ many times.

The addition of amine groups to UiO-66 not only led to significant decrease in surface area, pore volume, and pore radius, but also increased its adsorption capacity; at 1 bar and 303 K, storage of CO₂ and CH₄ increased two-fold.

Comparing with UiO-66, addition of NH₂ group enhanced the adsorption of CO₂ and CO₂/CH₄ mixture at low pressure but showed an adverse effect at high pressure. Meanwhile, it increased the adsorption of CH₄ even at high pressure.

The research group acknowledges the financial support for the program code VAST 03.01/13–14 from the Materials Science Council, Vietnam Academy of Science and Technology.