Synthesis of MOF-199 and application to CO₂ adsorption

MOF-199 was prepared by a modification of the synthesis procedure described by Yaghi and co-workers [10] as follows: the mixture including 1.36 mmol of different copper (II) salts and 1,3,5-benzenetricarboxylic acid (0.84 mmol) was dissolved in N,N-dimethylformamide (DMF), C₂H₅OH and H₂O (with ratio DMF:C₂H₅OH:H₂O = 1:1:1) in a 20 ml vial. The vial was tightly capped and heated at 85 °C for 24 h to becoming blue crystal. The solid product then was removed by decanting with mother liquor and washed with DMF. After that solvent exchange was carried out with CH₂Cl₂ or CH₃OH. The product was activated under atmospheric pressure condition (10⁻² Torr).

In order to determine the optimal conditions for MOF-199 preparation different precursors Cu(NO₃)₂, CuCl₂ and Cu(CH₃COOH)₂ were used; the volume of solvent varied from 6 to 12 ml. Activation time varied in the range of 8–15 h at 175 °C and in the range of 2–10 h at 200 °C.

The morphology of the synthesized products was obtained by using a scanning electron microscope (SEM, JEOL 7401). Powder x-ray diffraction (XRD) data was recorded on a Bruker D8 Advance Diffractometer. Also, the thermogravimetric analysis (TGA, TA—Q200) of samples was performed to determine the thermostability of the materials. BET and Langmuir surface areas and total pore volumes of the samples were determined from N₂ adsorption isotherms at 77 K on a Quantachrome NovaWin machine. Fourier transform infrared (FTIR) spectra (4000–400 cm⁻¹) were obtained from KBr pellets using a Bruker Vector 22 instrument.

The CO₂ adsorption capacity of MOF-199 was investigated using a high pressure volumetric analyzer (Micromeritics HPVA—100). Heat of adsorption (ΔH) was calculated by equation [11]

$$\begin{equation} n = {\rm{e}}^{{{ - \Delta H}/{RT}}} , \end{equation} \tag{ 1 }$$

where R is the gas constant (R = 8.314 J mol⁻¹ K⁻¹), T is temperature (T = 304 K) and n is the adsorbed amount (mol).

3.1.1. Influence of copper (II) precursors.

XRD patterns of MOF-199, synthesized from different copper (II) salts (figure 1(a)) are similar. Characteristic peaks of MOF-199 (at 2θ = 6.9°, 9.5°, 11.6°, 13.4°, 17.5°, 19.0° [5,11–14]) appeared in XRD patterns. Compared to MOF-199 prepared from CuCl₂ and Cu(CH₃COOH)₂, the sample obtained from Cu(NO₃)₂ precursor possessed stronger characteristic peaks. In addition, as seen in figure 1(b), this sample was characterized by the highest value of specific surface area (S_Langmuir = 1903 m² g⁻¹ and S_BET = 1305 m² g⁻¹), much higher compared to the other ones. So, Cu(NO₃)₂ has been shown to be the best precursor for the synthesis of MOF-199.

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3.1.2. Effect of solvent quantity.

As shown in figure 2(a), although the used amount of solvent did not lead to any changes in XRD spectrum of MOF-199, it significantly affected the intensity of characteristic peaks. When 9 ml of solvent was used, the obtained sample possessed the strongest intensity of characteristic peaks (figure 2(a)) and the largest specific surface area (figure 2(b)). One can suggest that a very small volume of solvent (6 ml), being not enough to neutralize all -COOH groups in the process of MOF creation, resulted in lower performance of crystals. However, when a large amount of solvent (12 ml) was used, the crystallization extent of MOF decreased due to the low concentration of reactants.

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3.1.3. Effect of activation temperature and activation time.

As seen in figure 3, the common trend in specific surface area of product is increasing with activation time, reaching a maximum at a certain value and then going down. According to figure 3, optimal activation time is reduced from 10 h down to 5 h when activation temperature increases from 175 to 200 °C. The samples obtained at the optimal activation times in both cases have approximately the same and the highest values of specific surface area (S_BET = 1306 m² g⁻¹ and S_Langmuir = 1955 m² g⁻¹), but these values are still lower than the value declared by Yaghi and co-workers [10]. This could be explained by the fact that in the obtained samples some amount of solvent (DMF) still remained in the structure of MOF-199.

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3.1.4. Effect of exchanged solvents.

Powder XRD pattern for MOF-199 is shown in figure 4. The sample showed good crystallinity and the diffraction peaks matched well with already published XRD patterns of MOF-199 [10–15]. As follows from figure 4, and according to Biemmi [15], one can propose that there was no Cu₂O existence in MOF-199. Crystalline size of MOF-199, calculated by the Sherrer equation [16] was 26 nm.

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All the vibration bands of IR spectra (figure 5) were in good agreement with the published data on MOF-199 [11]. The IR spectra of the MOF-199 exhibited the strong stretching vibration of carboxylate anions at 1643.86 cm⁻¹, proving the existence of the reaction of –COOH groups in the 1,3,5-benzenetricarboxylic acid with metal ions. The appearance of a broad band at 3500–2700 cm⁻¹ indicated the presence of water and –OH groups in the structure of the material.

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As follows from the SEM image (figure 6) the crystals of MOF-199 exist in the form of octahedral shape. The synthesized MOF-199 expressed high surface area (S_BET = 1448 m² g⁻¹ and S_Langmuir = 2028 m² g⁻¹); the porous size of 11.8 Å and the specific volume of 0.693 cm³ g⁻¹ were observed. The surface properties of synthesized MOF-199 were very positive. The specific surface area and porous volume are equivalent to that of MOF-199 obtained by Yaghi and co-workers [10], Liu et al [17] and much higher than the values obtained by Li and Yang [11], Chowdhury et al [12] and Chui et al [18].

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As is common for solid materials, the thermostability of the MOF-199 should be investigated. A differential thermogravimetric (DTG) diagram of MOF-199 (figure 7) had two main weight changes. The first weight loss of 13.35%, occurred between 30° and 180 °C due to vaporization of solvent (C₂H₂Cl₂­ and DMF), residing in the pore. The second step of weight loss was 54.64% in the temperature range of 332.64–660 °C due to decomposition of the material. So, MOF-199 is stable at temperature as high as 332.64 °C.

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Figure 8(a) reported the isotherms of carbon dioxide adsorption performed at 30 °C in the range from 0.9032 to 30 bar for MOF-199. The carbon dioxide storage capacity of the synthesized MOF-199 reached to 206.59 cm³ (STP) g⁻¹ at pressure of 25.76 bar, while the value previously reported by Yaghi was 246.4 cm³ (STP) g⁻¹ at 45 bar and 25 °C [7]. The reason for this difference may lie in the difference of adsorption conditions. However, the carbon dioxide uptake of the synthesized sample in this study was higher than the reported data in [5, 19].

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The dependence of adsorption heat of CO₂ on volumetric uptake of CO₂ calculated by equation (1) for MOF-199 is described in figure 8(b). It is notable that with increase of CO₂ uptake from 50.46 to 206.59 cm³ g⁻¹, the adsorption heat of CO₂ decreased from 15.08 to 11.83 kJ mol⁻¹.