Synthesis of Cu-BTC, from Cu and benzene-1,3,5-tricarboxylic acid (H₃BTC), by a green electrochemical method

3.1 Electrodeposition of Cu-BTC

Figure 1 presents the current intensity as a function of time during the synthesis process of Cu-BTC at 5 V/SCE in solution containing methanol+H₃BTC 0.05 M+TBATFB 0.05 M (DD2). At the beginning, the current increases quickly due to the formation of copper ions on the electrode surface; after that, the current reaches a stable value at 0.420 A corresponding to the synthesis process of Cu-BTC.

Figure 1:

Current as a function of time during the synthesis process of Cu-BTC, formed from Cu and benzene-1,3,5-tricarboxylic acid (H₃BTC), at E=5 V/saturated calomel reference electrode (SCE).

The IR spectrum of H₃BTC (Figure 2) shows the appearance of the peaks at 1720 cm⁻¹, 1180 cm⁻¹ and 1270 cm⁻¹ corresponding to stretching vibration of ν_C=O, ν_C-O and bending vibration of O-H, indicating the presence of an acid carboxylic group. The symmetric and antisymmetric stretching vibrations of O-C=O groups were in the range of 1400–1620 cm⁻¹ [8] (Table 2). With Cu-BTC, the peaks in the range 1380–1630 cm⁻¹ characterized for vibration of carboxylate groups. However, there are the appearance of peaks at 1710 cm⁻¹, 1180 cm⁻¹ and 1240 cm⁻¹ corresponding to the vibrations of C=O, C-O and O-H groups with low intensity.

Figure 2:

IR spectra of (A) benzene-1,3,5-tricarboxylic acid (H₃BTC) and (B) Cu-BTC, formed from Cu and H₃BTC.

Phase structure analysis results (Figure 3) show that synthesized Cu-BTC has a 3D structure which is identified at 2θ: 6.7°, 9.5°, 11.6°, and 13.5°. To form the 3D structure, H₃BTC needs to be deprotonated totally and the formed Cu²⁺ions to react with all of the three carboxylate groups. Thus, when dissolving in methanol, TBATFB, H₃BTC was fully deprotonated. The IR spectrum of Cu-BTC shows a peak at 1240 cm⁻¹ characteristic for the vibration of O-H groups. So, we can conclude that the obtained Cu-BTC still has impurities. In details, C₃H₅CuO₂ was observed in the X-ray diffraction pattern at 2θ=24.1°. However, the content of impurities is little, as shown by the low intensity of the diffraction peak corresponding with C₃H₅CuO₂. From the X-ray diffraction pattern, lattice parameter a=26.486 Ǻ was calculated [16].

Figure 3:

X-ray diffraction pattern of Cu-BTC, formed from Cu and benzene-1,3,5-tricarboxylic acid (2-Cu-BTC 3D; 3-C₃H₅CuO₂).

3.2 Influence of electrolyte

Solutions to synthesize Cu-BTC have a small conductivity and an electrolyte has to be added. In this part, we examine the influence of electrolyte NaNO₃ and TBATFB on the morphology and phase structure of Cu-BTC synthesized in methanol at an applied potential of 5 V. Figure 4A presents IR spectra of Cu-BTC synthesized in a solution containing NaNO₃ or TBATFB. The results show characteristic peaks for the vibration of groups in Cu-BTC (Table 2). The X-ray diffraction patterns (Figure 4B) indicate that, when using NaNO₃, the obtained Cu-BTC has a 1D structure corresponding to 2θ=9.3°, 10°, 11°, and 11.31°. With TBATFB, the synthesized Cu-BTC has a 3D structure with formula Cu₃(BTC)₂ and characteristic values 2θ=6.6°, 9.4°, 11.6°, and 13.2° shown in Table 3 [11]. The deprotonation of H₃BTC in solution was caused by the OH⁻ group of methanol. In addition, the two electrolytes NaNO₃ and TBATFB are soluble in methanol and generate two anions: NO₃⁻ and BF₄⁻, respectively. These ions do not only increase the conductivity of solution but also have a nucleophilic property and an H₃BTC deprotonation ability. Due to the stronger nucleophilic property of BF₄⁻ compared to NO₃⁻, H₃BTC was fully deprotonated in solution containing TBATFB, so that Cu²⁺ ions can react with 3 carboxyl groups of H₃BTC to form Cu-BTC 3D. Meanwhile, H₃BTC was only partly deprotonated in solution containing NaNO₃ leading to Cu-BTC with a 1D structure.

Figure 4:

(A) IR spectra and (B) X-ray diffraction pattern of Cu-BTC, formed from Cu and benzene-1,3,5-tricarboxylic acid, synthesized with different electrolytes (1-CuBTC 1D; 2-Cu-BTC 3D; 3-C₃H₅CuO₂).

From the X-ray diffraction pattern, the lattice parameter (a) of Cu-BTC 3D can be determined following Eq. (1), and the data are shown in Table 3. The lattice parameter (a) varies in the range 26.44–26.78 Ǻ, suitable with the result of Wojciech et al. [17] (26.346 Ǻ).

Figure 5 presents scanning electron microscopy images of Cu-BTC synthesized at 5 V with TBATFB or NaNO₃. Cu-BTC has a nonuniform block shape with size from 50 nm to 900 nm when synthesized in solution containing TBATFB (Figure 5A). In the presence of NaNO₃, the obtained Cu-BTC has a plate shape with large sizes from 100 nm to 3 µm (Figure 5B).

Figure 5:

Scanning electron microscopy (SEM) images of Cu-BTC, formed from Cu and benzene-1,3,5-tricarboxylic acid, synthesized with (A) tetrabutylammonium tetrafluoroborate (TBATFB) and (B) NaNO₃.

3.3 Influence of electrodeposition time

Electrodeposition time can affect the morphology and phase structure of Cu-BTC. Long synthesis time is advantageous for the growth of single crystals. Figure 6 presents X-ray diffraction patterns of Cu-BTC synthesized in DD2 during different times. All the characteristic peaks of Cu-BTC 3D structure can be observed (Table 4). This result shows that the electrodeposition time does not affect the phase structure of Cu-BTC 3D.

Figure 6:

X-ray diffraction patterns of Cu-BTC, formed from Cu and benzene-1,3,5-tricarboxylic acid, synthesized in DD2 with different times: 10 min (A), 15 min (B), 25 min (C) and 60 min (D) (2-Cu-BTC 3D, 3-C₃H₅CuO₂).

From the X-ray diffraction pattern, the lattice parameter (a) of Cu-BTC with different synthesis times can be calculated (Table 5). When the reaction time increases, the lattice parameter (a) increases slightly and the size of crystal rises.

To study the effect of the reaction time on the formation of Cu-BTC 1D, we electrodeposited Cu-BTC in DD1 with an applied potential of 5 V/SCE during 10 min and 60 min. The X-ray diffraction patterns of Cu-BTC synthesized in DD1 with different times are shown in Figure 7.

Figure 7:

X-ray diffraction patterns of Cu-BTC, formed from Cu and benzene-1,3,5-tricarboxylic acid, synthesized in DD1 with an applied potential of 5 V/saturated calomel reference electrode (SCE) during (A) 10 min and (B) 60 min (1-Cu-BTC 1D, 3-C₃H₅CuO₂, 4-C₈H₄CuO₄·3H₂0, 5-C₆H₅Cu, 6-C₄H₄CuO₄·2H₂O).

When the electrodeposition time was 10 min, Cu-BTC 1D was obtained with characteristic peaks at 2θ=9.23°, 9.9°, 10.9°, and 13.5°; in addition, there were peaks of C₃H₅CuO₂. However, for a long synthesis time (60 min), the product is not Cu-BTC, but a mixture of other compounds of copper: C₈H₄CuO₄·3H₂O, C₆H₅Cu, C₄H₄CuO₄·2H₂0. This can be explained that when the synthesis time is long, there is a reaction of H₂O that destroys the structure of Cu-BTC-1D [16].

3.4 Influence of hydration process on Cu-BTC 3D

Cu-BTC 3D which was synthesized in methanol+H₃BTC 0.05 M+TBATFB 0.05 M during 10 min at a potential of 5 V/SCE, was rinsed with methanol. After that one part was dried at 40°C (1) and other parts were immersed in water during different times: 0 min (2), 20 min (3), 30 min (4), 4 h (5), 13 h (6), 18 h (7) and 24 h (8). Thereafter, the samples were put in a vacuum for drying at room temperature, and pressure 250 mBar during 24 h. We observed that: Cu-BTC expands after immersing in water. The hydration step can increase the specific surface area, the size and the volume of the pores of Cu-BTC, but it can lead to a reaction of water with open copper sites and change the phase structure of Cu-BTC when the hydration time is long enough [18]. Figure 8 presents IR spectra of Cu-BTC with different hydration times. In general, IR spectra have similar shapes, with the appearance of characteristic peaks for the vibrations in molecular Cu-BTC as shown in Table 2.

Figure 8:

IR spectra of Cu-BTC, formed from Cu and benzene-1,3,5-tricarboxylic acid, under the effect of hydrate process [dry at 40°C (1), immersion 0 min (2), 20 min (3), 30 min (4), 4 h (5), 13 h (6), 18 h (7) and 24 h (8)].

X-ray diffraction patterns of Cu-BTC immersed in water (Figure 9) show that the hydration process affects the phase structure of Cu-BTC. Cu-BTC after filtering, rinsing, drying at 40°C (1) or immersing in water during short times [0 min (2), 20 min (3), 30 min (4)] still present a 3D structure with characteristic 2θ values. From the X-ray diffraction pattern d values at 2θ values corresponding to different (hkl) planes, the lattice parameter (a) of Cu-BTC 3D can be calculated (Table 6). These values are in agreement with the lattice parameter value 26.346 Ǻ of Wojciech et al. [17]. However, when the product is immersed in water during longer times: 4 h, 13 h, 18 h and 24 h, the phase structure of Cu-BTC was changed. In X-ray diffraction patterns, the characteristic peaks of Cu-BTC 3D do not appear. These peaks characterized for C₈H₄CuO₄.3H₂0, C₆H₅Cu and C₄H₄CuO₄·2H₂O at 2θ values: 7.81°, 9.07°, 11.36°, and 12.03° (Table 7) were, however, observed.

Figure 9:

X-ray diffraction patterns of hydrated Cu-BTC, formed from Cu and benzene-1,3,5-tricarboxylic acid, [dry at 40°C (A), immersion during 0 min (B), 20 min (C), 30 min (D), 4 h (E), 13 h (F), 18 h (G) and 24 h (H)] (2-Cu-BTC 3D; 3-C₃H₅CuO₂; 4-C₈H₄CuO₄·3H₂O; 5-C₆H₅Cu; 6-C₄H₄CuO₄·2H₂O).

To confirm the effect of the hydration process on the specific surface area, Cu-BTC 3D and Cu-BTC 3D synthesized at applied potential 5 V/SCE during 10 min were hydrated during 20 min and their specific surface area was determined by the adsorption of nitrogen (N₂).

Figure 10A and B present the N₂ adsorption isotherm curves of Cu-BTC 3D and Cu-BTC 3D hydrated during 20 min. The adsorption and desorption curves nearly match, therefore Cu-BTC may be considered as a type of microcapillary material. The specific surface area was determined from BET curves in the relative pressure range from 0.05 to 0.21 (Figure 10C and D). Cu-BTC 3D hydrated during 20 min has a specific surface area of 371 m²/g, 7.6 times higher than Cu-BTC without hydration treatment.

Figure 10:

The BET adsorption isotherm curves (A, B) and the BET curves (C, D) of Cu-BTC, formed from Cu and benzene-1,3,5-tricarboxylic acid, (A, C) and Cu-BTC immersed in water during 20 min (B, D) synthesized by the applied potential method.

Cu-BTC 3D synthesized in DD2 at an applied potential of 5 V/SCE during 10 min and immersed in water during 20 min shows a good thermal stability as shown by a TGA measurement performed in a range of temperature from 25°C to 600°C in atmosphere with a rate of 5°C/min (Figure 11).

Figure 11:

TGA, derivative thermogravimetry analysis (DrTGA), differential thermal analysis (DTA) plots of Cu-BTC, formed from Cu and benzene-1,3,5-tricarboxylic acid, synthesized in DD2, hydrated during 20 min.

These plots show three different parts:

Part 1: a first mass decrease (9.4%) between 50°C and 90°C, an exothermal process that shows the loss of physical water.

Part 2: between 100°C and 200°C, an exothermal process corresponding to the water lost in the structure of MOF with mass decrease of 11.3%.

Part 3: above 200°C, the MOF structure starts to be destroyed (52.8%). The mass loss is completed at 350°C, with an endothermic process corresponding to the phase transition to Cu₂O and CuO. After that, the sample is stable until 600°C and the remaining part is 26.57% [8], slightly higher than the value of 24% for Cu²⁺ in molecular Cu-BTC 3D [7].

3.5 Influence of dehydration process

The hydration process of Cu-BTC 3D can increase the specific surface area and the volume of pores. However, it can also change the phase structure of Cu-BTC as shown in part 3.4. Therefore, we investigated if the dehydration step was able to recover the initial phase structure of Cu-BTC. After hydration step of 4 h, the Cu-BTC 3D sample was dehydrated by drying at 110°C during 24 h. The X-ray diffraction pattern of the dehydrated Cu-BTC is shown in Figure 12. The pattern shows that the phase structure of Cu-BTC 3D cannot be recovered by the drying process. This result is agreement with literature.

Figure 12:

X-ray diffraction pattern of dehydrated Cu-BTC, formed from Cu and benzene-1,3,5-tricarboxylic acid, (4-C₄C₈H₄CuO₄·3H₂O, 5-C₆H₅Cu, 6-C₄H₄CuO₄·2H₂O).