Preparation and application of two kinds of supramolecular compounds with photodegradation of organic contaminants in wastewater

2.1 Materials and methods

The multivalent cation MAPB^2 + was prepared as bromides according to the literature, while the other chemicals and solvents were of reagent grade and used as purchased without further purification.

The IR spectra were measured on a Shimazu IR435 spectrometer adopting KBr pellets in the scale of 400–4000 cm^-1. Elemental analyses of C, H and N were performed using a Perkin–Elmer 240 elemental analyzer. UV–Vis diffuse reflectance spectra (DRS) were recorded with the aid of a Cary 5000 UV-Vis infrared spectrophotometer. UV-Vis absorption spectra were obtained using a UV-5500 PC spectrophotometer. Thermogravimetric analyses (TGA) were carried out on a model NETZSCHTG209 thermal analyzer in a flowing N₂ atmosphere of 20 mL min^-1 ata heating rate of 10° min^-1in the temperature range of 25–800° using platinum crucibles. Compounds 1 and 2 powder patterns were collected on a Philips X-pert X-ray diffractometer at a scanning rate of 4° min^-1 in the 2θ range from 6 to 58.46 (compound 1) and 7.422 to 134.15 (compound 2) with graphite monochromatized Mo-Kα radiation (λ= 0.71073 nm) or Cu-Kα radiation (λ= 1.54184 nm).

2.2 Compound of synthesis

2.3 X-ray crystallography study

The single crystal X-ray diffraction data of compound 1 and compound 2 were recorded at 293K on the Bruker SMART CCD diffractometer with a graphite-monochromated Mo-Kα radiation (λ= 0.71073 nm) or Cu-Kα radiation (λ= 1.54184 nm). The reduction and absorption of data is accomplished using the SADABS package. After absorption correction, use shelxtl-97, OLEX-2 and other packaging for analysis [1]. Table 1 summarizes the crystal structure of supramolecular compounds 1 and 2. Table 2 lists the selected bond lengths and bond angles of the crystal structures of supramolecules 1 and 2. Compound 1, CCDC reference numbers:1848446; Compound 2, CCDC reference numbers:1848448.

We choose MB as a contaminant model. Photocatalytic activity is determined by the decomposition rate of methylene blue in visible light. 50 mg of supramolecular compounds 1 and 2 were added to a 100 mL of previously prepared MB solution (concentration 1.0×10^-5 mol/L) and stirred in the dark for 30 min to achieve adsorption or desorption equilibrium. Photocatalysis uses a 500 W high pressure xenon lamp as a light source. Then use filter to remove ultraviolet light and irradiate. According to the different catalytic properties of each compound, select the appropriate time interval for sampling [20]. After removing the suspension and centrifuging, the supernatant was taken to measure the UV-Vis spectrum.

3.1 Description of crystal structures

3.1.1 Crystal structure of {(MAPB)[Ag₂(SCN)Br₃]} (1)

The crystals of supramolecular compound 1 belong to the monoclinic system, space group C2/c, and the unit cell parameters are a/Å = 24.660(2), b/Å = 12.8283(7), c/Å = 15.5185(10), β= 104.9950(8)°. Each silver atom coordinates with a bromine atom and a sulfur atom to form a cubane-like structure. As shown in Fig. 1a, a typical asymmetric cell contains a [Ag₂(SCN)Br₃]^2 - anion and a divalent cation. There exists silver-silver interaction between Ag(1) and Ag(2) with the distance of 3.352(12) Å, which is less than 3.44 Å of the van der Waals radius of silver. Adjacent anions and cations are connected by hydrogen bond N3-H3A⋯Br1¹ = 2.63(3) Å, N3-H3B⋯S1² = 2.66(4) Å, N5-H5B⋯N1³ = 2.33 Å. There are other weak forces between adjacent anions and cations in Mercury software. Figure 1b shows the spatial stacking structure of 1. In the Fig. 1c we can see that the distance between the centers of the pyridine rings of the adjacent two cations is 3.526 Å. And this distance is in the normal range of the face to face stacking [21]. Therefore, we can understand that the cation of supramolecular compound 1 has π–π stacking interaction (Fig. 1d).

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Fig.1

(a) Minimal repeat unit of supramolecular compound 1, hydrogen atoms, solvent molecules and water have been omitted for clarity (b) Stacked map looking from the b axis (c) The structural packing diagram of the supramolecular compound 1 in which the dashed red line represents the N3-H3A⋯Br1¹ hydrogen bond, the blue dashed line represents the N3-H3B⋯S1² hydrogen bond, and the green dotted line represents the N5-H5B⋯N1³ hydrogen key (d) π–π stacking of cations (MAPB^2 +), Anion ligands have been omitted for clarity.

3.1.2 Crystal structure of {(MAPB)₂[Ag₄I₈]CH₃CN} (2)

The single crystal diffraction data show that the crystal 2 of the supramolecular compound belongs to an orthorhombic system, the space group is Cmc2₁, and the unit cell parameter is a/Å = 19.6895(4), b/Å = 14.9590(4), and c/Å = 17.9275 (3), β= 90°, each silver atom coordinates with the iodine atom, forming a classical cubane-like structure. As shown in Fig. 2a for the minimal repeating unit structure, we can see that each Ag atom coordinates with six iodine (including five μ₃-I and one t-I) to form an asymmetry. From the stacking pattern of Fig. 2b, it can be seen that the supramolecular compound 2 have hydrogen bonds. An adjacent anion can form hydrogen bonds with bivalent cations. N-H⋯I hydrogen bond distance is 3.002 or 3.526 Å.

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Fig.2

(a) Minimal repeat unit of supramolecular compound 2, hydrogen atoms, solvent molecules and water have been omitted for clarity(b) b direction stacking (c) The structural packing diagram of the supramolecular compound 2 in which the dashed green line represents the N-H⋯I hydrogen bond (d) There are no π–π stacking interactions between pyridine rings.

Our group has studied the flexibility of MABP^2 + and the complexity of forming anionic ligands [22]. We can understand that the presence of anions in the form of cubane may be related to the radius of the metal ion in the anion or the position of dibenzyl in the cation template.The π–π stacking method in the literature is different. One is a π–π stacking between adjacent benzene rings, and the other is a π–π stacking between adjacent pyridine rings. So the cation packing may be related to the anion metal ion radius. Therefore, we also need more in-depth exploration. From Fig. 2(c) and Fig. 2d we can also see that the adjacent cations are deposited in different ways. Therefore, anionic ligands also have effects on adjacent cations.

In order to further explore the purity of compounds 1–2 and to provide a reliable guarantee for the follow-up property investigation. X-ray Diffraction of compounds 1–2 was done. As shown in Fig. 3, the experimental PXRD pattern of 1–2 corresponds well to the simulated X-ray diffraction pattern, indicating that the bulk phase material is isomorphous.

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Fig.3

(a) PXRD pattern of compound 1 (b) PXRD pattern of compound 2.

To investigate the stability of the synthesized compounds, we performed thermochemical experiments on the synthesized compounds. The TG analysis curve of compound 1–2 is shown in Fig. 4. From Fig. 4, we can see that compound 1 starts to lose weight from 275°C, compound 2 begins to lose weight from 350°C; the greater weight loss of compound 1 happen at 275–350°C, and compound 2 loses at 425–475°C. The biggest reason may be the volatile decomposition of organic cations.The decomposition of compounds 1–2 in the second part may be the decomposition of the cation causing the collapse of the anion structure. Therefore, the stability of compound 1 and compound 2 are relatively good.

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Fig.4

Thermogravimetric (TG) plot of Compound 1–2.

Through the UV-Vis diffuse reflectance spectra of these two compounds, we calculated their corresponding bandgap values (Eg). First, the spectrophotometer was used to obtain the spectrum with the reflectance R as the ordinate and the wavelength λ (nm) as the abscissa. Then, the ordinate is transformed according to the Kubelka–Munk equation: F = (1-R)²/2R, and the abscissa is transformed according to the energy formula E(g) = hc/λ. Finally, for the curves F and E, the energy value corresponding to the intersection of the tangent line and the abscissa of the curve where the slope is the maximum is the band gap value of the compound [23]. As shown in Fig. 4, the bandgap values of the two compounds are: 1.76±0.01 eV, 1.74±0.01 eV, respectively, lower than 3.2 eV of TiO₂, indicating that these compounds are potential semiconductor photocatalytic materials. In addition, the compound may be directly irradiated with sunlight, since the maximum photon flux of sunlight is about 1.7 e V [24]. Compounds 1–2 having the aforementioned bandgap values are all potential semiconductor materials and may possess better photocatalytic activity.

3.5 Photocatalysis

From the calculation of the bandgap value, we learned that compounds 1–2 are a potential semiconductor material and may have good catalytic properties. We explored the photocatalytic properties of compound 1–2 through the decomposition of organic dyes. We chose organic dye MB as a target pollutant for degradation experiments to investigate the photocatalytic properties of compounds 1 and 2. In the process of photocatalysis, 50 mg of 1 was added to MB (100 mL, 1.0×10^-5mol·L^-1) aqueous solution separately and magnetically stirred in the dark for about 30 min until equilibrium of the mixture. Then under the irradiation of a 500 W High-pressure mercury lamp, the solution was kept continuously stirring with the aid of a magnetic stirrer. Then, 500 mL high pressure mercury lamp was used to sample 3 mL every 30 mins and centrifuged to measure the UV absorption spectrum (UV absorption spectra at different times are shown in the Fig. (6). The characteristic absorption of MB at about 663 nm (MB) was chosen to monitor the photodegradation process. The percentage of degradation can be calculated using Lambert-Beer’s law; Finally, the data is processed using Origin 8.0 software.

As shown in Figs. 6, the absorption peak of MB decreases obviously along with the increase of the irradiation time.

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Fig.5

(a) The band gap value of compound 1 (b) The band gap value of compound 2.

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Fig.6

Absorption spectra of the MB solution during the decomposition reaction under UV light irradiation deal with (a) the control experiment without any catalyst (b) compound 1(c) compound 2. (d) Photodegradation ofsolution MB under sunlight irradiation with the use of 1–2 and the control experiment without any catalyst.

As shown in Fig. 6, it can be seen that compounds 1 and 2 have obvious photocatalytic activity. The catalytic effect of compounds 1 and 2 is relatively good. From Fig. 6,we can see that in the blank experiment without any catalyst, the MB degradation rate was only 30% within 150 min. In comparison, MB with Compound 1 had a degradation rate of 80% within 80 minutes, and MB with Compound 2 had a degradation rate of 78% within 120 minutes. Therefore, the photocatalytic activity of compound 1 was better than that of compound 2. The result reveals that compounds 1 and 2 may become potential catalysts in the photodegradation of some organic dyes existing in industrial wastewaters under visible light irradiation.