Metal-Organic Framework MIL-101: Synthesis and Photocatalytic Degradation of Remazol Black B Dye

Abstract

In the present paper, the synthesis of metal-organic framework MIL-101 and its application in the photocatalytic degradation of Remazol Black B (RBB) dye have been demonstrated. The obtained samples were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), and nitrogen adsorption/desorption isotherms at 77 K. It was found that MIL-101 synthesized under optimal conditions exhibited high crystallinity and specific surface area (3360 m²·g^-1). The obtained MIL-101 possessed high stability in water for 14 days and several solvents (benzene, ethanol, and water at boiling temperature). Its catalytic activities were evaluated by measuring the degradation of RBB in an aqueous solution under UV radiation. The findings show that MIL-101 was a heterogeneous photocatalyst in the degradation reaction of RBB. The mechanism of photocatalysis was considered to be achieved by the electron transfer from photoexcited organic ligands to metallic clusters in MIL-101. The kinetics of photocatalytic degradation reaction were analyzed by using the initial rate method and Langmuir-Hinshelwood model. The MIL-101 photocatalyst exhibited excellent catalytic recyclability and stability and can be a potential catalyst for the treatment of organic pollutants in aqueous solutions.

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1. Introduction

Textile and paint industries and dyestuff manufacturing release a considerable amount of wastewater with dyes. This has raised serious environmental concerns all over the world; thus, their removal is of interest to many scientists [1]. Dyes are difficult to treat along with municipal waste treatment operations due to their complicated chemical structures. Remazol Black B (RBB) is a popular diazo reactive dye and used widely in textile industries [2]. Various processes for eliminating RBB from aqueous solutions including adsorption, electrochemistry, and biosorption have been reported. Thi Thanh et al. [3] reported the efficient removal of RBB using iron-containing zeolite imidazole framework-8 (Fe-ZIF-8). Fe-ZIF-8 possessed high stability. After three cycles, the degradation yield was reduced slightly—95% compared to the initial catalyst. Soloman et al. [2] reported the degradation of hydrolyzed Remazol Black using the electrochemical approach. Brazilian pine-fruit shells (Araucaria angustifolia) in natural form are efficient adsorbents for the removal of RBB dye from aqueous effluents [4]. Biosorption of an azo dye by growing fungi (Aspergillus flavus) was reported in which the removal of chemical oxygen demand (COD) was found to be 90% at 100 mg·L^-1 initial concentration of dye [5]. The introduction of iron to ZIF-8 significantly enhanced the photocatalytic degradation of RBB Fe-ZIF-8 under visible light [6].

Metal-organic frameworks (MOFs) are porous materials formed via strong metal-ligand bonds between metal cations and organic linkers [7, 8]. MOFs have many applications in gas storage [9–15], separation [16, 17], and heterogeneous catalysis [18–21]. MIL-101 is a member of the large family of MOFs with the largest Langmuir surface area (4500 m²·g^-1), pore size (29–34 Å), and cell volume (702.000 Å). It was first reported by Férey et al. in 2005 [22], who synthesized it from HF-Cr(NO₃)₃-1,4-dicarboxylic acid- (H₂BDC-) H₂O. Since the discovery of large-pore MIL-101, several groups have tried to synthesize MIL-101 for gas adsorption. However, it is hard to obtain crystalline MIL-101 with a high BET surface of more than 3200 m²·g^-1 [18] because of the presence of H₂BDC residue or inorganic impurities in the pores as well as outside the pores. In the synthesis of MIL-101, it is complicated to remove most of the nonreacted H₂BDC present both outside and within the pores of MIL-101. Yang et al. [13] used various alkalis, such as potassium hydroxide (KOH), tetramethylammonium hydroxide (TMAOH), triethylamine ((C₂H₅)₃N), dimethylamine (C₂H₇N), methylamine (CH₅N), and ammonia (NH₃), to avoid recrystallization of H₂BDC. The TMAOH-Cr(NO₃)₃-H₂BDC-H₂O system was found to be suitable for obtaining MIL-101 with high surface properties. Hong et al. [18] reported a separation process in which huge amounts of H₂BDC were separated with a fritted glass filter, and then excess dissolution was done with a hot solvent (ethanol or N,N-dimethyl-formamide (NH₄F)).

The heterogeneous photocatalysis is one of the highly effective methods used for the treatment of a wide variety of organic pollutants owing to its ability to degrade the pollutants completely. Photocatalytic degradation is usually conducted for dissolved compounds in water, at mild temperature and pressure conditions, using UV radiation and photocatalytic semiconductors without any requirement of expensive oxidants. The use of metal-organic frameworks (MOFs) as photocatalysts is a new field of application for this material. Recently, some MOF materials such as MOF-5 [23–25], MIL-125 [26], and MIL-53(M) (M = Fe, Al, and Cr) [27] have been applied successfully in the decolorization of various dyes in aqueous solutions. Owing to its excellent porosities (high specific surface area, large pore volume, and uniform pores), MIL-101 is a good candidate in catalysis [28] and has been found to be a great candidate for many other applications [16, 17, 29–32]. Several papers have reported visible-light photocatalytic activity of modified MIL-101, e.g., N-K₂Ti₄O₉/MIL-101 composite [33] and Bi₂₅FeO₄₀/MIL-101/PTH [34]. To the best of our knowledge, no attention has ever been paid to the study of the photocatalytic properties of pure MIL-101 to date.

In the present paper, we focused on an Cr(NO₃)₃-H₂BDC-H₂O system and analyzed the effect of different conditions on the synthesis of MIL-101 and monitored the hydrothermal stability of MIL-101 in various solvents and conditions. Photocatalytic degradation of RBB was also investigated.

2. Experimental

2.1. Materials

Chromium(III) nitrate nonahydrate (Cr(NO₃)₃·9H₂O; 99%), benzene-1,4-dicarboxylic acid (C₆H₄(COOH)₂; >98%) (denoted as H₂BDC), and hydrogen fluoride (HF; 40%) were purchased from Merck, Germany. Remazol Black B (C₂₆H₂₁N₅Na₄O₁₉S₆, $\text{molecular}\text{weight}=991.82$) (denoted as RBB) was procured from the Thuy Duong Textile Company (Hue city, Vietnam). The structure of RBB is shown in Scheme 1.

[figure omitted; refer to PDF]

Ferrous ammonium sulfate (Fe(NH₄)₂(SO₄)₂6H₂O; >98%), potassium dichromate (K₂Cr₂O₇; >99%), ferrous sulfate heptahydrate (FeSO₄·7H₂O; >99%), silver sulfate (Ag₂SO₄; >99%), conc. H₂SO₄ (98%), and mercuric sulfate (HgSO₄; >98%) were supplied from Merck, Germany, and used to measure the COD of samples.

2.2. MIL-101 Synthesis

MIL-101 was synthesized according to an earlier report with some modifications [22]. The mixture of reactants including H₂BDC, Cr(NO₃)₃, HF, and H₂O was heated in a Teflon-lined stainless steel autoclave at 200°C for 8 h. The resulting green solid material was filtered using a 0.2 μm membrane and then extracted in ethanol with Soxhlet equipment for 12 h to remove residual amount of H₂BDC still present in the product. The effects of the molar ratio of chromium nitrate and water to H₂BDC on the formation of MIL-101 were also monitored. With a fixed water volume of 100 mL, the composition of the synthesized gel was calculated at the following molar ratio:

(i)

For the study on the effect of the molar ratio of Cr/H₂BDC, the molar composition of a reactant mixture of ${\text{H}}_2\text{BDC}:\text{Cr}{\left(\text{N}{\text{O}}_3\right)}_3·9{\text{H}}_2\text{O}:\text{HF}:{\text{H}}_2\text{O}=1.00:x:0.25:265$, with $x=0.5,0.75,1.00,1.25,1.50,\text{and}1.75$. The samples were denoted as M-0.5, M-0.75, M-1.00, M-1.25, M-1.50, and M-1.75

(ii)

For the study on the effect of the ratio of H₂O/H₂BDC, the molar composition of a reactant mixture of ${\text{H}}_2\text{BDC}:\text{Cr}{\left(\text{N}{\text{O}}_3\right)}_3·9{\text{H}}_2\text{O}:\text{HF}:{\text{H}}_2\text{O}=1.00:1.25:0.25:y$, with $y=200,265,350,400,500,\text{and}700$. The samples were denoted as M-200, M-265, M-350, M-400, M-500, and M-700

2.3. Photocatalytic Performance

The photocatalytic degradation of RBB was measured at ambient conditions using a set of home-made equipment. The source of UV light was UV-B313 30 W ($\lambda =310\text{nm}$).

The photocatalytic experiments were performed in a 1000 mL beaker containing 500 mL of aqueous suspensions of 10–50 ppm RBB and a 0.25 mg catalyst. The beaker was kept at 25°C with a thermostat. The UV lamp was focused on the beaker at 15 cm distance. All of the experiments were performed under natural pH conditions (around 7–7.5) unless specified otherwise. Then, 3 mL of the mixture was withdrawn at certain time intervals and centrifuged to remove the MIL-101 solid. The RBB concentration was monitored by means of spectroscopy at the maximum wavelength ($\lambda =600\text{nm}$). The experiments were replicated three times.

The chemical oxygen demand (COD) of the RBB solution was measured by the ASTM method [35]. The sample was oxidized by the boiling mixture of chromic and sulfuric acids. The sample was refluxed in a strongly acidic solution with a known excess of potassium dichromate (K₂Cr₂O₇). After digestion, the remaining unreduced K₂Cr₂O₇ was titrated with ferrous ammonium sulfate to determine the amount of K₂Cr₂O₇ consumed and the oxidizable matter was calculated in terms of the oxygen equivalent. The samples were analyzed in duplicate to yield reliable data.

X-ray diffraction (XRD) was carried on a D8 ADVANCE system (Bruker, Germany) (Hanoi city, Vietnam). Cu Kα radiation ($\lambda =1.5406\text{Å}$) was the light source with applied voltage of 35 kV and current of 40 mA. Transmission electron microscopy (TEM) was carried out using a JEOL JEM-2100F microscope (Hanoi city, Vietnam). Nitrogen adsorption/desorption isotherm measurements were conducted using a Micromeritics 2020 volumetric adsorption analyzer system (Hanoi city, Vietnam). The samples were pretreated by heating under vacuum at 150°C for 3 h. The BET (Brunauer–Emmett–Teller) model was used to calculate the specific surface area using adsorption data in the relative range 0–0.24. Total volume was obtained from the nitrogen volume adsorbed at a relative pressure of 0.99. The X-ray photoelectron spectroscopy (XPS) was conducted using a Shimadzu Kratos AXIS Ultra DLD spectrometer (Japan). Peak fitting was performed by CasaXPS software. The absorbance of RBB, methyl orange (MO), and methylene blue (MB) was measured at ${\lambda }_{\max }=600\text{nm},464\text{nm},\text{and}664\text{nm}$, respectively, using a Lambda 25 Spectrophotometer, PerkinElmer, Singapore (Hue city, Vietnam).

3. Results and Discussion

3.1. Synthesis of MIL-101 and Its Stability under Different Conditions

The XRD patterns of the MIL-101 samples synthesized with different Cr/H₂BDC molar ratios of 0.50, 0.75, 0.85, 1.00, 1.25, and 1.75 are shown in Figure 1(a). From the figure, it is found that the diffraction at a small angle of about 1.7° characterizing the mesoporous structure of MIL-101 was obtained depending on the Cr/H₂BDC ratio. At a low Cr/H₂BDC ratio, the diffraction at such a small angle was not observed. However, when the Cr/H₂BDC ratio was increased to a level such as M-1.25, M-1.5, or M-1.75, the XRD patterns were similar to the patterns of MIL-101 as reported earlier [22], in which sharp and strong diffraction at 2θ of around 1.7° was clearly observed. Therefore, a Cr/H₂BDC ratio larger than 1.25 could provide MIL-101 with high crystallinity. The textural properties of MIL-101 samples synthesized at different ratios of Cr/H₂BDC were investigated by the N₂ adsorption-desorption isotherms at 77 K as shown in Figure 1(b). The isothermal curves are of type IV, with pore-filling steps at $p/p^0\approx 0.2$ and $p/p^0\approx 0.3$, characteristic of the presence of two types of narrow mesopores [18]. The parameters characterizing the textural properties of the obtained MIL-101 samples are displayed in Table 1. The specific surface area tends to increase with the increase in the molar ratio of Cr/H₂BDC and reaches the highest value at the ratio of 1.25 and then decreases when this ratio continues to rise.

[figures omitted; refer to PDF]

Table 1

Textural properties of MIL-101 samples synthesized using different molar ratios of Cr/H₂BDC.

Samples	$S_{\text{BET}}$ (m2·g-1)	$S_{\text{Langmuir}}$ (m2·g-1)	$V_{\text{pore}}$ (cm3·g-1)	Particle size${}^{\ast }$$\text{M}\pm \text{SD}$(nm)	Crystallite size${}^{\ast \ast }$(nm)
M-0.75	1582	2426	0.79	$$231.5\pm 20.1$$	34.1
M-1.00	2328	3833	1.23	$$376\pm 15.4$$	29.3
M-1.25	2946	4776	1.53	$$216\pm 20.3$$	43.7
M-1.50	2642	4354	1.41	$$522\pm 20.0$$	45.8
M–1.75	2414	4057	1.28	$$573\pm 27.2$$	34.6

${}^{\ast }$Mean value (M) of particle size counted from 100 particles. SD: standard deviation. ${}^{\ast \ast }$Crystallite size calculated from the Scherrer equation using diffraction (753).

The morphologies of MIL-101 samples synthesized with different molar ratios of Cr/H₂BDC were observed by TEM images (Figure 2). It can be seen that the particles have an octahedron shape with different sizes, in the range of 230-570 nm, depending on the molar ratios of Cr/H₂BDC. The values of standard deviation (SD)/mean (4%-9%) were less than 10% in all cases indicating that particle size distributions were normal. The size of particles reaches a minimum at the Cr/H₂BDC ratio of 1.25 (M-1.25) (see Table 1). In fact, the effect of the Cr/H₂BDC ratio on the MIL-101 particle size was not clear as the Cr/H₂BDC ratio was between 0.75 and 1.25. The particle size increased significantly as the Cr/H₂BDC ratio is larger than 1.25. On the contrary, crystallite sizes obtained from the Scherrer equation (using (375) diffraction) was within 34.1 and 45.8 nm and they appeared to be less affected by the Cr/H₂BDC ratio. These results implied that particles consisted of several crystals. Based on the specific surface area and morphology, the suitable Cr/H₂BDC molar ratio for the synthesis of MIL-101 was found to be 1.25.

[figures omitted; refer to PDF]

Figure 3(a) displays the XRD results of MIL-101 samples synthesized with different molar ratios of H₂O/H₂BDC. The results show that the amount of water in the composition of reactants has a considerable effect on the structure of MIL-101. All the samples with an increase in molar ratios of H₂O/H₂BDC from 200 to 700 provided the characteristic diffractions of MIL-101. However, the peak at 2θ of about 1.7° characterizing the mesoporous structure did not appear for the samples having a high water content (M-500, M-700), while the same was observed clearly for the samples having a lower water content with molar ratios of H₂O/H₂BDC from 200 to 400. In addition, at a high water content in the reactant mixtures, a lower peak intensity was observed. Therefore, the water content in the reactant mixtures not only affected the structure of materials but also reduced their crystallinity.

[figures omitted; refer to PDF]

The isotherms of nitrogen adsorption/desorption and texture properties of MIL-101 synthesized with different molar ratios of H₂O/H₂BDC are illustrated in Figure 3(b) and Table 2. It was found that the specific surface area increased steadily when the molar ratio of H₂O/H₂BDC increased and peaked at the molar ratio of 350, but after that, it decreased when this ratio was increased. The water content in the reactant mixtures had a significant effect on the particle size of MIL-101 but had less effect on crystallite size. It can be seen in Figure 4 and Table 2 that the particle size decreased slightly with the increase in water content while crystallite sizes around 26.8–31.8 nm seemed to be unchangeable. A large amount of water resulted in irregular shapes and formation of the needle-shaped crystals of terephthalate acid. It can be inferred that a high water content probably reduced the crystal growth rates [36]. Therefore, a perfect MIL-101 crystal could not be achieved at high water contents in the reactant mixture, and the most perfect MIL-101 crystal was observed in the M350 sample.

Table 2

Textural properties of MIL-101 samples synthesized using different molar ratios of H₂O/H₂BDC.

Samples	$S_{\text{BET}}$ (m2·g-1)	$S_{\text{Langmuir}}$ (m2·g-1)	$V_{\text{pore}}$ (cm3·g-1)	Particle size${}^{\ast }$$\text{M}\pm \text{SD}$ (nm)	Crystallite size${}^{\ast \ast }$ (nm)
M-200	1618	2570	0.87	$$530.5\pm 78.2$$	26.8
M-265	2946	4776	1.53	$$520.3\pm 53.4$$	30.4
M-350	3360	5059	1.44	$$490.3\pm 19.4$$	31.8
M-400	2274	3664	1.25	$$250\pm 35$$	31.4
M-700	1708	2701	0.93	$$137\pm 25$$	30.0

${}^{\ast }$The mean value (M) of 50 particles counted from TEM images. SD: standard deviation. ${}^{\ast \ast }$Crystallite size calculated from the Scherrer equation using diffraction (753).

[figures omitted; refer to PDF]

As mentioned previously, a larger amount of nonreacted H₂BDC is present in MIL-101, resulting in a decrease in its surface area and pore volume. The previous studies are focused on the purification using hot ethanol, water, or fluoride–anion exchange using aqueous NH₄F solutions [18, 22, 32]. In the present work, we proposed the Soxhlet extraction using ethanol solvent to purify the H₂BDC. Table 3 shows the specific surface area and porous volume of the present MIL-101 material compared to some previous studies. The surface area and porous volume of MIL-101 synthesized in this study are smaller than those reported by Férey et al. [22] but much higher than recent studies. The proposed purification is time-consuming but is capable for completely removing H₂BDC, making the obtained MIL-101 have a high specific surface area.

Table 3

Comparison of the porosity of the present MIL-101 and the results published.

$S_{\text{BET}}$ (m2·g-1)	$S_{\text{Langmuir}}$ (m2·g-1)	$V$ (cm3·g-1)	Method/purification	References
3360	5059	1.4	Hydrothermal method/Soxhlet extraction using ethanol solvent	The present work
4100	5900	2.0	Hydrothermal method/ethanol and DMF	[22]
4230	—	2.2	Hydrothermal method/double filtration with glass and paper filters, water, ethanol, and NH4F	[18]
2345	3674	1.3	Microwave-assisted hydrothermal method/ethanol	[32]
3054	4443	2.	Microwave-assisted hydrothermal method/ethanol and DMF	[37]
3197	4546	1.7	Solvothermal process/TMAOH ((CH3)4NOH)	[13]
2220	—	1.1	Hydrothermal method/ethanol and DMF	[20]
2674	—	1.4	Hydrothermal method/ethanol and DMF	[9]
3360	4792	1.8	Microwave-assisted hydrothermal method/water, ethanol, and NH4F	[15]

DMF: N,N-dimethylformamide.

The results of stability testing of MIL-101 under ambient conditions over several days are given in Figures 5(a) and 5(b). Generally, the main characteristic peaks of MIL-101 could be observed in all XRD patterns. The peak at a small angle (2θ is about 1.7°) was not present in the samples exposed to ambient conditions for 15 to 30 days without drying before XRD measurements (Figure 5(a)). It is worth noting that this peak was observed clearly in the dried sample, although it was exposed to ambient conditions up to a year (Figure 5(b)). The peak at a small angle characteristic of a mesoporous structure seems to be unstable under moistened air. This explains why this peak which diffracted at 1.7° has been reported in some studies [22], while not in others [13, 15, 18]. The reason is not clear, but it could be possible that water in the moistened air blocks some of the pores resulting in the disappearance of the characteristic diffraction of the mesoporous structure at around 1.7°. Therefore, this peak was observed after the drying process to remove water vapor from the material structure.

[figures omitted; refer to PDF]

The stability of MIL-101 material in water at room temperature is illustrated in Figure 5(c). The results indicate that the characteristic diffractions of MIL-101 were obtained in all patterns. Notably, the characteristic diffraction of the mesoporous structure at around 1.7° still remained stable after the sample had been soaked in water for several days.

The stability of MIL-101 material in several solvents at boiling temperature for 8 h is shown in Figure 5(d). The results of XRD indicate that MIL-101 material was still stable after being soaked in boiling water continuously for 8 h, which is in good agreement with that reported by Hong et al. [18]. Moreover, the characteristic diffraction of MIL-101 still appeared with high intensity suggesting that its structure did not collapse after soaking in ethanol and benzene at boiling temperature for 8 h. In contrast, the MOFs that are used mostly, such as MOF-177 and MOF-5, have relatively high thermal and chemical stabilities; they are known to be unstable and to easily decompose in the presence of moisture [38, 39]. Therefore, MIL-101 is rather stable in both polar and nonpolar solvents at high temperatures, which makes MIL-101 an attractive candidate for various applications and catalysts.

Lin et al. [40] used X-ray absorption near edge structure (XANES) spectroscopy for characterizing the oxidation state of Cr in the MIL-101 catalyst and found that the chromium atom is in the divalent (Cr(II)) state in the MIL-101 crystals although the source of initial Cr is Cr(III) in Cr(NO₃)₃. In the present paper, the surface composition of the MIL-101 sample was analyzed by XPS (Figure 6(a)) and the spectra corresponding to C_1s and Cr_2p were collected. The binding energy values of 587 eV for Cr2p1/2 and 576 eV for Cr2p3/2 (Figure 6(b)) are typically assigned to Cr³⁺ [41]. It could be inferred that the oxidation state Cr(III) of the chromium in MIL-101 does not change during its synthesis.

[figures omitted; refer to PDF]

Figure 7(a) shows the DR-UV-Vis spectrum of MIL-101. There are three absorption peaks at 275, 425, and 610 nm. The absorption band at the UV region could be contributed by the electron transfer $n\to {\pi }^{\ast }$ in terephthalic acid. The absorption bands in the visible region should be related to the electron transfer in orbital 3d. The energy gaps based on Tauc’s plot were found to be 1.75, 2.27, and 3.74 eV (Figure 7(b)). Since the oxidation state of chromium in MIL-101 is 3+, it was believed that here the electron shift occurs in the 3d³ orbital of Cr³⁺ under the action of the terephthalate ligand field.

[figures omitted; refer to PDF]

In order to analyze this electron transfer, the Tanabe-Sugano d³ diagram was used (Figure 8) and, according to which, spin-allowed transition was as follows: $$\begin{array}{c}\text{(1)}& 4{\text{A}}_{2\text{g}}\to 4{\text{T}}_{2\text{g}}4{\text{A}}_{2\text{g}}\to 4{\text{T}}_{1\text{g}}\\ 4{\text{A}}_{2\text{g}}\to 4{\text{T}}_{1\text{g}}\left(\text{P}\right)\end{array}$$

[figure omitted; refer to PDF]

In Tauc’s plot (Figure 7), three energy levels could be seen to be excited corresponding to the wave number: ${\nu }_1=14104.37$ (cm^-1), ${\nu }_2=18281.54$ (cm^-1), and ${\nu }_3=30120.48$ (cm^-1). Since the ratio ${\nu }_2/{\nu }_1=18282.54/14104.37$ was found as 1.3, this ratio corresponded to ${\Delta }_{\text{o}}/B=36$ based on the Tanabe-Sugano diagram of the d³ system (Δ is the ligand field splitting parameter). Considering ${\Delta }_{\text{o}}/B=36$, the value of $E/B$ for the spin-allowed transition was determined to be ${\nu }_1/B=36$, ${\nu }_2/B=46$, and ${\nu }_3/B=76$.

Since ${\nu }_1$ was 14104.37 cm^-1, the value of $B$ calculated from the first spin-allowed transition was 391.8 cm^-1 and $\Delta$ was calculated as 14104.37 cm^-1 from the ratio $\Delta /B=36$.

It is worth noting that the Racah B parameter in MIL-101 is 391.8 cm^-1 while that in free Cr³⁺ is 1030 cm^-1 [42]. Thus, a reduction of about 62% in the Racah B parameter of MIL-101 compared to Cr₂O₃ indicates a strong influence of the terephthalate ligand. Figure 8 presents three spin-enabled electrons in the Tanabe-Sugano diagram of the d³ system corresponding to the three excited energy levels as follows:

(i)

4A_2g → 4T_2g with energy transfer of 1.75 eV corresponds to a wavelength of 709 nm

(ii)

4A2g → 4T_1g with energy transfer of 2.27 eV corresponds to a wavelength of 547 nm

(iii)

4A2g → 4T_1g (P) with energy transfer 3.74 eV corresponds to a wavelength of 332 nm

MIL-101 is constituted from the trimer units (Cr₃O₁₆), which are made up of CrO₆ clusters, wherein a central chromium atom is surrounded by 6 oxygen atoms [18, 22]. As suggested by Bordiga et al. [43] for MOF-5, it is supposed that Cr₃O₁₆ clusters in MIL-101, which behave as quantum dots surrounded by six terephthalate ligands, could act as light-absorbing antennae ($\text{h}\nu$) then transferring it to Cr₃O₁₆ clusters to irradiate the photons ($\text{h}{\nu }^{\prime }$) as can be observed in Figure 9. The photocatalysis of MIL-101 was considered to be achieved by the electron transfer from photoexcited organic ligands to metallic clusters in MIL-101, which was termed as a ligand-to-cluster charge transfer [23, 25].

[figure omitted; refer to PDF]

3.2. Photocatalytic Activity of MIL-101

Adsorption is one of the important aspects of photocatalysis. Hence, in most cases, the dark adsorption is often conducted to obtain an adsorption-desorption equilibrium before UV is irradiated. In the present case, that procedure could not be carried out because the dye molecules were adsorbed quickly on the MIL-101 surface and prevented the UV light from stimulating the MIL-101 catalyst, so that the UV light was irradiated at the same time as the catalyst was introduced as shown in Figure 10. Figure 10(a) shows the decolorization kinetics for RBB dye under different conditions. It was found that the color of the RBB solution with MIL-101 catalyst but without UV irradiation was decolorized around 43% within only 15 min and then stayed constant due to saturated adsorption while the RBB color was removed completely under UV irradiation/MIL-101 catalysis after 45 min. These experiments demonstrated that the decolorization of the solution was due to the photocatalytic effect rather than adsorption.

[figures omitted; refer to PDF]

The kinetics of decolorization of RBB on MIL-101 added with 0.01 g Cr(NO₃)₃ or Cr₂O₃ are shown in Figure 10(b). It was found that the Cr³⁺ ion or Cr₂O₃ did not affect decolorization. It could be inferred that the Cr³⁺ ion or Cr₂O₃ does not exhibit photocatalytic activity in this condition. In addition, the decolorization of RBB was not observed if UV was irradiated in the absence of MIL-101 suggesting that RBB was stable and did not undergo photolysis. A leaching experiment was also conducted in which the MIL-101 catalyst was filtered by centrifugation after 5 min of irradiation. The decolorization of dye was stopped despite the fact that UV light irradiation was still maintained. This indicates that there is no leaching of the active species, into the reaction solution, from the homogeneous catalyst. The above experimental results confirmed that MIL-101 was a heterogeneous catalyst in the degradation reaction of RBB.

The influence of the initial concentration of RBB on the photocatalytic decolorization rate in the presence of MIL-101 is shown in Figure 11. The results exhibited that when the dye concentration increased in the range of 10 ppm to 50 ppm, an increase in the decolorization rate was observed.

[figure omitted; refer to PDF]

The generalized rate equation for decolorization of dye can be written as $$\begin{array}{}\text{(2)}& r=-\frac{dC}{dt}=k·C^n,\end{array}$$where $C$ is the concentration of dye at time $t$ (the reaction time), $k$ is the kinetic rate constant, $n$ is the order of the reaction, and $r$ is the reaction rate.

In this paper, the initial rate method was used to determine $k$ and $n$ [44, 45].

The instantaneous reaction rate was calculated from the following equation: $$\begin{array}{}\text{(3)}& r_{\text{in}}=-\frac{dC}{dt}.\end{array}$$

Integrating equation (3) for the boundary conditions $t\to 0$, then $C\to C_0$ gives $$\begin{array}{}\text{(4)}& C_t=-r_{\text{in}}\cdot t+C_0,\end{array}$$where $C_0$ and $C_t$ are the initial concentration and the concentration at time $t$, respectively. The instantaneous rate ($r_{\text{in}}$) is determined from the plot of concentration versus time at time $t$. The initial rate ($r_0$) of a reaction is the instantaneous rate at the start of the reaction (when $t=0$). The initial rate is equal to the negative slope of the curve of reactant concentration versus time at $t=0$. From the slopes of the plots of $C_t$ against $t$ at $C_0$ ($t=0$), the values of $r_0$ corresponding to each initial concentration $C_0$ was obtained as shown in Figure 12(a).

[figures omitted; refer to PDF]

On the other hand, the initial rate for a reaction can be written as $$\begin{array}{}\text{(5)}& r_0=k_i·C_0^n,\end{array}$$where $k_i$ is the overall observed rate constant for the reaction and $n$ is the order of the reaction with respect to the concentration. The linearization of equation (5) by taking natural logarithms on both sizes yields $$\begin{array}{}\text{(6)}& \ln r_0=\ln k_i+n·\ln C_0.\end{array}$$

Therefore, the plot of the $\ln r_0$ against $\ln C_0$ gives a straight line with a slope corresponding to $n$ and the intercept on the ordinate gives $\ln k_i$ (Figure 12(a)). From the plot of $\ln r_{\text{Ao}}$ against $\ln C_{\text{Ao}}$, the slope, $n=0.604$, and $k=1.156$ were calculated, and the plot has an excellent correlation coefficient ($r^2=0.998$, $p\le 0.001$). The reaction order of photocatalytic degradation is unity in some cases [27, 46]. In the present paper, the value of $n$ less than unity could be due to the contribution of both adsorption and photocatalytic reaction.

The Langmuir-Hinshelwood (L-H) equation is widely used in studying the kinetics of photocatalytic reaction [46, 47]. In this model, the reaction rate depends on the percentage of the surface coverage, $\theta$, by the following equation: $$\begin{array}{}\text{(7)}& r_0=k_T·\theta =-\frac{dC}{dt}=\frac{k_T·K_{\text{LH}}·C_0}{1+K_{\text{LH}}·C_0},\end{array}$$where $K_{\text{LH}}$ is the Langmuir-Hinshelwood adsorption equilibrium constant (L·mg^-1) and $k_T$ is the reaction rate constant (mg·L^-1·min^-1). Equation (7) can be written in the linear form as $$\begin{array}{}\text{(8)}& \frac{1}{r_0}=\frac{1+K_{\text{LH}}·C_0}{k_T·K_{\text{LH}}·C_0}=\frac{1}{k_T·K_{\text{LH}}·C_0}+\frac{1}{k_T}.\end{array}$$

The plot of $1/r_0$ against $1/C_0$ (Figure 12(b)) gives a straight line with a good correlation ($r^2=0.987$, $p=0.001$); the values of $k_T$ and $K_{\text{LH}}$ were 17.857 mg·L^-1·min^-1 and 0.035 L·mg^-1, respectively. Akpan and Hameed [46] compared the adsorption and reaction rate relying on the ratio of $k_T:K_{\text{LH}}$. However, this comparison seems to be unclear because the units of $k_T$ and $K_{\text{LH}}$ are not similar.

The complete degradation of dyes with a cost-effective process is important for industrial applications. UV-Vis spectra are shown in Figure 13(a). The absorbance band at 310 nm was contributed by the $\pi \to {\pi }^{\ast }$ transition of the double bond in the aromatic ring. The absorbance band at 600 nm was assigned to $n\to {\pi }^{\ast }$ due to the double bond conjugation of N=N and C=C in the aromatic ring. The absorption peak decreased significantly with an increase in the irradiation time and disappeared after 45 min of irradiation. The chemical oxygen demand (COD) test is shown in Figure 13(b). The COD decreased from an initial value of 86.4 mg·L^-1 to around 10.0 mg·L^-1 after 80 min. The results indicate that RBB molecules were degraded into fragments and, subsequently, were completed with minerals.

[figures omitted; refer to PDF]

The photochemical degradation mechanism of RBB dye on MIL-101 can be interpreted by the semiconductor theory [25]. As proved earlier, MIL-101 was photoexcited leading to the electron transitions in the 3d³ orbital, followed by the formation of an electron (e^-) and hole (h⁺) pair on the surface of the catalyst. The high oxidation potential of the hole (h⁺) in the catalyst either permitted the direct oxidation of the dye or reacted with water molecules or hydroxyl ions (OH^-) to generate hydroxyl radicals (^⋅OH). These hydroxyl radicals oxidized the surface adsorbed organic molecules. On the other hand, the photogenerated electrons (e^-) reduced the dye or reacted with the O₂ adsorbed on the MIL-101 surface or dissolved in water forming a radical anion (^⋅O₂^-). This strong oxidation could degrade RBB. According to this, the photochemical degradation reactions of RBB on MIL-101 can be expressed as follows: $$\begin{array}{c}\text{(9)}& \left(\text{MIL}‐101\right)+\text{h}v\left(\text{MIL}‐101\right)\left({\text{e}}^{-}+{\text{h}}^{+}\right){\text{h}}^{+}+\text{dye}\to {\text{RBB}}^{\cdot }\to \text{degradation}\text{products}\\ {\text{h}}^{+}+{\text{H}}_2\text{O}\to {\text{H}}^{+}{+}^{\cdot }\text{OH}\\ {\text{h}}^{+}+\text{O}{\text{H}}^{-}{\to }^{\cdot }\text{OH}\\ {}^{\cdot }\text{OH}+\text{RB}{\text{B}}^{\cdot }\to \text{degradation}\text{products}\\ {\text{e}}^{-}+{\text{O}}_2{\to }^{\cdot }{{\text{O}}_2}^{-}\\ {}^{\cdot }{{\text{O}}_2}^{-}+\text{RBB}\to \text{degradation}\text{products}\end{array}$$

The reusability of the catalyst is an important concern in the application of heterogeneous catalysis. After the experiment, the MIL-101 material was collected by centrifugation and washed with water and ethanol for three times to remove RBB completely and dried at 120°C for 15 h and then reused. The photocatalytic degradation efficiency of MIL-101 after four cycles of usage was decreased slightly (only 5%) (Figure 14(a)). The XRD patterns of the recycled MIL-101 catalysts were unchanged (Figure 14(b)). It can be concluded that the MIL-101 photocatalyst exhibited excellent catalytic recyclability and stability in the tested conditions. It can be inferred that MIL-101 can be a potential catalyst for the treatment of organic pollutants in aqueous solutions.

[figures omitted; refer to PDF]

4. Conclusions

The suitable molar ratios of Cr/H₂BDC and H₂O/H₂BDC for the synthesis of MIL-101 with a large surface area and high crystallinity were 1.25 and 350, respectively. High water content in the synthesized gel significantly reduced particle size and crystallinity. MIL-101 can be exposed to ambient conditions for several months. The characteristic XRD diffraction pattern at 1.7° can be observed in the samples stored in dry conditions, whereas this diffraction disappeared when the samples were stored in moistened conditions. MIL-101 is stable in water, benzene, and toluene even at boiling point for several hours. MIL-101 exhibited excellent photodegradation of RBB in the UV region. The kinetics of the photocatalytic degradation reaction was fitted well with the Langmuir-Hinshelwood (L-H) equation. The initial rate method yielded the order of the reaction and the initial reaction rate constant to be 0.604 and 1.156 [(mg·L^-1)^0.396·min^-1], respectively.

Conflicts of Interest

The authors declare that they have no conflict of interest.

Acknowledgments

This research was sponsored by Hue University under Decision No. 1208/QĐ-DHH.

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