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New photocatalysts based on MIL-53 metal–organic frameworks for the decolorization of methylene blue dye
Journal of Hazardous Materials 190 (2011) 945–951
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New photocatalysts based on MIL-53 metal–organic frameworks for the decolorization of methylene blue dye Jing-Jing Du a , Yu-Peng Yuan a,∗ , Jia-Xin Sun a , Fu-Min Peng a , Xia Jiang a , Ling-Guang Qiu a,∗ , An-Jian Xie a , Yu-Hua Shen a , Jun-Fa Zhu b a b
Laboratory of Advanced Porous Materials and School of Chemistry and Chemical Engineering, Anhui University, Hefei 230039, China National Synchrocyclotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China
a r t i c l e
i n f o
Article history: Received 8 November 2010 Received in revised form 25 March 2011 Accepted 6 April 2011 Available online 12 April 2011 Keywords: Metal–organic frameworks MIL-53 Photocatalyst Methylene blue
a b s t r a c t The photocatalytic decolorization of methylene blue dye in aqueous solution using a novel photocatalyst MIL-53(Fe) metal–organic frameworks was investigated under UV–vis light and visible light irradiation. The effect of electron acceptor H2 O2 , KBrO3 and (NH4 )2 S2 O8 addition on the photocatalytic performance of MIL-53(Fe) was also evaluated. The results show that MIL-53(Fe) photocatalyst exhibited photocatalytic activity for MB decolorization both under UV–vis light and visible light irradiation, and the MB decolorization over MIL-53(Fe) photocatalyst followed the first-order kinetics. The addition of different electron acceptors all enhances the photocatalytic performance of MIL-53(Fe) photocatalyst, and the enhanced rate follows the order of H2 O2 > (NH4 )2 S2 O8 > KBrO3 under UV–vis light irradiation, while in the order of (NH4 )2 S2 O8 > H2 O2 > KBrO3 under visible light irradiation. Moreover, MIL-53(Fe) did not exhibit any obvious loss of the activity for MB decolorization during five repeated usages. The photocatalytic activities over MIL-53(M) (M = Al, Fe), the isostructure to MIL-53(Fe), indicate that the metal centers show nil effect on the photocatalytic activity of MIL-53(M) photocatalysts. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The ability of using solar energy to eliminate the organic pollutants makes photocatalysis a potential technology for solving environmental issues confronting mankind. Since the discovery of the first artificial photocatalytic system for pollutants degradation over TiO2 , many metal oxides and sulfides including ZnO, WO3 , CdS, ZnS have been identified as active photocatalysts for photodegradation of organic pollutants in gas or aqueous phase [1–6]. The quantum yield and solar energy conversion efficiency of these developed photocatalysts, however, are still low at present, thus limiting their practical applications in environmental purification. Consequently, it is of great interest to search for new photocatalysts with improved activities. Metal–organic frameworks (MOFs), which exhibit high surface area and large pore volume, have attracted considerable attention due to their elegant topology and potential applications in separation, gas storage, molecular sensing, and catalysis [7–10]. In addition, MOFs behave as semiconductors when exposed to light, thus making MOFs potentially be photocatalysts [11]. More
∗ Corresponding authors. Tel.: +86 551 5108212; fax: +86 551 5108212. E-mail addresses: [email protected] (Y.-P. Yuan), [email protected] (L.-G. Qiu). 0304-3894/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2011.04.029
recently, MOFs that can act as photocatalysts have attracted much attention for exploiting new applications of MOFs [12–18]. Garcia et al. firstly proposed MOF-5 to be an active photocatalyst for photodegradation of phenol. In contrast to a conventional photocatalyst of metal oxide or sulfide, MOF-5 displayed the reverse shape-selectivity in which small phenolic molecules that can diffuse freely into the micropores of MOF-5 are degraded more slowly than those that cannot access to the interior of MOF-5 [12]. Natarajan et al. used different MOFs based on Co, Ni, and Zn as photocatalysts to degrade organic dyes. The photocatalytic results show that all three MOFs are active for the photodegradation of four widely used dyes (orange G, rhodamine B, Remazol Brilliant Blue R, and methylene blue) in the textile industry. And the activities of three MOFs photocatalysts follow the reverse order of their band gap [13]. Gascon et al. employed isoreticular MOFs (IRMOF-1, IRMOF-2, IRMOF-7, IRMOF-8, and IRMOF-9) as photocatalysts for gas-phase photooxidation of propene. Particularly, IRMOF-8 displayed a higher activity than ZnO, a common photocatalyst [14]. These emerging researches demonstrate MOFs to be a potential new class of photocatalysts for environmental purification. In contrast to the conventional photocatalysts of metal oxides and sulfides, the photocatalytic properties of MOFs have remained unexplored. Herein, we report the photocatalytic activities of MIL-53(M) (M = Al, Cr, Fe) in photodegradation of methylene blue (MB)
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Scheme 1. The chemical structure of MIL-53(Fe) and the electron transfer processes that occur in MIL-53(Fe) when irradiated by light.
dye. Representatively, MOFs MIL-53(Fe) is three-dimensional porous solids built up by infinite one-dimensional linkage of –Fe–O–O–Fe–O–Fe–, cross-linked by bis-bidentate terephthalate (1,4-benzenedicarboxylate) linkers (see Scheme 1) [19–21]. Like TiO2 semiconductor whose conduction band was constructed by empty Ti 3d orbitals, MOFs MIL-53 containing transition metals as structural nodes are also expected to be semiconductors since the empty d metal orbitals mixed with the LUMOs of the organic linkers would formed the conduction band [14]. Upon light irradiation, electron excitation takes place in MOFs, followed by subsequent electron transfer (see Scheme 1) [18]. Therefore, MOFs are expected to be active photocatalysts, as reviewed above. Presently, MIL53(M) is a value-added material for separating gases, including CO2 , CH4 , H2 S and a variety of organic species [22–32]. To the best of our knowledge, no attention, however, has ever been paid to study the photocatalytic properties of MIL-53(M) to date. Recently, we found that MIL-53(Fe) exhibited photocatalytic activity for MB dye degradation under both UV–vis and visible light irradiation. The introduction of different electron acceptors in the MB aqueous solution greatly promoted the photocatalytic property of MIL-53(Fe). To our knowledge, this synergistic enhancement in the degradation of organic pollutants by the combination of different electron acceptors and MIL-53(Fe) photocatalyst has not yet been reported. Meanwhile, the photocatalytic activities of MIL-53(Al) and MIL53(Cr), the isostructure to MIL-53(Fe), were also investigated to provide an insight on the correlation between metal centers of a MOFs photocatalyst and its photocatalytic activity. 2. Materials and methods 2.1. Reagents and chemicals All reagents were analytical grade and used without further purification. Iron (III) chloride hexahydrate (99%), terephthalic acid (97%), N,N -dimethylformamide (99%) and hydrogen peroxide (30%) were purchased from Sinopharm Chemical Reagent Co. Ltd., China. Aluminum nitrate nonahydrate (99%) and chromium (III) nitrate nonahydrate (99%) were supplied by Tianjin Guangfu Fine Chemical Research Institute, China. De-ionized water obtained from a Milipore Milli-Q system was used to prepare aqueous solutions for MIL-53 synthesis and for the irradiation experiments.
53(Fe) powder was re-heated at 150 ◦ C overnight to remove the DMF inside the pores of MIL-53(Fe). After cooling down to room temperature, the solid was then washed with a large volume of de-ionized water (1 g of MIL-53 in 0.5 l of water) to give the final MIL-53(Fe) photocatalysts. MIL-53(Cr) was hydrothermally synthesized at 220 ◦ C for 3 days from a mixture of chromium (III) nitrate, terephthalic acid, hydrofluoric acid, and H2 O in the molar ratio 1:1:1:280 [20,21]. To remove traces of terephthalic acid, the obtained solid was then washed in 200 ml ethanol at 70 ◦ C four times. MIL-53(Al) was hydrothermally synthesized in a 60 ml Teflonlined stainless steel Parr bomb containing aluminum nitrate, terephthalic acid (BDC), and deionized water under autogenous pressure at 220 ◦ C for 3 days. The molar ratio of Al:BDC:H2 O was fixed at 1:0.5:80 [22]. The obtained product was followed to calcine at 280 ◦ C to remove the terephthalic acid residues and washed with deionized water for four times [23]. 2.3. Photocatalysts characterization X-ray powder diffraction (XRD) data of the obtained MIL-53(M) (M = Fe, Al, Cr) products was collected on a Rigaku Ultima III Xray powder diffractometer with monochromatic Cu K␣1 radiation (40 kV, 40 mA). The surface morphologies and size of MIL-53 products were observed by field emission scanning electron microscopy (SEM: S4800, Hitachi). UV–vis diffuse reflectance data was collected over the spectral range 210–850 nm with a spectrophotometer (UV-2550, Shimadzu) equipped with an integrated sphere. BaSO4 was used as a reference sample. X-ray photoelectron spectra (XPS) of the as-prepared MIL-53 were recorded on an ESCALAB 250 spectrometer (Thermo-VG Scientific) using Mg K␣ radiation (1253.6 eV) and the binding energy values were calibrated with respect to C (1s) peak (284.6 eV). 2.4. Photocatalytic degradation of MB The photocatalytic activities of MIL-53(M) (M = Fe, Al, Cr) photocatalysts were evaluated by the photodegradation of MB dye under a 500 W Xe lamp irradiation in open air and at room temperature. The distance between the light source and the beaker containing reaction mixture was fixed at 15 cm. 1 mg of MIL-53(M) (M = Fe, Al, Cr) photocatalyst was put into 100 ml of MB aqueous solution (4 × 10−4 mol/l) in a 250 ml beaker. The pH value of the suspension was adjusted to neutral (pH = 7.0). Prior to irradiation, the suspension was magnetically stirred in dark for 60 min to ensure the establishment of an adsorption/desorption equilibrium. During the photodegradation reaction, stirring was maintained to keep the mixture in suspension. Samples were withdrawn at regular intervals and immediately centrifuged to separate photocatalysts for analysis. The MB concentration was monitored by measuring the absorption intensity at its maximum absorbance wavelength of = 664 nm using a UV-visible spectrophotometer (UV-1800, Shimadzu) in a 1 cm path length spectrometric quartz cell. To measure the photocatalytic activity under UV–vis irradiation, a 420 nm cutoff filter was used to provide the visible light. 3. Results and discussion
2.2. Synthesis of MIL-53(Fe) photocatalyst
3.1. X-ray diffraction (XRD) analysis and electron microscopy (SEM)
MIL-53(Fe) photocatalyst was prepared according to the previous report by Férey et al. [21]. Typically, a mixture of iron chloride, terephthalic acid and N,N -dimethylformamide (DMF) with a molar ratio of 1:1:280 was transferred into a Telfon-lined stainless steel bomb and heated at 150 ◦ C for 15 h. The as-obtained yellow MIL-
The porous metal terephthalates MIL-53 solids are constructed by terephthalate anions and metal (III) ions, creating a threedimensional framework with a one-dimensional pore channel system [19–21]. Powder X-ray diffraction (PXRD) analysis showed that the as-prepared MIL-53(M) (M = Fe, Al, Cr) photocatalysts
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Fig. 1. Simulated patterns from the crystallographic data of (1) MIL-53(Fe), (3) MIL53(Al), (5) MIL-53(Cr) and powder X-ray diffraction pattern of (2) MIL-53(Fe), (4) MIL-53(Al), (6) MIL-53(Cr) synthesized using hydrothermal synthesis.
are crystallines and correspond to the known bulk phase MIII (OH)·[O2 C–C6 H4 –CO2 ] (M = Fe, Al, Cr), as shown in (Fig. 1), wherein the simulated PXRD patterns of MIL-53(M) (M = Fe, Al, Cr) are also provided for reference [33]. No impurity peaks were detected in the obtained MIL-53(M) (M = Fe, Al, Cr) photocatalysts, indicating that the obtained samples were single phase MIL-53(M) photocatalysts. Fig. 2 shows the SEM images of as-prepared MIL-53(M) (M = Fe, Al, Cr) photocatalysts that were used for XRD analysis. Wellcrystallized sphere-like particles with particle size distribution of 200–600 nm were dominative for MIL-53(Fe). A small fraction of sheet-like particles were also observed, as shown in Fig. 2a. By comparison, the morphology of MIL-53(Al) and MIL-53(Cr) is relatively homogeneous (see Fig. 2b and c). Such phenomenon should be ascribed to the different crystallization process, as described in Section 2. 3.2. Uv–vis diffuse reflectance spectroscopy The room temperature diffuse reflectance absorption spectra of MIL-53(M) (M = Fe, Al, Cr) photocatalysts, converted from reflectance to absorbance by Kubelka-Munk mehod, was illustrated in Fig. 3. The main optical absorption band is around 320, 395, 455 nm for MIL-53(Al), MIL-53(Cr), and MIL-53(Fe), respectively. These optical transitions can be assigned to ligand-to-metal charge transfer (LMCT) [15]. It can be observed that the absorption edge of the as-prepared MIL-53(M) photocatalysts is gradually shifted to longer wavelength by altering metal ions from Al to Cr and to Fe. This shift should be arisen from the variation of the inorganic composition in MIL-53(M). Theoretical calculations by FuentesCabrera et al. show that the band gap of the M-IRMOF-1 series is inert to the variation of metal (M) incorporated in the structure (M = Be, Mg, Ca, Zn, and Cd), where IRMOF-1 stands for isoreticular metal–organic framework MOF-5 [34]. Our result, however, is obviously different from the report by Fuentes-Cabrera et al. Considering that the electronegativity of Al3+ , Cr3+ and Fe3+ ions follows the order of Al3+ < Cr3+ < Fe3+ , the energy required for the LMCT is therefore in the reverse order. In simple terms, the low electronegativity of Al3+ would demand more energy for the transfer of the charge from the ligand to the Al3+ ion, thus resulting in the larger band gap than that of Cr3+ and Fe3+ ions. It is noteworthy to note that, in the case of Cr3+ , two clear additional peaks around 455 and 596 nm were observed. These two peaks at 455 and 596 nm would originate from the d–d transition of the Cr3+ ion [35]. The onset of the main absorption edge of MIL-53(Al), MIL-53(Cr) and MIL-53(Fe) was 320 nm, 395 nm and 455 nm, which corresponds
Fig. 2. SEM images of (a) MIL-53(Fe), (b) MIL-53(Cr) and (c) MIL-53(Al).
to the band gaps (Eg ) of 3.87, 3.20, 2.72 eV (Eg = 1240/wavelength), respectively. 3.3. Photocatalytic activities The photocatalytic degradation of MB was conducted to investigate the efficiency of the MIL-53(M) photocatalysts. In each experiment, the MIL-53(M) concentration was constant (0.01 g/l) because higher catalysts concentration always results in the total adsorption of MB dye on MIL-53(M). The photocatalytic activities of three MIL-53(M) photocatalysts were monitored from the variation of the color in the reaction system by measuring the maximum absorbance intensity of MB chromophoric group at
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Fig. 3. UV–vis absorption spectra of MIL-53(M) (M = Fe, Al, Cr) photocatalysts at ambient temperature. The inset shows the estimated band gap values.
max = 664 nm. For comparison, the photocatalytic performance of commercial TiO2 under UV–vis light irradiation was also assessed under the identical experimental conditions. Control experiments without photocatalysts and light irradiation were also performed. Fig. 4 illustrates the photodegradation profile of MB over MIL53(Fe) photocatalyst under the UV–vis and visible light irradiation, wherein the photocatalytic performance of TiO2 was also presented. Notably, MIL-53(Fe) shows a comparable activity with TiO2 under UV–vis light and visible light irradiation. No MB degradation was observed over MIL-53(Fe) photocatalyst without the light irradiation (see Fig. 4). In contrast, 4% and 1% MB were photodegradated after 40 min UV–vis and visible light irradiation in the absence of MIL-53(Fe) photocatalyst, respectively (see Fig. 4). After 40 min of UV–vis light and visible light irradiation, the MB removal over MIL53(Fe) photocatalyst is 11% and 3%, respectively. Thus, it can be concluded that MIL-53(Fe) shows the photocatalytic activity for MB degradation, although the photodegradation rate is low. The reaction mechanism for MB decoloration could be discussed based on semiconductor theory. Illumination of MIL-53(Fe) photocatalyst by photons with energy equal to or greater than its band gap excites electrons (e− ) from the valence band to the conduction band and
Fig. 4. MB degradation profile under the irradiation of () without light, () visible light, () UV–vis light, () visible light with the presence of MIL-53(Fe) photocatalysts, and () UV–vis light with the presence of MIL-53(Fe) photocatalysts. () UV–vis light with the presence of TiO2 photocatalysts, () visible light with the presence of TiO2 photocatalysts. The inset shows reaction kinetics over MIL-53(Fe) photocatalysts under the irradiation of UV–vis light and visible light, respectively. The plots were drawn based on the average values of three repeated experimental results.
produces holes (h+ ) in the valence band. The photogenerated holes (h+ ) with strong oxidant capacity can directly oxidize adsorbed organic molecules or react with water molecules or hydroxyl ion (OH− ) to generate hydroxyl radical (• OH). The formed hydroxyl radicals (• OH) also possess strong oxidation ability and can react readily with surface adsorbed organic molecules. Meanwhile, photogenerated electrons (e− ) can be trapped by molecular oxygen to form superoxide radical (• O2 − ), which also possesses strong oxidant ability to decolorize the MB molecules. The low efficiency of MB photodegradation over MIL-53(Fe) photocatalyst could be ascribed to the fast electron–hole recombination. The electron transfer process is more efficient if the molecules are preadsorbed on the surface within a reasonable range and with an appropriate orientation. However, in present case, the MB molecules preadsorbed on the surface of MIL-53(Fe) may exist in a manner that is not favor for the transfer of the photoexcited electrons since MOFs always display selective adsorption behavior [8]. Meanwhile, the electron of the excited state decays to its ground state very quickly. These two factors induce the low efficiency of MB photodegradation over MIL-53(Fe). More experimental and computational studies are needed to elucidate the present situation. The photodegradation of MB dye over MIL-53(Fe) photocatalyst follows first-order kinetics model (see the inset of Fig. 4). The first-order kinetics can be written as ln(C0 /C) = Kt according to the Langmuir–Hinshelwood model, wherein, C0 is the initial concentration of the MB, C is the t time’s concentration of the dyes, t is the reaction time, and K is the kinetic rate constant. The values of K were obtained from the slope and the intercept of the he linear plot. The rate constant of MB photodegradation was 0.0036 min−1 under visible light irradiation, which is only one-quarter of that under UV–vis light irradiation (0.0133 min−1 ) (see the inset of Fig. 4). The low rate of MB photodegradation should be reasonably ascribed to the decreased photons absorbed by MIL-53(Fe). The present finding indicates that MIL-53(Fe) can be used as a visible-light-response photocatalyst for removal of dye pollutants. The relatively low efficiency in photodegradation of MB over MIL-53(Fe) promotes us to search an approach that can circumvent this limitation. As stated above, the photocatalytic processes for dye degradation start from an electronic transition from the valence band (VB) to the conduction band (CB) with the generation of electron–hole pairs. The photogenerated holes (h+ ) either directly react with organic molecules or react with water molecules or hydroxyl ion (OH− ) to generate hydroxyl radical (• OH). The formed hydroxyl radicals (• OH) possess strong oxidation ability and can react readily with surface adsorbed organic molecules. The recombination of photogenerated holes and electrons, however, always results in the reduced holes for decolorization of organic dyes. Consequently, the introduction of external electron acceptor is expected to enhance the performance of MIL-53(Fe) photocatalyst. The addition of hydrogen peroxide (H2 O2 ), potassium bromate (KBrO3 ) and ammonium persulfate ((NH4 )2 S2 O8 ) electron acceptor can suppress the electron–hole pair recombination according to Eqs. (1)–(3), thus enhancing the photodegradation efficiency via generating more radical (• OH, BrO2 • and SO4 •− ) [36]. In fact, H2 O2 , KBrO3 and (NH4 )2 S2 O8 have been demonstrated to be the efficient electron acceptor in the reaction of the dye photodegradation [37,38]. In this connection, we have investigated the effect of electron acceptor addition on the photocatalytic performance of MIL-53(Fe) photocatalyst a period. Fig. 5 illustrates the time course of MB photodegradation in the presence of different electron acceptors over MIL-53(Fe) photocatalyst where all the electron acceptor concentration is constant (10−5 mol/l). It was observed that all the electron acceptors had beneficial effect on improving the rate for MB decolorization both under UV–vis light and visible light irradiation. And the enhanced rate follows the order of H2 O2 > (NH4 )2 S2 O8 > KBrO3 under UV–vis light irradiation (see
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Fig. 5. The effect of different electron acceptor additives on the MB photodegradation under the irradiation of (a) UV–vis light and (b) visible light, () with the presence of H2 O2 and MIL-53(Fe), () with the presence of H2 O2 and the absence of MIL-53(Fe), (䊉) with the presence of (NH4 )2 S2 O8 and MIL-53(Fe), () with the presence of (NH4 )2 S2 O8 and the absence of MIL-53(Fe), () with the presence of KBrO3 and MIL-53(Fe), () with the presence of KBrO3 but the absence of MIL-53(Fe). The inset shows the corresponding reaction kinetics over different electron acceptors and MIL-53(Fe) photocatalyst. The plots were drawn based on the average values of three repeated experimental results.
Fig. 6. Changes of the MB concentration during the five repeated processes over MIL-53(Fe) in the presence of H2 O2 (10−5 mol/l).
Fig. 5a), while in the order of (NH4 )2 S2 O8 > H2 O2 > KBrO3 under visible light irradiation (see Fig. 5b). The decreased electron–hole recombination is responsible for the improved rate for MB degradation. Notably, the addition of H2 O2 has shown pronounced effect on MB decolorization. Roughly 20% degradation of MB was observed after 20 min of visible light irradiation. Better result was obtained when irradiated under UV–vis light. In this case, 20 min
of UV–vis light irradiation causes the almost complete degradation of the MB molecules. The rate constant values in present case were 0.1250 min−1 , 0.0177 min−1 for UV–vis light and visible light irradiation, respectively (see the inset of Fig. 5a and b). These values are 9 times and 5 times higher than those in the absence of H2 O2 . The better degradation rate in the presence of H2 O2 than KBrO3 and (NH4 )2 S2 O8 could be attributed to the suitable electron trapping, thereby suppressing the recombination of e− and h− and thus increasing the number of OH• . Meanwhile, the more produced hydroxyl radicals (• OH) via Eq. (1) can also act as a strong oxidant for MB decolorization. These two factors induce the faster degradation rate of H2 O2 addition than that of KBrO3 and (NH4 )2 S2 O8 . Control experiments were conducted by irradiating the MB aqueous solution containing H2 O2 , KBrO3 and (NH4 )2 S2 O8 additives in the absence of MIL-53(Fe) photocatalyst (see Fig. 5). About 55%, 7% and 14% degradation of MB are observed after 20 min of UV–vis light irradiation, while 99%, 24% and 47% MB degradation are achieved at 20 min in the presence of MIL-53(Fe) and H2 O2 , KBrO3 and (NH4 )2 S2 O8 , respectively. Synergy index (SI), defined as SI = K(additive+MIL-53(Fe) /(KMIL-53(Fe) + Kadditive ), of 2.1, 1.2 and 1.1 indicates a synergic effect for the photocatalytic process over MIL-53(Fe) photocatalyst in the presence of electron acceptor additives. H2 O2 + e− → OH• + OH−
(1)
BrO3 − + 2H+ + e− → BrO2 • + H2 O
(2)
Fig. 7. (a) XRD patterns and (b) XPS spectrum of MIL-53(Fe) (2) before and (1) after MB decolorization for 60 min prolonged reaction.
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from Al to Cr and to Fe, the amount of the adsorbed photons should become larger because of the decreased band gap (see Fig. 2). Then, the MIL-53(Fe) with the narrowest band gap among three MIL-53 photocatalysts is expected to exhibit the highest rate for MB degradation. Contrary to this expectation, three MIL-53 photocatalysts with different band gap display the similar rate for MB degradation. More experimental and computational studies are needed to elucidate the present situation. 4. Conclusions
Fig. 8. MB degradation over () MIL-53(Al), (䊉) MIL-53(Fe), and () MIL-53(Cr) photocatalysts under UV–vis light irradiation.
S2 O8 2− + e− → SO4 2− + SO4 •−
(3)
For the purpose of the practical applications, it is essential to evaluate the long-term stability of a photocatalyst. In the present case, after each run of the MB total decolorization, the identical amount of MB was then followed to add into the suspension solution for another run reaction while kept other factors identical. Fig. 6 shows the time course of MB decolorization during five consecutive cycles. The MB decolorization rate constant for the 5 cycles was 0.1250 min−1 , 0.1123 min−1 , 0.1223 min−1 , 0.1323 min−1 and 0.1283 min−1 , respectively. No obvious loss of the activity for MB decolorization was observed over MIL-53(Fe) during five cycles, indicating that the MIL-53(Fe) possesses excellent long-term stability. The crystal structure and chemical states of MIL-53(Fe) before and after MB decolorization reaction were recorded by XRD and XPS analysis. In the present case, the concentration of MIL-53(Fe) was too low to be separated. For XRD and XPS analysis, the amount of MIL-53(Fe) and MB dye was proportionally increased and irradiated under UV–vis light for prolonged time. Fig. 7 shows the XRD and XPS patterns of MIL-53(Fe) before and after 60 min reaction. The almost identical diffraction pattern before and after prolonged usage illustrates that no structural transformation was happened, but with the obvious variations in the diffraction intensities (see Fig. 7a). Fig. 7b shows the Fe 2p binding energy spectra before and after multiple usages. The binding energy peak around 711 eV is ascribed to Fe 2p3/2 and the peak around 725 eV is assigned to Fe 2p1/2 . The peak separation, namely, = 2p1/2 − 2p3/2 = 14 eV, is very similar to those reported for Fe2 O3 [new references]. Therefore these peaks belong to Fe3+ of MIL-53(Fe). The XPS data did not vary after repeated use for 5 cycles, indicating the long-term stability of MIL-53(Fe) as a photocatalyst. Fig. 8 shows the photocatalytic activities of MIL-53(Al) and MIL-53(Cr), wherein, the photocatalytic activity of MIL-53(Fe) is also presented. MIL-53(Al) and MIL-53(Cr) are isostructural to MIL-53(Fe). Therefore, study on this series of isostructural photocatalysts would provide us valuable information of the effect of metal centers of a MOFs photocatalyst on its photocatalytic activity. Both MIL-53(M) (M = Al, Cr) photocatalysts display photocatalytic activities for MB decolorization. After 60 min of UV–vis light irradiation, the MB removals over MIL-53(Al) and MIL-53(Cr) are 30% and 32%, respectively. It can be noted that no obvious difference in photocatalytic activities of MIL-53(Al), MIL-53(Cr), and MIL-53(Fe) is observed. In photocatalysis research, it is generally believed that the larger the number of absorbed photons is, the higher the photocatalytic activity of a photocatalyst is. As altering the metal ion
We have shown a novel series of photocatalysts based on MIL-53(M) (M = Fe, Cr, Al) metal–organic frameworks for MB photodegradation. The photodegradation of MB over MIL-53(Fe) photocatalyst followed first-order kinetics, and the rate constant is 0.0133 min−1 and 0.0036 min−1 for UV–vis light and visible light irradiation, respectively. Furthermore, we present the large promoting effect of electron acceptor H2 O2 , KBrO3 and (NH4 )2 S2 O8 addition on the photocatalytic performance of MIL-53(Fe) photocatalyst. The metal centers of MIL-53 show nil effect on the photocatalytic activity for MB photodegradation. The present contribution clearly reveals the great potentialities of metal–organic frameworks as photocatalyst, a field that has remained unexplored so far. Acknowledgements This work was supported by the National Natural Science Foundation of China (20971001 and 51002001), the NSFC-CAS Joint Fund for Research Based on Large-Scale Scientific Facilities (10979014), the Program for New Century Excellent Talent in University, Ministry of Education, China (NCET-08-0617), the “Hundred Talents Program” of the Chinese Academy of Sciences, the Naturial Science Foundation of Anhui Province, China (Grant 090414164), and the “211 Project” of Anhui University. References [1] N.F. Steven, A.J. Bard, Heterogeneous photocatalytic oxidation of cyanide ion in aqueous solution at TiO2 powder, J. Am. Chem. Soc. 99 (1977) 303–304. [2] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96. [3] T.L. Thompson, J.T. Yates Jr., Surface science studies of the photoactivation of TiO2 – new photochemical processes, Chem. Rev. 106 (2006) 4428–4453. [4] A. Mills, S.L. Hunte, An overview of semiconductor photocatalysis, J. Photochem. Photobiol. A: Chem. 108 (1997) 1–35. [5] K. Ayoub, E.D. Van Hullebusch, M. Cassir, A. Bermond, Application of advanced oxidation processes for TNT removal: a review, J. Hazard. Mater. 178 (2010) 10–28. [6] U.G. Akpan, B.H. Hameed, Parameters affecting the photocatalytic degradation of dyes using TiO2 -based photocatalysts: a review, J. Hazard. Mater. 170 (2009) 520–529. [7] L.J. Murray, M. Dinca, J.R. Long, Hydrogen storage in metal–organic frameworks, Chem. Soc. Rev. 38 (2009) 1294–1314. [8] J.Y. Lee, O.K. Farha, J. Roberts, K.A. Scheidt, S.T. Nguyen, J.T. Hupp, Metal–organic framework materials as catalysts, Chem. Soc. Rev. 38 (2009) 1450–1459. [9] J.R. Li, R.J. Kuppler, H.C. Zhou, Selective gas adsorption and separation in metal–organic frameworks, Chem. Soc. Rev. 38 (2009) 1477–1504. [10] Y. Bai, G.J. He, Y.G. Zhao, C.Y. Duan, D.B. Dang, Q.J. Meng, Porous material for absorption and luminescent detection of aromatic molecules in water, Chem. Commun. (2006) 1530–1532. [11] C.G. Silva, A. Corma, H. Garcia, Metal–organic frameworks as semiconductors, J. Mater. Chem. 20 (2010) 3141–3156. [12] X.L.X. Francesc, A. Corma, H. Garcia, Applications for metal–organic frameworks (MOFs) as quantum dot semiconductors, J. Phys. Chem. C 111 (2007) 80–85. [13] P. Mahata, G. Madras, S. Natarajan, Novel photocatalysts for the decomposition of organic dyes based on metal–organic framework compounds, J. Phys. Chem. B 110 (2006) 13759–13768. [14] J. Gascon, M.D. Hernández-Alonso, A.R. Almeida, G.P.M. van Klink, F. Kapteijn, G. Mul, Isoreticular MOFs as efficient photocatalysts with tunable band gap: an operando FTIR study of the photoinduced oxidation of propylene, ChemSusChem 1 (2008) 981–983. [15] S. Bordiga, C. Lamberti, G. Ricchiardi, L. Regli, F. Bonino, A. Damin, K.P. Lillerud, M. Bjorgen, A. Zecchina, Electronic and vibrational properties of a MOF-5
J.-J. Du et al. / Journal of Hazardous Materials 190 (2011) 945–951
[16]
[17] [18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
metal–organic framework: ZnO quantum dot behaviour, Chem. Commun. (2004) 2300–2301. T. Tachikawa, J.R. Choi, M. Fujitsuka, T. Majima, Photoinduced chargetransfer process on MOF-5 nanoparticles: elucidating differences between metal–organic frameworks and semiconductor metal oxides, J. Phys. Chem. C 112 (2008) 14090–14101. P. Mahata, G. Madras, S. Natarajan, New photocatalysts based on mixed-mental pyridine dicarboxylates, Catal. Lett. 115 (2007) 27–32. M. Alvaro, E. Carbonell, B. Ferrer, X. Francesc, L. Xamena, H. Garcia, Semiconductor behavior of a metal–organic framework (MOF), Chem. Eur. J. 13 (2007) 5106–5112. T. Loiseau, C. Serre, C. Huguenard, F. Gerhard, F. Taulelle, M. Henry, T. Bataille, G. Férey, A rationale for the large breathing of the porous aluminum terephthalate (MIL-53) upon hydration, Chem. Eur. J. 10 (2004) 1373–1382. C. Serre, F. Millange, C. Thouvenot, M. Nogueès, G. Marsolier, D. Louër, G. Férey, Very large breathing effect in the first nanoporous chromium (III)-based solids: MIL-53 or CrIII (OH){O2 C–C6 H4 –CO2 }{HO2 C–C6 H4 –CO2 H} × H2 Oy , J. Am. Chem. Soc. 124 (2002) 13519–13526. P. Horcajada, C. Serre, G. Maurin, N.A. Ramsahye, F. Balas, M.V. Regí, M. Sebban, F. Taulelle, G. Férey, Flexible porous metal–organic frameworks for a controlled drug delivery, J. Am. Chem. Soc. 130 (2008) 6774–6780. T.K. Trung, P. Trens, N. Tanchoux, S. Bourrelly, P.L. Llewellyn, S.L. Serna, C. Serre, T. Loiseau, F. Fajula, G. Férey, Hydrocarbon adsorption in the flexible metal organic frameworks MIL-53(Al, Cr), J. Am. Chem. Soc. 130 (2008) 16926–16932. G. Férey, M. Latroche, C. Serre, F. Millange, T. Loiseau, A.G. Percheron, Hydrogen adsorption in the nanoporous metal-benzenedicarboxylate M(OH)(O2 C–C6 H4 –CO2 ) (M = Al, Cr), MIL-53, Chem. Commun. (2003) 2976–2977. S. Bourrelly, P.L. Llewellyn, C. Serre, F. Millange, T. Loiseau, G. Férey, Different Adsorption behaviors of methane and carbon dioxide in the isotypic nanoporous metal terephthalates MIL-53 and MIL-47, J. Am. Chem. Soc. 127 (2005) 13519–13521. C. Serre, S. Bourrelly, A. Vimont, N.A. Ramsahye, G. Maurin, P.L. Llewellyn, M. Daturi, Y. Filinchuk, O. Leynaud, P. Barnes, G. Férey, An explanation for the very large breathing effect of a metal–organic framework during CO2 . A desorption, Adv. Mater. 19 (2007) 2246–2251. P.L. Llewellyn, G. Maurin, T. Devic, S. Loera-Sern a, N. Rosenbach, C. Serre, S. Bourrelly, P. Horcajada, Y. Filinchuk, G. Férey, Prediction of the conditions for breathing of metal organic framework materials using a combination of X-ray powder diffraction, microcalorimetry, and molecular simulation, J. Am. Chem. Soc. 130 (2008) 12808–12814.
951
[27] L. Alaerts, M. Maes, L. Giebeler, P.A. Jacobs, J.A. Martens, J.F.M. Denayer, C.E.A. Kirschhock, D.E.D. Vos, Selective adsorption and separation of orthosubstituted alkylaromatics with the microporous aluminum terephthalate MIL-53, J. Am. Chem. Soc. 130 (2008) 14170–14178. [28] F. Millange, C. Serre, N. Guillou, G. Férey, R.I. Walton, Structural effects of solvents on the breathing of metal–organic frameworks: an in situ diffraction study, Angew. Chem. Int. Ed. 47 (2008) 4100–4105. [29] L. Hamon, C. Serre, T. Devic, T. Loiseau, F. Millange, G. Férey, G.D. Weireld, Comparative study of hydrogen sulfide adsorption in the MIL-53(Al, Cr, Fe), MIL-47(V), MIL-100(Cr), and MIL-101(Cr) metal organic frameworks at roomtemperature, J. Am. Chem. Soc. 131 (2009) 8775–8777. [30] V. Finsy, C.E.A. Kirschhock, G. Vedts, M. Maes, L. Alaerts, D.E.D. Vos, G.V. Baron, J.F.M. Denayer, Framework breathing in the vapour-phase adsorption and separation of xylene isomers with the metal–organic framework MIL-53, Chem. Eur. J. 15 (2009) 7724–7731. [31] V. Finsy, L. Ma, L. Alaerts, D.E.D. Vos, G.V. Baron, J.F.M. Denayer, Separation of CO2 /CH4 mixtures with the MIL-53(Al) metal–organic framework, Micropor. Mesopor. Mater. 120 (2009) 221–227. [32] M. Meilikhov, K. Yusenko, R.A. Fischer, The adsorbate structure of ferrocene inside [Al(OH)(bdc)]x (MIL-53): a powder X-ray diffraction study, Dalton Trans. 4 (2009) 600–602. [33] G. Férey, F. Millange, M. Morcrette, C. Serre, M.L. Doublet, J.M. Grenche, J.M. Tarascon, Mixed-valence Li/Fe-based metal–organic frameworks with both reversible redox and sorption properties, Angew. Chem. Int. Ed. 46 (2007) 3259–3263. [34] M.F. Cabrera, D.M. Nicholson, B.G. Sumpter, M. Widom, Electronic structure and properties of isoreticular metal–organic frameworks: the case of M-IRMOF1 (M = Zn, Cd, Be, Mg, and Ca), J. Chem. Phys. 123 (2005) 124713–124715. [35] J. Yin, Z.G. Zou, J.H. Ye, Photophysical and photocatalytic properties of new photocatalysts MCrO4 (M = Sr, Ba), Chem. Phys. Lett. 378 (2003) 24–28. [36] A. Khan, M.M. Haque, N.A. Mir, M. Muneer, C. Boxall, Heterogeneous photocatalysed degradation of an insecticide derivative acetamiprid in aqueous suspensions of semiconductor, Desalination 261 (2010) 169–174. [37] H.K. Singh, M. Saquib, M.M. Haque, M. Muneer, Heterogeneous photocatalysed degradation of 4-chlorophenoxyacetic acid in aqueous suspensions, J. Hazard. Mater. 142 (2007) 374–380. [38] R. Jain, M. Shrivastava, Photocatalytic removal of hazardous dye cyanosine from industrial waste using titanium dioxide, J. Hazard. Mater. 152 (2008) 216– 220.
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
New photocatalysts based on MIL-53 metal–organic frameworks for the decolorization of methylene blue dye Jing-Jing Du a , Yu-Peng Yuan a,∗ , Jia-Xin Sun a , Fu-Min Peng a , Xia Jiang a , Ling-Guang Qiu a,∗ , An-Jian Xie a , Yu-Hua Shen a , Jun-Fa Zhu b a b
Laboratory of Advanced Porous Materials and School of Chemistry and Chemical Engineering, Anhui University, Hefei 230039, China National Synchrocyclotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China
a r t i c l e
i n f o
Article history: Received 8 November 2010 Received in revised form 25 March 2011 Accepted 6 April 2011 Available online 12 April 2011 Keywords: Metal–organic frameworks MIL-53 Photocatalyst Methylene blue
a b s t r a c t The photocatalytic decolorization of methylene blue dye in aqueous solution using a novel photocatalyst MIL-53(Fe) metal–organic frameworks was investigated under UV–vis light and visible light irradiation. The effect of electron acceptor H2 O2 , KBrO3 and (NH4 )2 S2 O8 addition on the photocatalytic performance of MIL-53(Fe) was also evaluated. The results show that MIL-53(Fe) photocatalyst exhibited photocatalytic activity for MB decolorization both under UV–vis light and visible light irradiation, and the MB decolorization over MIL-53(Fe) photocatalyst followed the first-order kinetics. The addition of different electron acceptors all enhances the photocatalytic performance of MIL-53(Fe) photocatalyst, and the enhanced rate follows the order of H2 O2 > (NH4 )2 S2 O8 > KBrO3 under UV–vis light irradiation, while in the order of (NH4 )2 S2 O8 > H2 O2 > KBrO3 under visible light irradiation. Moreover, MIL-53(Fe) did not exhibit any obvious loss of the activity for MB decolorization during five repeated usages. The photocatalytic activities over MIL-53(M) (M = Al, Fe), the isostructure to MIL-53(Fe), indicate that the metal centers show nil effect on the photocatalytic activity of MIL-53(M) photocatalysts. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The ability of using solar energy to eliminate the organic pollutants makes photocatalysis a potential technology for solving environmental issues confronting mankind. Since the discovery of the first artificial photocatalytic system for pollutants degradation over TiO2 , many metal oxides and sulfides including ZnO, WO3 , CdS, ZnS have been identified as active photocatalysts for photodegradation of organic pollutants in gas or aqueous phase [1–6]. The quantum yield and solar energy conversion efficiency of these developed photocatalysts, however, are still low at present, thus limiting their practical applications in environmental purification. Consequently, it is of great interest to search for new photocatalysts with improved activities. Metal–organic frameworks (MOFs), which exhibit high surface area and large pore volume, have attracted considerable attention due to their elegant topology and potential applications in separation, gas storage, molecular sensing, and catalysis [7–10]. In addition, MOFs behave as semiconductors when exposed to light, thus making MOFs potentially be photocatalysts [11]. More
∗ Corresponding authors. Tel.: +86 551 5108212; fax: +86 551 5108212. E-mail addresses: [email protected] (Y.-P. Yuan), [email protected] (L.-G. Qiu). 0304-3894/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2011.04.029
recently, MOFs that can act as photocatalysts have attracted much attention for exploiting new applications of MOFs [12–18]. Garcia et al. firstly proposed MOF-5 to be an active photocatalyst for photodegradation of phenol. In contrast to a conventional photocatalyst of metal oxide or sulfide, MOF-5 displayed the reverse shape-selectivity in which small phenolic molecules that can diffuse freely into the micropores of MOF-5 are degraded more slowly than those that cannot access to the interior of MOF-5 [12]. Natarajan et al. used different MOFs based on Co, Ni, and Zn as photocatalysts to degrade organic dyes. The photocatalytic results show that all three MOFs are active for the photodegradation of four widely used dyes (orange G, rhodamine B, Remazol Brilliant Blue R, and methylene blue) in the textile industry. And the activities of three MOFs photocatalysts follow the reverse order of their band gap [13]. Gascon et al. employed isoreticular MOFs (IRMOF-1, IRMOF-2, IRMOF-7, IRMOF-8, and IRMOF-9) as photocatalysts for gas-phase photooxidation of propene. Particularly, IRMOF-8 displayed a higher activity than ZnO, a common photocatalyst [14]. These emerging researches demonstrate MOFs to be a potential new class of photocatalysts for environmental purification. In contrast to the conventional photocatalysts of metal oxides and sulfides, the photocatalytic properties of MOFs have remained unexplored. Herein, we report the photocatalytic activities of MIL-53(M) (M = Al, Cr, Fe) in photodegradation of methylene blue (MB)
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Scheme 1. The chemical structure of MIL-53(Fe) and the electron transfer processes that occur in MIL-53(Fe) when irradiated by light.
dye. Representatively, MOFs MIL-53(Fe) is three-dimensional porous solids built up by infinite one-dimensional linkage of –Fe–O–O–Fe–O–Fe–, cross-linked by bis-bidentate terephthalate (1,4-benzenedicarboxylate) linkers (see Scheme 1) [19–21]. Like TiO2 semiconductor whose conduction band was constructed by empty Ti 3d orbitals, MOFs MIL-53 containing transition metals as structural nodes are also expected to be semiconductors since the empty d metal orbitals mixed with the LUMOs of the organic linkers would formed the conduction band [14]. Upon light irradiation, electron excitation takes place in MOFs, followed by subsequent electron transfer (see Scheme 1) [18]. Therefore, MOFs are expected to be active photocatalysts, as reviewed above. Presently, MIL53(M) is a value-added material for separating gases, including CO2 , CH4 , H2 S and a variety of organic species [22–32]. To the best of our knowledge, no attention, however, has ever been paid to study the photocatalytic properties of MIL-53(M) to date. Recently, we found that MIL-53(Fe) exhibited photocatalytic activity for MB dye degradation under both UV–vis and visible light irradiation. The introduction of different electron acceptors in the MB aqueous solution greatly promoted the photocatalytic property of MIL-53(Fe). To our knowledge, this synergistic enhancement in the degradation of organic pollutants by the combination of different electron acceptors and MIL-53(Fe) photocatalyst has not yet been reported. Meanwhile, the photocatalytic activities of MIL-53(Al) and MIL53(Cr), the isostructure to MIL-53(Fe), were also investigated to provide an insight on the correlation between metal centers of a MOFs photocatalyst and its photocatalytic activity. 2. Materials and methods 2.1. Reagents and chemicals All reagents were analytical grade and used without further purification. Iron (III) chloride hexahydrate (99%), terephthalic acid (97%), N,N -dimethylformamide (99%) and hydrogen peroxide (30%) were purchased from Sinopharm Chemical Reagent Co. Ltd., China. Aluminum nitrate nonahydrate (99%) and chromium (III) nitrate nonahydrate (99%) were supplied by Tianjin Guangfu Fine Chemical Research Institute, China. De-ionized water obtained from a Milipore Milli-Q system was used to prepare aqueous solutions for MIL-53 synthesis and for the irradiation experiments.
53(Fe) powder was re-heated at 150 ◦ C overnight to remove the DMF inside the pores of MIL-53(Fe). After cooling down to room temperature, the solid was then washed with a large volume of de-ionized water (1 g of MIL-53 in 0.5 l of water) to give the final MIL-53(Fe) photocatalysts. MIL-53(Cr) was hydrothermally synthesized at 220 ◦ C for 3 days from a mixture of chromium (III) nitrate, terephthalic acid, hydrofluoric acid, and H2 O in the molar ratio 1:1:1:280 [20,21]. To remove traces of terephthalic acid, the obtained solid was then washed in 200 ml ethanol at 70 ◦ C four times. MIL-53(Al) was hydrothermally synthesized in a 60 ml Teflonlined stainless steel Parr bomb containing aluminum nitrate, terephthalic acid (BDC), and deionized water under autogenous pressure at 220 ◦ C for 3 days. The molar ratio of Al:BDC:H2 O was fixed at 1:0.5:80 [22]. The obtained product was followed to calcine at 280 ◦ C to remove the terephthalic acid residues and washed with deionized water for four times [23]. 2.3. Photocatalysts characterization X-ray powder diffraction (XRD) data of the obtained MIL-53(M) (M = Fe, Al, Cr) products was collected on a Rigaku Ultima III Xray powder diffractometer with monochromatic Cu K␣1 radiation (40 kV, 40 mA). The surface morphologies and size of MIL-53 products were observed by field emission scanning electron microscopy (SEM: S4800, Hitachi). UV–vis diffuse reflectance data was collected over the spectral range 210–850 nm with a spectrophotometer (UV-2550, Shimadzu) equipped with an integrated sphere. BaSO4 was used as a reference sample. X-ray photoelectron spectra (XPS) of the as-prepared MIL-53 were recorded on an ESCALAB 250 spectrometer (Thermo-VG Scientific) using Mg K␣ radiation (1253.6 eV) and the binding energy values were calibrated with respect to C (1s) peak (284.6 eV). 2.4. Photocatalytic degradation of MB The photocatalytic activities of MIL-53(M) (M = Fe, Al, Cr) photocatalysts were evaluated by the photodegradation of MB dye under a 500 W Xe lamp irradiation in open air and at room temperature. The distance between the light source and the beaker containing reaction mixture was fixed at 15 cm. 1 mg of MIL-53(M) (M = Fe, Al, Cr) photocatalyst was put into 100 ml of MB aqueous solution (4 × 10−4 mol/l) in a 250 ml beaker. The pH value of the suspension was adjusted to neutral (pH = 7.0). Prior to irradiation, the suspension was magnetically stirred in dark for 60 min to ensure the establishment of an adsorption/desorption equilibrium. During the photodegradation reaction, stirring was maintained to keep the mixture in suspension. Samples were withdrawn at regular intervals and immediately centrifuged to separate photocatalysts for analysis. The MB concentration was monitored by measuring the absorption intensity at its maximum absorbance wavelength of = 664 nm using a UV-visible spectrophotometer (UV-1800, Shimadzu) in a 1 cm path length spectrometric quartz cell. To measure the photocatalytic activity under UV–vis irradiation, a 420 nm cutoff filter was used to provide the visible light. 3. Results and discussion
2.2. Synthesis of MIL-53(Fe) photocatalyst
3.1. X-ray diffraction (XRD) analysis and electron microscopy (SEM)
MIL-53(Fe) photocatalyst was prepared according to the previous report by Férey et al. [21]. Typically, a mixture of iron chloride, terephthalic acid and N,N -dimethylformamide (DMF) with a molar ratio of 1:1:280 was transferred into a Telfon-lined stainless steel bomb and heated at 150 ◦ C for 15 h. The as-obtained yellow MIL-
The porous metal terephthalates MIL-53 solids are constructed by terephthalate anions and metal (III) ions, creating a threedimensional framework with a one-dimensional pore channel system [19–21]. Powder X-ray diffraction (PXRD) analysis showed that the as-prepared MIL-53(M) (M = Fe, Al, Cr) photocatalysts
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Fig. 1. Simulated patterns from the crystallographic data of (1) MIL-53(Fe), (3) MIL53(Al), (5) MIL-53(Cr) and powder X-ray diffraction pattern of (2) MIL-53(Fe), (4) MIL-53(Al), (6) MIL-53(Cr) synthesized using hydrothermal synthesis.
are crystallines and correspond to the known bulk phase MIII (OH)·[O2 C–C6 H4 –CO2 ] (M = Fe, Al, Cr), as shown in (Fig. 1), wherein the simulated PXRD patterns of MIL-53(M) (M = Fe, Al, Cr) are also provided for reference [33]. No impurity peaks were detected in the obtained MIL-53(M) (M = Fe, Al, Cr) photocatalysts, indicating that the obtained samples were single phase MIL-53(M) photocatalysts. Fig. 2 shows the SEM images of as-prepared MIL-53(M) (M = Fe, Al, Cr) photocatalysts that were used for XRD analysis. Wellcrystallized sphere-like particles with particle size distribution of 200–600 nm were dominative for MIL-53(Fe). A small fraction of sheet-like particles were also observed, as shown in Fig. 2a. By comparison, the morphology of MIL-53(Al) and MIL-53(Cr) is relatively homogeneous (see Fig. 2b and c). Such phenomenon should be ascribed to the different crystallization process, as described in Section 2. 3.2. Uv–vis diffuse reflectance spectroscopy The room temperature diffuse reflectance absorption spectra of MIL-53(M) (M = Fe, Al, Cr) photocatalysts, converted from reflectance to absorbance by Kubelka-Munk mehod, was illustrated in Fig. 3. The main optical absorption band is around 320, 395, 455 nm for MIL-53(Al), MIL-53(Cr), and MIL-53(Fe), respectively. These optical transitions can be assigned to ligand-to-metal charge transfer (LMCT) [15]. It can be observed that the absorption edge of the as-prepared MIL-53(M) photocatalysts is gradually shifted to longer wavelength by altering metal ions from Al to Cr and to Fe. This shift should be arisen from the variation of the inorganic composition in MIL-53(M). Theoretical calculations by FuentesCabrera et al. show that the band gap of the M-IRMOF-1 series is inert to the variation of metal (M) incorporated in the structure (M = Be, Mg, Ca, Zn, and Cd), where IRMOF-1 stands for isoreticular metal–organic framework MOF-5 [34]. Our result, however, is obviously different from the report by Fuentes-Cabrera et al. Considering that the electronegativity of Al3+ , Cr3+ and Fe3+ ions follows the order of Al3+ < Cr3+ < Fe3+ , the energy required for the LMCT is therefore in the reverse order. In simple terms, the low electronegativity of Al3+ would demand more energy for the transfer of the charge from the ligand to the Al3+ ion, thus resulting in the larger band gap than that of Cr3+ and Fe3+ ions. It is noteworthy to note that, in the case of Cr3+ , two clear additional peaks around 455 and 596 nm were observed. These two peaks at 455 and 596 nm would originate from the d–d transition of the Cr3+ ion [35]. The onset of the main absorption edge of MIL-53(Al), MIL-53(Cr) and MIL-53(Fe) was 320 nm, 395 nm and 455 nm, which corresponds
Fig. 2. SEM images of (a) MIL-53(Fe), (b) MIL-53(Cr) and (c) MIL-53(Al).
to the band gaps (Eg ) of 3.87, 3.20, 2.72 eV (Eg = 1240/wavelength), respectively. 3.3. Photocatalytic activities The photocatalytic degradation of MB was conducted to investigate the efficiency of the MIL-53(M) photocatalysts. In each experiment, the MIL-53(M) concentration was constant (0.01 g/l) because higher catalysts concentration always results in the total adsorption of MB dye on MIL-53(M). The photocatalytic activities of three MIL-53(M) photocatalysts were monitored from the variation of the color in the reaction system by measuring the maximum absorbance intensity of MB chromophoric group at
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Fig. 3. UV–vis absorption spectra of MIL-53(M) (M = Fe, Al, Cr) photocatalysts at ambient temperature. The inset shows the estimated band gap values.
max = 664 nm. For comparison, the photocatalytic performance of commercial TiO2 under UV–vis light irradiation was also assessed under the identical experimental conditions. Control experiments without photocatalysts and light irradiation were also performed. Fig. 4 illustrates the photodegradation profile of MB over MIL53(Fe) photocatalyst under the UV–vis and visible light irradiation, wherein the photocatalytic performance of TiO2 was also presented. Notably, MIL-53(Fe) shows a comparable activity with TiO2 under UV–vis light and visible light irradiation. No MB degradation was observed over MIL-53(Fe) photocatalyst without the light irradiation (see Fig. 4). In contrast, 4% and 1% MB were photodegradated after 40 min UV–vis and visible light irradiation in the absence of MIL-53(Fe) photocatalyst, respectively (see Fig. 4). After 40 min of UV–vis light and visible light irradiation, the MB removal over MIL53(Fe) photocatalyst is 11% and 3%, respectively. Thus, it can be concluded that MIL-53(Fe) shows the photocatalytic activity for MB degradation, although the photodegradation rate is low. The reaction mechanism for MB decoloration could be discussed based on semiconductor theory. Illumination of MIL-53(Fe) photocatalyst by photons with energy equal to or greater than its band gap excites electrons (e− ) from the valence band to the conduction band and
Fig. 4. MB degradation profile under the irradiation of () without light, () visible light, () UV–vis light, () visible light with the presence of MIL-53(Fe) photocatalysts, and () UV–vis light with the presence of MIL-53(Fe) photocatalysts. () UV–vis light with the presence of TiO2 photocatalysts, () visible light with the presence of TiO2 photocatalysts. The inset shows reaction kinetics over MIL-53(Fe) photocatalysts under the irradiation of UV–vis light and visible light, respectively. The plots were drawn based on the average values of three repeated experimental results.
produces holes (h+ ) in the valence band. The photogenerated holes (h+ ) with strong oxidant capacity can directly oxidize adsorbed organic molecules or react with water molecules or hydroxyl ion (OH− ) to generate hydroxyl radical (• OH). The formed hydroxyl radicals (• OH) also possess strong oxidation ability and can react readily with surface adsorbed organic molecules. Meanwhile, photogenerated electrons (e− ) can be trapped by molecular oxygen to form superoxide radical (• O2 − ), which also possesses strong oxidant ability to decolorize the MB molecules. The low efficiency of MB photodegradation over MIL-53(Fe) photocatalyst could be ascribed to the fast electron–hole recombination. The electron transfer process is more efficient if the molecules are preadsorbed on the surface within a reasonable range and with an appropriate orientation. However, in present case, the MB molecules preadsorbed on the surface of MIL-53(Fe) may exist in a manner that is not favor for the transfer of the photoexcited electrons since MOFs always display selective adsorption behavior [8]. Meanwhile, the electron of the excited state decays to its ground state very quickly. These two factors induce the low efficiency of MB photodegradation over MIL-53(Fe). More experimental and computational studies are needed to elucidate the present situation. The photodegradation of MB dye over MIL-53(Fe) photocatalyst follows first-order kinetics model (see the inset of Fig. 4). The first-order kinetics can be written as ln(C0 /C) = Kt according to the Langmuir–Hinshelwood model, wherein, C0 is the initial concentration of the MB, C is the t time’s concentration of the dyes, t is the reaction time, and K is the kinetic rate constant. The values of K were obtained from the slope and the intercept of the he linear plot. The rate constant of MB photodegradation was 0.0036 min−1 under visible light irradiation, which is only one-quarter of that under UV–vis light irradiation (0.0133 min−1 ) (see the inset of Fig. 4). The low rate of MB photodegradation should be reasonably ascribed to the decreased photons absorbed by MIL-53(Fe). The present finding indicates that MIL-53(Fe) can be used as a visible-light-response photocatalyst for removal of dye pollutants. The relatively low efficiency in photodegradation of MB over MIL-53(Fe) promotes us to search an approach that can circumvent this limitation. As stated above, the photocatalytic processes for dye degradation start from an electronic transition from the valence band (VB) to the conduction band (CB) with the generation of electron–hole pairs. The photogenerated holes (h+ ) either directly react with organic molecules or react with water molecules or hydroxyl ion (OH− ) to generate hydroxyl radical (• OH). The formed hydroxyl radicals (• OH) possess strong oxidation ability and can react readily with surface adsorbed organic molecules. The recombination of photogenerated holes and electrons, however, always results in the reduced holes for decolorization of organic dyes. Consequently, the introduction of external electron acceptor is expected to enhance the performance of MIL-53(Fe) photocatalyst. The addition of hydrogen peroxide (H2 O2 ), potassium bromate (KBrO3 ) and ammonium persulfate ((NH4 )2 S2 O8 ) electron acceptor can suppress the electron–hole pair recombination according to Eqs. (1)–(3), thus enhancing the photodegradation efficiency via generating more radical (• OH, BrO2 • and SO4 •− ) [36]. In fact, H2 O2 , KBrO3 and (NH4 )2 S2 O8 have been demonstrated to be the efficient electron acceptor in the reaction of the dye photodegradation [37,38]. In this connection, we have investigated the effect of electron acceptor addition on the photocatalytic performance of MIL-53(Fe) photocatalyst a period. Fig. 5 illustrates the time course of MB photodegradation in the presence of different electron acceptors over MIL-53(Fe) photocatalyst where all the electron acceptor concentration is constant (10−5 mol/l). It was observed that all the electron acceptors had beneficial effect on improving the rate for MB decolorization both under UV–vis light and visible light irradiation. And the enhanced rate follows the order of H2 O2 > (NH4 )2 S2 O8 > KBrO3 under UV–vis light irradiation (see
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Fig. 5. The effect of different electron acceptor additives on the MB photodegradation under the irradiation of (a) UV–vis light and (b) visible light, () with the presence of H2 O2 and MIL-53(Fe), () with the presence of H2 O2 and the absence of MIL-53(Fe), (䊉) with the presence of (NH4 )2 S2 O8 and MIL-53(Fe), () with the presence of (NH4 )2 S2 O8 and the absence of MIL-53(Fe), () with the presence of KBrO3 and MIL-53(Fe), () with the presence of KBrO3 but the absence of MIL-53(Fe). The inset shows the corresponding reaction kinetics over different electron acceptors and MIL-53(Fe) photocatalyst. The plots were drawn based on the average values of three repeated experimental results.
Fig. 6. Changes of the MB concentration during the five repeated processes over MIL-53(Fe) in the presence of H2 O2 (10−5 mol/l).
Fig. 5a), while in the order of (NH4 )2 S2 O8 > H2 O2 > KBrO3 under visible light irradiation (see Fig. 5b). The decreased electron–hole recombination is responsible for the improved rate for MB degradation. Notably, the addition of H2 O2 has shown pronounced effect on MB decolorization. Roughly 20% degradation of MB was observed after 20 min of visible light irradiation. Better result was obtained when irradiated under UV–vis light. In this case, 20 min
of UV–vis light irradiation causes the almost complete degradation of the MB molecules. The rate constant values in present case were 0.1250 min−1 , 0.0177 min−1 for UV–vis light and visible light irradiation, respectively (see the inset of Fig. 5a and b). These values are 9 times and 5 times higher than those in the absence of H2 O2 . The better degradation rate in the presence of H2 O2 than KBrO3 and (NH4 )2 S2 O8 could be attributed to the suitable electron trapping, thereby suppressing the recombination of e− and h− and thus increasing the number of OH• . Meanwhile, the more produced hydroxyl radicals (• OH) via Eq. (1) can also act as a strong oxidant for MB decolorization. These two factors induce the faster degradation rate of H2 O2 addition than that of KBrO3 and (NH4 )2 S2 O8 . Control experiments were conducted by irradiating the MB aqueous solution containing H2 O2 , KBrO3 and (NH4 )2 S2 O8 additives in the absence of MIL-53(Fe) photocatalyst (see Fig. 5). About 55%, 7% and 14% degradation of MB are observed after 20 min of UV–vis light irradiation, while 99%, 24% and 47% MB degradation are achieved at 20 min in the presence of MIL-53(Fe) and H2 O2 , KBrO3 and (NH4 )2 S2 O8 , respectively. Synergy index (SI), defined as SI = K(additive+MIL-53(Fe) /(KMIL-53(Fe) + Kadditive ), of 2.1, 1.2 and 1.1 indicates a synergic effect for the photocatalytic process over MIL-53(Fe) photocatalyst in the presence of electron acceptor additives. H2 O2 + e− → OH• + OH−
(1)
BrO3 − + 2H+ + e− → BrO2 • + H2 O
(2)
Fig. 7. (a) XRD patterns and (b) XPS spectrum of MIL-53(Fe) (2) before and (1) after MB decolorization for 60 min prolonged reaction.
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from Al to Cr and to Fe, the amount of the adsorbed photons should become larger because of the decreased band gap (see Fig. 2). Then, the MIL-53(Fe) with the narrowest band gap among three MIL-53 photocatalysts is expected to exhibit the highest rate for MB degradation. Contrary to this expectation, three MIL-53 photocatalysts with different band gap display the similar rate for MB degradation. More experimental and computational studies are needed to elucidate the present situation. 4. Conclusions
Fig. 8. MB degradation over () MIL-53(Al), (䊉) MIL-53(Fe), and () MIL-53(Cr) photocatalysts under UV–vis light irradiation.
S2 O8 2− + e− → SO4 2− + SO4 •−
(3)
For the purpose of the practical applications, it is essential to evaluate the long-term stability of a photocatalyst. In the present case, after each run of the MB total decolorization, the identical amount of MB was then followed to add into the suspension solution for another run reaction while kept other factors identical. Fig. 6 shows the time course of MB decolorization during five consecutive cycles. The MB decolorization rate constant for the 5 cycles was 0.1250 min−1 , 0.1123 min−1 , 0.1223 min−1 , 0.1323 min−1 and 0.1283 min−1 , respectively. No obvious loss of the activity for MB decolorization was observed over MIL-53(Fe) during five cycles, indicating that the MIL-53(Fe) possesses excellent long-term stability. The crystal structure and chemical states of MIL-53(Fe) before and after MB decolorization reaction were recorded by XRD and XPS analysis. In the present case, the concentration of MIL-53(Fe) was too low to be separated. For XRD and XPS analysis, the amount of MIL-53(Fe) and MB dye was proportionally increased and irradiated under UV–vis light for prolonged time. Fig. 7 shows the XRD and XPS patterns of MIL-53(Fe) before and after 60 min reaction. The almost identical diffraction pattern before and after prolonged usage illustrates that no structural transformation was happened, but with the obvious variations in the diffraction intensities (see Fig. 7a). Fig. 7b shows the Fe 2p binding energy spectra before and after multiple usages. The binding energy peak around 711 eV is ascribed to Fe 2p3/2 and the peak around 725 eV is assigned to Fe 2p1/2 . The peak separation, namely, = 2p1/2 − 2p3/2 = 14 eV, is very similar to those reported for Fe2 O3 [new references]. Therefore these peaks belong to Fe3+ of MIL-53(Fe). The XPS data did not vary after repeated use for 5 cycles, indicating the long-term stability of MIL-53(Fe) as a photocatalyst. Fig. 8 shows the photocatalytic activities of MIL-53(Al) and MIL-53(Cr), wherein, the photocatalytic activity of MIL-53(Fe) is also presented. MIL-53(Al) and MIL-53(Cr) are isostructural to MIL-53(Fe). Therefore, study on this series of isostructural photocatalysts would provide us valuable information of the effect of metal centers of a MOFs photocatalyst on its photocatalytic activity. Both MIL-53(M) (M = Al, Cr) photocatalysts display photocatalytic activities for MB decolorization. After 60 min of UV–vis light irradiation, the MB removals over MIL-53(Al) and MIL-53(Cr) are 30% and 32%, respectively. It can be noted that no obvious difference in photocatalytic activities of MIL-53(Al), MIL-53(Cr), and MIL-53(Fe) is observed. In photocatalysis research, it is generally believed that the larger the number of absorbed photons is, the higher the photocatalytic activity of a photocatalyst is. As altering the metal ion
We have shown a novel series of photocatalysts based on MIL-53(M) (M = Fe, Cr, Al) metal–organic frameworks for MB photodegradation. The photodegradation of MB over MIL-53(Fe) photocatalyst followed first-order kinetics, and the rate constant is 0.0133 min−1 and 0.0036 min−1 for UV–vis light and visible light irradiation, respectively. Furthermore, we present the large promoting effect of electron acceptor H2 O2 , KBrO3 and (NH4 )2 S2 O8 addition on the photocatalytic performance of MIL-53(Fe) photocatalyst. The metal centers of MIL-53 show nil effect on the photocatalytic activity for MB photodegradation. The present contribution clearly reveals the great potentialities of metal–organic frameworks as photocatalyst, a field that has remained unexplored so far. Acknowledgements This work was supported by the National Natural Science Foundation of China (20971001 and 51002001), the NSFC-CAS Joint Fund for Research Based on Large-Scale Scientific Facilities (10979014), the Program for New Century Excellent Talent in University, Ministry of Education, China (NCET-08-0617), the “Hundred Talents Program” of the Chinese Academy of Sciences, the Naturial Science Foundation of Anhui Province, China (Grant 090414164), and the “211 Project” of Anhui University. References [1] N.F. Steven, A.J. Bard, Heterogeneous photocatalytic oxidation of cyanide ion in aqueous solution at TiO2 powder, J. Am. Chem. Soc. 99 (1977) 303–304. [2] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96. [3] T.L. Thompson, J.T. Yates Jr., Surface science studies of the photoactivation of TiO2 – new photochemical processes, Chem. Rev. 106 (2006) 4428–4453. [4] A. Mills, S.L. Hunte, An overview of semiconductor photocatalysis, J. Photochem. Photobiol. A: Chem. 108 (1997) 1–35. [5] K. Ayoub, E.D. Van Hullebusch, M. Cassir, A. Bermond, Application of advanced oxidation processes for TNT removal: a review, J. Hazard. Mater. 178 (2010) 10–28. [6] U.G. Akpan, B.H. Hameed, Parameters affecting the photocatalytic degradation of dyes using TiO2 -based photocatalysts: a review, J. Hazard. Mater. 170 (2009) 520–529. [7] L.J. Murray, M. Dinca, J.R. Long, Hydrogen storage in metal–organic frameworks, Chem. Soc. Rev. 38 (2009) 1294–1314. [8] J.Y. Lee, O.K. Farha, J. Roberts, K.A. Scheidt, S.T. Nguyen, J.T. Hupp, Metal–organic framework materials as catalysts, Chem. Soc. Rev. 38 (2009) 1450–1459. [9] J.R. Li, R.J. Kuppler, H.C. Zhou, Selective gas adsorption and separation in metal–organic frameworks, Chem. Soc. Rev. 38 (2009) 1477–1504. [10] Y. Bai, G.J. He, Y.G. Zhao, C.Y. Duan, D.B. Dang, Q.J. Meng, Porous material for absorption and luminescent detection of aromatic molecules in water, Chem. Commun. (2006) 1530–1532. [11] C.G. Silva, A. Corma, H. Garcia, Metal–organic frameworks as semiconductors, J. Mater. Chem. 20 (2010) 3141–3156. [12] X.L.X. Francesc, A. Corma, H. Garcia, Applications for metal–organic frameworks (MOFs) as quantum dot semiconductors, J. Phys. Chem. C 111 (2007) 80–85. [13] P. Mahata, G. Madras, S. Natarajan, Novel photocatalysts for the decomposition of organic dyes based on metal–organic framework compounds, J. Phys. Chem. B 110 (2006) 13759–13768. [14] J. Gascon, M.D. Hernández-Alonso, A.R. Almeida, G.P.M. van Klink, F. Kapteijn, G. Mul, Isoreticular MOFs as efficient photocatalysts with tunable band gap: an operando FTIR study of the photoinduced oxidation of propylene, ChemSusChem 1 (2008) 981–983. [15] S. Bordiga, C. Lamberti, G. Ricchiardi, L. Regli, F. Bonino, A. Damin, K.P. Lillerud, M. Bjorgen, A. Zecchina, Electronic and vibrational properties of a MOF-5
J.-J. Du et al. / Journal of Hazardous Materials 190 (2011) 945–951
[16]
[17] [18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
metal–organic framework: ZnO quantum dot behaviour, Chem. Commun. (2004) 2300–2301. T. Tachikawa, J.R. Choi, M. Fujitsuka, T. Majima, Photoinduced chargetransfer process on MOF-5 nanoparticles: elucidating differences between metal–organic frameworks and semiconductor metal oxides, J. Phys. Chem. C 112 (2008) 14090–14101. P. Mahata, G. Madras, S. Natarajan, New photocatalysts based on mixed-mental pyridine dicarboxylates, Catal. Lett. 115 (2007) 27–32. M. Alvaro, E. Carbonell, B. Ferrer, X. Francesc, L. Xamena, H. Garcia, Semiconductor behavior of a metal–organic framework (MOF), Chem. Eur. J. 13 (2007) 5106–5112. T. Loiseau, C. Serre, C. Huguenard, F. Gerhard, F. Taulelle, M. Henry, T. Bataille, G. Férey, A rationale for the large breathing of the porous aluminum terephthalate (MIL-53) upon hydration, Chem. Eur. J. 10 (2004) 1373–1382. C. Serre, F. Millange, C. Thouvenot, M. Nogueès, G. Marsolier, D. Louër, G. Férey, Very large breathing effect in the first nanoporous chromium (III)-based solids: MIL-53 or CrIII (OH){O2 C–C6 H4 –CO2 }{HO2 C–C6 H4 –CO2 H} × H2 Oy , J. Am. Chem. Soc. 124 (2002) 13519–13526. P. Horcajada, C. Serre, G. Maurin, N.A. Ramsahye, F. Balas, M.V. Regí, M. Sebban, F. Taulelle, G. Férey, Flexible porous metal–organic frameworks for a controlled drug delivery, J. Am. Chem. Soc. 130 (2008) 6774–6780. T.K. Trung, P. Trens, N. Tanchoux, S. Bourrelly, P.L. Llewellyn, S.L. Serna, C. Serre, T. Loiseau, F. Fajula, G. Férey, Hydrocarbon adsorption in the flexible metal organic frameworks MIL-53(Al, Cr), J. Am. Chem. Soc. 130 (2008) 16926–16932. G. Férey, M. Latroche, C. Serre, F. Millange, T. Loiseau, A.G. Percheron, Hydrogen adsorption in the nanoporous metal-benzenedicarboxylate M(OH)(O2 C–C6 H4 –CO2 ) (M = Al, Cr), MIL-53, Chem. Commun. (2003) 2976–2977. S. Bourrelly, P.L. Llewellyn, C. Serre, F. Millange, T. Loiseau, G. Férey, Different Adsorption behaviors of methane and carbon dioxide in the isotypic nanoporous metal terephthalates MIL-53 and MIL-47, J. Am. Chem. Soc. 127 (2005) 13519–13521. C. Serre, S. Bourrelly, A. Vimont, N.A. Ramsahye, G. Maurin, P.L. Llewellyn, M. Daturi, Y. Filinchuk, O. Leynaud, P. Barnes, G. Férey, An explanation for the very large breathing effect of a metal–organic framework during CO2 . A desorption, Adv. Mater. 19 (2007) 2246–2251. P.L. Llewellyn, G. Maurin, T. Devic, S. Loera-Sern a, N. Rosenbach, C. Serre, S. Bourrelly, P. Horcajada, Y. Filinchuk, G. Férey, Prediction of the conditions for breathing of metal organic framework materials using a combination of X-ray powder diffraction, microcalorimetry, and molecular simulation, J. Am. Chem. Soc. 130 (2008) 12808–12814.
951
[27] L. Alaerts, M. Maes, L. Giebeler, P.A. Jacobs, J.A. Martens, J.F.M. Denayer, C.E.A. Kirschhock, D.E.D. Vos, Selective adsorption and separation of orthosubstituted alkylaromatics with the microporous aluminum terephthalate MIL-53, J. Am. Chem. Soc. 130 (2008) 14170–14178. [28] F. Millange, C. Serre, N. Guillou, G. Férey, R.I. Walton, Structural effects of solvents on the breathing of metal–organic frameworks: an in situ diffraction study, Angew. Chem. Int. Ed. 47 (2008) 4100–4105. [29] L. Hamon, C. Serre, T. Devic, T. Loiseau, F. Millange, G. Férey, G.D. Weireld, Comparative study of hydrogen sulfide adsorption in the MIL-53(Al, Cr, Fe), MIL-47(V), MIL-100(Cr), and MIL-101(Cr) metal organic frameworks at roomtemperature, J. Am. Chem. Soc. 131 (2009) 8775–8777. [30] V. Finsy, C.E.A. Kirschhock, G. Vedts, M. Maes, L. Alaerts, D.E.D. Vos, G.V. Baron, J.F.M. Denayer, Framework breathing in the vapour-phase adsorption and separation of xylene isomers with the metal–organic framework MIL-53, Chem. Eur. J. 15 (2009) 7724–7731. [31] V. Finsy, L. Ma, L. Alaerts, D.E.D. Vos, G.V. Baron, J.F.M. Denayer, Separation of CO2 /CH4 mixtures with the MIL-53(Al) metal–organic framework, Micropor. Mesopor. Mater. 120 (2009) 221–227. [32] M. Meilikhov, K. Yusenko, R.A. Fischer, The adsorbate structure of ferrocene inside [Al(OH)(bdc)]x (MIL-53): a powder X-ray diffraction study, Dalton Trans. 4 (2009) 600–602. [33] G. Férey, F. Millange, M. Morcrette, C. Serre, M.L. Doublet, J.M. Grenche, J.M. Tarascon, Mixed-valence Li/Fe-based metal–organic frameworks with both reversible redox and sorption properties, Angew. Chem. Int. Ed. 46 (2007) 3259–3263. [34] M.F. Cabrera, D.M. Nicholson, B.G. Sumpter, M. Widom, Electronic structure and properties of isoreticular metal–organic frameworks: the case of M-IRMOF1 (M = Zn, Cd, Be, Mg, and Ca), J. Chem. Phys. 123 (2005) 124713–124715. [35] J. Yin, Z.G. Zou, J.H. Ye, Photophysical and photocatalytic properties of new photocatalysts MCrO4 (M = Sr, Ba), Chem. Phys. Lett. 378 (2003) 24–28. [36] A. Khan, M.M. Haque, N.A. Mir, M. Muneer, C. Boxall, Heterogeneous photocatalysed degradation of an insecticide derivative acetamiprid in aqueous suspensions of semiconductor, Desalination 261 (2010) 169–174. [37] H.K. Singh, M. Saquib, M.M. Haque, M. Muneer, Heterogeneous photocatalysed degradation of 4-chlorophenoxyacetic acid in aqueous suspensions, J. Hazard. Mater. 142 (2007) 374–380. [38] R. Jain, M. Shrivastava, Photocatalytic removal of hazardous dye cyanosine from industrial waste using titanium dioxide, J. Hazard. Mater. 152 (2008) 216– 220.