Chinese Chemical Letters  2015, Vol.26 Issue (02):187-192   PDF    
Synthesis and characterization of a new catalyst for RhB degradation constructed by [SiMo12O40]4- anionic cluster
Hao Wanga, Yi-Ping Chena,b , Zhu-Chai Youa, Meng-Xi Zhoua, Ning Zhanga, Yan-Qiong Suna    
a College of Chemistry, Fuzhou University, Fuzhou 350108, China;
b State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China
Abstract: A Keggin-type polyoxomolybdate [H2biim]{Ni(biim)3(SiMo12O40)} [biim=1H,1'H-[2,2']biimidazolyl] has been synthesized under hydrothermal condition and characterized by XRD, temperature-dependent IR, TG, temperature-induced and magnetism-induced 2D infrared correlation spectroscopy (2D-IR COS) and UV-vis DRS in order to explore the relationship between structure and properties. Temperatureinduced 2D-IR COS spectroscopy indicates that the terminal Mo-Ou/v bonds are more sensitive than the bridging Mo=Ot bands to temperature variation, which is in agreement with the conclusion of temperature-dependent IR. Magnetism-dependent 2D-IR COS spectroscopy reveals the stretching vibration of the Mo=Ot occurs prior to the stretching vibration of the Mo-Ou/v, which is due to the coordination environment and the valence of the Siatom. The stability of compound 1 is investigated via TG and temperature-dependent IR. In RhB degradation, compound 1 shows good photocatalytic abilities.
Key words: 2D infrared correlation spectroscopy     Photocatalytic degradation of RhB     Silicon molybdenum polyoxometalate    
1. Introduction

Polyoxomolybdates are discrete anionic metal-oxygen clusters with a variety of possible MoOx units. Polyoxomolybdates consist of MoO6 octahedra which are linked through edge-sharing [1] and corner-sharing junctions [2] resulting in diverse possible geometric topologies. Six basic polyoxomolybdate anionic clusters have been identified so far: Keggin [3],Dawson [4],Anderson [5],Waugh [6],Silverton [7],and Lindqvist [8]. Anionic clusters are linked to each other by cations,including protonated organic ions [9], metallic ions [10],and metal ion complexes [11]. Anionic clusters especially in Keggin-type polyoxomolybdate can be coordinated with cation easily bracing crystal frame. Diverse structures in Keggin-type polyoxomolybdates lead to special properties useful in some fields,like electrochemistry [12, 13] and spectroscopy [14]. Keggin-type polyoxomolybdates have recently found many applications in the field of catalysis [15, 16, 17],which has become one of the interest areas of contemporary research for potential applications.

2D-IR COS,which was proposed by Noda [18],can not only enhance spectral resolution and identify overlapped peaks but also be used as a powerful tool to investigate intramolecular and intermolecular interactions. 2D-IR COS has been applied in many fields,like chemometrics [19],global phase angle [20],and kernel analysis [20]. This technique has been applied to the study of spectral phenomena in many fields except for molecule structure. Our team has introduced 2D-IR COS to molecule structure analysis in order to summarize spectra rules since 2005 [21]. Now,we report a new example to complement the spectra guidelines. In this paper,a new Keggin-type polyoxomolybdate was synthesized under hydrothermal condition and studied by various techniques,including 2D-IR COS. Compound 1 shows the photocatalytic property to RhB degradation.

2. Experimental 2.1. General procedures

All chemicals of reagent grade were purchased commercially and used without further purification. Powder X-ray diffractions of samples were collected on a Rigaku MiniFlex II (Kα (l = 1.54178Å ), 40 kV,40 mA,2θ = 5-55°; data recorded at 0.028/s step size, 40 s/step). 2D-IR COS spectra were obtained (as KBr pressed pellets) by using a Perkin-Elmer Spectrum 2000 FT-IR Fourier transforms spectrometer in the range of 4000-400 cm-1 at a total of 20 scans. Temperature variation was controlled by a portable programmable temperature controller (Love Control Corporation), when increasing the temperature from 50 ℃ to 120 ℃ at an interval of 10 ℃. The magnetism variation was corrected by CT50 Gaussmeter in the 5-50 mT range at an interval of 5 mT. 2D-IR COS spectra were obtained by treatment with the Hibert transform and MATLAB software which was provided by Tsinghua University. UV-vis DRS was performed on a Perkin-Elmer Lambda 900 in the range of 200-800 nm. TG analysis was measured on a Perkin-Elmer TGA 7 in nitrogen atmosphere from 60 ℃ to 750 ℃ at a heating rate of 10 ℃/min. The temperature-dependent IR spectra were recorded from KBr pellets in the range of 4000-400 cm-1 on a BRUKER TENSOR 27 IR spectrum 2000 as the samples were heated from 50 ℃ to 580 ℃ under nitrogen. Photocatalytic degradation tests were carried out with an UV high pressure [137TD$DIF]xenon lamp (PLSSXE300W). The UV-vis DRS of RhB were recorded on a Perkin- Elmer Lambda 900 in the range of 450-650 nm. Photocatalytic degradation tests were recorded on the maximum absorption wavelength of RhB (552 nm).

2.2. Synthesis of compound 1

A mixture of Na2MoO4·2H2O (0.0718 g,0.296 mmol),2,20- biimidazole (biim,0.0430 g,0.257 mmol),NiF2·4H2O (0.0095 g, 0.056 mmol),and Na2SiO3·9H2O (0.0708 g,0.249 mmol) in H2O (10 mL) was adjusted to pH 2.5 with HCl (1.0 mol/L solution in water) and heated at 150 ℃ for 48 h in a sealed 25 mL teflon-lined stainless vessels under autogenous pressure. After cooling to ambient temperature with 800 min,black block crystals (yield 50%,based on Na2MoO4·2H2O) were obtained.

2.3. X-ray crystallography

Crystals with dimensions 0.28 mm × 0.20 mm × 0.18 mmwere mounted on glass fibers for indexing and intensity data collected on a Rigaku Weissenberg IP single-crystal diffractometer with Mo/Ka radiation (λ = 0.71073Å ). The structure was solved by direct methods and refined by full-matrix least-squares on F2 with SHELXS-97 program [23]. For compound 1,anisotropic thermal parameters were used to refine all nickel,carbon,molybdenum, nitrogen,oxygen and silicon atoms. Hydrogen atoms were generated theoretically onto the specific atoms and refined isotropically with fixed thermal factors. A summary of crystal data and structure refinement for compound 1 is listed in Supporting information.

3. Results and discussion 3.1. Crystal structures of compound 1

As typical Keggin-type anions,the [SiMo12O40]4- anionic cluster has been reported before [22, 23, 24]. The [SiMo12O40]4- anionic cluster includes a central tetrahedron SiO4 surrounded by twelve triads {MoO6} arranged in four groups of three edge-sharing octahedral subunits {Mo3O3}. The groups of Mo3O3 are linked by sharing corners with each other to the central SiO4 tetrahedron (Fig. 1).

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Fig. 1. Polyhedral representation of [SiMo12O40]4- anionic cluster (a) and ORTEP drawing of the basic crystallographic unit and hydrogen atoms are omitted for clarity (b).

The Ni atom is surrounded by five nitrogen atoms and one oxygen atom,forming {MN5O1} octahedron units. The Ni atom is coordinated by one nitrogen atom of biim,four nitrogen atoms of two different biim,and one Ot oxygen atom pertaining to [SiMo12O40]4- anionic cluster. Only one nitrogen atom of biim is protonated and linked to [SiMo12O40]4- anionic cluster through hydrogen bond to form N-H···O (Table 1),which forms the 3D supramolecular architecture (Fig. 2) as well as the existence of π-π stacking between biim (Table 1 and Fig. 3). Owing to the existence of [Ni(biim)3]2+ and [H2biim]2+,compound 1 balances the charge of the [SiMo12O40]4- anionic cluster resulting in the stabilization of compound 1.

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Fig. 2. Packing diagram of compound 1 formed by H-bonding interaction (dashed lines) along c-axis.

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Fig. 3. The π-π stacking and H-bond interaction between the biim with irrelevant atoms omitted for clarity.

Table 1
Comparison of the experimental and simulated power XRD patterns for compound 1.
3.2. Characterization of the compound 1 3.2.1. XRD

The XRD (Fig. 4) patterns show that the synthesized compound 1 matches with the simulated one in the key positions,which indicates the phase purities of compound 1.

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Fig. 4. Comparison of the experimental and simulated power XRD patterns for compound 1.
3.2.2. IR spectrum

In the IR spectra of compound 1 (Fig. 5 and Table 2),the shark peaks at the band 1100-700 cm-1 are the characteristic peaks of [SiMo12O40]4- anionic cluster and the characteristic peaks of compound 1 are located at 1091 cm-1,961 cm-1,904 cm-1, 791 cm-1,and 754 cm-1 corresponding to ν(Si-O),νas(Mo=Ot), νs(Mo=Ot),νas(Mo-Oμ/ν),and νs(Mo-Oμ/ν) separately. Compared to biim,the intensity of the characteristic peaks of biim of compound 1 weakens. In addition,the peak at 3140 cm-1 (attributed to the vibration of C-H of biim rings) exhibit obvious blue shift phenomenon due to charge redistribution between Ni and biim. While the peak at 3315 cm-1 (assigned to the vibration of N-H of biim rings) exhibits drastic red shift because of the reduction of N-H polarity thorough N-H···O hydrogen bond.

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Fig. 5. The IR spectra of compound 1 and biim.

Table 2
The assignments of IR vibration frequency (cm-1) of compound 1 and biim.
3.2.3. 2D infrared correlation spectroscopy

Figs. 6 and 7 depict the synchronous and asynchronous 2D-IR COS spectra of compound 1 over the temperature range from 50 ℃ to 120 ℃ for different wavenumbers,which is calculated from the dynamic spectra. In the synchronous spectrum,two auto-peaks along the diagonal line can be detected at 793 cm-1 and 905 cm-1 unambiguously,which are assigned to the stretching vibration of Mo-Oμ/ν and the symmetry stretching vibration of the Mo=Ot separately. The positive cross-peaks (793 cm-1,905cm-1) reveal that the intensity change of the direction of νs(Mo=Ot) and ν(Mo-Oμ/ν) is consistent. Furthermore, it can be concluded that the stretching vibration of the Mo-Oμ/ν is as sensitive as the symmetry stretching vibration of the Mo=Ot to the temperature variation. However,the stretching vibration of the Mo-Oμ/ν responds to the temperature variation more weakly compared to the symmetry stretching vibration of the Mo=Ot. The stretching vibration of ν(Si-O) (1091 cm-1) is not observed,indicating that Si atoms are surrounded by four oxygen atoms forming {SiO4} tetrahedron unit. The stretching vibration of ν(Si-O) is not sensitive to temperature variation owing to the structural symmetry of the {SiO4} tetrahedron unit. In the asynchronous spectrum,the negative sign of the cross-peak (793 cm-1,905cm-1) reveals that the symmetry stretching vibration of the Mo=Ot occurs prior to the stretching vibration of the Mo-Oμ/ν in the dipole moment changes. In the synchronous spectrum (Fig. 7),autopeaks at 1425 cm-1,1525 cm-1,and 1635 cm-1 indicate that the vibration of biim aromatic ring suffer sensitivity to temperature. Meanwhile,strong auto-peaks at 3318 cm-1 reveals that biim linked to each other by hydrogen bonds are sensitive to temperature.

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Fig. 6. The synchronous and asynchronous 2D-IR COS spectra of compound 1 in the range of 700-1120 cm-1.

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Fig. 7. The synchronous 2D-IR COS spectra of compound 1 in the range of 1400- 1650 cm-1 and 3200-3450 cm-1.

Fig. 8 shows the magnetism-dependent 2D-IR COS spectra of compound 1 in the magnetism range from 5 to 50 mT. In the synchronous spectrum,there are lots of auto-peaks in the range of 1150-700 cm-1 including auto-peaks at 773 cm-1 (ascribed to the stretching vibration of the Mo-Oμ/ν),938 cm-1 (corresponding to the symmetry stretching vibration of the Mo=Ot),958 cm-1 (contributed to the asymmetry stretching vibration of the Mo=Ot), and 1090 cm-1 (attributed to the stretching vibration of the Si-O). Furthermore,it can be concluded that the stretching vibration of the Mo=Ot is more sensitive than the stretching vibration of the Mo-Oμ/ν to the magnetism variation. The intensity change of the stretching vibration of the Si-O indicates some sensitivity to magnetism,possibly because of the coordination environment and the valence of Si atom. In the synchronous spectrum,biim has two strong auto-peaks at 3050 cm-1 (assigned to the stretching vibration of C-H) and 3145 cm-1 (contributed to the stretching vibration of N-H),which results from the change of the flow of aromatic ring electrons in the magnetic field.

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Fig. 8. Synchronous 2D-IR COS spectra of compound 1 over a magnetic intensity from 5 mT to 50 mT.
3.2.4. TG and temperature-dependent IR

In the TG curve of compound 1 (Fig. 9),there is a nearly horizontal straight line between 60 and 230 ℃ indicating thermal stability of compound 1. There are two steps of weight loss in the range of 60-750 ℃. The first weight loss is 5.9% (calculated value 5.6%) in the temperature range of 240-430 ℃,which corresponds to the decomposition of biim between the planes. The second weight loss 32.3% (calculated value 31.7%) was attributed to the decomposition of coordinated biim and the [SiMo12O40]4- anionic cluster.

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Fig. 9. The TGA curve of compound 1.

In order to prove the thermal stability of compound 1,which conclude from TG and 2D-IR COS under thermal perturbation, temperature-dependent IR in the wavelength range from 4000 to 500 cm-1 (Fig. 10) were conducted. There are slight changes before 250 ℃at the peakofboththe stretching vibrationof skeletonandthe stretching vibration of the N-H of biim. When the temperature climbs to 430 ℃,the peak around 1450 cm-1 becomesweak. During the heating process of compound 1 from 410 ℃ to 430℃,the stretching vibration of Mo-Oμ/ν begins to disappear gradually and the characteristic peaks of Mo-Ot change significantly. It indicates that [SiMo12O40]4- cluster anions and biim begin to break down at the same time,which is in agreement with the conclusion of TG. When the temperature is up to 460 ℃,the stretching vibration of Si- O (about 1090 cm-1) changes significantly,which completely vanishes when the temperature rises up to 490 ℃. However,the asymmetry stretching vibration of Mo-Ot disappears at the temperature of 530 ℃,and the characteristic peaks of biim completely die away at the temperature of 550 ℃.

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Fig. 10. The thermal IR spectra of compound 1 with temperature range from 50 ℃ to 660 ℃.
3.2.5. UV-vis DRS

In the UV-vis spectrum of compound 1 and biim,there are two similar strong absorptions in the range of 200-800 nm (Fig. 11). These peaks at 269 nm and 279 nm are attributed to the transition of π→π* of biim. For compound 1,the broad absorption band at range of 320-420 nm and the absorption band about 500 nm are assigned to the LMCT bands of O→Mo and the d-d transfer absorption bands of Ni metallic ion.

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Fig. 11. The UV-DRS spectra of compound 1 and biim.
3.2.6. Photocatalytic degradation of rhodamine B dye

Degrading RhB through Keggin-type polyoxomolybdate catalysis has been studied before [25, 26]. In photocatalytic experiments,we take RhB solutions (0.02 mmol/L,30 mL) adjusted to pH 3.0 with HCl (1.0 mol/L solution in water), which have the maximum absorption wavelength at =6nm (Fig. 12),and the catalyst (0.01 mmol,24.2 mg) in a 100.0 mL beaker magnetically stirred at 850 r/min speed,then exposed it to the UV irradiation at a distance of 12 cm between the liquid surface and the lamp. Every 30 min,dye samples of about 3 mL were taken out from the test solution,and their absorbance (abs) were recorded at =6 nm using a spectrophotometer (Fig. 13). In addition,RhB solutions in the absence of the catalyst were also conducted in the same way (Fig. 14). As shown in Figs. 13 and 14,monitoring UV-vis absorbance (556 nm) of RhB (using compound 1 as the photocatalyst) indicates that UV-vis intensities of RhB decrease (Fig. 13). It shows that compound 1 has the ability to degrade RhB.

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Fig. 12. The UV-vis of 10 mg/L RhB solution.

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Fig. 13. The UV-vis absorption spectra changes of compound 1.

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Fig. 14. The UV-vis absorption spectra changes of RhB.
4. Conclusion

We have synthesized a new Keggin-type polyoxomolybdate through hydrothermal technique and structurally characterized it further. Compound 1 exhibits good thermal stability proved by temperature-dependent IR,TG,and 2D-IR COS under thermal perturbation. It indicates that π-π stacking and hydrogen bonds play an important role in the frameworks of compound 1. The sensitivity of compound 1 under magnetism perturbation may be due to the magnetism of compound 1. UV-vis DRS clarify the complex coordination situation of compound 1,revealing charge transfer in the molecule. In RhB degradation,compound 1 shows good photocatalytic ability. Compound 1 exhibits potential industrial application in RhB degradation.

Acknowledgments

This research was financially supported by the National Science Foundation of China (]FDI$DT341[]FDI$DT381[Nos. 21473030,J1103303),Fujian Provincial Natural Science Foundation (No. 2013J01042),Open Fund of State Key Laboratory of Structural Chemistry (No. 20130015) and Science. Appendix A. Supplementary data Supplementary data associated with this article can be found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2014.11.023.

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