Through a natural tree grain template and sol-gel technology, the heterogeneous catalytic materials based on polyoxometalate compounds H3[PM12O40] encapsulating SiO2: SiO2@H3[PM12O40] (SiO2@PM12, M = W, Mo) with core-shell structure had been prepared. The structure and morphology of the core-shell microspheres were characterized by the XRD, IR spectroscopy, UV-Vis absorbance, and SEM. These microsphere materials can be used as heterogeneous catalysts with high activity and stability for catalytic wet air oxidation of pollutant dyes safranine T (ST) at room condition. The results show that the catalysts have excellent catalytic activity in treatment of wastewater containing 10 mg/L ST, and 94% of color can be removed within 60 min. Under different cycling runs, it is shown that the catalysts are stable under such operating conditions and the leaching tests show negligible leaching effect owing to the lesser dissolution.

1. Introduction

Polyoxometalates (POMs) constitute a large class of inorganic compounds with considerably potential applications in catalysis, medicine, and material sciences [13]. Owing to their unique structural versatility and electronic properties, the self-assembly of such POMs building blocks to produce multifunctional materials is becoming a rapidly expanding area of research. Though POMs have high catalytic activity, due to their low-surface area and high solubility in polar solvent, the application of POMs as catalyst is restricted in many conditions. So, there has always been a demand for making high-surface area and insoluble POMs by means of incorporation into pillared clays of layered double hydroxides and on molecular sieves MCM-41, silica or silica nanoparticles, and polyaniline and polypyrrole films [4]. Gao et al. and coworkers have reported layer-by-layer assembly of polyoxometalates into the shell of the microcapsules, with POMs maintaining individual unique properties which can be used in medicine and inorganic chemistry [5]. Many efforts have been made to solve the leaching and solubility problem by modifying, doping, or exchanging with different metal ions [6]. Pollen is a ubiquitous and inexpensive material with a high degree of species-specific morphological complexity, of which the tough outer shell is amenable to inorganic mineralization without consequent loss of its fine structure [7]. Hall et al. described fabricating porous micron-sized particles of silica using pollen grain templates [8], which is a facile method for replicating the complex surface morphology of tree pollen grains, in the case of silica productions complex colloidal materials with surface areas higher than 800 m2 g−1. This material has a potential application as a drug or catalyst carrier. The synthesis of heterogeneous catalyst based on polyoxometalates impetus us to use this porous particle of silica to fabricate POMs micron-sized core-shell porous materials, which is suitable for catalysis design and industrial preparation. This way is suitable for large scale and industrial preparation.

Catalysis is one of the most important applications of POMs [2, 3, 9, 10]. As green photocatalysts, polyoxometalates (POTs) have been extensively studied for the degradation of organic pollutants in water [11, 12]. Recently, the H2 evolution activity of a few POTs has also been investigated [13]. However, few studies have been made to develop polyoxometalates catalyst for direct activation of molecular oxygen in degradation of organic pollutants. H2O2, O3, and air have been widely used as green oxidants in organic synthesis and environmental remediation because of their green byproduct, high content of active oxygen species [14]. Many POMs have been confirmed to be effective catalysts for activating H2O2, O3, and molecular oxygen in selected oxidation of various organic substrates in dark thermal reactions [1517]. Compared with H2O2, air is cheaper agent in industrial use which exhibits academic value as well as commercial value.

ST is a well-known textile colorant, which is harmful to human beings. A favored, promising, cleaner, and greener technology for the removal of this pollutant from water and wastewater has attracted considerable attention. By now, photochemical degradation of the hazardous dye ST using TiO2 catalyst has been carried out by Gupta group [18]. We synthesized the core-shell structural SiO2@PM12 heterogeneous oxidative catalyst in order to develop a new technique to completely degrade organics into water and CO2, without generating any harmful byproducts, which has popularized its role as a wastewater purifier.

In this paper, we described a facile method for assembly hollow inorganic porous polyoxometalates hybrid catalysts SiO2@PM12 (M = W and Mo) using pollen grains as template and demonstrated their potential applicability in degradation of dye ST catalyzed by these hollow core-shell SiO2@PM12 materials using molecular oxygen as oxidation agent. The catalytic activity of POMs in the hybrid catalysts was improved by the physical and chemical properties and the unusual core shell with higher surface areas. Moreover, the separation and recovery of the POMs from the reaction became easy. These catalysts are proved to be available heterogeneous catalysts to degrade the dye when using molecular oxygen as oxidant.

2. Experiment

2.1. Materials

All chemicals (Sinopharm Chemical Reagent Beijing Co., Ltd.) were of analytical reagent grade. All the aqueous solutions were prepared using ultrapure water (Milli-Q 18.2 MX cm, Millipore System). Pollen grain was bought from company Yi Yuan in Beijing. Other reagents were of AR grade and used without any treatment. H3PW12O4023H2O (PW12) and H3PMo12O4023H2O (PMo12) were prepared following a typical method [19], respectively, and identified by IR spectra.

2.2. Physical Measurements

IR spectra (2000–400 cm−1) of the microspheres were recorded in KBr discs on a Nicolet Magna 560 IR spectrometer. Absorption spectra were recorded on a UV-Vis spectroscopy was performed with a UV-2550 spectrophotometer (Shimadzu, Japan) at room temperature. Electron micrographs were recorded on a Hitachi H-800 scanning electron microscope (SEM) at 200 kV. The X-ray powder diffraction (XRD) patterns of the samples were collected on a Japan Rigaku D/max 2500 PC X-ray diffractometer equipped with graphite monochromatized Cu-Kα radiation 40 kV/40 mA (  =  0.15406 nm). Elemental analyses of materials were carried out using a Leeman Plasma Spec (I) ICP-AES.

2.3. Preparation of Silica Covering Pollens

The preparation of silica covering pollens was followed by a previously reported method [8]. The typical process is as follows: 0.5 g powder of pollen was soaked in a solution of silicic acid sol (Si(OH)4, 5 mL, pH = 7) for about 24 h. The silicic acid sol was prepared by passing a sodium silicate solution through an acidified cation-exchange column that was charged by flushing with hot distilled water, 2 M HCl aqueous solution, and then cold distilled water. And the final pH is 7. The soaked powder of pollen was centrifuged to remove the excess silicate. The resulting silica-coated pollen grains were decanted, washed with distilled deionized water, and dried in baking oven.

2.4. Preparation of H3PW12O40 Core-Shell Hollow Spheres

4.85 mmol H3PW12O4023H2O (PW12) was dissolved in 20 mL distilled water to form a clear solution. The upper prepared silica-coated pollen grains were soaking into the PW12 solution for about 24 h to induce polyoxometalate PW12 reacting with Si(OH)4 forming PW12 coated pollen grains. The powder was throughout washed with water and ethanol in order to remove the PW12 compound, which did not attach to the silicate pollen. Then the PW12 coated pollen grains were dried by airflow and then were calcined from ambient temperature to 350°C at rate of 1°C min−1 overnight to remove the original pollen grains. The greenish-yellow hollow PW12/SiO2 replicas of individual pollen grains were obtained with yield of 3 g. The preparation of H3PMo12O40 core-shell hollow spheres (PMo12/SiO2) is the same as that of H3PW12O40 core-shell spheres except H3PMo12O40 that was used instead of H3PW12O40.

2.5. Catalytic Procedure

The stoichiometry of reaction was determined by allowing 10 mg/L of ST to react with air at room temperature in the presence of the catalyst. A general procedure was carried out as follows: 0.2 g of catalyst SiO2@PM12 was suspended in a fresh aqueous dye solution (C0 = 10 mg/L). The air was inputted into the bottom of the suspension with the flow 0.08 m3/h. At given intervals of illumination, a sample of suspension was taken out by filter. The clear filtrate solution was tracked by UV-Vis spectroscopy using a 756 CRT UV-Vis spectrophotometer at 532 nm.

3. Results and Discussion

3.1. Preparation

As shown in Scheme 1, the assembly of core-shell POMs hybrids contains two main processes. Firstly, porous silica replicas of pollen grains were achieved. Secondly, using these silica replicas of pollen grains as template fabricated POMs coating materials. Then the obtained materials were calcined to remove pollen grain formed hollow core-shell SiO2@PM12 hybrid catalysts.

3.2. Spectra

The IR spectrum of the SiO2@PW12 was given in Figure 1 which showed several strong bands at 1080, 986, 889, and 800 cm−1. It has been widely reported that H3PW12 with Keggin structure gives four characteristic peaks which reflected the different vibrations of oxygen atoms at 1080, 960, 869, and 780 cm−1, being attributed to the asymmetry vibrations P-Oa, W-Od, W-Ob-W, and W-Oc-W. The bands of the hybrid SiO2@PW12 at 986, 886, and 799 cm−1 were attributed to the asymmetry vibrations W-Od, W-Ob, and W-Oc, confirming the existence of PW12 in SiO2@PW12 core-shell hybrid. It should be noted that the P-Oa band of PW12 fully overlapped with the band of Si-O-Si at 1030–1250 cm−1 [20]. The high-energy shift of the W-Oc vibration peaks of the Keggin anion before (780 cm−1) and after (800 cm−1) the adsorption to the silica supports the strong chemical interaction between silicate and heteropolyanion [21].

The IR spectrum of the SiO2@PMo12 was given in Figure 2. It is well known that the IR spectrum of PMo12 gives four strong typical IR bands at 1079 cm−1 (P-Oa), 988 cm−1 (Mo-Od), 889 cm−1, and 973 cm−1 (Mo-Ob-Mo and Mo-Oc-Mo), respectively. The IR spectrum of the SiO2@PMo12 gave three bands at 1080, 964, and 806 cm−1. The bands at 959 and 797 cm−1 were attributed to the asymmetry vibrations W-Od and W-Oc of Keggin structure polyoxometalate, indicating the presence of PMo12 in SiO2@PMo12 core-shell hybrid. It is also found that the P-Oa band of PMo12 fully overlapped with the band of Si-O-Si at 1030–1250 cm−1. The high-energy shift of the Mo-Oc vibration peaks of the Keggin anion before (793 cm−1) and after (800 cm−1) supports the strong chemical interaction between silicate and PMo12 anion [20].

In the UV-Vis spectrum (Figure 3(a)) of SiO2@PW12 core-shell hybrid, two characteristic absorbance bands at 203 nm and 260 nm, corresponding to oxygen-to-tungsten charge transfer Od→W and Ob/Oc→W, respectively, verify the presence of PW12 in the shell. The same phenomenon was also observed in the UV-Vis spectra of SiO2@PMo12 (Figure 3(b)) with two characteristic absorbance bands at 205 nm and 260 nm.

The XRD pattern of SiO2@PW12 and SiO2@PMo12 was shown in Figures 4(a) and 4(b), respectively. From Figure 4(a), the reflections are known to represent PW12O40 compared with the standard PDF card (JCPDS number 41-0369). And all the diffraction peaks in Figure 4(b) could be readily indexed to tetragonal PMo12O40 phase (JCPDS number 75-1588). The XRD results corresponding to Keggin unit in the SiO2@PW12 and SiO2@PMo12 particles implied their homogeneous dispersion in the silicate.

3.3. Morphology of the Material

The morphology and microstructure of the native pollen grains and prepared hollow core-shell SiO2@PM12 particles (Figure 5) were investigated with SEM. The native pollen grain (Figure 5(a)) was ca. 21 μm in length and exhibited a characteristic ellipsoidal morphology consisting of four longitudinal segments with foam-like surface structure. The SEM image of pollen grain covered by SiO2 and POMs (Figure 5(b)) showed that SiO2 and POMs form a denser shell covering the pollen grain. After the above materials were calcined, the obtained SiO2@PM12 particles (Figures 5(c) and 5(d)) with ellipsoidal morphology were replicated from native pollen grain structure. The SiO2@PW12 particles and SiO2@PMo12 particles are ca. 18 μm in length and ca. 17.2 μm in width, respectively.

3.4. The Catalytic Results

In order to test the potential application of these hollow core-shell SiO2@PM12 particles as heterogeneous catalysts for the degradation of dyes, the oxygenation degradation reaction of ST with molecular oxygen (air) catalyzed by SiO2@PM12 particles was examined. As shown in Scheme 2, the structure of ST was provided, which is shown as a mixture by these two compounds.

3.5. Degradation of ST

On flowing air into the aqueous solutions of ST without any catalyst, ST did not degrade. Figure 6 showed the results of UV-Vis spectra of degradation of ST with different time by SiO2@PM12 hybrid materials. Despite the absorbance of POMs at UV area, the peaks of ST were covered, and the absorbance of ST at visible area can reflect the degradation clearly. From Figure 6, the intensity of absorbance of ST decreased accompanied with the addition of reaction time. The ST can be degraded by SiO2@PW12 and SiO2@PMo12, respectively. In comparison, when the pure PW12 and PMo12 were added as catalyst, after 60 min, the conversion of ST only reached 74% and 78% (Figures 7(a) and 7(b)). However, apparent discolor of ST was observed when the SiO2@PM12 hybrid catalysts were introduced (Figures 7(a) and 7(b)); that is, the conversions of ST reached 91% and 94% corresponding to SiO2@PW12 and SiO2@PMo12, respectively, after flowing air for 60 min. Compared with the catalytic activity of their parents (PM12), SiO2@PM12 hybrids exhibited much higher activity. The result suggested that when forming core-shell hollow structure, the BET surface area was augmented and the activity of which was improved. It also could be seen that SiO2@PMo12 showed higher catalytic activity than SiO2@PW12 did.

3.6. Mechanism of Degradation

Generally, organic degradation by wet air oxidation was recognized as a free-radical mechanism and hydroxyl radical is an extremely potential oxidizing agent with a short life which is able to oxidize organic substrates and generate other free radicals [22]. So the mechanism of degradation can be deduced as a mainly two-stage redox cycle.

Firstly, the oxidized form of interacts with organic dye ST(s) (as an electron donor), leading to product(s) P and reduced form of catalyst , which is the formation of the heteropoly blue on the surface of SiO2. The latter stage is thought to be the rate-determining step throughout POM-catalyzed oxidation:

Thereafter, the catalytic cycle is completed by reoxidation of the reduced catalyst by molecular oxygen:

3.7. Separation and Recovery of the Catalysts

After finishing the reactions, the suspensions were centrifuged, and the catalysts were collected. Separation and recovery of the hybrid catalysts were easy. The amount of W determined by ICP-AES in the resulting clear solution was less than 1.2%, which confirmed the less solubility of the catalysts during the reaction process and also showed the chemical bond between PW12 and SiO2 preventing the removal of POMs molecules from it. The catalyst was reused for 4 times (Figure 8), during which the catalytic activity of SiO2@PW12 in the degradation of ST was maintained efficiently with slight decrease owing to the lesser dissolution. The results (Figure 8) show that the catalysts are stable under such operating conditions and the leaching tests show negligible leaching effect owing to the lesser dissolution.

4. Conclusion

The research shows that SiO2@PM12 with core-shell structure could be prepared using native pollen grains as templates associated with surface sol-gel method, which is a unique, convenient, low-cost, and green chemical pathway. We can prepare samples with different types of catalytic activity (such as acid, redox) by this method using different types of POMs. The results of catalytic reactions suggested that these samples are very active in degradation of organic dye and insoluble in water aqueous during the catalysis. This catalytic process is a green chemical pathway which exhibits potential industrial application in degradation of organic dye.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.


The authors gratefully acknowledge the financial support from the Department of Education of Heilongjiang Province (Project no. 12511141).