14394
J. Phys. Chem. C 2007, 111, 14394-14399
Structural and Catalytic Investigation of Mesoporous Iron Phosphate Dinghua Yu, Cheng Wu,† Yan Kong,† Nianhua Xue, Xuefeng Guo,* and Weiping Ding* Lab of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, China ReceiVed: April 13, 2007; In Final Form: July 26, 2007
The local structure of ordered mesoporous iron phosphate, prepared through coassembly of inorganic species and sodium dodecyl sulfate template under the assistance of HF, has been investigated with several physicochemical methods, including small-angle XRD, TG-DSC, and combined spectroscopic characterization of FT-IR, Far-IR, Raman, UV-vis, and XPS. The redox property has also been measured by using the H2TPR method. The results of TEM and nitrogen sorption show the mesoporous iron phosphate possesses a well-established mesostructure, narrow pore size distribution, and large surface area (423 m2/g). The results of the spectroscopic characterization reveal that the mesoporous iron phosphate has specific iron-oxygen and phosphorus-oxygen local environment, differing from those in its crystalline counterpart. Compared with a crystalline sample, the iron-oxygen polyhedrons in the mesoporous sample shows a characteristic of distorted configuration, while the phosphate roots have a shorter P-O bond. Fluorine species are detected by XPS measurement. These factors cause the reduction by hydrogen of the mesoporous iron phosphate to be difficult. The hydroxylation reaction of phenol in glacial acetic acid by hydrogen peroxide has been used as a probe reaction to test the catalytic property of the mesoporous iron phosphate. Better catalytic performance and unique distribution of products has been observed on the mesoporous sample, compared to the crystalline ion phosphate. Finally, a model about the surface structure and reaction network of the hydroxylation of phenol on iron phosphate is proposed.
1. Introduction With the discovery of the special categorical materials M41S,1 mesoporous materials have attracted considerable research attention because of their structural advantages, e.g., large internal surface areas, uniform pore sizes, and special local structures. They have been widely explored as catalysts, absorbents, and host materials. Supermolecular assembly pathways, developed in the synthesis of M41S, have been extended to the synthesis of a variety of mesoporous non-siliceous metal oxides and metallophosphates. As a very important class of nonsiliceous molecular sieves, conventional open-framework metallophosphates have been studied due to their rich compositional and structural diversity. Mesostructured metallophosphates have also attracted much research attention and several mesoporous metallophosphates have been reported recently.2-8 Compared with the relatively inert mesoporous silica, metallophosphates possess suitable mobility of lattice oxygen and mild acid-base properties, which benefit their use in oxidative reactions as selective catalysts. VPO, for example, as a wellknown selectively oxidative catalyst, is an important catalyst in practical use and receives continuing research attention. Iron phosphate, as a selectively oxidative dehydrogenation catalyst for saturated carboxylic acids9 or as an attractive cathode material for batteries,10 has also attracted research attention and much effort has been made to enhance its performance. We have reported the synthesis of well-organized mesoporous iron phosphate through a fluoride route.6 Its superiority as cathode * To whom correspondence should be addressed. E-mail: dingwp@ nju.edu.cn and
[email protected]. † Present address: School of Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China.
materials or catalytic materials has also been reported by several groups.13-18 Wang et al. have reported the improved catalytic performance of Fe-P-O loaded on mesoporous supports for selective oxidation reactions.11,12 The better performance of mesoporous iron phosphate should be attributed to not only the larger surface area but also its unique local structures. The documented results, however, predominantly emphasize the surfactant assembly techniques for synthesis or their performance in use. Their local structures are less involved, since the mesoporous iron phosphate materials are almost amorphous in the arrangment of atoms, although the mesostructures are proven. Here we present our recent careful characterization results on the mesoporous iron phosphate, including their pore structure, thermal stability, local structural arrangements, and redox properties with combined techniques such as X-ray diffraction (XRD), transmission electron microscopy (TEM), nitrogen sorption, spectroscopic methods (infrared and far-infrared, Raman, XPS, and UV-vis), and hydrogen temperature-programmed reduction (H2-TPR). The liquid hydroxylation of phenol by H2O2 was used as a probe reaction to test the catalytic performance of mesoporous iron phosphate and a possible model of the surface structure and catalytic pathways are proposed. 2. Experimental Section 2.1. Preparation of Mesoporous Iron Phosphate Catalysts. Mesoporous iron phosphate was synthesized by a modified procedure similar to that in the previous work.6 Fe(NO3)3‚9H2O (8.08 g) and Na2HPO4‚12H2O (7.16 g) were respectively dissolved in 60 g of distilled water. The two solutions were mixed under stirring. The resulting precipitate was recovered by centrifugation and was suspended in 10 mL of aqueous
10.1021/jp072893o CCC: $37.00 © 2007 American Chemical Society Published on Web 09/12/2007
Investigation of Mesoporous Iron Phosphate solution containing 2.88 g of sodium dodecyl sulfate. Then 1.2 mL of HF (40 wt %) was dropped into the suspension under vigorous stirring. The resulting transparent solution was continuously stirred at room temperature for another 6 h followed by heating at 333 K for 12 h. After the solution was cooled to room temperature, a light yellow precipitate was recovered by centrifugation, washed with water/acetone, and dried at room temperature. The removal of the template from the assynthesized solid was carried out by anion exchange in a manner similar to that used by Holland et al.19 in the preparation of mesoporous aluminophosphate. The as-synthesized sample (denoted as FePO-AP) was mixed with a 0.05 M ethanol solution of sodium acetate under stirring at room temperature for 4 h. The solid was then recovered by centrifugation, washed thoroughly with ethanol, and dried at room temperature. The resulting sample was labeled as FePO-A. Then, the FePO-A was calcined in a tubular furnace at 573 K for 6 h with flowing air, and the calcined sample was named FePO-B. In addition, bulky crystalline FePO4 was prepared by using the documented20 method for comparison and was named FePO-C. 2.2. Characterization. X-ray diffraction (XRD) analysis was performed on a Philips X’Pro X-ray diffractometer with Cu KR irradiation. The X-ray source was operated at 40 kV and 40 mA. Transmission electron microscopy (TEM) measurements were conducted with FEI TECNAI20, using an accelerating voltage of 200 kV. Samples for TEM measurements were suspended in ethanol and ultrasonically dispersed. Drops of the suspensions were applied to a carbon-coated copper grid. Specific surface areas and pore size distributions of the mesoporous iron phosphate samples were measured with nitrogen sorption, using Micromeritics ASAP2020 equipment. Before N2 adsorption, the sample was outgassed under vacuum at 573 K for 3 h. Transmission infrared spectra were acquired with a BioRad FTS-40 instrument equipped with a DTGS detector. The samples were mixed with KBr and pressed into pellets. Far-infrared spectra were acquired with the same instrument equipped with a DTGS polyethylene detector. The samples were mixed with paraffin. Raman spectra were recorded with a Renishaw Invia System. A laser at 514.5 nm was used as the excitation light, of which output power was set at 20 mW, and the maximum incident power at the sample was approximately 6 mW in each measurement. The UV-vis diffuse reflectance spectra (UVvis DRS) were recorded on a UV-2401PC spectrometer with BaSO4 as reference. X-ray photoelectron spectroscopy (XPS) was measured with an ESCALB MK-II (VG), using Mg KR irradiation. The binding energy was calibrated with a C1s photoelectron peak at 284.6 eV as a reference. The surface composition was determined from the peak areas and the sensitive factors. Temperature-programmed reduction with H2 (H2-TPR) was performed on a flow system equipped with a TCD detector. Typically, 100 mg of sample was first pretreated in a quartz reactor under flowing air at 523 K for 1 h followed by purging with pure N2, which will produce a clean surface before performing the H2-TPR. After cooling to ca. 298 K, a H2/Ar (5 vol % H2) mixture was introduced into the reactor, and the temperature was raised to 1273 K at a rate of 10 deg/min. 2.3. Catalytic Reaction. To explore the relationship between the local structure and catalytic property of the iron phosphates, phenol hydroxylation by hydrogen peroxide in the liquid phase was used as the probe reaction. The reaction was performed in a 50 mL round-bottomed flask equipped with a reflux condenser and a magnetic stirrer. In a typical procedure, 10 mmol of phenol
J. Phys. Chem. C, Vol. 111, No. 39, 2007 14395
Figure 1. Small-angle X-ray diffraction (SAXD) patterns of the mesoporous FePO4 synthesized with sodium dodecyl sulfate as the template.
was dissolved in 20 mL of glacial acetate acid and mixed with 50 mg of catalyst. After the mixture was heated to 333 K under vigorously stirring, 20 or 10 mmol of H2O2 (30 wt %) was added within 30 min followed by reacting for 6 h. After reaction, the products were centrifuged to remove the catalyst before analysis. In the present experiments an Agilent 1100 HPLC equipped with a reversed phase C18 column was used to determine the product distribution. 3. Results and Discussion 3.1. Morphology Characteristics of Mesoporous Iron Phosphate. The small-angle XRD patterns of the mesoporous iron phosphate are shown in Figure 1. The diffraction peak at 2θ ) 2.3° (d ) 4.4 nm) suggests the mesostructure of the assynthesized sample and the mesoporous structure remained for both template extracted and calcined samples, indicative of the stability of the meso FePO4. The transmission electron micrographs of the sample FePO-B are shown in Figure 2. The well-ordered hexagonal mesoporous structure can be seen with the pore diameter of ∼2.4 nm and the lattice period length of 4.5 nm, consistent with the d value calculated from the small-angle XRD patterns. By carefully controlling the synthetic parameters, the mesoporous iron phosphate with well-ordered structure can be obtained. Figure 3 shows the typical N2 sorption isothermals and the corresponding pore size distribution curves of the sample FePOB. The type-IV isothermal with a small hysteresis loop is observed. Quantitative calculation shows that the mesoporous iron phosphate possesses a BET surface area of 423 m2/g and a pore volume of 0.35 cm3/g. The inset shows the distribution of pore diameters centered at ∼2.3 nm. With the unit cell dimension (a0 ) 2d100/x3) disclosed by XRD and the pore diameters measured by N2 sorption, the wall thickness can be calculated as ca. 1.8 nm, in agreement with the results of TEM measurements. The TG-DSC curves of the as-synthesized mesostructured iron phosphate are shown in Figure 4. Thermogravimetric analysis of the FePO-AP sample mainly exhibits two characteristic weight loss stages. The total weight loss of the as-synthesized sample is about 55%. According to Trobajo et al.’s report,21 the initial weight loss occurring in the range 303464 K (36%) is due to water release, which indicates that the as-synthesized mesoporous sample is an analogue of ferric phosphate dehydrate (FePO4‚2H2O). The subsequent weight loss of about 20% occurring in the range of 464-723 K should be
14396 J. Phys. Chem. C, Vol. 111, No. 39, 2007
Yu et al.
Figure 2. Typical TEM images of the sample FePO-B.
Figure 3. Nitrogen sorption isothermals of the sample FePO-B at 77 K. The inset shows the corresponding BJH pore size distribution curves.
Figure 4. TG-DSC curves of FePO-AP in air.
assigned to the loss of template. The small exothermal peak at ∼780 K in the DSC curve may be attributed to the crystallization of iron phosphate. Considering the TG-DSC results, the calcinations at 650 K in air should be suitable for the template removal. After calcination at 650 K in air, however, the mesostructure of FePO-AP collapsed completely. Hence, ion exchange by acetic acid before calcination is necessary for mesostructure conservation. 3.2. Structural Characteristics of Mesoporous Iron Phosphate. Figure 5 shows the XPS spectra of Fe2p, P2p, and O1s for the FePO-B and FePO-C catalysts. The binding energies of Fe2p3/2, P2p, O1s, and F1s and the surface atomic ratios of the samples are summarized in Table 1. The binding energy of Fe2p3/2 for the mesoporous sample is relatively higher as shown in Table 1, which may reveal the change of tetrahedral iron in the crystalline sample to octahedral iron in the mesostructured
sample.22 Compared with the crystalline FePO4, the binding energy of P2p of the mesoporous sample shifts to a lower position. The shift in the phosphorus peak corresponds to the hydration of phosphates, resulting in an effective HPO42- ion,22 indicating the high surface unsaturated coordination status due to the very high surface area of the mesoporous iron phosphate. The surface Fe/P ratio is close to unit for the mesostructured sample but much lower for the crystalline sample. It is noteworthy that the surface of the mesoporous FePO contains many fluorine species with the ratio of F/Fe close to unit due to the strong interaction between the iron and fluorine in the synthesis. The fluorine species must influence the catalytic property of the mesostructured sample, such as acid/base and redox properties. The IR and far-IR spectra of the iron phosphate samples are shown in Figure 6. In the region of 400-4000 cm-1, the IR absorbance at ∼1000 cm-1 indicates the existence of PO43- units and all the spectra are dominated by the internal vibration mode (ν1-ν4) of the PO43- units.23 For the as-synthesized mesoporous iron phosphate, the band at 1250 cm-1 corresponds to R-OSO3and the IR bands at 2851 and 2924 cm-1 are assigned to the alkyl C-H bonds in the molecules of SDS.6 After ion-exchange and heat treatment, the IR spectra confirm the disappearance of the characteristic peaks of the organic species (1250 cm-1, ROSO3-) and acetate (1420 and 1548 cm-1, COO-). In the far-IR region, the external modes occur in the range of 200-600 cm-1 where translatory and vibrational motions of the phosphate ions and lattice motion of ferric or ferrous ions are recorded. The vibration below 400 cm-1 is correlated to iron-oxygen lattice vibration. IR absorbance bands at 320, ∼270, and 225 cm-1 correspond to tetrahedral Fe3+, octahedral Fe3+, and octahedral Fe2+ species, respectively.24 The far-IR spectra show obviously that the iron species exist mainly in the form of tetrahedral Fe3+ in crystalline iron phosphate, while the mesoporous iron phosphate has all three forms of iron species. Moreover, when the as-synthesized mesoporous iron phosphate is treated through ion-exchange and heat treatment, the IR peaks turn weaker and wider and it seems the environment of the iron species becomes not so typical. Additionally, the absorbance of FeIIIO4 species shifts to a higher wavenumber for mesoporous iron phosphate, reflecting the longer Fe-O bond distance and the weaker interaction between iron and phosphate groups. Raman spectra provide important structural information, especially at short distances. The Raman spectra of iron phosphate samples are shown in Figure 7. Iron phosphate samples exhibit two intense Raman bands at 1031 and 996 cm-1
Investigation of Mesoporous Iron Phosphate
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Figure 5. XPS spectra of Fe2p, P2p, and O1s.
Figure 6. FT-IR and far-IR spectra of the FePO4 samples.
TABLE 1: XPS Results of Mesoporous Iron Phosphate and Crystalline Iron Phosphate
TABLE 3: Distribution of Products of Phenol Oxidationa selectivity (%)
binding energy (eV) sample
Fe2p3/2
P2p
O1s
F1s
Fe/P
Fe/O
F/Fe
catalyst
FePO-B FePO-C
712.65 711.85
131.36 133.50
531.60 531.20
684.90
0.91 0.57
0.13 0.14
0.93
no catalyst FePO-C FePO-B FePO-C FePO-B
TABLE 2: P-O Bond Lengths of the Iron Phosphate Samples and Corresponding Stretching Wave Numbers Obtained from Raman Spectra sample
R/pm (ESD)
ν/cm-1
FePO-AP
150.73 152.80 154.08 151.81 153.88 155.95 154.40 150.48 153.61 154.64
1130 1059 1015 1093 1022 951 1004 1138.7 1031.3 995.7
FePO-A FePO-B FePO-C
along with weak bands at 1182 cm-1 and in the range of 400700 cm-1. It is generally accepted that the stretching and bending vibrations of phosphate groups occur at 1000-1200 and 400700 cm-1, respectively.25 The Raman peaks of crystalline sample are very sharp and well separated in the regions of 900-1200 and 100-400 cm-1. In contrast, the Raman peaks of mesoporous samples are wider and weaker and cannot be well-resolved for the vibrations from the phosphate roots or iron-oxygen. Very recently, Popovic´ et al. have developed a theory to correlate the Raman stretching (in wavenumber) of phosphorus-oxygen bonds with the bond lengths of inorganic phosphates.26 The correlation casts an invaluable insight into the structures of
ratio of conversion hydrobenzoH2O2/phenol (%) quinone catechol quinone 2:1 2:1 2:1 1:1 1:1
28.68 8.54 18.32 8.1 16.67
11.97 50.60 69.81 53.14 72.21
9.35 45.00 21.47 41.07 19.25
2.35 3.59 1.02 2.03 1.06
a For all cases, glacial acetic acid was used as solvent and HPLC was used to analyze the products. Balance carbon was unidentified tars. Quantitative calculations were performed by comparison with calibration curves obtained with known amounts of hydroquinone, catechol, and benzoquinone.
phosphate species for which diffraction or other spectroscopic techniques provide only incomplete structural information. Accordingly, the P-O bond lengths are calculated by using the method with the Raman results to disclose the local structures of the current FePO samples. The results are listed in Table 2. From the results listed in Table 2, the P-O bond distances in mesoporous iron phosphate are slightly shorter than its crystalline counterpart and, at the same time, the Raman peaks corresponding to the Fe-O lattice vibration move to lower wavenumbers. The results reveal the structural particularity of the synthesized mesoporous FePO, i.e., more rigid phosphate ions and weaker interaction between iron and phosphate ions, which must affect its performance as a catalyst. Diffuse reflectance UV-vis spectra can be used to study the metaloxygen surroundings in the solid sample. For the UV-vis spectrum of the mesoporous iron phosphate, shown in Figure 8 A, the absorbance for charge transfer moves to longer wave-
14398 J. Phys. Chem. C, Vol. 111, No. 39, 2007
Figure 7. Raman spectra of the samples.
Figure 8. Diffuse reflectance UV-vis spectra (A) and temperatureprogrammed reduction profiles (B) of the samples.
length by comparing with its crystalline counterpart, in agreement with the far-IR and Raman results. H2-TPR profiles for the mesoporous and crystalline Fe-P-O are shown in Figure 8B. The crystalline FePO4 exhibits two asymmetric reduction peaks with the maximum respectively at 523 and 619 °C, since the crystalline FePO4 used here contains a small amount of tridymite-like phase in addition to the main quartz-like phase, as revealed by XRD. Wang et al. have reported recently that the tridymite-like phase of FePO4 loaded on MCM-41 is easier to reduce than the quartz-like phase.11 Then the reduction peaks at 523 and 619 °C for the crystalline FePO4 can be respectively assigned to the reduction of the tridymite-like phase and the quartz-like phase of FePO4. The quantitative calculation indicates that these peaks correspond
Yu et al. to the reduction of bulk FePO4 to Fe2P2O7. Surprisingly, the reduction peaks for the mesoporous sample shift to much higher temperatures: the shifts surpass 150 °C. This may be attributed to the PO4 groups becoming more rigid in the mesoporous samples, and the formation of P2O7 groups is not so easy, compared with the crystalline iron phosphate, as revealed by the abovementioned Raman and IR measurements. 3.3. Catalytic Test. The liquid hydroxylation of phenol by H2O2 is selected as a probe reaction to test the catalytic performance of the mesoporous iron phosphate. The catalytic activity and product distributions are listed in Table 3. Generally, both the crystalline iron phosphate and the mesoporous iron phosphate shrink the conversion of phenol and promote the selectivity to hydroquinone. A low ratio of hydrogen peroxide to phenol benefits the selectivity to hydroquinone. And the mesoporous iron phosphate gives better catalytic performance. The active sites of iron phosphate catalyst in oxidative dehydrogenation reaction have been proposed in several references in gas-phase reactions with molecular oxygen or N2O as oxidant.11,12 But metal phosphates are seldom investigated as catalysts in liquid-phase oxidative reactions due to their weak oxidative ability. Rocha et al.27,28 have studied the catalytic properties of M4+-P-O for the liquid-phase hydroxylation of phenol. In glacial acetic acid, they showed that peroxyacetic acid was the major intermediate oxidant and the metal phosphates had no catalytic effect on its formation but had a significant effect on the oxidation of individual products, i.e., their existence altered the selectivity of the reaction. From the documented results, the titanium silicate is not so selective either for catechol or for hydroquinone in hydroxylation of phenol by hydrogen perxoxide.29 Cu-SBA-1529 or niobium phosphates30 was reported more selective to catechol. For the current results, the mesoporous iron phosphates show much higher selectivity to hydroquinone. There may be a different mechanism at work on the mesoporous iron phosphates. Vetter et al.31 have reported the hydroxylation of phenol over Ti-MCM41 and TS-1 with H2O2 as oxidant. Their results indicate that the exclusive p-selectivity is due to the different strengths of adsorption of the para and ortho isomers. Considering the unique structures of the mesostructured iron phosphate and the current characterization results, a schematic diagram for the surface reaction on the mesoporous iron phosphate for the hydroxylation of phenol is speculated (Figure 9).
Figure 9. A schematic diagram for the surface reaction of hydroxylation of phenol on the mesoporous iron phosphates
Investigation of Mesoporous Iron Phosphate Without any catalyst, the conversion of phenol is high, but the selectivity to useful products is very poor due to the very active peracetic acid oxidizing the products as tars. When iron phosphate is used as a catalyst, the direct reaction between phenol and peracetic acid in bulk solution is restrained and the surface-assisted reaction results in higher selectivity. The products penetrating the pores of catalyst are protected from deep oxidation. The fluorine adsorbed on the surface enhances the electrophilicity of Fe-O-P species and leads to a distorted iron oxygen polyhedron, which may be active for the reaction. Compared to the crystalline counterpart, the mesoporous iron phosphate has higher surface area and a big pore width, which benefit phenol molecule adsorption. These factors may account for the improved catalytic performance of the mesoporous FeP-O. 4. Conclusion The mesoporous iron phosphates with ordered mesostructure prepared with the HF assembly method are investigated with spectroscopic techniques. The small-angle X-ray diffraction, TEM, and low-temperature N2 sorption results show that the mesoporous iron phosphate has a narrow pore size distribution centered at 2.3 nm, and a remarkably large surface area of about 423 m2/g. The detailed local structural information of the mesoporous iron phosphate is disclosed by vibration spectra measured by IR, far-IR, Raman, and UV-vis DRS. The mesoporous iron phosphates possess specific phosphorusoxygen and iron-oxygen species, including tetrahedral Fe3+, octahedral Fe3+, and Fe2+. The phosphate roots are more rigid and meanwhile the Fe-O bonds become longer by comparison with the crystalline iron phosphate. These effects cause the mesoporous Fe-P-O to be difficult to reduce, as revealed by H2-TPR. For the liquid hydroxylation of phenol, the mesoporous iron phosphate shows better catalytic performance. Acknowledgment. The authors thank the MOST of China (Grant No. 2003CB615804) and the NSF of China (Grant No. 20403008 and 20673054) for financial support. References and Notes (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. J. Am. Chem. Soc. 1982, 104, 1146.
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