Mesoporous Nanotubes of Iron Phosphate: Synthesis

382

Langmuir 2007, 23, 382-386

Mesoporous Nanotubes of Iron Phosphate: Synthesis, Characterization, and Catalytic Property Dinghua Yu, Jieshu Qian, Nianhua Xue, Danyu Zhang, Chunyan Wang, Xuefeng Guo,* Weiping Ding,* and Yi Chen Lab of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, China ReceiVed July 20, 2006. In Final Form: NoVember 23, 2006 Iron phosphate nanotubes with mesoporous walls are solvothermally synthesized using sodium dodecyl sulfate (SDS) as a template. With different template concentrations, various shapes of nanosized iron phosphates can be obtained. When the concentration of SDS is set at the transition regions between the lamellar and the hexagonal mesophases, according to its phase diagram, the coassembly of iron phosphate precursor and SDS forms a flake-type mesoporous iron phosphate. Otherwise, nanoparticles or bulky sheets of iron phosphates are obtained. The followed solvothermal treatments on the mesoporous iron phosphate flakes produce iron phosphate nanotubes with mesoporous walls. The removal of the surfactant by acetate exchange and heat treatment results in the clean mesoporous nanotubes of iron phosphate with diameters of 50-400 nm and lengths of several microns. The nanotubular and mesoporous iron phosphate possesses a specific surface area of 232 m2/g and a bimodal distribution of pore sizes, corresponding to the size of mesopores in the walls and the diameter of the nanotubes, respectively. The novel nanotubular iron phosphate with composite meso-macroporous structure, in favor of the diffusion of reactive molecules, has been tested for direct hydroxylation of benzene with hydrogen peroxide and has shown better catalytic performance compared with the conventional particulate mesoporous iron phosphate.

Introduction Mesoporous metallophosphates (Al, Ti, V, Cr, Fe, Zr, and Sn),1-7 as non-siliceous porous inorganic materials, have attracted much research attention owing to their important application as advanced functional materials in catalysis, adsorption, and separation,.8 In particular, iron phosphate has shown good catalytic properties for the oxidation of glycolic acid to glyoxylic acid9 and for the selective oxidation of methane when it was supported by MCM-41 or SBA-15.10,11 In comparison to iron oxides, the lattice oxygen of iron phosphate is not so active, which should benefit the selectivity for oxidative catalytic reactions. Iron phosphate is also a useful material for lithium batteries.12 The mesoporous silica with hierarchically assembled structures has * To whom correspondence should be addressed. E-mail: dingwp@ nju.edu.cn or [email protected]. (1) (a) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. J. Am. Chem. Soc. 1982, 104, 1146-1147. (b) El Haskouri, J.; Guillem, C.; Latorre, J.; Beltran, A.; Beltran, D.; Amoros, P. Chem. Mater. 2004, 16, 4359-4372. (2) (a) Bhaumik, A.; Inagaki, S. J. Am. Chem. Soc. 2001, 123, 691-696. (b) Tian, B. Z.; Liu, X. Y.; Tu, B.; Yu, C. Z.; Fan, J.; Wang, L. M.; Xie, S. H.; Stucky, G. D.; Zhao, D. Y. Nat. Mater. 2003, 2, 159-163. (3) Mizuno, N.; Hatayama, H.; Uchida, S.; Taguchi, A. Chem. Mater. 2001, 13, 179-184. (4) Tarafdar, A.; Biswas, S.; Paramanik, N. K.; Pramanik, P. Microporous Mesoporous Mater. 2006, 89, 204-208. (5) Guo, X. F.; Ding, W. P.; Wang, X. G.; Yan, Q. J. Chem. Commun. 2001, 36, 709-710. (6) Jimenez-Jimenez, J.; Maireles-Torres, P.; Olivera-Pastor, P.; RodriguezCastellon, E.; Jimenez-Lopez, A.; Jones, D. J.; Roziere, J. AdV. Mater. 1998, 10, 812-815. (7) Mal, N. K.; Ichikawa, S.; Fujiwara, M. Chem. Commun. 2002, 37, 112113. (8) Corma, A. Chem. ReV. 1997, 97, 2373-2419. (9) Ai, M.; Ohdan, K. In 3rd World Congress on Oxidation Catalysis; Crassell, R. K., Oyama, S. T., Gaffney, A. M., Lyonsl, J. E. Eds.; Elsevier Science B.V.: Amsterdam, 1997; pp 527-534. (10) Wang, X. X.; Wang, Y.; Tang, Q. H.; Guo, Q. A.; Zhang, Q. H.; Wan, H. L. J. Catal. 2003, 217, 457-467. (11) Wang, Y.; Wang, X. X.; Su, Z.; Tang, Q. H.; Zhang, Q. H.; Wan, H. L. Catal. Today 2004, 93-95, 155-161. (12) (a) Zhu, S. M.; Zhou, H. S.; Miyoshi, T.; Hibino, M.; Honma, I.; Ichihara, M. AdV. Mater. 2004, 16, 2012-2017. (b) Nakamura, T.; Miwa, Y.; Tabuchi, M.; Yamada, Y. J. Electrochem. Soc. 2006, 153, A1108-A1114.

been a wide research topic in the recent years.13,14 Similarly, it is also a challenge to further tailor the morphologies of the metallophosphates for high-performance applications. The composite structures with mesopores and macropores are specially desired for their advantages in mass transfer.15,16 The nanotubes with mesoporous walls, possessing a unique morphology and a special local structure, as presented in this paper, should be very interesting. The mesoporous-nanotubular structures have never been reported, although some nanotubular metallophosphates (Ce and Ti) have been documented with solid tubular walls.17,18 Since the “rolling-up” mechanism for lamellar precursors to nanotubes has been widely demonstrated,19 the key precursor for the above-mentioned unique structure of mesoporous nanotubes may be a lamellar precursor with mesoporous structure, which would be reached by the surfactant assembling method. For example, the lamellar and hexagonal phases of SDS in aqueous solution are stable in a large range of concentrations, and very complicated structures are reported between the two phases.20 On the basis of the concepts developed in the previous synthesis of mesoporous FePO4,5 we envisage the possibility of coassembly of SDS and iron phosphate precursor to a flake mesoporous composite with a suitable SDS concentration, in which the lamellar and hexagonal phases coexist. By a solvothermal treatment on the flake-type precursors, mesoporous nanotubes can be obtained via the “rolling-up” mechanism.19 We describe here the synthesis (13) Wang, B.; Shan, W.; Zhang, Y. H.; Xia, J. C.; Yang, W. L.; Gao, Z.; Tang, Y. AdV. Mater. 2005, 17, 578-583. (14) Jin, H. Y.; Liu, Z.; Ohsuna, T.; Terasaki, O.; Inoue, Y.; Sakamoto, K.; Nakanishi, T.; Ariga, K.; Che, S. N. AdV. Mater. 2006, 18, 593-597. (15) (a) Huang, H.; Yin, S. C.; Nazaer, L. F. Electrochem. Solid-State Lett. 2001, 4, A170-A172. (b) Son, D.; Kim, E.; Kim, T. G.; MG, K.; Cho, J. H.; Park, B. D. Appl. Phys. Lett. 2004, 85, 5875-5877. (16) Carreon, M. A.; Guliants, V. V. Catal. Today 2005, 99, 137-142. (17) Tang, C. C.; Bando, Y.; Golberg, D.; Ma, R. Z. Angew. Chem. Int. Ed. 2005, 44, 576-579. (18) Yin, Z. L.; Sakamoto, Y.; Yu, J. H.; Sun, S. X.; Terasak, O.; Xu, R. R. J. Am. Chem. Soc. 2004, 126, 8882-8883. (19) Tremel, W. Angew. Chem. Int. Ed. 1999, 38, 2175-2179. (20) Kekicheff, P.; Grabielle-Madelmont, C.; Ollivon, M. J. Colloid Interface Sci. 1989, 131, 112-132.

10.1021/la062117s CCC: $37.00 © 2007 American Chemical Society Published on Web 12/08/2006

Letters

Langmuir, Vol. 23, No. 2, 2007 383

of mesoporous nanotubes of iron phosphate using a strategy of SDS-templating assembly. The method may be general for synthesis of mesoporous nanotubes with other materials. Experimental Section Sample Preparation. 4.04 g Fe(NO3)3‚9H2O was dissolved in 20 g distilled water, and 8.95 g Na2HPO4‚12H2O was dissolved in 60 g distilled water. The two solutions were mixed under stirring. The precipitate was recovered by centrifugation and was suspended in 10 mL 2.88 g sodium dodecyl sulfate containing aqueous solution. Then, 1.8 mL HF (40 wt %) was dropped into the suspension with vigorous stirring. The resulting transparent solution was continuously stirred at 283 K for 24 h and then soaked at 333 K for 12 h. After cooling to room temperature, a light yellow precipitate was observed in the solution and recovered by centrifugation, followed by washing with water and acetone and drying at room temperature. The sample was denoted FeP-S (the character S means “sheets”). The solid FeP-S was added into absolute ethanol and subjected to solvothermal treatment in a Teflon-lined autoclave at 423 K for 24 h. Then, the resultant was recovered by centrifugation, followed by washing with ethanol and drying at room temperature. The sample was denoted FeP-T. The surfactants included in the above products were further removed by ion exchange and heat treatment at 473 K for 5 h in air. The cleaned sample was denoted FeP-MNTs. Sample Characterization. X-ray diffraction (XRD) analysis was performed on a Philips X’Pro X-ray diffractometer with a Cu KR irradiation. The X-ray source was operated at 40 KV and 40 mA. Transmission electron microscopy (TEM) measurements were conducted with TECNAI20, using an accelerating voltage of 200 kV. Samples for TEM were prepared by dipping a Cu TEM grid, coated with holey carbon film, into a colloidal suspension of particles dispersed in ethanol, which were then air-dried and stored in a vacuum chamber. FT-IR spectra were recorded on Bruker Vector 22. Nitrogen sorption isotherms were measured at 77 K using Micromeritics ASAP2020 equipment. The BET (Brunauer-Emmett-Teller) method and BJH (Barrett-Joyner-Halenda) models are used for specific surface area calculation and porosity evaluation, respectively. All the samples were degassed at 473 K before the sorption measurement. The X-ray photoelectron spectroscopy (XPS) analyses were conducted on an ESCALAB MK-II spectrometer equipped with a Mg KR X-ray source. The C1s (Eb ) 284.6 eV) peak was chosen as a reference line for calibration of the energy scale. The UV-vis diffuse reflectance spectra (UV-vis DRS) were recorded using a UV-2401PC spectrometer and BaSO4 as reference. The temperatureprogrammed reduction (H2-TPR) was performed using a flow system equipped with a TCD detector. Typically, 100 mg of the samples was pretreated in flowing air at 573 K for 1 h. After cooling to ca. 300 K and purging by N2, 5% H2/Ar was introduced into the reactor, and the temperature was ramped to 1073 K at a rate of 10 K‚min-1. Catalytic Test. The direct hydroxylation of benzene was performed in the liquid phase in a 50 mL round-bottomed flask equipped with a reflux condenser and a magnetic stirrer. In a typical procedure, 20 mmol benzene was mixed with 0.1 g catalyst (FePMNTs or conventional mesoporous FePO4, synthesized as in ref 5, possessing the specific surface area of 258 m2/g, and denoted FePMPs) in 20 mL acetonitrile. After the mixture was heated to 333 K under vigorously stirring, 35 mmol H2O2 (30 wt %) were added within 30 min. Then, the mixture continued to react for 70 min. After the reaction, the mixture was separated by centrifugation and analyzed by gas chromatography. Quantitative calculation was achieved using o-cresol added after reaction as an internal standard.

Results and Discussion The small-angle X-ray powder-diffraction pattern of the sample FeP-S (Figure 1) shows the similar characteristic peak at 2.3° as the mesoporous iron phosphate,5 indicating the mesostructure of the as-synthesized FeP-S. The typical TEM images of the FeP-S, as shown in the insets of Figure 1, confirm their

Figure 1. Small-angle X-ray diffraction (SAXD) patterns of the mesoporous sheety FePO4 (MFeP-S). Insets show the TEM images of the sheety material with mesopores.

Figure 2. Typical TEM images of the as-synthesized iron phosphate nanotubes (FeP-T): (a) overview; (b) modified resolution to show the structure of the tube walls.

mesoporous structure and sheety shapes. The sheety morphologies are dominant for the product. When the suspension of the FeP-S in ethanol is subjected to solvothermal treatment, the solvothermal action dissociates the FeP-S to many Fe-P-O flakes and scrolls the latter into nanotubes. Figure 2 shows the TEM images of the FeP-T (surfactant-containing nanotubes of mesoporous Fe-P-O), which possesses primarily tubular structures with diameters of 50-400 nm and lengths of several microns. The walls of the nanotubes range from 20 to 40 nm in thickness. The selectedarea electron diffraction (SAED, not shown) on individual nanotubes reveals the amorphous nature of the walls of the asmade nanotubes. Besides the nanotubes, a small amount of flakes or particulates (Figure 2) are also observed. Generally, about 70% yield of nanotubes is estimated from the TEM images of different synthetic batches. The energy-dispersive X-ray analysis (EDX) performed on an individual nanotube detects the coexistence of Fe and P in a molar ratio close to unity, consistent with FePO4. In addition to Fe and P, S and Na are also detected, indicative of the existence of the surfactant SDS in the as-made nanotubes. The surfactant can be removed through acetate exchange and heating at 473 K for 5 h in air, resulting in clean products. The FT-IR measurements (Figure 3A) confirm the disappearance of the characteristic peaks of the surfactant (1250 cm-1, ROSO3-) and acetate (1420 and 1548 cm-1, COO-) after cleaning. The tubular structures are still reserved after the cleaning, revealed by TEM images (Figure 3B). The tubular walls possess wormlike mesoporous structures with pore diameters of ∼3 nm, as shown in the insets of Figure 3B. These results indicate that the final products obtained are mesoporous nanotubes of iron phosphate, which are novel one-dimensional (1D) nanostructures with hierarchical macro-mesoporous structures. The mesostructures

384 Langmuir, Vol. 23, No. 2, 2007

Letters

Figure 3. (A) FT-IR spectra: (a) as-synthesized FeP-T; (b) ion-exchanged; (c) heat-treated at 473 K for 5 h (FeP-MNTs). (B) TEM images of the FeP-MNTs heated at 473 K. Inset shows the TEM image of an individual heated nanotube consisting of wormlike mesoporous wall with higher resolution.

Figure 4. The nitrogen sorption isotherms of FeP-MNTs at 77 K. Inset shows the corresponding BJH pore size distribution curve calculated from desorption branch.

depicted in the Figures 1 and 3 appear to be similar in pore sizes, and the heat treatment at 473 K does not cause too much shrinkage in the pore sizes. Figure 4 shows the typical sorption isotherms and the corresponding pore size distribution (inset of Figure 4) of the FeP-MNTs. The type-IV isotherm with two separated hysteresis loops is obtained. The well-expressed H1 loop at the relative pressures P/P0 of 0.3-0.5 and the H3 loop at P/P0 of 0.8-1.0 represent the condensation in the mesopores and macropores, respectively, as defined by IUPAC.21 Accordingly, the pore size distribution curve displays a bimodal shape with two maxima at ∼3 nm and ∼50 nm, respectively. The first condensation step on the isotherm at P/P0 ) 0.3-0.5 is similar to the usual mesoporous materials, consistent with the pore diameter of ∼3 nm analyzed by TEM, indicating the presence of a welldeveloped mesoporous framework in the wall of the nanotubes. The second hysteresis loop at higher relative pressure (P/P0 > 0.8) is obviously a result of the capillary condensation of nitrogen in the nanotubes. The corresponding BJH pore sizes are distributed from several tens of nanometers to more than 100 nm, in agreement with the TEM observations. Quantitative calculation shows that the sample FeP-MNTs possesses a BET surface area of 232 m2/g and a pore volume of 0.25 cm3/g.

On the basis of the above results, the likely synthetic mechanism of the FeP-MNTs is proposed and schematically shown in Figure 5. The first step is the reaction of the iron phosphate precursors with HF molecules to generate [Fe-P-O-H]+ ions, and the later interact with DS- anions to form [Fe-P-O-H]-DS ion pairs. Secondly, the continuing condensation leads to mesostructured assemblies in very thin flake-type morphologies, confined by the lamellar structure of surfactant. The analysis has been supported by small-angle X-ray diffraction and TEM observations (Figure 1). In order to examine the effect of the template concentration on product morphology, we have also synthesized a series of samples with varied surfactant amount. For flake-type morphology, the most suitable amount of SDS is 2.75-3.0 g in 10 g water. It is a very narrow window, the varying amount of SDS cannot produce the flake-type mesoporous samples but rather particulates or bulky sheet samples (Figure 6A,B), indicating that the SDS concentration in the transition regions between the two stable phases is necessary. From the phase diagram of SDS in aqueous solution, its lamellar and hexagonal phases are stable in large spans of concentration, and a narrow transition region with complicated structures exists in between the two stable phases.20 Generally, lower SDS concentration has been used for the synthesis of inorganic mesostructured materials, and the inorganic species usually interact with SDS to form the cooperative template with lamellar or hexagonal phases.22 Both the concentrations of SDS and inorganic precursor, hence, are very important to produce such flake-shaped composite structures. For the current synthesis of FeP-MNTs, the synthetic parameters were optimized by adjusting the amount of FePO4 precursor, the surfactant concentration, and the synthetic temperature. Under the optimized synthetic conditions, the surfactant or the complex of surfactant and iron phosphate precursor exists as the template both for the lamellar structure and for the hexagonal mesophases. The suitable competition between the lamellar and hexagonal phases results in the flake shapes of FeP-S. This interesting but complicated phenomenon requires further investigation for thorough elucidation. In the following step, the iron phosphate nanoflakes are easy to roll up into tubes in the solvothermal treatment at ∼423 K due to their wiggly structures. These very thin flakes with mesopores (21) Kruk, M.; Jaroniec, M. Chem. Mater. 2001, 13, 3169-3183. (22) (a) Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schu¨th, F.; Stucky, G. D. Nature (London) 1994, 368, 317-321. (b) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schu¨th, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176-1191.

Letters

Langmuir, Vol. 23, No. 2, 2007 385

Figure 5. Schematic show of the mechanism for the typical FeP-MNTs synthetic process. (a,b,c) show the process of assembly of the inorganic precursor and the surfactant SDS to nanoflakes of Fe-P-O. (d) Solvothermal treatment induced rolling-up of the flakes. (e) Ion exchange and heat treatment result in clean Fe-P-O mesoporous nanotubes.

Figure 6. The TEM images of precursor morphology at different surfactant concentrations: (A) 1.44 g SDS in 10 g water; (B) 5.76 g SDS in 10 g water. Other conditions are the same as the typical synthetic process.

Figure 7. Diffuse reflectance UV-vis spectra (A) and temperature-programmed reduction profiles (B) of the samples.

are very flexible and unstable and easy to roll up into more stable tubes by thermal disturbance under solvothermal conditions, as described by Ma et al.23 Finally, ion exchange and heat treatment of the resulting nanotubes lead to clean mesoporous nanotubes of iron phosphate. The UV-vis diffuse reflectance spectra of the samples, i.e., mesoporous nanotubes of FePO4 (FeP-MNTs) and mesoporous particulates of FePO4 (FeP-MPs), are shown in Figure 7A. The charge transitions of P-O and Fe-O have attributed to the bands at 215 and 310 nm for FePO4 in the literature.24 For the current mesoporous Fe-P-O, however, the absorbance bands are very broad due to their amorphous local structures. The absorbance bands of FeP-MNTs samples shift to longer wavelength as shown in Figure 7A, indicating that the local structure of FePMNTs differs from that of FeP-MPs to some extent. Figure 7B depicts H2-TPR profiles of the two samples. (23) Ma, R.; Bando, Y.; Sasaki, T. J. Phys. Chem. B 2004, 108, 2115-2119. (24) Yuan, Q.; Zhang, Q. H.; Wang, Y. J. Catal. 2005, 233, 221-233.

Table 1. Surface Composition of the Samples (XPS) sample

Fe/P

Fe/O

F/Fe

FeP-MNTs FeP-MPs

0.66 0.91

0.13 0.13

0.98 0.93

In comparison to the unsupported and nonporous FePO4,10 both of the mesoporous samples underwent reduction by hydrogen at much higher temperatures, showing modified valence stability of the iron in the FeP-MPs and FeP-MNTs. It is an interesting phenomenon, considering the subtle structure of the mesorporous iron phosphates. A lot of fluorides, detected by XPS (Table 1), clinging to the surface of the two samples, may account for the special valence stability of iron, which may be good for some surface catalytic reaction by reducing the activity of lattice oxygen. Mesoporous iron phosphate has been reported as a highly active and recyclable heterogeneous catalyst for the selective synthesis of nopol by Prins condensation of β-pinene and paraformaldehyde

386 Langmuir, Vol. 23, No. 2, 2007

Letters

Conclusion

Table 2. Hydroxylation Reaction Results of FeP-MPs and FeP-MNTs catalyst

activity (nm-2‚h-1)

phenol selectivity (%)

no catalyst FeP-MPs FeP-MNTs

∼0 3.4 10.8

∼100 ∼100

in acetonitrile at 353 K.25 With special local structures and diffusion advantages, the current nanotubular and mesoporous iron phosphate (FeP-MNTs) is expected to be interesting as a catalyst. The direct hydroxylation reaction of benzene with H2O2 is selected to test the morphology effect, and the results are listed in Table 2. It can be seen, from Table 2, that the mesoporous nanotubes (FeP-MNTs) give better catalytic performance for the reaction. As mentioned above, the biggest difference between the two samples is their morphology. Because the activity has been normalized by surface area, the improved catalytic properties of FeP-MNTs should be mainly attributed to morphology effects. Reactive molecules have easier access to active sites of FePMNTs, compared to FeP-MPs. In addition, the fluids in the nanotubes of FeP-MNTs would be viewed under confined conditions. Gubbins et al. have simulated some chemical reactions under confined conditions of nanotubes. They have found that the fluid molecules in the nanotubes experience a strong interaction with the walls, which increases reaction efficiency.26,27 These factors account for the improved catalytic properties of FeP-MNTs. (25) Pillai, U. R.; Sahle-Demessie, E. Chem. Commun. 2004, 39, 826-827.

The mesoporous nanotubes of iron phosphate are synthesized by surfactant templating and solvothermal methods. When the concentration of the surfactant (SDS) is set at the transition region between its lamellar and hexagonal mesophases, the coassemblies of iron phosphate and SDS form a flake-type mesoporous iron phosphate. The following solvothermal treatment produces mesoporous nanotubes of FePO4 with diameters of 50-400 nm and lengths of several microns. The removal of surfactant by ion exchange and heat treatment produces the clean nanotubular and mesoporous iron phosphate, which possesses a specific surface area of 232 m2/g and a bimodal distribution of pore sizes, corresponding to the mesopores of the walls and the nanotube diameters, respectively. The novel tubular material with composite meso-macroporous structure, benefiting the diffusion of reactive molecules, has shown better catalytic performance for direct hydroxylation of benzene with hydrogen peroxide compared with conventional mesoporous iron phosphate. Acknowledgment. Authors thank financial support from the MOST of China (grant no. 2003CB615804) and the NSF of China (grant nos. 20403008 and 20673054). LA062117S (26) Turner, C. H.; Johnson, J. K.; Gubbins, K. E. J. Chem. Phys. 2001, 114, 1851. (27) Byl, O.; Kondratyuk, P.; Yates, J. T. J. Phy. Chem. B 2003, 107, 4277.