Syntheses of Mesoporous Hybrid Iron Oxophenyl Phosphate, Iron

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Syntheses of Mesoporous Hybrid Iron Oxophenyl Phosphate, Iron Oxophosphate, and Sulfonated Oxophenyl Phosphate Nawal Kishor Mal,†,| Asim Bhaumik,*,‡ Masahiko Matsukata,§ and Masahiro Fujiwara*,† Kansai Center, National Institute of AdVanced Industrial Science and Technology (AIST-Kansai), 1 Midorigaoka, Ikeda, Osaka 563-8577, Japan, Department of Materials Science, Indian Association for the CultiVation of Science, 2A & B Raja S. C. Mullick Road, JadaVpur, Kolkata 700 032, India, and Department of Applied Chemistry, Waseda UniVersity, Okubo, Tokyo 169-8555, Japan

A novel organic-inorganic hybrid mesoporous iron oxophenyl phosphate has been synthesized by using supramolecular assembly of sodium dodecyl sulfate molecules. X-ray diffraction, transmission electron microscopic studies, and N2 adsorption data indicated the wormhole-like disordered mesostructure in this sample. The 13C and 31P MAS NMR, FT IR, UV-visible spectroscopic data, and chemical analysis results indicated that all P atoms are attached to phenyl groups directly and combined with Fe atoms through O atoms. Calcination of this hybrid material produced organic-free mesoporous iron oxophosphate material, whereas sulfonation of the mesoporous iron oxophenyl phosphate resulted in sulfonated oxophenyl phosphate. The latter showed outstanding proton conductivity, which could be utilized in membrane or supports of anode and cathode materials in fuel cells. Introduction Phosphate-based mesoporous materials such as Ti,1,2 Al,3 Zr,4 Fe,5,6 Sn,7,8 Nb,9 V, In, W, and Ce10 have received widespread attention since the discovery of mesoporous MCM-4111 silicate in 1992 and metal oxides12 in 1994 by supramolecular templating mechanism. Both high surface area and unique pore morphology with narrow pore size distribution promote their use in a large variety of applications ranging from adsorbent, exchanger, catalyst, or host for the synthesis of optical,13 conducting,14 or magnetic nanomaterials.15 Although there are relatively few reports on the synthesis of mesoporous metallophosphate because of the difficulties that would come from the organization of two different inorganic units (metal and the phosphate moieties) around the surfactant assemblies, there is a huge possibility of transition metal phosphate based framework materials in optoelectronics and magnetic applications. One of the major drawbacks for these mesoporous materials is its hydrophilic surface properties, which is due to the presence of high surface defect groups (Si-OH, P-OH, etc.). Substitution of organic functional groups in the framework of these mesoporous materials drastically enhanced the hydrophobicity and thus the syntheses of organic-inorganic hybrid mesoporous silica materials have attracted widespread attention in recent times.16-21 These hybrid materials having microporous and mesoporous structures with exceptionally high surface area allows the binding of a large number of surface chemical moieties, leading to their unique surface properties and conversion to further advanced materials.16-28 On the other hand, the synthesis of stable mesoporous iron phosphate is difficult compared with other metal phosphates because the assynthesized mesostructured iron phosphates are either lamellar9 * To whom correspondence should be addressed. A. Bhaumik: Phone, +91-33-2473-4971; fax, +91-33-2473-2805; e-mail, msab@ mahendra.iacs.res.in. M. Fujiwara: Phone, +81-72-751-9253; fax, +8172-751-9628; e-mail, [email protected]. † AIST-Kansai. ‡ Indian Association for the Cultivation of Science. § Waseda University. | Present address: Material Laboratory, Sony Co. Ltd., Hodogaya, Yokohama 240-0036, Japan.

or other unstable phases, which readily collapse after calcination at 773 K.5 We have studied the preparations of various kinds of metal phosphate and phenylphosphonate materials.8,9,26 However, the preparation of mesoporous hybrid iron phosphate or oxophosphate has not been reported so far. Excessive surface defect P-OH groups in the phosphate-based mesoporous materials could be overcome by this surface organic modification. Apart from that, the mesoporous iron phosphate is known as a good catalyst for Prins condensation reaction.29 Here, we report the first synthesis of mesostructured hybrid iron oxophenyl phosphate using phenylphosphonic acid as a single source of phosphorus in the presence of sodium dodecyl sulfate (SDS) as surfactant. Furthermore, the sulfonated hybrid sample had high proton conductivity and could be a potential candidate of membrane material in fuel cell technology. Experimental Section In a typical synthesis the mesoporous hybrid iron oxophosphate was synthesized using the following molar composition: 1:(0.50-2.0):(0.50-2.0):(0.25-2.0):(100-500):(2-6) FeCl3: C6H5PO(OH)2:NaOH:SDS:H2O:NH4OH. In a typical procedure, 7.49 g of phenylphosphonic acid (45 mmol, 95%, Wako Chem.) and 13.66 g of SDS (45 mmol, 95%, Wako Chem.) were dissolved in 100 g of H2O at 313 K under stirring for 5 min. This clear solution was then added to 24.82 g of FeCl3‚6H2O (90 mmol, 98%, Aldrich) in 60 g of H2O at room temperature under stirring for 10 min. Finally, 1.80 g of NaOH (45 mmol, Wako Chem.) in 30 g of H2O and 30 g of aqueous NH4OH (30%) were added to the solution and stirred for 1 h. The resulting homogeneous gel was transferred into a Teflon-lined stainless steel autoclave and heated at 453 K for 15 h. After cooling, the solid product was filtered, washed with distilled water, and dried at 373 K for 24 h. One gram of as-synthesized material was treated in 100 mL of ethanol and 2 mL of HCl (2 M) at 353 K for 4 h, filtered, and dried at 373 K for 24 h. This procedure was repeated to ensure complete removal of surfactant. Calcination of this hybrid iron oxophenyl phosphate was carried out at 773 K for 2 h to obtain pure iron oxophosphate. For the sulfonation of hybrid iron oxophenyl phosphate, 1 g of

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Figure 1. XRD profiles of mesoporous hybrid iron oxophenyl phosphate (a) and calcined iron oxophosphate (b).

hybrid iron oxophenyl phosphate was dried at 393 K under vacuum and placed in a 10 mL two-necked flask at 313 K under nitrogen. Then 3.3 g of fuming sulfuric acid (28%) was added to the flask and stirred for 4 h. After the mixture was cooled to room temperature, 50 mL of diethyl ether was added. The insoluble material was filtered and washed with distilled water several times to remove all unreacted fuming sulfuring acid and decomposed sulfuric acid. Then the solid was dried at 333 K for 12 h under vacuum. To evaluate the amount of acid sites (SO3H) in the sulfonated sample, 90 mg of dried sample was mixed with 50 mL of H2O and stirred for 20 h at room temperature. Finally, it was titrated against 0.01 M NaOH using an automatic potentiometric titratior AT-510 (Kyoto Electronic MFG Co. Ltd.). Elemental analyses of Fe, P, and Na in the samples were measured by using an ICP analyzer (Shimadzu ICPV-1017). C, H, and N analyses were also performed using a carbon, hydrogen, and nitrogen analyzer. Cation-exchange capacity of the mesoporous hybrid and calcined iron oxophosphate was measured by using ICP. The samples were characterized using low and wide angle X-ray diffractometer (Cu KR radiation, R ) 0.15406 nm, Shimadzu XRD-6000), transmission electron microscope (Hitachi H-9000NA TEM operated at 300 kV), and N2 sorption at 77 K with a Belsorp 28 instrument. Prior to N2 adsorption, samples were degassed for 2 h at 353 K. For the Fourier transform infrared (FT IR) measurement, a NICOLET MAGNA IR 750 was used. 1H-13C CP/MAS NMR and 31P MAS NMR spectra were recorded on a JEOL CMX-400 machine at 100.54 MHz for 1H-13C and 161.84 MHz for 31P with a spinning rate of 8 kHz, pulse time (P1) of 3.0 µs, and a repetition time (D1) of 30 s for 31P NMR and 5.0 s for 1H-13C CP/MAS NMR. The total number of scans was 1248 times for 1H-13C NMR and 16 times for 31P NMR. Chemical shifts for 13C and 31P were measured with reference to hexamethylbenzene and triphenylphosphine, respectively. Results and Discussion The elemental analyses data of Fe, P, S, Na, C, and H for the mesoporous samples were used for the determination of the formula of these novel hybrid and organic-free materials. The comprehensive molecular formula for the hybrid sample was [(FeO1.5)0.68(C6H5PO2)0.32(H)0.92]. In this formula excess of hydrogen (0.92 mol) was present; it could be due to the presence of H2O and OH groups. Similarly, in the case of calcined iron oxophosphate the observed formula was [(FeO1.5)0.68(PO3)0.32]. XRD profiles of mesoporous hybrid iron oxophenyl phosphate and calcined iron oxophosphate samples are shown in Figure 1. The XRD pattern of the hybrid sample indicated a peak at

Figure 2. N2 adsorption-desorption isotherms of mesoporous hybrid iron oxophenyl phosphate (a) and calcined iron oxophosphate (b). Inset: Pore size distribution curve from the adsorption branch of isotherms calculated using the BJH method. Table 1. Physicochemical Properties of Iron Oxophenyl Phosphate Materials sample

molar composition

hybrid 0.68:0.32 FeO1.5:C6H5PO2 calcined 0.68:0.32 FeO1.5:PO2.5 a

VP PPDa SBET d (nm) (m2 g-1) (cm3 g-1) (nm) 5.1 6.7

236 173

0.17 0.24

2.1 3.2

PPD: Peak pore diameter from the adsorption branch of isotherms.

5.1 nm. After calcination the mesoporous structure was retained. It is noteworthy that interplanar spacing was increased to 6.7 nm after calcination. Interplanar spacing of many different mesoporous metal (Ti, Al, Zr, Nb, Ce, La, Sn, etc.) phosphates synthesized by using triblock polymer as template,10 which ranged between 8 and 25 nm, also showed an increase in their respective spacings after calcination. From the XRD patterns it is clear that no long-range order was observed in either of the samples. One of the interesting features of our materials is that the phosphorus atoms are homogeneously distributed throughout the samples, not allowing the segregation of the metal oxide (Fe2O3) species into crystallites even after calcination at 973 K. This is evidenced from the fact that mesoporous iron oxophosphate obtained after calcination of the hybrid sample showed no peak between 10 and 80° 2θ. In Figure 2 the N2 adsorption-desorption isotherms of both hybrid and calcined samples are shown. In both cases, type IV isotherms, which is a characteristic of mesoporous materials, were observed. Pore size distribution curves showed that the peak pore diameter of the hybrid sample shifted from 2.1 to 3.2 nm after calcination at 773 K. BET specific surface area and pore volume (VP) of the hybrid sample are 236 m2 g-1 and 0.17 cm3 g-1, respectively, and those of the calcined sample are 173 m2 g-1 and 0.24 cm3 g-1, respectively. Thus, the calcination treatment decreased the specific surface area, while increasing the pore size and volume of this material. Phenyl groups in the hybrid sample have occupied the space inside the mesopores. Thus, when the hybrid iron oxophenyl phosphate sample was calcined at high temperature, the phenyl groups burned away. This caused stretching of the bonds, leading to the increase in both pore volume and pore size. In Table 1 the molar composition of both the hybrid and the calcined samples and their surface area, pore volume, and peak pore diameters are shown. In Figure 3, a TEM image of the mesoporous hybrid iron oxophenyl phosphate is shown. It has a disordered wormholelike structure.30 The TEM image of the calcined sample also showed it to be similar in nature to the as-synthesized sample. Pore dimensions estimated from these images agree well with

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Figure 3. TEM image of mesoporous hybrid iron oxophenyl phosphate.

Figure 4. 13C and 31P MAS NMR spectra of mesoporous hybrid iron oxophenyl phosphate (*: spinning sidebands).

Figure 6. Titration of 90 mg of sulfonated hybrid mesoporous Fe oxophenyl phosphate sample in 50 g of H2O against 0.01 M NaOH solution at 298 K.

suggested the presence of phenyl group and Fe-O-P bonding in the hybrid sample. The number of acid sites (SO3H) in sulfonated hybrid mesoporous iron oxophenyl phosphate, obtained by sulfonation of iron oxophenyl phosphate with fuming sulfuric acid, was estimated to be 1.03 mmol of acid per g of the hybrid sample. In Figure 6 a titration curve of the sulfonated hybrid mesoporous Fe oxophenyl phosphate sample against 0.01 M NaOH solution at 298 K is shown. Proton conductivity of this sulfonated sample was 1.0 × 10-2 S/cm at 100% relative humidity, which is nearly the same as that observed for Nafion 117.33 In the case of Nafion 117, the number of acid sites is 0.91 mequiv/g of the sample.33 In our hybrid sample the high proton conductivity is obviously due to the high number of acid sites present in the sample. The ordered pore structure might be responsible for this high conductivity as well. This sulfonated hybrid sample is a potential candidate for developing the membrane and the support of cathode and anode materials in fuel cell technology.34 Conclusions

Figure 5. FT-IR spectra of surfactant-extracted as-synthesized mesoporous hybrid iron oxophenyl phosphates before (a) and after calcination at 773 K (b).

that observed from the N2 sorption measurements. In Figure 4a a 13C MAS NMR spectrum of mesoporous hybrid iron oxophenyl phosphate is shown. Two resonances at 131.9 and 128 ppm due to the presence of a phenyl group31 were observed. Peaks at 18.4 and 15.4 ppm in the 31P NMR spectrum (Figure 4b) could be attributed to the [C6H5P(OH)O2] and [C6H5PO3], respectively, further confirming the presence of phenyl group in the hybrid sample.26 All phenyl groups are attached to the phosphorus atoms because no other band was present in the 31P MAS NMR spectrum. In Figure 5, FT-IR spectra of ethanolwashed hybrid iron oxophenyl phosphate and calcined samples are shown. These three bands at 749, 721, and 692 cm-1 serve as an identity of phenyl ring in iron oxophenyl phosphate.26 Other characteristic bands for the phenyl group is at 1440 [ν(CdC) aromatic], 3054 [ν(C-H) aromatic], and 1146, 1082 (sh), 1042, 1012, and 994 cm-1 [ν(P-O)]. After calcinations, peaks are present at 653 (Fe-O-Fe), 1048 (Fe-O-P), 1630 (H-O-H), and 3436 cm-1 (PO-H).32 Thus, the FT-IR spectra

We reported here a novel method for the preparation of mesoporous hybrid iron oxophenyl phosphate using phenyl phosphonic acid as a single source of phosphorus in the presence of anionic surfactant under hydrothermal conditions. The hybrid iron oxophenyl phosphate is capable of retaining its structure after calcination at 773 K to produce pure mesoporous iron oxophosphate. Sulfonated hybrid iron oxophenyl phosphate has a potential to be used as a membrane, support of anode and cathode materials in fuel cell. Furthermore, their application in shape-selective acid-base catalysis could be expected because of their high surface area and narrow pore size distribution. Acknowledgment One of us, N.K.M., is grateful to JST and JISTEC for his STA fellowship. We also thank Dr. S. Ichikawa for TEM measurement and analysis. A.B. wishes to thank DST, New Delhi, for a Ramanna Fellowship grant. Literature Cited (1) Li, D.; Zhou, H.; Honma, I. Design and Synthesis of Self-ordered Mesoporous Nanocomposite through Controlled in-situ Crystallization. Nat. Mater. 2004, 3, 65. (2) Bhaumik, A.; Inagaki, S. Mesoporous Titanium Phosphate Molecular Sieves with High Ion-exchange Capacity. J. Am. Chem. Soc. 2001, 123, 691.

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ReceiVed for reView May 17, 2006 ReVised manuscript receiVed August 30, 2006 Accepted September 16, 2006 IE060609U