Novel organic–inorganic hybrid and organic-free mesoporous niobium oxophosphate synthesized in the presence of an anionic surfactant

Microporous and Mesoporous Materials 93 (2006) 40–45 www.elsevier.com/locate/micromeso

Novel organic–inorganic hybrid and organic-free mesoporous niobium oxophosphate synthesized in the presence of an anionic surfactant Nawal Kishor Mal a, Asim Bhaumik

b,*

, Masahiro Fujiwara

a,*

, Masahiko Matsukata

c

a b

Kansai Center, National Institute of Advanced Industrial Science and Technology (AIST-Kansai), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan Department of Materials Science, Indian Association for the Cultivation of Science, 2A and B Raja S.C. Mullick Road, Jadavpur, Kolkata 700 032, India c Department of Applied Chemistry, Waseda University, 3-4-1 Okubo, Tokyo 169-8555, Japan Received 6 October 2005; received in revised form 30 January 2006; accepted 1 February 2006 Available online 20 March 2006

Abstract A novel organic–inorganic hybrid mesoporous niobium oxophenylphosphate was synthesized by using supramolecular assembly of sodium dodecylsulfate molecules. X-ray diffraction, transmission electron microscopic image and N2 adsorption data indicated the formation of a wormhole-like disordered mesostructure in the sample. The 13C and 31P MAS NMR, FTIR, UV–visible spectroscopic data and chemical analysis results indicated that all P atoms are attached to phenyl groups directly and these are combined with Nb atoms through O atoms. Calcination of this hybrid material yielded an organic-free mesoporous niobium oxophosphate material. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Mesoporous materials; Nanostructures; Hydrothermal synthesis; Organic–inorganic hybrid composites; Niobium phosphates

1. Introduction Zeolites and related microporous and mesoporous materials [1] are one of the most studied inorganic materials known till date because of their interesting 2D and 3D open framework structures and widespread applications as ion-exchanger, adsorbent, support, film, acid–base, oxidation catalysts, etc. Among these, phosphate-based molecular sieves [2,3] have attracted considerable attention from academia and industry because of their high potential to various applications, especially in biological utilizations [4]. Although varieties of pure and binary mesoporous metal phosphates such as Ti [5], Al [6], Zr [7], Nb [8], Sn [9], In, W, Ce, V [10], etc. have been reported, only few phosphate based hybrid materials have been used in catalysis [5,11,12]. One of the major drawbacks of these phosphate-based mesoporous materials is due to the presence of excessive surface defect P–OH groups, which could over*

Corresponding authors. E-mail addresses: [email protected] (A. Bhaumik), [email protected] (M. Fujiwara). 1387-1811/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.02.001

come by surface organic modifications. The syntheses of organic–inorganic hybrid mesoporous silica materials have attracted widespread attention in this context [13–16]. These functionalized mesoporous materials are of great interest due to their range of potential applications in electronics, optics, catalysis, adsorption, etc. These hybrid materials having mesoporous structures with exceptionally high surface area allow the binding of a large number of surface chemical moieties leading to their unique properties associated with the surface functional groups. We have studied the preparation of various kinds of metal phosphate [8,17] and oxophenylphosphate materials [18]. As a consequence of these works, we have succeeded in the syntheses of microporous [8] and mesoporous [19] niobium phosphates, independently, by using neutral and cationic surfactants, respectively. However, synthesis of hybrid mesoporous niobium phosphate has not yet been reported to the best of our knowledge. Presence of phenyl group in the hybrid mesoporous material [20] may have many advantages like (a) hydrophobicity of the material could be increased, (b) after the sulfonation on the mesoporous niobium oxophenylphosphate, acid sites (SO3H) could be

N.K. Mal et al. / Microporous and Mesoporous Materials 93 (2006) 40–45

generated, which can be used as (c) proton conductor in anode or cathode catalyst, and as membrane in fuel cell technology, (d) shape selective acid catalysis is also expected because this material possesses definite pore size, which will allow the passage of molecules smaller than its pore size. In this paper, we wish to report the synthesis of mesoporous hybrid (organically modified) niobium oxophenylphosphate by using phenylphosphonic acid as the source of phosphorus in the presence of an anionic surfactant sodium dodecylsulfate (SDS). Calcination of this hybrid niobium oxophenylphosphonate material in air produced an organic-free pure mesoporous niobium oxophosphate. Detailed characterizations were performed to understand the nature of bonding and surface properties of these novel mesoporous materials. 2. Experimental In a typical synthesis, 1.25 g of phenylphosphonic acid (7.5 mmol, 95%, Wako Chem.) and 2.28 g of SDS (7.5 mmol, 95%, Wako Chem.) were dissolve in 20 g of H2O at 40 °C under stirring for 5 min. This clear solution was then added to a solution consisting of 4.09 g of NbCl5 (15 mmol, 99%, Aldrich Chem.) in 11 g of ethanol at room temperature under stirring for 10 min. Finally, 0.30 g of NaOH in 10 g of H2O and 5.53 g of aqueous NH4OH (30%) were added to the above mixture and stirred for 50 min. The resulting homogeneous gel was transferred into a Teflon lined stainless steel autoclave and heated at 180 °C for 15 h. After cooling (pH = 10.0), the solid product was filtered, washed with distilled water and dried at 100 °C for 1 D. Molar compositions of the gels were 1.0NbCl5:(0.15–1.0)C6H5PO(OH)2:(0.15–1.0)NaOH:(0.1– 1.0)SDS:(50–250)H2O:(3.0–10.0)NH4OH:(10–20)C2H5OH. One gram of as-synthesized material was extracted with 100 ml of ethanol and 4 ml of HCl (2 M) at 353 K for 6 h, filtered and dried at 373 K for 1 D. This procedure was repeated again to ensure complete removal of surfactant. Calcination of this hybrid niobium oxophenylphosphate was carried out at 773 K in air for 3 h to obtain pure niobium oxophosphate. Elemental analyses of Nb, P and Na in the samples were carried out using an ICP analyzer (Shimadzu ICPV-1017). C, H and N analyses were also performed using a carbon hydrogen and nitrogen analyzer. Cation exchange capacities of the mesoporous hybrid and calcined niobium oxophosphates were measured by using ICP. The samples were characterized using XRD (CuKa radiation, a = 0.15406 nm, Shimadzu XRD-6000), TEM (Hitachi H-9000NA, 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 (FTIR) 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 ls and a rep-

41

etition time (D1) of 30 s for 31P NMR and 5.0 s for 1H–13C CP/MAS NMR. The total number of scans were 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. Cation exchange capacities of the mesoporous hybrid and calcined niobium oxophosphate were measured by treating 1 g of sample (dried at 393 K under vacuum) with 500 ml of aqueous NH4NO3 solution (1 M) at 353 K for 12 h, then filtered and washed. The filtrate was collected in a measuring flask. This step was repeated and the collected solutions were analyzed using ICP to measure the amount of sodium ions. For sulfonation 1.00 g of ammonium exchanged hybrid niobium oxophenylphosphate was dried at 393 K under vacuum and placed in a 10 ml two necked flask at 313 K in nitrogen atmosphere. Four grams of fuming sulfuric acid (28%) was added and stirred for 4 h. After cooling to room temperature, 50 ml of diethyl ether was added and filtered. Finally, the solid mass was washed several times with distilled water to remove all un-reacted fuming sulfuric acid and decomposed sulfuric acid products. Then the solid was dried at 353 K for 12 h under vacuum. To evaluate the amount of acid sites (SO3H) in the sulfonated sample, 30 mg of the dried sample was suspended in 50 g of H2O and stirred for 15 h at room temperature. Finally, it was titrated against 0.01 M NaOH using an automatic potentiometric titration AT-510. For proton conductivity measurements the samples were made into the pallets of 4 mm diameter and 0.3 mm thickness by using the cold-pressing technique at pressure of 100 MPa for 2 min. Silver paste was applied on both sides of the pellet as ionic blocking electrodes. Conductivity was determined by the ac impedance method using a PC controlled SI 1260 impedance/gain-phase analyzer (Solarton) at 298 K. Impedance spectroscopic (IS) measurement was performed over the frequency range of 1 Hz–10 MHz at the oscillator amplitude of 10 mV. Proton conductivity was measured between 0% and 100% relative humidity. 3. Results and discussion XRD patterns of the hybrid mesoporous niobium oxophenylphosphate and the niobium oxophosphate samples are shown in Fig. 1. Inter-planar spacing d100 for the hybrid mesoporous niobium oxophenylphosphate was 5.0 nm, whereas after calcination at 773 K this spacing has been increased to 5.22 nm with improvement of ordering. This could be due to condensation of Nb–OH and C6H5P–OH groups. The transmission electron microscope (TEM) image of as-synthesized hybrid niobium oxophenylphosphate is shown in Fig. 2 indicated that it has uniform wormhole-like structure [21]. The TEM image of calcined sample is also similar in nature to the as-synthesized sample. The hybrid and the calcined mesoporous niobium oxophosphates had the type IV of isotherms, which are characteristics [2–10] of the presence of mesopores as shown in

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N.K. Mal et al. / Microporous and Mesoporous Materials 93 (2006) 40–45 1.0

150

10000

5.22 nm

b -1 -1

Dv(d) (cm nm g )

a

100

Intensity (a.u.)

6000

0.6

3

3 -1

V (cm g )

8000

0.8

50

close: adsorption open: desorption

0.4

b 0.2

5.00 nm 4000

0

0.0

0.0

0.2

0.4

0.6

0.8

0

P/P

A b

a

1.0

5

B

10

15

20

Pore size (nm)

Fig. 3. N2 adsorption–desorption isotherms (A) and pore size distributions from adsorption branch of isotherms calculated using BJH method (B) of the mesoporous hybrid niobium oxophenylphosphate (a) and mesoporous niobium oxophosphate after calcination at 773 K (b).

2000

a 0 2

4

6

8

2 theta Fig. 1. XRD profiles of mesoporous hybrid niobium oxophenylphosphate (a) and mesoporous niobium oxophosphate after calcination at 773 K (b).

Table 1 Physico-chemical properties of hybrid niobium oxophenylphosphonate Sample

d (nm)

SBET (m2 g 1)

VP (cm3 g 1)

PPDa (nm)

Na+ IECb (mmol g 1)

Hybrid Calcined

5.00 5.22

257 168

0.128 0.168

2.12 2.32

4.16 4.47

a b

Peak pore diameter from adsorption branch of isotherms. Na+ ion exchange capacity.

1

Fig. 2. TEM image of mesoporous hybrid niobium oxophenylphosphate.

Fig. 3. They possessed narrow pore size distributions as shown in Fig. 3B. Since the BJH method usually underestimate the pore size diameter smaller than 5 nm, therefore, pore size distribution curves indicated that the pore size of hybrid and calcined samples are bigger than 2 nm and probably closer to 3 nm. The physico-chemical properties for both the samples estimated from the N2 adsorption–desorption isotherms are summarized in Table 1. Surface area and pore volume of the hybrid sample were 257 m2 g 1 and 0.128 cm3 g 1, respectively. Surface area of the sample after calcinations was decreased to 168 m2 g 1, whereas the pore volume was increased to 0.168 cm3 g 1.

H–13C CP/MAS NMR and 31P MAS NMR spectra of the hybrid and the calcined mesoporous niobium oxophosphates are shown in Fig. 4. The hybrid sample exhibited two resonances at 131.6 and 128.0 ppm in 1H–13C CP/ MAS NMR spectrum due to the presence of phenyl group [22]. Absence of peak at 30 ppm in 1H–13C CP/MAS NMR indicates that all surfactants were removed during extraction. In 31P MAS NMR spectrum two peaks were observed at 18.5 [C6H5P(OH)O2] and 14.9 (C6H5PO3) ppm, further confirming the presence of phenyl group. Absence of any other band in this spectrum revealed that all phosphorus atoms are attached to the phenyl groups. Two resonance at 21.1 and 24.8 ppm after calcination indicate the formation of phosphate species, PO3(OH) and PO4, respectively in organic-free mesoporous niobium oxophosphate [8]. UV–visible absorption spectrum of the hybrid mesoporous Nb oxophenylphosphate shows two bands at 277 and 315 nm (Fig. 5). This could be assigned to the octahedral coordination of Nb. Mesoporous hexagonal niobium phosphate exhibited single band at 263 nm, where as mesoporous niobium oxide showed single band at 308 nm [9]. This assignment confirms that UV bands at 277 and 315 nm should be assigned to Nb–OPC6H5 and Nb–O–Nb units, respectively. No band was present below 250 nm confirming the absence of tetrahedral coordinated Nb species. The FTIR spectra of ethanol washed mesoporous hybrid niobium oxophenylphosphate (a) and calcined mesoporous

N.K. Mal et al. / Microporous and Mesoporous Materials 93 (2006) 40–45

136.6 ppm

128 ppm

20

160

120

Transmittance % (a.u.)

15

200

80

(a) Chemical shift (ppm)

185 ppm. [C6H5P(OH)2]

43

14.9 ppm (C6H5PO3)

a

*

b

*

10

5

*

*

0 50

(b)

0

-50

4000 3500 3000 2500 2000 1500 1000

Chemical shift (ppm)

500

-1

Wavenumber (cm ) - 21.1 ppm -24.8 ppm

*

Fig. 6. FTIR spectra of surfactant extracted as-synthesized mesoporous hybrid niobium oxophosphates before (a) and after calcination at 773 K (b).

*

-100

-50

0

50

(c) Chemical shift (ppm) Fig. 4. 1H–13C MAS NMR spectrum of mesoporous hybrid niobium oxophenyl phosphate (a), 31P MAS NMR spectra of hybrid niobium oxophenylphosphate (b) and calcined niobium oxophosphate (c).

Nb-O-PC6H 5 315 Nb-O-Nb

Absorption (a.u.)

277

200

300

400

500

600

700

800

Wavelength (nm) Fig. 5. UV–visible spectrum of ethanol washed mesoporous hybrid niobium oxophenylphosphate.

niobium oxophosphate (b) are presented in Fig. 6. These FTIR spectra of the mesoporous hybrid and the calcined

niobium oxophosphates further confirm the presence of phenyl group and Nb–O–P bond. Three bands at 750, 722 and 694 cm 1 served as an identity of phenyl group in niobium oxophenylphosphate [18]. Other characteristic bands for phenyl group are at 1440 (m(C@C) aromatic), 3055 (m(C–H) aromatic) and at 1148, 1083 (sh), 1044, 1012 and 994 cm 1 (m(P–O)) [17]. Presence of peak at 2900 cm 1 indicates that the trace amount of impurities for aliphatic hydrocarbon is still present in the solvent extracted sample. After calcination, the observed peaks at 657, 1051, 1631 and 3434 cm 1 were assigned as for Nb– O–Nb, Nb–O–P, H–O–H and O–H, respectively. The elemental analyses data of Nb, P, S, Na, C and H in the mesoporous sample could be used for the determination of the formula of this novel hybrid material. The weight of oxygen was calculated by subtracting the weight of all these elements from hundred. In addition, the ionexchange capacity measurement for Na+ ions was applied to confirm its presence outside the framework [23]. For example, in case of mesoporous hybrid Nb oxophenylphosphate, the observed wt.% of Nb, P, C, H and Na were found to be 48.15, 4.27, 9.92, 0.72 and 8.75 wt.%, respectively. Rest of the amount (i.e. 28.19 wt.%) was considered as weight of oxygen. When these values of wt.% were divided by their respective atomic weights and normalized (Nb + P = 1), the observed molecular formula was Nb0.79P0.21C1.26O2.686H1.1Na0.58. It was re-arranged to deduce a comprehensive molecular formula, Na0.58Nb0.79O2.055(O3PC6H5)0.21. In this case, sodium ion exchange capacity observed was 0.096 g Na per 1 g sample (4.16 mmol g 1), which is equivalent to 0.64 Na according to the proposed formula. In the case of calcined niobium oxophosphate, observed (experimental) weight for each

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N.K. Mal et al. / Microporous and Mesoporous Materials 93 (2006) 40–45

elements, Nb, P and Na atoms were found to be 53.23, 4.72 and 9.67 wt.%, respectively. Rest 32.38 wt.% was considered as weight of oxygen. The finally deduced formula was Nb0.79P0.21O2.79Na0.58. When the sodium ion exchange capacity was independently measured, the value observed was 0.103 g Na per 1 g sample (4.47 mmol g 1), which is equivalent to 0.62 Na in proposed formula. Thus the ion exchange capacity measurement agreed well with the estimated molecular formula for these mesoporous materials. The ion exchange capacity measurement confirms that all the Na+ ions are present outside the framework. In this formula, Nb is octahedrally coordinated as NbO3 to C6 H5 PO23 (hybrid sample) or PO34 (calcined sample), whereas phosphorus in C6 H5 PO23 is tetrahedrally coordinated to three Nb in hybrid sample and PO34 is attached to four Nb in calcined sample. Nb/P molar ratio in hybrid and calcined products was 3.76. Recently, TiO2/P2O5 molar ratio up to 4 was reported for a series of mesoporous titanium phosphates [5]. High thermal stability of mesoporous hybrid and calcined niobium oxophosphate is probably due to higher pore wall thickness. Titration curve for the acid sites (SO3H) in sulfonated hybrid mesoporous Nb oxophenylphosphate is shown in Fig. 7. The number of acid site was 0.81 mmol acid per 1 g of the sample. Fifty seven percentage of phenyl group was sulfonated in the hybrid material. Proton conductivity of the sample was nearly 10 2 S/cm at 100% relative humidity, which is nearly same as observed for Nafion 117 [24]. In case of Nafion 117, number of acid sites is 1 mmol per 1 g of the sample [24]. This sulfonated hybrid sample is a potential candidate for developing membrane and could be used as a support for cathode and anode

material in fuel-cell technology. Thus the sulfonated hybrid niobium oxophosphate is a good bi-functional catalyst and this could be used both as electron conductor and proton conductor, where as sulfonated silica derivative is proton conductor only. 4. Conclusions Here we reported a novel method for the preparation of mesoporous hybrid niobium oxophenylphosphate using phenylphosphonic acid as a single source of phosphorus in the presence of sodium dodecyl sulfate as SDA. The hybrid niobium oxophenylphosphate retained its structure even after calcination at 773 K to produce pure mesoporous niobium phosphate. Sulfonated hybrid niobium oxophosphate has the potential to be used as membrane, support for anode and cathode material in fuel cell. In addition to this, their use in shape-selective acid–base catalysis is expected due to their high surface area and narrow pore size distributions, which only allows the passage of molecules smaller than its pore size. Acknowledgments N.K.M. is grateful to JST and JISTEC for his STA fellowship. A.B. wishes to thank DST and CSIR, New Delhi for their extramural project grants. We also thank Dr. S. Ichikawa (AIST Kansai) for TEM measurement and analysis. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.micromeso. 2006.02.001.

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References 8

pH

6

4

2

0 0

2

4

6

8

10

Volume of NaOH (ml) Fig. 7. Titration of 30 mg of sulfonated hybrid mesoporous Nb oxophenylphosphate sample in 50 g of H2O against 0.01 M NaOH solution at 298 K.

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