Facile template-free synthesis of meso-macroporous titanium phosphate with hierarchical pore structure

Microporous and Mesoporous Materials 100 (2007) 139–145 www.elsevier.com/locate/micromeso

Facile template-free synthesis of meso-macroporous titanium phosphate with hierarchical pore structure Hailong Fei, Xiaoquan Zhou, Huijing Zhou, Zhurui Shen, Pingchuan Sun, Zhongyong Yuan, Tiehong Chen * College of Chemistry, Department of Materials Chemistry, Key Laboratory of Functional Polymer Materials of MOE, Nankai University, Tianjin 300071, China Received 17 November 2005; received in revised form 12 October 2006; accepted 12 October 2006 Available online 30 November 2006

Abstract Meso-macroporous titanium phosphate materials with hierarchical structure have been synthesized through a facile and template-free method using titanium n-butoxide (TBT) as titanium source. Both the as-synthesized titanium phosphate and the sample after calcination at 500 °C are amorphous, as proved by the XRD measurements. The co-existence of mesopores and macropores is identified by N2 adsorption–desorption measurements, SEM and TEM methods. The formation of Ti–O–P bonds during the synthesis process was supported by FT-IR spectroscopy, and was further verified by 31P MAS NMR spectra and XPS measurements. The BET surface area of the products can be adjusted by addition of n-butanol. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Hierarchical; Titanium phosphate; Mesoporous; Macroporous; Synthesis

1. Introduction Since the discovery of M41S family of mesoporous silica [1], much attention has been paid to mesoporous materials, which have potential applications to separation, catalysis and adsorption due to their high surface areas, pore volumes and controllable pore sizes. Among those, titaniumbased materials are extensively studied because of their UV–visible photocatalytic and photovoltaic properties. Titanium phosphate has the potential to be used not only as ion-exchanging reagent for cations and anions [2,3], but also as catalysts for liquid-phase partial oxidation of cyclohexene by H2O2 [4], heterogeneous photocatalysis [5], photocatalytic decomposition of water for hydrogen generation when mixed with zirconium phosphate [6] and reduction of carbon dioxide. It has also been reported that

*

Corresponding author. Tel./fax: +86 22 23507975. E-mail address: [email protected] (T. Chen).

1387-1811/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.10.019

titanium phosphate mixed with porous titania possesses high electrical conductivity [7]. There have been studies on the synthesis of novel organically templated mixedvalence [8] and 2-D layered titanium phosphates [9]. Ordered porous titanium phosphate with relatively high surface area (and/or high surface-to-volume ratio) and controlled crystalline framework is of great significance for practical applications. Hexagonally packed titanium oxo-phosphate with supermicropores (pore size between 1 and 2 nm) has been synthesized by using alkyltrimethylammonium bromide as template [10]. Mesoporous titanium phosphates with high surface area have been developed by using different titanium sources and surfactants [11]. Mesoporous fluorophosphate with semicrystalline inorganic framework has been synthesized in the presence of alkyltrimethylammonium or alkylamine surfactants [12]. Recently, the uniform titanium phosphate nanotubes were prepared via a microemulsion-based solvo-thermal method [13]. Ultrathin Ti(HPO4)2 film was prepared by layer-bylayer adsorption and reaction of phosphate group and

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Ti(SO4)2 [14]. A promising solid acid was obtained via functionalizing MCM-41 with titanium phosphate group [15]. However, there are few reports about the synthesis of meso-macroporous titanium phosphate with a hierarchical pore system [16]. Nowadays, much attention has been paid to the fabrication of hierarchically structured materials with meso-macropores because the mesopores would be helpful to adsorption and desorption and the macropores would enhance diffusion of the reactants. Many meso-macroporous and bimodal mesoporous materials have been prepared by different methods [17–24]. Recently, Smarsly and co-workers reported a kind of hierarchical porous silica materials with a trimodal pore system using poly(styrene) (PS), poly(ethylene-co-butylene)-block-poly(ethylene oxide) (KLE), and an ‘‘ionic liquid’’ (1-hexadecyl-3-methylimidazolium-chloride) as templates [25]. Vantomme et al. reported high surface area zirconium oxide with threelength-scaled pore structure synthesized with cetyltrimethylammonium bromide (CTABr) as template [26]. Zhang and Yu reported hierarchically porous titania spheres synthesized by sonochemical approach in the presence of poly(ethylene glycol)-block-poly(propylene glycol)-blockpoly(ethylene glycol) (PO20EO70PO20) [27]. However, many of the hierarchically porous materials reported above require polymers or surfactants to act as templates in the syntheses. It remains a challenge to fabricate hierarchical meso-macroporous materials with controllable pore size and pore structure by a simple and template-free way [28]. Here, we report the synthesis of meso-macroporous titanium phosphate in the absence of any surfactants or polymers. Only titanium n-butoxide (TBT), phosphoric acid, H2O and n-butanol were employed in the synthesis process. The effect of n-butanol on the control of the pore size is also investigated. This method is facile and the synthesized material would have potential applications on ionexchange, catalysis and separation.

2.2. Characterizations X-ray diffraction (XRD) patterns were obtained on a Rigaku D/Max-2500 diffractometer using Cu Ka radiation. XPS measurements were performed with a Kratos Axis Ultra DLD spectrometer employing a monochromated Al-Ka X-ray source (hm = 1486.6 eV). A pass energy of 160 eV was used for recording survey spectra, while 40 eV pass energies were used for high-resolution measurements. The FT-IR spectra of all samples were recorded on a Bruker Vector 22 spectrometer. 31P NMR was carried out by a Varian Infinityplus 400 MHz NMR spectrometer, with the chemical shift referring to 85% H3PO4 at 0 ppm. N2 adsorption and desorption isotherms were obtained on a Micromeritics Tristar 3000 system at 77 K. Nitrogen pore volumes were determined at P/P0 = 0.993. The micropore volume was determined by the t-plot analysis. Pore size distribution was calculated from the adsorption isotherm according to the BJH (Barrett–Joyner–Halenda) method. Scanning electron microscopy (SEM) was performed on a Philips XL-20 at 15 keV. Transmission electron microscopy (TEM) was carried out on a Philips Tecnai F20 electron microscopy instrument. 3. Results and discussion 3.1. XRD analysis X-ray diffraction (XRD) was used to identify the crystalline-phase and meso-structural order of the samples. As an example, Fig. 1 displays X-ray diffraction patterns of the as-synthesized and calcined materials (samples 2-d and 2c). The absence of low angle peak in the range of 1–3°, which is characteristic of ordered mesoporous materials, indicates that there is no long-range order in the as-synthesized material. There are broad peaks in the range of 15–40° in XRD patterns for both the as-synthesized and calcined materials, which implies the materials have

Intensity (cps)

2. Experimental

In a typical procedure, the mixture of titanium n-butoxide (TBT) and n-butanol with certain molar ratio was added dropwise to 30 ml of 0.1 M phosphoric acid solution under stirring at room temperature. After further stirring for 2 h, the obtained mixture was transferred into a teflon-lined autoclave and aged statically at 80 °C for 24 h. The product was filtered, washed with water, dried at 60 °C for 12 h and calcined at 500 °C for 2 h. The molar ratio of n-butanol to TBT varied and series of samples were synthesized. All the samples were denoted as R-d or R-c, respectively, where R denotes as the molar ratio of n-butanol to TBT, d denotes the as-synthesized and dried sample, c denotes the calcined sample.

Intensity (cps)

2.1. Preparation of titanium phosphates

2

4 8 6 2 Theta (degrees)

10

500oC as-synthesized

10

20

30

40 50 2 Theta (degrees)

60

70

Fig. 1. X-ray diffraction patterns of sample 2-d and sample 2-c.

amorphous framework. Therefore, the titanium phosphate existed in amorphous form, which is consistent with the results reported by Bhaumik et al. who synthesized amorphous titanium phosphate with high anion-exchanging capability [3]. 3.2. The existing form of phosphorus Fig. 2 shows the high-resolution XPS spectra of O, Ti and P measured on the surface of the as-synthesized and calcined samples (2-d and 2-c). The XPS spectra of the O1s region (Fig. 2a and d) was composed of three peaks at 529.9, 532.4 and 531.0 eV for calcined material and 529.9, 532.7 and 531.1 eV for as-synthesized material, corresponding to the O in Ti–O bond of TiO2, the O in P–O–H and the O in Ti–O–P and P = O, respectively, according to literature [5,14]. The component of O in Ti–O–P bonds, whose proportion is 68.0% for the as-synthesized sample and 67.4% for the calcined one, is dominant. The proportion of O in Ti–O bond of TiO2 is 19% and 18% for as-synthesized and calcined samples, respectively. The Ti2p peaks include 464.6 and 458.8 eV (shown in Fig. 2b and e), indicating that Ti ion are in octahedral environment [5]. The P2p signals at 133.2 eV for the as-synthesized material and 133.3 eV for the calcined material, are identified as in pentavalent-oxidation state (P5+) (shown in Fig. 2c and f) [5,16]. There is no indication of the Ti–P bond due to the absence of the characteristic peak at 128.6 eV [29]. FT-IR was performed to find out whether phosphorus was incorporated and in which form it existed. Fig. 3 is the FT-IR spectra of the as-synthesized and calcined titanium phosphates (samples 2-d and 2-c). According to the literature [3], the broad band at 3405 cm 1 can mainly be

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Transmittance [%]

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a

b

4000

3500

3000

2500

2000

1500

1 000

500

Wavenumber (cm-1) Fig. 3. FT-IR spectra of sample 2-d (a) and sample 2-c (b).

attributed to –OH stretching vibration of the residual water. The wide band at 1034 cm 1 is characteristic of Ti–O–P framework vibration [14]. The band at 1625 cm 1 is the deformation vibration for H–O–H bonds of the physically adsorbed water and the weak and sharp peak appeared at 1383 cm 1 might be assigned to traces of PO4 3 or polyphosphoric acid bound to the surface of titanium phosphates [30]. On the whole, the FT-IR data supports that phosphorus was incorporated into the framework in the form of Ti–O–P bonds. 31 P MAS NMR spectra of titanium phosphates are shown in Fig. 4, and relatively wide peaks can be found between 20 and 40 ppm. Signals appear at 8.6 ppm for the as-synthesized material (Fig. 4a) and 12.1 ppm for the calcined material (Fig. 4b). As it has been reported that the chemical shifts of P atoms change toward low

Fig. 2. XPS spectra of sample 2-d (a, b and c) and sample 2-c (d, e and f).

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resonance frequency depending on the number of metal atoms bonded to PO4 units [31], the 8.6 and 12.1 ppm signal could be assigned to PO4 units with one or two Ti atoms as neighbors [P(OTi)x(OH)4 x] (x = 1 or 2) and tetrahedral phosphorus environments connected with three O–Ti bonds [P(OTi)3OH], respectively [16]. 3.3. N2 adsorption–desorption isotherms

Fig. 4.

31

P MAS NMR spectra of sample 2-d (a) and sample 2-c (b).

N2 adsorption–desorption measurements were utilized to study the porous characteristics of both as-synthesized and calcined samples. The N2 adsorption–desorption isotherms and Barrett–Joyner–Halenda (BJH) pore size distribution (PSD) curves are shown in Figs. 5 and 6. All isotherms are type IV with hysteresis loops, indicating the existence of mesopores. In the PSD curves of the as-synthesized samples, there appeared two maximums whose values are listed in Table 1, however due to the broadness of the

Fig. 5. N2 adsorption–desorption isotherms (left) and BJH pore size distribution curves (right) of the as-synthesized samples with different molar ratios of n-butanol to TBT, as indicated in the figures.

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Fig. 6. N2 adsorption–desorption isotherms (left) and BJH pore size distribution curves (right) of calcined samples with various ratios of n-butanol to TBT as indicated in the figures.

Table 1 Textural characteristics of as-prepared and calcined titanium phosphate materials synthesized with different molar ratios of n-butanol to TBT Samples

2-d 11.5-d 27.5-d 2-c 11.5-c 27.5-c

BET surface area (m2/g)

Total pore volume (cm3/g)

Average pore size (nm)

Micropore volume (cm3/g)

315 391 436 218 156 188

0.576 0.760 0.984 0.541 0.395 0.584

2.6, 6.3 2.4, 6.2 4.5, 6.7 8.3 8.3 9.0

0.010 0.017 0.016 0.014 0.005 0.006

peaks they could not be well separated, thus those as-synthesized samples could not be regarded as bimodal mesoporous. There appears only one peak in the PSD curves of the calcined samples. Although the adsorption–desorption isotherms and PSD curves are similar regardless of the variation of the molar ratios of n-butanol to TBT, the BET surface areas vary obviously depending on the ratios (Table 1). This could be explained by the control

of n-butanol on the hydrolysis speed of TBT in the solution during aging process.

3.4. Morphologies of the materials The morphology of the as-synthesized products was characterized by scanning electron microscopy (SEM). Typical morphology of as-synthesized material with the molar ratio of n-butanol/TBT = 2 is displayed in Fig. 7a, showing blocks with non-uniform macropores. The magnified SEM image (Fig. 7b) clearly reveals that these macropores are formed by the fused blocks, which can be explained by micro-phase separation mechanism [32]. Transmission electronic spectroscopy (TEM) was also performed to characterize the as-synthesized and calcined materials (n-butanol/TBT = 2). As displayed in Fig. 8a and b, the as-synthesized sample consists of cross-linked particles of 200–400 nm. Each particle is mesoporous and the space between the cross-linked particles corresponds to macropores. For the calcined sample (Fig. 8c and d)

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Fig. 7. SEM images of sample 2-d.

Fig. 8. TEM images of sample 2-d (a, b) and sample 2-c (c, d).

the mesopores are more clearly shown, in accord with the larger pore size from the N2 adsorption measurements. The formation of the meso-macroporous structures is due to the hydrolysis of TBT in the phosphoric acid solution and the solvo-treatment. Nano-sized titanium phosphate particles would form under the different hydrolysis rate controlled by the addition of n-butanol. The accumulation of nano-particles will result in mesopores, as demonstrated by TEM images. Meanwhile, micro-phase separation regions of titanium phosphate nanoparticles and water/n-butanol forms, which gives rise to macropo-

rous structures, as described in the literature [32]. The solvo-thermal treatment could also play a decisive role in controlling the textural properties of titanium phosphate as it would enhance the micro-phase separation and promote accumulation of the nano-particles, as have been proved previously [33–35]. 4. Conclusion Meso-macroporous titanium phosphate with a hierarchical pore system was prepared via controlling the speed

H. Fei et al. / Microporous and Mesoporous Materials 100 (2007) 139–145

of the hydrolysis and condensation rates of the titanium butoxide in the presence of n-butanol and phosphoric acid. BET surface of the samples could be adjusted by the ratio of n-butanol/TBT. The synthesis method is facile and template-free and this kind of titanium phosphates with hierarchical pores would be promising materials for catalysis, separation and material science. Acknowledgments This work was supported by the National Science Foundation of China (Grant Nos. 20373029 and 20233030), Joint-Research Foundation of Nankai University and Tianjin University from the Chinese Ministry of Education and the Foundation for University Key Teacher by the Chinese Ministry of Education (GG-703-10055-1008). References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Breck, Nature 359 (1992) 710. [2] B.B. Sahu, K. Parida, J. Colloid Interface Sci. 248 (2002) 221. [3] A. Bhaumik, S. Inagaki, J. Am. Chem. Soc. 123 (2001) 691. [4] A. Bhaumik, Proc. Indian Acad. Sci. 114 (2002) 451. [5] J.C. Yu, L.Z. Zhang, Z. Zheng, J.C. Zhao, Chem. Mater. 15 (2003) 2280. [6] P.K. Mahendra, S. Inagaki, H. Yoshida, J. Phys. Chem. B 109 (2005) 9231. [7] T. Tsuru, Y. Yagi, Y. Kinoshita, M. Asada, Solid State Ionics 158 (2003) 343. [8] Y.N. Zhao, G.S. Zhu, X.L. Jiao, W. Liu, W.Q. Pang, J. Mater. Chem. 10 (2000) 463. [9] S. Ekambaram, C. Serre, G. Fe´rey, S.C. Sevov, Chem. Mater. 12 (2000) 444. [10] J. Blanchard, F. Schu¨th, P. Trens, M. Hudson, Micropor. Mesopor. Mater. 39 (2000) 163. [11] D.J. Jones, G. Aptel, M. Brandhorst, M. Jacquin, J. Jime´nez-Jime´nez, A. Jime´nez-Lo´pez, P. Maireles-Torres, I. Piwonski, E. Rodrı´guezCastello´n, J. Zajaca, J. Rozie`re, J. Mater. Chem. 10 (2000) 1957.

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