Mesoporous materials with high surface areas,pore volume and ordered pore channels have attracted much interest in recent years because of their potential applications in adsorption,catalysis and biomedicine ^{[1, 2, 3, 4, 5]}. Nevertheless,the neutral framework of pure mesoporous silica materials limits their application in catalysis. For the purpose of catalytic applications,it is necessary to modify the nature of the amorphous walls by incorporation of hetero elements. Up to now,a series of heteroatom-doped (such as Al-,Ti-, Co- and Ni-) silicate materials have been synthesized ^{[6, 7, 8, 9]}. Titanium-silicates have received more and more attention due to their catalytic performance in selective oxidation reactions.

The discovery of TS-1 ^[10] in the 1980s was a major breakthrough in the synthesis of titanium-containing zeolite materials. However,the small pore size of TS-1 limits its application for oxidation reactions involving bulky molecules. Ti-containing mesoporous materials such as MCM-41,MCM-48, HSM-6 and SBA-15 have been reported,with high surface area, large pore size and good stability. Three-dimensions interconnected pore system materials such as FDU-1,MCM-48,AMS-8 and SBA-16 are usually favorable to one dimensional pore system materials due to the better pore accessibility and faster diffusion of guest molecules ^{[11, 12, 13, 14]}. Through a simple,two-step prehydrolysis method,Shen et al. successfully prepared highly ordered Ti- SBA-16 mesoporous silica with cubic Im3m structure,which exhibited high surface area and large pore volume ^[15].

SBA-1 mesoporous silica possesses a cage-type structure with 3-D interconnected pores ^{[16, 17]}. Li et al. prepared the 3d-cubic Pm3n Ti-incorporated SBA-1 mesoporous molecular sieves under acidic conditions. The catalytic activity of the synthesized Ti-SBA-1 material was used in epoxidation of styrene and the catalyst exhibited relatively high activity and selectivity ^[18]. Vinu et al. prepared Ti-SBA-1 materials and found that the amount of Ti content and the structure of the Ti-SBA-1 can easily be controlled by the simple adjustment of n_HCl/n_Si ratio ^[19].

Recently,hierarchically porous materials with well-defined morphologies have attracted much attention because of the textural characteristics such as hierarchically porous structure, high surface area and large pore volume ^{[20, 21, 22, 23]}. Hence,the preparation of the Ti-SBA-1 materials with hierarchically porous structure will benefit the catalytic performance of the Ti-SBA-1 materials. To the best of our knowledge,there are few reports on the synthesis of hierarchically mesoporous Ti-SBA-1.

Herein,hierarchically nanoporous Ti-SBA-1 with well-ordered mesostructure was firstly synthesized by employing anionic polymer PAA and cationic surfactant CPC mesomorphous complexes as template,titanium ethoxide as Ti source.

The pristine siliceous SBA-1 was synthesized based on the reported method ^[24]. The Ti-SBA-1 samples with different Si/Ti molar ratio were synthesized via one step method under the same procedure as SBA-1 by using titanium ethoxide as Ti source. For a typical preparation,0.54 g of CPC was dissolved in 25 mL of deionized water under stirring,and then 3.0 g of PAA solution was added under vigorous stirring at room temperature to obtain a clear solution. Next,4.0 g of ammonia solution (25%) was added to the above solution under vigorous stirring. After further stirring for 20 min,2.08 g of tetraethylsiloxane (TEOS) and certain amount of titanium ethoxide (with the TEOS/titanium ethoxide molar ratio of 100,50 and 20) were added. The mixture was stirred for 3 h,and finally transferred into an autoclave which was left at 80 ℃ oven for 2 days. The final product was centrifugated,washed with deionized water,and dried at 60 ℃. The organic templates were removed by calcination at 550 ℃ for 5 h. The samples were denoted as Ti-SBA-1-x (x indicates the molar ratio of Si/Ti in the initial solution).

Small-angle X-ray scattering (SAXS) experiments were performed on a Bruker Nanostar small angle X-ray scattering system. Wide angle X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Focus with Cu Ka radiation (40 kV,40 mA). Scanning electron microscopy (SEM) images were obtained with a Shimadzu SS-550 instrument. Transmission electron microscopy (TEM) observations were performed on a Philips Tecnai F20 microscope, working at 200 kV. N₂ adsorption measurements were performed on a BELSORP-mini II sorption analyzer. Before measurements,the samples were dried under dry N₂ flow at 350 ℃ for 5 h. Elemental analysis for Si/Ti ratios was performed with inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a Thermo Jarrell-Ash ICP-9000 (N+M). Diffuse reflectance UV-vis spectra were measured on a UV-vis NIR scanning spectrophotometer (Shimadzu 2450) with BaSO₄ as an internal standard.

Fig. 1 showed the small-angle X-ray scattering (SAXS) patterns of the calcined pure silica SBA-1 and Ti-SBA-1 samples. As shown in Fig. 1a,the pure silica sample showed three well-resolved diffraction peaks,which were indexed to the (2 0 0),(2 1 0) and (2 1 1) diffractions of the cubic Pm3n mesostructure of SBA-1, respectively. The SAXS patterns of the Ti-SBA-1-100 (Fig. 1b) and the Ti-SBA-1-50 (Fig. 1c) were similar with the SBA-1,which indicated that the three-dimensional cubic cage type structure of the SBA-1 was well retained after the incorporation of titanium into the silica framework. However,when the amounts of titanium was further increased,the intensity of the three well-resolved diffraction peaks in the SAXS patterns of the Ti-SBA-1-20 sample was decreased and the diffraction peaks became broader (Fig. 1d), indicating that the structural order of the Ti-SBA-1 decreased. Moreover,it was observed that the incorporation of titanium in silica framework caused a shift of all reflections to lower angle, which revealed an increase in the d-spacing of the samples (Table 1). The unit cell parameter of the Ti-SBA-1-20 material (12.52 nm) calculated by using the formula was larger than that of the SBA-1 (11.18 nm). From the wide angle XRD pattern of Ti-SBA-1-20 (Fig. 2),no noticeable wide-angle diffraction pattern was observed,suggesting that no occluded nano-TiO2 particle in the Ti-SBA-1-20 sample and their pore walls were amorphous in nature.

Fig. 1

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Fig. 1.SAXS patterns of the calcined samples: (a) SBA-1, (b) Ti-SBA-1-100, (c) Ti-SBA-1-50 and (d) Ti-SBA-1-20.

Fig. 2

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Fig. 2.Wide angle XRD pattern of Ti-SBA-1-20.

SEM and TEM were utilized to characterize the morphology and interior mesostructure of the synthesized Ti-SBA-1 sample by choosing Ti-SBA-1-20 as the typical example. The SEM and TEM images of the SBA-1 and Ti-SBA-1-20 sample were shown in Fig. 3. The Ti-SBA-1-20 was composed dispersed spherical particles with submicrometer-sized and the particle size was in the range of 200-400 nm (Fig. 3b),which was similar to that of the pristine silica SBA-1 (Fig. 3a). Fig. 3d showed the TEM image of the Ti-SBA- 1-20 submicrometer spheres,the mesopores of the Ti-SBA-1-20 could be observed. Moreover,the interstitial nanopores were clearly visible and it was interesting to see that the mesopores exhibited same alignment within the entire particle,exhibiting a single-crystal-like feature. This was consistent with the interior mesostructure of the SBA-1 (Fig. 3c). This result indicated that the incorporation of titanium did not significantly disturb the interior texture and pore structure of the SBA-1 and the morphology of SBA-1 remained.

Fig. 3

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Fig. 3.SEM and TEM images of the calcined samples: (a, c) SBA-1 and (b, d) Ti-SBA-1-20.

The nitrogen adsorption-desorption isotherms and pore size distribution curves of the SBA-1 and Ti-SBA-1 samples were shown in Fig. 4,and they all exhibited Type IV isotherms with three distinct adsorption steps at relative pressures (p/p₀) of 0.3-0.5, 0.75-0.95,and 0.95-0.99,respectively. The first step at the relative pressure of 0.3-0.5,corresponding to the cagelike mesopores of SBA-1,gives rise to the relatively narrow peaks (~3 nm) in the pore size distribution curves. The second step at p/p₀ of 0.75-0.95, corresponding to the secondary interstitial nanopores,as observed in the TEM images (Fig. 3c and d),gives rise to a broad pore size distribution centered at 20-40 nm. The third adsorption step at p/p₀ = 0.95-0.99,corresponding to a broad pore size distribution centered at about 100 nm,was ascribed to the aggregated voids between the particles. The specific surface area,pore volume and pore size of the SBA-1 and Ti-SBA-1 samples are listed in Table 1. As can be seen,with the increase of the Ti content,the specific surface area and pore size increased. The Ti-SBA-1-20 sample possessed the highest specific surface area of 576 m²/g and pore volume of 1.10 cm³/g.

Fig. 4

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Fig. 4.(A) Nitrogen adsorption–desorption isotherms and (B) pore size distribution curves of (a) SBA-1, (b) Ti-SBA-1-100, (c) Ti-SBA-1-50 and (d) Ti-SBA-1-20.

The contents of the elements Si and Ti of Ti-SBA-1 were measured by ICP-AES and the results were presented in Table 1. The Ti amount of the calcined Ti-SBA-1 was increased with the increasing the Ti amount in the initial solution. With the Si/Ti molar ratio in the synthesized solution decreased from 100 to 50 and further to 20,the Si/Ti molar ratio in the calcined Ti-SBA-1-100 sample decreased from 180 to 154 and further to 78.

The coordination of the Ti species incorporated in the silica framework was characterized by diffuse reflectance UV-vis spectroscopy. Fig. 5 showed the UV-vis spectra of the calcined Ti-SBA-1 samples. According to the literature ^{[18, 19, 25]},the adsorption band at about 210-230 nm was ascribed to tetrahedral coordinated Ti in the silica framework,which was usually used as the direct proof of titanium atoms incorporated into the silicate framework. The broad band at about 260 nm was assigned to octahedral coordinated Ti species. The band at 350 nm was ascribed to the anatase TiO₂. As can be seen from the Fig. 5,the presence of the peak at 215 nm and the 260 nm indicated that the incorporated Ti atoms of the Ti-SBA-1 samples mainly existed in the form of tetrahedral and octahedral coordination. Moreover, the peak intensity at 215 nm and the 260 nm was increased monotonically with the increase of the Ti content,suggesting that the amount of the tetrahedral and octahedral coordinated Ti species was increased simultaneously. The absence of adsorption band at 350-400 nm revealed that there was no anatase formed after calcinations,which was consistent with the result of the wide angle XRD characterization.

Fig. 5

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Fig. 5.UV–vis diffuse reflectance spectra of the calcined (a) Ti-SBA-1-100, (b) Ti- SBA-1-50 and (c) Ti-SBA-1-20.

Hierarchical mesoporous titanosilicate Ti-SBA-1 was successfully fabricated. By adjusting the Si/Ti molar ratio in the synthesis,a series of Ti-SBA-1 samples with well-ordered cubic Pm¯3n mesostructure were obtained. This kind of material exhibited large surface area and pore volume. The Ti species was present in dispersed state and mainly existed in the form of tetrahedral and octahedral coordination. The synthesized hierarchically nanoporous titanosilicate Ti-SBA-1 provide the potential possibility as catalysts in selective oxidation.

This work was supported by National Science Foundation of China (Nos. 21373116 and 21421001),Tianjin Natural Science Research Fund (No. 13JCYBJC18300),RFDP (No. 20120031110005), the Technology Planning Project of Hunan Province (No. 2014SK2019),National Science Foundation for Post-doctoral Scientists of China (No. 2014T70774) and the Scientific Research Fund of Hunan Provincial Education Department (No. 14C0343).