XIAO Xue-Qing SUN Qin-Ping LIU Fng

ZHENG Yinga② LIN Jin-Huoaa (Fujian Key Laboratory of Polymer Materials,Fujian Normal University, Fuzhou 350007, China)b (College of Material Science and Engineering,Technology of Fujian University, Fuzhou 350108, China)

1 INTRODUCTION

Porous alumina has drawn much attention for various availabilities in the field of catalysis. There are numerous methods to obtain porous alumina, for example, hydrothermal, precipitation, semi sol-gel and evaporation-induced self-assembly with block co-polymers[1-5]. Especially, ordered mesoporous alumina (OMA) with much higher surface area and thermal stability has been applied in adsorption and catalysis[6-9]. Niesz et al. first used block co-polymers as the soft template to synthesize OMA successfully, which opens up a simple approach to synthesize OMA material. However, the experiments should be carried out under controlled conditions[10].A breakthrough in the synthesis of OMA has been achieved by adding citric or nitric acid as the pH regulator reported by Yuan and co-workers, ignoring the above extreme operating conditions[11]. Depending on such a significant achievement to prepare well-defined OMA, phthalic acid or p-aminobenzoic acid as a single interfacial protector, and mixed interfacial protectors such as citric and salicylic acids were utilized to protect the self-clustering of aluminum ions and avoid the interference from the chloride ions in our previous research work[12-14].The obtained materials possess very ordered mesoporous structure, high surface area and high thermal stability.

Now that, OMA material can be prepared through a facile and repeatable method, which has aroused extensive interest in obtaining OMA supported metal oxides with desirable properties[15-18]. Titanium dioxide (TiO2) is also an ideal candidate for its chemical stability, non-toxicity and numerous applications in photocatalysis and catalyst support[19-21]. Titanium-doped alumina material could not only modify the low activity and thermal stability of the alumina carrier but also overcome the shortcomings of TiO2,like limited thermal stability, low surface area and unsuitable mechanical properties[22-26]. Based on this,Morris et al. reported one-pot synthesis of aluminasupported metal oxides in the presence of nitric acid,including alumina-supported TiO2. The resultant materials had well-developed mesoporosity, relatively high surface area and crystalline pore walls[15,27].Then the effect of nitric acid concentration on the pore size in alumina-supported TiO2was also discussed in detail[28]. However, the existent form of titanium and the role of TiO2playing in the thermal stability of alumina supporter remain to investigate further.

In this work, titanium-doped OMA (Ti-OMA)sample has been synthesized by the evaporation induced self-assembly sol-gel method in the presence of citric and salicylic acids as the mixed interfacial protectors, using tri-block copolymer P123,tetrabutyl titanate and aluminum isopropoxide as the template, titanium source and alumina precursor,respectively. The products with highly ordered hexagonal mesostructure can be synthesized with different titanium loadings. The influences of dopant on the structure and thermal stability of OMA material are discussed. And the form of titanium existing in the OMA material is also studied in more details.

2 EXPERIMENTAL

2.1 Preparation of titanium-doped OMA material

All the chemicals used in this study were Grade AR. Tri-block copolymer P123 (EO20PO70EO20, EO= ethylene oxide, PO = propylene oxide) was used as the soft template. Approximate 1.00 g of P123 was dissolved in 20 mL absolute ethanol, followed by adding 1.0 mL 35 wt% hydrochloric acid under vigorous stirring to form a mixed solution. Then tetrabutyl titanate and aluminum isopropoxide were successively added until the mixed interfacial protectors (citric and salicylic acids) were completely dissolved, in which the molar ratio of salicylic to citric acid is controlled as 2. After stirring vigorously for 5 h at ambient temperature and then aging at 65 ℃ to form the xerogel, the resultant samples were calcined at 500 ℃ for 4 h, then further calcined at 700, 800 and 900 ℃ for 1 h, respectively.

2.2 Characterization

For convenience, the obtained titanium-doped OMA materials were named as Ti-OMA-m, where m is the molar ratio of aluminum to titanium(n(Al)/n(Ti)), and computed by the X-ray fluorescence (XRF) spectrometry. The results showed that the chemical components of the prepared samples were Ti-OMA-19.4, Ti-OMA-13.3, Ti-OMA-10.2,Ti-OMA-7.6, Ti-OMA-5.4 and Ti-OMA-3.8,respectively.

An X-ray diffractometer (XRD) was operated on a Phillips X'Pert SUPER powder with CuKα radiation(λ = 1.5418 ?) from 0.5° to 4.0° (small angle) and 20.0° to 75.0° (wide angle). The microcosmic appearance in the samples was observed by a Tecnai G2 F20 S-TWIN transmission electron microscope(TEM). Nitrogen adsorption/desorption isotherms were used to calculate the specific surface areas(SBET) based on the Brunauer-Emmett-Teller (BET)method using the Quanta Chrome Nova 4200 equipment. The pore volume (Pv) and pore diameter size distribution (PD) were derived from nitrogen desorption isotherms via the Barrett-Joyner-Halenda (BJH)method. Thermal stability associated with phase transformation was investigated by differential ther-mal analysis (DTA) on a Perkin-Elmer 1700 apparatus. The samples were heated in a 20 mL min-1dried air from room temperature up to 1200 ℃ at a rate of 5 ℃ min-1, using α-Al2O3as a reference.UV-vis diffuse reflectance spectra (DRS) were operated on a Perkin Elmer Lambda 950 in the range of 200～800 nm, with BaSO4as the reference compound. An RRNISHAW inVia Raman Microscope with the laser radiation (λexc= 532 nm) was operated to testify the presence of TiO2phase.

3 RESULTS AND DISCUSSION

The small- and wide-angle XRD patterns for Ti-OMA and OMA samples with different molar ratios of n(Al)/n(Ti) calcined at 500 and 900 ℃ are shown in Fig. 1. In the small-angle patterns calcined at 500 ℃ (Fig. 1A), the (100) diffraction peak is observed over the n(Al)/n(Ti) range of 19.4～3.8,suggesting the appearance of p6mm hexagonal symmetry in Ti-OMA samples according with the TEM observation. With the increasing addition of titanium, the intensity of the (100) diffraction peak enhances first and then weakens, while the (110) and(220) reflection peaks disappear. This phenomenon shows that excessive titanium can not be fully incorporated into the OMA material and may block the mesoporous structure to some extent, leading to a decline in the long-range order of the corresponding specimens. There is no evident diffraction peak observed in the OMA sample calcined at 900 ℃, suggesting the ordered structure has been destroyed to some extent. While for Ti-OMA-13.3 and Ti-OMA-10.2 samples, the (100) diffraction peak is reserved even calcined at 900 ℃ (Fig. 1B),indicating some ordered mesoporosity remains in the corresponding samples. And the Ti-OMA-10.2 sample keeps much stronger intensity of the (100)diffraction peak than other samples for its better ordering, so the optimal level of n(Al)/n(Ti) can be regarded as 10.2.

As seen from the wide-angle patterns (Fig. 1C),all the resultant samples show no evident diffraction calcined at 500 ℃, suggesting the mesostructure with amorphous wall. After further treatment at 900 ℃, the amorphous phases convert to γ-Al2O3phase with the characteristic reflections at about 2θ= 37o, 46o and 67o (JCPDS No. 10-0425), without any signs of α-Al2O3phase, indicating the resulting samples possess good thermal stability further testified by the DTA analysis. There is evidence of anatase TiO2and rutile TiO2phases in the Ti-OMA samples besides Ti-OMA-19.4 sample, as shown in Fig. 1D. In the case of sample Ti-OMA-19.4, the lack of diffraction peak for the titanic crystalline phases is most likely due to the insufficient titanium addition. The above results identify that titanium has been successfully doped along with TiO2crystalline phases, appearing mainly in the form of rutile TiO2.Compared with OMA sample calcined at 900 ℃,the intensities of γ-Al2O3diffraction peaks lower down as increasing the titanium concentration, while at the same time those of rutile TiO2phase enhance.The results suggest the doped titanium is able to improve the thermal stability of Ti-OMA samples by protecting γ-Al2O3from the further phase transition,and the cluster of TiO2gets together easily with high titanium content. Depending on the wide-angle XRD results, Ti-OMA material with good thermal stability has been successfully prepared. The improved thermal stability of OMA material may be ascribed to the formation of rutile TiO2, which may hold back the surface spread of aluminum ions that can inhibit the phase transition from γ-Al2O3to α-Al2O3.

To further verify the crystalline phase of incorporated titanium in the prepared Ti-OMA samples,the Raman spectra were recorded, as shown in Fig. 2.It can be noticed that the intensity of the bands at 143, 235, 448 and 610 cm-1gradually enhances when increasing the titanium content. The weak band at 143 cm-1is the typical band of anatase TiO2[24,29]. While the relatively strong bands around 235, 448 and 610 cm-1are the characteristic bands of rutile TiO2, which are attributed to the vibration of Ti=O bond, the twisting and axial antisymmetric vibration of O-Ti-O bond, respectively[29,30]. This appearance indicates that the doped titanium calcined at 900 ℃ mainly exists as rutile TiO2crystallites in the Ti-OMA samples except for sample Ti-OMA-19.4, which is in good agreement with the wide-angle XRD results.

Fig. 1. Small- and wide-angle XRD patterns of Ti-OMA and OMA samples calcined at (A and C) 500 ℃ and (B and D) 900 ℃, (a) OMA, (b) Ti-OMA-19.4,(c) Ti-OMA-13.3, (d) Ti-OMA-10.2, (e)Ti-OMA-7.6, (f) Ti-OMA-5.4 and (g) Ti-OMA-3.8

Fig. 2. Raman spectra of Ti-OMA samples calcined at 900 ℃. (a) Ti-OMA-19.4,(b) Ti-OMA-13.3, (c) Ti-OMA-10.2, (d) Ti-OMA-7.6, (e) Ti-OMA-5.4 and (f) Ti-OMA-3.8

In order to investigate the proper amount of doped titanium in the OMA material, the specific surface areas of calcined Ti-OMA samples are summarized in Table 1. The data show that doping titanium helps to enhance the specific surface area of the resultant samples when the molar ratio of n(Al)/n(Ti) is more than 10.2. However, a reduction appears in the surface area when the n(Al)/n(Ti) value is less than 10.2 because the adding titanium is too excessive to fully embed into the OMA material and then blocks the mesoporous pore canal. Therefore, Ti-OMA sample calcined at 900 ℃ with the highest surface area of 218 m2g-1is obtained with the molar ratio n(Al)/(Ti) of 10.2, which is considered as the right amount of added titanium.

Fig. 3 shows the TEM images of the optimal Ti-OMA-10.2 sample treated at 500 and 900 ℃. Welldefined ordered p6mm mesostructure can be clearly observed in sample Ti-OMA-10.2 calcined at 500 ℃, confirming highly ordered mesopores are success- fully synthesized. When the temperature rises to 900 ℃, though the uniform ordered mesostructure of Ti-OMA-10.2 sample becomes to decrease, certain ordered arrangement can be still observed (Fig. 3C and 3D). This observance further confirms the ordered mesopores and good thermal stability, suggesting the doped titanium can stabilize the OMA material without breaking the ordered mesoporous structure.

Fig. 3. TEM images of the optimal Ti-OMA-10.2 sample seen along the (100)and (110) orientations calcined at (A and B) 500 ℃ and (C and D) 900 ℃

Table 1. Specific Surface Areas (SBET/m2 g-1) of Samples Ti-OMA Calcined at Different Temperature

Nitrogen adsorption/desorption isotherms and pore size distributions for Ti-OMA-10.2 sample calcined at different temperature are shown in Fig. 4,and the structural properties are listed in Table 2. All samples show IV-typed isotherms with H1 hysteresis loops (Fig. 4A), which is characteristic for mesoporous material with the uniform cylindrical pore in agreement with the TEM observation. When calcined at 500 ℃, Ti-OMA-10.2 sample shows an identical pore size distribution with an average pore size of 4.6 nm (Fig. 4B). At the elevated temperature,the pore size distribution curves shift to the right,showing the pore size becomes large. And the specific surface areas and pore volumes also begin to decrease, seen from Table 2. The above results indicate that the structural properties of Ti-OMA-10.2 sample change with the different thermal annealing conditions. Though the reducing textural properties are emerged, the Ti-OMA-10.2 sample calcined at 900 ℃ has much higher surface area(218 m2g-1) and larger pore volume (0.42 cm3g-1)than those of the undoped OMA material, demonstrating the doped titanium helps improve the thermal stability of OMA material, which can be further proved by the DTA analysis[14].

Evidence for good thermal stability of Ti-OMA is provided by comparing the DTA profiles of the undoped OMA and Ti-OMA-10.2 samples calcined at 500 ℃, as shown in Fig. 5. There are three exothermic peaks in two samples. The first peak distributed at 460～580 ℃ is perceived as the decomposition of organic compound and the dehydroxylation of boehmite into transition alumina[13,31].And the other two exothermic peaks detected at 800～880 and 1040～1100 ℃ are ascribed to the formation of γ-Al2O3phase and the ultimate phase transformation to α-Al2O3, respectively. In the same temperature range, the titanium-doped OMA sample has higher temperature of phase transition than the undoped OMA, which further proves that titanium dopant helps increase the stability of OMA material.

Fig. 4. (A) Nitrogen adsorption/desorption isotherms and (B)pore size distributions for Ti-OMA-10.2 sample calcined at different temperature

Fig. 5. DTA profiles for (a) undoped OMA and (b) Ti-OMA-10.2 samples calcined at 500 ℃

Table 2. Textural Properties of the Ti-OMA-10.2 Sample Calcined at Different Temperature

The UV-vis DRS spectra of Ti-OMA-10.2 sample calcined at different temperature are displayed in Fig. 6.After treating at 500 ℃, a band centered at 228 nm is observed in the Ti-OMA-10.2 sample, which is assigned to O2-→Ti4+charge transfer transition relating isolated titanium atoms in the octahedral coordination[24,32]. Additionally, red shifts of this absorption are observed with the rise of temperature.When calcined at 900 ℃, this band moves to 265 nm associated with partially clustered titanium species in hexahedral coordination, indicating some Ti-O-Ti clusters co-existed in the Ti-OMA-10.2 sample[32].Combined with the former analysis results of wideangle XRD and Raman spectra, this band can be ascribed to a crystallization of TiO2phase.

Fig. 6. UV-vis DRS spectra of Ti-OMA-10.2 sample calcined at different temperature

4 CONCLUSION

Ti-OMA material with well-defined ordered mesoporosity and good thermal stability has been successfully prepared by the sol-gel method. The results show that the doped titanium does not influence the formation of ordered structure. But the amount of titanium dopant plays an important role in the structural properties and thermal stability. When the n(Al)/n(Ti) is adjusted as 10.2, the resultant sample keeps the highest surface area of 218 m2g-1after annealing at 900 ℃. Moreover, the doped titanium contributes to stabilizing the OMA material mainly due to the formation of rutile TiO2phase treated at 900 ℃, which can inhibit the phase transition from γ-Al2O3to α-Al2O3by preventing the surface diffusion of aluminum ions.

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