Elsevier

Journal of Alloys and Compounds

Volume 509, Issue 7, 17 February 2011, Pages 3430-3434
Journal of Alloys and Compounds

Ultrasound-assisted preparation of titania–alumina support with high surface area and large pore diameter by modified precipitation method

https://doi.org/10.1016/j.jallcom.2010.12.119Get rights and content

Abstract

Titania–alumina supports were prepared successfully using the ultrasound-assisted precipitation method. The resulting supports were characterized by fourier transform infrared analysis (FTIR), scanning electron microscope (SEM), X-ray diffraction (XRD) and N2 physisorption. The effects of precipitants, washing method and the addition of surfactant CTAB were studied. The supports prepared with NH4HCO3 showed better textural properties compared with that with NH3·H2O, which was attributed to the low NH4+ release rate of NH4HCO3. TiO2–Al2O3 support with SBET = 283 m2/g, Vp = 2.34 mL/g and Dp = 33.0 nm was obtained with this modified precipitation method.

Research highlights

▶ A new method for preparation of TiO2–Al2O3 support with high surface area and large pore diameter was developed. Ultrasound was introduced into the preparation of support. The water in as-prepared precipitate was replaced with anhydrous ethanol and then CTAB was added in as-prepared support before drying. ▶ TiO2–Al2O3 support with SBET = 283 m2/g, Vp = 2.34 mL/g and Dp = 33.0 nm was obtained with this modified precipitation method. ▶ This modified precipitation method was also suitable for the preparation of other supports such as TiO2, Al2O3, ZrO2 and ZrO2–Al2O3 with high surface area and large pore diameter.

Introduction

It is well known that the chemical and structural characteristics of the support are very important as well as the nature and the structure of the active phase in the supported catalysts. TiO2, being a good support candidate or catalyst, was applied into many catalysis fields such as reduction [1], oxidation [2], [3], hydrogenation [4], [5] and photocatalytic [6], [7], but they have no pore system and the relatively low specific surface area, limiting their extensive application in catalysis. Normal alumina has a relative large specific surface area (200 m2/g), but it still cannot meet the demand of chemical and petrochemical industry. The catalyst with a small pore diameter or a low specific surface area would decrease its catalytic activity by restricting the access of reactants to the catalytic sites or decreasing the numbers of activity sites per unit area. Rana et al. [8], [9] studied the effects of the catalyst textural properties on the hydrotreating of heavy Maya crude oil. They found that the hydrodemetallization and hydrodearomatization activities mostly depend on the catalyst pore size distribution and the high hydrodesulfurization activity was attributed to the high metal dispersion and the high surface area of the supported catalysts. Thus, it is significant to consider the special surface area and the pore size of the support as important parameters for the supported catalyst to minimize the mass transfer influence and put the active sites to maximum dispersion.

Usually, the conventional precipitation process is carried out into the aqueous solution. The precipitates tend to aggregate and cannot be dispersed sufficiently because the nanoparticles need to decrease their high surface energy, leading to the serious agglomeration of nanoparticles and the bad textural properties of materials. Ultrasound can create a special reaction environment to deagglomerate nanoparticles and improve the reaction greatly owing to the strong chemical and mechanical effects of acoustic cavitation. For example, Palani et al. [10] adopted the sonochemical method to prepare the mesoporous silica SBA-15. They found that synthesis route effectively could reduce the total synthesis time from days to a few hours and the resultant materials exhibited a hexagonally ordered mesostructure with high surface area over 560 m2/g, pore volume over 0.6 mL/g and large pore diameter in the range of 4.1–4.9 nm. The sonochemical processing has been developed to be a useful technique for the nanomaterial preparation [11], [12], [13], [14], [15], [16], [17]. Many nanomaterials such as TiO2 [18], [19], WO3 [20], MoS2 [21], Fe2O3 [22], [23] and Al2O3 [24] have been prepared using this process.

In order to obtain the support with high surface area and large pore diameter, many other technologies were also used in the support preparation. For example, Vargas et al. [25] adopted the freeze–drying technique to eliminate water before the Al2O3–TiO2 (20 wt% TiO2) support drying and obtained the mesoporous support with a narrow single model pore size distribution (the most probable pore size 3.5 nm) and a high specific surface area (SBET = 308 m2/g). This technology could avoid the negative effects of water in the drying and maintained the original structure of support. In addition, surfactant such as hexadecyl trimethyl ammonium bromide (CTAB) or P123 was added to the as-prepared support during the preparation process, which could also obtain the support with high surface area and large pore size [26], [27], [28], [29], [30]. The conventional methods for the preparation of titania–alumina were mainly focused on co-precipitation method [31], [32] and sol–gel method [33], [34]. The former method used the available inorganic precursors as the starting materials and the cost was cheap while the latter method could obtain the material with a high surface area. Padmaja et al. [35] used the sol–gel method to prepare the titania–alumina mixed oxide and obtained the titania–alumina (the molar ratio 1:1) mixed oxide with SBET = 200 m2/g, Vp = 0.27 mL/g and Dp = 5.3 nm after calcined at 873 K. In this study, a new modified precipitation method was developed. Titania–alumina support with high specific surface area and large pore diameter was prepared with this new method.

Section snippets

Synthesis of TiO2–Al2O3 supports

The detailed preparation of TiO2–Al2O3 (Ti/Al mole ratio = 1:2) support was described as follows. 13.35 g AlCl3 and 9.5 g TiCl4 were dissolved in a 500 mL water with pH 1. The precipitant solution such as NH3·H2O solution (A, 2.2 wt%) or NH4HCO3 solution (C, 10 wt%) was added to the above mixed solution to adjusted pH to 8 with vigorous agitation. This process was carried out in a commercial ultrasonic cleaning bath (from Kunshan Ultrasonic Instruments Co., China, model KQ2200DE, 40 kHz, 100 W) at 313 K

FTIR studies

Fig. 1 shows the FTIR spectra of CTAB, as-prepared supports and calcined supports. The bands around 3500 cm−1 and 1640 cm−1 were attributed to the stretching vibration and the bending vibration of adsorbed water. The bands around 960 cm−1, 1487 cm−1, 2853 cm−1 and 2923 cm−1 were corresponded to CTAB [28]. After the calcination, all of the bands of CTAB disappeared, suggesting that the surfactant was removed completely and there was not any CTAB in the calcined supports. However, the bands around 3400 

Conclusions

A new method for the preparation of support with high surface area and large pore diameter was developed. Ultrasound could produce the strong chemical and mechanical effects, which was beneficial to inhibit the agglomeration of as-prepared nanoparticles. Using anhydrous ethanol to replace the water in the as-prepared support could hinder the particles agglomeration and then increased the specific surface area and the pore diameter significantly. Adding CTAB in the as-prepared support before

Acknowledgements

This research was supported by Scientific Research Fund of Hunan Provincial Education Department (10K062) and Hunan Provincial Innovation Foundation for Postgraduate (CX2009B135).

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