Volume 9 Issue 2 - June 5, 2009
Fabrication of Pseudoboehmite and Alumina: Effects of Water and 1-Hexadecyl-2,3-dimethyl-imidazolium Chloride
Dian-Yi Li, Yi-Shiue Lin, Yu-Ching Li, Dong-Lin Shieh, and Jong-Liang Lin*

Department of Chemistry, College of Sciences, National Cheng Kung University

Microporous and Mesoporous Materials 108 (2007) 276-282.

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A porous material is a material containing pores. According to the pore diameter (ф), porous materials are classified into three categories, i.e., microporous (ф < 2 nm), mesoporous (2 nm < ф < 50 nm) and macroporous (ф > 50 nm) materials. Ordered, porous oxides were first demonstrated by the successful syntheses of aluminosilicate and silica with hexagonal pore array in 1992 [1]. The key step for this success was addition of surfactants that acted as structure-directing templates. This approach to mesostructured materials has been extended to other metal oxides. 1-Hexadecyl-2,3-dimethyl-imidazolium chloride (Figure 1), which is an ionic liquid, used in this study has a structural feature of cationic surfactants, a long alkyl chain and cation head group. Mesoporous aluminum oxides can provide high surface areas, and together with their unique physical and chemical properties, can have a wide range of applications in adsorption, separation, and catalysis. There are only few reports regarding the preparation of porous aluminum oxides using ionic liquid molecules [2]. In this article, we describe the outcomes of decomposition of aluminum tri-sec-butoxide in ethanol with controlled amount of water in the presence of 1-hexadecyl-2,3-dimethyl-imidazolium chloride.

Figure 1. Structure of 1-hexadecyl-2,3-dimethyl-imidazolium chloride.
In the preparation of the aluminum oxides, two reaction mixtures of ethanol with different molar ratios of Al/1-hexadecyl-2,3-dimethyl-imidazolium chloride/H2O (12/1/40 (water excess) and 12/1/1.5 (water deficient)) were investigated. The precipitates were collected after the solutions were stirred at 40˚C in air for 40 min and then kept in the sealed container at this temperature for 48 hr without agitation. The analysis for the precipitates is focused on crystal phase, morphology, and porous structure and the effect of calcination.

Figure 2. X-ray diffraction patterns of the as-prepared samples and the changes after calcination for the two reaction mixtures with excess (a) and (b) deficient water.

Figure 2 shows the X-ray diffraction patterns of the products from the two reaction mixtures. In the case of excess water, the as-prepared product with 2θ at 27.1, 49.6 and 64.8˚ is identified to be pseudoboehmite AlO(OH), which is a poorly crystallized form of boehmite and contains excess water. These peaks become smaller and broadened after 550˚C calcination, indicating that the pseudoboehmite domains are reduced in size. As the sample is heated to 800˚C, γ-Al2O3 (2θ = 45.8 and 67.0˚) is formed. In contrast to Figure 2a, the as-prepared and 550 ˚C-calcined samples are basically amorphous in the water-deficient case (Figure 2b), but Al2O3 is also generated after 800˚C calcination. Additional Fourier-transform infrared spectroscopy study shows that the as-prepared products contain the ionic liquid molecules, but they hardly exist after heating the products at 400˚C.

The morphology of the as-prepared products and the changes following 800˚C calcination examined by transmission electron microscopy are shown in Figure 3. The as-prepared product in the water-excess case consists of interconnecting fibrous particles, 2-5 nm in diameter. Elongated particles are observed after calcination at 800˚C. The surface area, total pore volume, and average pore diameter are 320 m2/g, 0.92 cm3/g, and 9.9 nm respectively for the 800˚C sample. In the water-deficient case, no fibrous framework is observed, but platelets with mesopores. After 800˚C calcination, the particles still have platelet shape and the material has a surface area of 260 m2/g, a total pore volume of 0.46 cm3/g, and an average pore diameter of 5.2 nm.

Figure 3. TEM images of the as-prepared samples and the changes after 800˚C calination for the two reaction mixtures with excess (a and b) and deficient (c, d) water.

In this study, the amount of water has profound effects on the particle morphology, crystal phase and porosity. In the presence of excess water, Al(OR)3 can be fully hydrolyzed (Al(OR)3 + 2H2O → AlOOH + 3ROH) to form fibrous pseudoboehmite nanoparticles. After comparing the results without 1-hexadecyl-2,3-dimethyl imidazolium chloride, it is found that the ionic-liquid molecules play a role promoting formation of the fibrous structure. In the water-deficient environment, incomplete hydrolysis may occur around the surface of self-assembly of the template molecules, forming particles with mesopores. In the absence of the template molecules, no mesoporous structure is observed.

[1] C.T. Kresge, M.E. Leonowize, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 1992,359, 710.
[2] N. Zˇ ilkova’, A. Zukal, J. Cˇ ejka, Micropor. Mesopor. Mater. 2006, 95, 176.
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