**3.2. Improvement of dispersion of active species of hollow silica-alumina composite spheres**

As described in the previous section, Brønsted acid sites are main active sites for hydrolytic dehydrogenation of ammonia borane. The acid sites can generally increase with the increase of four-coordinated aluminum species, which are well-dispersed active species. We investigated the influence of various preparation conditions such as promoters of sol-gel reaction for the formation of the shell of the hollow spheres, alcohol solvents, aluminum precursors, and so on, on the dispersion of the active aluminum species of hollow silica-alumina composite spheres and their activity for hydrolytic dehydrogenation of ammonia borane [60–62]. In the conditions, aluminum precursors significantly influenced on the dispersion and the activity. In this session, we especially introduce the investigation of the effects of aluminum precursors.

All the hollow silica-alumina composite spheres prepared using various aluminum precursors possessed similar morphology as shown in **Figure 3**. The shell thickness and diameter of all the hollow spheres were approximately 25 and 260 nm, respectively. The specific surface areas of the hollow spheres prepared using various aluminum precursors measured through nitrogen sorption using the Brunauer-Emmett-Teller (BET) methods were 436, 476, 483, and 523 m<sup>2</sup> g−1, respectively, indicating that the specific surface area does not significantly depend on the kind of aluminum precursors. On the other hand, the coordination numbers of the hollow spheres prepared using various aluminum precursors were quite different, and the ratio of four-coordinated aluminum species to all the aluminum species of the hollow spheres prepared using aluminum ethoxide, aluminum iso-propoxide, aluminum n-butoxide, and aluminum sec-butoxide calculated from the results of <sup>27</sup>Al MAS NMR spectra were 0.10, 0.33, 0.12, and 0.44, respectively [62]. The result indicates that the hollow spheres prepared using aluminum precursors with the branched alkyl groups exhibit larger proportion of four-coordinated aluminum species than those prepared using aluminum precursors with the normal alkyl groups. The dispersion of aluminum species increases with increase of the ratio of four-coordinated aluminum species [63]. The result indicates that the aluminum species of the hollow spheres prepared using aluminum precursors with the branched alkyl groups were well dispersed in the silica matrix. The acidic properties of the hollow spheres were measured using NH<sup>3</sup> -TPD. **Figure 4** shows NH<sup>3</sup> -TPD profiles of the hollow spheres prepared using various aluminum precursors. The assignment of NH<sup>3</sup> desorption peaks in this figure was same in Section 3.1, and the number of Brønsted acid sites was calculated using the area of the NH<sup>3</sup> desorption peaks of the hollow spheres. The number of Brønsted acid sites in the hollow spheres prepared using aluminum ethoxide, aluminum iso-propoxide, aluminum n-butoxide, and aluminum sec-butoxide were 0.08, 0.30, 0.12, and

in the silica-alumina composites depends on their morphology. According to the result, the amount of hydrogen evolution increases with the increase of the amount of Brønsted acid sites. The total amount of Brønsted acid sites in the hollow spheres is found to be 1.8 times higher than those in the fine particles. Moreover, the amount of hydrogen evolved in the presence of the hollow spheres is more than four times higher than that in the presence of the fine particles. Consequently, it is indicated that the morphology of silica-alumina composites influences their acidic properties and that the strong Brønsted acid sites are more effective for hydrolytic dehydrogenation of ammonia borane than the weak Brønsted acid sites. In addition, it is also suggested that the primary particles consisting of the shell of the hollow spheres formed micro- and/or meso-interparticles spacing, and the integrated surface acid


**3.2. Improvement of dispersion of active species of hollow silica-alumina composite** 

As described in the previous section, Brønsted acid sites are main active sites for hydrolytic dehydrogenation of ammonia borane. The acid sites can generally increase with the increase of four-coordinated aluminum species, which are well-dispersed active species. We investigated the influence of various preparation conditions such as promoters of sol-gel reaction for the formation of the shell of the hollow spheres, alcohol solvents, aluminum precursors, and so on, on the dispersion of the active aluminum species of hollow silica-alumina composite spheres and their activity for hydrolytic dehydrogenation of ammonia borane [60–62]. In the conditions, aluminum precursors significantly influenced on the dispersion and the activity. In this session, we especially introduce the investigation of the effects of aluminum

sites showed unexpectedly strong acid property.

**spheres**

**Figure 2.** NH<sup>3</sup>

226 Porosity - Process, Technologies and Applications

[52].

precursors.

**Figure 3.** TEM images of hollow silica-alumina composite spheres prepared using (a) aluminum ethoxide, (b) aluminum iso-propoxide, (c) aluminum n-butoxide, and (d) aluminum sec-butoxide [62].

of aluminum precursors. Hydrogen evolution of 1.0 mL was occurred in 3 min in the presence of the recycled hollow spheres prepared using aluminum sec-butoxide, indicating that the recycled hollow spheres exhibit much less hydrogen evolution than the fresh hollow spheres. The recycled hollow spheres prepared using aluminum ethoxide, aluminum isopropoxide, and aluminum n-butoxide exhibit similar results. Hydrogen evolution rate of the hollow spheres prepared using aluminum ethoxide, aluminum iso-propoxide, aluminum n-butoxide, and aluminum sec-butoxide calculated using data from the first 50% of the reaction was 1.5, 2.0, 1.5, 2.0, and 0.3 mL min−1, respectively, indicating that the rate of hydrogen evolution of all the hollow spheres was similar because they possess similar pore

Role of Interparticle Space in Hollow Spheres of Silica-Based Solid Acids…

http://dx.doi.org/10.5772/intechopen.71307

229

In order to evaluate the effect of the morphology and the dispersion of active aluminum species in the hollow spheres, the relation between the ratios of four-coordinated aluminum species and the activity for hydrogen evolution amount from aqueous solutions in the presence of various silica-aluminum composite particles. The relation calculated from some of our previous studies [52, 61, 62] was shown in **Figure 5**. From this figure, the hollow spheres showed unexpected high activity as compared with the fine particles, although the ratio of four-coordinated aluminum species in the hollow spheres was not significantly higher than the ratio of the fine particles. As described in Section 3.1, the larger amount of more effective strong Brønsted acid sites for hydrogen evolution from aqueous ammonia borane solution are in the hollow spheres than that in the fine particles. The micro- and/ or meso-interparticles spacing formed by the primary particles in the shell of the hollow spheres in which the integrated surface acid sites showed unexpectedly strong acid property were included. On the other hand, the dispersion of aluminum species included in the hollow spheres was controlled by adjusting the preparation conditions and the activity for hydrogen evolution from aqueous ammonia borane solution linearly depended on the

**Figure 5.** Relation between ratio of four-coordinated aluminum species and hydrogen evolution amount from aqueous

ammonia borane solution in the presence of various silica-alumina composite particles [52, 61, 62].

size distribution calculated from the results of nitrogen sorption measurement.

dispersion as shown in **Figure 5**.

**Figure 4.** NH<sup>3</sup> -TPD profiles of hollow silica-alumina composite spheres prepared using (a) aluminum ethoxide, (b) aluminum iso-propoxide, (c) aluminum n-butoxide, and (d) aluminum sec-butoxide [62].

0.34 mmol g−1, respectively. From the result, the hollow spheres prepared using aluminum precursors with the branched alkyl groups exhibit more Brønsted acid sites than those prepared using aluminum precursors with the normal alkyl groups. The result indicates that the number of Brønsted acid sites depends on the kind of aluminum precursors. In addition, the surface concentrations of Brønsted acid sites in the hollow spheres prepared using aluminum ethoxide, aluminum iso-propoxide, aluminum n-butoxide, and aluminum secbutoxide was 0.18, 0.63, 0.25, and 0.65 mmol m<sup>2</sup> , respectively. From the result, the hollow spheres prepared using aluminum precursors with the branched alkyl groups exhibit more surface concentration of Brønsted acid sites than those prepared using aluminum precursors with the normal alkyl groups. The result suggests that the aluminum species of the hollow spheres prepared using aluminum precursors with the branched alkyl groups were well dispersed in the silica matrix.

The activities of the hollow spheres prepared using various aluminum precursors for hydrolytic dehydrogenation of ammonia borane were compared. The hydrogen evolution of 5.0, 10.5, 6.0, and 11.5 mL was occurred in 40, 45, 45, and 35 min, respectively, in the presence of the hollow spheres prepared using aluminum ethoxide, aluminum iso-propoxide, aluminum n-butoxide, and aluminum sec-butoxide, respectively. The molar ratios of the hydrolytically evolved hydrogen to introduced ammonia borane were 1.3, 2.8, 1.5, and 3.0 in the presence of the hollow spheres prepared using aluminum ethoxide, aluminum iso-propoxide, aluminum n-butoxide, and aluminum sec-butoxide, respectively. The hollow spheres prepared using aluminum precursors with the branched alkyl groups exhibit more hydrogen evolution than those prepared using aluminum precursors with the normal alkyl groups. It has been reported that the branched alkyl groups exhibit lower sol-gel reaction in silicon precursors rate than the normal alkyl groups because of steric effects [64]. Consequently, the amount of hydrogen evolution increases as the sol-gel reaction rate decreases. The result indicates that the amount of hydrogen evolution depends on the kind of aluminum precursors. Hydrogen evolution of 1.0 mL was occurred in 3 min in the presence of the recycled hollow spheres prepared using aluminum sec-butoxide, indicating that the recycled hollow spheres exhibit much less hydrogen evolution than the fresh hollow spheres. The recycled hollow spheres prepared using aluminum ethoxide, aluminum isopropoxide, and aluminum n-butoxide exhibit similar results. Hydrogen evolution rate of the hollow spheres prepared using aluminum ethoxide, aluminum iso-propoxide, aluminum n-butoxide, and aluminum sec-butoxide calculated using data from the first 50% of the reaction was 1.5, 2.0, 1.5, 2.0, and 0.3 mL min−1, respectively, indicating that the rate of hydrogen evolution of all the hollow spheres was similar because they possess similar pore size distribution calculated from the results of nitrogen sorption measurement.

In order to evaluate the effect of the morphology and the dispersion of active aluminum species in the hollow spheres, the relation between the ratios of four-coordinated aluminum species and the activity for hydrogen evolution amount from aqueous solutions in the presence of various silica-aluminum composite particles. The relation calculated from some of our previous studies [52, 61, 62] was shown in **Figure 5**. From this figure, the hollow spheres showed unexpected high activity as compared with the fine particles, although the ratio of four-coordinated aluminum species in the hollow spheres was not significantly higher than the ratio of the fine particles. As described in Section 3.1, the larger amount of more effective strong Brønsted acid sites for hydrogen evolution from aqueous ammonia borane solution are in the hollow spheres than that in the fine particles. The micro- and/ or meso-interparticles spacing formed by the primary particles in the shell of the hollow spheres in which the integrated surface acid sites showed unexpectedly strong acid property were included. On the other hand, the dispersion of aluminum species included in the hollow spheres was controlled by adjusting the preparation conditions and the activity for hydrogen evolution from aqueous ammonia borane solution linearly depended on the dispersion as shown in **Figure 5**.

0.34 mmol g−1, respectively. From the result, the hollow spheres prepared using aluminum precursors with the branched alkyl groups exhibit more Brønsted acid sites than those prepared using aluminum precursors with the normal alkyl groups. The result indicates that the number of Brønsted acid sites depends on the kind of aluminum precursors. In addition, the surface concentrations of Brønsted acid sites in the hollow spheres prepared using aluminum ethoxide, aluminum iso-propoxide, aluminum n-butoxide, and aluminum sec-

aluminum iso-propoxide, (c) aluminum n-butoxide, and (d) aluminum sec-butoxide [62].


spheres prepared using aluminum precursors with the branched alkyl groups exhibit more surface concentration of Brønsted acid sites than those prepared using aluminum precursors with the normal alkyl groups. The result suggests that the aluminum species of the hollow spheres prepared using aluminum precursors with the branched alkyl groups were well

The activities of the hollow spheres prepared using various aluminum precursors for hydrolytic dehydrogenation of ammonia borane were compared. The hydrogen evolution of 5.0, 10.5, 6.0, and 11.5 mL was occurred in 40, 45, 45, and 35 min, respectively, in the presence of the hollow spheres prepared using aluminum ethoxide, aluminum iso-propoxide, aluminum n-butoxide, and aluminum sec-butoxide, respectively. The molar ratios of the hydrolytically evolved hydrogen to introduced ammonia borane were 1.3, 2.8, 1.5, and 3.0 in the presence of the hollow spheres prepared using aluminum ethoxide, aluminum iso-propoxide, aluminum n-butoxide, and aluminum sec-butoxide, respectively. The hollow spheres prepared using aluminum precursors with the branched alkyl groups exhibit more hydrogen evolution than those prepared using aluminum precursors with the normal alkyl groups. It has been reported that the branched alkyl groups exhibit lower sol-gel reaction in silicon precursors rate than the normal alkyl groups because of steric effects [64]. Consequently, the amount of hydrogen evolution increases as the sol-gel reaction rate decreases. The result indicates that the amount of hydrogen evolution depends on the kind

, respectively. From the result, the hollow

butoxide was 0.18, 0.63, 0.25, and 0.65 mmol m<sup>2</sup>

dispersed in the silica matrix.

228 Porosity - Process, Technologies and Applications

**Figure 4.** NH<sup>3</sup>

**Figure 5.** Relation between ratio of four-coordinated aluminum species and hydrogen evolution amount from aqueous ammonia borane solution in the presence of various silica-alumina composite particles [52, 61, 62].
