**3.1. Influence of the morphology on the activity for hydrolytic dehydrogenation of ammonia borane**

Homogeneous hollow spheres of silica-alumina composite particles were obtained by adjusting some factors such as calcination temperature and soaking time [47]. **Figure 1a** shows TEM images of typical hollow silica-alumina composite spheres. Homogeneous hollow spheres with the shell thickness of ca. 6 nm and the particle size of ca. 220 nm were observed in the TEM image. The sample was prepared with PS templates with the diameter of ca. 200 nm; thus, the size of the hollow voids of the hollow spheres reflected the size of PS templates. The shell thickness and particle size were also controlled by adjusting ratios of silica-alumina composite to the amount of PS templates and particle size of PS templates, respectively [48–50]. **Figure 1b** shows the TEM image of the fine particles prepared with similar method for preparation of the hollow spheres without PS templates. The sample consists of fine particles with the particle size of ca. 13 nm. Particle agglomeration was observed in some parts of this composite. The specific surface areas of the hollow spheres and the fine particles were found to be 393 and 295 m2 g−1, respectively, indicating that the specific surface areas do not significantly differ from each other, and the primary particles including the hollow spheres were slightly small compared with the particle size of the fine particles. On the other hand, both the hollow spheres and the fine particles consisted of a typical amorphous silica-alumina from the results of powder XRD measurements [51, 52].

The hollow silica-alumina composite spheres show unexpected high activity for hydrolytic dehydrogenation of ammonia borane compared with the silica-alumina composite

**Figure 1.** TEM images of (a) hollow silica-alumina composite spheres and (b) silica-alumina composite fine particles [52].

fine particles. The amount of evolved hydrogen from aqueous ammonia borane solution in the presence of the hollow spheres was 10 mL with the completion of the reaction in 12 min, while the evolution 2.5 mL of hydrogen with the completion of the reaction in 12 and 2 min, respectively. The molar ratios of the hydrolytically evolved hydrogen to the initial ammonia borane were 2.6 and 0.6 in the presence of the hollow spheres and the fine particles, respectively. These results indicate that the amount of hydrogen evolved in the presence of the hollow spheres was significantly higher than the amount of hydrogen evolved in the presence of the fine particles. It has been reported that the acidic protons on Brønsted acid sites promote the dissociation of the B–N bond and the hydrolysis of BH<sup>3</sup> species to produce borate ion species along with the hydrogen release Eq. (1) [47, 51, 53].

**2.3. Activity tests for hydrolytic dehydrogenation of NH3**

from the reaction was monitored using the gas burette.

**3. Results and discussion**

224 Porosity - Process, Technologies and Applications

of powder XRD measurements [51, 52].

**ammonia borane**

an addition funnel. The reaction was initiated by adding aqueous NH<sup>3</sup>

**BH3**

BH<sup>3</sup>

solution (3.5 mL,

The composites were placed in a two-necked round-bottomed flask under air at room temperature. One of the necks was connected to a gas burette and the other was connected to

0.14 wt.%; Aldrich, 90%) from the addition funnel to the composites. The evolution of gas

**3.1. Influence of the morphology on the activity for hydrolytic dehydrogenation of** 

Homogeneous hollow spheres of silica-alumina composite particles were obtained by adjusting some factors such as calcination temperature and soaking time [47]. **Figure 1a** shows TEM images of typical hollow silica-alumina composite spheres. Homogeneous hollow spheres with the shell thickness of ca. 6 nm and the particle size of ca. 220 nm were observed in the TEM image. The sample was prepared with PS templates with the diameter of ca. 200 nm; thus, the size of the hollow voids of the hollow spheres reflected the size of PS templates. The shell thickness and particle size were also controlled by adjusting ratios of silica-alumina composite to the amount of PS templates and particle size of PS templates, respectively [48–50]. **Figure 1b** shows the TEM image of the fine particles prepared with similar method for preparation of the hollow spheres without PS templates. The sample consists of fine particles with the particle size of ca. 13 nm. Particle agglomeration was observed in some parts of this composite. The specific surface areas of the hollow spheres and the fine particles were found to be 393 and 295 m2 g−1, respectively, indicating that the specific surface areas do not significantly differ from each other, and the primary particles including the hollow spheres were slightly small compared with the particle size of the fine particles. On the other hand, both the hollow spheres and the fine particles consisted of a typical amorphous silica-alumina from the results

The hollow silica-alumina composite spheres show unexpected high activity for hydrolytic dehydrogenation of ammonia borane compared with the silica-alumina composite

**Figure 1.** TEM images of (a) hollow silica-alumina composite spheres and (b) silica-alumina composite fine particles [52].

$$\rm NH\_3BH\_3 + 2H\_2O \rightarrow \rm NH\_4^+ + \rm BO\_2^- + 3H\_2 \tag{1}$$

However, the hydrolytic dehydrogenation of ammonia borane in the presence of the hollow spheres was incomplete. It is suggested that the H<sup>3</sup> BO<sup>3</sup> produced from the reaction of BO<sup>2</sup> − with the acidic protons on Brønsted acid sites shift the reaction shown in Eq. (2) to the right side [53].

$$\rm H^{+} + \rm BO\_{2}^{-} + \rm H\_{2}O \leftrightarrow \rm H\_{3}BO\_{3} \tag{2}$$

To determine the recycle ability of the composites, the activity of the recycled composites was compared. The recycled hollow spheres evolved 1.5 mL of hydrogen with the completion of the reaction in 2 min. On the other hand, the recycled fine particles showed no activity. The recycled hollow spheres were much lower amount of hydrogen evolution than the original hollow spheres because the acidic protons might be exchange into ammonium ion on the Brønsted acid sites of the hollow spheres during hydrolytic dehydrogenation of ammonia borane. Then, the recycled hollow spheres were calcined at 723 K at heating rate of 0.5 K min−1 and cooling down immediately after the designated temperature was reached. The recycled hollow spheres after calcination showed the same activity as the recycled hollow spheres. These results suggest that almost all the acidic protons on Brønsted acid sites of the hollow spheres were consumed by the hydrolytic dehydrogenation reaction. The acidic properties of the silica-alumina composites were measured using NH<sup>3</sup> -TPD. **Figure 2** shows NH<sup>3</sup> -TPD profiles of the hollow spheres and the fine particles. The NH<sup>3</sup> desorption from the hollow spheres showed two peaks: first peak at around 420 K (low-temperature peak) and a broad peak at around 580 K (high-temperature peak), whereas the NH<sup>3</sup> desorption from the fine particles showed peaks at around 420 and 430 K (low-temperature peaks), respectively. The low-temperature peaks can be attributed to Brønsted acid sites with weakly adsorbed NH<sup>3</sup> (weak Brønsted acid sites), whereas the high-temperature peak observed for the hollow spheres can be attributed to Brønsted acid sites with strongly absorbed NH<sup>3</sup> (strong Brønsted acid sites) [54–56]. These results indicate that the hollow spheres possess both weak and strong Brønsted acid sites, while the fine particles possess only weak Brønsted acid sites. The amount of Brønsted acid sites calculated from the areas under the peak in the temperature range 400–600 K [57–59] for the hollow spheres and the fine particles was 0.18 and 0.10 mmol g−1, respectively. The result indicates that the amount of Brønsted acid sites

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


desorption peaks of the hollow spheres. The number

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

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

low spheres prepared using various aluminum precursors. The assignment of NH<sup>3</sup>

tion peaks in this figure was same in Section 3.1, and the number of Brønsted acid sites was

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

**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].


desorp-

227

hollow spheres were measured using NH<sup>3</sup>

calculated using the area of the NH<sup>3</sup>

**Figure 2.** NH<sup>3</sup> -TPD profiles of (a) hollow silica-alumina composite spheres and (b) silica-alumina composite fine particles [52].

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 sites showed unexpectedly strong acid property.
