**2.1. Preparation of composites**

assembled superstructure [7, 8]. A small space made from pores or hollow structures leads to special properties such as the micropore filling of a pore for gas adsorption applications. If a special property were initiated into a solid-state material, a new possibility of material design would become available. Both porosity and hollow structures are very attractive for

Several papers have reported that mesoporous silica (MCM-41) itself acts as a typical acid catalyst for several reactions, including acetalization [9, 10], isomerization [11], and debenzylation [12]. Although the wall of mesoporous silica is made of amorphous silica and is generally believed to be neutral or slightly acidic, acetalization is generally believed to require strong or intermediate acids, such as HCl or proton zeolites, as catalysts [13–15]. These findings regarding the acidity of mesoporous silica [9–11, 16–21] raise important questions about

Hollow spheres have attracted much research and industrial interest due to their special shape, low density, and large fraction of voids. Hollow spheres possess a series of advantages such as tunable void volume, excellent flow performance, and large surface area. The large internal volume provides a storage space or artificial reaction cells that can serve many functions [22, 23]. The controlled synthesis of inorganic materials with well-designed structures at the nano-size level is extremely important in materials science. In particular, the preparation of hollow inorganic spheres with a defined structure has received increasing attention because of their broad potential applications ranging from drug delivery [24–26], ion exchange [27–29], sensors [30, 31], electro-optics [32, 33], and microreactors [34–36] to building block of photonic crystals [37]. Recently, this field has been advanced to the fabrication of hollow microspheres with holes on the shell wall, namely porous hollow microspheres. Because of their high specific surface area, low density, adsorption capacity, and ability to encapsulate actives, such materials are very useful in catalysis [38, 39], bioseparation [40, 41], tissue engineering [42–44], solar cells [45], and reaction separation

In this chapter, we review our previous works of hollow silica-alumina composite spheres. In our previous study, we investigated fabrication and morphology control of the hollow spheres and functionality of the hollow spheres. First, we discussed intrinsic properties of the hollow spheres compared with the conventional fine particles and their morphological effects on their acidic properties and activity for hydrolytic dehydrogenation of ammonia borane. Second, we also discussed the influence of dispersion of active sites on activity of the hollow spheres for hydrolytic dehydrogenation of ammonia borane. Four-coordinated aluminum species substituting silicon atom in silica lattice was assigned as Brønsted acid sites, and the sites were active species for hydrolysis dehydrogenation of ammonia borane. Thus, the increase of their dispersion is expected to improve the activity of silica-alumina composite particles. From the above two points of view, we reviewed our previous works on hollow

material functionalization.

222 Porosity - Process, Technologies and Applications

[46].

silica-alumina composite spheres.

how the surface of the material becomes acidic.

Hollow silica-alumina composite spheres were fabricated through the PS template method. Monodisperse PS particles were prepared by emulsifier-free emulsion polymerization using the following procedure. Styrene (9.0 mL; Kanto Chem. Co., >99.0%), poly(vinyl pyrrolidone) K30 (1.5 g; Fluka, Mw ≈ 40,000), and the cationic initiator 2,2′-azobis(2-metylpropionamidine) dihydrochloride (0.26 g; Wako Pure Chemical, >97.0%) were dissolved in ion exchanged water (100 mL) inside a 250-mL three-necked flask. The flask was equipped with a mechanical stirrer, a thermometer with a temperature controller, a nitrogen gas inlet, and a Graham condenser, and it was placed in an oil bath for heating. The reaction solution was deoxygenated by bubbling nitrogen gas at room temperature for 1 h and heated at 343 K for 24 h under stirring at 250 rpm. The final PS suspension was centrifuged at 6000 rpm for 5 min and washed three times with ethanol (30 mL; Kanto Chem.Co., >99.5%). The PS contents could be tailored through the addition of ethanol. Aluminum isopropoxide (0.0057 g; Aldrich, >98.0%), aqueous ammonia solution (3 mL; 28 wt.%, Kanto Chem. Co.), ethanol (40 mL), and tetraethoxysilane (0.1551 mL; TEOS, Kanto Chem. Co., >99.9%) were added to the PS suspension (15 g). The sol-gel reaction was carried out at 323 K for 1.5 h. The composite was dried overnight in a desiccator. Then, the composites were calcined in air at 873 K at a heating rate of 0.5 K min−1 and cooling down immediately after the designated temperature was reached. Fine silica-alumina composite particles were prepared by sol-gel method without the PS template particles. Aluminum isopropoxide (0.0057 g), aqueous ammonia solution (3 mL), and TEOS (0.1551 mL) were added to ethanol (40 mL). The sol-gel reaction was carried out at 323 K for 1.5 h. The composite was dried overnight in a desiccator. Then, the composite was calcined under the same conditions used for the hollow spheres.

#### **2.2. Characterization**

Morphology of the composite was identified by a transmission electron microscopy (TEM) using a Hitachi FE-2000 system operating at an acceleration voltage of 200 kV. Temperatureprogrammed desorption of ammonia (NH<sup>3</sup> -TPD) was carried out on a BELCAT-B instrument. The analysis was performed by loading 50 mg of the composites into a quartz reactor and drying them under a flow of pure He at 783 K for 1 h followed by purging with pure He at the same temperature for 1 h. The composites were allowed to cool to 373 K under the He flow and then exposed to NH<sup>3</sup> -He gas mixture (95 vol.% He) at 373 K for 1 h to allow NH<sup>3</sup> adsorption. The composites were then purged using pure He to allow for the accurate detection of the desorbed NH<sup>3</sup> . The NH<sup>3</sup> -TPD measurements were conducted by heating the composites from 373 to 773 K at a rate of 10 K min−1 under a flow of pure He. The desorbed NH<sup>3</sup> molecules were detected by a thermal conductivity detector (TCD).

#### **2.3. Activity tests for hydrolytic dehydrogenation of NH3 BH3**

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 an addition funnel. The reaction was initiated by adding aqueous NH<sup>3</sup> BH<sup>3</sup> solution (3.5 mL, 0.14 wt.%; Aldrich, 90%) from the addition funnel to the composites. The evolution of gas from the reaction was monitored using the gas burette.

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].

O → NH<sup>4</sup>

However, the hydrolytic dehydrogenation of ammonia borane in the presence of the hollow

the acidic protons on Brønsted acid sites shift the reaction shown in Eq. (2) to the right side [53].

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

hollow spheres showed two peaks: first peak at around 420 K (low-temperature peak) and a

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

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

low spheres can be attributed to Brønsted acid sites with strongly absorbed NH<sup>3</sup>

(weak Brønsted acid sites), whereas the high-temperature peak observed for the hol-

<sup>−</sup> + H<sup>2</sup>

properties of the silica-alumina composites were measured using NH<sup>3</sup>

broad peak at around 580 K (high-temperature peak), whereas the NH<sup>3</sup>


BO<sup>3</sup>

O ↔ H<sup>3</sup>

<sup>+</sup> + BO<sup>2</sup>

<sup>−</sup> + 3H<sup>2</sup> (1)

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

BO<sup>3</sup> (2)

− with 225


desorption from the

desorption from the

(strong

produced from the reaction of BO<sup>2</sup>

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

NH<sup>3</sup> BH<sup>3</sup> + 2H<sup>2</sup>

H<sup>+</sup> + BO<sup>2</sup>

NH<sup>3</sup>

NH<sup>3</sup>

spheres was incomplete. It is suggested that the H<sup>3</sup>
