**1. Introduction**

The ability to generate highly defined, hierarchically structured materials that range in size from a few nanometers to several micrometers is a key prerequisite for the fabrication of highly functional materials. Such structured materials have potential applications in the fields of energy conversion, energy storage, catalysis, and separation [1–4]. Bottom-up approaches that rely on the self-assembly of molecular or colloidal building blocks into superstructures of defined length scales and symmetries have been used to obtain these structures [5, 6]. The size and shape of these building blocks translate directly into the

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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 material functionalization.

**2. Typical experimental procedures**

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

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

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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. Temperature-

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

tion. The composites were then purged using pure He to allow for the accurate detection of

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>




adsorp-

molecules

**2.1. Preparation of composites**

hollow spheres.

**2.2. Characterization**

and then exposed to NH<sup>3</sup>

the desorbed NH<sup>3</sup>

programmed desorption of ammonia (NH<sup>3</sup>

. The NH<sup>3</sup>

were detected by a thermal conductivity detector (TCD).

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 how the surface of the material becomes acidic.

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 [46].

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 silica-alumina composite spheres.
