Synthesis of a novel Al₂O₃–SiO₂ composite aerogel with high specific surface area at elevated temperatures using inexpensive inorganic salt of aluminum

Abstract

We have developed a new sol–gel route to synthesize Al₂O₃–SiO₂ composite aerogel using inexpensive inorganic salt of aluminum. The approach, which is straightforward, inexpensive, and safe, can be employed to produce a monolithic mesoporous material with a high specific surface area, which is heat-treated at high temperatures. The results show that SiO₂ is essentially amorphous, whereas Al₂O₃ predominately exists as polycrystalline boehmite which consists of fibrous particles and web-like microstructures. As the heat treatment temperature increases to 600 °C, the structural transition from boehmite to γ-Al₂O₃ occurs within the composite. When the composite is heat-treated at 1100 °C, SiO₂ crystallization occurs and the mullite phase begins to appear. The specific surface area decreases with an increase in heat treatment temperatures; however, it remains 120 m²/g until 1200 °C. The heat treatment process improves the quality of the three-dimensional aerogel network, and the pore distribution becomes uniform after heat treatment.

Introduction

Alumina aerogels are nano-porous solid materials with low bulk densities, low thermal conductivities and amorphous network structures. Due to their high porosity and large specific surface area at elevated temperatures, they have broad application prospects in advanced aircraft, spacecraft and catalyst carriers at high temperatures [1]. However, the main problem for alumina aerogel is their high thermal conductivity compared with that of silica aerogel [2], [3], their limited heat resistance and a drastic decrease in the specific surface area after heat treatment at temperatures over 1000 °C due to phase transitions of alumina part. Therefore, maintaining the nanostructures and improving the specific surface areas at elevated temperatures are the main focus of alumina research. Two possible methods were reported for the suppression of the alumina phase transition: the first method involves the addition of other elements, such as Si, La, Ba, or Zr [4], [5], [6], whereas the second method involves a decrease in the bulk density of the alumina powders [7]. Thus, many researchers have focused on Al₂O₃–SiO₂ aerogel with a significant low bulk density and a silica-doped effect to obtain a composite with high heat resistance and a large specific surface area at elevated temperatures. Osaki et al. [8], [9], [10] proposed a method using aluminum isopropoxide as alumina source and tetraethyl orthosilicate as the silicon source to prepare Al₂O₃–SiO₂ aerogel. The addition of silica particles surrounding the γ-Al₂O₃ inhibited the contact of Al₂O₃ particles, decreased the probability of crystal nucleation, and significantly inhibited the α-Al₂O₃ phase transition. It possessed a large specific surface area of 47 m²/g at 1200 °C; however, the alumina alkoxides were very expensive and were readily hydrolyzed by water, which produced aluminum mono- or tri-hydroxides. Because the formed mono-hydroxide is the only material that can be peptized to a clear sol, a highly complex procedure is needed to obtain a clear solution [8], [9], [10]. Hurwitz et al. [11] directly employed boehmite powder as an alumina source to prepare Al₂O₃–SiO₂ aerogel. The specific surface area was as high as 33 m²/g at 1300 °C, whereas the phase transition of γ-Al₂O₃/η-Al₂O₃ occurred as early as 600 °C. The boehmite powder had to be dispersed in 0.09 M nitric acid solution or water and sonicated using an ultrasonic processor, which was also a complex procedure.

The objective of this study is to develop a new and convenient sol–gel method that facilitates the fabrication of Al₂O₃–SiO₂ composite aerogel, which possesses a high specific surface area at elevated temperatures. The monolithic Al₂O₃–SiO₂ composite aerogel was prepared by simply mixing the silica sol and alumina sol in one pot, followed by gel, aging, solvent exchange and C₂H₅OH supercritical drying. Al₂O₃–SiO₂ sols were synthesized using cheap inexpensive inorganic salt (AlCl₃·6H₂O) and tetraethoxysilane (TEOS) as precursors, ethanol (EtOH) as a solvent, and propylene oxide (PO) as a network forming agent; no catalyst was employed in the total procedure. Details of the synthesis and discussions of Al₂O₃–SiO₂ composite aerogel, which was heat-treated at different temperatures, are provided in the next section.