Optimization of instantaneous solvent exchange/surface modification process for ambient synthesis of monolithic silica aerogels
Graphical abstract
The effective synthetic conditions for crack-free silica aerogel monoliths via the ISE/SM process and ambient drying were successfully optimized.
Introduction
Silica aerogel has received significant attention due to its extraordinary properties (high specific surface area of 700–1200 m2/g, low density of 0.03–0.15 g/cm3, and low thermal conductivity of 0.01–0.025 W/m K) that originate from its mesoporous nanostructure (2–50 nm in pore diameter). Since the discovery by Kistler in 1931 [1], numerous applications of this material have been developed in both fundamental and advanced technological areas, from thermal insulation to life science [2], [3], [4]. However, conventional synthesis of silica aerogels has been limited by expensive and harmful starting materials such as alkoxides and by the required supercritical drying process which is expensive, dangerous, and difficult to use with mass-production techniques.
Thus far, many researchers have tried to reduce the cost of aerogel synthesis. In general, studies that have investigated the lowering of the costs for aerogels took one of two approaches. The first is the development of a new drying process at ambient pressure instead of the previous super/subcritical drying using an autoclave [5], [6], [7], [8], [9], [10], [11], [12]. The second involves an approach that uses inexpensive starting materials such as sodium silicate in the ambient drying procedure [13], [14], [15], [16], [17], [18].
The first synthesis of silica aerogel at ambient pressure was reported in 1992 by Smith et al. [5]. Ambient synthesis of aerogels is possible through the solvent exchange and surface modification (i.e., silylation) of the wet gels. Silanol groups on the silica surface that lead to the collapse of the gel by further condensation were modified into nonreactive organic radicals by silylation agents such as different types of alkoxy-chlorosilane. In particular, surface modification of hydrogels using a silylation agent requires an organic solvent environment. However, the solvent exchange of a wet gel requires a long processing time and involves troublesome steps as the solvent exchange proceeds only by the pore diffusion of liquids through a mesopore-sized channel. Furthermore, the solvent exchange of the wet gel bulks/monoliths requires a relatively long time and large quantity of exchanging solvents.
In 1998, Schwertfeger et al. suggested an epochal solution to these problems [13]. The basic idea was the design of a direct method for modifying a hydrogel to a surface-silylated gel that avoids a solvent-exchange step. As a silylation agent and a main solvent, they used trimethylchlorosilane (TMCS) and hexamethyldisiloxane (HMDSO), respectively. However, only powder-type or crushed pieces of aerogels could be synthesized using this method as it was difficult to prevent the cracks, and failure of the gel resulted, which was promoted by the violent reactions between TMCS and water in the pores of wet gels.
Our group had developed a similar process termed instantaneous solvent exchange/surface modification (ISE/SM) using isopropyl alcohol (IPA), TMCS, and n-hexane for the ambient synthesis of monolithic silica aerogels [15]. Further work by the authors [17] reported the effect of process variables such as the silica content of the starting material and the ratio of IPA and TMCS on the properties of the aerogel product.
In this work, we tried to describe the mechanism of the ISE/SM reactions more scrupulously and to optimize the process variables affecting the feasibility of crack-free silica aerogel monoliths. For these objectives, the important factors and phenomena such as reaction rate depending on temperatures during the ISE/SM step were investigated in detail, and effective synthetic conditions for crack-free aerogel monoliths with high production yields were elucidated.
Section snippets
Materials and methods
A schematic representation of the overall synthetic procedure for monolithic silica aerogels via the ISE/SM process with ambient drying is described in Fig. 1. As a starting material for preparing the silica wet gels in the present study, aqueous colloidal silica sol synthesized from an ion exchange of industrial sodium silicate solution (waterglass, Na2O⋅3.3SiO2⋅22.6H2O, Il-shin Chem., Korea) was used. Further information regarding the preparation methods and properties of the silica sol was
Results and discussion
The proposed mechanism of the ISE/SM process is described as a schematic representation in Fig. 2. When IPA and TMCS were immersed into the n-hexane phase containing silica hydrogels, several reactions occurred instantaneously at the interface of the n-hexane phase and the silica surface of the hydrogel. IPA and TMCS are partially reacted with each other, and isopropoxytrimethylsilane [IPTMS; (CH3)3SiOCH(CH3)2] and HCl then formed within the n-hexane phase (reaction (1)):IPA + TMCS → IPTMS +
Summary
The optimum conditions of an instantaneous solvent exchange/surface modification process for the ambient synthesis of monolithic silica aerogels were determined successfully. The yield for crack-free aerogel monoliths was improved to 80% as the reaction temperature of the ISE/SM process decreased below room temperature. The molar ratio of TMCS/H2O (pore water) and the volumetric ratio of n-hexane/TMCS were optimized in the range of 0.2500–0.3567 and 15–30, respectively, as the most effective
Acknowledgments
This work was financially supported by the Ministry of Commerce, Industry, and Energy, Korea Energy Management Corporation through the Energy Conversion Technology R&D program.
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Present address: Samsung Advanced Institute of Technology (SAIT), Nongseo-dong, Giheung-gu, Yongin-si Gyunggi-do 446-712, South Korea.