Chapter 8
Preparation of Functional Silica Aerogels Using Ionic Liquids as Solvents
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C. Y. 1
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Yuan ,
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S. Dai *, Y . Wei , and Y . W . Chen-Yang
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Chemical Technology Division, Oak Ridge National Laboratory, P . O . Box 2008, Oak Ridge, T N 37831-6181 Department of Chemistry, Drexel University, Philadelphia, P A 19104 Department of Chemistry, Chung Yuan Christian University, Chung-Li, Taiwan 320, Republic of China 2
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Abstract A new methodology has been developed to prepare functional silica aerogels using ionic liquids. This methodology makes use of the unique solvent properties of ionic liquids (negligible vapor pressure and tonicity), allowing application of long aging times without macroscopic phase segregation between ionic liquids and the silica network. The long ambient aging time also ensures development of stable silica networks so that the ionic liquids can be extracted and dried without collapse of the silica network.
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© 2002 American Chemical Society
Rogers and Seddon; Ionic Liquids ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
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Introduction The extraordinary properties of aerogels including high surface areas, low refractive indices, low dielectric constants, low thermal loss coefficients, and low sound velocities, lead to potential applications in areas such as ultra-low dielectric constant interlayer dielectrics, reflective and antireflective coatings, flat panel displays, sensors, catalyst supports, and sorbents. ' ' ' Thus far, the synthesis of silica-based aerogels has been accomplished mainly through the controlled condensation of small colloidal particles produced by sol-gel processing i n alcoholic aqueous solutions, followed by a supercritical drying process. A highly desirable goal in aerogel synthesis is the eUmination of the supercritical drying step, which is the most expensive and risky aspect of the process. The ambient-pressure route enables the preparation of aerogel materials i n a continuous process, which was previously impossible within the constraints of a supercritical autoclave. We have recently developed a new process to synthesize aerogels using ionic liquids as solvents. This ambient process makes use of the unique properties of ionic liquids, allowing the synthesis of stable silica aerogels without use of the supercritical drying step. The key properties of ionic liquids used i n our study are (1) negligible vapor pressure and (2) ionicity. ' ' ' ' ' · ' ' The former attribute allows a long aging process to be applied i n synthesis without concern about shrinkage of the network due to the vaporization of solvents, while the latter ensures that no phase separation between silica polymers and solvents. In the work reported here, we have extended our methodology to synthesize silica aerogels containing functional ligands. This new type of functional aerogels may find potential application as advanced sorbents.
Downloaded by CORNELL UNIV on May 23, 2017 | http://pubs.acs.org Publication Date: July 25, 2002 | doi: 10.1021/bk-2002-0818.ch008
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Experimental Synthesis: Functional aerogels were prepared by the sol-gel process using a room-temperature ionic liquid, l-butyl-3-methylimidazolium tetrafluoroborate (BuMelM+BF^), which was prepared by the metathesis of 1butyl-3-methylimidazolium chloride (BuMeEVfCl) and N a B F . ' Various mixtures of aminopropyltrimethoxylsilane [ ( M e O ^ S K C H ^ N F ^ = APTS, Aldrich] and tetramethylorthosilicate [Si(OM) = T M O S , Aldrich] were used as sol-gel precursors. The acid-catalyzed sol-gel process using formic acid (FA) was employed i n the aerogel preparation. In a typical run, T M O S , APTS, F A , and B u M e l M + B F ^ were mixed i n a plastic bottle. Table 1 gives the 16
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Downloaded by CORNELL UNIV on May 23, 2017 | http://pubs.acs.org Publication Date: July 25, 2002 | doi: 10.1021/bk-2002-0818.ch008
conditions for the preparation and the ratios of precursor solutions. In all cases, a monolithic gel was formed. The gelation times were all less than 1 min. The resulting gels were cured at ambient temperature for 2 weeks i n open air. Then, the entrapped ionic liquids were extracted by acetonitrile (Baker Chemical Co., H P L C grade) at reflux for 1 week. A l l the functional aerogels were dried i n a vacuum oven at 40°C. The final products were translucent. Characterization: FTIR spectra were recorded on a Bio-Rad F T S 3000 FTIR spectrophotometer. Nitrogen adsorption isotherms were measured with a Micromeritics Gemini 2375 surface area analyzer. The small-angle X-ray scattering (SAXS) measurements were conducted using the 5-m S A X S camera at the S A X S user facility of the Oak Ridge National Laboratory.
Table 1. Compositions of precursor solutions and gelation times. Sample a
0% 16% 28% 37% 44% 50%
TMOS (mmol) 6.72 6.72 6.72 6.72 6.72 6.72
APTS (mmol) 0 1.14 2.29 3.42 4.56 5.7
FA (mmol) 53.0 53.0 53.0 53.0 53.0 53.0
Ionic liquid (ml) 1 1 1 1 1 1
Gelation Time (min)
