Silica ionogels synthesized with imidazolium based ionic liquids in presence of supercritical CO2
Graphical abstract
Introduction
Silica aerogels are highly porous solid materials, which are having high specific surface area, low density, low refractive index and low thermal conductivity. The first silica aerogels were synthesized by Kistler in 1931. Kistler prepared aerogels also from many other precursors including different types of metal oxides [1]. Since then the route for synthesis of aerogels has been improved in order to be obtained shorter gelation times, tunable properties and faster production process. Nowadays aerogels are finding a lot of applications in many fields like thermal and acoustic isolation, supercapacitors, catalytic supports [2], [3]. Also different types of substance can be confined in the silica matrix using the sol–gel process.
Organic–inorganic nano composites are a novel class of hybrid materials. Silica ionogels are porous materials with intrinsic hybrid character, in which the ionic liquids are confined in a solid-like matrix. Ionic liquids are salts, which are having melting point below 100 °C. Ionic liquids usually consist of a large organic cation, which can be ammonium, phosphonium, pyridinium, imidazolium etc. and an ionorganic or organic anion. Because of the large variety of cations and anions combinations ionic liquids can be tuned for different kind of applications and are used e.g. as solvents and co-solvents, as catalysts as well as in electrochemistry [4]. Therefore the ionogel features derive from the combination of the properties of the ionic liquid and the component forming the solid network. These materials have found applications in areas of optical and electrochemical sensing, solid state electrolytes, etc. The most common method for the synthesis of silica ionogels is the sol–gel route. During this process two chemical reactions occur—hydrolysis and condensation. The gelation time of the ionogels depends on many factors like silica source, solvent, catalyst, temperature, nature of the ionic liquid etc. Also the properties of the final product are depending not only on the synthesis conditions but also on the aging time, drying procedure, etc. [5], [6], [7], [8].
The positive influence of the imidazolium based ionic liquids on the gelation time of silica ionogels was already reported in the literature. Karout and Pierre are reporting that a small amount of the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate can act as a catalyst, leading to shorter gelation times during the sol–gel process [9]. Similar results were reported also from Gupta et al. The authors were using the non-hydrolytic sol gel process for synthesis of ionogels with Tetraethylorthosilicate (TEOS) as a silica precursor. The used ionic liquid was 1-ethyl-3-methylimidazolium tetrafluoroborate. The synthesis temperatures were −10, 0 and 30 °C. It was proved that increasing the synthesis temperature in combination with ionic liquid leads to shorter gelation time. The obtained ionogels were with specific surface area between 125.5 and 320.2 m2/g [10].
The influence of the supercritical carbon dioxide on the gelation time of silica aerogels has been reported the first time from Smirnova and Arlt. It was used the two step sol–gel process and hydrochloric acid and ammonium hydroxide as gelation catalysts. It has been reported that the presence of supercritical CO2 during the synthesis leads to shorter gelation times enhancing the reactions speed. [11].
In the present work silica ionogels were prepared by the sol–gel process using imidazolium ionic liquids, containing different cations and anions. The influence of supercritical carbon dioxide on the gelation time and the properties of silica ionogels were investigated for first time in the present study.
Section snippets
Reagents
Tetramethyl orthosilicate (TMOS) 98% (Merck Millipore, Germany), ethanol 99.8% (Sigma-Aldrich), citric acid (CA) 99% from Roth and deionized water were used as received. The ionic liquids 1-butyl-3-methylimidazolium tetrafluoroborate [C4mim][BF4] purity >98%., 1-butyl-3-methylimidazolium hexaflurophosphate [C4mim][PF6] purity >98%, 1-butyl 3-methylimidazolium bis(trifluoromethyl-sulphonyl)imide [C4mim][Tf2N] purity >98%, 1-hexyl-3-methylimidazolium tetrafluoroborate [C6mim][BF4] purity >98%
Influence of the temperature and pressure on the gelation time of the reference sample
The gelation time for the reference sample was 240 min. To estimate the optimal synthesis temperature, the reference samples were made at Three different temperatures and room pressure. The gelation times for those samples are given in Table 1:
From the obtained results it is obvious that an increasing synthesis temperature shortens the gelation time, which is also in agreement with the study of Silva et al. [14]. The increase of the temperature accelerates the speed of the both reactions.
Conclusions
Imidazolium based ionic liquids [C4mim][BF4], [C4mim][PF6], [C4mim][Tf2N] and [C6mim][BF4] have been successfully applied as co–catalysts during the sol–gel process and can reduce the gelation time of one-pot synthesized silica ionogels. The presence of supercritical carbon dioxide decreases further the gelation time and has influence on the surface area of the obtained gels. The ionogels, containing the IL with the most hydrophobic anion [Tf2N]- show the longest gelation times and the samples
Acknowledgment
This work is supported by the Cluster of Excellence RESOLV (EXC 1069) funded by the Deutsche Forschungsgemeinschaft.
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