Silica aerogels formed from soluble silicates and methyl trimethoxysilane (MTMS) using CO2 gas as a gelation agent
Introduction
Silica aerogel is a nano-porous material consisting of nanoparticle building blocks, which has many unique properties, such as ultralow bulk density (0.03–0.35 g/cm3), high porosity (80–99%) and high surface area (500–1500 m2/g) [1]. Due to the excellent properties, silica aerogel has attracted a lot of attentions in many fields, such as thermal insulations, catalyst barriers, sensing, acoustic insulation and electrodes, and etc. Silica aerogel is commonly developed via the sol-gel process and dried in such a way to avoid pore collapse, leaving an intact solid nanostructure with 90–99% air inside by volume [2]. The conventional way of making aerogel is usually using silica alkoxides as the starting silica source and supercritical drying technique is widely adopted for drying process. However, silica alkoxides, such as Tetraethyl orthosilicate (TEOS) and Tetramethyl orthosilicate (TMOS), are expensive and more or less toxic, especially for TMOS. The supercritical dyring method requires a high temperature and/or high pressure vessel, which increases the manufacture cost and brings up safety issues. Therefore, large-scale manufacturing silica aerogel may have to rely on cost-effective silica precursors, such as water glass, and safer ambient pressure dyring technique. One of the the keypoints of ambient pressure dyring lies at the surface modification process, which controls the silica wet gel transferring from hydrophilic to hydrophobic. Researchers [3], [4], [5], [6] have proposed the method of silica aerogel fabrication via water glass, combined with ambient pressure dyring (APD) technique. In their work, water glass solution was filtrated through the ion exchange resin to remove the Na+ inside, forming silicic acids with PH = 2–3. The solution was then catalysed by diluted ammonia or sodium hydrate to PH = 4–6, after which gelation occurred spontaneously. The hydogel was aged, alcoholized and surface modified before dyring in an oven to obtain silica aerogel.
This work has first utilized CO2 gas as a gelation agent to produce silica aerogel with water glass as the precusor. In the process, CO2 reacts with water glass solution and gelation happens where a large amount of Na+ exists in the gel matrix. The Na+ inside the matrix was further removed by aging, solvent exchange and surface modification. The chlorotrimethylsilane (TMCS)/hexane/ethanol mixed solution was used as the surface modification agent. Afterwards, the hydrophobic silica aerogel has been derived through APD technique. However, results showed that the silica particles and pore diameters derived from this route were pretty large (>100 nm, Fig. S1(a)), which destroys the typical nanostructures of silica aerogels. Pan [7] et al. proposed developing silica aerogel using water glass/methyltrimethoxysilane (MTMS) as the co-precursors, and the introduction of MTMS can be a silylation agent which generates the hydrophobicity of the resulting silica aerogel. Rao [8] et al. developed a flexible silica aerogel using MTMS as the precusor, and characterization showed that the resulting aerogel exhibits a high contact angle (164°) with high thermal stability of 257 °C. Based on previous studies, MTMS has also been employed in this work to adjust the hydrolysis and condensation rate, which therefore modified the pore structures due to the synergistic reaction. The effects of the amount of MTMS, the volume ratio of TMCS/silica sol and the calcining temperature on the physicochemical properties of the resulting silica aerogels have been investigated.
Section snippets
Materials and synthesis methods
Water glass solution (Na2O, ~10.6% and SiO2, ~26.5%), trimethyl chlorosilane (TMCS), n-hexane, ethanol (EtOH), and deionized water (H2O) were purchased from Sigma-Aldrich corporation, USA. All of them were of analytical reagent grade and no further purification was carried out. CO2 used in this experiment was of high purity of 99.99%.
The technical development route of silica aerogel is shown in Fig. 1. 50 ml water glass solution was mixed with 160 ml water to make an 8 wt% silicate solution. The
Results and discussion
Water glass, which is synthesized commercially by reacting quartz and sodium hydroxide and/or sodium carbonate at elevated temperatures, has always been recognized as the cheapest source for silica aerogel fabrication [3], [4], [7]. The polar nature of molecule (Si-O- and Na+ ion pairs) makes it dissolve easily in water, and also prevents it form spontaneous agglomeration and gelation [9]. Water glass solution is unstable when the pH value is in the range of 4–10. When the pH value is greater
Conclusions
Hydrophobic silica aerogel is fabricated using CO2 gas as the gelation agent with the low-cost ambient pressure drying technique. In the gelation process, MTMS works as an effective hydrolysis-condensation rate control agent, and the MTMS/silica sol volume ratio has been determined at 1% based on specific surface area tests. Water wash and the following solvent exchange process removed Na+ existing in the matrix, and thus no ion exchange resin is involved throughout the entire aerogel
Acknowledgement
This project is supported by the funding from the Wyoming State Legislator through the School of Energy Resources, University of Wyoming, USA.
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