Synthesis and characterization of micrometer-sized silica aerogel nanoporous beads Pradip B. Sarawade, Dang Viet Quang, Askwar Hilonga, Sun Jeong Jeon, Hee Taik Kim ⁎ Department of Fine Chemical Engineering, Hanyang University, 1271 Sa3-dong, Sangnok-gu, Ansan-si, Gyeonggi-do 426–791, Republic of Korea abstract article info Article history: Received 8 March 2012 Accepted 23 April 2012 Available online 2 May 2012 Keywords: Micro-silica aerogel bead Ambient pressure drying Nanoporous Water-glass Here we report the preparation of micrometer-sized highly nanoporous, relatively trasperant silica aerogel beads with high surface area as well as large pore volume with sizes ranging from 165 to 395 μm. The wet micrometer-sized silica hydrogel beads were prepared through hydrolysis and polycondensation of sodium silicate as a silica precursor. A hydrophobic micro-silica aerogel nanoporous bead was synthesized by simultaneous solvent exchange surface modification process of as synthesized micron sized silica hydrogel bead at an ambient pressure. Hydrophilic micron-sized silica aerogel beads with relatively more textural properties (surface area, pore volume and pore size) with its counterpart were obtained by heating the synthesized hydrophobic micro-silica aerogel beads at 395 °C for an hour. This study demonstrates a robust approach to high porous hydrophobic and hydrophilic micro-silica aerogel beads with a myriad of potential applications in various fileds such as catalysis, biomolecule immobilization, chromatographic separation, and CO 2 absorption. This proposed synthesis, which exploits a low-cost silica source (water-glass), is suitable for large-scale industrial production of highly porous hydrophobic and hydrophilic micro-silica aerogel beads at an ambient pressure. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Materials with high porosity and low density have attracted much attention recently because they combine the advantages of high surface area and high pore volume as well as larger pore size with the accessible diffusion pathways associated with nanoporous structures. Aerogels are ultralight, non-hazardous, non-flammable, easy to discard, and extremely porous nanostructured materials are currently available. Silica aerogels modified with methyl (-CH 3 ) groups can function very well in the sorption of organics, and their adsorption capacities are 10 times higher than those of activated carbon [1]. Granular/silica aerogel beads (micron-sized) can easily be filled into hollow spaces and provide high thermal resistance, even without evacuation. Further, silica aerogels can serve as potential materials for capturing CO 2 as well as thermal super- insulators in solar energy systems, refrigerators, thermos flasks [2], internal confinement fusion (ICF) targets for thermonuclear fusion reactions [3], very efficient catalysts and catalytic supports [4], storage media for liquids in rocket propellants [5]. Despite these applications, aerogels are not routinely found in daily life because they fragile, collapse easily and are difficult to prepare in a large-scale industrial production setting. The unique characteristic features of silica aerogels arise from the fact that they are mainly composed of air. As the 3D skeletons of the silica aerogels that comprise their porous morphology are too thin, aerogels are brittle, and they must be dried with the utmost care using supercritical drying techniques. Conventionally, silica aerogels are prepared via sol–gel polymerization of high-cost hazardous alkoxides, such as tetraethoxysilane (TEOS), tetramethoxysilane (TMOS), and methyltrimethoxysilane (MTMS) through supercritical drying by removing the entrapped solvent from the wet gel while maintaining its integrity and the high porosity [6]. However, supercritical drying has limitations in terms of cost and safety as it involves heating and evacuation of flammable solvents at high temperature (260 °C) and pressure (100 bar). As a result, a method to produce silica aerogels using low cost inorganic precursors such as sodium silicates at low temperature and pressure is needed. Ambient pressure drying (APD) is competitive in terms of cost and safety, and the preparation of silica aerogels using this technique has been extensively studied over the last decade [7]. Ambient pressure drying is mainly based on solvent exchange and surface modification of wet gels [8]. Solvent exchange and surface modification processes are essential for preserving the porous network of the gel before APD. However, solvent exchange is a lengthy and tedious process that involves diffusion of a solution within a gel. Due to this lengthy process, drying of silica aerogels (large in size) at an ambient pressure can take several hours or even days, which limits industrial large-scale production. Solvent exchange processes depends on the surface tension of the solvent and the size (surface area) of the gel. Moreover, large-scale hydrogels are fairly weak and tend to crack during solvent exchange and an ambient pressure drying. Thus, the synthesis of large-scale sodium silicate-based monolithic silica aerogels at an ambient pressure has limitations. In contrast, if silica hydrogel beads (micrometer-size) are used, solvent exchange is much faster; Materials Letters 81 (2012) 37–40 ⁎ Corresponding author. Tel.: + 82 31 400 5274; fax: + 82 31 500 3579. E-mail address: khtaik@yahoo.com (H.T. Kim). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.04.110 Contents lists available at SciVerse ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet