Preparation of hydrophobic mesoporous silica powder with a high specific surface area by surface modification of a wet-gel slurry and spray-drying Pradip B. Sarawade a , Jong-Kil Kim b , Askwar Hilonga a , Hee Taik Kim a, ⁎ a Department of Chemical Engineering, Hanyang University, 1271 Sa 3-dong, Sangnok-gu, Ansan-si, Gyeonggi-do 426-791, Republic of Korea b E&B Nanotech. Co., Ltd, Republic of Korea abstract article info Article history: Received 16 June 2009 Received in revised form 15 September 2009 Accepted 10 October 2009 Available online 17 October 2009 Keywords: Mesoporous silica powder Sodium silicate Surface modification Spray-drying Surface area A hydrophobic mesoporous silica powder was prepared by surface modification of a sodium silicate-based wet-gel slurry. The effects of the volume percentage (%V) of trimethylchlorosilane (TMCS), used as surface- modifying agent, on the physicochemical properties of the silica powder were investigated. We observed that as the %V of TMCS in the simultaneous solvent exchange and surface modification process increased, so did the specific surface area and cumulative pore volume of the resulting silica powder. Hydrophobic silica powder with low tapping density (0.27 g/cm 3 ), high specific surface area (870 m 2 /g), and a large cumulative pore volume (2.2 cm 3 /g) was obtained at 10%V TMCS. Surface silanol groups of the wet-gel slurry were replaced by non-hydrolysable methyl groups (–CH 3 ), resulting in a hydrophobic silica powder as confirmed by FT-IR spectroscopy and contact angle measurements. We also employed FE-SEM, EDS, TG–DTA, and nitrogen physisorption studies to characterize the silica powders produced and to compare the properties of modified and unmodified silica powders. Moreover, we used a spray-dying technique in the present study, which significantly reduced the overall processing time, making our method suitable for economic and large- scale industrial production of silica powder. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Mesoporous materials with high surface area are used as dielectric materials, elastomeric materials (such as tires), in flat panel displays, sensors, filters for exhaust gases, filters for automobile exhaust systems, industrial pollutants, adsorbents, separations, biomedicine, censors, drug delivery systems, oil-spill clean-up [1–5], and hetero- geneous catalysts in various chemical reactions [6]. Aerogels are the most highly porous material available, and are normally prepared by supercritical extraction of pore liquid from wet gels [7–10]. However, economic large-scale industrial production of silica aerogels is hampered by three main factors: (i) the use of expensive autoclaves for the supercritical drying process that involves heating and evacuation of highly inflammable solvents; this is potentially risky because the process is performed at a high temperature (260 °C) and pressure (∼ 100 bars); (ii) the use of high-cost and hazardous alkoxides [tetraethoxysilane (TEOS) or trimethoxysilane (TMOS)] in the sol–gel synthesis, and (iii) the long synthesis period required (up to 7 days in some cases). Moreover, aerogels are very sensitive to moisture (hydrophilic); thus, it is essential to modify the surface of the aerogels using non-hydrolysable organic groups. The silanol (Si– OH) groups present on the gel structure are the main source of hydrophilicity and they promote a condensation reaction that produces H 2 O; the structure therefore deteriorates over time. The replacement of the hydrogen (H) in the Si–OH group with an hydrolytically stable organofunctional group such as Si–R (R=CH 3 , C 2 H 5 , etc.), alkoxysilanes, or alkylsilazanes and alkylchlorosilanes of the type R n SiX 4 - n , can be used to produce hydrophobic silica material that is not affected by moisture [11,12]. It is reasonable to predict that aerogels in powder form might have a high surface area and show a general improvement in undesirable properties caused by the presence of hydrophobic groups. However, this has not been thoroughly investigated, and powder-form aerogels have not been compared with conventional hydrophobic aerogels produced by a supercritical drying process or by ambient pressure drying [13–17]. To the best of our knowledge, few reports have investigated the versatile and cost–effect synthesis of hydrophobic silica powders. Kim et al. synthesized hydrophobic silica powders by surface modification of wet gels [18]; however, the authors used expensive silica alkoxides such as TEOS. Recently, Bhagat et al. synthesized silica powders using sodium silicate silica as a precursor and an expensive silylating agent (hexamethyldisilazane) using a derivatization method [19]. The silica powders they prepared had a low surface area (700 m 2 /g) and low pore volume (0.9 cm 3 /g). Furthermore, their method was fairly complicated and may not be suitable for economical large-scale industrial production of silica powder. To develop a versatile and cost-effective method to produce hydrophobic mesoporous silica powders with a high surface area and Powder Technology 197 (2010) 288–294 ⁎ Corresponding author. Tel.: +82 31 400 5493; fax: +82 31 500 3579. E-mail address: khtaik@yahoo.com (H.T. Kim). 0032-5910/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2009.10.006 Contents lists available at ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec