CO 2 adsorption on APTES functionalized mesocellular foams obtained from mesoporous silicas E. Vilarrasa-García a , J.A. Cecilia a , S.M.L. Santos b , C.L. Cavalcante Jr. b , J. Jiménez-Jiménez a , D.C.S. Azevedo b , E. Rodríguez-Castellón a,⇑ a Universidad de Málaga, Departamento de Química Inorgánica, Facultad de Ciencias, 29071 Málaga, Spain b Department of Chemical Engineering, Universidade Federal do Ceará, Campus do Pici, bl. 709, 60455-760 Fortaleza, Brazil article info Article history: Received 23 August 2013 Received in revised form 31 October 2013 Accepted 17 December 2013 Available online 31 December 2013 Keywords: APTES Grafted silicas CO 2 Chemisorption Pore expanding agents abstract The CO 2 adsorption capacity of different APTES-grafted mesoporous silicas of SBA-15 type has been inves- tigated and the influence of support textural properties and the role of the presence of silanol groups on the adsorption capacity are analyzed. Four adsorbents based on SBA-15 were prepared using tetraethyl orthosilicate (TEOS) as silicon source, with and without the addition of trimethyl-benzene (TMB) and n-heptane as swelling agents, and adding in some cases ammonium fluoride as a solubility enhancer. 3-(triethoxysilyl)propylamine (APTES) was then used as grafting agent by reaction with free silanol groups on the silica surface so as to provide pending amino groups for CO 2 capture. The adsorption behavior for all supports was adequately described by a Freundlich model, whereas for the APTES-grafted silica, a dual-site Langmuir model was applied, which allowed us to quantify and qualify two different adsorption sites. The addition of n-heptane as swelling agent led to pore sizes beyond 10 nm and improved significantly the grafting efficiency, leading to higher CO 2 uptakes as compared to the starting supports. At 1 bar and 25 °C and anhydrous conditions, CO 2 uptakes of 2.4 mmol g 1 or 0.64 mol CO 2 per mol N were achieved (which reveals a significant contribution of physisorption). This sample could be successfully regenerated at 100 °C, maintaining a constant capacity for 3 adsorption–desorption cycles. At 0.15 bar and 60 °C, anhydrous conditions, CO 2 uptake reaches 1.5 mmol g 1 . This value may be theo- retically doubled in the presence of humidity, and there is room for further improvement if supports with the same pore size (13 nm) and higher surface areas (e.g. 1000 m 2 /g) are successfully synthesized. Ó 2013 Elsevier Inc. All rights reserved. 1. Introduction Current industrial technologies for CO 2 capture involve the use of absorbing liquids such as monoethanolamine, diethanolamine and methyldiethanolamine. However, these absorption technolo- gies have several drawbacks due to the harmful nature of the amines, their corrosive activity, poor chemical stability and the high energy requirements for their regeneration. The use of adsorption in porous solids for CO 2 capture is an alternative that has been investigated along the last few years [1–31]. Adsorbents such as active carbons or carbon molecular sieves with very high adsorption capacity (8.6 mmol CO 2 g 1 ) at 0 °C and 1 bar have been reported [32,33], which is well above the maximum CO 2 adsorp- tion capacity for zeolites such as 13 and 5A at this temperature (5.2 and 4.1 mmol CO 2 g 1 , respectively). However, the adsorption capacity of these solids and the selectivity for CO 2 decrease dramatically at higher temperatures and in the presence of water vapor. Therefore, due to the relatively high temperatures of flue gases (45–75 °C), chemisorption may be more adequate than phys- isorption for CO 2 capture in such applications. Chemisorption pro- cesses based on amine functionalized mesoporous silicas have been investigated by many groups since some of these functional- ized porous solids exhibit adsorption capacities higher than 2.0 mmol CO 2 g 1 . These solids include siliceous supports such as MCM-41, SBA-15, HMS, MCM-48 [34–37]. Silica grafting with amines having different number of nitrogen atoms (3-(trimethoxy- silyl)propylamine, N[3-(trimethoxysilyl)propyl]ethylendiamine, N 1 [3-(trimethoxysilyl)propyl]diethylentriamine) have given rise to a class of materials with notably high adsorption capacities, for example, 2.7 mmol CO 2 g 1 at 45 °C and 1 bar for MCM-41 grafted with N[3-(trimetoxisilil)propyl]ethylendiamine [37]. The need of a robust porous support with high chemical and mechanic stability under the conditions of flue gas emissions oriented the research of Calleja and co-workers, who have used SBA-15 functionalized with different amine groups [38–40] for the capture of CO 2 at 45 °C and 1 bar. The best result (1.8 mmol CO 2 g 1 ) was obtained with a SBA-15 grafted with N 1 [3-(trimetoxisilil)pro- pyl]diethylentriamine) [39]. 1387-1811/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2013.12.023 ⇑ Corresponding author. Tel.: +34 952131873; fax: +34 952131870. E-mail address: castellon@uma.es (E. Rodríguez-Castellón). Microporous and Mesoporous Materials 187 (2014) 125–134 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso