Graphitic nanocrystals inside the pores of mesoporous silica: Synthesis, characterization and an adsorption study F. de Clippel b , A. Harkiolakis a , T. Vosch c , X. Ke d , L. Giebeler e , S. Oswald e , K. Houthoofd b , J. Jammaer a , G. Van Tendeloo d , J.A. Martens b , P.A. Jacobs b , G.V. Baron a , B.F. Sels b,⇑ , J.F.M. Denayer a,⇑ a Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, 1000 Brussel, Belgium b Centre for Surface Chemistry and Catalysis, Katholieke Universiteit Leuven, Kasteelpark Arenberg 23, 3001 Heverlee, Belgium c Molecular and Nanomaterials, Katholieke Universiteit Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium d Electron Microscopy for Materials Science, Universiteit Antwerpen, Groenenborgerlaan 171, 2020 Antwerpen, Belgium e Institute for Complex Materials, IFW Dresden e.V., Helmholtzstrabe 20, D-01069 Dresden, Germany article info Article history: Received 20 January 2011 Received in revised form 31 March 2011 Accepted 3 April 2011 Available online 8 April 2011 Keywords: Composite Mesoporous silica Carbon Adsorption Molecular sieving abstract This work presents a new carbon–silica hybrid material, denoted as CSM, with remarkable sorption prop- erties. It consists of intraporous graphitic nanocrystals grown in the pores of mesoporous silica. CSM is obtained by a subtle incipient wetness impregnation of Al-containing mesoporous silica with furfuryl alcohol (FA)/hemelitol solutions. Both the volume match of the impregnation solution with that of the silica template pore volume, and the presence of Al 3+ in the silica, are crucial to polymerize FA selectively inside the mesopores. Carbonization of the intraporous polymer was then performed by pyrolysis under He up to 1273 K. The resulting CSMs were examined by SEM, HRTEM, 27 Al MAS NMR, N 2 adsorption, XRD, TGA, TPD, XPS, pycnometry and Raman spectroscopy. Mildly oxidized graphitic-like carbon nanoblocks, consisting of a few graphene-like sheets, were thus identified inside the template mesopores. Random stacking of these carbon crystallites generates microporosity resulting in biporous materials at low car- bon content and microporous materials at high carbon loadings. Very narrow pore distributions were obtained when pyrolysis was carried out under slow heating rate, viz. 1 K min 1 . Adsorption and shape selective properties of the carbon filled mesoporous silica were studied by performing pulse chromatog- raphy and breakthrough experiments, and by measuring adsorption isotherms of linear and branched alkanes. Whereas the parent mesoporous silica shows unselective adsorption, their CSM analogues pref- erentially adsorb linear alkanes. The sorption capacity and selectivity can be adjusted by changing the pore size of the template or by varying the synthesis conditions. A relation between the carbon crystal- lites’ size and the shape selective behaviour of the corresponding CSM for instance is demonstrated. Most interestingly, CSM shows separation factors for linear and branched alkanes up to values comparable to those of zeolitic molecular sieves. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction During the past decades, extensive research efforts were deliv- ered in catalysis and sorption technology to develop microporous zeolites and mesoporous silica and alumina materials. Zeolites are crystalline and thermostable, and offer the possibility of molec- ular sieving. However, their pure microporous nature often causes diffusion problems and do not allow bigger molecules to enter the pores. Because of the limitations of zeolites, Mobil researchers developed an alternative mesoporous family of silicas [1]. These amorphous but ordered M41S materials are accessible to larger molecules but show a lack of molecular sieving capabilities. More recently, several research groups initiated developing biporous materials which combine both the advantages of meso- and micro- porous materials [2–7]. Although new zeolite topologies are discovered every year, their introduction in industrial applications seems to be decreasing. This observation demonstrates the lack of flexibility due to the ana- logue alumina–silica chemistry of all zeolites. New types of micro- porous materials exhibiting different surface chemistry could result in different adsorption selectivities and subsequently new applications and processes. Therefore, the search for alternative types of carbon based porous materials became pertinent again during the last few years. Commercial carbon materials are espe- cially common in wet gas stream separations, treatment of organic waste water (active carbon) and N 2 /CH 4 separations (carbon molecular sieves). Global demand of carbon materials is rapidly increasing as demonstrated for active carbon: its annual demand is expected to increase from 1.2 million tons/year in 2010 up to 1387-1811/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2011.04.003 ⇑ Corresponding authors. Tel.: +32 2 629 17 98; fax: +32 2 629 32 48. E-mail address: Joeri.Denayer@vub.ac.be (J.F.M. Denayer). Microporous and Mesoporous Materials 144 (2011) 120–133 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso