Bridging the gap between micropores and mesopores by the controlled transformation of bifunctional periodic mesoporous silicas S. Shylesh a , Ch. Srilakshmi b , A.P. Singh a, * , B.G. Anderson b a Inorganic and Catalysis Division, National Chemical Laboratory, Pune 411 008, India b Schuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600MB, Eindhoven, The Netherlands Received 7 July 2006; received in revised form 17 March 2007; accepted 19 March 2007 Available online 4 April 2007 Abstract Novel bifunctional periodic mesoporous silicas having high content of organic groups in the pore channels and chromium in the frame wall has been synthesized by direct co-condensation method, using organo trialkoxysilanes (chloro propyl, vinyl, methyl), chromium nitrate and tetraethyl orthosilicate, in presence of cetyltrimethyl ammonium surfactants. The obtained materials show one main peak in the XRD patterns up to a molar ratio of 1:1 between TEOS and organosilane in the synthesis gel and N 2 adsorption–desorption anal- ysis shows that the mesopore structure remains intact after the surfactant removal process. Among the organosilanes, chloro propyl and vinyl pendant chromium samples shows a shift in the main peak to higher 2-theta values with a corresponding decrease in pore volume and pore diameter, with the percentage of organosilane functionalization. Small-pore silicas and chromium silicas can be prepared from the large pore mesoporous MCM-41, by effectively removing the organic groups by calcination. By this method, the pore size of the material can be tailored into the super-microporous region, without changing the chain length of the surfactant used in the assembly process. The shrinkage in pore size is dependent on the nature and percentage of the organic pendant groups that the chloro propyl and vinyl pendant mesoporous materials show the maximum pore size shrinkage than the small chain length methyl units. Because of the tailorable pore size and with better textural characteristics, chromium-silica materials find applications in the field of shape selec- tive heterogeneous catalysis, as demonstrated in the oxidation reaction of cyclohexane, than the conventional metal-containing mesopor- ous materials. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Bifunctional silicas; MCM-41; Chromium; Cyclohexane; Oxidation 1. Introduction Soon after the discovery of mesoporous materials of the M41S family by Mobil researchers, ordered mesoporous materials possessing high content of organic groups consti- tute an active research area [1–6]. These organic–inorganic hybrid materials with reactive groups are unique as they offer potential advantages than the pristine silica analog for various surface modifications, and are widely explored in the field of catalysis, separation science, adsorption, sensing and so forth [7–12]. Importantly, the nature and the content of organic groups determine the specific prop- erties of these nanocomposites like the surface hydropho- bicity, hydrothermal, thermal and mechanical properties. However, the uniformity of the organic groups inside the pore channels affects the surface properties, functionalized organic group reactivity and the accessibility of the porous network for further modifications. There are two methods by which organic groups of desired functionality can be loaded on mesoporous support surfaces; viz. direct co-condensation method or post-syn- thesis grafting method [7–9]. Generally, post-synthesis method is the commonly and routinely used procedure for the synthesis of organic–inorganic hybrid mesoporous materials. In this process, a surface reaction of the silanol groups of mesoporous materials (surfactant-free) with the 1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.03.039 * Corresponding author. Tel.: +91 20 2589376; fax: +91 20 25893761. E-mail addresses: ap.singh@ncl.res.in (A.P. Singh), B.G.Anderson@ tue.nl (B.G. Anderson). www.elsevier.com/locate/micromeso Available online at www.sciencedirect.com Microporous and Mesoporous Materials 108 (2008) 29–40