Well ordered two-dimensional mesoporous CeSBA-15 synthesized with improved hydrothermal stability and catalytic activity M. Selvaraj a,⇑ , D.-W. Park a , C.S. Ha b a School of Chemical and Biomolecular Engineering, Pusan National University, Busan 609-735, Republic of Korea b Department of Polymer Science and Engineering, Pusan National University, Busan 609-735, Republic of Korea article info Article history: Received 26 February 2010 Received in revised form 23 September 2010 Accepted 26 September 2010 Available online 29 October 2010 Keywords: CeSBA-15 pH-adjusting direct hydrothermal method Tetrahedral cerium-ions Hydrothermal stability Oxidation of cyclohexane abstract Mesoporous CeSBA-15 catalysts with a variety of n Si /n Ce ratios have been synthesized under pH-adjusting direct hydrothermal (pH-aDH) method using cerium (IV) sulphate tetrahydrate and Pluronic P123 as the sources of cerium and template, respectively. For comparison studies, Ce/SBA-15(10) catalyst has been synthesized by an incipient wetness impregnation (IWI) method. The hydrothermal stabilities of CeS- BA-15 catalysts have been investigated in boiling water (373 K) at autogenous pressure for 168 h. The mesoporous CeSBA-15 catalysts synthesized by different methods have been characterized using the rel- evant techniques viz. ICP-AES, XRD, N 2 adsorption, UV–vis DRS, EPR, 29 Si MAS NMR, TEM and FE-SEM, and their characteristic results show that the CeSBA-15 catalysts synthesized by pH-aDH method have higher tetrahedral cerium-ions with two-dimensional mesostructures than the Ce/SBA-15(10) synthesized by IWI method. In addition, CeSBA-15(10) has higher hydrothermal stability than CeSBA-15(50). The CeS- BA-15 catalysts have been tested in the liquid phase oxidation of cyclohexane with hydrogen peroxide (H 2 O 2 ). Based on the catalytic activities of all catalysts, CeSBA-15(10) is found to be a highly active, recycle and promising heterogeneous catalyst for the oxidation of cyclohexane. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction Zeolites prepared by a variety of hydrothermal conditions are largely used for various catalytic applications because of their un- ique structural and textural properties. Unfortunately, the use of zeolites is restricted by their small pore sizes, which make them unsuitable for the production of bulky organic fine chemicals. Since the mesoporous M41s materials with large pore sizes are discov- ered by mobile scientists [1], they have gained significant attention and are used in the applications of catalysis and adsorption [2,3]. However, pure MCM-41 has limited to use in the catalytic reac- tions due to the lack of acidity, and it has low hydrothermal stabil- ity due to its thinner pore walls. Generally, the hydrothermal stabilities of mesoporous materials are very important in their cat- alytic applications. In 1998, a new mesoporous SBA-15 mesopor- ous molecular sieve is synthesized under strong acidic condition using Pluronic P123 as a structure-directing agent, and it has high- er hydrothermal stability than MCM-41 due to its thicker pore walls [1,4]. It is well known that, the pure mesoporous silica mate- rials show low catalytic activity due to the absence of heteroatomic active sites. Therefore, using different synthetic methods the het- eroatoms are incorporated into mesoporous silica materials, which are shown to exhibit considerable reactivity due to easily accessi- ble active sites within the mesoporous silica network [5]. However, it is very difficult to introduce the high amounts of metal species directly into the framework of SBA-15 under strong acidic hydro- thermal method because the formation of metal-oxo species in the mesoporous materials is much less. Even though the metal– O–Si bonds can form on the mesoporous silica network, they will be easily dissociated under this acidic condition. However, only few reports are available for the synthesis of MSBA-15 (M = transi- tions metals) materials [6–9]. The MSBA-15 materials are mainly prepared by post synthesis method, which would usually destroy the metal-framework of SBA-15. The post-synthetic process is complicated, and its condition is also stern with usage of solvent [10–12]. Additionally, in this method, the metal oxides easily de- posit on the pore walls of SBA-15, which makes always the forma- tion of non-framework metal oxides on the surface of SBA-15. The metal oxides irregularly distribute on the mesoporous silica net- work and block the pore channel of SBA-15, and it affects the cat- alytic activity [13]. So, the incorporation of cerium into the framework of SBA-15 is necessary using an efficient synthesis method. Generally, cerium incorporated into mesoporous material is powerful one-electron oxidant, and it has dual acid behavior such as Lewis and Brønsted acidity. An important advantage of this material is the reversible multi-electron redox transformation, 1387-1811/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2010.09.025 ⇑ Corresponding author. Tel.: +82 51 510 2397; fax: +82 51 512 8563. E-mail address: chems@pnu.edu (M. Selvaraj). Microporous and Mesoporous Materials 138 (2011) 94–101 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso