Ce-silica mesoporous SBA-15-type materials for oxidative catalysis: Synthesis, characterization, and catalytic application
Graphical abstract
Cerium-containing mesoporous materials have been synthesized by hydrothermal method and characterized by IR, DR-UV–vis and DRIFT spectroscopy, XRD and N2 adsorption methods. Important factors affecting the catalytic activity of Ce-SBA-15, namely the effect of cerium content, the state of cerium ions, the state of silanol groups on the surface of Ce-SBA-15 and stability of the catalyst have been studied in the cyclohexanol and cyclohexene oxidation with hydrogen peroxide. The reaction mechanism was discussed.
Introduction
Nowadays, cerium-containing materials are powerful one-electron oxidants that are used as catalysts for the selective oxidation of organic reagents [1], [2], [3], [4], [5], [6]. In order to improve activity and selectivity of the catalysts, the cerium-containing systems were intensively investigated. In spite of high activity of homogeneous catalytic systems, the current tendency is to replace the homogeneous catalysts by their immobilized counterparts, namely catalysts based on cerium ions incorporated into the network of micro- or mesoporous materials. Several methods have been developed to synthesize ceria-based oxides, such as precipitation, microemulsion, hydrothermal synthesis, etc. [7], [8], [9], [10], [11]. However, the examples of the incorporation of cerium ions into silica network are limited to few reports due to several difficulties connecting with the different size of silicon and cerium atoms [12], [13], [14], [15].
The main advantages of systems based on the cerium ions incorporated into the amorphous silica network are dual physicochemical properties such as Lewis and Brønsted acidity and reversible multielectron redox transformations. These properties can be useful for acid and redox catalysis as well. Thus, coordination of cerium ions in mesoporous materials can affect the catalytic properties because the incorporation of cerium atoms into the walls of mesoporous silica material allows creation of Lewis and Brønsted acid sites and preparation of materials with various acidities [15]. However, the details of location and coordination of cerium ions incorporated into the framework of mesoporous silicates and influence of these parameters on the catalytic activity are still unclear.
In the present paper, cerium-containing materials (Ce-SBA-15) were prepared by hydrothermal method. The synthesized materials are characterized in detail to insight of coordination environment of the cerium ions. The influence of the cerium state in Ce-SBA-15 on its catalytic properties and leaching features in cyclohexene and cyclohexanol oxidation with hydrogen peroxide are investigated in detail.
Section snippets
Materials
H2O2 (28 wt.% in water) was titrated iodometrically prior to use [16]. Cyclohexene (99%) from Aldrich was purified by distillation. Cyclohexene oxide, cyclohexanol, trans-cyclohexene-1,2-diol, 2-cyclohexene-1-ol, and 2-cyclohexene-1-on from Aldrich, a triblock copolymer, Pluronic P123 (EO20PO70EO20, Mav = 5800) from Sigma, sodium metasilicate (Na2SiO3·9H2O) and hydrochloric acid (37.6% HCl), cerium acetate (Ce(CH3CO2)3·H2O) from Fisher Scientific were used without further purification.
Synthesis of Ce-SBA-15
Ce-SBA-15
Textural characterization of Ce-containing mesoporous silica materials
The XRD and textural data (specific surface area (SBET), average pore diameters (D), and specific pore volume (Vpore)) of Ce-SBA-15 are summarized in Fig. 1 and Table 1, respectively. The XRD pattern of Si-SBA-15 shows the diffraction peaks at 0.99°, 1.63°, and 1.86° corresponding to p6mm hexagonal lattice symmetry, characteristic of SBA-15 structure (Fig. 1) [18], [19]. After insertion of cerium atoms into framework of SBA-15, the diffraction peaks corresponding to d100, d110, and d200 shift
Conclusion
Cerium-containing mesoporous SBA-15-type materials have been synthesized by hydrothermal method. The synthesized Ce-SBA-15 materials have been characterized by IR, DR-UV–vis and DRIFT spectroscopy, XRD and N2 adsorption isotherm. According to IR and DR-UV–vis spectroscopy, an agglomeration of cerium atoms takes place with the increasing of the metal content in SBA-15 samples.
Ce-SBA-15 materials were tested in cyclohexanol and cyclohexene oxidation with hydrogen peroxide. It was established that
Acknowledgements
This work was financially supported by Korean Research Institute of Chemical Technology (KK-0603-F0) and by Brain Pool program (grant 042-1-3 sponsored by KOFST of Korea). We thank Dr. E.B. Burgina, Dr. O.A. Kholdeeva, Dr. E.A. Paukshtis, Mr. A.B. Ayupov, and K.F. Obzherina for discussion and IR measurements.
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