Synthesis and Characterization of Dendrimer-Templated Mesoporous Oxidation Catalysts
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Catalysis Letters Vol. 98, Nos. 1, October 2004 (Ó 2004)
Synthesis and characterization of dendrimer-templated mesoporous oxidation catalysts Matt C. Rogers, Bamidele Adisa, and David A. Bruce* Department of Chemical Engineering, Clemson University, Clemson, SC 29634-0909
Received 11 March 2004; accepted 6 August 2004
We report here the use of 4th and 5th generation dendrimers poly(propylene)imine (CU-D32 and CU-D64) as templating agents for the synthesis of mesoporous titanosilicate and vanadosilicate oxidation catalysts via sol–gel techniques. The physical properties of these mesoporous materials were characterized by TGA, BET, PXD and SEM/EDX analyses and these showed that the transition metals are evenly distributed throughout these silicates, which have interconnected spherical pores (approx. 12 A˚ in diameter) and high surface areas of about 650 m2 g1 . Kinetic studies showed that all transition metal-doped catalysts were highly selective at oxidizing cyclohexene to the corresponding epoxide. Additionally, CU-D64-templated catalysts were more catalytically active for cyclohexene epoxidation than CU-D32-templated catalysts as a result of differences in pore size. All CU-D64-templated catalysts exhibited epoxidation catalytic activity comparable to that of titanium doped MCM-41 materials. KEY WORDS: dendrimer, mesoporous, oxidation catalyst, sol–gel, vanadosilicate, titanosilicate, cyclohexene, epoxidation.
1. Introduction The selective oxidation of an olefinic bond is an essential step in the production of numerous fine chemicals and pharmaceuticals [1–6]. In recent years, several heterogeneous and homogeneous catalysts have been shown to be effective in the selective oxidation of alkenes [7–11]. Of the heterogeneous catalysts studied, mesoporous oxide materials have in many cases proven to be ideal because of their high surface areas, large pore diameters, mechanical strength, and controllable surface properties [12–13]. One of the most widely studied groups of mesoporous oxidation catalysts are derived from the silica-based family of highly ordered mesoporous materials known as M41S [14]. These materials, such as MCM-41, are prepared via sol–gel methods using surfactant structure-directing agents [14]. The catalytically active forms of these mesoporous oxides are readily synthesized via the impregnation of transition metals to yield metal-supported catalysts with high metal dispersions [15–21]. For example, researchers have shown that MCM-41 can be impregnated with small amounts of Ti (2 wt%) to produce an effective catalyst for the selective oxidation of alkenes under mild conditions [20–22]. It is well known that structural changes in a catalyst can lead to significant changes in product selectivity [23– 27]. Since the final structure of sol–gel-based materials is dependent upon the structure of the template that was used to create it [14], a change in the templating agent * To whom correspondence should be addressed. E-mail: [email protected]
can lead to significant changes in the physical structure an
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