TiO overlayers on MCM-48 silica by consecutive grafting

Autor: Klaus Köhler, Stefan Grasser, Reiner Anwander, Markus Widenmeyer
Rok vydání: 2001
Předmět:
Zdroj: Microporous and Mesoporous Materials. :327-336
ISSN: 1387-1811
DOI: 10.1016/s1387-1811(01)00199-8
Popis: High-quality MCM-48 materials were prepared by employing C16-12-16 and C22-12-22 gemini surfactants, respectively, and hydrothermal post-treatment methods. Metalorganic titanium reagents comprising alkoxide and amide complexes were grafted onto the dehydrated MCM-48 samples in dry hexane to give (SiO)4−xTi(L)x (L=NMe2, OiPr). The inorganic/metalorganic hybrid materials were characterized by elemental analysis, FTIR spectroscopy, nitrogen physisorption, and a catalytic test reaction. The ratio of monopodally and bipodally anchored species can be controlled by variation of the metalorganic precursor and the distribution of surface OH groups (type of MCM-48 sample). For L=NMe2, and the pore-enlarged MCM-48 sample, the number of mono (x=3) and bipodally anchored (x=2) titanium species could be calculated as 1.4 and 1.0 mmol g−1, respectively, by correlating the titanium content and the carbon content of a HN(SiHMe2)2-silylated parent material. Isolated TiOH groups were obtained after mild hydrolysis of the surface-attached titanium complexes. After mild calcination and dehydration the number of the TiOH groups and the TiOTi connectivities of the TiO(OH)x overlayer could be estimated. A second grafting step further increased the Ti loading (≈15 wt.% Ti) to form an increasingly interconnected “TiO(OH)” network. The influence of the molecular surface structure on the catalytic behavior was examined for the epoxidation of cyclohexene with TBHP. The consecutively grafted material displayed high average catalytic activity (27 h−1 TOF) during the first 2 h and high selectivity (>90%).
Databáze: OpenAIRE
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