TiO_x overlayers on MCM-48 silica by consecutive grafting

High-quality MCM-48 materials were prepared by employing C_16-12-16 and C_22-12-22 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-x Ti(L)_x (L = NMe₂, O/Pr). The inorganic/metalorganic hybrid materials were characterized by elemental analysis. FTIR spectroscopy, nitrogen physi-sorption, 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%).