TiOx overlayers on MCM-48 silica by consecutive grafting

High-quality MCM-48 materials were prepared by employing C-16.12.16 and C-1 2.17.22 gemini surfactants, respectively, and hydrothermal post-treatment m ethods. Metalorganic titanium reagents comprising alkoxide and amide comple xes were grafted onto the dehydrated MCM-48 samples in dry hexane to give ( SiO)(4-x),Ti(L)(x) (L = NMe2, OiPr). The inorganic/metalorganic hybrid mate rials 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 metalorga nic precursor and the distribution of surface OH groups (type of MCM-48 sam ple). For L = NMe2, and the pore-enlarged MCM-48 sample, the number of mono (x = 2) and bipodally anchored (x = 2) titanium species could be calculate d as 1.4 and 1.0 mmolg(-1), respectively, by correlating the titanium conte nt and the carbon content of a HN(SiHMe2)(2)-silylated parent material. Iso lated TiOH groups were obtained after mild hydrolysis of the surface-attach ed titanium complexes. After mild calcination and dehydration the number of the TiOH groups and the TiOTi connectivities of the TiO(OH)(3) overlayer c ould be estimated. A second grafting step further increased the Ti loading (approximate to 15 wt.% Ti) to form an increasingly interconnected "TiO(OH) " network. The influence of the molecular surface structure on the catalyti c behavior was examined for the epoxidation of cyclohexene with TBHP. The c onsecutively grafted material displayed high average catalytic activity (27 h(-1) TOF) during the first 2 h and high selectivity (> 90%). (C) 2001 Els evier Science B.V. All rights reserved.