RESUMEN
Titanium/silica systems were prepared by grafting a titanium alkoxide (titanium isopropoxide and titanium (triethanolaminate) isopropoxide) precursor onto amorphous silica. The grafting process, which consisted of the hydrolysis of the Ti precursor by the hydroxyl groups on the silica surface, yielded samples containing Ti-loadings of 1-1.6 wt %. The as synthesized and calcined TiO(2)-SiO(2) samples were characterized by UV-vis, FTIR, XPS, and XANES spectroscopic techniques. These systems were tested in the liquid-phase epoxidation of oct-1-ene with hydrogen peroxide reaction. Spectroscopic data indicated that titanium anchoring takes place by reaction between the alkoxide precursor and surface OH groups of the silica substrate. The nature of surface titanium species generated by chemical grafting depends largely on the titanium precursor employed. Thus, the titanium isopropoxide precursor yields tetrahedrally coordinated polymeric titanium species, which give rise to a low-efficiency catalyst. However, if an atrane precursor (titanium (triethanolaminate) isopropoxide) is employed, isolated titanium species are obtained. The fact that these species remain isolated even after calcination is due to the protective effect of the triethanolaminate ligand that avoids titanium polymerization. These differences in the titanium environment have a pivotal role in the performance of these systems in the epoxidation of alkenes with hydrogen peroxide.
RESUMEN
A commercial mesoporous silica (Grace Davison) was chemically grafted with trimethylsilyl chloride (TMSCl) and hexamethyldisilanaze (HMDS). The silylation process brought about some reduction in the specific BET area, the pore volume, and the pore sizes of the samples. Thermogravimetric studies of the silylated samples revealed that the grafting process is kinetically controlled at short reaction times. In the kinetic regime, increasing concentrations of the silylant agent up to 2 wt% in the solvent led to an increase of the extent of the silylated surface, although this limitation disappeared at higher concentrations. Silylation was confirmed by diffuse reflectance infrared Fourier transform (DRIFTS), (29)Si CP-MAS NMR, and photoelectron (XPS) spectroscopic techniques. Solid-state (29)Si MAS-NMR spectra of the silylated samples revealed the presence of -SiCH(3) groups (9.5 ppm) together with two resonances, Q3 (approximately equal to -104 ppm) and Q4 (approximately equal to -114 ppm), coming from siloxane [Qn approximately Si(OSi)n(OH)(4-n), n approximately 2-4] groups, the Q3 signal decreasing upon silylation. The DRIFT spectra of the silylated samples exhibited two well defined bands at 2970 and 2907 cm(-1), due to stretching vibration modes of the C-H bonds in surface -CH(3) groups formed during the silylation process, and also the disappearance of the band at 3740 cm(-1). This observation indicates the complete removal of terminal and geminal hydroxyl groups by grafting with the silylating agent. Similarly, high-resolution photoelectron spectra of the Si2p core levels showed a high binding-energy component (103.5 eV) in all the samples, coming from the Si coordinated with oxide anions in SiO(2), together with a second component at 102.1 eV, which is the fingerprint of Si coordinated by oxide anions and an organic group. Finally, the samples were ranked according to their hydrophobicity, as determined from the temperature-programmed desorption profiles of adsorbed water and 2-methylbutane.
