Structure of hybrid (organic/inorganic) TiO 2–SiO 2 xerogels II: thermal behavior as monitored by temperature-programmed techniques and spectroscopy

Abstract

Titanium-containing xerogels made by acid pre-hydrolysis of tetraethylorthosilicate (TEOS) and dimethyl-dimethoxysilane (SiMe 2(OMe) 2), followed by reaction with dichloro-diisopropyl titanium (TiCl 2( i-OPr) 2) were characterized by means of several techniques. The decomposition patterns of these xerogels, both in the presence and in the absence of oxygen are discussed in terms of the gaseous products' evolution under temperature-programmed (TP) reaction conditions, and diffuse reflectance infrared spectroscopy (DRIFTS). The primary decomposition routes of residual alkoxide groups are: (i) auto-redox (CO, CO 2) and dehydration (to olefins) under pyrolysis conditions; (ii) oxidative (CO, CO 2) under flowing air. The decomposition of the Si–C bond upon heating, as monitored by all three techniques, follows a pattern independent of that of (CH) species from −OR groups. The 29Si MAS/NMR of these materials is dominated by the so-called D 2 (Me 2Si(OX) 2), Q 3 ((OR)Si(OX) 3) and Q 4 (Si(OX) 4) resonances. Fairly homogeneous proton envelopes around Si centers are evident from the NMR spectra of these materials in the proton cross-polarization mode.