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NIRE Annual Report
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 1999

Titanium-Containing Porous Silica Prepared by Supramolecular Templating Approach

Siliceous Materials Division
Materials Processing Department
Objectives
Titanium-containing porous silica in which Si4+ is isomorphously substituted by Ti4+ ions in the silicate framework is of great interest from both academic and industrial viewpoints due to their extraordinary potential as oxidation catalysts and many other applications. In the present work, we investigated the synthesis of high surface area as well as high Ti-containing porous silica with mesostructural units as secondary pore walls at ambient temperature by a modified sol-gel procedure. The physico-chemical properties of the products were characterized by nitrogen adsorption, 29Si solid state MAS NMR, XRD, IR, X-ray absorption, Raman and UV-vis spectroscopy. Furthermore, the catalytic activities were evaluated for the peroxide epoxidation of olefins.
Results
Ti-containing porous silica (MTP) has been synthesized by a simple modified sol-gel method with aqueous octylamine as template and alkoxides of TEOS and TEOT as framework sources. The transparent gel formed by partial co-hydrolysis of the alkoxides with a substoichiometric amount of water in the presence of a large quantity of octylamine resulted in high Ti-incorporation in the silicate framework. This contrasted with the maximum of 3 mol % in earlier attempts at titanium incorporation for Ti-zeolite and Ti-containing mesoporous materials. In addition, MTP has mesostructered units as secondary pore walls, which results in a higher specific surface area of MTP compared to the Ti-Si mixed amorphous oxides prepared by a conventional sol-gel procedure. The increase in Ti in the silicate network is accompanied by the change in the framework porosity from mesoporous to microporous properties. Microporous MTP has been formed by incorporation of Ti above 10 mol% in the framework and has a uniform micropore diameter of 0.84-0.90nm. The degree of framework cross-linking of MTP is found to be remarkably improved by Ti substitution based on the 29Si MAS NMR spectra. The XANES, EXAFS, Raman and UV-vis results indicate that most Ti atoms are incorporated and site-isolated into the silicate framework up to 15mol % Ti. An additional small amount of a titania phase might be formed in the silica matrix in the case of a higher Ti content, > 15 mol % Ti in silicate framework. The catalytic results for the liquid phase peroxide oxidations of olefins with TBHP reveal that MTP exhibits catalytic activities in olefin oxidation.
Hence, this synthetic approach to Ti-containing porous silica has resulted in a wide composition range, a high specific surface area and a high degree of homogeneous Ti dispersion. These new materials should have applications in a variety of areas, including the liquid-phase catalytic reaction reported here, molecular adsorption, coating and membranes.
Selected Publications
1) Titanium-Containing Porous Silica Prepared by Modified Sol-Gel Method, Journal of Physical Chemistry (in press).
2) Synthesis of Ti-containing Porous Silica with High Photocatalytic Activity, Chemistry Letters, 1999, 9.

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