Extensions in the synthesis and catalytic application of titanium silicalite-1

Titanium silicalite-1 (TS-1) was prepared by microwave heating of a SiO₂–TiO₂ xerogel, which was dry-impregnated with tetrapropylammonium hydroxide (TPAOH). Highly crystalline product was obtained, with the yields higher than 90%, within 30 min after microwave irradiation. These are significant advantages over the conventional oven heating using alkoxide precursors in liquid phase, which requires 1-2 day crystallization time with low product yields. Sub-micron sized SiO₂–TiO₂ prepared by thermal plasma process and Ti-containing mesoporous silica, Ti-HMS, were also tested as the solid phase precursor for TS-1. These were found inferior as precursor due to difficulties in uniformly wetting the surface with TPAOH. It was possible to prepare TS-1 using diethoxysiloxane-ethyltitanate (DESET, 93.75 mol Si:6.25 mol Ti) as a single mixed alkoxide precursor, but titanium species in the sample were highly unstable and large fraction of them came out from the framework to form TiO₂ clusters upon calcination. In addition, TS-1 monolith was prepared utilizing polyurethane foam as an infiltration medium of the synthesis gel. The prepared TS-1 monolith exhibited properties similar to those of TS-1 powders, but the elementary unit of the former was made of 1–2 micron-sized hexagonal shaped crystals, causing diffusion problems. Catalytic activities of all the prepared catalysts were evaluated using 1-hexene epoxidation as a probe reaction, and 2,5-dihydrofuran epoxidation was also examined using TS-1 as an alternative process to make 3,4-epoxytetrahydrofuran.