Rapid Condensation-Regulated Crystallization of Titanium Silicalite-1 with a High Content of Active Ti

Abstract

Titanium silicalite-1 (TS-1) attracts wide attention as an effective oxidative catalyst. However, the preparation of TS-1 typically requires rigorous synthesis procedures with a relatively long synthesis time and generates a substantial amount of wastewater. This study discovers that, in the presence of tetrapropylammonium hydroxide (TPAOH), Si and Ti sources can form a homogenous liquid solution that is highly stable although the two typically undergo hydrolysis at very different rates individually. With such understanding, this work develops a straightforward one-pot synthesis strategy that eliminates the cumbersome procedures aimed at suppressing the hydrolysis of the Ti source in TS-1 synthesis and demonstrates that the crystallization process of TS-1 can be accelerated by reducing the molar ratio of water to Si species (n(H2O/Si)) in the synthesis solution. In propene epoxidation with H2O2, an excellent catalytic activity is obtained over the TS-1 synthesized at n(H2O/Si) = 3.5 and crystallized for only 6 h. This low n(H2O/Si) synthesis regime results in a significantly high yield per batch (12 g TS-1/100 mL batch) and, at the same time, produces almost no wastewater. The superior catalytic performance of TS-3.5-6 synthesized is attributed to its high total Ti content and the presence of catalytically active Ti(OSi)2(OH)4.