Crystallization of Ti-Rich∗BEA Zeolites by the Combined Strategy of Using Ti-Si Mixed Oxide Composites and Intentional Aluminum Addition/Post-Synthesis Dealumination

Titanosilicate zeolites are well-known catalysts for selective oxidation using hydrogen peroxide, an environmentally friendly oxidant. To effectively synthesize these materials with high Ti contents, we have focused on using a Ti-Si mixed oxide composite as the ingredient along with intentional addition of an aluminum source to promote crystallization. Ti-beta, a∗BEA-type zeolite containing titanium at the framework sites, was chosen as a model zeolite. First, (Ti, Al)-beta, a∗BEA-type zeolite containing both Ti and Al, was prepared; the occluded aluminum inside the product was subsequently removed by an acid treatment. This treatment not only lead to the reduction of the aluminum content to trace levels but also improved the states of the titanium species to the desired tetrahedral coordination state. Thus, Ti-beta zeolites with little extra-framework Ti were successfully obtained with molar compositions up to Ti/(Ti + Si) = 4.0 mol %. As a titanosilicate zeolite catalyst, high functionality was demonstrated based on the oxidation of cyclooctene, confirming the positive impact of having high titanium content with low aluminum content. Finally, investigation of the intermediates during the crystallization process was performed to understand the behavior of titanium species throughout the crystallization and to propose the critical factors for achieving efficient Ti introduction.