Improving the hydrophobicity and oxidation activity of Ti-MWW by reversible structural rearrangement

The postsynthesis treatment of Ti-MWW having the three-dimensional (3D) MWW structure with aqueous amine solutions has been carried out with the purpose to improve its hydrophobicity and catalytic activity in the liquid-phase oxidations. The treatment with piperidine (PI) or hexamethyleneimine (HMI) converted the 3D MWW structure into the corresponding lamellar precursor, which returned reversibly to the 3D MWW structure by further calcination. The treatments with other amines, however, caused a structural collapse or crystalline transfer to other phases. In the case of PI treatment, the structural conversion from 3D MWW to the MWW lamellar precursor occurred readily at 443 K at a PI/SiO₂ molar ratio of > 0.1 within 1 day for the Ti-MWW samples with various Si/Ti ratios. The structural interchange did not alter the amount as well as the coordination states of the Ti active sites, but removed the internal silanols by ca. 40%, leading to a defectless Ti-MWW catalyst with a more rigid and hydrophobic framework. This kind of structural rearrangement improved the catalytic activity by up to 20% in the ammoximation of ketones and also in the epoxidation of a wide range of alkenes with various molecular dimensions.