Controllable construction of highly active Ti species in TS-1 zeotype by organic base treatment†

Abstract

Regulating the microenvironment of Ti active centers in titanosilicates is of great significance in both theoretical research and practical application of titanosilicate/H₂O₂ systems. Herein, a novel six-coordinated Ti species containing an organic amine ligand, Ti(OSi)₂(OH)₂(H₂O)TPA, was constructed in the TS-1 zeotype in the process of dissolution and recrystallization by hydrothermal post-treatment with tetrapropylammonium hydroxide and ammonium chloride. The newly formed hexa-coordinated Ti species promoted the activation of H₂O₂ significantly, responsible for the superior catalytic activity in 1-hexene epoxidation with a conversion of 35.5% compared with the untreated TS-1 (18.0%). After removal of organics by calcination, Ti(OSi)₂(OH)₂(H₂O)TPA was transformed into Ti(OSi)₂(OH)₂(H₂O)₂ sites, which also exhibited higher epoxidation activity compared to the original framework Ti(OSi)₄ sites. In addition, the acid sites of Si–OH in TS-1 zeotype were quenched by the basic amine molecules, which effectively inhibited the occurrence of side reactions such as epoxide ring opening, leading to a high epoxide selectivity of 98.6%. With the construction of highly active Ti(OSi)₂(OH)₂(H₂O)TPA sites and without further calcination process, the obtained catalyst (denoted as TS-PN-am) exhibited not only excellent catalytic capacity but also application potential in continuous liquid-phase epoxidation.