New mesoporous titanosilicate MCM-36 material synthesized by pillaring layered ERB-1 precursor

Abstract

A new Ti-containing MCM-36 material (abbreviated Si/Ti-MCM-36(E)) was prepared through sequential steps using ERB-1 precursor, it is a borosilicate and aluminum-free analogue of MCM-22 precursor, possessing an MWW layer structure. The layered precursors were swollen with hexadecyltrimethylammonium bromide and tetrapropylammonium hydroxide, followed by pillaring the sheets with titanosilicate formed via the hydrolysis of the tetraethylorthosilicate and tetrabutylorthotitanate interlayer. In comparison with MCM-22, ERB-1 precursor was more effective in forming ordered stacking structures and generating 2 nm mesopores, and the resultant Si/Ti-MCM-36(E) was of lower acidity. The Ti K-edge X-ray absorption spectra indicated that Ti could be distributed well in Si/Ti-MCM-36(E) without the formation of TiO₂ clusters by controlling the Si/Ti ratio during pillaring. The subsequent acid treatment mainly removed octahedrally coordinated Ti and left tetrahedrally coordinated Ti species as the active centers in the catalytic epoxidation of cyclohexene. Acid treatment further decreased the acidity of the sample and enhanced the surface hydrophobicity. In comparison with microporous Ti-YNU-1, Ti-MWW and TS-1, the 2 nm mesopores in Si/Ti-MCM-36(E) effectively improved the accessibility of bulky molecules to the active Ti sites, and the surface hydrophobicity significantly enhanced the epoxide selectivity.