Structural state of boron in MFI-titanium boralites and their catalytic properties

Abstract

A series of titanium boralites with the MFI structure has been synthesized. The catalysts were characterized using X-ray diffraction (XRD), IR and magic-angle spinning (MAS) NMR spectroscopic techniques and the incorporation of boron into the framework in the presence of titanium has been investigated in as-synthesized and in calcined samples. Tetrahedral framework boron was determined quantitatively by ¹¹B MAS NMR. The activity of titanium boralites in the oxidation of hexane using hydrogen peroxide as oxidizing agent was evaluated. In the as-synthesized samples, boron and titanium were simultaneously incorporated into the siliceous MFI framework. At constant boron loading, a higher titanium loading leads to a higher fraction of boron removed from its framework position after calcination. The lower activity and H₂O₂ selectivity toward oxyfunctionalization products when boron is present are related to the mild acidic Brønsted sites of the boralite. Acid leaching selectively removes boron and allows a partial recovery of the activity of the titanium silicalite.