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

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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.

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