Transformation of SiO2 in Titanium Silicalite-1/SiO2 extrudates during tetrapropylammonium hydroxide treatment and improvement of catalytic properties for propylene epoxidation

• The SiO2 of TS-1/SiO2 can dissolve and recrystallize during TPAOH treatment.
• The SiO2 showed a protection to the framework Ti species during TPAOH treatment.
• The Si/Ti(B), crystallinity, and hydrophobicity of the treated catalysts increased.
• The treated TS-1/SiO2 showed an excellent stability in a fixed-bed reactor.

Titanium Silicalite-1 (TS-1)/SiO2 extrudates were treated with tetrapropylammonium hydroxide (TPAOH) solutions under hydrothermal conditions. The catalysts were characterized using various analysis techniques including X-ray diffraction (XRD), ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis), UV Raman, X-ray photoelectron spectroscopy (XPS), nitrogen physisorption, X-ray fluorescence (XRF), 29Si magic-angle-spinning nuclear magnetic resonance (29Si MAS NMR) spectroscopy, and transmission electron microscopy (TEM). The transformation of amorphous SiO2 in TS-1/SiO2 catalysts during TPAOH treatment was studied in detail. The results show that the SiO2 dissolved and then crystallized during TPAOH treatment, which increased the hydrophobicity and the ratio of Si/Ti on the external surface of TS-1. Moreover, the existence of SiO2 inhibited the formation of anatase TiO2 during TPAOH treatment. The changes in physical and chemical properties not only improved the catalytic activity but also improved the stability of catalysts after TPAOH treatment. The experimental results show that the treated TS-1/SiO2 catalysts exhibited higher catalytic activity and stability than the untreated counterparts. Especially for the TS-1/SiO2 catalyst treated with 0.06 mol/L TPAOH solution for 72 h, the H2O2 conversion still remained above 90% and the propylene oxide selectivity was as high as 99% after a long-term run with time on stream (TOS) of 648 h.

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