Epoxidation of Propylene over Small-Crystal TS-1 Extrudate in a Fixed-Bed Reactor

A small-crystal titanium silicalite-1 (TS-1) with a size of 600 nm x 400 nm x 250 nm was synthesized using a nano-sized TS-1 mother liquor as the seed in a tetrapropyl ammonium bromide (TPABr)-ethylamine hydrothermal system, and was extruded with silica sol. The obtained TS-1 extrudate was characterized by X-ray powder diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, ultraviolet-visible (UV-Vis) diffuse reflectance spectroscopy, and nitrogen physisorption. The reaction conditions, including temperature, pressure, molar ratio of propylene/H2O2 (n(C3H6)/n(H2O2)), the weight hourly space velocities (WHSVs) of propylene, methanol and H2O2, and the concentration of NH3 center dot H2O, were systematically studied to identify ideal conditions for propylene epoxidation over small-crystal TS-1 extrudate. The reaction temperature had little effect on the propylene oxide (PO) yield under the conditions examined. The highest PO yield was obtained when the pressure was 2.0 MPa and the n(C3H6)/n(H2O2) was 4. The PO content in the product was maximized when the WHSVs of propylene, methanol and H2O2 were 0.93, 2.5, and 0.25 h(-1), respectively. A low concentration of NH3 center dot H2O was beneficial for a high PO yield. Under the optimized condition, we compared catalytic performances of TS-1 with different crystal sizes and performed a long-term test over the small-crystal TS-1. The tests indicated that H2O2 conversion and PO selectivity could both reach 95%, even after 1000 h on stream.