Preparation, Characterization Of Ag/TS-1 Catalyst And Its Catalytic Properties On The Gas-Phase Epoxidation Of Propylene

The epoxidation of propylene was carried out over silver modified titanium silicalites (TS-1) in the presence of hydrogen and oxygen at atmospheric pressure. The effects of the factors involved in the catalysts preparation and reaction conditions as well as the modification of Ag/TS-l catalyst on the catalytic properties have been investigated; Function of the internal and external silver species of the Ag/TS-1 catalyst and the titanium species for propylene epoxidation have also been investigated; The deactivation and regeneration of the Ag/TS-1 catalyst have been preliminarily discussed; at the same time the catalysts have been characterized by XRD?UV-Vis?FT-IR?TEM?ESR?ICP and GC-MS. The results show that:1. There is a synergy between Ag species and TS-1. The catalyst prepared by deposition-precipitation (DP) method is optimum, however the impregnation method can also get a moderate selective catalyst to propylene oxide (PO). The key factors for the two catalysts are the different interactions between silver species and TS-1 support.2. The optimum preparation conditions are: DP preparation method; Ag loading, 2wt%; the nsi/nTi of TS-1, 64; The Ag/TS-1 catalysts were calcined at 450℃ in air for 5h. The optimum reaction conditions are: 150℃; gas velocity 4000h-1; n (C3H6)/ n (O2)/ n (H2)/ n (N2) = 1:2:3:12. Moreover, hydrogen plays an important role in the gas-phase epoxidation of propylene.3. As a silver precursor, AgNO3 is better than Ag2SO4 and CH3COOAg; the selectivity to propylene oxide (PO) decreases drastically with a little increase in.the conversion of propylene, when CH3COOAg was used. Alkali metal carbonate, especially K2CO3, is favorable as the precipitator. Both the propylene conversion and the selectivity to PO can be improved with the existence of K+. Using 0.07mol/L K2CO3 as the precipitator, 1.9% propylene conversion with 90.9% PO selectivity is obtained over 2wt%Ag/TS-l (nsi/nTi-33) catalyst, when reacted at the optimum reaction conditions.4. Both internal and external silver species have catalytic properties, however, the internal silver species of Ag/TS-1 plays an important role in the gas-phase epoxidation of propylene.5. The framework titanium species of TS-1 in synergy with Ag plays an important role in the gas-phase epoxidation of propylene over Ag/TS-1 catalyst. Both anatase type and rutile type TiO2 are not effective supports for the propylene epoxidation; however, the extra framework titanium species (280~290nm in UV-Vis spectra) together with Ag also shows weak epoxidation activity, and the selectivity to PO decreases with large amount of it in the gas-phase epoxidation of propylene.6. During the direct gas-phase epoxidation of propylene, the catalytic properties of Ag/TS-1 can be increased when TS-1 was silylated. When 2wt%Si02 was loaded on TS-1, 2.1% propylene conversion with 98% selectivity to PO is obtained over Ag/TS-1 catalyst.7. Apart from the polymers of propylene or propylene oxide being the cause of the catalyst deactivation, the presence of water may be one of the reasons resulted in the deactivation of the Ag/TS-1 catalysts. The catalytic properties of the spent catalysts can be recovered to the original value, when the catalysts were calcined at450 in air.8. Neither big (larger than 10nm) nor small silver particles (less than 5nm) areIVbeneficial to the formation of PO. The small silver particles less than 5nm do not show any catalytic activities, however, the metal silver particles bigger than 10nm and large amount of metal silver can decrease the PO selectivity. The oxidized silver ions are the effective silver species for the gas-phase epoxidation of propylene. In UV-Vis spectra, that the shift of the silver species' absorbance ranged at 400~800nm to a longer wavelength (longer than 450nm) does not favor the formation of PO. The longer the absorbance wavelength of the silver species has, the poorer the catalytic properties of the catalysts exhibit.