Direct Gas Phase Epoxidation Of Propylene Over Nano Gold Catalysts

Epoxidation of propylene over nano gold catalysts with hydrogen and oxygen is an environmental friendly and economic process.The commonly used supports for the catalysts are Ti-containing mesoporous or microporous zeolites.Currently,the main restrictions of these catalysts in the reaction lie in two two aspects:On one hand,Strong adsorption or agglomerization of PO occurs on the catalyst surface due to the strong acidity of the Ti-containing mesoporous support,which results in the catalyst deactivation.On the other hand,the Au catalysts supported on microporous TS-1 have good stability,however,due to the strict tetrahedral coordination of Ti in the framework of TS-1,the amount of more catalytically active defective Ti sites is insufficient,which hinders the catalytic activity.Thus the development of highly active and stable Au catalysts for proplylene epoxidation is very changeling.It is believed that the modification of the Ti-containing support is crucial for the improvement of the catalytic performance.Therefore,in this thesis,Au supported on three different Ti-containing porous materials were prepared and tested for propylene epoxidation.The supports include a conventional mesoporous Ti-MCM-41,a microporous TS-1 and a Ti-MCM-41 with hybrid mesoporous-microporous structure synthesized using TS-1 as the building units.Various characterizations were conducted to illustrate the different Ti coordination situations in these materials.In situ spectroscopic study was performed in order to distinguish the adsorption/desorption behaviors of propylene.Kinetic investigations were also carried out to depict intrinsic behaviors of the catalysts.The detailed results are as follows:1.Gas phase propylene epoxidation over Au supported on titanosilicates with different Ti chemical environments.Three Ti-containing porous materials-a mesoporous Ti-MCM-41 with distorted tetrahedral framework Ti sites?denoted as Ti-MCM-41-C?,a microporous titanium silicate TS-1 with tetrahedral framework Ti sites and a hybrid Ti-MCM-41 containing TS-1 microstructure?denoted as Ti-MCM-41-H?were used to prepare supported Au catalysts for gas phase propylene epoxidation in the presence of H₂ and O₂.Both catalyst structures and catalytic reaction kinetics were investigated in detail.The kinetic results show that the apparent activation energies for both PO and CO₂ formation follow the order of Au/Ti-MCM-41-C<Au/Ti-MCM-41-H<Au/TS-1.Besides,the adsorption behaviors of propylene on the catalysts were strongly affected by the chemical environment of the framework Ti sites and pore structure in the supports.The adsorption capacity follows the order of Au/Ti-MCM-41-H>Au/TS-1>Au/Ti-MCM-41-C,in consistence with the order of propylene conversion rate,and the adsorption stability follows the order of Au/TS-1>Au/Ti-MCM-41-H>Au/Ti-MCM-41-C,in consistence with the order of apparent activation energy.These results demonstrated that the adsorption strength of propylene on the titanosilicates supports strongly affected the catalytic behavior,and implied that the support with balanced defective Ti sites and mesoporous/microporous structure may be a promising approach in the preparation of high-performance supported Au catalysts.2.Preliminary investigation on the effect of alkali/alkaline metal promoters on the propylene epoxidation over Au/Ti-TUD catalysts.Ti-TUD with wormlike 3D porous structure and supported Au/Ti-TUD catalysts were prepared.The effects of alkali/alkaline metal promoters such as K,Cs,Ca and Ba on the catalytic performance were investigated.It was found that the addition of Ca and Ba improved the activity of the catalyst;while the addition of K and Cs resulted in a decline in catalytic activity,but a stable conversion of propylene.