| dc.description.abstract | The distribution of metals in a silica bead plays a crucial role in many industry-oriented applications and an understanding of its preparation method is helpful for the improvement of catalytic efficiency of catalyst. The main purpose of this study was to develop a method to prepare Pd-Cu/SiO2 bead catalyst with egg-shell structure for the production of allyl acetate. Propyl acetate is an important chemical in industry. The acetoxylation of olefins was discovered in the case of the liquid-phase synthesis of vinyl acetate in the presence of palladium dichloride. Subsequent modification of the liquid-phase process and then to the development of the gas-phase acetoxylation of ethylene and propylene was developed. PdCu/SiO2 was used as the catalyst for this reaction in industry. However, since the reaction medium is acidic, Pd and Cu would migrate and sinter, causing severe deactivation. How to make PdCu nano alloy in the thin shell is crucial. In this study, natural and synthetic spherical silica with 5 and 3 mm size, respectively, was used as the carrier. Five preparation methods with various steps were tested. In principle, metal salts were added by impregnation, and then using alkali solution to push metal cations to shell side. Drying, calicneation, and concentration of alkali solution influence the distribution of metal in silica beads significantly. All the samples were characterized by nitrogen sorption, optical microscope, X-ray diffraction, transmission electron microscopy, and high resolution-transmission electron microscopy. Natural silica bead contains sodium cation, it was not stable in acidic condition during preparation process, and resulted in loss of pore structure. The synthetic silica does not have sodium cation, the pore structure is very stable in preparation. Using weak basic solution in preparation did not form egg-shell structure. The amount of aqueous solutions of Pd and Cu cations also affected the particle size of metals. The catalyst prepared by the following method had the thinnest shell among all catalysts: (i) impregnation Pd and Cu metal cations solution, (ii) drying at 110 oC, (iii) adding basic solution in moderation to push metal cations to shell side, (iv) calcination at 450 oC. | en_US |