Preparation Of Nano-Silver Based 4A-Zeolite Catalyst And Their Performances For Styrene Epoxidation

Styrene oxide is an important intermediate of organic synthesis, and it is widely applied in the spice synthesis, pharmacy, fine chemicals, cosmetics and other fields. The catalysts which have many kinds play an important role in the process of the styrene epoxidation, and the silver-based catalyst is one of them. The reunion and sintering phenomenon of silver nanoparticles are prone to appear, which will lead to a drop in activity sites, and the catalyst can not be effective used which affects the catalytic activity and product selectivity. Therefore, it is necessary to develop a highly active and highly selective silver-based catalyst, in which the silver nanoparticles have a good dispersion and it is difficult to sintering and reunion.In this paper, it was investigated that the technological conditions of preparing silver nanoparticles/4A-zeolite composite catalyst via the impregnation method, one-pot hydrothermal technology and supercritical carbon dioxide technology. Furthermore, the composite catalysts were used to catalyze the styrene epoxidation reaction for the investigation of catalytic properties. The concrete investigation contents are as follows:1. 4A-zeolite was prepared by hydrothermal synthesis method, and the silver nanoparticles/4A-zeolite composite catalyst was prepared through the combination of impregnation and liquid phase reduction. FT-IR、UV-vis、SEM、TEM、BET、XRD and XPS were used to characterize the prepared catalysts, and the results indicate that the4A-zeolite had a regular cubic crystal morphology and a excellent crystallinity. The silver nanoparticles were evenly dispersed on the 4A-zeolite, and nano-silver was the zero valent face-centered cubic crystal. The composite catalysts showed an excellent catalytic effect for styrene epoxidation. The catalyst which was prepared by adsorption of silver ion and hydrazine hydrate reduction showed an excellent catalytic performance. The conversion of styrene was 84.9 %, and the selectivity of SO was 57.7 % in the conditions of 82 ℃.2. The silver nanoparticles/4A-zeolite composite catalyst was prepared through one-pot hydrothermal technology, which was the combination of solvothermal and in situ reduction. Then, the catalytic performances of the composite catalyst were investigated.The composite catalyst was characterized by SEM、TEM、BET、XRD、UV-vis、FT-IR and XPS, in which the silver nanoparticles were excellently dispersed on the 4A-zeolite.Furthermore, nano-silver was the zero valent face-centered cubic crystal in the composite catalyst. The catalytic results indicated that the catalytic performance of the composite catalyst was optimal when acetonitrile was used as solvent. In this solvent, the conversion of styrene was 80.8 % and the selectivity of styrene oxide was 89.2 %(catalyst amount:0.04 g, reaction time: 48 h). In adition, the composite catalyst showed an excellent recycling performance.3. The silver nanoparticles/4A-zeolite composite catalyst was prepared by using supercritical carbon dioxide technology on the basis of traditional impregnation method,and the catalytic performances of the catalysts were inveatigated. The composite catalysts were prepared by two ways. The first method as follows: Firstly, the 4A-zeolite which supported silver was prepared by the supercritical carbon dioxide extraction after the mixture of 4A-zeolite and nano-silver colloid. Then, the silver nanoparticles/4A-zeolite composite catalyst was prepared after removing PVP which was used as the stabilizer of nano-silver via 450 ℃ calcination. The second method as follows: Firstly, silver ions were fixed on the 4A-zeolite by supercritical carbon dioxide technology. Then, silver nanoparticles/4A-zeolite composite catalyst was prepared by high-pressure hydrogenation.The composite catalysts were prepared by the two ways were characterized via SEM 、TEM、UV-vis、BET、XRD and XPS, and it showed an excellent morphology. Two kinds of composite catalysts were used to catalyze the styrene epoxidation for testing their catalytic properties. The catalytic results indicates that the composite catalysts which were prepared by the two methods have an excellent catalytic performances. After 26 h, the conversion of styrene reached 97.8 % and the selectivity of SO reached 90.4 % when 0.05 g catalyst which was prepared by the second method was used in the condition of 82 ℃, and the acetonitrile was as solvent in the reaction. Compared with our previous two chapters, the catalytic activity of the catalysts had an obvious ascension. The selectivity of products could be controlled by changing the solvents and oxidants. The selectivity of BZ was 76.2% when TBHP was used as oxidants, and dimethyl sulfoxide was used as solvent. After 10 h, the conversion of styrene reached 97.6 % and the selectivity of BZ reached 91.6 % when H2O2 was used as oxidant, and the solvent was acetonitrile.The investigation of this paper will provide a new idea for green synthesis the high catalytic activity and easily recycled nano-silver catalyst.