Synthesis, Characterization And Application Of The Core/Shell Structure Zeolite

Catalytic pyrolysis technology to produce propylene from heavy oil is concerned widely because of its many advantage like broad-range supply of feed stock and low cost. The ZSM-5 zeolite catalyst which has higher propylene selectivity is one of the dominating catalysts. However, the conversion of heavy oil is poor due to its small pore, which can be improved by modifying carrier, increasing ZSM-5 zeolite content or adding USY zeolite. However, as the content of USY in catalyst increases, the yield of alkane increases and the yield of propylene decreases. Furthermore, the mechanical strength of catalyst is damaged and the cost of catalyst goes up when catalyst contains higher level of active components. Secondary reaction, which comes from farther diffusion distance between two different zeolites in mechanical mixed catalyst, decreases synergistic effect in most occasions. Based on the above analysis, the ZSM-5/SAPO-5 composite zeolites were researched in present work. In order to decrease hydrogen transfer reaction and to increase conversion of heavy oil, the SAPO-5 zeolite, which is of lower acidic density and larger pore size, was made as the shell of composite zeolites. In order to increase the yield of propylene, the ZSM-5 zeolite, which has higher ratio of silicon to aluminium was made as the core.Different methods were used to synthesize the ZSM-5/SAPO-5 core-shell structure composite zeolites in the paper. Firstly, the composite zeolites were synthesized by one pot method. The effects of synthesis condition, the mass ratio of ZSM-5 to SAPO-5 and the components of SAPO-5 gel on the composite zeolite structure were studied. The results showed that the composite contained amorphous zolite, which is attributed to the effect of the anion and primary structure in mother liquor, and it didn't meet the requirement of producing propylene because of higher SAPO-5 content. In order to remove the interference, ZSM-5 powder was mixed with SAPO-5 gel and crystallised to obtain composite zeolite by embedding method. The method can obtain ZSM-5/SAPO-5 composite zeolite. As a result, the quality of composite zeolite was improved through adjusting crystallization mode, content of crystal seeds and properties of solvent. But the composite zeolite shows bigger particle size, more aggregate and SAPO-5 zeolite of independence growth, because so much water in gel decreases the chance of contact and combination between ZSM-5 and phosphorus-aluminum ligands. Furthermore, the zeolite synthesized by hydrothermal synthesis method usually aggregates together. The vapor-phase transport method improves quality of composite zeolite, reduces particle size and aggregation, but it not able to avoid independence growth of SAPO-5 zeolite.On the basis of above experiment results and analysis, a new method called sowing seed method was designed for synthesis of ZSM-5/SAPO-5 composite zeolite with core-shell structure. This procedure of method is following. Fistly, the ZSM-5 zeolites were coated with one or several kinds of synthesis source of SAPO-5 zeolite. Secondly, calcination at appropriate temperature made synthesis sources of SAPO-5 to be firmly fixed to the ZSM-5 surface. Finally, other synthesis source of SAPO-5 zeolite was added, and crystallized at proper conditions to obtain composite zeolite. The sowing source that includes phosphorus, aluminum and silicon source, and sowing condition that includes temperature and time were detailedly researched. The sowing phosphorus source method decreases aggregate bewteen two different zeolites, reduces particle size of composite zelite from 10 to 4~6μm and avoids independence growth of SAPO-5 zeolite on the whole. At the same time, the core/shell composite zeolites with different shell thickness were synthesized by sowing phosphorus source method through adjusting crystallization condition and source quality. Finally, the cracking propertier of differen zeolite catalysts from heavy oil to propylene were invetigated. Compared with mechanical mixture zeolite catalyst, the composite zeolite catalyst synthesized by embedding method can't increase the conversion of heavy oil. However, the composite zeolite synthesized by sowing phosphorus source can increase the conversion of heavy oil about 2.5 percent and yield of propylene percent 1.3 percent.