Preparation And Modification Of ZSM-5 Zeolite For Its Catalytic Application

With the rapid development of world economy and the downstream industries of propylene,the gap between supply and demand of propylene is greatly increasing.In view of those existing propylene production technology,methanol to propylene（MTP）and fluid catalytic cracking（FCC）are regard as the most two effective ways to obtain propylene.ZSM-5 zeolite commonly utilized in the above two reactions owing to its superior physico-chemical properties such as excellent shape-selectivity and thermal-hydrothermal stability,and suitable acid features.In view of those above two application,ZSM-5 zeolites with high and low silica-alumina ratio were prepared,respectively,and sequential Fe modification was introduced to get Fe-/ZSM-5 zeolites to increase propylene selectivity and catalysts stability in MTP or FCC reactions.The results manifest that:（1）The suitable conditions for synthesizing high-silicon ultrafine ZSM-5 zeolite by dry-gel method are SiO₂:Al₂O₃=100:0.4,crystallization at 170°C for 24h.Base on that,ultrafine Fe-ZSM-5 zeolites with framework heteroatoms were in-situ synthesized by adding Fe directly to the dry gel.MTP evaluation results show that Fe-ZSM-5zeolite with appropriate amount of Fe could not only increase the diffusion efficiency of products,but also reduce the occurrence of side reactions such as hydrogen transfer and aromatization,which improved propylene selectivity and catalyst stability.（2）The low-silicon ZSM-5 zeolites were synthesized directly by dry-gel method without adding any template or crystal seed,and the optimum conditions were obtained.Base on that,Fe-ZSM-5 zeolites with framework heteroatoms were in-situ synthesized directly by adding Fe to the dry gel.FCC evaluation results shew that,compared with Fe/ZSM-5 zeolite prepared by impregnation method,Fe-ZSM-5 zeolite as FCC additive reduced the dehydrogenation reaction while increasing the deep cracking of the heavy oil,so that the selectivity of propylene in the product increased and the coke yield decreased.（3）Fe-ZSM-5 zeolite prepared by in-situ synthesis method contains three kinds of Fe species,and most of Fe attributes to isolated Fe³⁺in tetrahedral coordination and the rest are exist as oligomerized FeO_x and Fe₂O₃ particles.But Fe/ZSM-5 zeolite prepared by impregnation method mainly contains latter two forms of Fe species with a little of Fe³⁺in the framework of the zeolite.