Green Synthesis Of ZSM-5 Zeolite From Natural Minerals By Amine-free Method And Its Catalytic Application

ZSM-5 zeolite is commonly utilized in petrochemical processes,environmental protection and adsorption separation etc,owing to its unique pore structure,adjustable acidity and excellent hydrothermal stability.However,products obtained from natural mineral always possess low additional value and poor economic performance.Besides,the synthesis of zeolites from chemical materials can greatly increase the cost and cause environmental issues.In order to solve these problems,a green route for synthesis of high value-added Fe-ZSM-5zeolites from natural minerals under amine-free system was put forward.In this green route,silica-rich mineral（diatomite）and alumina-rich mineral（kaolin）were applied as all the source of silicon,aluminum and iron.The impacts of factors on the activation of natural mineral and the synthesis of Fe-ZSM-5 zeolites were systematically investigated.In addition,the catalytic pyrolysis performance of synthesized Fe-ZSM-5 zeolite under the optimal conditions was evaluated using n-heptane as model compounds.The results manifest that:（1）the dissolution rate can reach 90%when we adapt liquid alkali-solution method at a low temperature,which is also effective to remove the quartz and other inert impurities.Besides,kaolin can be activated effectively at a low temperature via solid alkali-fusion method（the optimal activation temperature is 250 ^oC for 30 min）.Comepared with traditional thermal activation method,solid alkali-fusion method can significantly reduce energy consumption and short the activation time.（2）Amine-free method,including crystal seed method and direct method,has been successfully used to synthesis ZSM-5 zeolite with high specific surface area,high crystallinity and the pure crystal phase using activated natural mineral as raw materials.In crystal seed method,the synthesis path using heterogeneous seed is similar to direct method,while homogeneous seed can accelerate the synthesis of target zeolite and inhibit the formation of mixed crystal because it has the same secondary structural unit with target zeolite.Comepared with crystal seed method,direct method is economical because natural minarals can be transferred into ZSM-5 zeolites without any additives.（3）In the synthesis process of Fe-ZSM-5,most of Fe attributes to isolated Fe³⁺in tetrahedral coordination,while a small amount Fe is oligomeric Fe_x³⁺O_y clusters and large Fe₂O₃ particles.Catalytic cracking evaluation result demonstrates that the introduction of the framework Fe to ZSM-5 zeolite can improve activity of catalytic pyrolysis and selectivity of propylene.Overall,the reaction of n-heptane catalytic pyrolysis is mainly monomolecular cracking path,and with the increase of Fe/Al,the trend becomes more obvious.