Synthesis And Phosphorus Modification Of ZSM-5 Zeolite And Its Catalytic Performance In The MTO Reaction

Methanol to olefin（MTO）reaction,which serves as an alternative route for the preparation of light olefins from non-petrochemical routes,holds research significance given the current high demand for light olefins and the background of"dual carbon".The key to the MTO reaction lies in the design and development of catalyst.ZSM-5zeolite is utilized as MTO catalyst due to its unique pore structure and appropriate acidity.However,the use of ZSM-5 catalyst in the MTO reaction is limited by its crystal structure and acidity,resulting in low selectivity for light olefins.The crystal structure can be controlled by inhibiting b-axis growth,constructing hierarchical pore structure and reducing grain size,thereby improving the catalytic performance.The regulation of acidity can be achieved through element modification,as phosphorus modification of ZSM-5 zeolite,which not only adjusts acidity,but also exhibits good hydrothermal stability and selectivity towards light olefin.In this thesis,the morphology of ZSM-5zeolite was controlled by adding crystal growth inhibitor.Subsequently,b-axis oriented ZSM-5 zeolite was synthesized in solvent-free system and modified by phosphorus.The specific results are as follows:（1）Hierarchical H-type ZSM-5 zeolite was successfully synthesized through one-step crystallization by adding urea and PEG2000.It was observed that urea could inhibit the b-axis growth of ZSM-5 zeolite,and the NH₄⁺released from urea decomposition helped balance the negative charge of the framework,resulting in the direct formation of H-type ZSM-5 zeolite without the need for ion exchange directly after calcination.Meanwhile,PEG2000 served as a soft template to regulate the pore structure,leading the synthesis of continuous micro-mesoporous hierarchical structure.Compared with the traditional preparation of ZSM-5 followed by ion exchange to obtain H-type zeolite,the hierarchical H-type ZSM-5 zeolite（PZ5-4）synthesized by one-step method exhibited a 19.0%increase in light olefin selectivity.（2）ZSM-5 zeolite was synthesized by quasi-solid-state method,and b-axis oriented ZSM-5 zeolite was synthesized by adding a small amount of ammonia as a crystal growth inhibitor.The entire reaction was conducted without the addition of extra solvents,making it a greener,safer,and more environmentally friendly process.The synthesized product showed high selectivity for light olefins during the MTO catalysis process,with a remarkable selectivity of 40.7%for propylene.（3）H-type ZSM-5 zeolite was successfully synthesized with the aid of trace ammonia in the system without extra solvent.Additionally,the b-axis oriented H-type ZSM-5 zeolite was synthesized using urea in the system without extra solvent.,achieving both green synthesis and morphology control of ZSM-5 zeolite.The catalytic lifetime of the ZSM-5 zeolite（NHZ-2-U2）synthesized by this method was twice as long as that of the H-type ZSM-5 zeolite（PZ5-4）synthesized in the hydrothermal system mentioned above.（4）Phosphorus modified ZSM-5 zeolite with a lamellar stacking morphology was synthesized by an in-situ solvent-free method using ammonium dihydrogen phosphate（NH₄H₂PO₄）as phosphorus source.The MTO catalytic lifetime of phosphorus-modified ZSM-5 zeolites was nearly 30 times longer than that of unmodified ZSM-5zeolites.Moreover,the light olefin selectivity of phosphorus-modified ZSM-5 zeolites synthesized through this method was approximately 10.0%higher than those synthesized in the hydrothermal system.This method not only reduced the discharge of sewage,but also introduced phosphorus species into zeolite in a simple and effective manner,offering a novel approach for phosphorus-modification of ZSM-5 zeolite.（5）The H-type ZSM-5 zeolite modified with phosphorus was synthesized in a solvent-free system by adding NH₄H₂PO_4.NH₄H₂PO₄ served not only a phosphorus source for zeolite modification,but also as a provider of H⁺and NH₄⁺through its decomposition.These ions helped balance the negative charge of the framework,enabling the direct synthesis of H-type ZSM-5 zeolite.As a result,the ion-exchange step was omitted,simplifying the synthesis process.Additionally,by controlling the amount of template under solvent-free conditions,submicron ZSM-5 zeolite aggregates,consisting of small grains at around 100 nm.These aggregates increased the diffusion rate and significantly prolong the catalytic lifetime effectively when employed in MTO catalysis.