Synthesis And Characterization Of Nano Hierarchical ZSM-5 Zeolite

ZSM-5 zeolite is crystalline aluminosilicates,due to its high internal surface area,favorable thermal stability and inherent microporous system,it has been widely employed in industry as heterogeneous catalysts.However,because the conventional micron-sized ZSM-5zeolite has a long microporous channel and a small pore size,reactant molecules or product molecules diffused slowly in the channel and deactivation caused by substance blockage in micropores.The ways to overcome diffusion limitation are to reduce the crystal size of zeolites to the nanometer scale or to introduce the mesoporous structure into the microporous zeolites.However,nano zeolites have the problem of difficult separation and low yield in the synthesis process.the introduction of mesoporous structures by the conventional etching method is associated with destruction of the crystal structure,poor connectivity of mesopores,lots of energy,the pollution of environment and so on.Therefore,hydrothermal synthesis(HTS)and steam-assisted conversion(SAC)were used to synthesize nano hierarchical ZSM-5 zeolite.Based on the investigation into synthesis rules and confirmation of optimal reaction conditions,the methanol-to-propylene catalytic reaction performances of synthetic samples was investigated.First,the nano hierarchical ZSM-5 zeolites are hydrothermally synthesized by adding the seed.The structure,morphology,acid properties,and specific surface area and pore structure properties of ZSM-5 zeolites were systematically studied by various characterization methods such as XRD,SEM,TEM,NH₃-TPD,Py-FTIR.The experimental results show that the addition of seed crystals is beneficial to reduce particle size,increase specific surface area and mesoporous pore volume.The addition of self-synthesized seed crystal can synthesize a nanocrystalline agglomerated hierarchical ZSM-5 zeolite with a particle size of about 80 nm and a mesoporous pore volume of 0.237 cm³/g.The MTP reaction performance results show that the zeolite synthesized by the self-synthesized seed crystal has the largest propylene yield.The high crystallinity nano hierarchical ZSM-5 zeolites can be synthesized rapidly(160?,3 h)by the steam assisted conversion method.Compared with the traditional hydrothermal synthesis method,it reduces the pollution and energy consumption in the production process.The change of water content also directly affects the autogenous pressure inside the crystallizer,the concentration of template on the surface of dry gel and the p H value.Molecular sieves can not be synthesized with too little water.The addition of PEG400 can enhance the structure of nanoparticle aggregates.Different additives showed different results in the process of synthesizing zeolites.Spherical ground grains were synthesized by adding PEG4000 and PEG20000 with longer chain length.When 1,6-hexanediol was used as additive,the synthesized zeolite was the secondary agglomeration of nanoparticles.Adding 1-octadecanol was conducive to synthesizing small particle zeolites with particle size ranging from 100 to 300 nm,but the formed mesoporous encapsulation was in the range of 100-300 nm.The interconnection between crystals is poor.The addition of PVP-K30 is beneficial to the formation of two-scale zeolite particles,but the mesoporous content is small;the addition of carbon nanotubes can synthesize zeolites with large mesoporous content and good connectivity,and the mesoporous volume is as high as 0.562 cm³/g.The MTP reaction results of nano hierarchical ZSM-5zeolites synthesized by different additives have large differences in selectivity both propylene and ethylene,mainly due to the acid amount of zeolites,the change of B/L acid and the change of pore structure properties.