Synthesis Of ZSM-5 Zeolites By Steam-assisted Method And Catalytic Performance

ZSM-5 is a silica-aluminate zeolite with a unique pore structure,good thermal stability and acid resistance,which has been widely used in coal chemical,petrochemical,ion exchange and adsorption fields.ZSM-5 zeolite is usually synthesized by hydrothermal method in a sealed autoclave,which requires large amount of water as solvent.However,the hydrothermal synthesis method has certain safety and environmental problems,such as high autogenous pressure,low energy efficiency,and the discharge of large amounts of wastewater and loss of silicate/aluminosilicate,which do not meet the requirements of green chemistry.In order to overcome the shortcomings of the above-mentioned traditional hydrothermal synthesis method,this thesis combines the advantages of solid grinding method and steam-assisted crystallization method,and synthesizes ZSM-5 zeolites by steam-assisted crystallization strategy based on grinding solid raw materials,i.e.,solid-phase steam method,and loads metals on its basis as well as conducts in-depth research on the graded pore structure,and the specific research results are as follows:The ZSM-5 zeolite was prepared by mixing and grinding solid raw materials and crystalline seeds in the presence of water vapor by high temperature crystallization of the solid mixture without organic templating agent.The direct use of ground and mixed solid raw materials instead of preparing dry gel precursors made the whole synthesis process simpler and greener than the conventional steam-assisted crystallization method.The solid-phase vapor method can improve the mobility of the solid mixture compared with the conventional solvent-free solid grinding method,accelerate the hydrolysis and condensation of silica and/or aluminosilicate species,effectively reduce the crystallization temperature and time,and obtain ZSM-5 zeolites with high silica-aluminum ratio,high crystallinity and multi-acid sites,and exhibit excellent catalytic activity in the cyclohexanone acetalization reaction.Fe_xCu_（1-x）@ZSM-5 zeolites with different Fe and Cu contents were synthesized in situ by a steam-assisted method based on solid grinding using ethylenediamine as an organic ligand to stabilize the metal species.The catalysts with a lower Fe content（1.43%）exhibited excellent catalytic properties and stability in the degradation of methylene blue compared to the impregnation method.ZSM-5 zeolite was beneficial to increase the dispersion of metals and the number of active sites,resulting in a uniform distribution of active sites,in addition to facilitating the rapid diffusion of reactants and products,thus effectively increasing the degradation rate.Finally,catalysts loaded with other metal species（e.g.Co and Ni）using ZSM-5 zeolite as a carrier were successfully prepared using this synthetic strategy,which proved the universality of this synthetic strategy.Biodegradable and inexpensive hydroxypropyl cellulose was used as a mesoporous directing agent to prepare hierarchical ZSM-5 zeolites by solid-phase vapor method.When 0.1 g of hydroxypropyl cellulose was added,the prepared multistage pore ZSM-5 zeolite had the highest crystallinity,specific surface area and pore volume.Due to the introduction of intermediate pores of ZSM-5 zeolite,the active sites of catalytic reactions and the mass transfer efficiency of reactants were effectively increased.Compared with the conventional microporous ZSM-5 zeolite,the conversion of cyclohexanone in the cyclohexanone acetalation reaction was higher with the multistage pore ZSM-5 zeolite.