Green Synthesis Of HZSM-5 Zeolite And Its Catalytic Performance For Methanol/n-Heptane To Light Olefins

Light olefins are very important basic organic chemical raw materials and are widely used.At present,light olefins are mainly obtained by naphtha steam cracking and heavy oil catalytic cracking.Based on the energy status of"rich in coal,short in oil and short in gas"in China,the process of producing light olefins（MTO）from coal-based methanol is of great significance for ensuring China’s energy security.Zeolite catalyst plays an important role in the reaction of methanol to olefins.Among them,ZSM-5 zeolite has been widely studied and applied in the fields of hydrocarbon catalytic cracking and methanol to olefins because of its unique MFI pore structure,adjustable acidity,good hydrothermal stability and carbon deposition resistance.Sodium-type Na ZSM-5 zeolite synthesized by traditional hydrothermal synthesis method and dry gel conversion method needs many times of ion exchange and roasting to become hydrogen-type HZSM-5 which can be used in the above reaction.This treatment process will produce a lot of wastewater and increase energy consumption.In order to realize the green synthesis of ZSM-5 zeolite,reduce wastewater discharge and save energy,this study directly synthesized HZSM-5 zeolite with ammonia as mineralizer without adding alkali metal cations,and then prepared ZSM-5 zeolite with low silicon-aluminum ratio by hydrothermal synthesis.X-ray diffraction analysis（XRD）,Fourier transform infrared spectroscopy（FT-IR）,ammonia temperature programmed desorption（NH₃-TPD）,nitrogen adsorption and desorption,scanning electron microscope（SEM）,solid-state nuclear magnetic resonance（MAS-NMR）and MTO and n-heptane catalytic cracking reactions were used to study the effects of physical and chemical properties of zeolites on catalytic performance.Firstly,HZSM-5 zeolite was synthesized by steam-assisted crystallization（SAC）,and the molar composition of the initial gel was changed:100 Si O₂:1.0 Al₂O₃:y TPAOH:x NH₃·H₂O.The effects of ammonia consumption,crystallization time,template dosage and raw materials on the crystallization process of the zeolite were investigated.The results showed that the optimal ammonia consumption was x=25,the optimal crystallization time was 72 h,the optimum dosage of template agent y=5.5.The comparison between steam-assisted crystallization and hydrothermal crystallization shows that both solid phase transition mechanism and liquid phase transition mechanism exist in the crystallization process of ZSM-5 zeolite.ZSM-5 zeolites with different Si/Al ratios were prepared under optimized synthesis conditions.It was found that the lower the Si/Al ratio,the higher the acid content,but the more difficult it was to synthesize.The catalytic performance of ZSM-5 zeolite with Si/Al ratio of 100,200 and 300 for n-heptane cracking showed that the conversion rate of n-heptane was the highest when the reaction temperature was 550℃.In the MTO reaction,when the reaction temperature is 470℃,the methanol conversion rate is 100%and the lifetime of the zeolite catalyst with the silicon-aluminum ratio of 300 is the longest.The selectivity of light olefins can be improved when the common feed of n-heptane and methanol is used.When the molar ratio of mixed feed is 1:1,the selectivity of light olefins reaches the maximum（54.83%）,but at this time,the catalyst is more prone to carbon deposition and deactivation faster.Finally,ZSM-5 zeolite with low Si/Al ratio was synthesized by hydrothermal synthesis,and the effects of raw material composition,ammonia consumption,crystallization time and mineralizer on the crystallization process of zeolite were investigated.The results show that tetraethyl orthosilicate（TEOS）and pseudo-boehmite are more favorable for the synthesis of ZSM-5 zeolite with low silica-alumina ratio.The crystallization study was carried out by using ammonia water dosage z=50（molar ratio）and crystallization time 72 h.It was found that the lower the ratio of silicon to aluminum,the more difficult it was for aluminum to enter the skeleton.With the decrease of Si/Al ratio,the acid B of zeolite decreased,the acid L first increased and then decreased,and the total acid amount decreased.When the ratio of silicon to aluminum is 20,adding NH₄⁺and Zn²⁺can promote aluminum to enter the skeleton.When Si O₂/NH₄⁺=40,the lowest ratio of silicon to aluminum in the skeleton can be obtained（26.41）.Under these conditions,the zeolite catalyst synthesized has the highest conversion activity for n-heptane,and the conversion rate of n-heptane is 92.24%.