Sodium-free Preparation Of ZSM-5 Zeolite And Its Catalytic Cracking Performance

Zeolite materials have the characteristics of large specific surface area,high hydrothermal stability and many catalytic active sites,and they are widely used in traditional and emerging fields such as petrochemical industry,medicine,and environmental protection.Usually,in the synthesis process of ZSM-5 zeolites,it is necessary to introduce alkali metal ions such as sodium to provide the basicity of the synthesis system and balance the charge of the skeleton.Then,to make the zeolite acidity,the processes of ammonium exchange,roasting and deammonium to remove the alkali metal ions are required,but these process not only make the production process complicated,but also discharge a large amount of waste water and exhaust gas,which causes great pressure on the ecological environment.In this paper,the facile methods for synthesis of rich mesoporous ZSM-5 zeolites,low silicon-aluminum ratio ZSM-5zeolites and small grain ZSM-5 zeolites under sodium-free system are carried out,and the reaction conditions and raw materials in the synthesis process are investigated.The influence factors such as Si/Al ratio,temperature etc.on synthetic zeolites,and the performance of synthetic zeolites in n-hexane catalytic cracking were evaluated.The main progress of the paper is as follows:In the sodium-free system,the specific surface area was 421 m~2/g and the external specific surface area was Mesopore-rich ZSM-5 zeolite with a total pore volume of 165m~2/g and a total pore volume of 0.42 cm~3/g.It shows that the addition of organic amine TPAOH can balance the charge of the skeleton and provide an alkaline environment for the synthesis system.The raw materials used are low-priced industrial raw materials,the silicon source is coarse-pored silica gel,and the aluminum source is pseudo-boehmite.The n-hexane catalytic cracking performance was evaluated for the rich mesoporous ZSM-5 zeolite.When the reaction temperature was 600°C,the n-hexane conversion rate was 99%,the yield of triene was 56.06%,of which the yield of ethylene was 28.32%,and the yield of propylene was 28.32%.The yield was 21.27%,showing good performance of n-hexane catalytic cracking to produce light olefins.In the process of synthesizing ZSM-5 zeolite with low silicon-aluminum ratio without sodium,it is found that it is difficult to synthesize products with high crystallinity using TPAOH as a single template agent.It is beneficial to improve the crystallinity of the product.When the template agent is changed from TPAOH to n-butylamine,ZSM-5 zeolite with low silicon-alumina ratio with better crystallinity can be obtained by crystallization at 170℃for 48h.Finally,ZSM-5 zeolite(TP-1)with a silicon-aluminum ratio of 29 was synthesized with TPAOH as a template agent,and ZSM-5 zeolite(ZD-1)with a silicon-aluminum ratio of 24 was synthesized with n-butylamine as a template agent.).When the reaction temperature is 600°C,the conversion of n-hexane on both catalysts can reach more than 98%;the yield of triene on TP-1 catalyst is 54.44%,of which the yield of ethylene is 23.80%,and the yield of propylene is the yield of triene on the ZD-1 catalyst was 56.08%,of which the yield of ethylene was 27.65%and the yield of propylene was 21.63%.In the sodium-free synthesis system,small-grain ZSM-5 zeolite was successfully synthesized.The grain size of zeolite can be controlled by adjusting the amount of seed crystals and the crystallization temperature,and the grain size is 100 nm～1μm.Small-grain ZSM-5 zeolite can also be synthesized in a wide range of silicon-aluminum ratios,and ZSM-5 zeolite with a grain size of about 200 nm can be synthesized with a silicon-aluminum ratio from 40 to 120.The performance of the synthesized small-grain ZSM-5 zeolites was evaluated in the reaction of n-hexane catalytic cracking.When the reaction temperature was 600℃,the n-hexane conversion rate could reach 99%,the triene yield could reach 54.41%,and the ethylene yield was 24.25%.%,the propylene yield is 23.79%,with the ratio of ethylene/propylene more than 1.It has good performance of cracking n-hexane,showing the characteristics of high production of ethylene while taking into account the characteristics of high production of propylene.