Synthesis Of ZSM-5 Zeolite Nanosheets And Its Performance In Catalytic Cracking

Light olefins are important basic chemical raw materials and occupy a very significant position in the development of the national economy.Traditional steam cracking process could mainly produce ethylene and coproduce propylene,which has the disadvantages of less raw materials available,large energy consumption and equipment investment,serious environmental pollution and low productivity.In contrast,catalytic cracking technology,due to the introduction of high-efficiency catalysts,can not only reduce the reaction temperature but also increase the selectivity of ethylene and propylene.Conventional ZSM-5 zeolite has the advantages of high catalytic activity and strong hydrothermal stability,but the full microporous system also reduces the diffusion rate of the reaction molecules,affects the reaction rate,and generates coke deposits severely,resulting in deactivation of the catalyst and a decrease in the life cycle.In recent years,the synthesis and application of ZSM-5 zeolite nanosheets have received extensive attention.It has a large specific surface area and mesoporous volume,and the ordered lamellar structure shortens the diffusion path drastically and increases the amount of acid sites in the intersection.Used in the catalytic cracking of light hydrocarbons,it could increase reactant conversion,yield of olefins,and catalytic stability.A di-quaternary ammonium-type surfactant was synthesized in this dissertation,and its molecular structure was confirmed by NMR.Using this bifunctional surfactant as the template,tetraethyl orthosilicate as the silicon source and aluminum sulfate 18-hydrate as the aluminum source,ZSM-5 zeolite nanosheets were synthesized by hydrothermal crystallization method successfully.Multiple sets of exploratory experiments were designed to analyze the effects of some factors on the synthesis of ZSM-5 zeolite nanosheets,including filling degree,alkalinity,amount of templating agent,crystallization temperature and crystallization time.The results showed that increasing the filling degree appropriately can increase the crystallization pressure and then increase the crystallinity of the zeolite.However,when the filling degree is too large,it will affect the lamella structure to destroy the zeolite.Increasing the alkalinity could speed up the decomposition reaction of silicon source and aluminum source,therefore accelerate the crystallization rate,but,excess alkalinity can cause agglomeration of zeolite.If the amount of templating agent is too low,it cannot fully guide and synthesis zeolite,On the contrary,it will affect badly the ordered structure of ZSM-5 zeolite nanosheets.If the crystallization temperature is too low,it is not sufficient to crystallize.If the temperature is too high,the templating agent will partially decompose.The crystallization time can be increased within a suitable range to increase the crystallinity of zeolite.Finally,the optimal synthesis conditions for ZSM-5 zeolite nanosheets under static conditions were confirmed.Namely,the filling degree was 50%,the crystallization temperature was 150?,the crystallization time was 6 days,and the molar ratio of the raw materials in the gel solution was 30Na2O:1Al2O3:100SiO2:10C22-6-6Br2:18H2SO4:4000H2O.The synthesized ZSM-5 zeolite nanosheets are spherical particles stacked by a disk structure with a uniform shape and have a particle size of about 2.5 ?m.Under the optimized synthesis conditions,ZSM-5 zeolite nanosheets with high crystallinity and single morphology were synthesized successfully.Catalytic cracking of n-heptane was used as a model reaction to evaluate its catalytic activity and similar stability,and coke deposition on the zeolite after the reaction was further analyzed.Compared with conventional ZSM-5 zeolite,ZSM-5 zeolite nanosheets have higher catalytic activity and similar stability;the longer the crystallization time is,the larger specific surface area and mesopores volume of ZSM-5 zeolite nanosheets have,as well as the higher its catalytic activity is.Coke deposition amount of ZSM-5 zeolite nanosheets prepared under different crystallization time conditions is similar.With the increase of crystallinity of zeolite,distribution of coke deposits in micropores gradually decreased.The coke content of the conventional ZSM-5 zeolite is twice that of ZSM-5 zeolite nanosheets and is mostly distributed in the micropores.During the reaction,coke deposition in the micropores is more likely to cause the acidic sites to be covered and the pores to be blocked,resulting in a decrease in activity.