Optimization Of Preparation Conditions For Fly Ash Based Cu-SSZ-13 Molecular Sieve

Fly ash is a kind of bulk solid waste.Its main chemical composition is SiO₂ and Al₂O₃,the chemical composition of which is very similar to the molecular sieve.Activating fly ash to prepare molecular sieve is an effective way to comprehend sive utilization of fly ash.SSZ-13 is a CHA molecular sieve with high Si-Al ratio,which has many advantages such as high specific surface area,good hydrothermal stability,adjustable acidity,excellent ion exchange performance and so on.SSZ-13 has attracted extensive attention in the research field of NO_x removal from automobile exhaust.However,due to its high preparation cost,it has not been used in large scale industry.Therefore,using fly ash as raw material to synthesize SSZ-13 molecular sieve,on one hand,can reduce the raw material cost,on the other hand,can turn waste into valuable metarials,to achieve high value utilization of fly ash.The previous study on SSZ-13 molecular sieve showed that the Si-Al ratio,the content of active metal components and the drop position in the framework of SSZ-13 molecular sieve played important roles in broadening the window of denitration activity and its hydrothermal stability.In addition,the literature shows that the intra-crystalline reactions of the molecular sieve skeleton are affected by the different particle sizes of the samples.The previous researches of our group have successfully prepared initial zeolite using fly ash as raw material through hydrothermal activation.Then,using the initial zeolite as raw material,adding template agent and silicon source,a series of SSZ-13 zeolite were synthesized by hydrothermal crystallization method.Based on this,a series of Cu-SSZ-13 catalysts were prepared in this paper with fly ash as the main raw material and kaolin,fused mullite and sintered mullite as raw materials.And through the evaluation of denitration activity,the Cu-SSZ-13 catalyst with better performance was screened out.The denitration activity and hydrothermal stability of the catalyst with different Si-Al ratio were investigated,and the best Si/Al ratio was optimized.On this basis,SSZ-13 molecular sieve was synthesized by seed method.The particle size of catalyst was controlled by adjusting the addition amount of seed,and Cu-SSZ-13 catalyst with wide denitration activity window and good hydrothermal stability was prepared by adjusting the p H of the initial gel and the loading amount of Cu ions.XRD,ICP-OES,N₂ adsorption and desorption,H₂-TPR,NH₃-TPD were used to analyze the physical and chemical properties of Cu-SSZ-13 catalyst.The specific research contents are as follows:（1）A series of Cu-SSZ-13 catalysts were prepared using fly ash,kaolin,fused mullite and sintered mullite as raw materials.The denitration performance of the catalysts prepared with four kinds of raw materials were investigated on a fixed bed catalyst activity evaluation device.The experimental results show that the Cu-SSZ-13 catalyst prepared from fused mullite has the highest specific surface area,followed by the catalyst prepared from fly ash.Cu-SSZ-13 catalysts prepared from the four kinds of raw materials all have good denitration activity（The conversion rate of NO reaches over 90%in the corresponding temperature range）,and the Cu-SSZ-13 catalyst prepared from fly ash has the widest activity temperature window which may be related to the highest copper loading（2）Using fly ash as raw material,Cu-SSZ-13 catalysts with different Si-Al ratios were prepared by adjusting the Si-Al ratio during the crystallization process,and the effects of Si/Al ratio on the denitration activity and hydrothermal stability of the catalysts were investigated.The experimental results show that the Si/Al molar ratio of Cu-SSZ-13catalyst increases with the increase of Si/Al ratio in feeding.The specific surface area and copper loading of Cu-SSZ-13 catalyst decreased with the increase of Si/Al molar ratio.The conversion rate of Cu-SSZ-13 catalyst with a Si/Al molar ratio of 15.1 is close to 100%in the range of 210-520℃,which may be related to the higher specific surface area and copper loading.The hydrothermal aging resistance of all samples is not ideal and needs to be improved.（3）Adding seed in the process of hydrothermal crystallization can greatly reduce the dosage of template agent and improve the properties of the product.A series of fly ash based Cu-SSZ-13 catalysts were prepared by adding crystal seeds and adjusting the p H value of the reaction system on the basis of SiO₂:Al₂O₃ ratio of 30,and the denitration and hydrothermal aging resistance of the catalysts were investigated.The specific surface area of Cu-SSZ-13 catalyst was greatly increased with the addition of seed crystals,and the maximum surface area was about 695.8 cm³/g.When only 0.1 wt.%seed crystal（The total mass of SiO₂ in the raw material was taken as the benchmark）was added,the denitration activity of the sample was higher,and the range of active temperature window 210-580℃.After hydrothermal aging,the denitration performance of the samples prepared by adding seed crystals is slightly better than that of the samples prepared by adding seed crystals.In addition,it was found that increasing p H appropriately could Reduce the agglomeration of catalyst grains...