Synthesis Of Fe-based FER Zeolites And The NH₃-SCR Catalytic Performance Study

Nitrogen oxides emitted from vehicle exhausts and industrial plants have become a major contributor to photochemical smog,ozone depletion and acid rain in atmospheres.Selective catalytic reduction of NO_x by ammonia（NH₃-SCR）technology has become one of the most-widely used denitration（de NO_x）technology due to its low-cost and high-efficiency.Our previous researches demonstrated that iron-based MCM-22 zeolite synthesized by one-pot method is one of the potential NH₃-SCR catalyst with high catalytic activity in a wide temperature window.However,the poor hydrothermal stability of iron-based MCM-22 zeolite limits its further application in NH₃-SCR,as H₂O generated in NH₃-SCR could result in the destruction of zeolite framework and finally the deactivation of catalyst.Therefore,preparation and studying catalyst with both high catalytic activity and good hydrothermal stability is of great significance.In this thesis,one-pot synthesis of iron-based FER zeolite（[Fe,Al]-FER）by interzeolite conversion method（from MCM-22 to FER）was successfully achieved by exploring the synthesis condition of zeolites.In addition,the influences of preparation method on the catalytic activity and hydrothermal stability of Fe-based FER catalysts for NH₃-SCR was detailly investigated.The main results are as follows:（1）One-pot synthesis of[Fe,Al]-FER zeolites with high Al and Fe content（Si/Al of8-18 and Si/Fe of 11-13）were successfully achieved through interzeolite conversion method（from MWW to FER topology）by optimizing the zeolite synthesis conditions such as synthesis gel composition（Si/Al and Na OH/Si O₂ molar ratios）,crystallization time（72-144h）,crystallization temperature（150-180℃）,adding different content（1-5wt%）of MWW or FER seeds.Crystallization temperature or alkalinity significantly affects the direction of interzeolite conversion process.High crystallization temperature（180℃）or high alkalinity（Na OH/Si O₂≥0.5）results in the formation of MOR phase,with a possible interzoelite conversion process of MWW→MOR→FER→α-quartz.Therefore,the optimal crystallization temperature and alkalinity for one-pot synthesis of[Fe,Al]-FER in this work are 170℃ and Na OH/Si O₂ molar ratio of 0.4,respectively.When crystallizing at 150-170℃ without seeds,the interzeolite conversion of MWW to FER gradually occurred with prolonging crystallization time and increasing crystallization temperature.Adding MWW or FER zeolites as seeds facilitates the interzeolite conversion process.The higher the seeds content,the faster the interzeolite conversion occurs.Finally,pure phase[Fe,Al]-FER zeolite can be successfully synthesized by adding either MWW or FER seeds,though shorter crystallization time and lesser seed contents are needed when using FER as seeds.SEM images illustrate that the crystal particle sizes of[Fe,Al]-FER zeolite can be adjusted by adding different amounts of MWW or FER seeds.UV-vis experiments demonstrate that iron species exist in[Fe,Al]-FER are mainly isolated Fe³⁺species,only little aggregated Fe_xO_y nanoparticles species can be formed.Catalytic results in NH₃-SCR demonstrate that[Fe,Al]-FER has a similar catalytic activity with[Fe,Al]-MCM-22 zeolite while an obviously higher hydrothermal stability due to the higher framework stability of FER than MWW,which suggests that[Fe,Al]-FER synthesized by one-pot method is a potential NH₃-SCR catalyst with both high activity and good hydrothermal stability.（2）Three Fe-based FER zeolites with similar iron content（4.5-5.2wt%）were prepared by solid state ion exchange method（SSIE-Fe/FER）,incipient wetness impregnation method（IM-Fe/FER）and mechanical mixing method（MM-Fe/FER）in order to study the effect of preparation methods on the NH₃-SCR catalytic performances of catalysts.The catalytic results of NH₃-SCR showed that the activity of OP-Fe/FER was significantly higher than that of the other three catalysts over the whole temperature range（100-550℃）.XRD,SEM,N₂ physical adsorption,and NH₃-TPD experiments showed that the four Fe-based FER zeolites prepared through different methods had similar relative crystallinity,morphology,specific surface areas,pore volumes and acid density.However,UV-vis,XPS and H₂-TPR experiments suggested that the distribution of active iron species on four Fe-based FER zeolites was significantly different.OP-Fe/FER prepared by one-pot method has the highest proportion of isolated Fe³⁺species（56%）which are highly active in NH₃-SCR,while the lowest percentage of Fe₂O₃ nanoparticles（24%）that are not conducive to NH₃-SCR reaction.In contrast,the isolated Fe³⁺species only account for 10～14%in SSIE-Fe/FER,IM-Fe/FER and MM-Fe/FER zeolites prepared by post-synthesis method.In addition,large portions of Fe₂O₃ nanoparticles（57-71%）are existed in SSIE-Fe/FER,IM-Fe/FER and MM-Fe/FER zeolites.Ultimately,highly dispersed isolated Fe³⁺species in OP-Fe/FER guarantee the highest NH₃-SCR catalytic activity of OP-Fe/FER among the four Fe-based zeolites in a wide temperature window.The above results demonstrate that one-pot synthesis of Fe-based zeolites by interzeolite conversion is an effective and high-efficient way to adjust the distribution of iron species in zeolites.Based on this synthesis method,the uniform distribution of isolated Fe³⁺species with high activity in NH₃-SCR can be achieved,which is favorable for the improvement of NH₃-SCR catalytic activity of Fe-based zeolite catalysts.