Synthesis Of FeAPSO-34 Molecular Sieves And Its Performance On Selective Catalytic Reduction Of Nitrogen Oxides With Ammonia

Nitrogen oxides(NO,NO2,and N2O)are well-known as the major air pollutants which have a harmful effect on the environment,such as photochemical smog,acid rain,ozone depletion,and the greenhouse effect.One of the most effective technologies for abating NOx is the selective catalytic reduction(SCR)of NOx by NH3,which has been used for stationary power plants,industrial processes and tail gas treatment of automobiles.In recent years,copper-based chabazite(Cu/CHA)type molecular sieve is developed as a new generation of diesel SCR catalysts with excellent NOx selective reduction ability,hydrothermal durability,and carbon deposition resistance.However,the Cu/CHA catalysts exist the problem of poor activity at high temperature.It is well known that Fe-based molecular sieves catalysts have excellent high-temperature activity.Nevertheless,the study of the iron based zeolite(Fe-CHA)just started recently,and most of the studies have focused on the SSZ-13 materials.In this paper,FeAPSO-34 molecular sieves catalysts are prepared through in-situ hydrothermal synthesis.The catalysts are characterized by XRD,SEM,XRF,temperature programmed reduction and desorption,the effects of preparation conditions on the catalytic performance of NH3-SCR catalys are studied,including the effect of dosage of Fe and Si,crystallization time and calcination temperature.There we give the following conclusions:(1)A series of FeAPSO-34 molecular sieves catalysts with different Fe contents(0.91,1.51,2.58 and 3.22 wt.%)are prepared by in-situ hydrothermal synthesis.Because the radius of Fe atom is larger than Si,Al and P atoms,the doping of Fe cause the shift of the main peak from(100)to a low angle,especially for Fe APSO-34-4(Fe content of 3.22 wt.%).Fe doping can also lead to the decrease of the specific surface area and pore volume of Fe APSO-34 catalysts,and the pore structure changes from micropore to mesopore.The formation of Fe-OH on the surface of molecular sieve increases the number of weak acid sites.However,the doping of Fe can lead to incomplete use of raw materials,which is not conducive to the formation of strong acid sites.FeAPSO-34-3(Fe content of 2.58 wt.%)sample has the best conversion rate for NOx(96.2%)at 500 oC,the reasons maybe the better crystallinity of the FeAPSO-34-3 sample,the largest acid content of the weak acid sites and the maximum Fe doping amount in the crystal structure without destroying the molecular sieve.(2)The effects of SiO2/Al2O3 molar ratios(0.6,0.8 and 1)on Fe APSO-34 are investigated.When the molar ratio of SiO2/Al2O3 is 0.8 and 1,the peak intensity of XRD diffraction will be weakened due to the formation of silicon island in the molecular sieve framework.At the same time,the doping of Fe can also lead to the expansion of the crystal lattice,which will cause the decrease of diffraction peak intensity.The NH3-TPD analysis shows that when the ratio of silicon to aluminum is 1,the high temperature desorption peak is not obvious due to the incomplete formation of the silicon island.With the increase of Si content,the Br?nsted acid sites are more and more evident.And with the increase of Fe doping,the acid sites are shifted to the lower temperature region.The difference of the acidity,the existence of the active species and the contents of Fe APSO-34 catalyst result in the difference in the performance of the catalysts.When the molar ratio of SiO2/Al2O3 is 1,the activity of SCR is the best.(3)During the 12 h induction period,the molecular sieve crystal is mainly amorphous.After the crystallization of 12 h,cube structure appears.With the extension of crystallization time,the crystal structure is more and more complete,until the crystallization time(36 h).Prolonging the crystallization time will not change the crystal structure.So it is necessary to consider the energy consumption and save the molecular sieve preparation time.The best crystallization time is 36 h.(4)The four calcination temperatures are discussed by the thermal thermogravimetric curves,which are respectively 550,600,650 and 700 oC.The XRD results show that the different calcination temperature does not change the crystal structure of the Fe APSO-34 molecular sieves.But the diffraction peak intensity is slightly stronger than that of other samples when the calcination temperature is 600 oC.And when the calcination temperature is 700 oC,the crystallinity of zeolite will decrease,and the activity will be affected.(5)The results show that the NH3-SCR activity of FeAPSO-34 catalyst decreases with the increase of hydrothermal aging temperature.During the investigation of the low temperature water resistance of Fe APSO-34 catalyst,it is found that after the operation of water bath immersion of 80 oC the crystallinity of the sample reduces rapidly and the amorphous structure is formed.