Modified Synthesis Of CHA-type Molecular Sieves And Their Applications In NH₃-SCR

Nitrogen Oxides(NOx)is a normal hazardous pollution in the atmosphere.One of the main sources is the emission of diesel exhaust.To meet the requirements of air environment and living standards,wide applications of diesel vehicles highly depend on the effective NOx reduction strategies.Among them,ammonia selective catalytic reduction(NH3-SCR)is an efficient NOx abatement route.Considering the real driving conditions and tightening legislation,the NH3-SCR catalysts are supposed to possess excellent catalytic performalce as well as stability.Currently,Cu2+exchanged SAPO34 and SSZ-13(Cu/SAPO-34 and Cu/SSZ-13)have been regarded as two promising catalysts resulting from their outstanding NH3-SCR performance.However,they still suffer from their own limitations.Based on these,investigations in this thesis have improved the shortcomings of both catalysts from the perspective of optimizing the synthesis parameters unraveled from structural characterizations.It consists of three aspects as follows:1.The Bronsted acids(BAs)in SAPO-34 play important roles in NH3-SCR.Exploring the acid properties,including acid density and strength,is of great significance for improving the low temperature hydrothermal stability.In this case,the acid properties of four series of SAPO-34 molecular sieves,which were synthesized by different OSDAs,such as tetraethylammonium hydroxide(TEAOH),diisopropylamine(DIPA),n-Butylamine(nBA),and morpholine(MOR)with different Si contents,were investigated by solid-state nuclear magnetic resonance(ss-NMR),Rietveld refinement and Thermogravimetry(TG).The final results show that the density of BAs is determined by the amounts of protonated OSD As in each cha cage while the strength is associated with the Si contents and the amount of Si atoms at the edge of Si islands.2.Cu/SAPO-34,once considered a dream catalyst for NH3-SCR,is limited by its disappointing low-temperature hydrothermal instability.It is believed that the fragile Cu/SAPO-34 at low-temperature hydrothermal condition is influenced by several parameters,such as Si contents,Si environments and Cu2+contents.Based on investigations in the last section,SAPO-34s with different Si contents and environments were successfully synthesized by employing DIPAs as OSDAs.A further direct ion exchanged(DIE)method was performed to obtain Cu/SAPO-34 for NH3SCR tests.The results demonstrate that Cu/SAPO-34 with low Si contents(8.8%in molar ratio),distinct Si(4Al)environment and moderate Cu contents(2.4 wt%)displays the most promising catalytic performance.We further summarize criteria for constructing robust NH3-SCR catalysts,which enable us to identify another three commercialized OSDAs for synthesizing robust Cu/SAPO-34 catalyst.3.Cu/SSZ-13 is a commercialized catalyst for NH3-SCR at present.However,the huge budgets from the preparation of initial SSZ-13 impede the further investigation.Therefore,it is important to develop novel methods to reduce costs of Cu/SSZ-13.Since SAPO-34 and SSZ-13 are closely related from the structural point of view,it would be effective to attempt to fully or partially utilize commercialized OSDAs used for SAPO-34 to synthesize SSZ-13.Herein,DIPA was employed to partially replace expensive N,N,N-trimethy ladamantammonium hydroxide(TMAdaOH)in the synthesis of SSZ-13.As a result,the as-synthesized SSZ-13 zeolites possess a tunable range of silicon-to-alumina ratio(SAR)and Cu/SSZ-13 catalysts show excellent NH3SCR performance.The addition of DIPA would also accelerate the crystallization process,improve the yields,and prevent the formation of amorphous phase.Besides,other OSDAs(DPA and nBA)from SAPO-34 are alternatives for the synthesis of SSZ13 in this method.