Effect Of Crystallization Modes On SSZ-13 Zeolite For Catalytic Removal Of NO_X In Vehicle Exhaust

As nitrogen oxide（NO_x） emissions increased gradually, urea/ ammoniaselective catalytic reduction（urea / NH₃-SCR） has become one of the most effective method for removing NOx under enriched-oxygen conditions. Recently, SSZ-13 zeolite with the CHA structure, owing to its excellent catalytic activity, selectivity and hydrothermal stability, become a significant research object of zeolite catalysts reducing NOx from diesel vehicles. The systhesis of SSZ-13 zeolite was mainly restricted to traditional static hydrothermal crystallization, however, the crystallization modes had a great influence to its structure. Therefore, it is particularly important for choosing appropriate crystallization mode to further improve the deNOx performance of SSZ-13. In this paper, the crystallization process and the effects of Si/Al values of raw material on crystals morphology of SSZ-13 zeolite synthesized by microwave, dynamic and static hydrothermal methods,and the difference of catalytic performance of Cu-SSZ-13 catalysts by Cu ion exchange were investigated. Meanwhile, We systematically studied the effects of microwave hydrothermal crystallization conditions（time and temperature） on catalytic activity. This work will also provide a rapid way to synthesize CHA zeolite and related catalysts, and more importantly, provide the theoretical basis data for optimizing SSZ-13 catalysts deNOx performance. The main conclusions of this work are as follows:（1） Both microwave and dynamic hydrothermal method could shorten the SSZ-13 zeolite crystallization period（including induction, nucleation and crystal growth period） of the molecular sieve, and the crystallization rate of microwave hydrothermal method was the fastest. As the crystallization time increased, to samples prepared by microwave and static hydrothermal method, cube particle dispersion increased and its size abated, however, cube particle dispersion and size of samples prepared by dynamic hydrothermal method showed a trend of first increased and then decreased, and the rule was consistent with the change of crystallinity of samples. The SSZ-13 zeolite catalysts prepared by microwave and dynamic hydrothermal method displayed more developed pore structure, stable Cu2+ions, and stronger NH₃ and NO adsorption capacity.（2） The SSZ-13 zeolite could be fast synthesized with microwave hydrothermal method in relatively wide temperature range（145-190 oC）. The low temperature activity and hydrothermal stability of SSZ-13 zeolite catalysts prepared by microwave hydrothermal method were improved greatly, compared with the samples prepared by static hydrothermal methods, which can be attributed to the improvement of the zeolite structure. SSZ-13 catalysts which was quickly prepared by microwave hydrothermal method exhibited outstanding ion exchange properties, and the quantity of weak and strong acid were enhanced to some extent.The SSZ-13 catalysts prepared under low temperature crystallization（145-160 oC） condition displayed excellent low-temperature and high-temperature NH₃-SCR activity, the samples obtained at temperature of 190 oC showed better reaction selectivity; and the samples prepared at 175 oC exhibited excellent hydrothermal stability. The reason for above differences were largely due to the greater influence of crystallization temperature on skeleton structure, active metal loading and location of SSZ-13 catalysts.The crystallization time almost has no effect on the activity, selectivity and hydrothermal stability of the catalysts when it exceed 4 h. This phenomenon was mainly attribute to the incomplete skeleton of samples prepared by crystallization 2 h, resulting in a low load active ingredient, poor stability and less acid sites of skeleton and bad catalytic activity.（3） With the increase of Si/Al value, the crystallinity of samples prepared by microwave and dynamic hydrothermal method gradually enhanced but showed stable state finally, but the role of the latter was weak, and the samples prepared by static hydrothermal method crystallinity decreased first and then increased. The changing Si/Al value of raw material had different effects on the morphology of the samples. The particle size the samples with lower Si/Al value was small, and its also appeared local agglomeration phenomenon. When the Si/Al value was 5, the samples obtained by microwave hydrothermal method was amorphous. At higher Si/Al value, to samples prepared by dynamic and static hydrothermal method, the regularity of morphology of decreased, aggregation degree increased, and the larger size particles formed.