In Situ Hydrothermal Synthesis Of Copper Based CHA Type Zeolites And Catalytic Performance In NH₃-SCR Reaction

Nitrogen oxides?NO_x?from the exhaust of vehicles is considered as one kind of hazardous air pollutants that can cause photochemical smog,acid rain and do harm to human's health.The selective catalytic reduction of NO_x with NH₃?NH₃-SCR?has been recognized as one of the most promising technologies to remove NO_x owing to its high efficiency and less secondary pollution.Transition metal-exchanged zeolites?e.g.,Cu/ZSM-5,Cu/Beta?have attracted growing attention in the last several years for NH₃-SCR reaction.However,the chemical structures of these catalysts are prone to be collapsed under hydrothermal conditions.Recently,SCR catalysts based on small-pore Cu-exchanged Chabazite?Cu-CHA?zeolites,such as Cu/SSZ-13 and Cu/SAPO-34 have been widely studied because of their excellent catalytic activity and hydrothermal durability.So far,various synthetic methods have been employed to prepare Cu/SAPO-34catalysts,including impregnation method,traditional solution ion-exchange method,solid-state ion exchange and in situ hydrothermal synthesis method.Among them,the in situ hydrothermal method is the most economic and simplest route for preparation of Cu/CHA catalysts.The NH₃-SCR performance is mainly affected by the hydrothermal stability of catalysts,and the state and content of active sites.Typically,there are two kinds of active sites in Cu/CHA catalysts,i.e.,Lewis acid?from Cu ions?and Br?nsted acid?from zeolites?sites.It was generally accepted that the isolated Cu ions located in six-ring of zeolites contributed to producing N₂,while the Cu ions in the eight-ring led to side product N₂O,which decreased the selectivity of N₂.On the other hand,Br?nsted acid sites could also mediate the SCR performance by affecting dispersion of Cu ions and NH₃ storage capacity.There are various methods to modify Cu/CHA catalysts,such as adding other metals,modulating the acidity and active center of the catalysts,or changing the morphology which promote the adsorption of NH₃ to improve the catalytic activity.In this thesis,the catalytic performance of small pore Cu/CHA?Cu/SSZ-13 and Cu/SAPO-34?zeolites with excellent catalytic properties have been prepared and studied.Morphology,acidity,metallic species are characterized and catalytic performance are evaluated.The main results are as follows:1.A series of nanosized Cu/SAPO-34 catalysts are successfully prepared by in situ synthesis method using tetraethylammonium hydroxide?TEAOH?and copper-tetraethylenepentamine?Cu-TEPA?as co-templates.Meanwhile,the influence of another synthetic method,Br?nsted acidity,as well as crystal size of Cu/SAPO-34 catalysts on the NH₃-SCR performance are systematically investigated.Note that the in situ preparation strategy and nano size of catalysts are beneficial for the dispersion of active Cu species and thus promote the NH₃-SCR catalytic activity.Particularly,the in situ prepared nanosized Cu/SAPO-34 catalyst with the strongest Br?nsted acidity exhibits the best catalytic performance in reducing NO_x.2.A hydrothermal synthesis method is developed to directly synthesize bimetallic CuNi/SSZ-13 zeolites using nickel-diethylenetriamine?Ni-DETA?and Cu-TPEA as co-templates.Compared with the single metal Cu/SSZ-13 or Ni/SSZ-13,the obtained bimetallic CuNi/SSZ-13 catalyst exhibits more excellent catalytic performance in the NH₃-SCR reaction.The introduction of Ni leads to a certain catalytic performance improvement at high temperature.As Ni/SSZ-13zeolite is completely inactive,nickel might play a synergistic effect with Cu in CuNi/SSZ-13,which promotes the improvement of catalytic performance.The molar ratio of Cu/Ni can significantly affect the catalytic activity of the catalyst.Among the obtained catalysts,the one with Cu/Ni ratio of 5.33 exhibits the highest catalytic performance,and the NO_x conversion rate is above 80%at the temperature window range of 190-520°C.In summary,we have developed in situ hydrothermal method to directly prepare Cu/SAPO-34 and CuNi/SSZ-13 catalysts by introducing metal complex or bimetal complexes into the zeolite synthetic system.The synthetic method is simple and straightforward,which avoids the aggregation of metal species in zeolites introduced by post-treatment methods.The resultant catalysts exhibit high performance in NH₃-SCR reaction.