Synthesis Of CHA-Type Zeolites And Their Catalytic Properties In MTO/DeNO_x Reactions

Chabazite,a zeolite with the CHA topology,has attracted much attention because of its outstanding performances in energy and environmental catalyses.The development of cheap,green,and high-performance CHA zeolites is of great significance for overcoming the application limitations like high manufacture cost and poor economy,and also for realizing large-scale industrial applications.This thesis is aimed to develop the CHA zeolite catalysts with practical application value from following four aspects,that is,acid site regulation,pore opening,optimized synthesis,and design of new organic structure-directing agent(OSDA)for CHA zeolite synthesis.The effect of changes in the microenvironment of the active sites on the methanol-to-olefin(MTO)reaction and the catalytic propeties of cheap Cu-CHA catalyst obtained after Cu²⁺ion-exchange in the nitrogen oxide elimination(DeNO_x)reaction have both been systematically studied.This research is expected to provide theoretical reference and new technical methods for the design and synthesis of cheap,green and high-performance CHA-type zeolites for promising industrial applications in MTO and DeNO_x.The main works of this thesis are as follows:(1)CHA aluminosilicate synthesized without any OSDA is a green zeolite material,but possessing excess acid sites,structural instability and limited applications due to a high aluminum content.A novel stepwise strategy including dealumination and stabilization was proposed,which not only increased the Si/Al ratio from 2.4 to 5.0,but also stabilized the framework structure.Simultaneously,the removal of Al atoms introduced mesopores into the CHA framework and enhanced the diffusion property.Compared with the pristine low-silica CHA,the obtained high-silica CHA zeolite displayed attractive catalytic performance in methanol-to-olefin(MTO)reaction,showing a methanol conversion above 80%for more than 360 min at a maximum selectivity of 87%for C₂⁼-C₄⁼light olefins olefins.(2)The acidity and reaction space of CHA have been balanced by tuning the residual K⁺cations in the cavity via controlled ammonium ion-exchange.During the exchange process,the K⁺cations at the eight-membered ring position in the CHA structure is preferentially exchanged and the resultant CH A zeolites exhibit a weak acidity,while the K⁺cations at the six-membered ring position are more difficult to be exchanged,but the resultant zeolites exhibit a strong acidity.The completely exchanged CHA(>98%K⁺cation exchange level)deactivated rapidly in MTO,whereas CHA(65%K⁺cation exchange level)with moderate acidity showed the best catalytic performance.The moderate acidity in the open cage facilitated the formation of higher methylbenzene intermediates,giving propylene(selectivity up to 45%)as the predominant product.(3)In view of the high-cost of high-silica CHA zeolite,the synthesis route is optimized,so as to try to obtain relatively cheap high-silica CHA zeolite.In this study,high-silica silicon-alumina sources were obtained by dealumination of low-cost Na Y and Al-rich Beta precursors as raw materials,and then under the guidance of low-cost or low-content organic structure directing agents,high-silica CHA zeolites were successfully synthesized through the inter-zeolitic conversion route.After ion-exchange with Cu²⁺,the Cu-CHA zeolites showed superior performance in the NH₃-SCR reaction,which can be attributed to their high surface area,abundant acidic sites.The in situ IR analysis revealed the reaction intermediates of NH₃-SCR on the low-cost Cu-CHA catalysts.(4)In this study,a cheap new quaternary ammonium N,N,N trimethylisobutylammonium hydroxide(TMAib OH)was designed and prepared,and under the guidance of it,silicoaluminophosphate zeolites with CHA structure were successfully prepared.The Si content in the CHA zeolite synthesized under the guidance of the new template TMAib OH is widely adjustable.Among them,the CHA zeolite with less Si content is outstanding in the MTO reaction,maintaining a methanol conversion rate of more than 90%for 850 minutes,and the selectivity of light olefins(C₂⁼-C₄⁼)is as high as 90%.