A Solid-state NMR Study On The Microstructures Of Cation-exchanged SSZ-13 Zeolite

SSZ-13 zeolites with CHA structure are widely used as the support in the DeNOx catalyst and directly catalyze methanol to olefins.Variations on Al distributions,locations of corresponding nonframework cations and porous structures have significant influences on their catalytic performances,hydrothermal stabilities of SSZ-13 zeolites.Multinuclear solid-state NMR is a powerful tool to detect the local structures of solid materials.In this thesis,the Al distribution of SSZ-13 zeolites by ²³Na MAS NMR and DFT calculations,as well as the locations of Y and Cu cations and changes in pore size due to dealumination by¹²⁹Xe NMR were studied.The main findings are listed as follows:（1）The locations of Na cations related to Al distributions in SSZ-13 zeolites were investigated by two-dimentional ²³Na MQ MAS NMR techniques combined with DFT calculations.The isolated Al and Al pair in 6-membered ring（MR）distributions and corresponding Na⁺locations are clearly identified based on the only one crystallographically inequivalent T site in CHA cage.In high-silica Na-SSZ-13 zeolite,Na⁺ions prefer to locate at 6-MR SIIa0 site corresponding to one Al substitution in the 6-MR.However,a small amount of Al pair at 6-MR is still observable even with Si/Al ratio of 48,corresponding to two Na⁺ions locating at 6-MR SIIa1 and 8-MR SIII’a1 sites,respectively.With more Al substitutions in the CHA cage,the signal at SIII’b site is attributed to a portion of Na⁺ions experiencing high mobility among different SII sites.In Al-rich SSZ-13 zeolite（Si/Al=4）,the presence of the appreciable amount of Si（2Al）and minor Si（3Al）in this sample may lead to Na⁺at 6-MR SIIa2 and the other Na⁺site at 8-MR SIII’a2 with overlapped NMR signal of SIII’a1 site.（2）High-pressure ¹²⁹Xe NMR was employed to investigate the porous structures of SSZ-13 zeolites with 8-MR and their variations after exchanged Cu and rare-earth Y cations.The locations of extra-framework cations were also detected.It is found that ¹²⁹Xe NMR can identify the supercages in SSZ-13 with or without dealumination.Y ions tend to locate near the 8-MR of SSZ-13 when increasing their loadings or introducing Cu cations.The paramagnetism of Cu²⁺ions causes the disappearance of ¹²⁹Xe NMR signal in the supercages,but has no effect on the ¹²⁹Xe NMR signal in the dealuminated supercages.After reduction,the interaction of Cu⁺with Xe atom in the supercages reduces the ¹²⁹Xe NMR chemical shift.