Exploring Microstructures And Acid Properties Of SSZ-39 And ITQ-13 Zeolite By Solid-state NMR Spectroscopy

SSZ-39 zeolite with AEI structure has been proved to be an excellent catalyst for NH3-SCR and MTO reactions,while ITQ-13 zeolite with ITH structure is also widely used in MTP and MTH reactions,and the new Ge-free ITQ-13 zeolite has brought more research direction.The outstanding catalytic performance of these two zeolites is closely related to their specific microstructures and acid properties,and solid state NMR combined with theoretical calculation is an effective method to study the structural properties of porous materials.So in this study,27Al/23Na MQ MAS NMR and DFT calculations were used to investigate the framework A1 distribution and Na+cations locations of SSZ-39 and ITQ-13 zeolites,and 1H MAS NMR was employed to explore the differences in the acidity of samples with different Al content.The main findings are listed as follows:(1)Combined with 23Na MQ MAS NMR and DFT theoretical calculations,isolated A1 and AlSiSiAl distributions in SSZ-39 zeolite were differentiated,and the SIa0 signal in the spectra was formed by Na+in the hexadecromatic prism.The SⅡa0 and SⅢ’a0 sites belong to the Na+in the 6MR and the 8MR with 1A1 substitution at the T3 position,respectively,while the priority is slightly higher in the 8MR.For AlSiSiAl substitution,the A1 pair occupies the counter position of the 6MR(T3-T3),and the SIIal and SⅢ’al sites correspond to two related Na+cations.Some free Na+caused by structural destruction may form notable SⅢ’b signal.The 1H MAS NMR of d3-acetonitrile adsorbed on H-SSZ-39 zeolite were used to determine that the amount of B acid in Al-rich H-SSZ-39 zeolite was larger and the strength of B acid was weaker.Theoretical calculations verified that the decrease of acid strength was caused by the increase of A1 pairs in the structure.(2)27A1 MQ MAS NMR and DFT calculation are adopted to prove that the T5 and T7 positions are preferentially occupied by isolated A1 atoms in ITQ-13 zeolite,and the T5 position is slightly stable.At the same time,the locations of Na+was investigated by 23Na MQ MAS NMR,the Na+attached to the T5 or T7 aluminum atom located on the 9MR and 10MR formed the S’c signal.S’b1 and S’b2 signals are caused by the Na+with strong mobility in the zeolite structure,these Na+tend to move into the rings at high temperature.When the Na content of zeolite is high enough,the Na+signal in the Lau structure will appear,namely S’a site.Two kinds of B acid signals were identified in H-ITQ-13 zeolite by 1H MAS NMR,and the amount of B acid in the Al-rich sample was higher.The difference of B acid strength of the sample could not be compared by adsorption of d5-pyridine,because d5-pyridine was not sensitive to the change of B acid strength.Through NH3-TPD characterization,it was determined that the acid strength of H-ITQ-13 zeolites with higher Si/Al was stronger,which was contrary to the acid properties of the former Si-Ge-Al ITQ-13 zeolites.