Calculating the Concentration of SiOHAl Species in Ferrierites by²⁹Si and²⁷Al NMR Spectroscopy

Two NH 4 -ferrierites (NH 4 -FER) and their calcined H-FER forms with Si/Al ratios of 10 and 27.5 were characterized by 27 Al and 29 Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The 27 Al and 29 Si NMR spectral resolutions of two NH 4 -FER samples with Si/Al = 8.4 and 30.0 were taken from the literature. The quantitative contributions of the Al sites, defect-free Si( n Al), and silanol groups to the different periodical building units (PerBUs) of the FER zeolite framework were calculated from the various Si/Al ratios and relative 27 Al and 29 Si NMR signal intensities. Two possible PerBU models and a dealumination route were suggested by the FER structure, and the distribution of PerBUs followed the composition of the framework with appropriate precision in the studied composition range (Si/Al = 8.4−30.0). The independently measured [Fourier transform infrared (FTIR) spectroscopy, NH 3 temperature-programmed desorption (TPD)] extraframework aluminum (EFAl) concentrations and those measured by 27 Al NMR spectroscopy were in a good agreement. The concentration of strong Brønsted acidic SiOHAl groups calculated with the SiOHAl PerBU model revealed that one-quarter to one-third of all OH groups were present in these SiOHAl sites in the ferrierites with Si/Al = 10 and one-half of them were in these sites in the NH 4 -FER sample with Si/Al = 27.5. The former zeolites were dealuminated at a higher degree than the latter. Tetrahedral aluminum species occupied the SiOHAl sites, and both deformed and octahedral Al species were present in the EFAl positions.