J Phys Chem C, vol 113, issues 2 and 3

Calculating the Concentration of SiOHAl Species in Ferrierites by 29Si and 27Al NMR Spectroscopy
Tamás I. Korányi*†‡ and János B.Nagy‡
J. Phys. Chem. C, 2009, 113 (3), pp 907–913

Abstract:Two NH4-ferrierites (NH4-FER) and their calcined H-FER forms with Si/Al ratios of 10 and 27.5 were characterized by 27Al and 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The 27Al and 29Si NMR spectral resolutions of two NH4-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(nAl), 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 27Al and 29Si 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, NH3 temperature-programmed desorption (TPD)] extraframework aluminum (EFAl) concentrations and those measured by 27Al 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 NH4-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.