| dc.description.abstract | The dealumination of zeolite H-b by ammonium hexafluorosilicate (i.e., (NH4)2SiF6, AHFS) treatment was investigated by 27Al and 19F magic angle spinning (MAS) NMR, combined with double resonance NMR. Our results demonstrated that the operating conditions of AHFS dealumination, that is, in the presence and absence of ammonium acetate (NH4Ac), strongly affect the amount, state, and nature of extraframework aluminum species (EFAl). Different aluminum fluoro-complexes after dealumination were detected. The correlation between 19F and 27Al spins was made with the use of double resonance methods such as 19F{27Al} TRAPDOR and 27Al{19F} REDOR NMR. There are significant differences in the 27Al and 19F NMR spectra of dealuminated H-b samples treated by AHFS with and without NH4Ac. For AHFS treated H-b in the presence of NH4Ac, 27Al MAS NMR revealed that tetrahedral aluminum at 54 ppm was mostly expelled from the zeolitic framework, resulting in the formation of two types of EFAl at 13 and 0 ppm. The EFAl at 0 ppm, corresponding to the 19F resonance at –143 ppm, can be assigned to (NH4)3AlF6. In the absence of NH4Ac, besides the EFAl species observed at 13 and 0 ppm, a new broad powder pattern spread from –20 to –90 ppm were observed. The 27Al NMR results demonstrated that at least two different forms of aluminum fluoro-complexes other than (NH4)3AlF6 were present after dealumination. These aluminum fluoro-complexes showed multiple lines located in the range of –150 to –158 ppm in the corresponding 19F spectra. It was found that the formation of fluorinated species after AHFS treatment strongly depends on the pH of the solution, i.e., in the presence and absence of NH4Ac. Complementary characterizations with 27Al and 19F NMR as a function of AHFS content are useful to make peak assignments, and to elucidate possible dealumination mechanisms. | en_US |