²⁷Al and ¹⁹F Solid-State NMR Studies of Zeolite H-β Dealuminated with Ammonium Hexafluorosilicate

Abstract

The dealumination of zeolite H-β by ammonium hexafluorosilicate (i.e., (NH 4 ) 2 SiF 6, AHFS) treatment was investigated by 27 Al and 19 F solid-state NMR, combined with 27 Al/ 19 F double resonance NMR. The NMR results demonstrated that the experimental conditions of AHFS dealumination, that is, in the presence and absence of ammonium acetate (NH 4 OAc), strongly affected the amount, state, and nature of extraframework aluminum (EFAl) species. Different aluminum fluoro-complexes after dealumination were detected. 19 F and 27 Al NMR results revealed that the essential part of the dealumination process carried on with AHFS in the presence of NH 4 OAc was that most of the extracted Al 3+ reacted with F - to form (NH 4 ) 3 AlF 6 species that were evident from the signals at 0 ppm in the 27 Al spectrum and −143 ppm in the 19 F spectrum, respectively. In the absence of NH 4 OAc, a resonance at 13 ppm and a broad pattern spread from −20 to −90 ppm in the 27 Al spectrum were observed, indicating the presence of two different forms of aluminum fluoro-complexes other than (NH 4 ) 3 AlF 6 after dealumination. There were significant differences in their 27 Al chemical shifts and spectral line shapes despite the geometry of all these complexes corresponding to a hexacoordination. These aluminum fluoro-complexes showed multiple lines located in the range of −150 to −156 ppm in the 19 F spectrum. Discrimination between these different species was made by monitoring the change of 19 F peak shift as a function of AHFS content. The correlation between 19 F and 27 Al spins was also made with the use of double resonance methods such as 19 F{ 27 Al} TRAPDOR and 27 Al{ 19 F} REDOR NMR. Complementary characterizations with 27 Al and 19 F NMR as a function of AHFS content have been useful to make peak assignments, and to elucidate possible reaction pathways during AHFS dealumination.