27Al and 19F solid-state NMR studies of zeolite H-β dealuminated with ammonium hexafluorosilicate

Hsien Ming Kao, Yun Chu Chen

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Abstract

The dealumination of zeolite H-β by ammonium hexafluorosilicate (i.e., (NH4)2SiF6, AHFS) treatment was investigated by 27Al and 19F solid-state NMR, combined with 27Al /19F double resonance NMR. The NMR results demonstrated that the experimental conditions of AHFS dealumination, that is, in the presence and absence of ammonium acetate (NH4OAc), strongly affected the amount, state, and nature of extraframework aluminum (EFAl) species. Different aluminum fluoro-complexes after dealumination were detected. 19F and 27Al NMR results revealed that the essential part of the dealumination process carried on with AHFS in the presence of NH4OAc was that most of the extracted Al3+ reacted with F- to form (NH4)3AlF6 species that were evident from the signals at 0 ppm in the 27Al spectrum and -143 ppm in the 19F spectrum, respectively. In the absence of NH4OAc, a resonance at 13 ppm and a broad pattern spread from -20 to -90 ppm in the 27Al spectrum were observed, indicating the presence of two different forms of aluminum fluoro-complexes other than (NH4)3AlF6 after dealumination. There were significant differences in their 27Al 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 19F spectrum. Discrimination between these different species was made by monitoring the change of 19F peak shift as a function of AHFS content. The correlation between 19F and 27Al spins was also made with the use of double resonance methods such as 19F{27Al} TRAPDOR and 27Al{19F} REDOR NMR. Complementary characterizations with 27Al and 19F NMR as a function of AHFS content have been useful to make peak assignments, and to elucidate possible reaction pathways during AHFS dealumination.

Original languageEnglish
Pages (from-to)3367-3375
Number of pages9
JournalJournal of Physical Chemistry B
Volume107
Issue number15
DOIs
StatePublished - 17 Apr 2003

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