| dc.description.abstract | The dealumination of Y zeolites with different cations by ammonium hexafluorosilicate (i.e., (NH4)2SiF6, AHFS) treatment was investigated by 27Al and 19F magic angle spinning (MAS) NMR, combined with double resonance MAS NMR. Our results demonstrated that the operating conditions of AHFS dealumination, that is, with and without pH adjustment, strongly affect the amount, state, and nature of extraframework aluminum species (EFAl). Different aluminum fluoro-complexes after dealumination were detected. First, for AHFS treated HY with pH adjustment, XRD revealed that tetrahedral aluminum was expelled from the zeolitic framework, resulting in the formation of (NH4)3AlF6, which was evident from the peak at 0 ppm in the 27Al MAS NMR and at -141 ppm in the 19F MAS NMR. Without pH adjustment, we observed that the Y zeolite structure were progressively destroyed with increasing the amounts of AHFS. Besides (NH4)3AlF6, another phase structure was observed and identified as NH4AlF4 by XRD and MAS NMR. A new broad powder pattern spreads from -20 to -90 ppm (NH4AlF4) is observed at SiAHFS/AlHY > 0.40. The NH4AlF4 crystalline phase observed exhibited two 19F resonance peaks at -151 ppm and -166 ppm, which are assigned, for the first time, to the fluorine atoms in the terminal Al-F and bridging Al-F-Al groups, respectively.
Secondly, for AHFS treated NaY with pH adjustment, MAS NMR revealed that tetrahedral aluminum was expelled from the zeolitic framework, resulting in the formation of Na3AlF6, which shows the characteristic peaks at -1.7 and -10 ppm in the 23Na MAS NMR and at -190 ppm in the 19F MAS NMR spectra. Without pH adjustment, the Y zeolite structure was again destroyed with increasing the amount of AHFS. A new NaxAlFy structure was observed with 19F MAS NMR for both treatments.
Finally, for AHFS treated NH4Y with pH adjustment, XRD revealed that tetrahedral aluminum was expelled from the zeolitic framework and mainly formed (NH4)3AlF6. The Y zeolite structure was still retained even at a higher SiAHFS/AlHY ratio (~ 1.0) with pH adjustment. Without pH adjustment, on the other hand, the products of dealumination of NH4Y are various AlO6-xFx complexes. | en_US |