H-1 MAS and H-1/Al-27 TRAPDOR NMR studies of oxygen-zeolite interactions at low temperatures: Probing Bronsted acid site accessibility

Sorption of oxygen in the pores of zeolite HY and H-1 MAS NMR has been used to determine which Bronsted acid sites are accessible to oxygen. Large inc reases in the H-1 spinning-sideband manifolds are observed at low temperatu res for the supercage protons that can directly interact with the oxygen mo lecules; a much smaller increase in sideband intensity is seen for the soda lite protons. Dramatic reductions of the T-1's of all the protons, of appro ximately 2 orders of magnitude, are observed at -150 degrees C, in comparis on to T-1 measurements made in air at room temperature, or at - 150 degrees C for samples with adsorbed N-2. The T-1's of the supercage protons are, h owever, shorter than those measured for the sodalite protons. Both the decr ease in the T-1's, and the large sideband manifolds, are due to the dipolar coupling interactions with the unpaired electrons present on the O-2 molec ules. Second moment (M-2) analysis is used to quantify the increase in the width of the H-1 spinning sideband manifolds, in samples of zeolites HY and HZSM-5. Comparison between samples run in air and with fixed loading level s of oxygen allows changes in loading level with temperature to be detected and oxygen loading levels to be estimated. The contribution to Mt of the p rotons due to bulk magnetic susceptibility effects and from oxygen molecule s that do not directly interact with the protons was estimated from the val ues of M-2 extracted from the Si-29 MAS NMR of the same samples. The contri bution to M-2 from the direct interaction of the protons with the oxygen mo lecules could then be estimated.