1H MAS and 1H/27Al 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 increases in the H-1 spinning-sideband manifolds are observed at low temperatures for the supercage protons that can directly interact with the oxygen molecules; a much smaller increase in sideband intensity is seen for the sodalite protons. Dramatic reductions of the T-1's of all the protons, of approximately 2 orders of magnitude, are observed at -150 degrees C, in comparison 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, however, shorter than those measured for the sodalite protons. Both the decrease 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 molecules. 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 levels 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 protons due to bulk magnetic susceptibility effects and from oxygen molecules that do not directly interact with the protons was estimated from the values of M-2 extracted from the Si-29 MAS NMR of the same samples. The contribution to M-2 from the direct interaction of the protons with the oxygen molecules could then be estimated.