H MAS and 1H/27Al TRAPDOR NMR Studies of Oxygen−Zeolite Interactions at Low Temperatures: Probing Bronsted Acid Site Accessibility

Abstract

Sorption of oxygen in the pores of zeolite HY and 1H MAS NMR has been used to determine which Bronsted acid sites are accessible to oxygen. Large increases in the 1H 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 T1's of all the protons, of approximately 2 orders of magnitude, are observed at −150 °C, in comparison to T1 measurements made in air at room temperature, or at −150 °C for samples with adsorbed N2. The T1's of the supercage protons are, however, shorter than those measured for the sodalite protons. Both the decrease in the T1's, and the large sideband manifolds, are due to the dipolar coupling interactions with the unpaired electrons present on the O2 molecules. Second moment (M2) analysis is used to quantify the increase in the width of the 1H spinning sideband manifolds, in samples of zeolites H...