Dehydration Studies of a High-Surface-Area Alumina (Pseudo-boehmite) Using Solid-State1H and27Al NMR

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Abstract

1H NMR based on the CRAMPS technique has been used to identify and monitor the protons of surface Al−OH groups and “physisorbed” water associated with a high-surface-area (230 m2/g) pseudo-boehmite material following dehydration in the 110−1100 °C temperature range. Three distinguishable 1H CRAMPS peaks were identified: a broad peak at 4.0 ± 0.2 ppm attributed to the protons of “physisorbed” water and two peaks at 8.2 ± 0.3 and 2.3 ± 0.2 ppm associated with the protons of structural Al−OH groups. The 1H CRAMPS results are interpreted in relationship to two important regions of the experimental dehydration weight-loss profile for this material, a lower temperature region (110−300 °C), in which desorption of “physisorbed” water occurs, and an intermediate temperature region (350−550 °C), where condensation of adjacent Al−OH groups occurs. The combination of heating between 110 and 300 °C and room temperature evacuation were found to eliminate the “physisorbed” water peak, permitting the observation of the two resonances associated with the structural Al−OH sites. Dipolar dephasing experiments indicate that the 8.2 ppm peak is associated with highly coupled, “clustered” Al2OH groups, while the 3.0 ppm resonance is associated with terminal, “isolated” AlOH groups. 1H CRAMPS evidence shows that upon heat treatment the Al2OH groups condense at lower temperatures (350 °C) than the AlOH groups (550 °C). Three mechanisms are proposed for the condensation of the proton-containing surface Al−OH groups that occur in this material, based on crystalline boehmite as a structural model. In addition to 1H CRAMPS studies, 27Al MAS NMR spectra at 14 T of samples dehydrated from 100 to 1100 °C provide structural information about the aluminums in the high-surface-area pseudo-boehmite. This material dehydrates by condensation of both Al2OH and AlOH groups to form distorted, hydrogen-bearing 4-, 5-, and 6-coordinate aluminum-containing intermediates in the 350−500 °C range. At 1100 °C, this hydrogen-bearing γ- or δ-alumina material is converted to a material consisting of primarily α-Al2O3.

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