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

1 H 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 m 2 /g) pseudo-boehmite material following dehydration in the 110−1100 °C temperature range. Three distinguishable 1 H 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 1 H 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” Al 2 OH groups, while the 3.0 ppm resonance is associated with terminal, “isolated” AlOH groups. 1 H CRAMPS evidence shows that upon heat treatment the Al 2 OH 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 1 H CRAMPS studies, 27 Al 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 Al 2 OH 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-Al 2 O 3 .