Solid-State 1H NMR Studies of Aluminum Oxide Hydroxides and Hydroxides

1 H CRAMPS (Combined Rotation and Multiple Pulse Spectroscopy) investigations have been used to identify the protons of Al−OH groups associated with low-surface-area aluminum oxide hydroxide and hydroxide materials. The 1 H CRAMPS spectrum of boehmite (γ-AlOOH) shows a single peak at 8.8 ppm due to protons associated with Al 2 OH groups containing six-coordinate aluminum and three-coordinate oxygen, whereas the 1 H CRAMPS spectrum of diaspore (α-AlOOH) depicts a sharp peak at 10.8 ppm that is assigned to the protons associated with Al 3 OH groups containing six-coordinate aluminum and four-coordinate oxygen. The 1 H CRAMPS spectra of aluminum hydroxides show three 1 H NMR peaks, in approximate 3:2:1 ratios, that are attributed to the protons associated with three different types of Al 2 OH groups. In gibbsite (γ-Al(OH) 3 ), these 1 H NMR peaks appear at 5.8, 4.3, and 2.9 ppm and are assigned to protons (a) H(3), H(5), and H(6), (b) H(1) and H(4), and (c) H(2), respectively, on the basis of X-ray diffraction and neutron diffraction data. In bayerite (α-Al(OH) 3 ), these resonances are at 5.0, 4.3, and 2.9 ppm and are assigned to protons (a) H(1), H(2), and H(3), (b) H(4) and H(6), and (c) H(5), respectively. Dipolar dephasing experiments indicate that the protons associated with Al 2 OH groups containing three-coordinate oxygens are involved in stronger 1 H− 1 H dipolar interactions than the protons associated with the Al 3 OH groups containing four-coordinate oxygens.