Characterization of gamma-alumina and berated alumina catalysts

The acidity and surface structure of pure gamma-alumina and a berated alumi na (AB), containing a low amount of boria (1.9 wt.%), are investigated by a variety of techniques. The coordination states of B, and the distribution of hydroxyls on the surface, are studied by B-11 MAS NMR and UV-Vis-NIR spe ctroscopies, respectively. Partially hydrated AB has trigonal boron (BO3) o n the surface, as found in B2O3 (via B-11 quadrupolar parameters), and a sm all part (ca. 50 mu mol/g of AB, or 10% mole) of the boron nuclei exhibit B -11 resonances narrowed by exchange with water (BO4). Boria introduction cr eates new types of surface hydroxyl groups, giving rise to a B-OHNIR band a t 1382 nm. However, quantification via NIR bands reveals no significant cha nge in the total number of hydroxyl groups. The structural types of berate surface species on dried ABI consistent with this finding, are presented. P yrrole adsorption shows that AB contains no basic sites. The acidities (qua ntity, strength) of dehydrated samples are evaluated by IR spectroscopy and static volumetric adsorption using pyridine and ammonia as basic probes, r espectively. In-situ C-13 NMR is also used to study the acid/base strength by monitoring the low energy model reactions (at 25 degrees C) of 1-butene double-bond isomerization (DBI) and isobutene dimerization. All three metho ds concur that dried AB has greater acidity than gamma-alumina due to Lewis acid sites with greater strength. Volumetry and in-situ NMR find that only ca. 6% of the boron nuclei (34 mu mol/g) on dried AB furnish (Lewis acid) chemisorption sites for butene. This corresponds closely to the number of s ites in AB adsorbing water (forming BO4 by B-11 NMR) and to those chemisorb ing pyridine strongly (desorption above 400 degrees C). W-Vis-NIR spectra r eveal that the Lewis sites of AB perturb the double bond of chemisorbed ole fins (partial hydride transfer) and the surface hydroxyl groups physisorb o lefins by H-bonding. (C) 1999 Elsevier Science B.V. All rights reserved.