Characterization of γ-alumina and borated alumina catalysts

Abstract

The acidity and surface structure of pure γ-alumina and a borated alumina (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 11B MAS NMR and UV–Vis-NIR spectroscopies, respectively. Partially hydrated AB has trigonal boron (BO 3) on the surface, as found in B 2O 3 (via 11B quadrupolar parameters), and a small part (ca. 50 μmol/g of AB, or 10% mole) of the boron nuclei exhibit 11B resonances narrowed by exchange with water (BO 4). Boria introduction creates new types of surface hydroxyl groups, giving rise to a B–OHNIR band at 1382 nm. However, quantification via NIR bands reveals no significant change in the total number of hydroxyl groups. The structural types of borate surface species on dried AB, consistent with this finding, are presented. Pyrrole adsorption shows that AB contains no basic sites. The acidities (quantity, strength) of dehydrated samples are evaluated by IR spectroscopy and static volumetric adsorption using pyridine and ammonia as basic probes, respectively. In-situ 13C NMR is also used to study the acid/base strength by monitoring the low energy model reactions (at 25°C) of 1-butene double-bond isomerization (DBI) and isobutene dimerization. All three methods concur that dried AB has greater acidity than γ-alumina due to Lewis acid sites with greater strength. Volumetry and in-situ NMR find that only ca. 6% of the boron nuclei (34 μmol/g) on dried AB furnish (Lewis acid) chemisorption sites for butene. This corresponds closely to the number of sites in AB adsorbing water (forming BO 4 by 11B NMR) and to those chemisorbing pyridine strongly (desorption above 400°C). UV–Vis-NIR spectra reveal that the Lewis sites of AB perturb the double bond of chemisorbed olefins (partial hydride transfer) and the surface hydroxyl groups physisorb olefins by H-bonding.