ABSTRACT
A combined use of DFT periodic calculations and spectroscopic studies (IR and solid-state NMR) shows that a gamma-alumina treated at 500 degrees C under high vacuum contains surface defects, which are very reactive toward H2 or CH4. The reaction of H2 on defect sites occurs at low temperature (ca. 25 degrees C) on two types of Al atoms of low coordination numbers, AlIII or AlIV, to give AlIV-H and AlV-H, respectively. The amount of defects as titrated by H2 at 25 and 150 degrees C is 0.043 and 0.069 site/nm2, respectively, in comparison with 4 OH/nm2). In contrast, CH4 reacts selectively at 100-150 degrees C on the most reactive AlIII sites to form the corresponding AlIV-CH3 (0.030 site/nm2). The difference of reactivity of H2 and CH4 is fully consistent with calculations (reaction and activation energy, DeltaE and DeltaE++).
ABSTRACT
The nature and structure of grafted organometallic complexes on gamma-alumina are studied from a combination of experimental data (mass balance analysis, IR, NMR) and density functional theory calculations. The chemisorptive interactions of two complexes are analyzed and compared. The reaction of [Zr(CH2tBu)4] with alumina dehydroxylated at 500 degrees C gives {[(AlsO)2Zr(CH2tBu)]+[(tBuCH2)(Als)]-}, a bisgrafted cationic complex as major surface species. The DFT calculations show that the reaction with surface hydroxyls is very exothermic and that alkyl transfer on Al atoms is favored. In contrast, [W(CtBu)(CH2tBu)3] reacts with an alumina treated under identical conditions to give selectively a monografted neutral surface complex, [(AlsO)W(CtBu)(CH2tBu)2]. This was inferred by the evolution of 1 equiv of tBuCH3 per grafted W and the presence of remaining hydroxyls. The calculations show that the reaction of [W(CtBu)(CH2tBu)3] with surface hydroxyls is in fact less exothermic and has a considerably higher activation barrier than the one of the Zr complex. Additionally, the transfer of an alkyl ligand onto an adjacent Al center is disfavored, and hence cationic species are not formed. Some ligands of this monoaluminoxy surface complex interact with remaining surface hydroxyls, which explains the complexity of the experimental NMR and IR data.
ABSTRACT
The knowledge of the properties of gamma-alumina is of great importance in order to control its surface for numerous applications. We investigate the kinetic behavior of the hydrated alumina (110) surface toward water desorption: the minimum energy path is presented for successive desorption steps starting from the completely hydrated surface toward the dehydrated one. It appears that water desorption is a non activated process. A kinetic model is proposed based on an extension of the Eyring theory. This model is a useful tool to understand the evolution of water coverage during the pretreatment of alumina. It is then used to model temperature programmed desorption experiments for various heating rates. The shape of the desorption curve is qualitatively reproduced, and the differences between theory and experiments are discussed.
