Structure and surface and catalytic properties of Mg-Al basic oxides

摘要

Mg-Al mixed oxides with Mg/Al molar ratios of 0.5-9.0 were obtained by thermal decomposition of precipitated hydrotalcite precursors. The effect of composition on structure and surface and catalytic properties was studied by combining several characterization methods with ethanol conversion reactions. The nature, density, and strength of surface basic sites depended on the Al content. On pure MgO, strong basic sites consisted predominantly of O2- anions. Calcined hydrotalcites contained surface sites of low (OH- groups), medium (Mg-O pairs), and strong (O2- anions) basicity. The relative abundance of low and medium strength basic sites increased with the Al content. The addition of small amounts of Al to MgO diminished drastically the density of surface basic sites because of a significant Al surface enrichment. Formation of surface amorphous AlOy structures in samples with low Al content (Mg/Al > 5) partially covered the Mg-O pairs and decreased the concentration of surface O2- anions. At higher Al contents (5 > Mg/Al > 1), the basic site density increased because the Al3+ cations within the MgO lattice created a defect in order to compensate the positive charge generated, and the adjacent oxygen anions became coordinatively unsaturated. In samples with Mg/Al < 1, segregation of bulk MgAl2O4 spinels occurred and caused the basic site density to diminish. The catalyst activity and selectivity of Mg-Al mixed oxides in ethanol conversion reactions depended on composition. The dehydrogenation of ethanol to acetaldehyde and the aldol condensation to n-butanol both involved the initial surface ethoxide formation on a Lewis acid-strong base pair. Pure MgO exhibited poor activity because the predominant presence of isolated O2- basic centers hindered formation of the ethoxide intermediate by ethanol dissociative adsorption. The incorporation of small amounts of Al3+ cations to MgO drastically increased the acetaldehyde formation rate because of the generation of new surface Lewis acid-strong base pair sites. Acetaldehyde condensation toward n-butanol is a bimolecular reaction between adjacent adsorbed acetaldehyde species that requires not only acid-strong base pair sites but also a high density of basic sites; these pathways were favored on Mg-Al samples with higher Al content (5 1 Mg/Al > 1). The dehydration of ethanol to ethylene, and the coupling and dehydration to diethyl ether increased with Al content, probably reflecting the density increase of both Al3+ - O2- pairs and low- and medium-strength basic sites. Pure Al2O3 displayed the highest dehydration activity. (C) 1998 Academic Press. [References: 41]