STRUCTURE AND SURFACE AND CATALYTIC PROPERTIES OF MG-AL BASIC OXIDES

Mg-Al mixed oxides with Mg/Al molar ratios of 0.5-9.0 were obtained by thermal decomposition of precipitated hydrotalcite precursors. The ef fect 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 ba sic sites depended on the Al content. On pure MgO, strong basic sites consisted predominantly of O2- anions. Calcined hydrotalcites containe d surface sites of low (OH- groups), medium (Mg-O pairs), and strong ( O2- anions) basicity. The relative abundance of low and medium strengt h basic sites increased with the Al content. The addition of small amo unts of Al to MgO diminished drastically the density of surface basic sites because of a significant Al surface enrichment. Formation of sur face amorphous AlOy structures in samples with low Al content (Mg/Al > 5) partially covered the Mg-O pairs and decreased the concentration o f surface O2- anions. At higher Al contents (5 > Mg/Al > 1), the basic site density increased because the Al3+ cations within the MgO lattic e created a defect in order to compensate the positive charge generate d, and the adjacent oxygen anions became coordinatively unsaturated. I n 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 acetaldehyd e and the aldol condensation to n-butanol both involved the initial su rface ethoxide formation on a Lewis acid-strong base pair. Pure MgO ex hibited poor activity because the predominant presence of isolated O2- basic centers hindered formation of the ethoxide intermediate by etha nol 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 rea ction between adjacent adsorbed acetaldehyde species that requires not only acid-strong base pair sites but also a high density of basic sit es; these pathways were favored on Mg-Al samples with higher Al conten t (5 1 Mg/Al > 1). The dehydration of ethanol to ethylene, and the cou pling and dehydration to diethyl ether increased with Al content, prob ably reflecting the density increase of both Al3+ - O2- pairs and low- and medium-strength basic sites. Pure Al2O3 displayed the highest deh ydration activity. (C) 1998 Academic Press.