Model reactions as probe of the acid–base properties of aluminas: Nature and strength of active sites. Correlation with physicochemical characterization Original Research Article

Acetonylacetone conversion at 250 and 350 °C was carried out over various transition aluminas (η- to θ-Al2O3) in order to estimate simultaneously their acid–base properties. The formation of 2,5-dimethylfuran (DMF) was associated to the presence of acid site, and the formation of 2-methyl-3-cyclopenten-1-one (MCP) associated to basic site. First, from carbon dioxide adsorption measurement, hydroxyl basic group was found to be the active site for MCP formation and 2,6-dimethylpyridine (DMP) adsorption measurement shows that Brønsted acid site is active site for DMF formation. Secondly, from catalytic results, we show that θ-Al2O3 possesses a basic/acid ratio (MCP/DMF = 16) higher than the other aluminas (MCP/DMF < 10), and from comparison with cyclopentanol/cyclohexanone mixture transformation (acid alcohol dehydration and acid–base hydrogen transfer between alcohol and ketone), we also show that acetonylacetone cyclization are a more difficult reaction. Finally, the combination of model reactions and CO2 and DMP adsorption allows us to propose a description of the nature and the strength of the various aluminas surface.