Ethanol catalytic condensation over Mg–Al mixed oxides derived from hydrotalcites

Different Mg–Al mixed oxides derived from hydrotalcites were prepared and tested for ethanol catalytic condensation. Different procedures for preparing the catalysts lead to different distributions of acid and basic sites, and therefore to different reactivity. The studied catalysts are active for both dehydration reaction yielding ethylene (because of the presence of acid sites) and hydrogenation reactions yielding acetaldehyde (catalysed by medium-strength basic sites). Acetaldehyde is the key reactant for condensation reaction, yielding 2-butenal as primary condensation product. This reaction is catalysed by the strongest basic sites. This unsaturated aldehyde undergoes successive hydrogen transfer reactions and/or dehydrations yielding different C4 chemicals: 2-buten-1-ol, butanal, 1-butanol, 1,3-butadiene and 1-butene. Although the total conversion obtained with the different materials is rather similar, important differences are observed in the obtained selectivities. Those materials with higher concentration and strength of the basic sites are those more selective for C4 fractions, whereas the presence of acid sites promotes ethanol dehydration, decreasing the efficiency for condensation reactions.