On the irrelevance of acetaldehyde self-aldolization during ethanol condensation at high temperatures over basic heterogeneous catalysts

The condensation of ethanol to butanol was investigated over a commercial hydroxyapatite catalyst in the 350-410°C temperature range. An analysis of thermodynamic and kinetic data, including the measure of the concentration of water and dihydrogen formed during the reaction, unambiguously revealed that the pathway involving acetaldehyde self-aldol condensation is irrelevant at such high temperatures for the present catalyst. Two reaction pathways are suggested. The main pathway (so-called “direct” route) refers to the condensation of two ethanol molecules with apparently no intermediate gaseous compounds. A minor “indirect” route involves the condensation of ethanol with acetaldehyde (formed from ethanol dehydrogenation) to form butenol, which is subsequently converted to butanol by hydrogen transfer from a sacrificial ethanol molecule. This minor route would be less selective, resulting in the formation of acetaldehyde and H2 as by-products. The alcohol condensation mechanism(s) taking place over basic oxides at high temperatures would therefore be fundamentally different from that taking place over bi-functional solids (containing both metallic and basic sites) at lower temperatures. In a more general context, this work underlines the benefits of considering thermodynamic data when assessing the relevance of potential reaction pathways.