Thermodynamics conditions for Guerbet ethanol reaction

Ethanol (EtOH) is now produced from fermentation of carbohydrates [1]. Guerbet reaction is a possible way to couple C-C bonds of aliphatic alcohols towards higher carbon skeleton molecules that are suitable for diesel, gasoline, aviation fuel and chemicals [2–4]. When vaporized EtOH is contacted to solids at temperatures near 673.15 K, a lot of different products are obtained containing olefins, paraffins, longer alcohols, diolefins, ketones, aldehydes, ethers, etc. Some H2 is produced and carbonaceous deposits are formed [5, 6]. On acidic catalyst, gas phase conversion of EtOH to aromatics hydrocarbons can be performed (zeolites) at 673-723K. This reaction involves the dehydration of EtOH to ethylene, and then ethylene is converted to unsaturated hydrocarbons and hydrogen [7]. The drawbacks of this strategy is that the mixture should be vaporized (heat-consuming) and that some hydrogen atoms abandon the carbon skeleton (loosing energy yield in drop-in fuels), however it could be used to obtain biobased aromatics hydrocarbons from EtOH platform. Very acid catalyst accelerates excessive dehydration, producing short hydrocarbons without OH functionality capable of undergo C-C coupling. For that reason basic or multifunctional catalysts are employed for EtOH conversion to higher hydrocarbons [6, 8, 9]. The most accepted mechanism for Guerbet reaction using basic catalyst is the aldol mechanism (Scheme 1) [3, 9]. In the first step (Scheme 1, R1), EtOH is dehydrogenated by a solid catalyst to produce acetaldehyde (AcCO). Then AcCO can undertake acid-base catalyzed aldol addition (Scheme 1, R2). H2O is then released from the molecule. Basic catalysts have the capacity to store hydrogen by heterolytic dissociation. In a desired Guerbet reaction, adsorbed hydrogen is re-inserted into carbon skeleton (1butenal to 1-butanol in Scheme 1, R4) to generate a higher alcohol that can re-start the catalytic cycle. C H3 OH Site C H3 O + +