The Role of Aldol Condensation and Hydrogen Transfer Reactions in the Guerbet Coupling Reaction of Ethanol over Acid-Base Catalysts

Guerbet coupling is a well-known reaction that couples two short chain alcohols into a longer chain alcohol. Guerbet coupling is believed to consist of many reactions including dehydrogenation, aldol condensation, dehydration, and hydrogenation. Recently, the calcium phosphate hydroxyapatite (HAP) has been shown to have a higher activity and selectivity towards butanol when compared to a more traditional Guerbet coupling catalyst such as MgO. In an attempt to study the overall Guerbet coupling reaction network in more detail, as well as to understand what makes HAP such an effective catalyst, the constituent reactions of the Guerbet coupling reaction network were investigated. Specifically, aldol condensation of acetaldehyde, dehydrogenation of benzyl alcohol, and hydrogenation of ethene and acetone using both H2 and ethanol were studied. The aldol condensation reaction was tested over anatase titania (TiO2), hydroxyapatite (HAP), and magnesia (MgO) in an attempt to determine the mechanism of aldol condensation over these catalysts. Conclusions based on reaction kinetics and isotope studies indicated enolate formation was fast while both adsorption of acetaldehyde and product desorption were kinetically relevant. The role that hydrogen plays in the coupling network was also studied in detail over HAP and MgO. Guerbet coupling in the presence of D2 revealed that deuterium was incorporated into the product butanol formed over MgO but not over HAP. Reactions involving H2 with ethene and acetone showed that H2 was unable to hydrogenate C=C or C=O bonds. The main route for hydrogenation reactions instead occurs through a Meerwein-Ponndorf-Verley-like (MPV-like) mechanism. The lack of C=C bond hydrogenation with either H2 or ethanol indicates that crotonaldehyde hydrogenation proceeds through the MPV-like hydrogenation to crotyl alcohol, followed by rapid double bond isomerization and keto/enol tautomerization to butanal. Butanal will then be hydrogenated by an MPV-like reaction with ethanol. In this way, the acetaldehyde that was consumed during aldol condensation is regenerated in the hydrogenation steps. The initial alcohol dehydrogenation reaction that must occur in the Guerbet reaction network was probed by measuring benzyl alcohol dehydrogenation. This reaction occurred over both MgO and HAP, but required much higher temperatures when compared to aldol condensation and hydrogenation. The efficiency with which the rapid aldol condensation/crotonaldehyde hydrogenation cycle takes place seems to control the overall activity and selectivity of the Guerbet coupling of ethanol.