Innovative catalytic systems for the Guerbet reaction driven by a molecular Ru-NHC complex

The Ru-NHC catalyst /NaOEt system is known to catalyze the Guerbet reaction, enabling the synthesis of longer-chain alcohols from ethanol. However, the process is complicated by the presence of side reactions and a partial loss of hydrogen from the catalytic cycle. This study aims to optimize the reaction's performance by evaluating different co-catalysts and additives. Alternative basic co-catalysts, such as Cs2CO3, K2CO3, Na2CO3, K3PO4, were investigated using an autoclave vessel. Among these, the Ru-NHC/Cs2CO3 system showed the most promising results, prompting a preliminary kinetic investigation to determine the optimal reaction conditions. The effect of additives known to improve the reaction, such as benzoquinone, which reduces hydrogen loss and increases yield, was also explored. K3PO4, which was not entirely suitable as a co-catalyst, was also tested as an additive in combination with the Ru-NHC/NaOEt system on a Schlenk scale. Subsequently, the effects of benzoquinone on the two most effective systems, Ru-NHC/NaOEt and Ru-NHC/Cs2CO3, were studied using a larger reaction volume in an autoclave. To further mitigate carbon loss and side reactions, the effects of a hydrogen atmosphere were examined. After a preliminary study aimed at reducing catalyst loading and increasing the reaction volume, the use of molecular sieves to absorb water was also investigated to reduce the side reactions and, consequently, reduce the carbon-loss. Finally, in order to assess the catalyst's potential for recovery and recycling, its resting state was examined using mass spectrometry, ¹H-NMR, and IR spectroscopy.