(291f) How Basic Sites Strength Influences the Guerbet Reaction

n-Butanol is a widely used additive in every-day use products (such as paints, skin care products, ...) and an important chemical intermediate to more valuable compounds (e.g. maleic anhydride, methyl meta-acrylate, ...). Moreover, thanks to its physico-chemical properties, n-butanol is a likely alternative to gasoline and can be used in cars without any modification to the engine.

To date, n-butanol is mainly produced via oxo-synthesis, which is a petrochemical process. However, the need for processes to upgrade biomass has been the thrust behind a renewed interest in the ABE (Acetone Butanol Ethanol) fermentation. In fact, the latter is the only industrial green alternative currently available. Unfortunately, it suffers from low concentrations in the product stream and low productivity due to its fermentative nature. Consequently, the demand for an efficient green chemical route to n-butanol has reignited the interest on the Guerbet condensation of alcohols. This reaction can be conducted in both liquid and gas phase using heterogeneous catalysts, the most used being hydrotalcites, hydroxyapatites and basic oxides.

The reaction mechanism of this reaction is still subject of debate. The widely accepted pathway that involves a first dehydrogenation step followed by aldol condensation and hydrogenation has been recently discarded on MgO and on basic zeolites. In fact, in 2015 Cavani and coworkers proposed a mechanism comprising the direct condensation of two ethanol molecules via a carbanionic intermediate. This hypothesis was supported by in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) experiments and Density Functional Theory (DFT) calculations. To date, a fine-tuning of the acid-base catalyst properties is thought to be the key aspect to be optimized to improve n-butanol selectivity.

The work we undertook in this context aimed at understanding the influence of catalyst basicity on the reaction, with the purpose of verifying that the direct condensation is the dominant mechanism on other basic catalytic systems. Hence, a screening of the reaction performances on the alkaline-earth oxides MgO, CaO, and SrO was conducted. The catalyst performances were studied in a continuous flow quartz reactor. The three catalysts showed different product distributions and the hydroxylation degree affected the catalytic performances of SrO and CaO. The possible reaction pathways were investigated by means of DRIFTS and DFT calculations. The latter suggested a decrease in the relative stability of the carbanion upon decreasing the acidity of the metal cation, and an increasingly more facile aldolic pathway upon increasing the oxide basicity. In this work important insight on the possible reaction pathways are given and the basicity is directly related to the selectivity to n-butanol.