(560ai) Optimization of the Upgrading of Ethanol to n-Butanol Using in-Situ IR Spectroscopy in the Guerbet Reaction Network

As the crude oil reserves are finite, the intensified use of renewable sources for fuels is necessary. The most common renewable fuels used today are ethanol from fermentation processes and fatty acid methyl esters (biodiesel). In comparison with ethanol, higher alcohols are advantageous fuels because of their higher energy density. A suitable reaction for the upgrading of ethanol to higher alcohols is the Guerbet-reaction which is formally a condensation of two alcohols under elimination of water. [1] Either pure ethanol or a blend of ethanol and methanol are preferably used as substrates, resulting in either n-butanol or iso-butanol as desirable Guerbet-alcohols. In homogeneously catalyzed systems, the reaction proceeds over three steps (see Scheme 1): (1) dehydrogenation of two alcohol molecules to aldehydes, (2) aldol condensation, (3) hydrogenation of the α,β-unsaturated aldehyde to the higher alcohol. The redox steps are catalyzed by a transition metal catalyst, while the aldol condensation is catalyzed by a strong base. The most important side-reaction is the over-oxidation of acetaldehyde. The resulting ethyl acetate or acetic acid neutralizes the base and thereby inhibits the aldol condensation. [2]

Scheme 1: Reaction network of the Guerbet condensation from ethanol to n-butanol (upper row) including the side reaction to acetate.

In this work, we present an in-situ IR-spectroscopic study on the Guerbet-reaction of ethanol, catalyzed by the redox catalyst Ru‑MACHO^® and KOtBu. The aim is to prevent the over‑oxidation of acetaldehyde in the reaction system to reach higher yield and selectivity of n‑butanol. Ru‑MACHO^® was first employed for the hydrogenation of esters to alcohols and is proven to be a suitable catalyst for the acceptorless dehydrogenation of alcohols for hydrogen production. The variable parameters of the reaction are the catalyst loading (<1 mol-%), the base concentration (1-10 mol-%) and the temperature (120-150 °C). Furthermore, this work focuses on the effect of hydrogen pressure on the reaction, since the equilibrium between hydrogenated and dehydrogenated catalyst has a great impact on the reaction rates of the two redox steps of the Guerbet reaction (see Scheme 2).

Scheme 2: Hydrogenation/dehydrogenation equilibrium of the Ru-MACHO-catalyst.

The use of in-situ IR-spectroscopy enables to analyze the kinetic behavior of the reaction by resolving the time-dependent concentrations of the present species. In particular, the high time resolution of the method makes details visible that are often overlooked, e.g. accumulating intermediates or non-ideal kinetic behavior. [3]

[1] H. Aitchison, R. L. Wingad, D. F. Wass, ACS Catal. 6 (2016) 7125-7132.

[2] R. Mazzoni, C. Cesari, V. Zanotti, C. Lucarelli, T. Tabanelli, F. Puzzo, F. Passarini, E. Neri, G. Marani, R. Prati, F. Vigano, A. Conservano, F. Cavani, ACS Sustainable Chem. Eng. 7 (2019) 224-237.

[3] A. Ohligschläger, C. Gertig, D. Coenen, S. Brosch, D. Firaha, K. Leonhard, M. A. Liauw, Chem. Eng. J. 368 (2019) 649-658.