On the Chemistry of Ethanol on Basic Oxides: Revising Mechanisms and Intermediates in the Lebedev and Guerbet reactions

Abstract

A common way to convert ethanol into chemicals is by upgrading it over oxide catalysts with basic features; this method makes it possible to obtain important chemicals such as 1-butanol (Guerbet reaction) and 1,3-butadiene (Lebedev reaction). Despite their long history in chemistry, the details of the close inter-relationship of these reactions have yet to be discussed properly. Our present study focuses on reactivity tests, in situ diffuse reflectance infrared Fourier transform spectroscopy, MS analysis, and theoretical modeling. We used MgO as a reference catalyst with pure basic features to explore ethanol conversion from its very early stages. Based on the obtained results, we formulate a new mechanistic theory able to explain not only our results but also most of the scientific literature on Lebedev and Guerbet chemistry. This provides a rational description of the intermediates shared by the two reaction pathways as well as an innovative perspective on the catalyst requirements to direct the reaction pathway toward 1-butanol or butadiene.

Review of catalytic systems and thermodynamics for the Guerbet condensation reaction and challenges for biomass valorization

“…In a recent paper by Chieregato et al, 114 the direct condensation mechanism was further supported by kinetic experiments in combination with in situ infrared spectroscopy (DRIFTS), MS and DFT calculations. Based on their findings, the authors assume the formation of a carbanion on the MgO surface.…”

“…According to the authors, these alkenols will preferably dehydrate to 1,3-butadiene or rearrange into 3-buten-2-ol, but are not considered intermediates for n-butanol formation. Note that these direct mechanisms have been so far only suggested for the specific Guerbet condensation of ethanol to n-butanol on purely basic heterogeneous catalysts (MgO, 67,114 hydroxyapatite, 71 and RbLi-X…”

Ethanol to 1,3‐Butadiene Conversion by using ZrZn‐Containing MgO/SiO2 Systems Prepared by Co‐precipitation and Effect of Catalyst Acidity Modification

“…However, some reports have recently ruled out the aldol condensation as the main path, suggesting instead that crotyl alcohol is produced through the reaction between an activated form of ethanol and acetaldehyde. [18,19] Due to the specific features of the catalyst that are required for this cascade reaction, materials with multifunctional properties have been studied, especially MgO-SiO2 systems. [2,5] This is related to the fact that Mg-O pairs might act as Lewis acid-Brønsted basic sites and the silanol functionality as a Brønsted acid which are necessary for ethanol dehydrogenation, [20] acetaldehyde condensation, [21] crotonaldehyde reduction and the further crotyl alcohol dehydration to 1,3-BD.…”

Olefins from Biomass Intermediates: A Review

“…Several research projects suggest that aldol condensation of acetaldehyde is a key step in ethanol conversion into C4H6 [103]. On the other hand, latest works, including DRIFTS (Diffuse Reflection Infrared Spectroscopy) analyses and DFT (Density Functional Theory) calculations, propose that ethanol carbanionic intermediates are of key importance for the reaction mechanism [104]. Apart from bio ethanol, C4H6 can be formed from other biomass derived oxygenates such as butanols and butanediols, produced through biomass fermentation or gasification [105,106].…”