Pathways to Selectively Form Individual Aromatic Products from Ethanol

Ethanol derived from the fermentation of biomass can be catalytically converted into higher molecular weight hydrocarbons and oxygenates to produce advanced biofuels and lubricants. The Guerbet reaction, which involves dehydrogenation, aldol condensation, and hydrogenation, is one pathway for this process. Cascades of subsequent self- and cross-condensation reactions between ethanol and C_≥4­ alcohol derived intermediates provide a wide-range of possible higher carbon number (C_n) products. Successful production of these products requires high selectivities for dehydrogenation and condensation reactions to avoid the loss of carbon by forming undesirable products (e.g., C₁-, C₂-_­hydrocarbons) but also requires the ability to form specific molecules which can serve as fuels and platform chemicals.

Here, we show how bifunctional catalysts, comprised of amphoteric oxides (TiO₂, hydroxyapatites, or hydrotalcites) and Cu clusters promote these networks of reactions by measuring formation rates of more than 30 distinct products as functions of reactant concentrations, temperature (523 - 623 K), and the ratios of the numbers active sites for hydrogen transfer and those for condensation reactions. The selectivities with which the C₂-units of ethanol are incorporated into growing chains is limited by the selectivity of both catalytic functions, while the isomeric structure of the products is determined by the concentrations of the unsaturated oxygenates present on the oxide surfaces and differences between the deprotonation energies of C-atoms α to the carbonyl groups of these species. Reactions among product mixtures that predominantly contain alcohol and aldehyde reactants form broad distributions of fatty alcohols whose product selectivities follow predictions from step-growth polymerization models. These diverse mixtures, however, are only suitable as fuels.

Aldol condensations directly form unsaturated aldehydes, which exist at much higher concentrations than their saturated analogs in the absence of effective hydrogen transfer catalysts. Large differences in the deprotonation energies between α C-atoms for aldehydes and unsaturated aldehydes allow these product mixtures to be exploited to form a very small set of C₈ aromatic species (ortho- and para-tolualdehydes). The structure of the resulting products effectively halts further condensation reactions and provides a unique opportunity to make a desirable platform chemical within this complex network of reactions. The isomeric structure of the C₈ aromatics are sensitive to the presence or absence of steric hindrance about the acid-base site pairs that catalyze the reaction. Coupling hydroxyapatite, which lacks microporosity produces predominantly ortho-C₈ products with minor (< 10%) amounts of the para-isomers. These selectivities can be inverted, however, when the reactant stream include ethylene (derived from in situ dehydration of ethanol or added intentionally). The competing pathways to ortho- and para-tolualdehydes reflect the relative rates of aldol and Diels Alder additions between hexadienal and either acetaldehyde or ethylene co-reactants.

These findings provide evidence that the Guerbet reaction proceeds by dehydrogenation and aldol condensation steps and elucidates and the complex sets of surface reactions that dictate the formation rates of certain categories of products. This knowledge will help guide further research into reaction schemes to convert alcohols into larger oxygenates and hydrocarbons.