UC Berkeley

The condensation of mixtures of acetone, butanol and ethanol (ABE), derived from

fermentation processes has recently been reported as a viable pathway to diesel fuel

precursor molecules. This process is catalyzed by bifunctional catalysts that include a metal functionality to dehydrogenate the alcohols and a basic functionality to catalyze the aldol condensation between the aldehydes and the acetone. However, the currently used catalysts are poorly understood, in terms of the driving forces of their selectivity, reactivity and stability.

Based on screening reactions, we show that monometallic Pd and Cu catalysts supported on mixed metal oxides derived from hydrotalcites are the most active catalysts for this reaction. Kinetic analysis of these catalysts reveals that the dehydrogenation over the metal catalysts proceeds via a rate‐determining C‐H bond scission. On the other hand, the aldol condensation over the mixed oxide support takes place via an equilibrated enolate formation and its subsequent abstraction from the surface by an aldehyde. However, the decarbonylation of aldehydes over Pd and the formation of esters over Cu via the Tishchenko reaction limits the usability of monometallic catalysts.

Supporting Pd and Cu on hydrotalcite‐derived supports results in the elimination of the

Tishchenko side reaction and a reduction of the decarbonylation rates. Complete elimination of decarbonylation was not possible with these supports, due to the incorporation of Cu (II) ions in the Mg‐Al oxide structure. For this reason, TiO2 was used as a support; this resulted in complete alloying of Cu and Pd and, in turn, in significant selectivity improvements, combined with enhanced stability in the liquid phase.

The liquid phase stability is due to the absence of any transformation from the anatase to the rutile phase. Kinetic analysis of the C‐C bond formation reactions over TiO2 reveals anatase to be the active phase for these reactions over TiO2. In contrast to the C‐C bond formation reactions over the mixed Mg‐Al oxide, the aldol condensation reaction over TiO2 proceeds via a rate‐determining alpha‐hydrogen abstraction and enolate formation over an acid‐base pair on the TiO2 surface. Similar analysis for a ZrO2 catalyst shows that the enolate formation is also rate‐determining over ZrO2.

Apart from the ABE mixture, fuel precursors can also be derived by condensation of furfural and alcohols. These reactions proceed over transition‐metal‐free basic oxides via a rate-determining hydride transfer from a butoxide to an adsorbed furfural molecule, as shown by kinetic experiments in the gas and the liquid phase, as well as kinetic isotope effect experiments.