(665c) Design of Bimetallic Catalysts for the Acetone-Butanol-Ethanol Condensation

Current methods to produce diesel replacements from biomass include the transesterification of plant-derived triglycerides [1] with methanol to form fatty acid methyl esters (FAME) and the decarboxylation [2] of triglycerides to form long-chain alkenes and alkanes. Still, large-scale use of these processes is problematic, because of the low yields per acre in the case of rapeseed and soybean, and the sustainability challenges that palm oil plantations face [3].

Avoiding the use of oleiferous seeds as feedstock for a diesel replacement production process is therefore desirable. A promising process for biodiesel production is the catalytic condensation of fermentation-derived acetone, butanol and ethanol mixtures, to form C₇-C₁₅ ketones [4]. Previous publications have reported high yields for this reaction, but with long reaction times and poor recyclability [4]. Improvements were achieved using hydrotalcite-supported Pd and Cu catalysts [5]. However, side reactions over the Pd and Cu catalysts, namely the decarbonylation of aldehydes and the Tishchenko reaction to form esters, respectively, necessitate the development of a catalyst that combines high rates with high selectivity.

In this work, we propose alloying Pd with Cu to reduce decarbonylation over Pd-based catalysts. Using PdCu catalysts supported on Mg-Al metal oxides for the ABE reaction results in modest selectivity improvements compared to equivalent Pd-based catalysts. The reason for that is the formation of a solid Cu-Mg-Al oxide solution, which results in poor reducibility of the Cu. More significant improvements of the selectivity require the use of a support that does not form solid solutions with Cu. For this, a size or chemical mismatch of the support with Cu²⁺ is required. Based on this, we demonstrate that using hydroxyapatite, titanium dioxide or carbon as support results in the complete reduction of Cu and a commensurate increase in selectivity.

Particle size and surface segregation of Cu can also influence selectivity. Larger Pd and PdCu nanoclusters can be shown to catalyze the dehydrogenation of the alcohols preferentially over the decarbonylation of the aldehydes. Also, increasing the surface segregation of Cu, as measured by EXAFS fitting of the Pd and Cu edges of the catalyst, results in further reduction of decarbonylation reactions.

Measurement of the intrinsic rates of the hydrogenation and decarbonylation reactions reveals that they proceed on a pair of adjacent Pd or Pd and Cu sites. This suggests that the reason for the increased selectivity demonstrated by PdCu catalysts is due to the disruption of the Pd-Pd ensembles necessary for decarbonylation. In order to better understand this catalyst, it is important to distinguish between the geometric and electronic influences. This is achieved by comparing the activation energies of the reactions over Pd and PdCu catalysts.