(61a) Mechanistic Insights into the Guerbet Coupling of Ethanol into Butanol over Hydroxyapatite Catalysts
Drawbacks posed by corn-based bioethanol as a gasoline fuel additive have called attention to its catalytic transformation into a higher-value fuel or chemical, such as butanol. Butanol is an important industrial solvent and is a more desirable transportation fuel additive compared to ethanol because of its high volumetric energy density and lower solubility in water.
The catalytic transformation of ethanol into butanol occurs via the Guerbet coupling reaction – a well-known industrial route for higher alcohol synthesis. The most commonly-accepted reaction path involves the initial dehydrogenation of ethanol to form acetaldehyde, which then undergoes an aldol condensation reaction, followed by hydrogenation of the unsaturated condensation product to give butanol [1,2]. Since the Guerbet coupling of ethanol into butanol likely occurs on an acid/base site pair, bifunctional surfaces such as hydroxyapatite (HAP; Ca10(PO4)6(OH)2) have recently been reported as highly active and selective catalysts for this reaction . The specific acid/base sites on HAP materials that are active for ethanol coupling however have not been identified and therefore a detailed understanding of the reaction mechanism is lacking. The goals of this work are to explore the active sites on HAP surfaces for the conversion of ethanol into butanol and to evaluate key kinetic parameters that will help to elucidate the reaction mechanism at the active-site level.
Experiments involving the co-feeding of dihydrogen, acetaldehyde and water during ethanol conversion over HAP and MgO catalysts at atmospheric pressures in a gas-phase, fixed-bed reactor system were performed. Relationships between catalyst composition and performance in Guerbet coupling will be discussed in light of the characterization results and catalytic activity measurements.
 C. Carlini, M. Di Girolamo, A. Macinai, M. Marchionna, M. Noviello, A. M. Raspolli Galletti, and G. Sbrana, “Selective Synthesis of Isobutanol by Means of the Guerbet Reaction Part 2. Reaction of Methanol/Ethanol and Methanol/Ethanol/n-Proposal Mixtures over Copper Based/MeONa Catalytic Systems,” J. Mol. Catal. A-Chem. 200 (2003) 137-146.
 T. Tsuchida, S. Sakuma, T. Takeguchi, and W. Ueda, "Direct Synthesis of n-Butanol from Ethanol over Nonstoichiometric Hydroxyapatite," Ind. Eng. Chem. Res. 45 (2006) 8634-8642.