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, dehydration, and hydrogenation, is one pathway for this process. The catalyst hydroxyapatite (HAP, Ca10(PO4)6(OH)2) exposes both acid and base sites, and therefore, HAP gives better selectivities and shows improved stabilities for the reaction of ethanol Guerbet than basic metal oxides or ion-exchanged zeolites.1,2 Cascades of primary and secondary Guerbet reactions give broad distributions of heavier products; however, most prior work has been focused on the formation of short-chain alcohol or secondary products. Little is known about how the acid-base properties of catalysts affect the distribution of carbon numbers (Cn) or branching of the products. We need to understand what determines the relative rates of parallel growth pathways in order to create catalysts that will give heavier and more linear products. Here, we use steady-state rate measurements, taken as a function of reactant concentrations (1-10 kPa EtOH, 0-90 kPa H2) and temperature (573 – 673K), to study the reaction network for ethanol Guerbet on HAP materials (Ca/P ratio from 1.55 to 1.7) that produces C4-C14 alcohols via cascades of coupling steps. The ratios of branched and linear alcohols in the product distribution reflect ratios of rates for deprotonating acetaldehyde or C2+ aldehyde intermediates. The step-growth polymerization model is used to estimate the carbon number and molecular weight distribution of the products at high conversion, and probability of chain growth for any given Cn depends on the exact Ca/P of the material. These findings will be useful for future catalyst design for ethanol upgrading to advanced chemicals via Guerbet reaction.
(1) Tsuchida, T.; Kubo, J.; Yoshioka, T.; Sakuma, S.; Takeguchi, T.; Ueda, W. J. Catal. 2008, 259, 183.
(2) Tsuchida, T.; Sakuma, S.; Takeguchi, T.; Ueda, W. Ind. Eng. Chem. Res. 2006, 45, 8634
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