(629g) Kinetic Modeling of Ethanol Oligomerization over Calcium Hydroxyapatite

Ethanol is currently the most common liquid fuel derived from biomass in the world, used primarily as a blendstock in gasoline-powered vehicles. In the next 20 years, demand is anticipated to grow in heavier, C_8-22 distillate-range fuels such as diesel and jet while that of gasoline is projected to decrease,¹ motivating research efforts into the conversion ethanol to heavier fuels.² Coupling of ethanol via the Guerbet pathway has been examined mainly as a means to produce 1-butanol, which has improved gasoline fuel properties compared to ethanol.^3-4 Ethanol coupling beyond 1-butanol is possible, resulting in both linear and branched C₆₊ alcohols.^5-6 The potential of using alcohol coupling to produce distillate-range alcohols from ethanol is largely unknown since coupling beyond 1-butanol is seldom-studied and complicated by several complexities which arise at elevated conversions (>30%). Here we discuss the use of kinetic modeling to understand how inhibition by byproduct water and differences in alcohol reactivity affect conversion rates, alcohol distributions, and selectivities to undesired side-reactions (e.g. dehydration) in ethanol oligomerization over calcium hydroxyapatite.

A kinetic model has been developed within a MATLAB framework which allows for the modeling of over 70 species which may participate in over 150 reactions. Rate expressions were developed for alcohol dehydration to mono-alkenes, Guerbet coupling to higher alcohols, and Lebedev condensation to dienes based on prior literature and our own findings. Reactions with various feed mixtures allow for the fitting of kinetic parameters include rate constants and adsorption constants. Our findings show that water inhibition can fully account for the declining rates observed with increasing conversion. Additionally, stronger inhibiting effects for desirable base-catalyzed reactions versus undesirable acid-catalyzed reactions are responsible for the decrease in alcohol selectivity observed with increasing conversion. We also discuss how the observed alcohol distributions reflect both the nucleophilicity and electrophilicity of the species involved in condensation. Lastly, we show how these results can be used to predict catalytic performance at elevated conversions and the implications of various parameters on the product distributions observed.

References

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