Kinetic Modeling of Alcohol Oligomerization over Calcium Hydroxyapatite

Guerbet coupling to oligomerize ethanol to heavy alcohols over a hydroxyapatite (HAP) catalyst is explored through the development and application of a reaction kinetics model that describes coupling to higher alcohols, condensation to interrupted coupling products such as dienes and aromatics, and direct a. unimolecular dehydration to monoenes. The kinetic data collected to develop this model includes ethanol, 1-butanol, and water-ethanol feeds at 350 degrees C, 1 bar, and between 3 and 20% conversion over HAP. The model consists of nine kinetic parameters describing 647 reactions among 185 alcohols. Inhibition from byproduct water leads to decreases in reaction rates and alcohol selectivities as conversion increases, which the model can accurately predict at 60% conversion at over 8 times the highest contact time used in model development. The modified step growth kinetics underlying the model also accurately predict alcohol distributions at this conversion. Further extrapolations clarify how these kinetics (modified step growth wherein branched alcohols do not act nucleophilically) in addition to inhibition and side reactions affect the potential use of direct ethanol oligomerization to selectively produce heavy alcohols with HAP. Further deviations from step-growth kinetics can be modeled via manipulation of the apparent nucleophilicity and electrophilicity of ethanol relative to higher alcohols. Catalysts that could increase the nucleophilicity would lead to higher product linearities, while catalysts that decrease the electrophilicity would lead to heavier alcohol distributions at fixed conversion.