Mechanism, ab initio calculations and microkinetics of straight-chain alcohol, ether, ester, aldehyde and carboxylic acid hydrodeoxygenation over Ni-Mo catalyst

Abstract

Hydrotreatment of bio-based model compounds was investigated in a three-phase slurry reactor over pre-sulphided NiMo/γ-Al2O3 bifunctional catalyst. Experimental results were supported by comprehensive in silico studies. Mathematical microkinetic model was developed for mass transfer phenomena, adsorption, desorption and surface kinetics description, while quantum chemical calculations were performed for the proposed reaction mechanism validation. Every moiety followed the same chemical reduction sequence as its predecessor down the oxidation ladder, except of aldehyde. Experiments using hexanal as a model compound resulted in its extraordinary high concentrations in liquid phase and on the catalyst surface, which led to mutual aldol condensation. However, this reaction was not observed when aldehyde was formed as an intermediate from hexanoic acid or methyl hexanoate, due to its low concentration and high hydrogen availability which promoted further hydrogenation into hexanol. C12/C6 (aldol condensation/hydrogenation) product ratio increased with higher temperature, reflecting higher activation energy (EA) for hexanal condensation compared to negligible hydrogenation energy barrier, confirmed by density functional theory (DFT) calculations. Primary alcohols are more resistant to HDO compared to secondary counterpart (studied in previous work) over NiMo/γ-Al2O3, specifically; the activation energy of 1-hexanol deoxygenation to olefin was 43% higher compared to secondary hexanols according to microkinetic model, and 45% higher according to quantum mechanics calculations. Primary alcohols are also extensively dehydrated into ethers, which was never the case for secondary alcohols at tested reaction conditions, while aldehydes are much easier to hydrogenate than ketones, which cannot undergo CC coupling reactions.