Mechanism, ab initio calculations and microkinetics of hydrogenation, hydrodeoxygenation, double bond migration and cis-trans isomerisation during hydrotreatment of C6 secondary alcohol species and ketones

- Experimental and in-silico study of carbonyl hydrogenation and hydroxyl group HDO.
- Hexanol deoxygenation by dehydration yielded all possible cis/trans hexene isomers.
- Measured equilibria between hexene isomers were in accordance with DFT calculations.
- A microkinetic model accounts for adsorption, desorption, surface and bulk kinetics.
- DFT revealed different active sites for hydrogenation and deoxygenation.

Hydrotreatment of secondary hexanone and hexanol, and primary hexene species was investigated over the sulphide-form NiMo/γ-Al2O3 heterogeneous catalyst within the process temperature range 200-275 °C. The mechanistic microkinetic model for a three-phase slurry reactor was developed, comprising the mass transfer flux from the dispersed gaseous H2 bubbles to liquid solvent bulk, the external convective resistance at catalytic surface interface, material lattice adsorption and desorption, and intrinsic conversion kinetics (homogeneous and catalysed). It reported the reaction rate constants and activation energies for ketones and alcohols. Intermediate alkene isomers were identified and quantified, demonstrating the same reactive selectivity regardless of the cascade reactant compound. Furthermore, the C6 olefin isomerisation studies under pressurised hydrogen and nitrogen (up to 9.5 times slower) atmospheres indicated a similar equilibrium distribution of the concentrations of 1-hexene, cis-2-hexene, trans-2-hexene, cis-3-hexene and trans-3-hexene. The position of the oxygen-containing (heteroatom) functional group on an aliphatic hydrocarbon chain exhibited only a minor kinetic effect on parallel and serial hydrogenation or de-hydroxylation steps. Quantum chemical (QC) calculations utilising density functional theory (DFT) computational framework were performed to elucidate the mechanism of hydrodeoxygenation (HDO). The competing main and side pathways were considered in calculating transition state barriers. Simulations showed a very good agreement with measured experimental data. Deoxygenation valorisation routes were examined as they are vital upon converting ligno-cellulosic biomass resources and for the production of the bio-based platform oxygenates in bio-refineries. Hexose(s), for example, are (poly)alcohols, the monomer building blocks of cellulose, which is besides lignin and hemicellulose the principal wood, grass and straw constituent.

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