Levulinic acid hydrodeoxygenation, decarboxylation and oligmerization over NiMo/Al2O3 catalyst to bio-based value-added chemicals: Modelling of mass transfer, thermodynamics and micro-kinetics

Abstract

Levulinic acid (LA) hydrotreatment at solvent-free conditions was studied in a slurry reactor over the sulphide form of NiMo/Al2O3 catalyst. The influence of process parameters on the H2 equilibrium solubility in the liquid (calculated by Soave–Redlich–Kwong, EOS), on the external and internal mass transfer limitations, adsorption and desorption rates and the rates of surface and homognenous reactions were determined experimentally and computationally by utilizing a newly-developed micro-kinetic model. Main catalytic hydrodeoxygenation (HDO) product γ-valerolactone (GVL) was formed on NiMoSx phase (surface sites concentration of 0.33μmolm−2 was determined) exclusively by LA hydrogenation to hydroxypentanoic acid, and its subsequent intramolecular esterification, while cyclisation of LA to angelica lactones and their saturation was negligible. Activation energies determined by the regression analysis for the rate-determining step of catalytic LA HDO (Ea 39kJmol−1) and competitive LA decarboxylation in bulk liquid (Ea 134kJmol−1) show, that the temperature increase from 225°C to 275°C accelerated the decarboxylation rates much faster than HDO, which reflected in the drop of selectivity towards GVL formation from 90 to 42%, while its yield was not significantly affected (2.5±3molL−1 within 60min at final temperature). LA oligomerization by aldol addition and subsequent dehydrative cyclisation is reported for the first time and its rate was independent of the catalyst loading or H2/N2 pressure. Internal mass transfer had no influence on the global reaction rate, while the external mass transfer was eliminated at high agitation rate.