Hydrodeoxygenation of Lignocellulosic Fast Pyrolysis Bio-Oil: Characterization of the Products and Effect of the Catalyst Loading Ratio

The hydrodeoxygenation (HDO) of bio-oil at 350 °C and 200 bar in a batch reactor over a Ru/C catalyst has been studied experimentally with the aim of contributing to the understanding of the HDO reaction and its effect on the physicochemical properties of the organic liquid fraction obtained. Moreover, the effect of the catalyst loading ratio used in the HDO treatment and a previous stabilization stage carried out at 250 °C have also been assessed. Under the studied operational conditions, reactions of decarboxylation, HDO, polymerization, decarbonylation, methanation, demethylation, and pyrolytic lignin depolymerization took place during the HDO process. In these experiments, O was removed from the bio-oil mainly in the form of CO₂ (15–26 g of CO₂·100 g–¹ of dry bio-oil) and also as H₂O (1.8–5.8 g of H₂O·100 g–¹ of dry bio-oil). The consumption of H₂ was between 0.75 and 1.0 g·100 g–¹ of dry bio-oil. A comparison of the physicochemical properties of the raw bio-oil and the HDO organic phases shows that the major effects of HDO are a reduction in the O content from 34 to 13 wt %, an increase in the higher heating value (dry basis) from 24.3 to 35.5 MJ·kg–¹, lower polarity of the organic compounds determined by the significant increase in the hexane solubility, lower corrosiveness evidenced by the smaller total acid number and acid concentrations, and a marked change in the gas chromatography–mass spectrometry detectable compounds, increasing the presence of monophenols and cyclic ketones and decreasing the presence of levoglucosan, methoxyphenols, and furans. Electrospray ionization(±)–FTMS analyses of the raw bio-oil and the HDO liquid fractions show a widespread reduction of the O/C molar ratio of the compounds, an efficient deoxygenation and depolymerization of pyrolytic lignin, and a nondesirable increase in the range of molecular weights of the organic molecules after the HDO treatment.