Hydrodeoxygenation of lignocellulose-related compounds with supported noble-metal catalysts

Lignocellulose is a renewable feedstock that can potentially be processed in biorefineries into transportation biofuels. This work comprises two case studies representing the downstream valorization of biorefinery streams via hydrodeoxygenation (HDO). In HDO, the organic feedstock reacts with gaseous H2 in the presence of a catalyst to remove chemically bound oxygen, yielding deoxygenated products for biofuel production. The goals of the case studies were to investigate the effects of different noble-metal catalysts and of the reaction conditions on the HDO of representative compounds. The first case study looked into the HDO of a potential biorefinery platform stream, Levulinic acid (LA) dimers. The experiments were performed in batch and solvent free. Firstly, model compound γ-nonalactone (GNL) was tested in HDO with different noble-metal (Ru, Rh, Pd, and Pt) catalysts supported on ZrO2. This study revealed that Ru/ZrO2 was the most effective of the tested catalysts at hydrocarbon production, although with the drawback of its tendency to crack carbon chains. The GNL tests also showed that hydrocarbons were promoted at higher temperatures. Therefore, the HDO of LA dimers was studied with Ru/ZrO2. The dimers had a strong tendency to react thermally, although it was clear that the catalyst promoted hydrogenation. Unfortunately, the thermal production of CO2 hindered the feeding of sufficient H2 into the batch reactor. Higher reaction temperatures resulted in more deoxygenation, but also in more CO2 formation. The hydrocarbon yields from LA dimers were low, and included saturated, unsaturated, cyclic, and aromatic hydrocarbons. The second case study represented the upgrading of pyrolysis or liquefaction biocrudes and focused on the HDO of 4-prophylphenol to produce propylbenzene. The experiments were performed in batch and using n-C14H30 as a solvent. The tested catalysts were Pt/Nb2O5, Pt/TiO2, and Pt/ZrO2. The results indicated that the catalysts with the reducible supports (Nb2O5 and TiO2) were more selective to deoxygenated products than with the non-reducible support (ZrO2), and that the Nb2O5 catalyst was more selective to propylbenzene than the TiO2 catalyst. Thermodynamic calculations supported by experiments suggested that the effects of temperature and pressure on aromatic selectivity were strongly subject to limitations imposed by the thermodynamic equilibrium. Furthermore, the H2 pressure determines the temperature at which the equilibrium transitions from favoring a cycloalkane product (low temperature) to favoring an aromatic product (high temperature). For future research on LA dimer HDO, it is recommended to ensure sufficient H2 availability and to target catalysts that are active at low temperatures (≤250 °C) to avoid the thermal reactions. Future research on phenolic compounds with niobia-supported catalysts could be promising; the thermodynamic equilibria should be considered.