Hydrodeoxygenation of Biomass-Derived Liquids over Transition-Metal-Sulfide Catalysts
Bio-oil production through flash pyrolysis of biomass has been identified as one of the most feasible routes for the production of renewable fuels, as these biomass-derived liquids possess high oxygen content, and therefore, low stability over time as well as a low heating value. Upgrading is needed to decrease the oxygen content, and thus render a product resembling crude oil. One of the general routes for bio-oil upgrading is hydrodeoxygenation (HDO), a high-pressure operation in which hydrogen is used to exclude the oxygen functionalities from the bio-oil, giving a high-grade oil product equivalent to crude oil. Catalysts for the reaction are traditionally hydrodesulfurization (HDS) catalysts, such as CoMo sulfides. However, catalyst lifetimes of more than 200 h have not been achieved with any of the current catalysts due to carbon deposition. So far, two main routes have been proposed, considering many studies on the laboratory-scale and others from industry: catalytic hydrotreatment (HDT), mainly by hydrodeoxygenation (HDO), and catalytic cracking, technologies that are already present in today's refinery configurations. HDO has been performed at high hydrogen pressure, using catalysts based on those typically applied in conventional hydrotreating. This chapter focuses on the upgrading possibilities of renewable feedstocks, obtained from biomass fast pyrolysis or nonedible oils, towards the production of second-generation biofuels. It also includes some recent studies concerning the coprocessing of bio-oils together with petroleum. In fact, although all these raw materials have the potential to be directly converted into transportation fuels in dedicated units, it seems more attractive to upgrade them in combination with conventional oil feeds.