Recent Advances in Hydroliquefaction of Biomass for Bio-oil Production Using In Situ Hydrogen Donors

With the rapid growth of energy demand and environmental concerns associated with traditional fossil fuels, renewable and sustainable energy sources have attracted intensive research attention in recent years. Biomass is considered as one of the most promising alternative energy resources due to its many advantages including abundance, renewability, carbon neutrality, and worldwide distribution. Liquefaction is an efficient thermochemical conversion route for the production of bioderived fuels and chemicals under mild reaction conditions. In addition, this process does not require energy-intensive drying as a preprocessing step of the wet biomass feedstocks. Nevertheless, subsequent hydrogenation and upgrading treatment of the bio-oil with high oxygen content are imperative for practical applications mainly for improving the calorific value of the bio-oil. The cost and safety issues of external hydrogen are main obstacles for the liquefaction-upgrading route, which can be somewhat offset by the use of in situ hydrogen. The research on in situ hydrogen generation and use for biomass liquefaction is nascent. Therefore, the latest research developments on hydroliquefaction of biomass feedstocks in the presence of various in situ hydrogen donors are reviewed in this article. Several commonly applied in situ hydrogen donors including formic acid, alcohols (isopropyl alcohol, methanol and ethanol), and zerovalent metals (zinc, aluminum, iron, etc.) are discussed in detail focusing mainly on their influence on the distribution of liquefied products and the mechanisms of hydrogen-donation/hydrogenation reactions. Moreover, future research directions toward the commercial applications of biomass liquefaction technology with in situ hydrogenation strategies are presented.