Removal of oxygen from bio-oil through the addition of hydrogen is called hydrodeoxygenation (or hydrotreating and hydrogenation). Oxygen is removed in the form of water (H2O). This process takes place at temperatures of 300-400°C and pressures of 80-300 bars. The catalysts used for the dehydrogenation are Co-Mo, Ni-Mo, and their sulfides or oxides or loaded on Al2O3 (Zhang et al. 2006, 2007). This process results in naphtha­like product, which can be used in traditional petroleum refining process. The main advantage of hydrodeoxygenation is that removal of oxygen in the form of H2O retains biomass carbon for formation of hydrocarbon and improves energy content of the product. However, since hydrogen is expensive, high consumption of H2 makes this process economically less attractive. High carbon content (C/H ratio) in bio-oil and resulting products also lead to severe coking of the catalysts. Overall, hydrodeoxygenation reaction can be represented in the following equation with respect to carbon of the bio-oil (Mortensen et al. 2011).

CH14O056 + 0.7 H2^1 CH2 + 0.4 H2O (1)

Where CH1.4O0.56 and CH2 represent bio-oil and hydrocarbon product, respectively.