In the previous sections the significance that hydrogenolysis reactions have and will have in the future bio-refineries has been highlighted. In fact, they will be essential in fuel and chemical manufacturing. Hydrogenolysis involves chemical bond dissociation in an organic substrate and simultaneous addition of hydrogen. Therefore, hydrogen is required as reac­tant in all hydrogenolysis reactions. This is the reason why, most of the literature works re­ferred to hydrogenolysis report experiments conducted under molecular hydrogen (H2) atmosphere. Nevertheless, the use of molecular hydrogen has some important drawbacks:

i. Liquid phase processes are preferred to gas phase processes as they are more ener­gy efficient. However, H2 presents really low solubility on aqueous or organic solu­tions. As a consequence, when operating in liquid phase it is necessary to operate at elevated hydrogen pressures to obtain significant hydrogen concentrations near the catalysts. This, on one hand, notably increases the cost of design and building of the future plants, and on the other hand, increases the operating cost related to safety measures, as hydrogen is easily ignited and shows high diffusivity.

ii. Most of the nowadays available hydrogen gas is produced from fossil fuels by ener­gy intensive processes. Therefore, if sustainability is the goal it is a contradiction that the main reactant in most of the biorefinery processes is based on fossil resources.

iii. The low density and high diffusivity of hydrogen make problematic and expensive its transportation and storage. This problem is more relevant for small size biomass conversion facilities.

Hydrogen from non fossil origin will surely be a reality in the oncoming years, as reforming processes from various renewable compounds (like biomethane, glycerol or ethanol) and water splitting processes using solar light are being intensively developed. Nonetheless, the problems of transportation, storage and low solubility in liquid solutions will remain. One interesting option that could solve the problems associated to the use of molecular hydrogen is to directly generate the required hydrogen in the active sites of the catalyst.