Catalytic hydroprocessing of liquid biomass is a technology currently un­der developed and there is a lot of room for optimization. For example there are not many commercial catalysts specifically designed and devel­oped for such applications, while conventional commercial catalysts, em­ployed for catalytic hydroprocessing of refinery streams, are used instead. Common hydrotreating catalysts employed contain active metals on alu­mina substrate with increased surface area. The most known commercial catalysts employ Cobalt and Molybdenum (CoMo) or Nikel and Molybde­num (NiMo) in alumina substrate (Al2O3) as shown in Figure 4.

Hydrotreating catalysts are dual action catalytic material, triggering both hydrogenation and cracking/isomerization reactions. On one hand hydrogenation takes place on the active metals (Mo, Ni, Co, Pd, Pt) which catalyze the feedstock molecules rendering them more active when subject to cracking and heteroatom removal, while limiting coke formation on the catalyst. Furthermore hydrogenation supports cracking by forming an active olefinic intermediate molecule via dehydrogenation. On the other hand both cracking and isomerization reactions take place in acidic envi­ronment such as amorphous oxides (SiO2 — Al2O3) or crystalline zeolites (mainly z-zeolites) or mixtures of zeolites with amorphous oxides.

During the first contact of the feedstock molecules with the catalyst, a temperature increase is likely to develop due to the exothermic reactions that occur. However, during the continuous utilization of the catalyst and coke deposition, the catalyst activity eventually reduces from 1/3 to 1/2 of its initial one. The catalyst deactivation rate mainly depends on tempera­ture and hydrogen partial pressure. Increased temperatures accelerate cata­lyst deactivation while high hydrogen partial pressure tends to mitigate catalyst deactivation rate. Most of the catalyst activity can be recovered by catalyst regeneration.

The selection of a suitable hydroprocessing catalyst is a critical step defining the hydroprocessing product yield and quality as well as the operating cycle time of the process in petroleum industry [5]. However the hydrotreating catalyst selection for biomass applications is particu­larly crucial and challenging for two reasons: a) catalyst activity varies significantly, as commercial catalysts are designed for different feed­stocks, i. e. feedstocks with high sulfur concentration, heavy feedstocks (containing large molecules), feedstocks with high oxygen concentration etc, and b) there are currently no commercial hydroprocessing catalysts available for lipid feedstocks and other intermediate products of biomass conversion processes (e. g. pyrolysis biooil), and thus commercial hy­drotreating catalysts need to be explored and evaluated as different cat­alyst have different yields (Figure 5) and different degradation rate [8].

Nevertheless, significant efforts have been directed towards developing special hydrotreating catalysts for converting/upgrading liquid biomass to biofuels [9-12].