Pyrolysis of biomass with catalysts has been widely studied and they are sometimes used to tailor the yields of the pyrolysis products. However, catalysis chemistry is extremely complex and only a very few research groups in the world can explain catalysis mechanism for specific reactions and in highly controlled conditions. Thus, understanding (in a fundamental mechanistic way) the effects of adding catalysts on the evolution of pyrolysis products distribution and their composition is not currently feasible. The effects of catalysis on pyrolysis reactions are therefore determined empirically and undesired behaviours were often observed: significant drops in liquid yield have been the most common [47].