A sulphiding agent added to maintain catalyst activity affects hydrodeoxygenation (HDO) of bio-oils. Aromatic compounds are dominant oxygenates in wood-based biofuels, and aliphatic oxygenates in vegetable oils and animal fats. The effects of H2S on the HDO of aromatic and aliphatic oxygenates on sulphided NiMo/γ-Al2O3 and CoMo/γ-Al2O3 catalysts were compared in liquid and gas phases.
Direct hydrogenolysis and hydrogenation reactions played a major role in the HDO of phenol. H2S suppressed these reactions on the NiMo catalyst and mainly direct hydrogenolysis on the CoMo catalyst, inhibiting thereby the HDO of phenol. The inhibition by H2S was attributed to the competitive adsorption of phenol and H2S. The effect was in contrast to the promoting effect of H2S on the HDO of aliphatic oxygenates, in which acid-catalysed and hydrogenation reactions predominate. The differences were attributed to the different reaction mechanisms due to the distinct molecular and electronic structures of aromatic and aliphatic oxygenates. Phenol was found to be less reactive than aliphatic oxygenates. Further, the NiMo catalyst exhibited lower activity for the HDO of phenol than did the CoMo catalyst, while the reverse was the case for the HDO of aliphatic oxygenates.