Hydrodeoxygenation of aliphatic and aromatic oxygenates on sulphided catalysts for production of second generation biofuels (2007)

Environmental concerns and diminishing petroleum reserves have increased the importance of biofuels for traffic fuel applications. Second generation biofuels produced from wood, vegetable oils and animal fats have been considered promising for delivering biofuels in large amount with low production cost. The abundance of oxygen in the form of various aliphatic and aromatic oxygenates decreases the quality of biofuels, however, and therefore the oxygen content of biofuels must be reduced. Upgrading of biofuels can be achieved by hydrodeoxygenation (HDO), which is similar to hydrodesulphurisation in oil refining. In HDO, oxygen-containing compounds are converted to hydrocarbons by eliminating oxygen in the form of water in the presence of hydrogen and a sulphided catalyst. Due to the low sulphur content of biofuels, a sulphiding agent is typically added to the HDO feed to maintain activity and stability of the catalyst.\n\nThe aim of this work was to investigate HDO using aliphatic and aromatic oxygenates as model compounds on sulphided NiMo/γ-Al2O3 and CoMo/γ-Al2O3 catalysts. The effects of side product, water, and of sulphiding agents, H2S and CS2, on HDO were determined. The primary focus was on the HDO of aliphatic oxygenates, because a reasonable amount of data regarding the HDO of aromatic oxygenates already exists.\n\nThe HDO of aliphatic esters produced hydrocarbons from intermediate alcohol, carboxylic acid, aldehyde and ether compounds. A few sulphur-containing compounds were also detected in trace amounts, and their formation caused desulphurisation of the catalysts. Hydrogenation reactions and acid-catalysed reactions (dehydration, hydrolysis, esterification, E2 elimination and SN2 nucleophilic substitution) played a major role in the HDO of aliphatic oxygenates. The NiMo catalyst showed a higher activity for HDO and hydrogenation reactions than the CoMo catalyst, but both catalysts became deactivated because of desulphurisation and coking. Water inhibited the HDO, but the addition of H2S effectively eliminated the inhibition. The addition of H2S enhanced HDO and stabilised the selectivities but did not prevent deactivation of the catalysts. The effect of H2S was explained in terms of promotion of the acid-catalysed reactions due to enhanced catalyst acidity. Water and the sulphiding agents added to the HDO feed suppressed hydrogenation reactions on the NiMo catalyst but did not affect them on the CoMo catalyst. The addition of H2S resulted in less hydrogen consumption and coke formation than the addition of CS2, but the product distribution was shifted such that the carbon efficiency decreased. It was concluded that, for the HDO of aliphatic oxygenates, H2S was superior to CS2 as a sulphiding agent.\n\nThe HDO of phenol, used as a model aromatic oxygenate, produced aromatic and alicyclic hydrocarbons in parallel routes in which the primary reactions were direct hydrogenolysis and hydrogenation, respectively. The addition of H2S on both catalysts inhibited the HDO due to competitive adsorption of phenol and H2S, and affected the hydrogenation reactions in the same way as in the HDO of aliphatic oxygenates. The opposite effects of H2S on the HDO of aliphatic and aromatic oxygenates were attributed to the different reaction mechanisms for oxygen elimination (dehydration, hydrolysis, elimination and direct hydrogenolysis reactions). The different molecular and electronic structures of the aliphatic and aromatic oxygenates are likely the reason for the different reaction mechanisms. Under the identical conditions, phenol was less reactive than aliphatic oxygenates on the sulphided catalysts. In contrast to the situation in the HDO of aliphatic oxygenates, the NiMo catalyst was less active for the HDO of phenol than was the CoMo catalyst.\n\nThis work illustrates that the composition of biofuels determines the overall performance of the HDO process and the effect of sulphiding agent on the HDO. The HDO performance may be lower for wood-based biofuels, which contain mainly aromatic oxygenates, than for vegetable oils and animal fats, which contain aliphatic oxygenates. This conclusion further implies that operating conditions in an industrial process need to be more severe for upgrading of wood-based biofuels than upgrading of vegetable oils and animal fats. The addition of a sulphiding agent to the HDO feed will probably affect the total HDO of wood-based biofuels negatively and that of vegetable oils and animal fats positively.