Role of transition metals on MoS2-based supported catalysts for hydrodeoxygenation (HDO) of Propylguaiacol

Transition metal sulfides (TMSs) are typically used in the traditional petroleum refining industry for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) applications. Bio-oils require an upgrading process like catalytic hydrodeoxygenation (HDO) to produce advanced biofuels and chemicals. Herein, MoS2/γ-Al2O3 promoted by transition metals like nickel (Ni), copper (Cu), zinc (Zn), and iron (Fe) was evaluated for the HDO of a bio-oil model compound, 4-propylguaiacol (PG) in a batch reactor at 340 °C under 50 bar H2 pressure. The catalyst screening results showed that the sulfided Ni-promoted catalyst gave a high 94% yield of deoxygenated cycloalkanes, however for the sulfided Cu-promoted catalyst, 42% of phenolics remain in the reaction medium after 5 h. The results also revealed that the sulfided Zn and Fe-promoted catalysts gave a final yield of 16% and 19% at full PG conversion, respectively, for deoxygenated aromatics. A kinetic model considering the main side reactions was developed to elucidate the reaction pathway of demethoxylation and dehydroxylation of PG. The developed kinetic model was able to describe the experimental results well with a coefficient of determination of 97% for the Ni-promoted catalyst system. The absence of intermediates like 4-propylcyclohexanone and 4-propylcyclohexanol during the reaction implies that direct deoxygenation (DDO) is the dominant pathway in the deoxygenation of PG employing sulfided catalysts. The current work also demonstrated that the activity of the transition metal promoters sulfides for HDO of PG could be correlated to the yield of deoxygenated products from the hydrotreatment of Kraft lignin.