Hydrotreatment of lignin and its bio-oils over transition metal sulfide-based supported and unsupported catalysts

Licentiatavhandling, 2021

The scarcity of fossil feedstocks and the deterioration of the current global climate condition have prompted the search for reliable alternatives for fossil fuel replacement. Biomass feedstocks such as lignin can be used to produce renewable bio-oils that can fill the gap left by fossil-derived oils. Such bio-oils require an upgrading process, such as catalytic hydrodeoxygenation (HDO), to improve their quality for use as advanced biofuels and chemicals. Transition metal sulfides (TMS) are typically used in the traditional petroleum refining industry for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) applications. This thesis focuses on the hydrotreatment of a model bio-oil compound, propylguaiacol (PG), and an actual bio-feedstock, Kraft lignin (KL), over TMS-based supported and unsupported catalysts. 

In the first study, catalysts based on MoS2 supported on γ-Al2O3 and promoted by transition metals, such as Nickel (Ni), Copper (Cu), Zinc (Zn), and Iron (Fe) were evaluated for the HDO of PG in a batch reactor setup. The catalyst screening results showed that the sulfided Ni-promoted catalyst gave a 94% yield of deoxygenated cycloalkanes, however, 42% of the phenolics remained in the reaction medium after 5 h for the sulfided Cu-promoted catalyst. It was also found that the sulfided Zn- and Fe-promoted catalysts gave a final yield of 19% and 16% at full PG conversion, respectively, for deoxygenated aromatics. A pseudo-first kinetic model that took into consideration the main side reactions was developed to elucidate the deoxygenation routes for the HDO of PG using sulfided catalysts. 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. This work also demonstrated that the activity of the transition metal promoters for the HDO of PG correlated to the yield of deoxygenated products from the hydrotreatment of Kraft lignin.

The main focus of the second study was on the effect of the annealing treatment of a hydrothermally synthesized unsupported MoS2 catalyst. The prepared unsupported catalysts were studied and evaluated for the HDO of PG. The annealing treatment of the as-synthesized catalyst under N2 flow at 400 °C for 2 h was found to enhance the HDO activity of PG. The effect on catalysts activity of hydrothermal synthesis time and acid addition combined with the annealing treatment was also studied for the same model reaction. The annealed MoS2 with a synthesis time of 12 h in an acidic environment was found to have improved crystallinity and to exhibit the highest degree of deoxygenation of all the studied catalysts, moreover, giving a full PG conversion after 4 h and a final 4-propylbenzene selectivity of 23.4 %. An acidic environment during the synthesis was found to be crucial in facilitating the growth of MoS2 micelles, resulting in smaller particles that affected HDO activity. The annealed unsupported MoS2 that gave the best performance for HDO of PG was further evaluated for the hydrotreatment of KL. The annealed unsupported MoS2 demonstrated a high capacity for deoxygenation with a selectivity of 78.6% and 20.1% for cycloalkanes and aromatics from KL, respectively. The results also indicate that a catalyst with high activity for deoxygenation and hydrogenation reactions can suppress char formation and favor a high lignin bio-oil yield.