Slurry Hydrotreatment of Biomass Materials over Metal Sulfide-based Supported and Unsupported Catalysts

Doctoral thesis, 2023

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 are abundant, carbon-rich, and renewable bioresources that 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. In this thesis, we have explored the use of unsupported and supported metal sulfides in the hydrotreatment of Propylguaiacol (PG), a bio-oil model compound, Kraft lignin (KL), and pyrolysis bio-oil. In the recent work, the co-processing of the Kraft lignin and pyrolysis oil over the unsupported NiMoS was also performed.

Firstly, MoS2 supported on γ-Al2O3 catalysts 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 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. It was 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 KL. Further, the effect of the annealing treatment of a hydrothermally synthesized unsupported MoS2 dispersed catalyst was 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 annealed unsupported MoS2 demonstrated a high capacity for deoxygenation with a selectivity of 78.6% and 20.1% for cycloalkanes and aromatics from KL hydrotreatment, 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.

The main hurdle during Kraft lignin liquefaction was the occurrence of repolymerization reactions during depolymerization that lead to the production of undesired solid char residues and subsequently cause low bio-oil yield. In this regard, the combination of NiMo sulfides with various ultra-stable Y zeolites (USY) for the KL hydrotreatment was studied. The use of the physical mixture of the unsupported NiMoS and the USY support was also studied to better understand the role of the catalyst components, and their interactions during lignin depolymerization, HDO, and also repolymerization of the reactive lignin intermediates. Further work was then extended to the co-hydrotreatment of KL and pyrolysis oil over the unsupported NiMo Sulfides. The synergistic effect between the complex feedstocks (KL and pyrolysis oil) was further explored by investigating the effect of supplementing various bio-oil monomers during KL liquefaction. It was found that the strategy of co-feeding bio-derived monomers and pyrolysis oil in the KL hydrotreatment presented an insight for co-processing and also the role of second co-feed was able to facilitate efficient lignin depolymerization increasing the desired bio-liquid yield and limiting lignin condensation. Further, a two-stage fast pyrolysis bio-oils (FPBO) processing concept that involves first a stabilization step in the slurry hydrocracker over an unsupported NiMoS and then followed by downstream fixed-bed hydrotreating producing renewable hydrocarbon was studied. The liquid products were thoroughly analyzed to understand their chemical and physical properties.