Development of Novel Catalysts for Bio-Oil Upgradation
Creators: Bathla, Sagar
Publisher: name: University of Alberta Library
Year: 2024

<p>Transition metal carbide catalysts (TMCs), particularly Molybdenum carbide (Mo2C), have emerged as a cost-effective and promising alternative to precious-metal-based catalysts (Pt, Ru) for the hydrodeoxygenation (HDO) of biomass-derived species, including bio-oil. However, low selectivity towards deoxygenation and stability against oxygen poisoning are major limitations of monometallic carbide catalysts. This work primarily focuses on developing Mo2C-based bimetallic carbide catalysts and provides structural and mechanistic insights into HDO mechanism, as these insights are pivotal in developing more selective and stable catalysts for HDO.  Understanding the atomic/molecular structure of the catalyst is crucial, hence, using a combination of density functional theory (DFT) calculations of mixing energies and relative stabilities with experimental characterization data, we predicted the most stable microstructure with a Tungsten to Molybdenum ratio (W/Mo) of 5:3, a mixed metal carbide bulk with overlayers of metallic tungsten (MoWC) is the most stable microstructure. Further investigation into the reaction mechanisms and pathways for the HDO of guaiacol revealed that the presence of tungsten overlayers on bulk MoWC catalyst creates a step-like structure which selectively favors direct deoxygenation (DDO) pathway in comparison to its monometallic counterpart Mo₂C on which both DDO and hydrogenation (HYD) pathways are competitive. These results explain the experimentally observed higher selectivity towards benzene on MoWC, as opposed to the monometallic Mo₂C.  Bio-oils from lignocellulosic biomass are complex mixtures of oxygenated organic compounds, making catalyst performance crucial for effective hydrodeoxygenation (HDO). We compared monometallic Mo₂C with W-doped Mo₂C (MoWC) using DFT calculations and bio-oil components. Our study found that MoWC, due to tungsten&#39;s enhanced oxophilicity, is more effective for HDO than Mo₂C. MoWC can cleave both single and double C–O/C=O bonds, while Mo₂C only cleaves single C–O bonds. MoWC also outperforms Mo₂C in HDO of aromatic and carbohydrate components.<br>
While MoWC bimetallic carbide catalysts show improved activity and selectivity, stability against oxygen poisoning remains a challenge. We screened 45 Mo₂C-based bimetallic carbides (Co, Cr, Fe, Mn, Nb, Ni, Ti, and Zr) using phenol as a model compound and evaluating three key reactions: C–O dissociation, oxygen removal, and ring hydrogenation. Among these, optimal candidates for direct deoxygenation (DDO) include Mo-terminated Fe₃Mo₃C, Mn-terminated Mn₃Mo₃C, and Ni-terminated Ni₃Mo₃C, while triclinic Mo-terminated Fe₁₁MoC₄ and Ni-terminated Ni₆Mo₆C excel in hydrogenation (HYD). We identified an optimal oxygen binding energy (OBE) range of -100 to -200 kJ/mol for effective DDO, balancing oxygen binding with removal. This OBE descriptor and the proposed parameter combining activity, stability, and selectivity offer a basis for preliminary screening of catalysts in large databases for DDO and HYD processes.<br>
We investigated solvent effects on HDO reactions of bio-oil using a combination of catalytic experiments, advanced characterization, and quantum mechanical simulations. Ruthenium (Ru), known for its high catalytic activity, served as a benchmark to compare noble-metal TMCs. We found that Ru&#39;s selectivity shifts from deoxygenation in the gas phase to ring hydrogenation in the condensed phase. In aqueous conditions, water partially dissociates into hydroxyl fragments and H atoms, which are critical for this shift. Experiments showed &gt;99% conversion and &gt;75% selectivity towards hydrogenated products in the presence of water. Computational results supported these findings, showing reduced kinetic barriers for hydrogenation (70 kJ/mol) and increased barriers for dehydroxylation (63 kJ/mol to 202 kJ/mol). Water not only provides H for hydrogenation but also regenerates with external hydrogen supply, acting as a hydrogen shuttle. This study highlights how solvent effects can tune Ru catalyst selectivity towards hydrogenation.</p>