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Abstract:
Molybdenum carbides are considered as promising replacements for expensive noble metal catalysts in bio-refining reactions. The following thesis provides insights into novel preparation methods for nanostructured molybdenum carbide catalysts and their catalytic behavior in deoxygenation reactions. Based on nanostructured phenolic resins, scalable procedures for the preparation of well-defined catalysts consisting of carbon supported molybdenum carbide nanoparticles have been developed. The catalysts were tested for the deoxygenation of both, lignin model compounds and fast-pyrolysis bio-oil, revealing a high selectivity for the direct deoxygenation of phenolics into aromatics, which is in perfect agreement with calculated barriers for phenol HDO. The catalytic mechanisms of phenol hydrodeoxygenation over Mo2C and the effect of water on the catalytic activity were investigated by a combined experimental and computational investigation, revealing detrimental effects of water at increased water partial pressure. Based on these results a mechanistic model for the deactivation of molybdenum carbide catalysts during phenol HDO in the presence of water has been proposed. The catalyst was optimized by its doping with small amounts of nickel, leading to overall superior activity compared with a non-doped catalyst, especially in the presence of water. The better performance was attributed to a change of the favored reaction pathway from a direct deoxygenation towards an initial hydrogenation of the aromatic ring, leading to a decrease of the C–O bond dissociation energy. While the production of shaped catalyst bodies starting from powdered carbon materials often requires the addition of a binder, the carbonization of pre-shaped phenolic resin monoliths allows circumventing the powder form and therefore the use of binders, which was successfully demonstrated by using casting molds for pellet production. In addition, on the basis of porous phenolic resins, a scalable method for the mold- and binder-free production of porous and ash-free carbon beads has been developed.
