Synthesis and characterization of molybdenum nitride hydrodenitrogenation catalysts

摘要

Details concerning the relationships between the structural, chemical and catalytic properties of Mo nitrides have been elucidated. A series of Mo nitride catalysts were prepared by the temperature programmed reaction of MoO3 with NH3. The structural properties of these nitrides were complex functions of the heating rates and space velocities employed. Two reaction sequences were proposed to account for the synthesis of high, medium and low surface area materials. An interesting conclusion was that the degree of reduction of the molybdate precursor or intermediate governed the structural properties of the product. Some evidence is also presented to suggest that the nucleation and growth rates involved in the transformation of the oxide to the nitride were significantly influenced by the synthesis conditions. The Mo nitrides proved to be exceptional pyridine hydrodenitrogenation catalysts. Their catalytic properties were superior to those of a commercial sulfided Co-Mo hydrotreatment catalyst, having higher activities and better C-N bond hydrogenolysis selectivities. Hydrodenitrogenation over the Mo nitrides appeared to be structure-sensitive. While detailed relationships between the catalytic activity and surface stoichiometry could not be ascertained, there did appear to be a correlation between the activity, and the particle size and grain boundary length. We proposed that at least two types of HDN sites existed on the Mo nitride surfaces; modest activity sites on the particles and high activity sites at grain boundaries. The N/Mo stoichiometry of the highest activity catalyst was near unity suggesting that MoN was present perhaps localized at the grain boundaries. Finally structures near or at the surface were markedly different from those of the bulk. While the predominant bulk phase was [gamma]-Mo2N, the surface appeared to consist of either non-stoichiometric [beta]-Mo16N7 or mixtures of Mo and [beta]-Mo16N7.