Preparation Of Tungsten - Based Catalysts And Their Effects On Hydrogenation And Denitrification

Nitrogen compounds in diesel are deleterious to the stability of diesel, and result in the NOx emissions exceeding regulatory levels. Hydrotreating is the most effective way to reduce nitrides in diesel. It is accepted that an effective way to prepare highly active hydrotreating catalyst is improving the dispersion of active metal and weakening the metal-support interaction. To achieve this aim, W/Al2O3 and NiW/Al2O3 catalysts were prepared by hydrothermal method. Oxalic acid (OA) was used as additive to weaken the W-Al interaction. The catalysts with same metal contents were prepared by impregnation method and used as reference catalysts. Their physicochemical properties and hydrodenitrogenation (HDN) activity for pyridine were characterized and tested. Moreover, phosphorus was introduced to prepare tungsten-based catalyst with both high WO3 dispersion and further weakened W-Al interaction. The effect of phosphorus source and content on physicochemical properties and HDN activity of catalysts were investigated. The results are given as below.1. The W/Al2O3 catalysts prepared by hydrothermal method showed higher tungsten dispersion and weaker tungsten-alumina interaction than those prepared by impregnation method. The lower resistance to mass transfer of the solution under hydrothermal conditions can accelerate the diffusion of tungsten species, and improve their dispersion. The reaction between OA and alumina could weaken the interaction of tungsten and alumina, which was favorable to the sulfidation of tungsten. The HDN conversion of the W/Al2O3 catalyst prepared by hydrothermal method was 41.8% higher than that of the W/Al2O3 catalyst prepared by impregnation method.2. The introduction of Ni is favorable to forming Ni-W-S Type â…¡ active phases and thus helpful to improve the HDN activity. The highly dispersed WO3 prepared by hydrothermal method facilitates dispersing Ni homogenously. Moreover, NiW/Al2O3 prepared by hydrothermal method showed weaker W-Al interaction than that prepared by impregnation method, and the HDN conversion of the former was 54.9% higher than that of the latter.3. The introduction of phosphorus into WP/Al2O3 and NiWP/Al2O3 prepared by hydrothermal method could further weaken the W-Al interaction, leading to the increased octahedrally coordinated tungsten species with strong reducibility and the decreased tetrahedrally coordinated tungsten species with weak reducibility. Among three phosphorus precursors, phosphoric acid, triethyl phosphate, hydroxy ethidene diphosphonic acid (HEDP), HEDP could weaken W-Al interaction as well as keep highly dispersed tungsten oxide by forming hydrogen-bond with H2WO4 to prevent them from aggregation. The HDN conversion of WP/Al2O3 containing 2% P2O5 and prepared by the hydrothermal method was 23.1% higher than that of the catalyst without phosphorus, and 95.4% higher than that of the catalyst with the same phosphorus content prepared by the impregnation method. Moreover, the NiWP/Al2O3 catalyst containing 4% P2O5 and prepared by the hydrothermal method showed 21.2% and 64.1% higher HDN conversion than the NiW/Al2O3 without phosphorus and the NiWP/Al2O3 with the same phosphorus content prepared by the impregnation method, respectively.