Studies On Catalytic Activity Of Ni₂P/TiO₂ In Hydrodesulfurization And Dehydrogenation

The quality of gasoline and diesel fuel in our country now is low for high sulfur and nitrogen content. Demands for a cleaner environment have led to a global tightening in the allowed sulfur content in fuels and increased restrictions on the release of oxides. For this reason, there are considerable efforts being expended to develop new hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) catalysts for the production of clean fuels. Nickel phosphide of chemical formula Ni₂P has recently been reported as a new class of high-activity HDS and HDN catalyst. The surface area of unsupported Ni₂P catalyst is rather low (< 1 m²/g). In order to increase the active surface area, Ni₂P is dispersed on a high-surface support. Titania is a new class of support for hydrotreating catalysts,it has attracted attention in view of the higher HDS activity displayed by molybdenum-based sulfide supported on this oxide than on traditional Al₂O₃ and SiO₂. Therefore, the focus of this work was the preparation of TiO₂ supported Ni2P catalysts and their activities in HDS of thiophene. In this thesis, nickel phosphide was successfully prepared in dispersion form on TiO₂ support by means of temperature-programmed reduction of the corresponding phosphate. The phase purity of the catalyst was established by x-ray diffraction (XRD), and the surface property was determined by N₂ BET specific surface area measurement. Results showed that Ni2P was the main phase on the TiO₂ supported catalysts. And there was Ni₁₂P₅ phase on the Ni₂P/TiO₂ catalysts with higher Ni loading and lower P loading. With Ni and P content increasing, specific surface areas of catalysts were decreasing. The activities of the TiO₂ supported Ni₂P catalysts were tested in a high-pressure fixed bed reactor for the HDS of thiophene using a model liquid feed at realistic conditions (30 atm, 370℃). The reactivity studies showed that Ni₂P/TiO₂ was an active catalyst. The most active composition was found to have a initial synthesis Ni/P mole ratio close to 1/2, and the best Ni loading of this sample on TiO₂ was observed to be 15 wt%. Ni₂P/TiO₂ had substantially high thiophene HDS activity (almost 100 %) and stability (more than 200 h). Reaction temperature had more effect on HDS. When reaction temperature was higher than 300 oC, HDS activity was close to 100 %. While, reaction pressure, liquid VHSV and H2/mixed oil ratio had less effect on HDS. In addition, Ni2P/TiO2 catalyst was studied in dehydrogenation of cyclohexane and methylcyclohexane for the first time. Hydrogen storage and release can be achieved by liquid organic hydride and hydrogen reversible reaction, i.e. hydrogenation and dehydrogenation reaction. The reversible hydrogen storage technology based on Cyclohexane-Benzene-Hydrogen system (CBH) and Methylcyclohexane-Toluene-Hydrogen system (MTH) was reported recently. The conventional catalysts for hydrogenation and dehydrogenation are sensitive to sulfur-containing compounds, which leads to their irreversible deactivation. However, Ni2P catalysts have favorable sulfur resistance capability. So, it is much significant that supported Ni2P catalysts are applied in hydrocarbons reactions to avoid sulfur poisoning. Previously, one literature has reported catalyst showed the good activity in benzene and toluene hydrogenation. Hydrogenation and dehydrogenation reaction are reversible. To explore the real possibilities of the use in dehydrogenation of Ni2P/TiO2 catalyst, it was used in cyclohexane and methylcyclohexane dehydrogenation. The activities of the TiO2 supported Ni2P catalysts were tested in a fixed bed reactor for the dehydrogenation of cyclohexane and methylcyclohexane at realistic conditions (1 atm, 300 oC). With the increase of reaction temperature and the decrease of reaction H2 flow, dehydrogenation activity was better. The most active composition was found to have a initial synthesis Ni/P mole ratio close to 1/1, and the best Ni loading of this sample on TiO2 was observed to be 12 wt%. Ni2P/TiO2 had over 60 % dehydrogenation activity and 100 % product selectivity. In comparison with Ni2P/SiO2 and Ni/TiO2, Ni2P/TiO2 catalyst had much better activity in dehydrogenation reaction.