Optimization of active metal Ni d charge density for efficient phenanthrene hydrogenation over Ni2P/Al2O3 catalyst
The content of phenanthrene in high temperature coal tar is relatively high.The hydrogenation saturation of phenanthrene through catalytic not only expands the utilization of phenanthrene,but also obtains perhydrophenanthrene,which is an ideal component for jet fuel due to its high density and calorific value.However,the competitive adsorption between phenanthrene and intermediate hydrogena-tion products in the process of phenanthrene hydrogenation is not conducive to the adsorption and activation of phenanthrene on the cata-lyst,and the further hydrogenation of symmetric octahydrophenanthrene is the speed control step in the hydrogenation saturation process of phenanthrene.Due to the difficulty in adsorption and activation of symmetric octahydrophenanthrene on the catalyst,the catalyst activity is often difficult to meet the hydrogenation demand.According to the π-complex adsorption mechanism between polycyclic aromatic hydro-carbon and transition metals,polycyclic aromatic hydrocarbon and active metals act as electron donors and electron accepters respectively in the process of reactant adsorption and activation.Therefore,when the active metal Ni in the Ni-based catalyst is in the electron-defi-cient state,it is conducive to the formation of perhydrophenanthrene.However,the reason why the amount of Ni electron deficiency and its electronic structure affect the performance of the catalyst for hydrogenation of phenanthrene and symmetric octahydrophenanthrene needs to be further explored.In addition,considering the advantages of the supported Ni2P catalysts,such as high stability,strong sulfur resistance and nitrogen resistance,Ni2P/Al2O3 catalysts with different d charge densities were prepared by adjusting the P/Ni amount-of-substance ratios using hypophosphite disproportionation method,to investigate the influence of Ni d charge density on the adsorption and reaction performance of phenanthrene and symmetric octahydrophenanthrene.The results show that under reaction conditions of 320 ℃,5 MPa,and Weight Hourly Space Velocity(WHSV)of 1 309 h-1,the Ni-2.5P/Al2O3 catalyst obtains the highest turnover-frequency(fT0)of 44.64×10-3 s-1.By describing the interaction strength between phenanthrene,symmetric octahydrophene,and catalyst surfaces through adsorption activation entropy,it was found that the adsorption strength of phenanthrene and symmetric octahydrophenanthrene on the sur-face of Ni-xP/Al2O3 catalysts with different P/Ni molar ratios are different.Further,by quantitatively calculating the Ni d charge density,it was identified that the suitable Ni d charge density for the hydrogenation of phenanthrene over Ni-xP/Al2O3 catalyst is about-0.24 e,and the suitable Ni d charge density for the hydrogenation of symmetric octahydrophenanthrene is about-0.05 e.
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