Structure And Performance Adjustment Of Ni Based Catalyst For Phenanthrene Saturated Hydrogenation

The production of valuable chemicals from the refining of coal tar is an important method for the clean and efficient utilization of coal tar.Phenanthrene is such a chemical obtained from coal tar in a large scale(theoretically,?1 million tons)but struggles to find the effective utilization methods.Intriguingly,the hydrogenation saturated product of phenanthrene(perhydrophenanthrene)can be used as an ideal component of jet fuel due to its higher energy density and higher thermal stability.By the conversion of phenanthrene to perhydrophenanthrene,the value of coal tar can be enhanced along with the provision given for the development of China's aerospace industry.The success of this process primarily depends on the preparation of high-performance hydrogenation catalysts.Despite the low cost and the stability,the traditional metal sulfide catalysts lack the hydrogenation saturation capacity.On the other hand,noble metal catalysts have high activity but their price is high and sulfur resistance is poor.Intriguingly,Ni-based catalysts have shown the excellent aromatic hydrogenation saturation ability but mostly suffers with poor stability.Among the Ni-based catalysts,Ni₂P displays good stability but exhibits a less phenanthrene hydrogenation saturation efficiency.Therefore,designing and preparing the low cost,stable and highly efficient phenanthrene hydrogenation saturation catalyst is very important and challenging.With this background,this study aims to prepare the Ni-based catalyst with good stability and a higher hydrogenation saturation activity in the rate limiting step of phenanthrene hydrogenation.This aim is attained by tuning the structure and electronic properties of Ni-based catalyst.Initially,reaction conditions of phenanthrene hydrogenation are optimized followed by the analysis of factors inhibiting the hydrogenation saturation stability of Ni-based catalyst(Ni/Al₂O₃).Ni-based catalyst is modified by the electron withdrawing ability of P.The influence of P on the structure and hydrogenation performance of Ni-based catalyst(Ni₂P/Al₂O₃)is discussed;To further improve the perhydrophenanthrene selectivity of Ni₂P/Al₂O₃,Al₂O₃was doped and modified with Ti O₂.The influence of amount of Ti O₂ on the structure and performance of Ni₂P/Al₂O₃ was investigated.In this study,the following key results are observed.(1)The optimized reaction temperature of phenanthrene hydrogenation is 320°C at 5 MPa.The mass flow rate of reactants below 3.93×10^-2 g·h^-1 and catalyst particle size higher than 0.38mm are not useful as they lead to internal and external diffusion during the reaction.(2)Under the chosen reaction conditions(temperature=320°C,pressure=5 MPa,and weight hourly space velocity(WHSV)=52 h^-1),Ni/Al₂O₃ catalyst displays 98%initial phenanthrene conversion with 98%perhydrophenanthrene product selectivity.But after 6 hours,the phenanthrene conversion of Ni/Al₂O₃ is dropped to 75%with very poor perhydrophenanthrene selectivity(5%).The structural comparison of fresh and spent catalysts reveal that Ni/Al₂O₃ is deactivated mainly by S-poisoning.In the case of Ni₂P/Al₂O₃,though the selectivity of perhydrophenanthrene is dropped from 98%to 49%,95%of initial activity is retained even after 5 hours that indicates a better stability of Ni₂P/Al₂O₃.The increase in catalyst stability is attributed to the electron transfer from Ni to P.The decrease in Ni electron density weakens the interaction between Ni and S,inhibiting the S-poisoning.(3)Under the optimized reaction conditions(temperature=320°C,pressure=5 MPa,and WHSV=52 h^-1),compared to Ni₂P/Al₂O₃,the selectivity of perhydrophenanthrene over Ni₂P/Ti O₂-Al₂O₃ is increased from 49%to 68%.After 5 hours of reaction,Ni₂P/Ti O₂-Al₂O₃retains 95%of its initial selectivity.The characterization studies indicate that Ti O₂ transfers the electrons to Ni in Ni₂P which tunes the electron-deficient state of Ni more appropriate for the adsorption of phenanthrene over the catalyst.The Ti O₂ doping eventually improves the catalyst's selectivity to yield the higher amount of perhydrophenanthrene.