| The similarity between the aromatic carbon number(C8-C15)of coal tar and the carbon number of ideal jet fuel highlights the coal tar as a valuable feedstock.Accordingly,the low-temperature hydrogenation of coal tar has received much attention in order to prepare the ideal components of jet fuel.The success on the low-temperature hydrogenation of coal tar highly depends on the efficiency and selectivity of the catalysts.Therefore,a remarkable attention has been continuously given for design and preparation of the catalysts.As the most widely used catalyst in industry,transition metal sulfides have great sulfur resistance,but its hydrogenation activity is lower.Noble metals show excellent hydrogen activation ability in all the hydrogenation catalysts,and its anti-poisoning ability could be improved by doping second metal,however,the wide utilization of the noble catalyst was limited by its high cost.In the recent years,Ni2P has been recognized as a significant hydrogenation catalyst due to its special morphology and electronic structure.However,the unsupported Ni2P catalysts generally possessed low specific surface area,poor dispersion of active components,less mechanical strength and poor heat dissipation,which resulted in low activity.Thus,support is widely employed to disperse Ni2P particles.The supports used in Ni2P catalysts mainly include Al2O3?activated carbon?SiO2 and zeolites.Interestingly,like the choice of supports,the selection of preparation method is also equally important to accomplish the best results during the hydrogenation.So far,TPR,hypophosphite decomposition and liquid phosphating are the frequently reported methods for preparing the supported Ni2P catalysts.However,even the same active component is used,the different preparation methods and support could yield the different particles distribution,dispersion and micro structure of the resultant catalysts.Thus,in this work,Ni2P/SiO2 catalyst was prepared by TPR method and hypophosphite disproportionation method so as to obtain a suitable preparation method for Ni2P/SiO2 catalyst.Based on the previous research result in our group,SiO2 was employed to modify Ni2P/Al2O3 catalyst and the influence of SiO2 coating on its structure and hydrogenation performance were investigated.Naphthalene and tetrahydronaphthalene were utilized in this work as the model compounds representing the aromatic hydrocarbon components of coal tar.The hydrogenation reaction was studied at a higher weight hourly space velocity(20.8 h-1)than the usual case(3-10 h-1).The conversion rates of the chosen aromatic compounds and selectivity ratio of the obtained products were calculated and discussed.The main observations of the current work are summarized as below:(1)The influence of internal and external diffusion on the reaction could be excluded when the weight hourly space velocity(WHSV)is 20.8 h-1,the mass flow rate was higher than 13.2×10-2g·h-1 and the catalyst particle size was0.25-0.38 mm.(2)The effects of preparation methods on the structure of Ni2P/SiO2 catalyst and the performance of naphthalene hydrogenation were investigated.Results showed that Ni2P/SiO2 catalyst with pure Ni2P crystal phase could be successfully prepared by the TPR method and the hypophosphite disproportionation method.Among the attempted two preparation methods,TPR method not only installed the higher number of Ni2P species over SiO2 and smaller particle size of Ni2P,but also obtained higher-level electron deficiency of Ni?+and more weak acid sites.As a result,more active sites on the surface of Cat-T catalyst were accomplished,which promoted the adsorption of aromatic hydrocarbons and subsequently a higher hydrogenation activity.Therefore,TPR method is recommended as a better method to synthesize Ni2P/SiO2 with the most desired properties.(3)Ni2P/Al2O3 catalyst with SiO2 coating on?-Al2O3 surface was prepared by hypophosphite disproportionation method and the influence of SiO2 coating on its structure and hydrogenation performance were investigated.It is found that all the Cat-x Si catalysts showed better naphthalene conversion(higher than65%)than that of Cat-0Si catalyst(56%),which signifies the importance of SiO2coating.The coating of SiO2 over?-Al2O3 promoted the formation of more Ni2P particles in the Cat-x Si catalyst,and achieved the better dispersion of Ni2P sites.Furthermore,the SiO2 coating lead to the formation of small-sized Ni2P particles with the more exposed Ni(2)sites and appearance of higher-level electron deficiency of Ni?+in Ni2P.The Cat-16.0Si catalyst contained more amount of Ni2P particles and smaller size,also the highest-level electron deficiency of Ni?+,which contributed to the best aromatic hydrogenation activity than the other catalysts in the present study. |
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