Study On The Preparation And Hydrotreating Performance Of The Transition-metal Phosphides With Sulfur

Transition-metal phosphides,like Ni2P,MoP,and WP,are able to exhibit a metallic character and good performance on a variety of reactions such as hydrotreating,hydrogenation,hydrodeoxygenation,and hydrogen evolution.Temperature-programmed reduction(TPR)is the common method to prepare transition metal phosphide,but it is inefficient due to the harsh reduction conditions.As the bond energy of P-O is stronger than that of P-S,in the present study,a new method for preparing Ni2P and MoP catalysts by replacing phosphorous salts as oxygen-free nickel phosphorus(Ni2P2S6)or phosphorus salt((NH4)4P2S6)and ammonium molybdate mixture was proposed,and the performance of sulfur-containing phosphide catalysts was studied.The surface active phase of WP during hydrodesulfurization(HDS)reaction and the inhibition effect of nitrogen-containing compounds on the HDS performance of transition-metal phosphides were further investigated.An alternative approach to synthesizing Ni2P,by reduction of nickel hexathiodiphosphate(Ni2P2S6),was presented.Conversion of Ni2P2S6 into Ni2P occurred at 220 0C,a temperature much lower than that required by the conventional TPR method(>500 ?).A sulfur-containing layer with a thickness of about 4.7 nm,composed of tiny crystallites was observed at the surface of the obtained Ni2P catalyst(Ni2P-S).This is a direct observation of the sulfur-containing active phase of Ni2P.Both the hydrogenation activity and the C-N bond cleavage activity of Ni2P were enhanced after the introduction of sulfur.The thiolate or thiol species was detected as the major sulfur-containing species in the surface of the spent Ni2P-S after the hydrodenotrogeneration(HDN)reaction,which can be an indication of the participation of sulfur in the HDN reactions.A bulk MoP catalyst(MoP-S)was prepared by the low-temperature reduction of a mixture of phosphorus salt((NH4)4P2S6)and ammonium molybdate at low H2 pressure(0.1MPa)and 450 ?.During the reduction,the sulfur embedded in the lattice of MoP phase,and the sulfur content in the bulk was much more than that on the surface over MoP-S,suggesting that the entire MoP-S catalyst was a metal phosphosulfide phase.Compared with MoP-TPR,MoP-S exhibits high HDS activity of dibenzothiophene(DBT)and inclines to happen through hydrogenation pathway,whereas the sulfurized MoP-TPR increases the selectivity of direct desulfurization pathway.In addition,MoP-S catalyst also showed high activity and selectivity in hydrogenation reactions of unsaturated hydrocarbons such as alkynes,olefins,polycyclic aromatic hydrocarbons,halogenated nitrobenzenes,aromatic aldehydes and so on.The method of using a mixture of(NH4)4P2S6 and ammonium metatungstate as precursor couldn't get WP catalyst,but the tungsten.The surface structure of freash and spent WP catalyst prepared by TPR method after HDS reaction was investigated.It is demonstrated that the actual active phase of WP in the HDS was a sulfur-containing phase.Compared with Ni2P and MoP,the surface sulfidation of WP was faster and the sulfur/metal ratio was lower.NH3 was the dominant nitrogen-containing species detected in the WP and Ni2P catalyst after simultaneous HDS and HDN reactions.This indicated that NH3 was one of the important factors inhibiting the activity of phosphide catalyst.