Study On The Mechanism Of Hydrodesulfurization Of Tetrahydrothiophene Catalyzed By Nickel Phosphide

With the increasing consumption of energy,the production of conventional light crude oil is insufficient.Heavy oil,natural gas and various new energy would become the main energy that countries have high expectations on.The existence of sulfur-containing organic compounds in heavy oil causes environmental pollution by reducing the quality of oil products.Due to environmental pressure,international environmental regulations are becoming more and more stringent.It is necessary and urgent to reduce the viscosity of heavy oil and sulfur content in heavy oil.In this paper,the mechanism of hydrodesulfurization（HDS）of tetrahydrothiophene（THT）catalyzed by nickel phosphide（Ni₂P）catalyst was studied.The reaction mechanism of hydrodesulfurization of tetrahydrothiophene with nickel phosphide catalyst was investigated by combining theoretical calculation with experimental verification.The adsorption state,electron transfer and energy change of tetrahydrothiophene on the surface of nickel phosphide were analyzed,and the THT in phosphorus was calculated.The activation energy of HDS reaction on nickel phosphide surface was determined.The active sites of thiophene adsorption on nickel phosphide surface,the composition of reaction pathway products and the rate-controlling steps were determined.Using nickel phosphide,hydrogen and tetrahydrothiophene as experimental materials,the hydrodesulfurization reaction of tetrahydrothiophene at 200～300 C was studied experimentally.The gas and liquid phase products were detected.Combining theoretical calculation and experimental analysis,the conclusions are as follows,（1）By changing the number of fixed atomic layers,the thickness of vacuum layer and the number of atomic layers of flat model,48 kinds of flat model were calculated.Finally,by convergence test analysis,it was determined that when the chemical composition of the terminal surface of Ni₃P₂was used as the surface of flat model,its surface energy could be lower,that is,the flat model was more stable.（2）Twenty-seven adsorption sites of tetrahydrothiophene on Ni₂P（001）surface were calculated.The adsorption energy of v-Ni-Hcp1 under vertical adsorption mode was calculated,and the adsorption model was more stable than others.After adsorption,the C-S bond is elongated and the C-C bond is shortened.In addition,except for the adsorption models of Ni-Hcp3,Ni-Top3,P-Top1 and V-P-Top1,the C-S bonds of tetrahydrothiophene all increased under the other 23 adsorption modes,while the C-S bonds in the optimal adsorption mode changed the most,and the C-S bonds broke first in the study of hydrodesulfurization.（3）Mulliken population distribution and differential charge density analysis of tetrahydrothiophene show that the electrons are transferred to Ni₂P（001）surface through tetrahydrothiophene molecule during the whole adsorption process.The polarity of C-S bond is strong,and the shift of electrons between carbon and sulfur is obvious.（4）The activation energy of HDS unit reaction of THT on nickel phosphide surface was calculated.According to the calculation results of reaction energy,f is the most possible route for hydrodesulfurization of tetrahydrothiophene on Ni2P（001）surface.In other words,hydrogen atoms first combine with the fractured alpha-C to saturate the carbon chain.From Figure 3-15RS7,it can be seen that in the presence of hydrogen,-SH migrates to the Ni-top position and continues to combine with the surrounding hydrogen atoms to form hydrogen sulfide,while the carbon chain combines with hydrogen atoms to form butane.（5）After hydrodesulfurization of tetrahydrothiophene,the presence of H2S was detected in the gas phase products,and the concentration of H2S was much higher than that of hydrogen sulfide produced by pyrolysis under the same conditions,indicating that the degree of pyrolysis was smaller than that of hydrodesulfurization under the experimental conditions in this paper.（6）According to the most probable reaction path of the simulated hydrodesulfurization of tetrahydrothiophene,the most likely final products are hydrogen sulfide and butane,which occupy the majority of gases.Combined with the analysis of hydrodesulfurization experiments,a large number of saturated hydrocarbons are generated,which correspond to the simulation results.