| The existence of aromatic compounds and organic sulfides in the fuel oil isnot only bring down the quality of oil and cause environmental pollution, but alsomake a main contribution to the poisoning deactivation of oil refining catalysts,and new environmental regulations have made stricter limits for the content ofthem. In order to solve the problem, a research thoughts of the development of abifunctional catalyst which can be used for hydrodearomatization (HDA) andhydrodesulfurization (HDS) was adopted in this paper. The way combined withexperiment and theoretic calculation was used to study the structure,hydrogenation performance and stability of a transition metal nitrides catalyst.And the results of adsorption state, electron transfer and energy change ofthiophene and benzene adsorbed on γ-Mo2N(100) and Ni2Mo3N(110) surfacewere analyzed, include the configuration of intermediate products and activationenergy for elementary reactions during thiophene HDS on γ-Mo2N(100). Theactive sites on the surface, reaction path, composition of products and ratecontrolling step of thiophene HDS on γ-Mo2N(100) also be confirmed over thecourse of the study.With ammonium molybdate and hexamethylenetetramine (HMT) as rawmaterial, the Mo2N catalyst was prepared by coordination compounddecomposition method, and the reduction temperature of Mo2N is500oCdetermined by H2-TPR. The performance evaluation of the catalytic was operatedin a continuous flow tubular fixed bed reactor, the results showed that the thiophene with a concentration of1wt.%conversion reached to more than99%and remained stable activity under360oC and3MPa, and the C4products had aconcentration gradient of2-butene>1-butene> butane and H2S was detected ingas phase products. After the reaction, the crystal pattern of Mo2N had no anychange by XRD (X-Ray Powder Diffraction) analysis, while the S species(thiophene and sulfide) on the surface of catalyst was detected by XPS (X-RayPhotoelectron Spectroscopy). In the experiment of HDA, Mo2N showed a weakercatalytic hydrodearomatization performance with a benzene conversion of18%.With the same method, the Ni-Mo2N was prepared by nickel acetate tetrahydrate,ammonium molybdate and HMT. The addition of Ni greatly improved theefficiency of catalytic HDA with a stable HDS conversion, and the results ofperformance evaluation showed that benzene had a conversion of97.5%with ahexane selectivity of100%, and thiophene had a conversion of95%. What’s more,the conversion of benzene could reach81%in the presence of0.01wt.%thiopheneunder360oC and3MPa, and there were no significant change on both body andsurface of the catalyst after reaction by XRD (X-Ray Powder Diffraction) andXPS (X-Ray Photoelectron Spectroscopy) analysis, which indicated thatNi-Mo2N is a highly efficient catalyst for the hydrogenation of aromatichydrocarbons and organic sulfides.The microscopic properties of catalysts and catalytic hydrogenation processwere further analyzed, and the results of density functional theory (DFT) showedthat benzene has a weak adsorption on γ-Mo2N(100) surface with an adsorptionenergy of–0.062eV, which was responsible for the low activity of HDA. After thegeometric optimization, the η5-Mo2N configuration was the most stableadsorption model, where thiophene absorbed on the hcp site paralleling to thesurface through S atom being bonded to a Mo2atom, and hcp site is the active sitefor thiophene HDS reaction. The process of thiophene HDS on γ-Mo2N(100)surface can be divided into two parts, the removal of S atom and the hydrogenation of C4species, and the activation energy of elementary reactionswere confirmed by transition state search. The addition of the first H needs ahighest activation energy of1.69eV, which is the rate-determining step forthiophene HDS. The results of geometry optimization showed that butadiene and–SH butadiene are firstly formed after the hydrogenation of thiophene, thereaction energy barrier for C4species formation have a rank of2-butene <butadiene <butane, and2-butene is the main C4product due to a lower activationenergy and a weaker adsorption. The active surface of Ni2Mo3N was determinedby TEM (Transmission Electron Microscopy) as Ni2Mo3N(110), surface energyof different terminal surface and the change of catalyst caused by Ni wereanalyzed to investigate the adsorption state, electron transfer of thiophene andbenzene adsorbed on Ni2Mo3N(110) surfaces, and the results showed that theNi2Mo3N(110) surface has a stronger adsorption and activation for benzene. |
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