Investigation On The Catalytic Performance Of Ni₂Mo₃N Catalyst In Aromatics Hydrogenation

The high content of aromatic hydrocarbons and heteroatoms such as nitrogenand sulfur in coal liquefaction seriously affect the quality of oil as well as thedownstream process, so reduce the aromatics content by the way of hydrogenationrefining is well worthy and significant. The bimetallic nitrides Ni2Mo3N werechosen as hydrogenation catalyst, the influence of the preparation conditions onthe composition of catalyst and the catalytic activity as well as sulfur tolerancewas investigated, respectively. The deactivation of Ni2Mo3N catalyst by sulfurwas in-depth analyzed. According to the design principle of sulfur-resistentcatalysts, the introduction of alkali metal K and surfactant P123and PEG-20000to improve the sulfur tolerance of catalyst was investigated. The main results andconclusions are as follows.1. The effects of HMT content, growth of crystal nucleus and calcinationtemperature on the formation of Ni2Mo3N phase was investigated. The resultsindicated Ni2Mo3N phase was the main component of product barked at650oCof the precursor obtained dissolved the starting material in NH3·H2O in water bathtemperature of40oC under stirring rate of300r/min. Using deionized water asthe solvent, Ni2Mo3N was the single active phase of the products barked at650oC of the precursor with different HMT content. According to the compositioncharacteristics of coal liquefaction products, benzene was chosen as a targetreactant and thiophene as a model compound, the catalytic activity of Ni2Mo3Nused for hydrodearomatization (HDA) and hydrodesulfurization (HDS) wasinvestigated. The evaluation results indicated Ni2Mo3N converted98%benzeneto cyclohexane in the sulfur-free feed under250oC and3MPa and the catalyticactivity exceeded that of Pt/Al2O3. Under the same reaction conditions, the catalytic activity of Ni2Mo3N dropped obviously in the presence of100ppmthiophene. XPS results hinted the surface of Ni2Mo3N was sulfided. TPD-MSanalysis showed that the strong adsorption of thiophene and its HDS productscovered the hydrogenation activity sites on the catalyst surface. Increase thereaction temperature to360oC, the benzene hydrogenation activity was decreaseddue to the thermodynamic nature of exothermic reaction, but the catalytichydrodesulfurization performance of thiophene increased and initial activities ofbenzene hydrogenation increased that is the sulfur tolerance was improvedsignificantly.2. Based on the inhibition of metal-s bond, K promoted Ni2Mo3N wasdesigned and synthesized through adding alkali additives and the hydrogenationperformance of K promoted Ni2Mo3N was evaluated. The results manifested theadding of small amount (0.01wt.%~0.07wt.%)K, the intial activity of K-promotedcatalysts was not affected by thiophene, particularly when the K content is0.05wt.%, after9hrs reaction, benzene hydrogenation activity is still above90%under250oC and3MPa attributed to adding of K weakened the chemicaladsorption capacity between thiophene and active metal. Nevertheless when thecontent of K increased to0.08wt.%, conversion of benzene decreased to80%inabsence of thiophene, this is because the excessive K accumulated on the surfacecovered the activite sites.3. Based on the design priciples of sulfur resistant catlysts and preparationof nano material, bimetallic nitride catalyst with addition of surfactant PEG-20000and P123was designed from the perspective of reducing the crystallite size. Thecatalytic activity and sulfur resistance was analyzed under different conditions.XRD and TEM results showed the introduction of surfactant minished thecrystallite size of Ni2Mo3N with the pore structure tending to be uniform. Theevaluation results uncovered the catalytic activity of benzene was99%under250oC and3MPa without thiophene in the feed, increased the reaction temperatureto360oC, the conversion of benzene was85%while the thiophene conversion was95%. In the presence of thiophene, the initial conversion of benzene oncatalyst modified by PEG-20000was97.6%, after the rection went on8hrs, thereaction of hydrogenation activity began to decline slowly under250oC. Raisethe reaction temperature to360oC, benzene hydrogenation activity was stable at84%after18hrs on stream. Herein, declining the crystallite size and raisingreaction temperature seemed to enhance the sulfur resistance capacity.4. Supported Ni2Mo3N/γ-Al2O3with different loadings were prepared byincipient wetness impregnation. The results showed that monolayer dispersion ofcatalyst system was in the range from10wt.%to15wt.%and the highest activitywas about50wt.%, when the load is25wt.%, the benzene conversion is99%.Compared to the non-supported catalysts, the crystallite size of Ni2Mo3Ndecreased significantly and the sulfur tolerance was improved largely.