Preparation And Catalytic Performance Of Catalysts For Naphthalene Hydrocracking

As an unconventional oil,coal tar is rich in resource.It is of great practical significance to make full use of coal tar resources and convert heavy polycyclic aromatic fraction into basic organic chemical products such as benzene,toluene,ethylbenzene and xylene（BTEX）by catalytic hydrocracking.In this paper,the preparation and process conditions of bifunctional catalyst for naphthalene hydrocracking to BTEX were studied.According to the reaction mechanism of naphthalene hydrocracking to monocyclic aromatic hydrocarbons,the metal hydrogenation active components suitable for naphthalene selective hydrogenation to tetralin intermediates and acidic molecular sieves suitable for tetralin catalytic cracking were screened,and the catalysts suitable for naphthalene hydrocracking to BTEX monocyclic aromatic hydrocarbons were prepared.The performance of the catalysts was characterized by XRD,SEM,H₂-TPR,TPD and BET The physical and chemical properties were characterized.The results shows:Raney Ni/Al₂O₃ catalysts supported by metallic nickel prepared by using nickel-aluminum alloy powder and pseudo-boehmite as supports,and MoO₃-Raney Ni/Al₂O₃ hydrogenation catalysts modified by MoO₃were prepared.The XRD characterization results show that it has the presence of dispersed skeletal nickel,its low-temperature active hydrogen activity is excellent,and it has strong hydrogenation activity when catalyzing the conversion of naphthalene.It can completely convert naphthalene to decalin under the experimental conditions investigated,and the selectivity of tetrahydronaphthalene is very low.However,a series of x NiO·y MoO₃/γ-Al₂O₃ catalysts with 25%total metal oxide loading prepared by ultrasonic equal volume impregnation method have better catalytic activity and selectivity for hydrogenation of naphthalene to tetrahydronaphthalene than metallic nickel catalysts.The NiO·7MoO₃/γ-Al₂O₃catalyst was prepared by loading NiO/MoO₃=1:7.Under the conditions of hydrogen oil volume ratio 120:1,reaction temperature 300℃,reaction pressure 3 MPa and liquid hourly space velocity 2 h^-1,the conversion of naphthalene and selectivity of tetralin can reach 98.14%and 90.3%.Respectively,the results show that it is not suitable to use metallic nickel as the active component of naphthalene hydrogenation,and the oxidized nickel is more suitable for naphthalene hydrogenation to tetralin intermediate.Moreover,as the promoter of hydrogenation catalyst,the loading of MoO₃ has a significant effect on the selectivity of naphthalene hydrogenation to tetralin.Acidic molecular sieve is used as the cracking active component,mixed with pseudo-boehmite to prepare the carrier of the hydrogenation active component,and NiO/MoO₃=1:7,the total amount of metal oxide is25%loaded on the prepared carrier,the tetralin hydrocracking catalyst was obtained.The effects of different acidic molecular sieve cracking active components of HY,HZSM-5,and Hβon the performance of hydrogenation of tetralin to BTEX monocyclic aromatics were emphatically studied.Results have shown that different molecular sieves have a significant impact on the catalytic cracking activity of tetralin and the selectivity of BTEX due to their different pore structures and acid properties.The relatively small pore size and relatively strong acidic center of HZSM-5 molecular sieve are not conducive to the conversion of tetralin hydrocracking into BTEX,The weaker acidic HY molecular sieve has a low catalytic conversion rate of tetralin due to its weak acidity.The Hβmolecular sieve is more suitable for the cracking reaction of tetralin because of its proper acid strength and proper pore size.The NiO·7MoO₃·Hβ/γ-Al₂O₃ catalyst prepared with Hβmolecular sieve as the cracking active component has obvious advantages in its activity in catalyzing the hydrocracking of tetralin into monocyclic aromatic hydrocarbons and the selectivity of BTEX.Under the conditions of hydrogen to oil volume ratio 180:1,temperature 350°C,pressure 4 MPa,and liquid hourly space velocity 2 h^-1,the catalytic tetralin hydrocracking conversion rate is 93%,and the BTEX selectivity is 60%.The paper also investigated the reaction conditions under which the NiO·7MoO₃·Hβ/γ-Al₂O₃catalyst with excellent activity for catalyzing tetralin is directly used to catalyze the hydrocracking of naphthalene to BTEX,and the reaction conditions are obtained suitable for the hydrocracking of naphthalene.Since the hydrocracking reaction of naphthalene undergoes two main reactions,the hydrogenation of naphthalene is converted to tetralin intermediate and the hydrocracking of tetralin is converted to BTEX.Among them,the hydrogenation process of naphthalene is an exothermic reaction with a decrease in the number of molecules,while the cracking and isomerization of tetralin in the latter is a typical endothermic reaction with an increase in the number of molecules.Therefore,the hydrocracking reaction of naphthalene needs to comprehensively consider the process conditions of the two main reactions in series,to improve the conversion rate of naphthalene and the selectivity of BTEX monocyclic aromatic hydrocarbons.The study found that the reaction temperature and reaction pressure of naphthalene hydrocracking have obvious effects on the conversion rate of naphthalene and the selectivity of BTEX.When the volume ratio of hydrogen to oil is controlled to 180:1,the reaction temperature is 400℃,the pressure is 5MPa,and the liquid hourly space velocity is 2 h^-1,the conversion rate of naphthalene is 96%,and the selectivity of BTEX can reach 63%.