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Study On Hydrodealkylation Of Heavy Aromatics To Produce BTX Over Zeolite-Supported Metal Oxide And Noble Metal Catalysts

Posted on:2011-04-10Degree:DoctorType:Dissertation
Country:ChinaCandidate:Q B ShenFull Text:PDF
GTID:1101360305969140Subject:Chemical processes
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Catalytic hydrodealkylation of heavy aromatics to produce benzene (B), toluene (T) and xylenes (X) which are the three basic aromatic starting reactants is an important approach to improve the utilization of heavy aromatics. In this thesis, hydrodealkylation of C9+ heavy aromatics to produce BTX over zeolite-supported metal oxide and noble metal catalysts was studied. Firstly, the thermodynamics analysis for the hydrodealkylation system of C9+heavy aromatics was accomplished. Then the comparation in catalytic performance between HZSM-5-supported metal oxide catalysts andγ-Al2O3-supported metal oxide catalysts applied to this reaction system was carried out. Based on the preferred support type, the effect of zeolite support, active component, loading of active component, recombination of active components and impregnation sequence on physical and structural properties and catalytic performance of the employed hydrodealkylation catalysts was systematically investigated by means of XRD, H2-TPR, N2 adsorption and desorption, NH3-TPD and FTIR spectrum of adsorbed pyridine. And the NiO/HMCM-56 catalyst was developed and selected with its excellent catalytic performance. Moreover,1,2,4-trimethylbenzene was employed as the model reactant and the reaction mechanism of hydrodealkylation of 1,2,4-trimethylbenzene over the NiO/HMCM-56 catalyst was further studied.The thermodynamics analysis indicates that the main reactions of hydrodealkylation are irreversible reactions and most liable to occur with their equilibrium constants far greater than those of isomerization, transalkylation and disproportionation among the hydrodealkylation system of C9+heavy aromatics.For the hydrodealkylation process of C9+heavy aromatics feedstock employed in this thesis, the catalytic performance of metal oxide catalysts with HZSM-5 zeolite as support is superior to that withγ-Al2O3 as support.The catalytic performance of zeolite-supported nickel oxide and molybdenum oxide catalysts indicates that the HMCM-56 zeolite is an excellent support used to hydrodealkylation of C9+heavy aromatics. For zeolite-supported nickel oxide catalysts, the samples with the moderate interaction between NiO and zeolite exhibit the relatively high selectivity. And for zeolite-supported molybdenum oxide catalysts, the presence of the molybdenum species located in the channels of zeolite and associated with Br(?)nsted acid site causes the remarkable reduction of selectivity of BTX. The presence of Br(?)nsted acid sites, the growth of the strength of Lewis acid sites, the increase of acid amount and the accretion of specific surface area can all enhance the conversion of C9+aromatics in hydrodealkylation of heavy aromatics.The experimental results about investigation of active components indicate that among the HMCM-56-supported catalysts with different metal oxides as active components in this thesis, the NiO/HMCM-56 catalyst shows the greatest overall catalytic performance. The effect of NiO loading on the catalytic activity of NiO/HMCM-56 is related to the dehydrogenation-hydrogenation function of NiO, the acidity of catalysts and the dispersion state of NiO on the surface of HMCM-56. And the suitable NiO loading is 3wt%-6wt%. For the 6wt% NiO/HMCM-56 catalyst, the conversion of C9+aromatics is 72.75mol%, the selectivity of BTX is 78.51mol% and the yield of BTX is 57.11mol%. For the binary composite catalyst of Re-NiO/HMCM-56 containing noble metal Re, the recombination of Re and NiO active components has a great effect on physical and structural properties and catalytic performance of Re-NiO/HMCM-56, whereas the impregnation sequence of Re and NiO has little effect.The reaction mechanism of hydrodealkylation of 1,2,4-trimethylbenzene over the NiO/HMCM-56 catalyst shows that the main reaction of hydrodealkylation is catalyzed by Lewis acid sites and accomplished through a radical mechanism. Disproportionation is catalyzed by Br(?)nsted acid sites and accomplished through a bimolecular carbenium ion chain mechanism while isomerization is catalyzed by Br(?)nsted acid sites and accomplished through monomolecular and bimolecular carbenium ion chain mechanisms. And catalytic hydrocracking requires the hydrogenation of the aromatic ring over metal sites in a first step to produce naphthenes, which rapidly undergo cracking on the Bronsted acid sites and Lewis acid sites to produce light paraffins.
Keywords/Search Tags:heavy aromatics, hydrodealkylation, zeolite-supported catalyst, metal oxide, reaction mechanism
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