ZSM-5 Zeolite Modified With Zn Is Used In The Study Of Catalytic Conversion Of N-butane

Light alkenes such as ethene,propene and aromatic hydrocarbons are the raw materials for chemical industry and their output and productive capacity presents the development level of chemical industry of a country.It could be a feasible pathway to produce light olefins and BTX by catalytic cracking of C4 hydrocarbons,which are the by-products of fluid catalytic cracking process in oil refinery.n-Butane is one kind of the C4 hydrocarbons,which is not efficiently utilized in chemical industry due to its higher C-C bond energy,more stable chemical properties,and more difficult activation than other C4 hydrocarbons.It is an effective way to promote the catalytic conversion of n-butane by introducing the metal into zeolite to form bifuncational catalysts.In this thesis,highly efficient metal-zeolite bifuncational catalysts were prepared for the catalytic conversion of n-butane to high value-added raw materials of light olefins and BTX.In the first part of this study,Zn modified ZSM-5 catalysts were synthesized by strong electrostatic absorption between the[Zn（NH₃）₄]（NO₃）₂ and the hydroxyl group of zeolite and the as-prepared samples were tested in catalytic conversion of n-butane.The characterization results showed that the Zn species were successfully introduced into ZSM-5 zeolite in the form of ZnOH⁺,meanwhile,the micropore of zeolite was not obviously blocked in all the samples.In the catalytic test of catalytic conversion of n-butane,a small amount of Zn could largely promoted the reaction,and the conversion of n-butane and the yield of light olefins and BTX reached 78.0%and69.4%over 1.0Zn/CHZ5（40）catalyst at 575℃,respectively,which were 60.1 and 57.5percentage higher than those of commercial ZSM-5 zeolite,respectively.The XPS and NH₃-TPD characterization results revealed that the formed ZnOH⁺were the active sites of dehydrogenation reaction,and the superior catalytic performance could be attributed to the synergetic effect of dehydrogenation of Zn species and cracking of acid sites,which ensured that n-butane could be dehydrogenated firstly on the metal sites to be converted into butene,and the obtained butene could be transformed into light olefins and BTX on acid sites of ZSM-5 zeolite.Furthermore,in order to investigate the effect of the distance between the dehydrogenation active sites and the acid sites on bifuncational catalysts on catalytic cracking of n-butane,tartaric acid was employed as a post-treatment agent to remove aluminum atoms located on the surface of ZSM-5 zeolite,followed by the introduction of Zn species on the surface of zeolite in ethanol media under reflux condition.The XRD,SEM and N₂ adsorption-desorption characterizations results illustrated that the crystallinity,morphology,specific surface area and pore volume showed no obvious change.Combining with the ICP and XPS results,introduced Zn species may be enriched on the surface of ZSM-5 zeolite.The NH₃-TPD results showed that the desorbed temperature of weak acid sites was shifted slightly to the lower temperature,while the amount of the weak acid sites and the property of the strong acid sites had not changed.In the catalytic test of n-butane,the superior catalytic performance was observed over 1.0Zn-TAT-CHZ5（40）sample,and the conversion of n-butane is 10.7and 8.3 percentage higher than those of commercial one at 625℃ and 650℃ respectively,meanwhile,the yield of ethene and propene was 8.0 and 5.9 percentage higher than those of commercial ZSM-5,respectively.The superior catalytic performance was ascribed to the co-effect of Zn species located on ZSM-5 zeolite surface and the increased distance between dehydrogenation active sites and acidic sites which promoted the conversion of n-butane and decreased the aromatization reaction simultaneously.