Effects Of Structural Properties Of ZSM-5 Molecular Sieve On Cracking And Aromatization Of Chain Hydrocarbons

In recent years,the quality of gasoline is required strictly by environmental protection laws.The decrease of the olefin content in commercial gasoline by converting it into the aromatic hydrocarbon with high octane number,which is the main method to upgrade gasoline.The linear hydrocarbons with low octane number in Fischer-Tropsch products are converted into the aromatic hydrocarbons with high octane number herein,which is based on the molecular structure characteristics of Fischer-Tropsch oils.According to the literatures,the molecular sieve supported with metal exhibits high aromatic selectivity for aromatization reactions of low carbon number hydrocarbons and zinc（Zn）is widely used as the modified metal in such catalysts.The cracking reactions are the necessary process for the aromatization of linear hydrocarbons,however,the effect of the structural characteristics of the molecular sieves on the conversion rate of linear hydrocarbons in Fischer-Tropsch products is not clear.Therefore,it is significant to study the effect of the structural characteristics of molecular sieves on the aromatization of linear olefins in Fischer-Tropsch products and the conversion rate of linear hydrocarbon cracking reactions.In this paper,ZSM-5 molecular sieves with different Si O₂/Al₂O₃ mole ratios,different crystallite sizes,and mesoporous structure were synthesized and these corresponding catalysts were prepared by the loading of zinc.The acidity and pore structure of ZSM-5 molecular sieves were characterized by N₂ adsorption-desorption,X-ray diffraction（XRD）,NH₃ temperature programmed desorption（NH₃-TPD）and pyridie infrared（Py-IR）.The results obtained in this paper are as follows.The effect of the Si O₂/Al₂O₃ mole ratio of ZSM-5 molecular sieves on the aromatization of linear olefins was investigated.As the Si O₂/Al₂O₃ mole ratio of the ZSM-5 molecular sieve increases,the contents of aromatics,propane and butane in the aromatization products decrease.Moreover,the production of aromatic hydrocarbons through hydrogen transfer reaction gradually decreases while through direct dehydrogenation gradually increases.It is because the cracking,oligomerization and cyclization of the olefin and the hydrogen transfer reaction occur on the Br?nsted（B）acid sites.After the loading of zinc,the dehydrogenation reaction occurs on the formed Zn-Lewis acid sites.The amount of B acid sites is determined by the Si O₂/Al₂O₃ mole ratio of the ZSM-5 molecular sieve.With the increase of the Si O₂/Al₂O₃ mole ratio,the amount of B acid sites decreases,which is not beneficial to the cracking,oligomerization and cyclization of olefins and the hydrogen transfer reaction,resulting in the inferior production of aromatic hydrocarbons.ZSM-5 molecular sieves with different crystallite sizes were synthesized and characterized.The effect of the crystallite size of ZSM-5 molecular sieves on the aromatization of linear olefins was investigated.ZSM-5 molecular sieve with small crystallites based catalyst presents low selectivities to benzene,toluene,and xylene（BTX）in the aromatization products of linear olefins.Monocyclic aromatic hydrocarbons are more likely to be generated in the micropores of ZSM-5 due to the confinement effect of pore channels.If the cracking reaction of long-chain hydrocarbons occurs on the external surface of ZSM-5,the small olefins are generated and they enter the micropores of ZSM-5 to dimerize and generate monocyclic aromatic hydrocarbons.If the cracking reaction of long-chain hydrocarbons occurs inside the micropores of the ZSM-5,aromatic hydrocarbons are most likely generated by dimerization due to the confinement effect of pore channels.However,ZSM-5molecular sieves with small crystallites has a large number of intercrystalline mesopores,which leads to the small olefins difficult to dimerize inside the micropores of the ZSM-5,resulting in the inferior production of monocyclic aromatic hydrocarbons.The aromatization experiments over ZSM-5 molecular sieves with intracrystalline mesopores based catalyst were performed.The results show that although ZSM-5molecular sieve after alkali treatment and the counterpart have the almost identical Si O₂/Al₂O₃ mole ratio,the mesoporous ZSM-5 molecular sieves based catalyst presents lower aromatization activity,aromatics content in the aromatization products and cyclization activity than the counterpart catalyst.This is because the mesopores produced by alkali treatment limit the generation of monocyclic aromatic hydrocarbons inside the micropores of ZSM-5.With the increasing amount of mesopores,the aromatization of long-chain hydrocarbons inside the micropores of ZSM-5 decreases,which leads to a lower aromatization efficiency.The effect of reaction conditions on aromatization over ZSM-5 molecular sieve based catalyst was investigated.As the reaction time increases,the aromatic content in the aromatization products gradually decreases.As the reaction temperature increases,the aromatic content of the aromatization products increases.The increase of the reaction pressure causes the increase of the alkane and the aromatic content in the aromatization products.It is because the increase of the reaction temperature accelerates the cracking reaction and dehydrogenation reaction of linear olefins,which promotes the generation of aromatic hydrocarbons.The increase of the reaction pressure inhibits the cracking reaction of linear olefins while promotes the hydrogen transfer reaction,resulting in the increase of aromatic and the alkane content.The effect of structure characteristics of ZSM-5 molecular sieve on the conversion rate of linear hydrocarbons cracking reactions using n-dodecane as the feedstock was investigated.With the increase of Si O₂/Al₂O₃ mole ratio of ZSM-5,the n-dodecane conversion and the aromatic content in the products decrease.It is because the cracking reaction of linear hydrocarbons occurs on the B acid sites and the increase of Si O₂/Al₂O₃ mole ratio of the ZSM-5 results in the decrease of the amount of B acid sites,and thereby the n-dodecane conversion decreases.Among the ZSM-5 molecular sieves with different crystallite sizes,the ZSM-5 molecular sieve with a crystallite size of700-800 nm has the most amount of B acid sites,and thus,the corresponding catalyst presents the highest n-dodecane conversion.