Study On Catalysts And Reaction Process For Aromatization Of Long Chain Alkanes

In today’s era of excess refining capacity and high demand for high value-added chemicals,a technical route to convert excess refining capacity into aromatics has been proposed,expanding the sources of aromatics.Taking n-dodecane as a model compound,the aromatization catalyst and reaction process of long-chain alkanes were studied,hoping to obtain high aromatics yield while producing more alkenes.The pore size of catalysts affects the diffusion of reactants and products.The aromatization effects of HY,Hβ,HZSM-5 and other catalysts were compared in experiments,and it was found that HZSM-5 zeolite with the excellent anti-carbon ability of its ten-membered ring three-dimensional pore system has the most potential as a catalyst for the aromatization of long-chain alkanes.The Si/Al ratio has a direct effect on the acidity of the molecular sieves,and its effect on the aromatization reaction was investigated.Molecular sieves with low silica-alumina ratio were conducive to the generation of dry gas and carbon deposits due to their excessive acid content,especially the excessive strong acid centers;while molecular sieves with high silica-alumina ratio lacked enough acid centers to convert olefin intermediates in time.All of them were not conducive to the improvement of BTX yield,and the highest BTX yield was obtained on HZSM-5 with a silicon-aluminum ratio of 38,which was 24.30%.The effects of temperature and space velocity on the reaction under normal pressure were investigated.The yield of BTX increased with the increase of temperature,and reached the maximum value at 2 h^-1.The aromatization effects of HZSM-5（38）catalysts supported by different elements（Mg,Cr,Mn,Fe,Co,Ni,Cu,Zn,Ga）were compared,and it was found that the BTX yield of Zn and Ga supported catalysts improved significantly,44.81%and 35.99%,respectively.In view of the advantages of Zn element,which is cheap and easy to obtain,low in toxicity and less harmful,further studies on Zn-supported catalysts were carried out.The effect of reaction conditions on the aromatization performance of the zinc-modified catalyst was investigated,and it was found that the highest BTX yield was 46.54%at 500℃and 2 h^-1.Comparing the influence of the Si/Al ratio of molecular sieve,the catalyst with Si/Al ratio of 38 still has the highest BTX yield,and it was pointed out that the poor catalytic effect of molecular sieve with a Si/Al ratio of 50 was the reason for the mismatch of molecular sieve pores（small specific surface area and lacked of mesoporous structure were not conducive to product diffusion）,catalyst acidity（larger proportion of medium and strong acid sites favored dry gas and carbon deposition）and Zn active sites（denser Zn active sites leaded to a more intense hydrogenolysis reaction）.By changing the exchange times of the ion exchange method and the impregnation time of the incipient-wetness impregnation method,the catalytic effects of the catalysts prepared by loading Zn on the HZSM-5（38）carrier in different ways were compared.The results showed that the effect of different preparation methods on aromatization products was highly consistent with the changing trend of Zn loading.Subsequently,the aromatization effects of catalysts with different Zn loadings were studied,and it was found that the direct dehydrogenation ratio was insufficient at low loadings,and a large amount of olefins were still converted by hydrogen transfer;the catalysts with higher loadings had stronger direct dehydrogenation capacity and hydrogenolysis ability,resulting in more carbon deposition and the generation of methane and ethane,were both detrimental to the BTX yield,and the optimal Zn loading was2 wt%.In order to accelerate the recombination desorption of hydrogen atoms to improve the olefin yield and reduce the dry gas generation,Pt was continuously introduced on the zinc-supported catalyst.It was found that the introduction of Pt alone enhanced the hydrogenolysis ability of the catalyst.The hydrogenolysis was inhibited after improving the dispersion of Pt with Sn additives,but the structure of the molecular sieve was greatly damaged by multiple impregnation and calcination,and the yield of BTX was not improved.Finally,the carbon species on the surface of the catalysts after the reaction were studied by FT-IR,showing the absorption peaks of aromatic rings,methyl and methylene groups.