The Study On The Preparation Of HZSM-5@mesoSiO₂ Based Catalysts And Their Catalytic Performance In Catalytic Cracking Of N-Butane

Light olefins(ethylene,propylene and butenes)are key building block for the petrochemical industry.Especially,the output of ethylene has become an important symbol to measure the development level of petrochemical industry of a country.Nowadays,more than 70% ethylene is produced through the steam cracking of naphtha in China.However,this industrial process has disadvantages such as large project investment,high reaction temperature,and low yield of propylene.At the same time,there are a large amount of C4 alkanes that have not been effectively utilized in China.From above,developing C4 alkanes catalytic cracking technology for ethylene and propylene production is of great importance.The key point of developing this technology is to design highly efficient cracking catalyst.Among the catalysts for catalytic cracking of hydrocarbons,HZSM-5 has raised great attention owing to its moderate acidic property and special pore structure.In this dissertation,we aim for developing highly efficient catalyst for n-butane catalytic cracking from the following two aspects.Firstly,constructing a hierarchical pore system on the basis of zeolite which can enhance diffusion property.Secondly,designing metal-zeolite bifunctional catalyst containing dehydrogenation function and cracking function.Core-shell structured composite materials consisting of HZSM-5 zeolite as core and mesoporous silica as shell were synthesized through sol-gel process by using tetraethyl orthosilicate(TEOS)as the shell precursor.As a result,micro/mesoporous zeolitic composites with zeolite structure maintained were successfully obtained.During the process of preparing the core-shell structure,mesoporous silica shell with different thickness was tuned by varying the mass ratio of zeolite to TEOS.Transmission electron microscopy images show the shell thickness of as-prepared core-shell materials is uniform,and the mesopores are nearly perpendicular to the surface of zeolite core.Meanwhile,the diffusion properties of n-butane in core-shell materials and HZSM-5 have been investigated by zero length column(ZLC)technique.The study indicated that the diffusion of n-butane in core-shell structured materials is faster than that in HZSM-5.For n-butane catalytic cracking reaction,core-shell structured materials have higher n-butane conversion and light olefins yield compared to HZSM-5.When the reaction temperature is 550 oC,the cracking conversion of n-butane over the optimized core-shell structured material is nearly 10 percentage points higher than that of HZSM-5.On the basis of the above-mentioned core-shell catalysts,Pt with dehydrogenation function was introduced into the catalyst,and the effect of the interaction between the dehydrogenation function of Pt and the cracking function of zeolite on the catalytic cracking performance of n-butane was investigated.The optimized core-shell structured composite and HZSM-5 can be act as supports to prepare bifunctional catalysts,0.5Pt/HZSM-5 and xPt/ZSC,by impregnation method.Transmission electron microscopy images and CO-FTIR results show the core-shell material could efficiently promote Pt dispersion more homogeneously than that on HZSM-5.The statistical mean size of Pt particles in 0.5Pt/HZSM-5 was much larger than that over xPt/ZSC.Owing to the introduction of Pt,both 0.5Pt/HZSM-5 and xPt/ZSC show a remarkably improved activity in the n-butane catalytic cracking compared with HZSM-5 and ZSC respectively,in which the activity of 0.5Pt/ZSC increased by a large extent.When the reaction temperature is 575 oC,the n-butane conversion is ca.78% for 0.8Pt/ZSC with the improvement of 11 percentage points compared to that of ZSC,and the yield of ethylene,propene,butene plus BTX is ca.54.3% with the enhancement of 10 percentage points.