The Preparation Of ZSM-5-based Hierarchical Zeolites And The Performances For Catalytic Cracking Of N-octane To Light Olefins

There is no doubt that ethylene and propylene play a significant role in the industrial development of a country.In terms of polymers or chemicals,the demand for these two kinds of light olefin is far greater than the current supply level.It would continue in short supply situation in the coming period.As the main application in industrial production,the steam cracking itself has some disadvantages such as high reaction temperature,high energy consumption and severe limitation for feedstocks.In contrast,the catalytic cracking technology has an excellent prospect to overcome the disadvantages of the steam cracking,and eliminate the huge dependence waste of water resource in MTO technology.ZSM-5 zeolite has been widely used in petrochemical and fine chemical catalysis now.However,the micropore of ZSM has its own shortcomings because of the small scale pore diameter,easy carbon deposition and the limited diffusion.Hierarchical zeolites materials can provide multidimensional spatial networks on different length scales with improved characteristics relevant to molecular diffusion and coking,and they also have good accessability to the reactants.It is becoming the research focus that preparing high performance hierarchical ZSM-5 zeolite and quantitatively evaluating its catalytic activity with pore structure.The main study has two aspects in this dissertation.One is preparing two kinds of hierarchical catalysts:?1?combining mesoporous materials and nano ZSM-5 to form micro-mesoporous composite catalysts,?2?using mesoporous template agent to synthesize hierarchical ZSM-5 zeolite.The other is to investigate the performances for catalytic cracking of n-octane over two kinds of zeolites,and using hierarchy factor to discuss the hierarchical ZSM-5 with single phase and explore the quantitative evaluation method of pore structure to evaluate the relationship between the pore structure and the catalytic activity.In this study,KIT-5/ZSM-5 meso-microporous composite materials were synthesized from zeolite nano ZSM-5 by self-assembly method using F127 as the mesostructured directing agent under the acidic condition in this dissertation.The optimal conditions?the adding amount of HCl,the adding sequences of nano ZSM-5 and the adding amount of nano ZSM-5?for the synthesis of KIT-5/ZSM-5 were systematically investigated and obtained as follows: n_TEOS:n_HCl=1.07,m_ZSM-5:m_SiO2=2,and nano ZSM-5 and silicon source of KIT-5 were simultaneously added.The results showed that with the change of hydrochloric acid content,the microporous volumes of samples were decreased,which was related to the hydrochloric acid inhibiting the crystallization of ZSM-5.K/Z-H?1.07?sample showed higher catalytic activity.At the same time,the different adding orders of nano ZSM-5 have a certain influence in catalytic activity.Since ZSM-5 zeolite could provide more acid sites in the catalytic cracking,the ZSM-5 with content of m_ZSM-5:m_SiO2=2 exhibited better catalytic activity.By evaluating the performances of as-synthesized composite catalyst,the conversion of n-octane can reach 99%,which is larger than those of the mechanical mixed K-Z and nano ZSM-5 sample?94% and 90%?.At the reaction temperature of 675?,the yield of ethylene plus propylene and total olefin of the composite catalyst were 66% and 77%,which are higher than the 56% and 69% of nano ZSM-5 respectively.Composited K/Z showed higher ethylene plus propylene selectivity.At the same time,though K/Z and mechanical mixed K-Z had similar acidity,they exhibited a big difference in catalytic activity,which revealed that K/Z composite material had more reasonable connectivity and good composite effect.In the second study system,a series of hierarchical ZSM-5 zeolites with single phase were synthesized firstly by changing the addition amount of mesoporous template agent CTAB.The typical physico-chemical properties of the as-synthesized catalysts were characterized by the means of XRD,BET,SEM,TEM and NH₃-TPD,and the hierarchy factor of each catalyst was calculated.The performances of the as-prepared hierarchical ZSM-5 zeolites for the catalytic cracking of n-octane were also evaluated in a fixed bed microreactor.The results revealed that the ZSM-5 zeolite with mesopores was successfully synthesized.The hierarchical ZSM-5 zeolites with adding content of 0.14-0.20 of CTAB show a more regular,cubic morphology and the typical rough surface of the mesoporous zeolite.The pore diameter distribution was relatively concentrated and the mesopores diameter was also uniform.Among the series of catalysts,the sample with the adding content of CTAB of 0.20 during preparation has the highest catalytic activity,with the conversion above than 99% and the yield of ethylene plus propylene of 70%.The hierarchy factor values of samples were associated with the catalytic activity,which showed that the hierarchy factor values of as-prepared catalysts through dual template distributed between 0.10 to 0.14.In the case of similar acidity of each sample,during 400-550?,the hierarchy factor values had positive correlation with the conversion of noctane,selectivity and the yield of ethylene plus propylene,and the yield of aromatics.In contrast,in the high-temperature region,the conversion of n-octane remained stably,the hierarchy factor values had negative correlation with selectivity and the yield of ethylene plus propylene,but positive correlation with the yield of aromatics.