| Light olefins such as ethylene and propylene are very important bulk organic chemical materials,they are mainly produced by naphtha steam cracking technology.Nowadays,with the surge in propylene market demand,traditional steam cracking co-production of propylene is difficult to meet market demand,and has problems such as high energy consumption and equipment investment,heavy environmental pollution and limited adjustment range of light olefins ratio.Therefore,as one of the popular technologies to solve the mentioned problems,the naphtha catalytic cracking process has been developed rapidly in recent years.The ZSM-5 zeolites with rich acid sites are widely used cracking catalysts,the design for their structure and acid properties has always been the focus of researchers.Thus,in this dissertation we have used ZSM-5 zeolites as the catalysts for the naphtha cracking reaction(n-heptane as a model compound),and introduced mesopores into zeolite structure through the soft template method to improve the mass transfer performance.On this basis,we have prepared hierarchical zeolites with suitable acidity through a variety of modification methods,so as to enhance the yield of light olefins and further explore the reaction mechanism.Firstly,a series of hierarchical ZSM-5 zeolites with similar porous structures and different acid properties in external environment have been successfully prepared through chemical vapor deposition method(CVD)by adjusting the reaction temperature and treatment time.In the experiment of adjusting the acid strength,we found that external environment is an important place for primary cracking reaction and bimolecular reaction.The weakening of the acid strength is conducive to increasing the ratio of the cracking rate to the hydrogen transfer rate;At the same time,the change of acid strength has a great influence on the selectivity of C3 and C4 products,but it hardly affects the selectivity of ethylene,indicating that the microporous pores are the main place to generate ethylene compared with external environment which lacks the confinement effect.In the experiment of adjusting the acid amount in external environment,studies have proved that reducing the acid amount will effectively inhibit the occurrence of side reactions and improve the selectivity of light olefins.However,too long treatment time will also lead to the failure to make full use of the acid sites in the internal pores,resulting in the rapid decline of the activity and stability of the catalyst.Finally,through the stability experiments and coke analysis,it can be seen that the reduction of acid amount or acid strength in the external environment can help to reduce the probability of deep reaction of ethylene and propylene and effectively inhibit the coke formation.Secondly,a variety of post treatment methods have been attempted to obtain hierarchical ZSM-5 zeolites with significantly changed acid types without damaging the structure of zeolites.Studies have found that mild oxalic acid treatment can significantly strengthen the acidity,the proportion of strong and weak acids increased from 0.9 to 1.3,while the treatment of H2O2 and NH4Cl can increase the ratio of Br(?)nsted/Lewis acid amount of zeolites from 2.9 to 4.3 and 4.5,respectively.By comparing the reaction performance of all samples,we have found that the improvement in acid strength or Br(?)nsted acid amount can push the cracking activity up,thereby increasing the yield of light olefins.Finally,a series of zeolite samples with a gradually increasing ratio of Br(?)nsted/Lewis acid amount were obtained by extending the H2O2 treatment time.The reaction results show that the increase of Br(?)nsted acid content within a certain range helps to improve the selectivity of light olefins.Compared with commercial ZSM-5 zeolite with the same Si/Al ratio,the total yield of ethylene and propylene has increased by 13%. |
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