Cracking Reaction Mechanism and Reaction Pathway of 1-Hexene Over H-ZSM-5 Catalyst at High Temperature
The cracking reactions of 1-hexene over H-ZSM-5 zeolite and quartz sand at reaction temperatures from 500 to 750 ℃ were investigated in a fixed bed reactor.On this basis,a catalytic/thermal cracking proportion model was established to quantitatively discuss the relationship between catalytic cracking and thermal cracking at high temperatures.In addition,the reaction pathway was deduced and estimated according to the products distribution of 1-hexene cracking over H-ZSM-5 zeolite.The results indicate that the high temperature reaction of 1-hexene over H-ZSM-5 zeolite is dominated by catalytic cracking.Even at 750 ℃,the ratio of 1-hexene feed converted by the catalytic cracking reaction is still as high as 91.32％.Methane,ethylene,and propylene in the products are mainly derived from catalytic cracking rather than thermal cracking reaction.It is found by estimating the cracking pathway of 1-hexene that the proportion of bimolecular oligomerization cracking reaction has been decreased from 74％at 500 ℃ to 0 at 700 ℃.Monomolecular direct cracking is beneficial to the formation of small molecule olefins such as ethylene and propylene,while bimolecular oligomerization cracking is advantageous to the formation of larger molecule olefins.The reason for the high yield of ethylene and propylene at high temperatures may be that high temperature can promote the monomolecular direct cracking of 1-hexene.
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