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The relative rates of cracking and resultant product distributions for cracking C5–C8olefins over ZSM-5 at 510°C were quantified and rationalized in terms of carbenium ion mechanisms. Conditions were chosen to minimize bimolecular reactions. Cracking rates increase more dramatically with carbon number for olefins than for monomolecular cracking of paraffins, as more energetically favorable modes become available for β-scission of the carbenium ion formed by proton donation to the olefin. Product distributions were used to determine the relative rates of various modes of β-scission, as classified by the types of carbenium ions involved. For hexene and heptene feeds, the most-favorable β-scission mode available (C-type, involving just secondary carbenium ions, for hexene feed; B-type, involving secondary plus tertiary carbenium ions for heptene) accounted for 70–80% of the cracking. Product distribution was independent of which hexene or heptene isomer was fed, since double-bond and skeletal isomerization precedes significant cracking. For 1-octene feed, however, the olefin was nearly all cracked via secondary-tertiary and tertiary-secondary β-scission (after isomerizing to a dimethylhexene) before it isomerized further to the 2,4,4-trimethylpentene isomer, which would be required to undergo the most energetically favored (tertiary-tertiary) form of cracking. A semiquantitative prediction of rates and product distribution for 1-octene cracking could be made, using rates for the various types of β-scission calculated from results with C6–C7feeds. |
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