MECHANISTIC CONSIDERATIONS IN ACID-CATALYZED CRACKING OF OLEFINS

The relative rates of cracking and resultant product distributions for cracking C-5-C-8 olefins over ZSM-5 at 510 degrees C were quantified and rationalized in terms of carbenim ion mechanisms. Conditions were chosen to minimize bimolecular reactions. Cracking rates increase more dramatically with carbon number for olefins than for monomolecular cr acking of paraffins, as more energetically favorable modes become avai lable for beta-scission of the carbenium ion formed by proton donation to the olefin. Product distributions were used to determine the relat ive rates of various modes of beta-scission, as classified by the type s of carbenium ions involved. For hexene and heptene feeds, the most-f avorable beta-scission mode available (C-type, involving just secondar y carbenium ions, for hexene feed; B-type, involving secondary plus te rtiary carbenium ions for heptene) accounted for 70-80% of the crackin g. Product distribution was independent of which hexene or heptene iso mer was fed, since double-bond and skeletal isomerization precedes sig nificant cracking. For 1-octene feed, however, the olefin was nearly a ll cracked via secondary-tertiary and tertiary-secondary beta-scission (after isomerizing to a dimethylhexene) before it isomerized further to the 2,4,4-trimethylpentene isomer, which would be required to under go 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 beta-scission calculated from results with C-6-C-7 feeds. (C) 1996 Aca demic Press, Inc.