Catalytic isomerization of 2-pentene in H-ZSM-22 - A DFT investigation

The skeletal isomerization of a 2-pentene molecule catalyzed by acidic ZSM-22 was investigated by ab initio DFT studies. Two different scenarios proposed in the literature were tested. First a reaction including an alkyl shift was considered: a methyl or ethyl group is detached from the carbenium ion chain and reattached at another site in the residual hydrocarbon chain. However, this mechanism is rather unlikely, since the alkyl ion is a high-energy species, so its detachment from the carbenium ion induces a high activation energy. We find that the more likely pathway for skeletal isomerization inside the channels of ZSM-22 involves the rearrangement of the carbenium ion into a protonated dimethylcyclopropane and implies the formation of relatively stable secondary carbenium ions as transient intermediates. (C) 2003 Elsevier Science (USA). All rights reserved.