| Abstract
| - The most commonly cited example of a transition state shape selective reaction, m-xylenedisproportionation in zeolites, is examined to determine if the local spatial environment of a reaction cansignificantly alter selectivity. In the studied reaction, ZPE-corrected rate limiting energy barriers are 136kJ/mol for the methoxide-mediated pathway and 109 to 145 kJ/mol for the diphenylmethane-mediatedpathway. Both pathways are likely to contribute to selectivity and disfavor one product isomer (1,3,5-trimethylbenzene), but relative selectivity to the other two isomers varies with pore geometry, mechanisticpathway, and inclusion of entropic effects. Most importantly, study of one pathway in three different commonzeolite framework types (FAU, MFI, and MOR) allows explicit and practically oriented consideration ofpore shape. Variation of the environment shape at the critical transition states is thus shown to affect thecourse of reaction. Barrier height shifts on the order of 10−20 kJ/mol are achievable. Observed selectivitiesdo not agree with the transition state characteristics calculated here and, hence, are most likely due toproduct shape selectivity. Further examination of the pathways highlights the importance of mechanisticsteps that do not result in isomer-defining bonds and leads to a more robust definition of transition stateshape selectivity.
|
