| Abstract
| - Ab initio calculations were performed to study the H/H exchange between light alkanes (methane, ethane,propane, and isobutane) in protonated zeolites. The Brönsted acid site of the zeolite was represented by a T3cluster (T = Si, Al). The results of the calculations, at the B3LYP/6-31G** and MP2/6-31G**//HF/6-31G**levels, indicated that the transition state resembles a pentacoordinated carbonium ion. The enthalpy of activationwas similar, regardless of the alkane and the type of hydrogen being exchanged. The ΔH⧧ values, at roomtemperature, ranged from 32.2 kcal/mol for methane to 36.2 kcal/mol for the exchange of the tertiary hydrogenof isobutane, both at the B3LYP/6-31G** level. These results are not in complete agreement with experiments,as it has been shown that for isobutane only the primary hydrogens exchange at temperatures near ambient.This disagreement may be attributed to the fact that the cluster model employed in the calculations neitherincludes the electrostatic effects of the zeolite cavity nor takes into account steric repulsion associated withthe framework.
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