Proton exchange of small hydrocarbons over acidic chabazite: Ab initio study of entropic effects

The proton-exchange reaction of a series of short hydrocarbons over an acidic zeolite (chabazite) was studied using periodic density functional theory (DFT) calculations. It was found that the chain length of hydrocarbons does not have a significant effect on the height of the potential-energy barrier. The experimentally observed regioselectivity between methyl and methylene groups in propane and between methyl and methine groups in isobutane was shown to be an entropic effect. In addition to the direct H-exchange, a mechanism mediated by a methylpropene molecule recently suggested by experimentalists was explored. It was found that entropy plays a very important role in driving the reaction.