PNNL

Abstract

Substantial controversy surrounds the role of the isopropyl cation in the reactions of propene on acidic zeolites. Here we report new experimental and theoretical results that together provide a more detailed description of the chemistry of propene on zeolite solid acids. At cryogenic temperatures, propene is weakly bound to the Brønsted site as a p complex. Density functional theory (DFT) calculations indicate the binding energy of the p complex is only 2.6 kcal/mol, with essentially no charge transfer between the acid site and the alkene. At higher temperature, propene reacts with the acid site to form a framework-bound isopropoxyl. DFT predicts this isopropoxyl has a charge of +0.44 |e|, far less than the unit positive charge of a free carbenium ion. Assignment of p complex and framework bound species is supported by the excellent agreement between experimental and theoretical 13 C chemical shifts. NMR experiments indicate propene does not undergo complete label scrambling on zeolite HZSM-5, demonstrating that the isopropyl cation does not exist on this catalyst (under these conditions). In contrast, complete label scrambling does occur for isopropyl bromide on true superacids such as SbF5 and AlBr3 .