NMR AND THEORETICAL-STUDIES OF SOLID ACIDS - SUPER AND OTHERWISE

Substantial controversy surrounds the role of the isopropyl cation in the reactions of propene on acidic zeolites. Here we report new experi mental and theoretical results that together provide a more detailed d escription of the chemistry of propene on zeolite solid acids. At cryo genic temperatures, propene is weakly bound to the Bronsted site as a pi complex. Density functional theory (DFT) calculations indicate the binding energy of the pi complex is only 2.6 kcal/mol, with essentiall y no charge transfer between the acid site and the alkene. At higher t emperature, propene reacts with the acid site to form a framework-boun d isopropoxyl. DFT predicts this isopropoxyl has a charge of +0.44\e\, far less than the unit positive charge of a free carbenium ion. Assig nment of pi complex and framework bound species is supported by the ex cellent agreement between experimental and theoretical C-13 chemical s hifts. NMR experiments indicate propene does not undergo complete labe l scrambling on zeolite HZSM-5, demonstrating that the isopropyl catio n does not exist on this catalyst (under these conditions). In contras t, complete label scrambling does occur for isopropyl bromide on true superacids such as SbF5 and AlBr3.