Using in situ IR Spectroscopy to Understand Selective Alkylation of Aromatics over Zeolites

In situ IR spectroscopy in combination with kinetic measurements was used to clarify the mechanism of the methylation of toluene over zeolitic catalysts. Over basic zeolites (i.e. Rb-X), the main surface species under reaction conditions is toluene strongly bound with the aromatic ring to the cation and with the methyl group to the lattice oxygens of the zeolite. The activated carbon atom of the methyl group reacts with the positively activated C-atom of methanol (formaldehyde) to yield styrene as the main product. Over acidic zeolites (i.e. HZSM-5), methanol is preferentially adsorbed over toluene, the rate of methylation is directly proportional to the surface concentration of the activated methanol species (methoxonium ions at low and methoxy groups at high reaction temperatures). The aromatic ring of toluene is attacked by the reactive alcohol methyl group, giving the xylene isomers as reaction products.