Active sites in the alkylation of toluene with methanol : a study by selective acid-base poisoning

Selective acid–base poisoning of the alkylation of toluene with methanol was studied over alkali and alkaline-earth exchanged Y zeolites. Surface acid–base properties of the samples were determined by infrared spectroscopy using carbon dioxide and pyridine as probe molecules. Selective poisoning experiments were performed by co-feeding either acetic acid or 3,5-dimethyl pyridine. Kinetic deactivation parameters were determined by modeling experimental data with a deactivation model with residual activity (DMRA). MgY produced essentially xylenes and alkylated C9+ compounds. This zeolite was deactivated only by co-feeding 3,5-dimethyl pyridine, thereby indicating that exclusively acid sites are involved in the ring alkylation of toluene with methanol. In contrast, zeolite CsY formed mainly styrene and ethylbenzene, and was strongly deactivated by the addition of either acid or base compounds. Infrared characterization showed that zeolite CsY contains Lewis acid (Cs+)–base (O2-) pairs capable of adsorbing bidentate carbonates but does not exhibit Brönsted acidity. Thus, selective poisoning results and sample characterization showed that bimolecular side-chain alkylation reaction on CsY requires a specific surface acid–base pairs configuration not only for activating toluene and methanol, but also for promoting the rate-limiting step.