The accessibility of sites active in the dissociative adsorption of aromatic hydrocarbons in FeZSM-5 zeolite

The competitive adsorption of aromatic hydrocarbons (benzene, methyl- and ethyl-benzene) and water on FeZSM-5 zeolites have been investigated by means of temperature-programmed desorption coupled with mass spectrometry (TPD/MS). The incorporation of iron in zeolite was done by aqueous ion exchange using dilute solutions of Fe complexes (ferric citrate and ferrous oxalate) and ferric nitrate. Diffuse reflectance UV-Vis spectroscopy and temperature-programmed reduction (TPR) were applied to characterize active sites on investigated zeolites. The existence of different iron species on FeZSM-5 zeolites was revealed. It has been demonstrated that the activity of the Fe exchanged zeolite depends on the iron salt used for ion exchange. The isolated, dispersed ions, which are often considered to be essential for adsorption and catalysis, were obtained with high yield only by ion exchange in the presence of ferrous oxalate. TPD/MS measurements show that aromatic hydrocarbons adsorb on specific, strong active sites in hydrated zeolites. The binding occurred when organic molecules replace water previously adsorbed at the same sites. Benzene showed non-dissociative adsorption/desorption, while new mass fragments were recorded during methyl-benzene and ethyl-benzene desorption implying their dissociative adsorption/desorption on active sites in hydrated zeolites.