Temperature-programmed desorption of aromatic hydrocarbons on silicalite-I and ZSM-5-type zeolites

Abstract

Temperature-programmed desorption (t.p.d.) of benzene, toluene, p-xylene, o-xylene, m-xylene, and mesitylene on HZSM-5 in a helium stream from 353 to 673 K at different heating rates (2.5-25 K. mine^-1) has been studied using a gas chromatographic technique and the heat of sorption of the hydrocarbons evaluated from the t.p.d. data. The influence of the Si/Al ratio, degree of Na⁺-exchange, calcination temperature (or dehydroxylation), and poisoning of strong acid sites on the t.p.d. and heat of sorption of benzene on the zeolite has also been investigated. The decrease in the heat of sorption of benzene with the increase in the Si/Al ratio, extent of dehydroxylation, and poisoning of the strong protonic acid sites with pyridine has indicated the involvement of surface hydroxyl groups responsible for protonic acidity in the sorption of HZSM-5. The increase in the heat of sorption of aromatic hydrocarbons with the decrease in their ionization potential (or with the increase in the number of CH₃ groups) has indicated the occurrence of strong interactions between the p-electrons of benzene nucleus and the hydroxyl groups. The higher heat of sorption of benzene in Na-containing ZSM-5 has revealed that the interaction of the benzene molecule with the Na⁺ cation is much more stronger than that with the protonic acid site in HZSM-5. Further, the t.p.d. of benzene at different sorbate loadings has indicated site heterogeneity on HZSM-5.