In situ 1H MAS NMR investigations of the H/D exchange of alkylaromatic hydrocarbons on zeolites H-Y, La,Na-Y, and H-ZSM-5

In situ ¹H MAS NMR spectroscopy was used to investigate the H/D exchange behavior of benzene-d₆, ethylbenzene-d₁₀, toluene-d₈, and p-xylene-d₁₀ with Brønsted acid sites in different zeolite catalysts. In the temperature range of 303 to 393 K, the H/D exchange takes place exclusively between zeolitic bridging OH groups and deuterons bound to the aromatic rings of adsorbed compounds. The activation energies E_a of the H/D exchange of same deuterated aromatics adsorbed on different zeolites have the sequence: H-Y ≥ La,Na-Y > H-ZSM-5. The decrease of the E_a values correlates well with the increase of the strength of Brønsted acid sites determined via the low-field shift of the ¹H MAS NMR signal of bridging OH groups upon adsorption of deuterated acetonitrile (CD₃CN). The E_a values determined for the different deuterated aromatic hydrocarbons adsorbed on the zeolites under study show the sequence: benzene (E_a = 46-76 kJ/mol) > ethylbenzene (E_a = 29-41 kJ/mol) > toluene (E_a = 25-32 kJ/mol) >p-xylene (E_a = 20-27 kJ/mol). Generally, this sequence demonstrates the electron donation effect of alkyl groups bound to the aromatic rings (+I effect). The larger E_a value of ethylbenzene in comparison with that of toluene indicates an interaction of ethylbenzene with the zeolite framework via its ethyl group leading to a lower electron density and a decrease of the +I effect of this alkyl group towards the aromatic ring.