In situ1H MAS NMR investigations of the H/D exchange of alkylaromatic hydrocarbons on zeolites H-Y, La,Na-Y, and H-ZSM-5

In situ1H 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 Ea 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 Ea 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 Ea values determined for the different deuterated aromatic hydrocarbons adsorbed on the zeolites under study show the sequence: benzene (Ea = 46–76 kJ/mol) > ethylbenzene (Ea = 29–41 kJ/mol) > toluene (Ea = 25–32 kJ/mol) >p-xylene (Ea = 20–27 kJ/mol). Generally, this sequence demonstrates the electron donation effect of alkyl groups bound to the aromatic rings (+I effect). The larger Ea 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.