Reactivity of zeolite hydroxy groups toward σ-donor bases. H–D exchange with 3-methylpentane

Abstract

The H–D exchange of deuteriated Y zeolites, silica–alumina and γ-alumina with 3-methylpentane has been studied by IR spectroscopy. At 373 K exchange ocurred only in the zeolites. Silica–alumina required higher temperatures (523–623 K) and exchange did not occur in γ-alumina in the range of temperature studied. Ultrastable Y zeolite with non-framework alumina (NFA), showed a higher reactivity for H–D exchange and hexane cracking than Y zeolite dealuminated by (NH₄)₂SiF₆. These findings were explained by the difference in acid strength between the zeolites, probably due to the presence of NFA as proposed in the literature, and stress the dependence of H–D exchange on acid strength. The IR study of H–D exchange between 3-methylpentane and Y zeolites having pyridine pre-adsorbed, neutralizing some of the hydroxy groups, showed that the high-frequency (hf) hydroxy group is responsible for the exchange. The low-frequency (If) and NFA hydroxy groups appear to exchange via hydrogen bridges with the hf hydroxy group, rather than by direct interaction with the hydrocarbon. The H–D exchange between solid acids and alkanes could be a qualitative test for acid strength, since only very strong acids would attack σ bonds.