Kinetics of H/D Exchange for n-Butane on Zeolite H-ZSM-5 Studied with 1H MAS NMR In Situ

Abstract

The kinetics of hydrogen (H/D) exchange between Brønsted acid sites of zeolite H-ZSM-5 and deuterated n-butanes (n-butane-d10 and n-butane-1,1,1,4,4,4-d6) has been monitored by 1H magic-angle spinning (MAS) NMR spectroscopy in situ within the temperature range of 423−448 K. The initial part of the kinetics is defined mainly by the hydrogen exchange, whereas the final part is strongly influenced by the chemical transformation of the alkane. Analysis of the initial part has been performed on the basis of consecutive, parallel, and cyclic kinetic schemes of the H/D exchange. It has been found that both the methyl and methylene groups of n-butane are directly involved in the exchange with acidic SiOHAl groups of the zeolite. No intramolecular hydrogen exchange between the methyl and the methylene groups of the adsorbed n-butane has been detected. Similar rates of the direct exchange of either the methyl or methylene group with acidic SiOHAl groups and the apparent activation energy of 108 kJ mol−1 are rationalized in terms of the carbonium ion mechanism of the exchange with the involvement of a pentacoordinated carbon atom in a transition state.