Isomerization of n-butane over H-mordenites under nitrogen and hydrogen: Influence of the acid site density

摘要

n-Butane transformation was investigated in a flow reactor at 523 K over a series of H-mordenite samples with Si/Al ratios of 6.6, 10, 20, 60, and 80, Under nitrogen, butane isomerization is accompanied by the formation in significant amounts of propane and pentanes. The reaction order with respect to n-butane is close to 2 for isobutane, for propane, and for pentane formation. These observations are in favor of a bimolecular mode of n-butane transformation involving successively formation of C-8 carbenium ions, isomerization, and cracking of these C-8 carbenium ions, A large positive effect of the density of acid sites is found, which can be related to the demanding character of this bimolecular process: two acid sites are required for the formation of the two sec-butyl carbenium ions involved as intermediates, Hydrogen strongly inhibits the transformation of n-butane, which can be explained by a decrease in the concentration of sec-butyl carbenium ions, An increase in the selectivity to isobutane and a decrease in the reaction order are observed, which seems to indicate participation of the very difficult monomolecular process (through the very unstable isobutyl carbenium ion) in butane isomerization. (C) 1998 Academic Press. [References: 31]