Improving the stability of H-mordenite for n-butane isomerization

The conversion of n-butane over mordenite-based catalysts in the presence of hydrogen and water was investigated for reaction temperatures between 523 and 623 K. Special attention was given to the influence of Pt upon catalytic activity, selectivity, and stability. With parent mordenite the catalytic activity for n-butane conversion decreased markedly after a short time on stream. Deactivation can be minimized by hydrogen (in the presence of Pt) and water addition. Both measures are thought to reduce the concentration of intermediate olefins in the zeolite pores. The best results with respect to selective conversion of n-butane to isobutane were obtained for 0.25 wt% Pt on mordenite in the presence of hydrogen. Higher concentrations of Pt in the catalyst are shown to be detrimental for n-butane isomerization, because of increasing selectivity to hydrogenolysis. A detailed mechanistic scheme for n-butane conversion over Pt-containing mordenites is presented. n-Butane conversion is concluded to occur via a bimolecular mechanism involving a complex network of hydrogen transfer, oligomerization/cracking, isomerization, hydrogenation/dehydrogenation, and hydrogenolysis. (C) 1997 Academic Press.