Dehydroisomerization of n-butane over Pt-ZSM5 (I): Effect of the metal loading and acid site concentration

The dehydroisomerization of n-butane to isobutene over Pt-ZSM5 catalysts wi th a high Si/Al ratio was studied. The catalytic activity increases with in creasing metal loading. Butenes formed via dehydrogenation over the metalli c particles are converted to isobutene over the Bronsted acid sites. The mo lar fraction of isobutene tin all butenes), which can be taken as a measure for the isomerization activity, increases parallel to the acid site concen tration, but is independent of the metal loading. The highest yields of iso butene achieved at 830 K, at 1.8 bar, and with a feed of 10% n-butane and 2 0% hydrogen were approximately 12.5%. The thermodynamic limit under these c onditions is about 22%. The inability to reach the thermodynamic limit is c aused by consumption of the primarily formed n-butene by secondary side rea ctions. The major side reactions are oligomerization and cracking of butene s over Bronsted acid sites leading to propene and pentene. Propene that is formed via this route is hydrogenated to propane over Pt. Consequently, pro pane is the dominant by-product at high conversions. The metal loading has only a minor influence on the selectivity of the catalyst. (C) 1999 Academi c Press.