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 with a high Si/Al ratio was studied. The catalytic activity increases with increasing metal loading. Butenes formed via dehydrogenation over the metallic particles are converted to isobutene over the Brønsted acid sites. The molar fraction of isobutene (in all butenes), which can be taken as a measure for the isomerization activity, increases parallel to the acid site concentration, but is independent of the metal loading. The highest yields of isobutene achieved at 830 K, at 1.8 bar, and with a feed of 10% n-butane and 20% hydrogen were approximately 12.5%. The thermodynamic limit under these conditions is about 22%. The inability to reach the thermodynamic limit is caused by consumption of the primarily formed n-butene by secondary side reactions. The major side reactions are oligomerization and cracking of butenes over Brønsted acid sites leading to propene and pentene. Propene that is formed via this route is hydrogenated to propane over Pt. Consequently, propane is the dominant by-product at high conversions. The metal loading has only a minor influence on the selectivity of the catalyst.