PENTANE AND BUTANE ISOMERIZATION OVER PLATINUM PROMOTED SULFATED ZIRCONIA CATALYSTS

Skeletal isomerizations of n-pentane and n-butane have been studied at 11 bar in a microflow reactor over platinum containing sulfated zirco nia (Pt-SZ) catalysts. The isomerization rate of n-pentane is about se ven times higher than that of n-butane; selectivity to i-pentane is 91 .6% at p(H-2) = 2.2 bar, Hexanes and butanes are the main byproducts o f pentane isomerization; the C-6/C-4 ratio is 0.91. Upon increasing th e H-2 pressure from 2.2 to 6.1 bar, the isomerization rate of n-butane decreases markedly (n(H2) < - 1.1), but that of n-pentane only margin ally ((n(H2) approximate to 0). Both changes are perfectly reversible. Kinetic analysis reveals that the major mechanism of C-4 isomerizatio n is inter-molecular, but that of C-5 isomerization is intra-molecular . With n-pentane, small amounts of hexanes and butanes are formed in a parallel, apparently inter-molecular reaction, one presumably involvi ng a C-10 intermediate. The qualitative difference between the isomeri zation mechanisms of butane and pentane parallels the known chemistry in liquid superacids. Catalysis via ''chemisorbed carbenium ions'' on heterogeneous catalysts thus strongly resembles catalysis via ''solvat ed carbenium ions'' in liquid acids.