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 zirconia (Pt-SZ) catalysts. The isomerization rate of n-pentane is about seven 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 of pentane isomerization; the C-6/C-4 ratio is 0.91. Upon increasing the 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 marginally ((n(H2) approximate to 0). Both changes are perfectly reversible. Kinetic analysis reveals that the major mechanism of C-4 isomerization 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 involving a C-10 intermediate. The qualitative difference between the isomerization 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 ''solvated carbenium ions'' in liquid acids.