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Skeletal isomerizations of n-pentane and n-butane have been studied at 11 bar in a microflow reactor over platinum containing sulfated zirconia (PtSZ) 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(H2)= 2.2 bar. Hexanes and butanes are the main byproducts of pentane isomerization; the C6/C4 ratio is 0.91. Upon increasing the H2 pressure from 2.2 to 6.1 bar, the isomerization rate of n-butane decreases markedly (nH2< −1.1), but that of n-pentane only marginally ((nH2≈ 0). Both changes are perfectly reversible. Kinetic analysis reveals that the major mechanism of C4 isomerization is inter-molecular, but that of C5 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 C10 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. |
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