Butane isomerization over platinum promoted sulfated zirconia catalysts

The isomerization of n-butane to i-butane has been studied at 11 bar in a microflow reactor over sulfated zirconia (SZ) and platinum containing sulfated zirconia (Pt-SZ) catalysts. In the presence of H-2 a significantly higher temperature is required for isomerization over SZ than in its absence. The rate over SZ is higher with n-butane containing 33 ppm butene as an impurity than with a feed that is pre-equilibrated over a Pt/SiO2 catalyst to a much lower butene content. Over Pt-SZ the reaction rate is higher, because any butene consumed is rapidly regenerated; the conversion is perfectly stable in 83 h runs, selectivity to i-butane is 95%; i-pentane and propane are the main byproducts. The activation energy is 53 kJ mol(-1). Upon increasing the pressure of H-2 from 1.1 to 6.6 bar, the reaction rate was found to decrease in a perfectly reversible fashion. Kinetic analysis reveals that the reaction order is negative in H-2 (-1.1 to -1.3 depending on the temperature) and positive in n-butane (+1.3 to + 1.6), indicating that the mechanism of this isomerization is intermolecular: butene is formed and reacts with adsorbed C-4-carbenium ions to adsorbed C-8 intermediates which isomerize and undergo beta-fission to fragments with i-C-4 structure. This mechanism is confirmed over Pt-SZ by isotopic labelling experiments, though at much lower pressure, using double labelled (CH3)-C-13-CH2-CH2-(CH3)-C-13. The primary reaction product consists of i-butane molecules, containing zero, one, two, three and four C-13 atoms in a binomial distribution.