n-butane isomerization over transition metal-promoted sulfated zirconia catalysts: effect of metal and sulfate content

n-Butane isomerization has been investigated over transition metal-promoted sulfated zirconia catalysts. Fe alone was a more efficient promoter than Mn and Cr, and mixtures of Fe-Mn, Fe-Cr, and Fe-V. Promotion of a commercial sulfated zirconia (4.8% sulfate) with iron amounts in excess of 2% depressed the conversion and increased the deactivation rate. Increasing reaction temperatures improved the conversion and decreased the induction period. At 70 degreesC and above, the induction period was suppressed. The influence of activation temperature was studied over a Fe-sulfated zirconia catalyst (with 2% Fe and 4.8% SO42- pre-calcined at 600 degreesC). The best conversion was achieved when the catalyst was activated at 350 degreesC in air. This temperature was apparently sufficient to generate the redox sites active at low reaction temperature. Activation in flowing helium depressed the catalytic activity and increased the induction period. The sulfate content had a significant effect on the catalyst performance, For 2% Fe-promoted zirconia, a maximum conversion was found for 7.5% SO42-, probably related with a better balance (or synergism) of the redox and acid sites involved in a bimolecular mechanism. The time required to reach a maximum of conversion (induction period) decreased with increasing total acidity, i.e, with sulfate content. The series of catalysts with different amounts of sulfate has been characterized by X-ray diffraction, nitrogen adsorption-desorption isotherms, TGA, ammonia-TPD, DRIFT, Raman, and X-ray photoelectron spectroscopy. (C) 2001 Elsevier Science B.V. All rights reserved