Hydroisomerization of n-hexane over Pt/sulfated zirconia: activity, reversible deactivation, and surface analysis Original Research Article

Transformations of n-hexane on a calcined Pt/ZrO2/SO42− catalyst were studied in a laboratory reactor in hydrogen excess between 420 and 600 K below atmospheric pressure. Methylpentanes (MPs) were formed with selectivities between 80 and 100%. Typical dimethylbutane (DMB) selectivities were around 10%. Higher temperatures increased the overall rates with slightly lower isomer and higher fragment selectivities. No selectivity changes were observed at 483 K in long runs up to 5 h; deactivation (with a marked drop of isomerization at the expense of fragmentation) was, in turn, significant at 603 K. Regeneration by oxygen could restore the initial activity in a reproducible way. Nevertheless, a gradual activity decrease appeared after a summarized length of use lasting 21 h. The parallel selectivity changes were less marked. X-ray photoelectron spectroscopy (XPS) indicated carbon accumulation and a slight removal of sulfur after this period. The valence states of the Pt, O, Zr, S components remained approximately the same. We attribute reversible deactivation during long runs to carbon accumulation. Regeneration with oxygen must have removed most of that carbon. At the same time, it oxidized a part of the residual surface carbon to COOH entities. These carboxylic species could serve as hydrogen catchers hampering the hydrogen traffic between metallic and acidic sites, causing the deterioration of isomerization selectivity at lower hydrogen pressures and higher temperatures.