Electron spectroscopy of sulfated zirconia, its activity in n-hexane conversion and possible reasons of its deactivation

Sulfated zirconia catalysts were prepared and characterized by X-ray photoelectron spectroscopy taken in the dried state (fresh) and after calcination at 900 K (calc.). A maximum activity was observed as a function of the calcination temperature. The Zr 3d region showed that any Zr hydroxide in the dried catalyst transformed into zirconium oxide upon calcination. The O 1s peak could be fitted by two components corresponding to ZrO2 and sulfate, respectively. Sulfur was present as sulfate. Both catalysts showed activity in n-hexane conversion (including isomerization) between 300 and 473 K. The activity of the calcined catalyst was much higher. The main products were isopentane and isobutane, along with 2-methyl- and 3-methylpentane. The activity was not stable and only a limited amount of n-hexane transformed before final deactivation. This observation pointed to a limited amount of active sites able to start the reaction. The activity could be fully regenerated by oxygen treatment. Thus, the "oxidative" start of the reaction [A. Ghenciu, D. Farcasiu, Catal. Lett. 44 (1997) 29] may have also played a role apart from those on strong acid sites. Deactivation may have been due to a partial reduction of sulfate groups rather than to carbon accumulation, as shown also by the minor amounts of S4+ detected by XPS. Parallel isomerization and splitting of hexane into two C-3 units may occur, followed by the formation of surface C-9 units, the latter being intermediate of larger fragments. (C) 2002 Elsevier Science B.V. All rights reserved.