In Situ Spectroscopic Study of Isomerisation of Light Alkanes over Sulfated Zirconia Catalysts

In situ DRIFT and DR-UV/Vis spectroscopies were performed during n-butane (1 or 5 kPa partial pressure, 358 – 453 K) and n-pentane (1 kPa, 298 – 323 K) isomerization in the presence of two different sulfated zirconia catalysts: a sulfate containing ordered mesoporous zirconia of the MCM-41 structure and a conventional sulfated zirconia catalyst with a tetragonal zirconia bulk structure. After a 100 min induction period, the mesoporous zirconia deactivates slowly during n-butane isomerization at 453 K while an absorption band at 285 nm grows, indicating the formation of unsaturated surface species. The conventional catalyst which is more active and produces the same maximum rate already at 378 K deactivates rapidly and a band grows at 310 nm, indicating the formation of allylic carbocations on the surface. During n-pentane isomerization, both catalysts deactivate rapidly while bands at 285 and 310-320 nm (mesoporous) and 335 nm (conventional) are formed. The spectra clearly show that the surface species on the two catalysts differ, although in principle the same gas phase products are observed, indicating an influence of the underlying wall or tetragonal bulk structure, respectively