In situ spectroscopic investigation of activation, start-up and deactivation of promoted sulfated zirconia catalysts

Sulfated zirconia (SZ), unpromoted and promoted with 0.5–2.0 wt.% Mn or Fe, was investigated by in situ IR and UV–vis spectroscopy during activation at 723–773 K and interaction with n-butane at 298–323 K. During start-up of the catalysts, the isomerization rate increases with the amount of water that is being formed, consistent with an activation of n-butane via oxidative dehydrogenation. Sulfate is the most likely oxidizing agent because activity is also observed for unpromoted SZ and without reduction of Mn. Isomerized species were also found on the surface of Fe-promoted SZ after exposure to n-butane without water formation indicating other yet unknown initiation pathways. During isomerization, Lewis acid sites on Fe-promoted SZ are blocked by water; the valence of Mn decreases only if the catalyst was previously activated in O2. Stable species absorbing at 2820–2840 cm−1 detected on the surface of SZ and Fe-promoted SZ after contact with n-butane at 298 K are tentatively ascribed to alkoxides, which may be side-products or intermediates. These species differ slightly for the two catalysts, reflecting the different structural and electronic properties of the materials.