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

Abstract

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 Fepromoted 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 O 2 . 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.

Alkane isomerization over sulfated zirconia solid acid system

“…Later, Klose et al also confirmed the formation and adsorption of water under isomerization reaction conditions by in situ IR investigations . Interestingly, they found that the isomerization rate was in direct proportion to the amount of water accumulated on the surface, which further confirmed the oxidative activation of alkane by sulfate species.…”

“…Later, Klose et al also confirmed the formation and adsorption of water under isomerization reaction conditions by in situ IR investigations. 70 Interestingly, they found that the isomerization rate was in direct proportion to the amount of water accumulated on the surface, 6 which further confirmed the oxidative activation of alkane by sulfate species. Although the oxidative activation pathway can well explain the formation of carbenium ions in alkane isomerization over SZ catalyst, there are several questions still under debate.…”

“…More importantly, the adsorption of above molecules all results in a decrease in the covalency degree of neighboring S═O bonds, since it is observed that the FTIR band of S═O bonds (approximately 1390 cm −1 ) significantly shifted to the low frequency. [6][7][8]23,84,94 This indicates the decrease in electron-withdrawing and polarization ability of sulfate species on O─H bonds in adjacent Zr─OH groups, resulting in the decrease of acid strength. Namely, the oxidizing ability and superacidity are interacted.…”

“…For example, the in situ Fourier transform infrared (FTIR) investigations conducted by several groups unequivocally confirmed the possibility of oxidative activation of n-alkane on SZ catalysts. [5][6][7][8] Additionally, the advanced theoretical calculations give a new approach to reveal the surface structure and the nature of active sites. 5,[9][10][11] Haase and Sauer found that the most stable configurations of sulfur species on SZ surface were tridentate sulfate anions or tridentate SO 3 complexes on the (1 0 1) and (0 1 1) surface of tetragonal zirconia.…”

The Balance Between Reactivity and Stability of Modified Oxide Surfaces Illustrated by the Behavior of Sulfated Zirconia Catalysts

“…The observed variations in color from bluish to grayish indicate a change in the manganese valence, and shrinkage of the unit cell volume possibly indicates more oxygen vacancies. Manganese in these catalysts can be reduced during heating in inert gas [42,81]; it is thus conceivable that reduction takes place over months in the O 2 -depleted atmosphere of a glovebox. Activation was performed in inert gas so that a reduction would not be reversed.…”