Sulfated Fe2O3 catalysts impregnated with different molybdenum contents varied from 2 to 42 wt.%, were examined by
temperature-programmed pyridine desorption via differential scanning calorimetry (DSC) and thermogravimetry (TG)
techniques. The acidity of these catalysts were investigated and compared with those corresponding to SO4
2ÿ
/free Fe2O3 ones.
DH values (obtained from DSC) and amounts of pyridine irreversibly adsorbed (obtained from TG) showed that the
concentration of Lewis acid sites decreased with increasing Mo loading. On the other hand, the sulfated Fe2O3 supported Mo
catalysts showed an increase in the concentration of acid sites to almost 10 times that of the corresponding SO4
2ÿ
/free Fe2O3
catalysts. In addition, the enthalpies of pyridine desorption from SO4
2ÿ
/Fe2O3 supported Mo catalysts were higher than those
for the SO4
2ÿ
/free Fe2O3 catalysts. This indicated the strength of acid sites on the former samples compared with those on the
later ones. Comparative FT-IR spectroscopic analyses of pyridine adsorption have been investigated to study the nature of the
acid sites and to assess the effect of SO4
2ÿ
species on the surface acidity. It was found that the presence of SO4
2ÿ
species
enhanced the surface acidity and helped the formation of Brùnsted sites earlier at the 2 wt.% Mo loading. These were
manifested by the presence of vibrational bands at 1635 and 1540 cm
ÿ1. These bands were only detected at a loading of
5 wt.% Mo when the support was SO4
2ÿ
/free Fe2O3. Important correlations and shifts in the IR absorption bands were
evaluated and discussed.