Acidic properties of sulfated iron oxide supported molybdenum catalysts: a differential scanning calorimetry, thermogravimetry and Fourier transform-infrared study

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 SO42−/free Fe2O3 ones. ΔH 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 SO42−/free Fe2O3 catalysts. In addition, the enthalpies of pyridine desorption from SO42−/Fe2O3 supported Mo catalysts were higher than those for the SO42−/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 SO42− species on the surface acidity. It was found that the presence of SO42− species enhanced the surface acidity and helped the formation of Brønsted sites earlier at the 2 wt.% Mo loading. These were manifested by the presence of vibrational bands at 1635 and 1540 cm−1. These bands were only detected at a loading of 5 wt.% Mo when the support was SO42−/free Fe2O3. Important correlations and shifts in the IR absorption bands were evaluated and discussed.