Abstract
The catalytic performances of Mo–ZSM-5 catalysts (6 wt% of Mo, Si/Al = 26), prepared by solid–state ion exchange from different molybdenum precursors, were evaluated in the ammoxidation of ethane and ethylene into acetonitrile in the temperature range of 425–500 °C. The catalysts were characterized by chemical analysis, N2–physisorption, XRD, FTIR, 27Al MAS NMR, DRIFT, UV/Vis DR, Raman, and XPS spectroscopies, NH3–TPD, and H2–TPR. Starting from MoCl5, Mo is stabilized in the dimeric form and only small crystallites of MoO3 were formed. However, the oxygenated precursors, i.e. MoO3, (NH4)6Mo7O24·4H2O, and MoO2(C5H7O2)2 are favorable for the agglomeration of amorphous MoO3. In the studied reaction, the required active sites are (MoO4)2–, (Mo2O7)2–, (Mo7O24)6–, and small crystallites of MoO3, while the undesired amorphous MoO3 inhibited the diffusion of reactants to the active sites and/or enhanced the hydrocarbon combustion. Upon the catalyst issued from MoCl5, UV/Vis DRS revealed the abundance of dimeric Mo at the detriment of mono and polymeric species. The former specie played a key role in the ammoxidation since the two Mo atoms in (Mo2O7)2– are spatially too separated and the steric hindrance between intermediate molecules is therefore limited.