Kinetics of Selective Oxidation of Dimethyl Ether to Formaldehyde Over Al2O3-Supported VO x and MoO x Catalysts

The object of this work is to study the effect of hydrothermal treatment on the TS-1/SiO₂ catalyst in propylene epoxidation in fixed bed. The TS-1/SiO₂ catalysts before and after hydrothermal treatment were characterized by means of XRD, XRF, BET, UV–Vis and EPR techniques. It was found by EPR characterization that two types of Ti(IV)-superoxide radicals, A (g _z = 2.0271; g _y = 2.0074; g _x = 2.0010) and B (g _z = 2.0247; g _y = 2.0074; g _x = 2.0010), were observed for the TS-1/SiO₂ catalyst. The superoxo species A was converted to B after the TS-1/SiO₂ catalyst was hydrothermally treated. The results show that hydrothermal treatment temperature and time have marked effects on the activity and the PO selectivity. The optimal hydrothermal treatment temperature and time are 170 and 4 h, respectively. In the long-term propylene epoxidation reaction, about 95% H₂O₂ conversion and above 94% PO selectivity are obtained over TS-1/SiO₂ catalyst hydrothermally treated at 170° C for 4 h.