STUDY ON ISOMERIZATION MECHANISM OF PROPY LENE-OXIDE ON METAL-OXIDES BY SEMIEMPIRICAL CALCULATION METHOD

On the isomerization of propylene oxide (PO) to propionaldehyde (PA), acetone (AC), and allyl alcohol (AA) on five oxide catalysts (SiO2, Al 2O3, MgO, ZnO, CdO), the mechanism and selectivity-controlling factors of the isomerization were studied by the MOPAC-PM3 semi-empirical cal culation method. A reasonable mechanism was derived from requirement t hat the selectivity difference of the catalysts should be consistently interpreted by the calculated electronic states of PO. With regard to the selectivity-controlling factors, it was found that PA and AC sele ctivities correlate closely with net charges of Lewis acid site (surfa ce metal ion) and basic site (surface oxide ion) on oxide clusters hav ing formal charges of 1+ (Lewis acid site) and 1- (basic site), respec tively. There was no correlation concerning AA selectivity. As the res ult, it was supported by computational chemistry that PA and AC select ivities are used as measures of surface acidity and basicity, respecti vely, of oxide catalysts. These results suggest that it is a simple an d effective method to derive a reasonable reaction mechanism by compar ative study of semi-empirical calculations with results of experiments on a series of catalysts.