Effect of alkali metals added to Cu ion-exchanged Y-type zeolite catalysts in the gas-phase catalytic oxidation of benzyl alcohol

Abstract

The effect of alkali metals added to Cu ion-exchanged Y-type (Cu-NaY) zeolite catalysts on the oxidation activity and the selectivity is investigated using the gas-phase catalytic oxidation of benzyl alcohol as a model reaction. The added alkali metals such as K have been found to promote significantly the catalytic oxidation activity of the Cu-NaY catalysts. In particular, the addition of alkali metals to the Cu-NaY zeolites caused an increase in the yield of the partial oxidation product, benzaldehyde. Thus, the alkali metals added to the Cu-NaY zeolites were found to increase the selectivity of the partial oxidation of benzyl alcohol. The influences of the percentage of Cu ion-exchange, the type of added alkali metal, the amount of the added alkali metal and the atomic ratio of added alkali metal/Cu on the oxidation activity and/or the selectivity of the partial oxidation product are investigated in detail. The absence of gaseous oxygen molecules leads to no deep oxidation products (carbon oxides, CO₂+CO). On the other hand, benzaldehyde, the partial oxygen product, is produced, though the yield is lower and decreases with time-on-stream, even after halting the oxygen supply following steady state benzyl alcohol oxidation. The yield of benzaldehyde in the absence of oxygen catalyzed by alkali metal added Cu-NaY is found to be considerably higher than that catalyzed by Cu-NaY without added alkali metal. The amount of oxygen uptake of the prereduced alkali metal added Cu-NaY zeolite is found to be higher than that of the prereduced Cu-NaY without added alkali metal. One of the roles of the alkali metal added to the Cu-NaY zeolites is to increase the adsorbed oxygen species, which plays an important role in the partial oxidation of benzyl alcohol to benzaldehyde.