Palladium-catalysed vapour phase aerobic acetoxylation of toluene to benzyl acetate

Supported Pd–Sb acetoxylation catalysts with different Pd and Sb amounts as well as varying supports were prepared by impregnation technique. The contents of Pd and Sb are varied over a wide range, for instance Pd is varied from 0.5 to 20 wt% by keeping Sb content at 8 wt%. In a similar way, Sb content is varied from 4 to 20 wt% by keeping Pd loading constant at 10 wt%. Four different supports such as TiO2, γ-Al2O3, SiO2 and ZrO2 are applied at constant Pd (10 wt%) and Sb (8 wt%) contents. Catalytic performance of these solids is evaluated for the gas phase acetoxylation of toluene to benzyl acetate (BA) at T = 210 °C and p = 2 bar. XPS revealed a considerable loss of both Pd and Sb in the near-surface region in the used catalysts. TEM showed that Pd particles exhibit spherical morphology and their size increased dramatically in the spent catalysts compared to their corresponding fresh ones. Monometallic catalysts showed very poor acetoxylation performance but high total oxidation, which results in an increase of the yield of COx up to ca. 50%. However, combination of both Pd and Sb was found to suppress total oxidation and thereby enhance the acetoxylation performance with high BA selectivity of ≥85%. Catalytic activity was observed to increase continuously with increase in Pd loading. The catalyst with the highest Pd loading (20 wt% Pd) displayed the best performance (toluene conversion = > 90%, BA yield = > 75%). The activity is however decreasing with time-on-stream due to coke deposits. Nevertheless, the deactivated samples can be regenerated in air to restore their maximum activity. Nature of support, content of co-components showed strong influence on the catalytic performance.