Effect of Sb loading on Pd nanoparticles and its influence on the catalytic performance of Sb–Pd/TiO2 solids for acetoxylation of toluene

Gas-phase acetoxylation is a green technology for the direct synthesis of aromatic and heteroaromatic acetates in a single step from their corresponding alkyl aromatics and heteroaromatics. Moreover, aromatic as well as heteroaromatic alcohols are easily accessible by subsequent hydrolysis. We observed in an earlier study that the addition of Sb to Pd catalysts is responsible for enhancing the yield of benzyl acetate in the acetoxylation of toluene. In the present contribution, we report for the first time the dependence of Pd particle size on Sb loading and its influence on catalytic performance. Furthermore, to gain deeper insight into the role of Sb, a series of Sb–Pd/TiO2 catalysts with different Sb loadings (4–20 wt%) was prepared with constant Pd proportion (10 wt%), characterised by N2 adsorption, XRD, TEM, and XPS and tested at 210 °C and 2 bar. BET surface areas and pore volumes were observed to decrease considerably, from 96 to 39 m2/g and 0.164 to 0.058 cm3/g with increasing Sb loading from 4 to 20 wt%. XPS results showed that the presence of Sb influences both the valence state of Pd and the amount of coke deposition during the reaction. TEM analysis revealed that Sb is found to be always present together with titania (support). No separated free Sb particles and no intermetallic formation between Sb and Pd is seen even with higher Sb loading (20 wt%). On the other hand, free Pd particles with spherical morphology are clearly seen. The initial size of Pd particles in the fresh catalysts is approximately 1–10 nm depending on the Sb loading. However, the samples used exhibited much larger Pd particles (up to 90–100 nm), indicating significant growth during the course of the reaction. The presence of Sb revealed a significant influence on increasing the size of Pd nanoparticles, which in turn showed an appreciable influence on catalytic performance. The activity of the catalysts is found to increase continuously with increasing Sb loading up to 8 wt% and then decrease slightly with further increase. The catalytic activity is observed to change more or less in a similar fashion as that of changes in Pd particle size of the fresh catalysts (i.e., enhanced catalytic performance accompanied by the formation of larger Pd particles). The catalyst with a Sb loading of 8 wt% exhibited the best performance among all other solids of this series, giving 68% conversion of toluene and 85% selectivity of benzyl acetate. Interestingly, all fresh catalysts displayed very low initial activity (X-Tol=<5%=<5%), which increased with time until steady-state conditions were attained, remained stable for few more hours, and then declined with further increases in reaction time. The increased catalytic activity is attributed to increased Pd particle size, whereas the decreased activity is due to coke deposits, as evidenced from TEM and carbon analyses. But these solids can be readily regenerated in air to restore their maximum activity. The correlation between the C/Pd ratio obtained by XPS and the catalytic results suggests that the carbon–palladium interaction is the main reason for catalyst deactivation and depends on the size and shape of the Pd particles. An attempt has been made through the current study to describe the present state of understanding of the potential effects on catalytic performance induced by varying the Sb loading.