Gas-phase Oxidative Acetoxylation Of Toluene Over Pd-Sn-K/SiO₂ Catalyst

The gas phase oxidative acetoxylation of toluene (OAT) with acetic acid over Pd-Sn-K/SiO2 catalysts was studied in a fixcd-bcd flow reactor. The catalysts were prepared by multi-step incipient wetness method. The loading amount of the active material of Pd and promoter, tin oxide, supported on the carrier of SiO2 was optimized. The action of the individual components of the catalyst in the reaction, and the preparation method were also investigated.The results show that palladium in the metal state supported on SiO2 is the high active component for the toluene acetoxylation reaction. When other noble metal, such as Pt, Au etc, instead of Pd, or some transitional oxides, such as cobalt oxide, cerium oxide, manganese oxide were used as active component, no detectable amount of oxidative products can be found in the reaction at 180 ℃. In the investigation, we found that the presence of tin oxides in the catalyst of Pd-Sn-K/SiO2 drastically increases benzyl acetate yield in the reaction proceeding in the gas/solid conditions, and the catalytic performance of Pd-Sn-K/SiO2 catalysts are strongly dependent on its preparation methods. The higher activity was obtained when the catalyst was prepared by hydrogen reduction of the palladium component on SiO2 before tin oxides loading. The XRD result reveals that when the reduction of palladium with hydrogen followed by tin oxides loading, the intermetallic compounds of PdSn or Pd20Sn13 phases are formed in the Pd-Sn-K/SiO2 catalyst, leading to lower catalytic activity for the reaction. At the reaction conditions of 180 ℃, toluene: acetic acid: O2 : N2 =1 : 4 : 1 : 4 (mole ratio), and GHSV of 1680 h-1, 25.3% of toluene conversion with 91% of selectivity to benzyl acetate has been achieved over the Pd-Sn-K/SiO2 catalyst (Pd/Sn/SiO2 (wt.) ratio of 0.015/0.05/1 and n(K)/n(Pd) ratio of 27/1) prepared with a optimized method. On the other hand, lower activity and selectivity were obtained when TiO2 or ZrO2 was used as a carrier instead of SiO2, or othernoble metals were used instead of palladium as active component. No other valence-variable metal oxides were found? to be better than tin oxides as promoters for the toluene oxidative acetoxylation reaction. In addition, the reaction mechanism of toluene acetoxylation in the presence of acetic acid over the Pd-Sn-K/SiO2 catalyst was proposed on the basis of experimental results.