Abstract
The optimization of the oxidative esterification of propionaldehyde to methyl propionate using a supported palladium catalyst in methanol under heavy-metal-free and pressurized-oxygen conditions, which we recently reported in a previous paper, were carried out together with a study of the reaction route, the nature of the catalytic active sites, and the effect of the support. In our previous paper, we reported significantly improved activity for oxidative esterification using commercially available 5% Pd/Al2O3 at 1.5 MPa of O2 gas and 333 K and emphasized that the doping of 5% Pd/Al2O3 with lead was not needed for the reaction system, but we could not improve the activity that was reported when using 5% Pd/γ-Al2O3 doped with 5% Pb (a 93.2% conversion of propionaldehyde, 76.8% selectivity for methyl propionate and a 71.6% yield of methyl propionate) at 0.3 MPa of O2 gas and 353 K, as reported by another laboratory. In the present study, however, we exceeded those values and obtained a 98.3% conversion of propionaldehyde, 75.3% selectivity for methyl propionate and a 74.0% yield of methyl propionate using 5% Pd/γ-Al2O3 undoped with Pb at 1.5 MPa of O2 gas and 333 K. It should be noted that, in the preparation of the present 5% Pd/γ-Al2O3, Pd was doped onto Al2O3 that had been previously treated with aqueous NaOH. Another active alumina support, η-Al2O3, prepared from boehmite, afforded activity that was substantially lower than that of γ-Al2O3 and depended on the calcination temperature of boehmite to η-Al2O3. When using various concentrations of CH3OH in the aqueous reaction solution, the oxidative esterification proceeded through the formation of propionic acid. To determine why the Al2O3 support afforded better activity than the active carbon support, Pd/Al2O3 and Pd/C catalysts were examined by XAFS (X-ray absorption fine structure). XAFS revealed that Pd on Al2O3 shows a better redox nature than Pd on C, which resulted in activity on Pd/Al2O3 that was better than that on Pd/C.
