PD SN CATALYST FOR TOLUENE ACETOXYLATION

Silica supported Pd-Sn catalysts were prepared by controlled surface p recipitation of SnCl2 with subsequent redox-deposition of Pd over the tin hydroxide layer. The variables studied were the pH level attained after the tin hydroxide precipitation, the effect of the Pd:Sn ratio, the palladium loading and the temperature of the final He heat treatme nt. The samples were characterized by DRFT-IR, physisorption, hydrogen , oxygen and CO chemisorption, single turnover reaction procedure and acetoxylation activity measurements. The results of this study suggest that the initial precipitation of the tin hydroxide takes place witho ut significant interaction between the partially hydrolyzed SnCl2 and the surface silanol groups. The final pH in this step controls the str ength of attachment and the distribution of the tin hydroxide species on the silica surface and apparently the size of the immobilized palla dium particles. The DRFT-IR spectra of the samples taken at various st ages of preparation suggest that the PdSn(x) coverage is completely fo rmed in the drying stage and subsequent sintering in He at 300-degrees -C only increases the number of the bridging Sn-O-Si bonds. The optimi zed system requires a pH level of 4.2-4.7 in the tin precipitation ste p, a Pd:Sn ratio of 0.5, a Pd loading of 1-1.25% by weight and a heliu m sintering temperature of 250-270-degrees-C. The ex-situ prepared cat alyst displayed higher acetoxylation activity than did the in-situ for med catalysts which were prepared from Sn(OH)2, Pd(OAc)2 and KOAc.