Pd/Sn catalyst for toluene acetoxylation

Abstract

Silica supported PdSn catalysts were prepared by controlled surface precipitation of SnCl 2 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 treatment. The samples were characterized by DRFI-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 without significant interaction between the partially hydrolyzed SnCl 2 and the surface silanol groups. The final pH in this step controls the strength of attachment and the distribution of the tin hydroxide species on the silica surface and apparently the size of the immobilized palladium particles. The DRFT-IR spectra of the samples taken at various stages of preparation suggest that the PdSn x coverage is completely formed in the drying stage and subsequent sintering in He at 300°C only increases the number of the bridging SnOSi bonds. The optimized system requires a pH level of 4.2–4.7 in the tin precipitation step, a Pd:Sn ratio of 0.5, a Pd loading of 1–1.25% by weight and a helium sintering temperature of 250–270°C. The ex-situ prepared catalyst displayed higher acetoxylation activity than did the in-situ formed catalysts which were prepared from Sn(OH) 2, Pd(OAc) 2 and KOAc.