Acetoxylation of toluene catalyzed by supported Pd-Sn catalysts

Abstract

The toluene acetoxylation run in the presence of Pd(OAc) 2, Sn(OH) 2 and KOAc was found to proceed through three characteristic stages of the oxygen absorption. In the first stage, the system produces a Pd(II) [Sn(II)] 2(OAc) 6 complex which remains homogeneous until the end of that phase. In the second region of the oxygen uptake curve there is an absorption of one equivalent of oxygen after which a heterogeneous palladium-tin species precipitates on the surface of the silica carrier present in the system. In the third region the oxygen adsorption is related only to the oxidation of the toluene to benzyl acetate and benzylidene diacetate which are formed in a 6.7/1 ratio. The turnover frequencies determined on the basis of the reaction rates were 3–4 times higher than observed with the reference system which used Sn(OAc) 2 as the tin precursor. An additional triple increase in the rate was observed for the same optimized system when the oxidation was begun in the presence of ICN 60 silica. The activity of these in-situ supported Pd/Sn species depends strongly on the porosity of the silica carriers. The optimal average pore diameter was found to be near 20–30 Å. The samples with variable Sn/Pd ratios and with variable loading exhibit strongly suppressed hydrogen chemisorption and single turnover (STO) surface reactivity which is thought to result from an SMSI state of the metallic palladium caused by the co-adsorbed tin.