Structural investigation of solid-acid-promoted Pd/SDB catalysts for ethyl acetate production from ethanol

Catalyzed by styrene-divinylbenzene copolymer (SDB)-supported Pd (Pd/SDB) catalysts, ethyl acetate can be formed from water-containing ethanol via concomitant partial oxidation and esterification reaction, The partial oxidation reaction is carried out over palladium clusters on the catalysts. The esterification reaction is catalyzed by protons formed from the spontaneous dissociation of acetic acid and can be enhanced by addition of solid acid catalysts. To investigate the nature of solid acid in promoting the catalytic properties of Pd/SDB catalysts, extended X-ray absorption fine structure (EXAFS) and inductively coupled plasma optical emission (ICP) spectroscopy were used to characterize the reaction-induced morphology change of Pd clusters and metal loss. After the reaction, there is no evidence of palladium oxide formation, suggesting that the oxidation reaction takes place via the adsorbed oxygen. In addition, concomitantly with Pd leaching, increase of average Pd cluster size was observed. Pd-Pd coordination number increases from 3.0 to 7.9 and the Pd content decreases from 1.0 to 0.55 wt %. Addition of acid catalysts reduces leaching and growth of Pd clusters. This effect increases with an increase in acid-to-Pd catalyst ratio of the mixed catalysts. As the acid-to-Pd catalyst ratio increases from 1.0 to 4.0, Pd-Pd coordination number decreases from 7.2 to 6.5 and the Pd content increases from 0.63 to 0.71 wt %.