Aluminosilicate palladium

The industrially important acetoxylation consists of the aerobic oxidation of ethylene into vinyl acetate in the presence of acetic acid and acetate. The catalytic cycle can be closed in the same way as with the homogeneous Wacker acetaldehyde catalyst, at least in the older liquid-phase processes (320). Current gas-phase processes invariably use promoted supported palladium particles. Related fundamental work describes the use of palladium with additional activators on a wide variety of supports, such as silica, alumina, aluminosilicates, or activated carbon (321-324). In the presence of promotors, the catalysts are stable for several years (320), but they deactivate when the palladium particles sinter and gradually lose their metal surface area. To compensate for the loss of acetate, it is continuously added to the feed. The commercially used catalysts are Pd/Cd on acid-treated bentonite (montmorillonite) and Pd/Au on silica (320). 

In addition to palladium, the catalysts used commercially always contain alkali salts, preferably potassium acetate. Additional activators include gold, cadmium, platinum, rhodium, barium, while supports such as silica, alumina, aluminosilicates or carbon are used. The catalysts remain in operation for several years but undergo deactivation. The drop in activity is due to a gradual sintering of the palladium particles which causes the catalytically active area to decrease progressively. Under reaction conditions potassium acetate is slowly lost from the catalyst and must continuously be replaced. 

The palladium catalyst supported on aluminosilicate is prepared by exchanging the surface protons of aluminosilicate with palladium-ammine complex cations, followed by washing with water, thermal decomposition, and reduction with hydrogen. This reduction easily transforms the exchanged palladiumammine complex cations into metallic palladium particles which are fine spheres and homogeneously dispersed through a cloud of the fine particles of aluminosilicate. 

Figure 26 is a plot of ko (mole/hour/atm/gm Pd) at 150° versus the exchanged amount of palladium in aluminosilicate, Pd in PdSA. 

Fig. 24. Palladium on aluminosilicate prepared by cation-exchange method. Palladium content, 1.24 meq/gm.

Fio. 26. Catalytic activity of palladium on aluminosilicate catalyst prepared by the cation-exchange method versus amount of palladium in the catalyst, the reaction being hydrogenation of benzene at 160°. 

Flo. 27. Frequency factor (A) for hydrogenation of benzene, aize of crystallite (Z),) and lattice imperfection rj) of palladium on aluminosilicate catalyst prepared by cation-exchange method versus temperatrue of calcination of the catalyst. Reduction temperature after calcination was kept constwt at 300°. 

Fio. 29. Life of palladium catalyst supported on aluminosilicate with and without promoter for hydrogenation of benzene. 

Refer to Figure 9.9 in your textbook. Cd and Pb are classified as chalcophiles and give soft cations. Thus, they will be found as sulfides. Rb and Sr are lithophiles and are hard they can be found in aluminosilicate minerals. Cr and Pd are siderophiles and give cations of intermediate hardness. As such, they cEin be found as both oxides and sulphides. Palladium can also be found in elemental form. 

---