Palladium vapor pressure, high temperature

Under high pressures and temperatures, iodine reacts with oxygen to form iodine pentoxide [12029-98-0] (44). The reaction of iodine with carbon monoxide under acidic conditions is catalyzed by palladium salts (45). Phosphorous vapor and iodine react to form phosphoms trHodide [13455-01 -17, PI (46). 

Finally, selective hydrogenation of the olefinic bond in mesityl oxide is conducted over a fixed-bed catalyst in either the Hquid or vapor phase. In the hquid phase the reaction takes place at 150°C and 0.69 MPa, in the vapor phase the reaction can be conducted at atmospheric pressure and temperatures of 150—170°C. The reaction is highly exothermic and yields 8.37 kJ/mol (65). To prevent temperature mnaways and obtain high selectivity, the conversion per pass is limited in the Hquid phase, and in the vapor phase inert gases often are used to dilute the reactants. The catalysts employed in both vapor- and Hquid-phase processes include nickel (66—76), palladium (77—79), copper (80,81), and rhodium hydride complexes (82). Complete conversion of mesityl oxide can be obtained at selectivities of 95—98%. 

The catalyst should have a very low vapor pressure at the operating temperature in order to avoid loss of catalytic material. The vapor pressure of palladium at 1300°C is about an order of magnitude lower than that of platinum. Computer modeling [24] shows that at 1300 °C 50% of platinum will be lost over 30 min and 50% of palladium over 30 h. Clearly, neither of these metals would be usable at this temperature. Loss of oxygen is a problem with oxides such as C03O4 at high temperatures and can also be a problem with mixed oxides such as the perovskites, due to the presence of simple oxides as impurities. 

Since, as shown in equation (34), palladium metal is precipitated as a byproduct of the reaction, it is necessary to reoxidize it back to the Pd " " state. This is accomplished with a palladium-copper couple, as depicted in equations (35) and (36), which is driven by oxygen. The reaction is carried out by contacting a mixture of ethylene and oxygen with a mixture of acetic acid, lithium acetate, and the palladium-copper couple at temperatures of 80 to 150 °C. The vapor-phase process is carried out under pressure at high temperatures (120 to 150 °C) using a fixed-bed palladium catalyst [218]. The oxidative acylation of ethylene can also be used for the preparation of the higher vinyl esters, although it is not currently used for that purpose, due to the low demand for those materials. 

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