Palladium catalysts for hydrogenation

Palladium oxide is used to prepare palladium catalyst for hydrogenation. The oxide is readily reduced by hydrogen to metal. 

Microwave irradiation also shows a beneficial effect in the preparation of solid-supported palladium catalysts for hydrogenation reactions. Thus, alumina- and silica-supported palladium catalysts were synthesised by conventional and microwave heating, and their physical properties and catalytic activity in the hydrogenation of benzene were compared. The alumina-based system prepared under microwave conditions showed turnover numbers an order of magnitude higher than the conventionally prepared catalysts28. 

In spite of the successful use of NHCs in a number of palladium-catalyzed reactions, no system for hydrogenation was reported until 2005. This can be easily explained as it had been observed that hydridopalladium-carbene species decompose due to attack of the hydride on the carbene, which results in its reductive elimination to yield the corresponding imidazolium salt [ 190]. However, Cavell and co-workers recently showed that the oxidative addition of imidazolium salts to bis-carbenic palladium complexes leads to isolable NHC-hydridopalladium complexes [191]. This elegant work evidenced the remarkable stabilizing effect of NHC ligands in otherwise reactive species and led to the development of the first NHC-palladium catalyst for hydrogenation. 

Pd-0803 E1/16 ". [M THarshaw] Palladium catalyst for hydrogenation of aOcenes in aromatic streams. 

A variety of catalysts including copper, nickel, cobalt, and the platinum metals group have been used successfully in carbonyl reduction. Palladium, an excellent catalyst for hydrogenation of aromatic carbonyls is relatively ineffective for aliphatic carbonyls this latter group has a low strength of adsorption on palladium relative to other metals (72,91). Nonetheless, palladium can be used very well with aliphatic carbonyls with sufficient patience, as illustrated by the difficult-to-reduce vinylogous amide I to 2 (9). 

The rhodium complexes are excellent catalysts for hydrogenation of NBR. At low temperature and pressure, high catalyst concentrations are used to obtain a better rate of reactions. Due to higher selectivity of the reaction, pressure and temperature can be increased to very high values. Consequently the rhodium concentration can be greatly reduced, which leads to high turnover rates. The only practical drawback of Rh complex is its high cost. This has initiated the development of techniques for catalyst removal and recovery (see Section VU), as well as alternate catalyst systems based on cheaper noble metals, such as ruthenium or palladium (see Sections IV.A and B). 

Palladium is used as a catalyst for hydrogenation reactions in the food industry, and a rhodium catalyst is used in... 

Rossi, L.M., Silva, F.P., Vono, L.L.R., Kiyohara, P.K., Duarte, E.L., Itri, R., Landers, R. and Machado, G. (2007) Superparamagnetic nanopartide-supported palladium a highly stable magnetically recoverable and reusable catalyst for hydrogenation reactions. Green Chemistry, 9 (4), 379-385. 

The most commonly used catalysts for hydrogenation (finely divided platinum, nickel, palladium, rhodium, and ruthenium) apparently serve to adsorb hydrogen molecules on their surface. 

Cathode Platinum metals, especially platinum and palladium, achieve the lowest known overvoltages for hydrogen. Moreover, they are effective catalysts for hydrogenation reactions [15]. 

To date, reports have involved palladium catalysts for Suzuki and Sono-gashira coupling reactions [63-66], rhodium catalysts for silylations of alcohols by trialkylsilanes [67,68], and tin-, hafnium-, and scandium-based Lewis acid catalysts for Baeyer-Villiger and Diels-Alder reactions [69]. Regardless of exact mechanism, this recovery strategy represents an important direction for future research and applications development. Finally, a particularly elegant protocol where CO2 pressure is used instead of temperature to desorb a fluorous rhodium hydrogenation catalyst from fluorous silica gel deserves emphasis [28]. 

Biemeyer N, Ley SV, Ramarao C, Shirley IM, Smith SC. Palladium acetate in pol3mrea microcapsules a recoverable and reusable catalyst for hydrogenations. Synlett 2002 1843-1844. 

Stalder, C. J., Chao, S., Summers, D. P., and Wrighton, M. S. (1983). Supported Palladium Catalysts for the Reduction of Sodium Bicarbonate to Sodium Formate in Aqueous Solution at Room Temperature and One Atmosphere of Hydrogen. Journal of the American Chemical Society, 105, 6318-6320. 

Membrane is the catalyst Cation exchange membranes for esterification reactions Palladium membranes for hydrogenation/dehydrogenation reactions... 

The catalyst in each of these three reductions is a different metal. Palladium and platinum are the most commonly used metal catalysts for hydrogenation, but hydrogenation can also work with nickel, rhodium, or ruthenium. The choice of catalyst depends on the compound to be reduced. 

From the standpoint of structure and bonding, explain why transition metals such as palladium and nickel are often used as catalysts for hydrogenation reactions. 

Riesz and Weber compared the selectivities of commercial platinum, palladium, rhodium, and nickel catalysts for hydrogenation of linolenic components in soybean oil.110 Representative results are summarized in Table 3.8. Certain platinum metal catalysts showed higher selectivities than nickel catalysts, as indicated by the values of SL (A in Scheme 3.13) = 2.4-2.7. Generally, nickel catalysts showed selectivities... 

---