Palladium selective hydrogenation

From acid chlorides by selective hydrogenation in the presence of a catalyst (palladium deposited upon a carrier, which is usually barium sulphate but is... 

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%. 

For more selective hydrogenations, supported 5—10 wt % palladium on activated carbon is preferred for reductions in which ring hydrogenation is not wanted. Mild conditions, a neutral solvent, and a stoichiometric amount of hydrogen are used to avoid ring hydrogenation. There are also appHcations for 35—40 wt % cobalt on kieselguhr, copper chromite (nonpromoted or promoted with barium), 5—10 wt % platinum on activated carbon, platinum (IV) oxide (Adams catalyst), and rhenium heptasulfide. Alcohol yields can sometimes be increased by the use of nonpolar (nonacidic) solvents and small amounts of bases, such as tertiary amines, which act as catalyst inhibitors. 

Hydrogenation of cinnamaldehyde has been studied extensively since selectivity has often been an issue. Under mild conditions the carbonyl group is reduced giving cinnamyl alcohol, whereas at elevated temperatures complete reduction to 3-phenylpropanol [122-97 ] results. It is possible to saturate the double bond without concomitant reduction of the carbonyl group through selective hydrogenation with a ferrous chloride-activated palladium catalyst (30), thereby producing 3-phenylpropanol [104-53-0]. 

The depropanizer overhead, Cj and lighter feed is compressed to about 300 psi and then passed over a fixed bed of acetylene removal catalyst, generally palladium on alumina. Because of the very large amount of hydrogen contained in this stream, the operating conditions are critical to selectively hydrogenate the acetylene without degrading the valuable ethylene to ethane. 

Many workers (5,6,7,87) have compared various metals for the selective hydrogenation of lower acetylenes to olefins, and it was always found that palladium was by far the most selective. This conclusion concurs with the usual synthetic experience, but under special circumstances other metals, such as platinum, may prove more useful (35,63). The catalyst support may also have an influence (21,65). Carbon, calcium carbonate, and barium sulfate are frequently used supports. Examples of some differences are noted later,... 

Formation of diamines from dinitro compounds, which are unable to interact intramolecularly, presents no problem. Very large volumes of diaminotoluene, a precursor to toluene diisocyanate, are produced by hydrogenation of dinitrotoluene over either nickel or palladium-on-carbon. Selective hydrogenation of one or the other of two nitro groups is much more of a challenge, but a number of outstanding successes have been recorded. A case in point is the hydrogenation of 2,4-dinitroaniline (11) to 4-nitro-l,2-benzenediamine (12) (2) or to 2-nitro-l,4-benzenediamine (10). 

It can be summarized from the available data in Table 3 that supported palladium catalysts selectively hydrogenated carbon-carbon double bonds in the presence of the nitrile group in NBR. However, there is no detailed fundamental study on heterogeneous catalytic hydrogenation of nitrile rubber in the literature that can provide an insight into the reaction. The available information is limited since most of the literature is patented. 

The available data in Table 6 reveal that palladium complexes are excellent catalysts for selective hydrogenation of C=C in NBR. Recent attempts to recover the catalyst (see Section VII) after hydrogenation and lower the cost of the metal make it an attractive supplement in the industrial production of HNBR. 

The disadvantages of the general method using supported palladium and hydrogen are a lack of selectivity and overreduction. Where selectivity is not an important requirement, Pd/C is a widely used catalyst for the hydrogenolysis of the C-O bond in benzyl ethers. 

Thus, [HRh(C0)(TPPTS)3]/H20/silica (TPPTS = sodium salt of tri(m-sulfophenyl)phopshine) catalyzes the hydroformylation of heavy and functionalized olefins,118-122 the selective hydrogenation of a,/3-unsaturated aldehydes,84 and the asymmetric hydrogenation of 2-(6 -methoxy-2 -naphthyl)acrylic add (a precursor of naproxen).123,124 More recently, this methodology was tested for the palladium-catalyzed Trost Tsuji (allylic substitution) and Heck (olefin arylation) reactions.125-127... 

Ralthydrierung [German, meaning cold hydrogenation] A process for selectively hydrogenating pyrolysis gasoline, a petroleum refining byproduct, at temperatures below 100°C. A palladium catalyst is used. 

SHP A process for purifying 1-butene by selective hydrogenation of C4 streams in petroleum refineries. A hetrogeneous palladium catalyst is used. Developed in Htils and used in 1989 in Germany, the United States, and Japan. In 1991 the licensing rights were acquired by UOP. 

It was concluded that the high selectivity observed in the hydrogenation experiments using 26 b is explained by the relatively strong coordination of the alkyne to the palladium center, which only allows for the presence of small amounts of alkene complexes. Only the latter are responsible for the observed minor amounts of ( )-alkene, which was shown to be a secondary reaction product formed by a subsequent palladium-catalyzed, hydrogen-assisted isomerization reaction. Since no n-octane was detected in the reaction mixture, only a tiny... 

Table 14.4 Selective hydrogenation of 1,3-cyclooctadiene to cyclooctene with various palladium complexes.

Similarly, 1,2-cyclononadiene in methanol with 10% palladium on carbon catalyst gave cis-cyclononene122. The cis isomer is not necessarily the primary product of allene hydrogenation, since the initially formed trans isomer is rapidly isomerized under the reaction conditions. Bond and Sheridan showed that allene resembles acetylene in its ease of hydrogenation123. They suggested that it is selectively adsorbed and held more strongly by the catalyst than 1-propene. Allene was selectively hydrogenated with Pd, Pt and Ni in the presence of 1-propene without its further reduction. 

Ermilova, M. M., N. V. Orekhova, L. D. Gogua and L. S. Morosova. 1981. Selective hydrogenation of diene hydrocarbons on a palladium-ruthenium membrane catalyst. Met. i Kak Membran. Katal. M. 82-100. 

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