Org. Synth. 1946, 26, 77
DOI: 10.15227/orgsyn.026.0077
PALLADIUM CATALYSTS
Submitted by Ralph Mozingo
Checked by Homer Adkins and James E. Carnahan.
1. Procedure
(Note 1)
A. Palladium on barium sulfate catalyst (5% Pd). A solution (Note 2) of 8.2 g. of palladium chloride (0.046 mole) in 20 ml. (0.24 mole) of concentrated hydrochloric acid and 50 ml. of water is prepared. To a rapidly stirred, hot (80°) solution of 126.2 g. (0.4 mole) of reagent barium hydroxide octahydrate in 1.2 l. of distilled water contained in a 4-l. beaker (Note 3) and (Note 4) is added all at once 120 ml. (0.36 mole) of 6 N sulfuric acid. More 6 N sulfuric acid is added to make the suspension just acid to litmus (Note 5). To this hot barium sulfate suspension (Note 6) are added the palladium solution and 8 ml. (0.1 mole) of 37% formaldehyde solution. The suspension is then made slightly alkaline to litmus with 30% sodium hydroxide solution, constant stirring being maintained. The suspension is stirred 5 minutes longer, and then the catalyst is allowed to settle (Note 7). The clear supernatant liquid is decanted and replaced by water, and the catalyst is resuspended. The catalyst is washed by decantation eight to ten times. After the final decantation, the catalyst is collected on a 90-mm. medium-porosity sintered-glass funnel (Note 8). Most of the water is removed from the cake, but not enough to cause the cake to break or channel. The filter cake is washed with 250 ml. of water in five portions, the last being removed as completely as possible by filtration. The funnel and its contents are then placed in an oven at 80° until the catalyst is dry. The catalyst (93–98 g.) is powdered and stored in a tightly closed bottle (Note 9).
B. Palladium on carbon catalyst (5% Pd). A suspension of 93 g. of nitric acid-washed Darco G-60 (Note 10) in 1.2 l. of water contained in a 4-l. beaker (Note 3) and (Note 4) is heated to 80°. To this is added a solution of 8.2 g. (0.046 mole) of palladium chloride in 20 ml. (0.24 mole) of concentrated hydrochloric acid and 50 ml. of water (Note 2). Eight milliliters (0.1 mole) of 37% formaldehyde solution is added. The suspension is made slightly alkaline to litmus with 30% sodium hydroxide solution, constant stirring being maintained. The suspension is stirred 5 minutes longer. The catalyst is collected on a filter and washed ten times with 250-ml. portions of water. After removal of as much water as possible by filtration, the filter cake is dried (Note 11), first in air at room temperature, and then over potassium hydroxide in a desiccator. The dry catalyst (93–98 g.) is stored in a tightly closed bottle.
C. Palladium chloride on carbon (5% Pd). A solution of 8.2 g. (0.046 mole) of palladium chloride in 20 ml. (0.24 mole) of concentrated hydrochloric acid and 50 ml. of water is prepared (Note 2). The solution is diluted with 140 ml. of water and poured over 92 g. of nitric acid-washed Darco G-60 (Note 10) in an 8-in. evaporating dish (Note 3). After the palladium chloride solution has been thoroughly mixed with the carbon, the whole mixture is dried, first on a steam bath and then in an oven at 100°, with occasional mixing until completely dry. The mass (98–100 g.) is powdered and stored in a closed bottle.
The required quantity of palladium chloride on carbon is transferred to a hydrogenation bottle and reduced with hydrogen in the solvent to be used for the hydrogenation (Note 12) and (Note 13). When no more hydrogen is absorbed by the catalyst, it is collected (Note 14) on a sintered-glass funnel and washed with more of the solvent to remove the hydrogen chloride, and then returned to the reduction bottle, the last being washed in with the solvent. The material to be hydrogenated is then added and the hydrogenation is completed in the usual way.
D. Palladium on carbon catalyst (10% Pd). A solution of 8.33 g. of palladium chloride in 5.5 ml. of concentrated hydrochloric acid and 40 ml. of water is prepared by heating the mixture on a steam bath (Note 2) and (Note 15). The resulting solution is poured into a solution of 135 g. of sodium acetate trihydrate in 500 ml. of water contained in a 1-l. reduction bottle (Note 16). Forty-five grams of Norit (Note 10) is added, and the mixture is hydrogenated until absorption ceases after 1–2 hours. The catalyst is collected on a Büchner funnel and washed with 2 l. of water in five portions. The filter cake, after removal of most of the water, is dried in air and then in a desiccator over calcium chloride (Note 11). The catalyst (48–50 g.) is stored, after being powdered, in a tightly closed bottle.
2. Notes
1.
The four procedures given for the preparation of
palladium catalysts differ in that in A the support is
barium sulfate or
barium carbonate whereas in the others the support is
carbon. In procedures A and B, alkaline
formaldehyde is the reducing agent; in C and D,
hydrogen is used. The catalysts A, B, and D are prepared and stored until required with the
palladium in the reduced form ready for use. In C,
1 the palladium salt is reduced to the metal as needed, so that there is no loss of activity during storage. Catalyst A is similar to that usually recommended for Rosenmund reductions; D is essentially that developed by Hartung
2 and extensively used by Cope
3 and others. Catalyst D carries twice as much
palladium per unit weight as the others.
Catalysts reduced with formaldehyde carry no adsorbed hydrogen and are less pyrophoric. Barium carbonate as a support may sometimes be advantageous in that the neutrality of the hydrogenation mixture may be maintained. Barium sulfate or barium carbonate may be a better support than carbon, which may, in some instances, so strongly adsorb the derived product that recovery is difficult or incomplete. Palladium may be more completely and easily recovered from a spent catalyst where carbon rather than barium sulfate is the support. In general, the submitter prefers a catalyst prepared according to procedure C.
2.
Since
palladium chloride dissolves rather slowly in aqueous acid, the mixture is heated on a steam bath for about 2 hours, or until solution is complete. If the
dihydrate of palladium chloride is used the quantity should be increased to 9.9–10.0 g.
3.
The entire preparation is carried out with all-glass or porcelain equipment in order to prevent contamination with
iron or other metals.
4.
The catalyst may be prepared in ten times the amount given here, with a
20-l. battery jar in place of the beaker.
5.
The rapid addition of
sulfuric acid is made to give finely divided
barium sulfate.
6.
An equal weight of precipitated
barium carbonate (93 g.) may be substituted for the
barium hydroxide and
sulfuric acid to give a
palladium on
barium carbonate catalyst. The amount of
hydrochloric acid should then be reduced to 8.2 ml.7.
After 5 minutes, the solution is colorless and free of
palladium chloride.
8.
A Büchner funnel may be used, but filtration through paper is very slow. The washing process may be carried out by centrifugation instead of filtration.
9.
The
palladium may be conveniently separated from the
barium sulfate by solution in aqua regia. The used catalyst is collected from the reaction mixture on a sintered-glass funnel. The organic material is removed with suitable solvents, and the solvents are replaced by water. The
palladium is dissolved in aqua regia and is washed out with dilute
hydrochloric acid, the solutions being collected for recovery of the metal. For recovery of the
palladium from
carbon, the mass is ignited and the ash is extracted with aqua regia for several hours. The
palladium solution is filtered, and any residue is reignited and then treated with alkaline
formaldehyde solution to reduce any
oxides of palladium which may have been formed, and which are only slowly soluble in aqua regia. The solids are collected on a filter, and the
palladium is extracted with aqua regia.
10.
Norit, Darco, or other carbons may be used. The
carbon is heated on a steam bath with
10% nitric acid for 2–3 hours, washed free of acid with water, and dried at 100–110° before use.
11.
The
palladium on
carbon catalysts should be dried at room temperature or the
carbon may ignite. These catalysts are first dried in air and then over
potassium hydroxide (or
calcium chloride) in a desiccator.
12.
The solvent is conveniently that in which the hydrogenation is to be done. During the reduction of the
palladium chloride, a neutral solvent is to be preferred; any acid or alkali needed for the hydrogenation is added after reduction of the catalyst.
13.
The presence of
hydrogen chloride during the hydrogenation of many organic compounds is desirable or without effect, so that the washing operations may be omitted in such cases. Thus, the
palladium chloride on
carbon may be used in the same manner as the prereduced catalysts, i.e., simply added before reduction to the solvent and the hydrogen acceptor.
14.
The catalyst should be kept wet with the solvent during the washing process, as it is pyrophoric.
15.
The resulting solution is approximately equivalent to
50 ml. of the commercial palladium chloride solution
(p. 385) suggested by Hartung and Cope.
316.
The checkers reduced the
palladium chloride, in three batches, in a
500-ml. bottle. The bottle was not shaken, but the contents were rapidly stirred under a pressure of 1.1 atmospheres of
hydrogen. The reduction of each batch required about 5 hours.
3. Discussion
Palladium catalysts have been prepared by fusion of
palladium chloride in
sodium nitrate to give
palladium oxide;
4,5 by reduction of palladium salts by alkaline
formaldehyde6,7,8 or
sodium formate,
9 by
hydrazine,
10 and by the reduction of palladium salts with
hydrogen.
11 The metal has been prepared in the form of
palladium black,
6,9 and in colloidal form in water containing a protective material,
10 as well as upon supports. The supports commonly used are asbestos
12 barium carbonate,
13 barium sulfate,
1,7,8,14 calcium carbonate,
15 carbon,
1,11,14,16 kieselguhr,
14,16 silica-gel,
17 and
strontium carbonate.
18 The catalysts described here are prepared by modifications of the methods of Schmidt,
8 Rosenmund and Langer,
14 Mannich and Thiele,
11 and Hartung.
2,3
Polyvinyl alcohol19 and
aluminum oxide20 have been used as supports for
palladium catalysts.
This preparation is referenced from:
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
palladium black
dihydrate of palladium chloride
oxides of palladium
Polyvinyl alcohol
calcium chloride (10043-52-4)
sulfuric acid (7664-93-9)
hydrogen chloride,
hydrochloric acid (7647-01-0)
hydrogen (1333-74-0)
sodium hydroxide (1310-73-2)
formaldehyde (50-00-0)
iron (7439-89-6)
nitric acid (7697-37-2)
calcium carbonate (471-34-1)
barium sulfate (7727-43-7)
carbon,
Norit (7782-42-5)
potassium hydroxide (1310-58-3)
palladium (7440-05-3)
barium hydroxide (17194-00-2)
barium carbonate (513-77-9)
barium hydroxide octahydrate (12230-71-6)
hydrazine (302-01-2)
sodium formate
sodium nitrate
palladium oxide
palladium chloride (7647-10-1)
aluminum oxide (1344-28-1)
sodium acetate trihydrate (6131-90-4)
strontium carbonate (1633-05-2)
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