Urushibara Catalysts

General Characteristics of the Urushibara Catalysts

As we have seen in the preceding paragraph, there are many varieties of Urushibara catalyst. Of these catalysts, U-Ni-Aand U-Ni-B are most frequently used, the former being by far the most important catalyst, since it is useful for almost every reduction. U-Ni-A, as well as U-Ni-A(s), serves for almost every liquid-phase hydrogenation; U-Ni-AA is especially effective for vapor-phase catalytic reductions. Other catalysts are prepared for particular purposes, and are not always efficient for all reductions.

The general characteristics common to these Urushibara catalysts are as follows:

1. They may be used for miscellaneous reductions, such as the hydrogenation of ethylenic and acetylenic compounds and benzene rings, and the reduction of carbonyl compounds, oximes, nitriles, and nitro compounds, etc. Their activities are very similar to those of Raney catalysts. As far as the author is aware, all reductions which Raney catalysts promote are effected as well in the presence of an Urushibara catalyst, and the activities of the Urushibara catalysts are, under both ordinary and high pressures, by no means lower than displayed by Raney catalysts.
2. Urushibara catalysts are also used in dehydrogenation, reductive desulfurization, and reductive alkylation, and have proved to be as active for these reactions as Raney catalysts (see Chapter 8).
3. In general, the preparations of Raney catalysts or other earlier catalysts are very involved, and require a long time. Moreover, they must be treated with great caution. On the other hand, the preparation of Urushibara catalysts is very simple and rapid. They can be prepared from commercial materials by simple operations in only an hour or less. Simple operations allow an untrained person to easily obtain catalysts of high activity.
4. As for U-Ni-A and U-Ni-B, it is worth noting that the more vigorous the reaction between the nickel chloride solution and the zinc dust, the more likely the activity of the catalyst will be promoted; a mild and lengthy reaction which is brought about by the gradual addition of the reagents will reduce the activity. This is one of the unique characteristics of the Urushibara catalysts.
5. As Urushibara catalysts are not very sensitive to impurities, they can conveniently be produced from commercial chemicals of ordinary grade without careful choice. Reagents such as nickel chloride, zinc dust, aluminum, sodium hydroxide, and acetic acid may be "chemically pure" grade commercial materials. City water is sufficient for use in preparing the precipitated metals. (If, however, it contains poisoning metal ions or other poisoning materials, purified water must be used.) Purified water should be used in the digestion process.
6. Dried precipitated metal can be stored for a long time without decreasing the activity of the catalyst which is to be prepared. The catalyst prepared from the stored precipitated metal does not differ in activity from that prepared from freshly precipitated metal. Preparation time can be greatly reduced if a large quantity of the precipitated nickel is prepared in advance from nickel chloride and zinc dust and stored; from this the requisite amount can be occasionally taken out to be digested with an acid or alkali. The time required for digestion with acid or alkali almost never exceeds twenty minutes.
7. The handling of an Urushibara catalyst is safe and easy, as it is not inflammable upon contact with air, in contrast to Raney catalyst which is pyrophoric. Newly prepared U-Ni-A does ignite spontaneously when dried after washing with ether; however, it does not ignite when washed with methanol or ethanol and left standing wet in the air for a short time. U-Ni-B never ignites even if washed with ether and dried. (6) Waste catalyst does not ignite when left standing after being filtered from the reaction mixture, washed with water or other solvents, and dried. Therefore, used catalyst can easily be recovered and re-generated. For regeneration, the waste catalyst is converted to its salt, nickel chloride for example, from which a new U-Ni catalyst is produced. An alternative and simpler method for regenerating a U-Ni-A catalyst of high activity has been discovered, in which a spent catalyst is treated with acetic acid in the presence or absence of a small amount of zinc dust (see Section 6.4.2). The operation permits the repeated use of the catalyst.
8. As the materials used in preparing the catalyst are inexpensive, they may be used in large quantity without much expense. In laboratory scale production one need not worry about recovery and regeneration.
9. Urushibara catalysts preserve their activities for considerably long periods when stored carefully in a solvent free from dissolved air. Long storage, however, is not really necessary, since the catalysts can be prepared easily in a short time whenever required. For maximum convenience, therefore, storage in the state of the precipitated metal is recommended instead.
10. The durability of the Urushibara catalyst during a reaction is not quite satisfactory; this is also the case with the Raney catalyst. This matters little in practical use, as the Urushibara catalyst is inexpensive and its preparation, recovery, and regeneration can be carried out very easily.
11. The catalytic activities of the Urushibara catalysts with respect to a particular substance differ according to the type of catalyst. In addition, there are a few cases where the Urushibara catalyst shows a lower activity than the Raney catalyst for a particular reduction. Therefore, selective reduction can easily be programmed by choosing the catalyst or the reaction conditions. For instance the use of the Urushibara iron catalyst particularly favors the partial hydrogenation of acetylenic compounds to ethylenes.
12. Though Urushibara catalysts are usually employed in liquid phase reduction under both ordinary and high pressures, they can also be used in vapor-phase reduction, provided appropriate apparatus is employed. Specifically, U-Ni-AA is the most convenient catalyst for vapor-phase reduction. The reduction can even be carried out in the usual Sabatier apparatus for ordinary vapor-phase reduction.

3.3. Details On Individual Urushibara Catalysts

1. U-Ni-A and U-Ni-B

   Of all the Urushibara catalysts, U-Ni-A and U-Ni-B are the most commonly used and have the widest applications. Either catalyst will serve for the same catalytic reduction, as there is no substantial difference in activity between the two catalysts. It sometimes happens, however, that one of them is to be preferred according to the kind and purity of the substance to be reduced, or according to the reaction conditions. Both U-Ni-A and U-Ni-B are produced from the same precipitated nickel that is deposited by the reaction between nickel salt solution and zinc dust. The precipitated nickel digested with acetic acid (or propionic acid) gives rise to U-Ni-A, and that digested with sodium hydroxide gives rise to U-Ni-B. In the latter case, the activity of the catalyst is somewhat reduced when the precipitated nickel is treated with an alkali solution of too high a concentration at too high a temperature, or when digestion is continued for a long time until the evolution of hydrogen subsides. Highly active U-Ni-B is obtained when the precipitated nickel is warmed with an approximately 10% solution of caustic alkali for 15 minutes. It contains considerable amounts of undissolved zinc and zinc oxide. In contrast, a good result is obtained in acetic acid treatment only when digestion is continued to such an extent that the zinc and zinc compounds almost completely dissolve away, and a small portion of nickel itself is dissolved to make the solution greenish. U-Ni-A consists of 70-80% nickel, together with a small amount of zinc folded into the former, and little, if any, zinc compounds are contained in it. The same amount of precipitated nickel prepared from nickel chloride and containing 1 g of nickel metal gives rise to Urushibara catalysts of different quantities; the gross weight of U-Ni-A is 1.1 - 1.4 g (nickel content ca. 0.85 g), whereas that of U-Ni-B amounts to as much as 5-10 g (nickel content nearly 1 g), the latter being far more bulky. A neutral catalyst is readily obtained when U-Ni-A is washed a few times with water after preparation. In contrast, alkali combines with U-Ni-B so firmly that a trace amount of alkali can not be removed without much difficulty. When alkali treated precipitated nickel, which is washed twice with water and twice with ethanol, is used in catalytic reduction, the solution in which the reduction takes place be- comes weakly alkaline (pH 9-10), as indicated by the pink color of phenolphthalein. Alkali-treated precipitated nickel, which is washed 5-6 times either with water or with ethanol, makes the solution only faintly alkaline, almost neutral to phenolphthalein (pH 8-9). Trace amounts of alkali are ultimately removed only when the catalyst is washed with water ten times or more, and subsequently many times with ethanol (or other solvent to be used in the reduction process to replace the wash-water.

   U-Ni-B is effectively employed in catalytic reductions for which the presence of alkali is favorable; whereas U-Ni-A is appropriate for those reductions where the presence of alkali interferes. For example, a trace amount of alkali favors the reduction of ketones, aldehydes, nitriles, and oximes, for which the use of U-Ni-B is desirable. On the other hand, the reduction of aromatic nitro compounds is hindered by the presence of alkali and U-Ni-A can conveniently be used in this case.

   It is nevertheless true that U-Ni-B can be used in neutral reductions, if it is washed thoroughly with water to completely remove alkali, and U-Ni-A can be used in the same reductions that are effectively conducted in the presence of U-Ni-B, provided a small amount of alkali is added. This is compatible with the fact that the activities of these two catalysts do not differ substantially from each other.

   The above difference in the natures of U-Ni-A and U-Ni-B is successfully applied to selective reduction. For instance, m-nitroacetophenone gives m-aminoacetophenone in good yield in the presence of U-Ni-A or thoroughly washed U-Ni-B.

   U-Ni-A is far less bulky than U-Ni-B, and has the apparent advantage that it can be readily dispersed into solution when used in liquid phase reduction. This matters little in a reduction under atmospheric pressure, as the reaction vessel can be shaken as vigorously as necessary to obtain thorough dispersion. In a high-pressure reduction, especially in a large scale reduction in a high capacity autoclave, however, the efficiency of reduction is mainly governed by the dispersability of fine catalyst particles into the solution. In such cases, U-Ni-B has a disadvantage, and in liquid-phase reduction at high pressure the use of U-Ni-A is always preferable. Addition of an appropriate amount of alkali may, however, be required in certain cases.



2. U-Ni-BA

   U-Ni-BA is prepared by the alkali treatment of the precipitated nickel which is obtained from nickel chloride solution and aluminum, instead of zinc dust. As the preparation of the precipitated nickel in this case requires much time, it is advisable to prepare and store in advance a large quantity of precipitated nickel, from which the required amount is occasionally removed and treated with alkali.

   The U-Ni-BA catalyst consists primarily of nickel metal, along with a small amount of contaminant, so that the bulk of the catalyst is greatly reduced, and is even smaller than that of U-Ni-A for the same nickel content.

   The main characteristic of U-Ni-BA is its ability to hydrogenate aromatic rings of benzene and naphthalene and their derivatives. U-Ni-A and U-Ni-B can hydrogenate phenols to cyclohexanols, but are inactive to other aromatic compounds. On the contrary, U-Ni-BA is effective for almost every aromatic compound, including phenols.

   U-Ni-BA, like U-Ni-A and U-Ni-B, can be used for the hydrogenation of olefins, or the reduction of carbonyl and nitro compounds, but it gives somewhat poorer results. We can see this in the high-pressure reduction of acetophenone. For general purposes other than for benzene ring hydrogenation, the use of U-Ni-A or U-Ni-B, with their high activity, stability, and availability is recommended.



3. U-Ni-AA

   U-Ni-AA is prepared by warming the precipitated nickel which is deposited from nickel chloride solution by aluminum grains, with acetic acid saturated with sodium chloride. It consists of aluminum grains coated by nickel, as the nickel metal deposited on aluminum grains does not separate from the latter, which remain undissolved after acetic acid treatment owing to the mild reaction between aluminum metal and acetic acid. As the aluminum grains which support the nickel metal act as a carrier, U-Ni-AA can most conveniently be used for a vapor phase reduction.

   It is to be noted that the use of U-Ni-AA is not confined to vapor phase reduction; it is also suitable for liquid-phase hydrogenation at room temperature and atmospheric pressure. This can be seen from the reduction of nitrobenzene in ethanol which, in the presence of U-Ni-AA, gives aniline in a 70% yield.



4. U-Ni-C Catalysts

   To promote the activity of U-Ni-A or U-Ni-B, it is desirable to reduce the particle size of the catalyst so as to facilitate its dispersion into solution. U-Ni-C catalysts comply with this requirement.

   The particle size of Urushibara catalysts is seemingly determined by that of the precipitated nickel. To reduce the particle size of the precipitated nickel, it is necessary to retard the velocity of the ion exchange reaction; that is, to retard the speed of deposition of nickel. The optimum conditions for the slow and uniform deposition of nickel on zinc dust have been established. The precipitated nickel which is prepared by the reaction of zinc dust with nickel chloride solution of an appropriate concentration, either at room temperature or with cooling by running water or ice, is treated again in the cold with acetic acid or sodium hydroxide solution. In this way we obtain U-Ni-CA and U-Ni-CB. These catalysts, compared with ordinary Urushibara nickel catalysts, have a smaller particle size and exhibit higher activity, especially for liquid-phase reduction in an autoclave. These catalysts, however, require much time for preparation, and therefore lack the distinguishing characteristic of the general Urushibara nickel catalysts namely, speed in preparation.



5. U-Ni(s) Catalysts

   Another modified preparation of Urushibara nickel catalysts consists of the addition of nickel chloride crystals, instead of a nickel chloride solution, to zinc dust mixed with a small amount of water. Owing to the heat of reaction, local reaction is promoted and precipitated nickel is formed in a few minutes. It is digested with acid or alkali in the usual way, giving rise to two kinds of catalyst. They are U-Ni-A(s) and U-Ni-B(s).

   The extreme ease with which the precipitated nickel is produced allows the U-Ni(s) catalysts to be prepared very quickly. This ease itself might mislead one to presume that the catalysts would have impaired activities. Nevertheless, they are sufficiently active for use in almost every reduction, and their activities are very close to, although not as high as, those of ordinary Urushibara nickel catalysts. For practical purposes, they may be widely used because of their simple preparation and high activities.



6. U-Ni-A (HCl)

   The use of hydrochloric acid in the digestion of the precipitated nickel greatly reduces catalytic activity. Therefore, U-Ni-A(HCl), which is obtained by treating the precipitated nickel with hydrochloric acid, is inadequate for ordinary purposes, except in certain instances where high activity is not desirable. Though the catalyst gives fairly good results in the partial hydrogenation of acetylenic compounds (see Section 6.5.4), it is practically of little use because the Urushibara iron catalyst is most profitable for such partial hydrogenations.

   U-Ni-A(HCl) and U-Ni-A(s) (HCl) may be used for the reduction of benzoin under high pressure, but their activities can not compare with those of other U-Ni-A catalysts. The cause of the impaired activity is apparently that a considerable amount of nickel is dissolved when the precipitated nickel is treated with hydrochloric acid, so that the nickel content of the catalyst is diminished, or that some chlorine compound is adsorbed on the surface of the catalyst and behaves as a kind of poison.



7. U-Ni-NH₃

   Precipitated nickel also gives rise to an Urushibara nickel catalyst on treatment with aqueous ammonia instead of sodium hydroxide. This is U-Ni-NH₃. A considerable amount of ammonia remains even after washing with water. Ammonia is adsorbed on the catalyst far more strongly than is sodium hydroxide on U-Ni-B.

   Catalytic reduction of nitriles or oximes yields primary amines together with secondary amines. The use of ammonia especially favors the formation of the former. Therefore, the yields of primary amines in the catalytic reduction of these compounds are higher in the presence of U-Ni-NH₃ than in the presence of U-Ni-B.



8. U-Co Catalysts

   Urushibara cobalt catalysts are modifications of the Urushibara nickel catalysts, just as Raney cobalt is a congener of Raney nickel. Their activities compare with that of Raney cobalt and have properties very similar to the corresponding useful nickel catalysts. However, they are less useful for general purposes, as their activities are somewhat less and they are more expensive. In catalytic reduction of nitriles or oximes Raney cobalt surpasses Raney nickel in depressing the formation of accompanying undesirable secondary amines. Likewise, the U-Co-B catalyst was used for the same purpose and proved to be more effective than U-Ni-B. In general, the cobalt catalysts are known to be less active than the nickel catalysts for hydrogenation of the ethylenic bond, U-Co-B can effect the selective reduction of unsaturated nitriles to unsaturated amines, leaving the ethylenic linkage intact.

   It should be remembered that U-Co-B is entirely inactive when used in ethanol saturated with ammonia. Therefore, ammonia should be precluded from U-Co-B, even in the reduction of nitriles for which the presence of ammonia is usually preferable.



9. U-Cu Catalysts

   In general, copper catalysts have an outstanding disadvantage in hydrogenation. This is the case with the Urushibara copper catalyst, which, like the Raney copper catalyst, is far less active than the corresponding nickel or cobalt catalysts. The Urushibara copper catalyst is quite unfit for catalytic reduction under atmospheric pressure, but can be used in reduction under higher pressure, provided the reaction is carried out at a temperature higher than would be appropriate for the nickel catalyst.



10. U-Fe Catalysts

    Crystals of iron(II) or iron(III) chloride are added to zinc dust mixed with a small amount of water, affording precipitated iron which, on treatment with acetic acid, furnishes U-Fe(II) or U-Fe(III), respectively. These Urushibara iron catalysts, like the Raney iron catalyst, have a distinct specificity in favoring the partial hydrogenation of acetylenic compounds to ethylenic compounds. Either U-Fe(II) or U-Fe(III) is suitable for this purpose. It must be remembered, however, that U-Fe-BA, which is prepared from iron(III) chloride solution and aluminum grains, is completely inactive in the hydrogenation of acetylenic compounds.

Preparation of the Urushibara Catalysts

Urushibara Nickel Catalysts

Preparation of Urushibara catalysts is carried out in two stages. The first stage involves the deposition of nickel metal by reaction between a soluble nickel salt and a metal which is more electropositive than nickel. The second stage consists in the treatment of the precipitated nickel with alkali or acid to yield an active catalyst; treatment with a base gives rise to U-Ni-B, whereas treatment with an acid gives rise to U-Ni-A. It is established that the reaction conditions in the first stage, where the precipitated nickel is prepared, have a striking influence upon the activity of the catalyst which is produced.

Zinc, aluminum, and magnesium have been tested for use in precipitating nickel metal from its salt, but as zinc dust has the greatest advantage in the ease with which it is handled, it is being used exclusively in the preparation of ordinary Urushibara catalysts.

As for the soluble nickel salt, the chloride, nitrate, sulfate, and acetate were successively employed, and nickel chloride was found to be the most appropriate for obtaining a catalyst of high activity. Nickel nitrate solution hardly reacts with zinc dust, and nickel sulfate solution yields a catalyst of rather low activity. Nickel acetate, on the contrary, readily yields precipitated nickel, which proves to give as good a catalyst as that obtainable from nickel chloride.

Two methods are available in effecting the reaction between nickel chloride solution and zinc dust. One is the addition of zinc dust to the nickel chloride solution, and the other is the addition of nickel chloride solution to the zinc dust. In an early stage of investigation, the precipitated nickel was prepared exclusively by way of the first method, but it gradually turned out that when zinc dust was used in large excess relative to the amount of nickel chloride, the second method was preferable in that it gave a catalyst of striking activity. Later, a simplified procedure was devised in which nickel chloride crystals were added with stirring to the zinc dust mixed with a small amount of water, giving rise to an Urushibara nickel catalyst of comparatively high activity.

Sodium hydroxide is usually employed in the alkali treatment and activation of precipitated nickel. Potassium hydroxide gives a catalyst of as high an activity, although it requires a longer digestion time. Aqueous ammonia may be used instead of caustic alkali, but the resultant activity is somewhat lower.

Acetic acid is most commonly used as the digesting agent for U-Ni-A. Formic, propionic, and butyric acids were also examined; the details will be described later. In spite of the advantage of propionic over acetic acid in yielding a better catalyst, the latter has the most general application because of its practical convenience. Hydrochloric acid was also examined, but the activity brought about was quite small.

The activity of the Urushibara nickel catalyst gradually increased as successive examinations and improvements were applied to the preparative method. We shall first present the preparative method for U-Ni-B which was employed in an early period of this investigation.

4.1.1. Optimum Conditions for the Preparation of Catalysts

1. Relative Amounts of Nickel and Zinc

   To establish the optimum conditions for obtaining a catalyst of high activity, the amount of zinc dust to be brought into reaction with a fixed amount of nickel chloride has been examined. Several batches of precipitated nickel were prepared from 4.04 g of NiCl₂·6H₂O and amounts of zinc dust varying from 5 to 10 g. They were digested with 10% sodium hydroxide solution, yielding different amounts of U-Ni-B, each of which contained 1 g of nickel metal. A comparison of the activities of these U-Ni-B catalysts, by utilizing them in the reduction of cyclohexanone, revealed that the addition of 9-10 g of zinc dust to 1 g of nickel gave the best result. This experiment was carried out together with that in the next paragraph, and the combined data are illustrated in Table 4-1.



2. Alkali Treatment of Precipitated Nickel

   The amount of 10% sodium hydroxide solution to be used for activating the precipitated nickel has been examined. The experiment was carried out together with that described in the preceding paragraph and each batch of precipitated nickel described above was digested with varying amounts of 10% sodium hydroxide solution to yield various U-Ni-B catalysts. An ion-exchange reaction between nickel chloride solution and zinc dust was carried out on a boiling water bath and alkali digestion was carried out at 50-60°C.

   To compare the activities, a solution of 0.04 mole (3.92 g) of cyclohexanone in 25 ml of ethanol was reduced in the presence of each catalyst at 25-28°C, and the amount of hydrogen absorbed during the first ten minutes was determined. The results are summarized in Table 4-1. We see that 9-10 g of zinc dust to 1 g of nickel combined with 80 ml of 10% sodium hydroxide solution for digestion gives the best U-Ni-B. This catalyst, however, contains large quantities of zinc, zinc hydroxide, and zinc oxide, and the unwieldy bulk of the catalyst, weighing as much as 10 g, limits its application to reductions.

   To obtain a catalyst of less bulk by reducing the amounts of zinc and zinc hydroxide in the catalyst, it is sufficient to raise the concentration of alkali or the temperature of digestion; however, this procedure causes an inevitable reduction in catalytic activity. A catalyst for practical use which is less bulky, though somewhat less active, than the best catalyst mentioned above is prepared by choosing the reaction conditions so as to make the gross weight of the catalyst amount to 6-7 g per gram of nickel. The present standard preparation of U-Ni-B involves 160 ml of 10% sodium hydroxide solution for digesting the precipitated nickel which contains 1 g of nickel.



3. Washing U-Ni-B and Catalytic Activity

   U-Ni-B obtained by digesting the precipitated nickel with sodium hydroxide solution is used for reduction after being washed free from alkali with water. For substances such as ketones, nitriles, oximes, and phenols, which are rapidly reduced in the presence of alkali, U-Ni-B should be prepared without thorough washing, so that a finite amount of alkali remains in the catalyst. On the other hand, for reductions in which the presence of alkali interferes, we must wash the alkali-treated catalyst thoroughly with water.

   Care should be taken to preserve catalytic activity during the washing. Washing in a stream of hydrogen, as in Adkins and Billica's method of preparing Raney nickel W-6,^* which is probably the most advisable process, is, however, very troublesome for practical application. Information is available regarding the effect of the washing temperature on catalytic activity. It describes the change in activity for the reduction of nitrobenzene according to the temperature at which the catalyst is washed in air. The catalyst is washed with distilled water, which is boiled once and then brought to the temperature specified. The activity of the catalyst, which is realized after washing with 50 ml portions of water either ten times or until the wash-water is no longer alkaline to phenolphthalein, is illustrated in Fig. 4-1. Here the activity refers to the amount of hydrogen absorbed during the first 20 minutes, when 0.02 mole (2.46 g) of nitrobenzene in 20 ml of ethanol is reduced at 25°C under atmospheric pressure. We see that good activity is secured when distilled water at 50-60°C is used for washing, the temperature being the same as that of alkali digestion.

   * J. Am. Chem. Soc. 70, 695 (1948); Org. Synth. 29, 24 (1949); Org. Synth. Coll. Vol. 3, 176 (1955)



4. Treatment of Precipitated Nickel with Acids

   Information is also available regarding the change in catalytic activity with the amount or concentration of acetic acid used to digest the precipitated nickel. Several portions of the precipitated nickel, each of which was prepared by adding 5 g of zinc dust to a nickel chloride solution containing 0.5 g of nickel, were warmed with varying amounts of acetic acid. When the reaction had nearly subsided (3-5 minutes), the remaining solids were washed with distilled water at 50-60°C. A solution of 0.02 mole (2.46 g) of nitrobenzene in 20 ml of ethanol was reduced at 25°C under atmospheric pressure in the presence of each catalyst, and the amount of hydrogen absorbed during the first 10 minutes was recorded. The results are shown in Table 4-2.

   It turns out that an insufficient amount of acetic acid causes a reduction in catalytic activity, whereas enough acid, sufficient not only for the complete dissolution of zinc metal but also for the partial dissolution of the nickel itself, gives rise to a catalyst of high activity. The highest activity is obtained when 80 ml of ca. 13% solution is used for digestion.

   Pertinent data for catalysts prepared by digesting the precipitated nickel with formic acid, propionic acid, butyric acid, and hydrochloric acid is listed in Table 4-3.

   We see that high activity is obtained when acetic or propionic acid is used to digest the precipitated nickel, the latter giving the best catalyst. Formic and butyric acids do not activate the catalyst to any great extent. A strong acid, such as hydrochloric acid, greatly reduces activity which, however, can be recovered to a considerable extent when the catalyst is treated subsequently with caustic alkali. It is supposed that hydrochloric acid treatment causes deposition of an unidentified poisonous chemical species containing chlorine and that the catalytic activity is recovered when this is removed by alkali treatment.

   U-Ni-A catalyst prepared by way of acetic acid or propionic acid retains as good as activity as an alkali-treated N-Ni-B would show. For example, 265 ml of hydrogen is absorbed during the first ten minutes when nitrobenzene is reduced in the presence of the best U-Ni-B catalyst under the conditions indicated in Table 4-3, the value being in good accord with that obtained for U-Ni-A.



5. Conditions for the Preparation of Precipitated Nickel

   As we shall see later, an intimate relation exists between the catalytic activity and the crystal structure of the catalyst metal. It is generally believed that the crystal structure of an Urushibara nickel catalyst is determined at the stage where nickel is deposited on the surface of the zinc metal. Hence the activity of a catalyst will depend largely on the conditions under which the precipitated metal is produced. This relation was noticed in an early period of the investigation of Urushibara nickel catalysts, and was established by later work, in which the activities of different U-Ni-A catalysts, prepared from different precipitated nickels, were compared with each other under the same conditions. The results are illustrated in Table 4-4, for which a number of precipitated nickels were produced by varying the concentration of the original nickel chloride solution, or by varying the temperature at which the ion exchange reaction took place. The values, which refer to activities, are the amounts of hydrogen absorbed during the first 5 minutes of the reduction of nitrobenzene in the presence of these catalysts under atmospheric pressure. They are averaged over a number of runs.

   We can see that the more vigorous the reaction between nickel chloride solution and zinc dust, the more active the catalyst produced. The recommended procedure is as follows: Nickel chloride solution of maximally high concentration is added with strong stirring to an excess amount of zinc dust mixed with a small amount of water. The resultant mixture is then placed on a boiling water bath. A high reaction temperature and a short preparation time favor high activity.

4.1.2. Standard Method of Preparation of U-Ni-B and U-Ni-A

The preceding discussion has led to a standardized method for the preparation of U-Ni-B and U-Ni-A. Catalysts so prepared exhibit the highest activities and are the most profitable for practical use. The procedure for the preparation of the precipitated nickel is the same for both catalysts; they are different because of their activation processes. We shall present in the following a standard method for preparing Urushibara nickel catalysts, each containing 1 g of nickel.

Notes:

1. The zinc dust should be as fine as possible; its purity, however, need not be of the highest grade. Commercially available zinc dust is sufficient. The presence of zinc oxide is permissible. Zinc dust containing 10% or more of zinc oxide has been used with success. However, as catalysts obtained from zinc dusts of different grades may have different activities, zinc dust from the same package should be used in order to obtain catalysts of like activity, as are required in comparative studies.
2. If too small a flask is used, the contents may run over wizen a vigorous ion-exchange reaction takes place. An ordinary round-bottomed flask may be used; a wide-necked "egg-shaped" flask is more appropriate in that the contents can be removed more easily. A three necked flask is also convenient, especially for comparatively large scale preparation of precipitated nickel.
3. Nickel chloride may be commercially available material of a chemically pure grade. One gram of nickel corresponds to 4.04 g of the nickel chloride crystals NiCl₂·6H₂O; however, as the crystals are hygroscopic, direct weighing is meaningless. To obtain precipitated nickel of known nickel content, as is required in a comparative study of catalysts prepared under various conditions, it is recommended that the solution be prepared in large quantities by direct weighing so that about 1 g of nickel is contained in 10 ml of solution, the exact concentration of which is then determined with dimethylglyoxime. In this method, the appropriate amount of solution is pipetted prior to use. For ordinary reductions, however, an exact determination of the nickel content is not necessary, and a catalyst containing roughly 1 g of nickel is obtained from 4 g of nickel chloride crystals dissolved in 10 ml of water.
4. An ordinary pipette is not convenient for adding the nickel chloride solution to the zinc dust, as discharge is time consuming, which inevitably causes a reduction in catalytic activity. A pipette of which the tip has been cut off allows the solution to run out quickly and uniformly. The apparatus shown in Fig. 4-2, equipped with an inlet tube through which the nickel chloride solution can be poured from a beaker, allows the uniform addition of solution in 2 to 3 seconds.
5. The product is a dark gray powder-like mass. The wet precipitated nickel, as it stands, is usually subjected to the subsequent digestion process. It has been found, however, that the precipitated nickel always gives an active catalyst on treatment with alkalis or acids, even if it is dried previously at about 100°C and stored in air. Therefore, a large amount of precipitated nickel may be prepared and stored dry; the requisite amount is then occasionally taken out to produce a catalyst, of which the activity is by no means unsatisfactory for practical purposes.
6. 80 ml of 10% sodium hydroxide solution instead of 160 ml yields a more active catalyst; however, it is more bulky and the gross weight amounts to as much as 10 g, because the zinc is dissolved to a lesser extent.
7. The procedure was designed so as to avoid contact with air. However, as it has been verified that drying of the precipitated nickel has little effect upon catalytic activity, one may take an alternative procedure, in which the precipitated nickel is first placed in a beaker, and sodium hydroxide solution is added to it with stirring. The latter process is preferred because liquid overflow due to the vigorous reaction can easily be controlled.
8. The activity of the catalyst is reduced when the temperature at which the precipitated nickel is warmed with sodium hydroxide solution is too high, or when digestion continues so long that the evolution of hydrogen gas subsides.
9. A trace of alkali remains in the catalyst, as revealed by the pink color of phenolphthalein, if rinsing is carried out only 2 or 3 times. In cases where the presence of alkali is undesirable, according to the kind of material to be reduced, washing should be repeated many times until the wash-water is no longer alkaline to phenolphthalein. In such cases, adsorbed alkali can be removed with considerable ease if the catalyst is first washed with two or three portions of warm water, then with saturated sodium chloride solution, and finally several times again with warm water.
10. A catalyst of somewhat higher activity is obtained when 160 ml of 20% propionic acid is used instead of acetic acid, and digestion is continued for 4-5 minutes at 50°C.
11. An alternative procedure may be employed, in which the precipitated nickel is first put into the beaker and then aqueous acetic acid is added with stirring (see Note 7).
12. Digestion may be carried out at 40°C on a water bath for 4 minutes.
13. Part of the nickel dissolves in acetic acid and produces a green color. When the solution is colorless, stirring should be continued until the color develops. In order to obtain a catalyst of high activity, it is necessary to allow digestion to proceed until green coloring develops. However, the catalyst should be filtered off as soon as the coloring appears, because a longer digestion diminishes the nickel content.
14. Acetic acid adsorbed on U-Ni-A can be removed by washing far more easily than can alkali.
15. An alternative method may be used as well, in which the catalyst is transferred to the reaction vessel by means of a small amount of distilled water after washing is completed, and the wash-water is replaced with solvent by decantation.

4.1.3. U-Ni Catalysts Prepared from Nickel Acetate

Nickel acetate, in place of nickel chloride, gives an Urushibara nickel catalyst of like activity. As the reaction between nickel acetate and zinc dust is more violent than that between nickel chloride and zinc dust and is accompanied by strong bubbling of the reaction mixture, it must be carried out in a larger vessel with strong stirring.

Regarding the reduction of benzophenone, it has been established that U-Ni-B prepared from nickel acetate is somewhat more active than that from nickel chloride, whereas the reverse is true with U-Ni-A.

4.1.4. U-Ni-C Catalyst

To obtain a precipitated metal of small particle size, it is necessary to retard the rate of the ion-exchange reaction. The rate of the ion exchange reaction depends on several factors, such as the difference in the standard electrode potentials of zinc and the catalyst metal, the fineness of the zinc dust, the concentration of the solution of metal chloride, the temperature at which the precipitated metal is prepared, and the efficiency of agitation. Of these, only the temperature can be freely controlled; the other factors are mainly decided by the nature or quality of the chemicals. At low temperatures, the ion-exchange reaction takes place slowly and the metal separates out uniformly on the surface of the zinc dust, giving thereby a catalyst of small particle size (see the microphotographs shown in Fig. 5-2, Chapter 5). The precipitated nickel prepared at low temperatures gives a highly active Urushibara nickel catalyst. Its preparation, however, requires so much time that speed of preparation, the unique characteristic of the Urushibara catalysts, is lost.

4.1.5. Simplified Preparations of Urushibara Nickel Catalysts

The reaction between nickel chloride solution and zinc dust is exothermic. Attempts were made to make use of the heat of reaction in accelerating the reaction and in simplifying the procedure. Attempts in which a concentrated nickel chloride solution was added to dry zinc dust failed, as the reaction took place too vigorously to permit thorough agitation, so that a uniform deposit of nickel metal onto the zinc could not be obtained. After a number of trials, it was found that the practicable method consisted of adding crystals of nickel chloride to zinc dust mixed with a small amount of water. By this method, good precipitated nickel is obtained in a few minutes.

Urushibara nickel catalysts prepared from this precipitated nickel are distinguished from ordinary catalysts by adding the bracketed letter(s) after the name, as in U-Ni-B(s) or U-Ni-A(s). In spite of their simple preparative method, the activities of the catalysts are by no means poor, and the hydrogenation of ketones has assured that they are sufficient for practical use.

Notes:

1. The reaction begins at room temperature. As a vigorous reaction takes place, the temperature of the reaction mixture rises to 60-70°C owing to the heat of reaction. In large scale production, the temperature may rise to as high as 100°C, and often most of the water evaporates.

4.1.6. Urushibara Nickel Catalyst Prepared with Aq. Ammonia

Precipitated nickel treated with aqueous ammonia in place of sodium hydroxide gives U-Ni-NH₃. A fairly large amount of ammonia remains in the catalyst even after washing with water.

Notes:

1. In contrast to the case of U-Ni-B, a large amount of aqueous ammonia, sufficient to dissolve away most of the zinc, does not paralyze the catalytic activity.

4. 1. 7. Urushibara Catalyst Prepared with Hydrochloric Acid

It is to be understood that digestion is the process where the precipitated nickel is activated by alkali or acid. It involves the removal of deactivating substances and the erosion of the nickel surfaces. We have seen in a preceding chapter that acetic and propionic acids are good digesting agents, whereas formic, butyric, and hydrochloric acids give poorer results. The reduction of ketones has revealed that U-Ni-A(HCl) is much less active than any other U-Ni-A. Either a considerable amount of nickel may be lost from the catalyst during strong acid treatment, or some chlorine compound may be adsorbed on the surface of the catalyst to behave as a kind of poison.

4.1.8. U-Ni-BA

Aluminum can be used in place of zinc dust for precipitating nickel metal from its salt solution. In this case, nickel chloride is almost exclusively employed as the starting material. The catalyst U-Ni-BA obtained by treating the precipitated nickel with sodium hydroxide solution, shows a specific activity for aromatic ring hydrogenation.

Preparation of U-Ni-BA with Aluminum Powder

Commercially available aluminum powder reacts with nickel chloride solution. However, the reaction is extremely vigorous and the solution foams up, carrying aluminum powder onto its surface together with the froth, which often flows out of the vessel. Practically, the reaction can not be controlled and treatment becomes extremely troublesome. A small amount of a surface-active agent may be added to suppress frothing, but this inevitably brings about a considerable reduction in activity. Therefore, U-Ni-BA obtained in this way is not a good catalyst, though it is still applicable to vapor-phase hydrogenation.

Chips of aluminum wire of proper diameter are also useful, but the chip size should be as small as possible in order to obtain good results. Aluminum grains or chips undergo a violent exothermic reaction with nickel chloride solution, giving precipitated nickel. U-Ni-BA is obtained from this precipitated nickel via a method similar to that established for U-Ni-B.

Catalysts prepared under varying conditions have been compared with each other by applying them to the reduction of acetone, and a standard procedure for the preparation of U-Ni-BA catalyst, as shown below, has been established. The usual U-Ni-B and U-Ni-A can be prepared in a short time, but the preparation of U-Ni-BA requires somewhat longer. This matters little, however, because it turns out that the nickel precipitated from nickel chloride and aluminum grains can be dried and stored; from this precipitated nickel a requisite amount may occasionally be taken out and digested with alkali, giving U-Ni-BA of reserved activity.

Notes:

1. A 500 ml beaker may also be used.
2. Usually fine particles will not settle down completely. As long contact with alkali reduces catalytic activity, the supernatant liquor should properly be removed just before it becomes clear.
3. A one liter vessel may be used.
4. The precipitated nickel may be dried in air. Drying under reduced pressure is desirable, but not necessary.

Modified Preparation of U-Ni-BA

In Preparation 15, we have seen that the ion-exchange reaction is too violent to be controlled by external cooling, so that it fails to give U-Ni-BA catalysts of uniform activity even when the fixed procedure for the preparation is carefully followed. For this reason, the procedure may be modified as shown in Preparation 17.

Pre-treatment of aluminum grains by alkali has proved not to affect catalytic activity. It, however, requires thorough washing before use, to such an extent that the wash-water is completely neutral. The process requires a rather long time, but this can be reduced by employing dilute hydrochloric acid instead of alkali.

The activity of the U-Ni-BA catalyst, just as those of U-Ni-B and U-Ni-A, is mainly determined by the reaction conditions under which the precipitated nickel is produced, and is influenced little by the digestion process.

Experiments were carried out regarding the variation in catalytic activities according to the change in concentration of nickel chloride solution or according to the change in temperature during the ion-exchange reaction. When a solution of 4.04 g of nickel chloride crystals in 10 ml of water is heated to boiling, and is added to aluminum grains in a reaction vessel on a boiling water bath, the reaction is too vigorous from the beginning to be controlled by external cooling with water. This is still the case when the nickel chloride solution is diluted to half of its original concentration. Cooling is only attained by pouring water into the reacting solution. It has been established that when more dilute nickel chloride solution is heated to about 65°C (not to boiling), and added to aluminum grains at room temperature, one can control the reaction temperature at 70-80°C, which is appropriate for obtaining a catalyst of high activity.

As a practical procedure, it is better to add comparatively dilute nickel chloride solution to aluminum grains at room temperature, then warm slightly on a water bath to start the reaction.

A mild exchange reaction gives a black nickel precipitate, and a more violent reaction tends to give a gray to dark gray nickel precipitate; which often exhibits a metallic luster. The color of the precipitated nickel clearly demonstrates that a violent reaction promotes crystallization of the nickel, which tends to decrease the catalytic activity.

In line with the above, a modified method of preparing U-Ni-BA was established. The catalyst was tried in the high pressure reduction of ethyl salicylate and proved to have a uniform activity. This modified procedure, though free from the drawback of non-uniform activity, requires a somewhat longer time for preparation.

Notes:

1. The wide-necked round-bottomed flask may be replaced by a beaker.
2. The temperature can be conveniently regulated by using ice-water and hot water baths alternately.
3. If no heat is applied at the end of the reaction, the green color will not disappear and a catalyst of high activity can not be produced. At this stage, the mixture may safely be heated to 90-100°C.
4. Fine particles will not settle easily. Therefore, the supernatant liquor, should be removed by decantation while still turbid. as long contact with alkali reduces catalytic activity.

4.1.9. U-Ni-AA

U-Ni-AA is a catalyst prepared by treating with acetic acid the precipitated nickel obtained from nickel chloride solution and aluminum grains. The precipitated nickel is usually obtained in the same way as U-Ni-BA.

As aluminum reacts with acetic acid very slowly, special techniques, such as the addition of an appropriate inorganic salt, are required to promote the reaction. It is suggested that 40% acetic acid saturated with sodium chloride is most advisable for practical purposes. In acetic acid treatment, in contrast to alkali treatment, nickel precipitated on the surface of aluminum grains, does not separate from the latter, thereby preventing the aluminum grains from dissolving away. Therefore, the remaining aluminum grains can act as a carrier for the catalyst; this makes U-Ni-AA particularly appropriate for vapor-phase hydrogenation.

Notes:

1. The precipitated nickel is usually prepared by a procedure similar to Preparation 17, but the preparation of U-Ni-AA for vapor phase reduction requires the aluminum grains to be of somewhat larger mesh, and the concentration of nickel chloride solution to be the same as in Preparation 15.
2. The amount of sodium chloride required to saturate this volume of 40% acetic acid.

Related Information

• The Zinc-Nickel Couple in the Hydrogenation of Organic Compounds
• Urushibara Cobalt Catalysts
• Reduction of Nitroalkenes to amines using Urushibara Nickel