New Hydrogenation Catalysts: Urushibara Catalystsby Kazuo Hata, 1971.
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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:
3.3. Details On Individual Urushibara Catalysts
(a) 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.
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.
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.
(d) 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.
(e) 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.
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.
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.
(h) 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.
(i) 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.
(j) 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.
Preparation 1: U-Ni-B
Ten ml of solution prepared from 2 g of crystalline nickel chloride is warmed to 80-90°C and added, over a period of 1-2 minutes with stirring, to 5 g of zinc dust, which has been mixed with a small amount of water and placed in a water bath of the same temperature. Immediately afterward, the precipitate is filtered off with a sintered glass filter and washed with a small amount of hot distilled water. It is then plunged into 100 ml of 10% sodium hydroxide solution as quickly as possible and left standing for 15-25 minutes on a water bath at 50-60°C with occasional stirring. The supernatant liquid is decanted, and the remainder is washed with two 40 ml portions of distilled water at 50-60°C. Then the wash-water is replaced by ethanol. The catalyst, containing 0.45 g of nickel adhering to 2 g of zinc, is thus obtained. The last step, in which ethanol replaces the wash-water, can be omitted when reduction is to be carried out in an aqueous solution.
4. 1. 1. Optimum Conditions for the Preparation of Catalysts
(a) 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 · 6H20 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-l.
(b) 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.
(c) 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, *1 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-l. 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.
(d) 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.
(e) 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.
Preparation 2: The Precipitated Nickel
Place 10 g of zinc dust (Note 1) in a 100 ml round-bottomed flask (Note 2) and add 3 ml of distilled water. A good stirrer, extending almost to the bottom, is fitted and the flask is heated on a boiling water bath. Dissolve nickel chloride crystals in distilled water in such a way that a 10 ml solution containing 1 g of nickel is obtained (Note 3). Heat to boiling in a beaker and pour within a few seconds (Note 4) into the flask containing the slurry of zinc dust and water with vigorous stirring. As a vigorous reaction takes place, the contents should be carefully watched for flooding. The vigorous reaction will soon subside. Stop stirring and filter with suction the solid contents of the flask with a sintered glass filter and wash with about 200 ml of hot water (Note 5). Place the solid mass with a stainless steel spatula into a 300 ml beaker or Erlenmeyer flask for digestion. Washing on the sintered glass filter may be replaced by decantation with several portions of hot water in a beaker.
Preparation 3: U-Ni-B
To a 300 ml beaker or Erlenmeyer flask containing 160 ml of 10% sodium hydroxide solution (Note 6) is added (Note 7) the precipitated nickel (Preparation 2). It is recommended that a part of the sodium hydroxide solution be reserved for washing out the precipitated nickel on a sintered glass filter. As the precipitated nickel reacts with the sodium hydroxide solution with the violent evolution of hydrogen, care should be taken that the contents do not run over. The reaction vessel is heated on a water bath at 50-55°C with gentle stirring for 15-20 minutes (Note 8). The supernatant liquid is decanted, and the remainder washed with two or three 40 ml portions of distilled water, which have been boiled in advance and cooled to 50-60°C. Each time the wash-water is decanted (Note 9), and the remainder is washed with the solvent, e.g., ethanol, to be used in the subsequent reduction, and is then transferred together with the solvent to the reduction vessel. The solid should always be covered with water or solvent after the alkali treatment, so that it is protected from contact with the air. The product is U-Ni-B, a dark gray powder-like solid. U-Ni-B, prepared from nickel chloride containing 1 g of nickel, consists of about 0.95 g of nickel and 4-5 g of zinc, together with small amounts of zinc oxide and zinc hydroxide, the total weight amounting to 5-7 g.
Preparation 4: U-Ni-A
To a 300 ml beaker or Erlenmeyer flask containing 160 ml of 13% acetic acid (Note 10) is added (Note 11) the precipitated nickel (Preparation 2). It is recommended that a part of the aqueous acetic acid be reserved for washing out the precipitated nickel on a sintered glass filter. As the addition of precipitated nickel to acetic acid results in the violent evolution of hydrogen, the contents should be carefully watched to prevent running over. After stirring for 4-6 minutes at room temperature (Note 12), the evolution of hydrogen gas gradually subsides and most of the zinc and zinc compounds dissolve away, a black powder like solid having adsorbed hydrogen appearing on the surface of the solution. When the solution becomes greenish (Note 13), it is carefully filtered and the black solid is collected on a sintered glass filter. It is washed with 200 ml of distilled water (Note 14), which has been boiled in advance and cooled to 50-60°C, then with the solvent to be used in the subsequent reduction (ethanol, for example), to replace the wash-water. The whole of the catalyst is put into the reduction vessel together with the solvent (Note 15). The catalyst on the sintered glass filter should be protected from contact with air, and washing repeated in such a way that the wash-liquid on the catalyst is not exhausted.
The U-Ni-A obtained is a black powder-like solid. U-Ni-A from nickel chloride containing 1 g of nickel contains 0.8-0.85 g of nickel, together with a small amount of contaminants, such as zinc, and weighs 1.3-1.4 g.
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.
Preparation 5 : U-Ni-B from Nickel Acetate
On a boiling water bath, 4.24g of nickel acetate, Ni(CH3CO2)z·4H20, is dissolved in 20 ml of water. The hot solution is added all at once with stirring to a hot mixture of 10 g of zinc dust and 10 ml of water, which is placed in a 500 ml beaker and warmed on another boiling water bath. As a vigorous reaction takes place and the reaction mixture begins to inflate, strong agitation is required to prevent the contents from running over. When the reaction subsides, 200 g of 10% sodium hydroxide solution is cautiously added to the reaction product with stirring. The temperature of the mixture is kept at 50-55°C for 15 minutes with occasional stirring. When the solid matter settles, the supernatant liquor is decanted and the solid is washed with two 100 ml portions of hot water, then with two 50 ml portions of the solvent to be used in the reduction, e.g., ethanol. In this way a bulky catalyst, weighing as much as 8.5-10.5 g, is obtained. The catalyst contains about 1 g of nickel, together with considerable amounts of zinc and zinc oxide and a very small amount of alkali.
Preparation 6 : U-Ni-A from Nickel Acetate
To prepare U-Ni-A from nickel acetate, Preparation 5 should be modified as follows: To the precipitated nickel prepared in Preparation 5 is added, with stirring, 160 ml of 13% acetic acid instead of caustic alkali. The mixture is left standing with occasional stirring until the evolution of hydrogen ceases and a solid rises to the surface of the greenish solution. The solid is collected on a sintered glass filter and washed with 200 ml of hot water, then with 100 ml of ethanol. The catalyst contains only small quantities of zinc and zinc oxide and weighs about 0.7 g.
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.
Preparation 7 : U-Ni-CB
To a 100 ml flask containing 10 g of zinc dust and 4 ml of water is added 10 ml of an aqueous solution containing 4.04 g of nickel chloride, NiCl2 · 6H20. The mixture is stirred at room temperature, or while being cooled with water or ice, until the green color of the nickel ion disappears. The stirring may be interrupted after the first hour, as the ion-exchange reaction is practically complete after this period. The mixture should be left standing thereafter until the remaining faint. color disappears. The whole process requires 3-4 hours. To prepare precipitated nickel on a large scale, it is advisable to cool the mixture. with ice during the ion-exchange process. The slushy precipitate is transferred to a 300 ml beaker and washed with 200 ml of cold water. To digest the precipitated nickel, 160 g of cold 10% sodium hydroxide solution is added to the beaker and the mixture is stirred for an hour. Cooling with water or ice is often required. When most of the solid. settles, the upper liquor is carefully decanted and the solid is washed with two 100 ml portions of cold water and then with two 50 ml portions of solvent. The catalyst contains about 1 g of nickel; zinc, zinc oxide, and a trace amount of alkali are always present in the catalyst. Weight 8-11 g.
Preparation 8: U-Ni-CA
The precipitated nickel (Preparation 7) is transferred to a 500 ml beaker and is carefully treated with 200 ml of 10% acetic acid while being cooled with water. After about 5 minutes, the liberation of hydrogen subsides and a solid comes to the surface of the green solution. The solid is collected on a sintered glass filter and washed with 200 ml of cold water, and then with 100 ml of solvent. The catalyst is a fine powder and weighs 0.6 to 0.8 g. It contains 0.4-0.5 g of nickel together with small amounts of zinc and zinc oxide.
4.1.5. Simplified Methods for the Preparation 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.
Preparation 9: Precipitated Nickel (Simplified Method)
4.04 g of commercial nickel chloride crystals (NiCl2 · 6H20) is added all at once to a 50 ml beaker containing 10 g of zinc dust which is mixed well with 4 ml of water, and the mixture is stirred with a glass rod. Reaction takes place and abruptly becomes violent. The reaction goes on for a few minutes and the mixture inflates into a slushy mass (Note 1). It is then washed with 200 ml of cold water, and the wash-water is removed by filtration or decantation. The precipitated nickel weighs about 13.5 g and contains about 1 g of nickel, together with zinc, zinc oxide, and zinc hydroxide chloride.
Preparation 10: U-Ni-B(s)
To precipitated nickel (Preparation 9) placed in a 300 ml beaker is added 160 g of 10% sodium hydroxide solution with stirring. The resultant mixture is heated to 50° C on a water bath and stirred gently for 15 minutes. The supernatant liquor is decanted and the residue is washed with two 100 ml portions of water, then with the same amounts of solvent. Each time the wash-liquid is removed by decantation. The catalyst is a black powder and is less bulky than ordinary U-Ni-B (the latter is a grayish powder).
Preparation 11: U-Ni-A(s)
To precipitated nickel (Preparation 9) placed in a 300 ml beaker is added 150 ml of 20% acetic acid and the mixture is stirred at room temperature. In a few minutes the evolution of hydrogen subsides and a black solid comes to the surface, when a green color should develop in the solution. At this time, the solid is collected on a sintered glass filter and washed with 200 ml of distilled water at 50-60° C. Before the wash-water is completely drained off, the solid is transferred to a 100 ml beaker with 50 ml of ethanol and the wash-liquid is decanted. The catalyst is further washed with two 50 ml portions of ethanol, and each time the supernatant liquor is decanted. The catalyst should be protected from air as carefully as possible after digestion.
4. 1. 6. Urushibara Nickel Catalyst Prepared with Aqueous 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.
Preparation 12 : U-Ni-NH3
Precipitated nickel containing about 1 g of nickel (Preparation 2) is added to 100 ml of 14% aqueous ammonia (Note 1), and the mixture is stirred gently on a water bath at 50-60° C. After 15-20 minutes of digestion, the evolution of hydrogen gradually subsides. The mixture is left standing for a while and the supernatant liquor is decanted.
The solid is washed with two 20 ml portions of methanol or ethanol. In each case the mixture should be stirred well before the wash-liquid is decanted. The U-Ni-NH3 obtained in this way is a grayish black powder (darker than U-Ni-B) and weighs about 8.6 g. It contains large amounts of zinc and zinc compounds.
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.
Preparation 13: U-Ni-A(HCl)
0.75 N Hydrochloric acid (480 ml) is added to precipitated nickel containing about 1 g of nickel (Preparation 2 or Preparation 9, simplified method) and the mixture is stirred at room temperature. Violent evolution of hydrogen takes place. After approximately one minute of agitation the solution becomes greenish, and a black solid comes to the surface of the solution. At this time, the solid is collected on a sintered glass filter and washed with 400 ml of distilled water. Particular care should be taken to prevent the solid from coming into contact with air. Before the wash-water is drained off completely, the catalyst is transferred into a 100 ml beaker with 50 ml of ethanol, and washed further with two 50 ml portions of ethanol. Each time the supernatant liquor is decanted.
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.
(a) 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.
Preparation 14: U-Ni-BA
(a) for Vapor-phase Hydrogenation
Ten grams of aluminum powder (200 mesh) are suspended in a small amount of water and the mixture is heated on a boiling water bath. Ten ml of nickel chloride solution at 90° C containing 4 g of NiCl2 · 6H20 crystals is then added to the hot suspension. When the vigorous exchange reaction subsides, the contents are heated to dryness and 200 ml of 20% sodium hydroxide solution is added gradually. The contents should be stirred well and cooled with running water, as a vigorous exothermic reaction takes place while the aluminum is dissolved in a short time. The mixture is further heated for 5 minutes. The supernatant liquid is decanted and the residue is washed several times with water at 50-60° C, until the washing water is no longer alkaline to litmus. It is then washed thoroughly with methanol. The catalyst contains about 1 g of nickel.
(b) with Aluminum Grains
Aluminum grains are best employed for preparing precipitated nickel from nickel chloride. As commercial aluminum grains are not uniform, they must be sifted to obtain grains of proper size. Grains of 40 to 80 mesh can best be used; a large mesh often makes the ion exchange reaction difficult to control, giving a catalyst of non-uniform activity. As the commercial product is often stained, it must be treated with about 3% sodium hydroxide solution to clean the surface before being used in preparing precipitated nickel.
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.
Preparation 15: U-Ni-BA
Ten g of aluminum grains (ca. 100 mesh) are washed well with water and 50 ml of 3% sodium hydroxide solution is added. A vigorous reaction takes place, with the liberation of hydrogen, and the solution becomes frothy with the elevation of temperature. Care should be taken to prevent overflow of the contents. When the clean surface of the grains appears, cold water is added to suppress frothing. The supernatant liquor is decanted and the residue is washed several times with water, until the wash-water is no longer alkaline to phenolphthalein.
The purified aluminum grains are transferred with 5-6 ml of water to a 500 ml wide-necked round-bottomed flask (Note 1) and heated on a boiling water bath. In another vessel 8.08 g of nickel chloride crystals, NiCl2 · 6H2O, (corresponding to 2 g of nickel) is dissolved in water to a total volume of 20 ml. This solution is heated to boiling and is poured all at once on the aluminum grains. A violent reaction takes place and the solution froths, with fuming. It is left standing and stirred occasionally until nickel metal deposits on the surface of the aluminum grains, which become black and come up in part to the surface of the solution. The reaction mixture becomes slimy, and then slushy, and the green color disappears. Water evaporates on account of the heat of reaction until the whole mixture forms a viscous semi-solid, which, on cooling becomes nearly solid. The solid is crushed with a glass rod or stainless steel spatula, and washed two or three times with water to remove water-soluble products.
A small amount of water is added to the precipitated nickel, and it is cooled on an ice bath. To the well-cooled mixture, 250 g of 20% sodium hydroxide solution is added in small portions with rapid stirring. Particular care should be taken to prevent the contents from overflowing, by cooling the mixture thoroughly with ice and by adding the alkali as slowly as possible while stirring well. The initial addition of even a small amount of alkali very often causes a violent reaction with a sudden evolution of hydrogen. Therefore, the portions of alkali to be added should be as small as possible, and later additions should be made at proper time intervals. The speed of addition should be controlled to maintain the temperature below 60° C. About 10 minutes are required for the addition of half the total amount of alkali. As the reaction gradually subsides, the other half may be added at once, whereupon the mixture should be stirred until the evolution of hydrogen ceases; warm to 50°C if necessary. The solution looks black on account of the suspension of fine black particles. Stirring is interrupted at this stage and the mixture is left standing for a few minutes. When the solid has almost settled, the supernatant liquor is decanted (Note 2) and the solid is washed with five 100 ml portions of water at 50-60° C, then with three 50 ml portions of ethanol. It is then transferred to the reduction vessel with the aid of the solvent. The catalyst is carefully protected from contact with air after alkali treatment.
Washing should be carried out with distilled water, which is removed each time by decantation.
The product, U-Ni-BA, contains about 2 g of nickel and a small amount of aluminum, together with a trace of alkali. It is a black powder-like solid, and its appearance resembles that of U-Ni-A. It contains very fine particles.Preparation 16: U-Ni-BA (from Stored Precipitated Nickel)
The precipitated nickel prepared according to Preparation 15 (Note 3) from 50 g of aluminum grains (100 mesh) and 100 ml of solution containing 40 g of NiCl2·6H20 (corresponding to ca. 10 g of nickel) is washed well with water. The slushy solid is collected on a Buchner funnel and dried and stored (Note 4). Its gross weight amounts to about 70 g, but changes more or less according to experimental conditions. To obtain U-Ni-BA containing about 2 g of nickel, one-fifth (about 14 g) of the dry precipitate is digested, following Preparation 15, with 250 g of 20% sodium hydroxide solution.
(c) 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.
Preparation 17 : U-Ni-BA (Modified Method)
Place 50 g of aluminum grains (40-80 mesh) in a beaker, wash well with water, and add 50 ml of 6 N hydrochloric acid on a water bath. When the surface of the grains has become clean, the upper liquid is decanted and the aluminum is washed several times with water. It is then transferred to a 1 l wide-necked round-bottomed flask (Note 1), and 200 ml of solution containing 40.4 g of NiCl2·6H20 (corresponding to 10 g of nickel) is poured onto the aluminum grains all at once. The mixture is gently heated for a short time on a water bath to start a mild reaction. The temperature should be maintained below 70° C (Note 2) to prevent the reaction from getting out of control, and the mixture is stirred occasionally with a stainless steel spatula. The aluminum grains gradually turn black as nickel deposits on them, and the reaction mixture becomes a viscous slush. When the reaction subsides, the mixture is heated on a boiling water bath (Note 3). A violent reaction begins again and the whole mixture becomes a massive gel with the green color of the nickel ion disappearing. The semi-solid product is washed several times with water to remove water-soluble matter and the washings are decanted. After the gel-like substance is removed, the resultant slushy solid is collected on a Buchner funnel and dried. The precipitated nickel obtained weighs 65-70 g, differing slightly according to the case, and contains about 10 g of nickel. The precipitated nickel can be stored in a moisture-free vessel. In all of the above procedures, tap water may be used for washing.
To obtain U-Ni-BA containing about 2 g of nickel, one-fifth of the above precipitated nickel is treated with sodium hydroxide solution. The dry precipitated nickel is added in small portions with vigorous stirring to a 1 l or larger three-necked round-bottomed flask equipped with a good stirrer and a thermometer, and containing 250 g of 20% sodium hydroxide solution. As a violent reaction takes place with the evolution of hydrogen, the flask should be cooled in an ice bath with vigorous stirring to maintain the temperature at 50-55° C. Addition of the entire amount of precipitated nickel requires 10-15 minutes. Stirring is continued until the evolution of hydrogen ceases, with the occasional application of heat, if necessary, on a water bath to maintain the temperature at about 50° C. When the reaction is complete, the mixture is left standing for a few minutes to allow the black particles to settle (Note 4), and the upper liquor is decanted. The black matter is transferred to a 100 ml beaker with distilled water and washed with 200 ml of warm distilled water divided into several portions. At the end of this operation, the wash-water should be neutral to phenolphthalein. The solid is washed with the solvent to be used for the hydrogenation, e.g., ethanol, and transferred to the reduction vessel.
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.
Preparation 18: U-Ni-AA
An 80 ml nickel chloride solution made from 32 g of NiCl2·6H20 crystals is heated to 50-60°C and is added with stirring to 60g of aluminum grains (45 mesh) mixed with a small amount of water. An ion exchange reaction takes place, and nickel begins to deposit onto the surface of the aluminum. The reaction is controlled by occasional cooling or heating either with cold or hot water (Note 1). The precipitated nickel is washed with cold water and 385 ml of 40% acetic acid (70° C) containing 89 g (Note 2) of sodium chloride is added. After 3 to 7 minutes standing, the acid is decanted and the residue is washed with about 21 of water (50-60° C), and then with an appropriate amount of ethanol. The catalyst contains 8 g of nickel.
The Zinc-Nickel Couple in the Hydrogenation of Organic Compounds
Reduction of Nitroalkenes to amines using Urushibara Nickel