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.
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.
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.
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.
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.
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.
(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.
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.
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.
Notes:
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.
(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.
Notes:
1) The wide-necked round-bottomed flask may be replaced by a beaker.
2) The temperature can be conveniently regulated by using a hot water bath and an ice-water bath 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.
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 60 g 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.
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.
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