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HILTONA. SMITH,ANDREWJ. CHADWELL, JR.,AND S. S. KIRSLIS
Vol. 59
THE ROLE OF HYDROGEN I N RANEY NICKEL CATALYST BY HILTONA. SMITH,ANDREW J. CHADWELL, JR.,AND S. S. KIRSLIS Contribution No. l S 4 from the University of Tennessee, Department of Chemistry, Knoxville, Tenn. Received February 66,1966
The hydrogen content of Raney nickel has been found by direct analysis to vary between one-half and one atom of hydrogen per atom of nickel. The activity of a sample of this catalyst has been shown to be pro ortional to its hydrogen content. The surface area decreases linearly with loss of hydrogen until about 70% is removed; &en it decreases more rapidly. If the hydrogen is released rapidly, heat is evolved which results in an explosion of the catalyst. The best ex lanation for these phenomena appears to be based on the assumption that the hydrogen is in the form of atoms attached to t i e nickel in a metastable state. If desorption of the hydrogen is rapid, the highly exothermic recombination of the hydrogen atoms becomes explosive.
In the reaction of nickel-aluminum alloy to form Raney nickel catalyst, a considerable amount of hydrogen is formed. The amount and character of the hydrogen in the catalyst has been the subject of some controversy. Raney' describes the catalytic material as a hydride of the formula NiH2. Bougnult and co-workers2 also concluded from analytical experiments that this catalyst was a hydride or mixture of hydrides. Vandae13 measured the quantity of hydrogen liberated from the hot leaching bath, and also claims that Raney nickel is a hydride of formula NiH2. Difference methods such as those used by these workers may be subject to considerable error. Freidlin and Ziminova4 found that one gram of Raney nickel contained 95.3 ml. of hydrogen. This corresponds approximately to the formula Ni2H. The catalyst lost its activity when all of the hydrogen was removed. These authors do not consider the catalyst to be a hydride, but rather skeletal nickel promoted with hydrogen. They consider that part of the hydrogen is adsorbed and part dissolved.6 Furthermore, it, is difficult to replace the dissolved hydrogen during a hydrogenation reaction.6 These workers postulate that the loss of activity of the catalyst under the usual hydrogenation conditions is caused by destruction of active sites due to dehydrogenation and blocking.' Aubrys also states that the activity of Raney nickel is due to adsorbed and absorbed hydrogen, and that the catalyst behaves as a reversible electrode to hydrogen. I n the pH range from 5 to 14, Raney nickel follows potential changes in the same manner as a hydrogen e l e ~ t r o d e . ~ The influence of a number of factors in the preparation have been studied by Adkins and coworkers, who designat)e a series of catalysts of different activity depending on the particular conditions used.10 It has been claimed11 that the various (1) M. Raney, I n d . Eno. Chem., 32, 1199 (1940). (2) J. Bougault, E. Cattelain a n d P. Chabrier, Bull. S O C .chim. France, 151 5 , I699 (1938). (3) C. Vandael, I n d . Chim. Belg., 17, 581 (1952). (4) L. K. Freidlin a n d N. I. Ziminova, Dokladg Akad. Nauk S.S.S.R., 7 4 , 955 (1950); Izuest. A k a d . Nauk S.S.S.R., Oldel K h i m . N a u k , 659 (1950). (5) L. K. Freidlin and N. I. Ziminova, ibid., 145 (1951). (6) L. K. Freidlin a n d K. G . Rudneva, Dokladv Akad. Nauk S.8.S.R.,81, 59 (1951). (7) L. K. Freidlin and K. G . Rudneva, Izuest. Akad. Nauk S.S.S.R., Otdel Khim. Nauk, 1111 (1953). ( 8 ) J. Aubry, Bull. S O C . chim., [5] 5 , 1333 (1938). (9) A. Travers a n d J. Aubry, A l l i X e congr. intern. chim., 2 , 546 (1938). (10) (a) A. Pavlic and H. Adkins, J . A m . Chem. S O C . ,68, 1471 (1946); (b) 69, 3039 (1947); (0) H. Adkins a n d H. Billica, ibid., 70,
conditions of preparation have little influence on the activity of Raney nickel as a catalyst in the hydrogenation of d-limonene. In the procedure of making these tests, the alcohol under which the catalyst was stored was evaporated from the catalyst under vacuum after which the d-limonene was added. Lieber and Morritz12 suggest that with the probable loss of hydrogen under these conditions, it is doubtful that the more active types of Raney nickel were being investigated. In view of the many controversies concerning the hydrogen in this catalyst, it was decided to determine by a direct method the hydrogen content of Raney nickel. In addition, the activity of this catalyst toward the hydrogenation of benzene and its ability to adsorb palmitic acid13were determined as a function of its hydrogen content.
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Experimental Raney nickel alloy was the commercial product, reported to have the following analysis: Ni, 51.06; AI, 48.19;. Si, Fe and Cu combined, 0.75. The alloy was converted into Raney nickel catalyst by treatment with sodium hydroxide according to the directions of Adkins and Billica,'oo except that 60 grams of alloy was treated with 300 g. of alkali in 1000 ml. of water instead of 125 g. of alloy with a solution of 160 g. of alkali in 600 ml. of water. In addition, hydrogen was bubbled through the wash water for 10-15 min. prior to its use, but no excess hydrogen was maintained over the mechanical washer. The use of a hydrogen atmosphere in the washing process made no difference in the activity of the catalyst toward the hydrogenation of benzene. The catalyst was stored in a refrigerator under absolute ethanol, and all measurements were made within 72 hours from the time of preparation of the catalyst sample. A C.P. grade of benzene was purified by fractionation in a five-foot helix-packed column. Eastman Kodak Co., white-label palmitic acid was used for adsorption studies. The total hydrogen content of the nickel catalyst was determined as follows: A sample of Raney nickel was placed into one arm of a tared, U-shaped vessel. The vessel was then cooled to -80" and the arm sealed off. The cold vessel was then attached to the hydrogen collecting system by means of a ball joint. A coarse fritted glass disk sealed between the catalyst and the joint prevented loss of nickel particles. The vacuum line led through a cold trap ( -80') which could be by-passed after all of the alcohol was condensed, through a mercury diffusion p ' f ~ " followed by a cold trap, and then through a sealed &hco Pressovac" pump which discharged the hydrogen into a gas buret. The whole system was carefully tested for leaks. It was necessary to start the diffusion and mechanical pumps for 695 (1948); (d) H. Adkins a n d H. Billica, O w . Syntheaea, 29, 24 (1949); (e) H. Adkins a n d G . Krsek, J . A m . Chem. Soc., 70, 412 (1948). (11) H. A. Smith, W. C. Bedoit, Jr., a n d J. F. Fuzek, ibid., 71, 3769 (1949). (12) E. Lieber and F. L. Morritz i n "Advances in Catalysis," Vol. V, Academic Press Inc., New York, N. Y., 1953, p. 419. (13) H. A. Smith and J. F. Fuzek, J . A m . Chem. Soc., 68, 229 (1946).
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THEROLEOF HYDROGEN I N RANEY NICKELCATALYST
Sept., 1955
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% ' of hydrogen removed from Raney nickel catalyst. Fig. 1.-Catalytic activity of Raney nickel catalyst for the hydrogenation of benzene at. 80' as a function of hydrogen content of the nickel. several hours before each run in order to attain temperature equilibrium. The usefulness of this system was verified by attaching a calibrated bulb of hydrogen and comparing the volume collected with that known to have been present. The results always checked within 0.5%. The catalyst bulb was opened to the system and thoroughly evacuated. The cooling bath around the bulb was then removed, and the alcohol allowed to distil from the bulb to the cold trap. NO hydrogen was evolved until the alcohol had been removed. At this point, the cold trap bypass was opened, and the catalyst heatled with a Bunsen burner. Hydrogen was evolved with increasing rapidity until the catalyst virtually e ~ p l o d e d . ' ~At this point, heat was produced so rapidly that the nickel attained red heat even though the outside of the glass bulb was relatively cool. Heating was continued for 15 minutes, and the volume of evolved hydrogen recorded. The vessel was then allowed t o cool and a stopcock between the ball joint and glass frit closed. The catalyst bulb was removed and weighed. Next, dry oxygen was carefully admitted to the catalyst vessel, and a stream of oxy en subsequently led over the nickel. The bulb was carefulfy heated in the presence of oxygen until the entire sample was converted to the oxide. Excess oxygen was led through a tared absorption tube containing anhydrous magnesium perchlorate. When the highly exothermic reaction had subsided, the vessel was heated with the full flame of a Bunsen burner. The amount of moisture formed in the combustion process was determined gravimetrically. When it was desired to remove only part of the hydrogen from a catalyst sample, a similar procedure was followed; an evacuating and filling apparatus similar to that previously describedI3 was employed. A sample was placed in the adsorption-type bulb which was then attached to the vacuum system. The bulb was cooled t o -80" and degassed. I t was then warmed to room temperature, the alcohol removed, and the desired amount of hydrogen collected. The bulb could be immersed in warm water to speed up hydrogen evolution. However if the temperatu;? was raised above loo", the catalyst would "explode. Evolution of hydrogen was stopped by cooling the bulb to -SO", and the catalyst covered with alcohol before admitting atmospheric pressure. A sample of the nickel was withdrawn for catalytic activity measurements. The aluminum content of the Raney nickel was determined by the method of Willard and Tang.I5 The adsorption of palmitic acid on the nickel samples was Reaction measured by methods previously described rate constants were determined for the hydrogenation of benzene. The kinetics of this reaction show first-order dependence on hydrogen pressure. Quantities of catalyst used ranged from 0.2 to 0.3 g., initial pressures were around .11013
(14) Mr. Raney (ref. 1) reports t h a t the catalyst which has been dried under acetone will decompose with a flash when warmed gently. (15) H. H. Willard and N. K. Tang, I n d . Enu. Chem., Anal. E d . , 9, 357 (1937).
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% of hydrogen removed from Raney nickel catalyst. Fig. 2.-Surface
area of Raney nickel catalyst as a function of hydrogen content.
1000 p.s.i. and 10 ml. of benzene was employed in a bomb of 65 ml. total void. The shaking rate (46 cycles per minute) was such that the rate of the reaction was a linear function of catalyst weight.
Results The amount of hydrogen obtained from various samples of catalyst all prepared in the same manner showed considerable variation. Values ranged from 92 to 157 standard milliliters per gram of catalyst. The moisture collected after oxidation of four different catalyst samples from which the hydrogen had been removed by pumping was equivalent to 1.4, 1.3, 1.3 and 0.4% of total hydrogen. Three analyses for total aluminum in the catalyst showed 11.5 0.3% as A1 or 21.8 i 0.6% as A1203. The catalytic activity of a sample of the nickel as a function of its hydrogen content is shown in Fig. 1, while the influence of hydrogen in the catalyst on the adsorption of palmitic acid is given by Fig. 2. Discussion The amounts of hydrogen found on the various catalyst preparations corresponded to atomic ratios of hydrogen to nickel between one-haIf and one. In no case was there any indication that a hydride of the formula NiHz was present. The variation in the hydrogen associated with different catalyst preparations is in line with the concept of adsorbed hydrogen as suggested by Freidlin' and by Aubry.s The amount of hydrogen in Raney nickel catalyst which can be made to react with readily reducible organic compounds4 is comparable to the amounts reported here. The activity of any given catalyst varies linearly with the amount of hydrcgen associated with the nickel. This is readily seen from Fig. 1. The loss of activity of the catalyst when all of the hydrogen is removed was also noted by Freidlin and Z i m i n ~ v a . ~The small amounts of hydrogen remaining after pumping a t high temperatures were apparently trapped in the catalyst pores, perhaps by alumina formed during the catalyst preparation, Figure 2 shows that the surface area as determined by palmitic acid adsorption is a linear func-
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HILTONA. SMITH,ANDREW J. CHADWELL, JR.,AND S. S. KIRSLIS
tion of hydrogen content until some 70% of the hydrogen is removed. This may well explain the parallelism between surface area and activity of a catalyst as previously rep0rted.l' The catalyst is known to loose hydrogen slowly on standing. After 70% hydrogen evolution, the ability of the catalyst to adsorb palmitic acid decreases rapidly. When all of the hydrogen has been removed, the surface area of the catalyst is about one-fifth of its original value. The total aluminum content found for the catalyst studied is compatible with that reported by Ipatieff and Pines16 who found that W-6 nickel contained 12.72% total aluminum. However, their statement that the catalyst contained 21.13% aluminum oxide has been challenged by Watt and Parker." These authors also find some 12 to 13% total aluminum in W-6 nickel, but state that the maximum present as the oxide does not exceed 1%. (16) V. N. Ipatieff and H. Pines, J . Am. Chem. Soc., 72,5320 ( 1 9 5 0 ) . (17) G. W. Watt and S. G . Parker, ibid., 74, 1103 (1952).
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No attempt was made in this research to determine the manner of combination of the aluminum. The evidence presented here may be explained by the concept that Raney catalyst is a form of nickel promoted by adsorbed hydrogen. This hydrogen is evidently in the form of atoms attached to the nickel in a metastable state. If this is to be considered a hydride, its composition is rather indefinite. The hydrogen atoms are slowly desorbed on standing. This process may be accelerated by pumping, although the hydrogen evolution is still rather slow. The process may be greatly accelerated by increasing the temperature. I n fact, the desorption may be so rapid that the highly exothermic recombination of the hydrogen atoms becomes explosive. The catalytic activity of the nickel toward the hydrogenation of benzene is proportional to its hydrogen content. Acknowledgment.-The authors are grateful to the Office of Ordnance Research, United States Army, for the sponsorship of this research.
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