tablished in this work because of difficulties with impurities in the toluene, but it is less than three-halves for toluene of the highest purity as judged by the fastest rate. The proposed mechanism suggests a catalyst concentration order greater than 1.
Conclusions The initial rates (up to 20y0 conversion) of hydrogenation of toluene with Raney nickel catalyst are approximately zero order in toluene, half order in hydrogen, and three-halves order in catalyst concentration for unpurified analytical reagent grade toluene. The temperature dependence of the rates could be expressed by the usual Arrhenius equation with an activation energy of 20 kcal. per gram mole. The differences in the absolute rates of hydrogenation among different batches of toluene under identical conditions
Catalyst Structure and Properties
I
HOWARD LITTMAN' and DAVID DEW-HUGHES2 Yale University, New Haven, Conn.
T o
PROVE that Raney nickel catalyst does not age or change its structure during hydrogenation, it was analyzed by metallographic and x-ray techniques. The nature of the yellow and blue molybdenum trioxides was also investigated. In an x-ray characterization ( 3 ) diffuse nickel lines were reported. The reason for these was not investigated but excessive amounts of hydrogen in iron, nickel, and chromium have been reported to give rise to line broadening ( 5 ) . Because the catalyst used contained about 100 cc. of hydrogen per gram of catalyst, this hypothesis is not unreasonable. This large amount of hydrogen comes from the caustic leaching process, in which aluminum is removed from the original nickel-aluminum alloy.
Experimental Raney nickel catalyst, purchased from the Davison Chemical Co., was prepared 1 Present address, Syracuse University, Syracuse, N. Y . a Present address, IBM Research Laboratories, Poughkeepsie, N. Y .
662
were never wholly resolved, but a treatment involving partial hydrogenation minimized them and resulted in high catalyst activity. Use of purified toluene does not appear to change the toluene or hydrogen orders but lowers the catalyst concentration order. A chain mechanism has been proposed for the hydrogenation whose support rests primarily on the experiments using molybdenum trioxide. It is consistent with the orders in toluene and hydrogen and suggests a catalyst concentration order greater than 1.
Nomenclature (Cc), = initial catalyst concentration (Ch)o = initial hydrogen concentration in liquid phase (CJ, = initial toluene concentration P = pressure R, = initial rate of reaction T = absolute temperature a and /3 = constants e = time
from approximately 50-50 nickel-aluminum alloy by caustic leaching and supplied in water. The water was replaced with toluene by distillation in a 30-plate Oldershaw column. The catalyst lost its pyrophoric character when refluxed in toluene for 18 hours. This nonpyrophoric catalyst was used in all the hydrogenation studies. Its activity was good; 4 grams of catalyst hydrogenated 1 liter of toluene at an initial rate of about 970 per hour at 130' C. and 1000 psi. Activity of a Raney nickel catalyst does not depend on its pyrophoric character. The nonpyrophoric catalyst has great advantages in handling.
Analysis of the Catalyst wt. % Ni 96.3 A1 2.7 Fe 1.0 Traces of Cu and Co
Element
Yellow molybdenum trioxide M as prepared according to the method of Liebert ( 4 ) . Apparatus and Procedure. The Raney nickel catalyst in toluene was examined under a microscope, both in toluene and in a dry condition. AS there was no apparent difference, the dry powder was mounted in Bakelite, polished by normal metallographic methods, and examined under a metallographic microscope. A photomicrograph was taken. The specimen was then etched with a 1 to 1 mixture of concentrated nitric and glacial acetic acids in methanol and a photomicrograph was taken. The dry catalyst was subjected to three treatments. After each treatment,
INDUSTRIAL AND ENGINEERING CHEMISTRY
Acknowledgment The fellowship assistance of Allied Chemical and Dye Corp. is gratefully acknowledged. literature Cited (1) Baker, R. B., Scheutz, R. D., J . A m . Chem. SOC.69, 1250 (1946). (2) Cook, M. W., U. S. Atomic Energy Comm., UCRL-2459 (1954). (3) Littman, H., Ph.D. thesis, Yale University, 1956. (4) Lozovoi, A. V., D'yakova, M. D., J . Gen. Chem., U.S.S.R.7, 2764 (1937). (5) Melville, H. W., Robb, J. C., Proc. Roy. SOC.(London) A196, 445 (1949). (6) Ruiter, E. de, Jungers, J. C., Bull. SOL. chim. Belges 5 8 , 210 (1949). (7) Smith, H. A., Meriwether, H. T., J. Am. Chem. Sac. 71. 413 (1949). (8) Smith, H. A., Peinekamp, E. F. H., Zbid., 67, 276 (1945). RECEIVED for review April 8, 1958 ACCEPTED December 15, 1958 Based on a thesis submitted by Howard Littman to the graduate school of Yale University in partial fulfillment of the requirements for the Ph.D. degree.
the dry catalyst was polished and etched and photomicrographs were taken.
1. The catalyst was annealed at 400'C. IongLenough to drive off the hydrogen (usually 30 minutes). This hydrogen was collected and measured; the hydrogen content amounted to about 100 cc. per gram of catalyst. 2. The catalyst was annealed at 1200" C. for 18 hours in a vacuum. 3. A portion of the catalyst from the second treatment was heated at 400' C. for 6 hours in an autoclave under a pressure of 2100 p.s.i. of hydrogen. The catalyst took up about 50 cc. of hydrogen per gram of catalyst. This treatment is referred to as "rehydrogenation." The material from these three treatments and the dried catalyst were xrayed. Photographs were taken with a Philips back-reflection camera of 60mm. radius, an iron target at 50 kv. and 10 ma. The exposure times were 6 hours, and a manganese filter was used. The catalyst was stored in toluene. To determine whether it had aged or changed during hydrogenation, x-ray photographs were taken of tlvo samples taken from storage 1 month apart and two samples taken during and after hydrogenation. Yellow niolybdenum trioxide and a mixture of the blued oxide and Raney nickel catalyst from a hydrogenation run were also x-rayed. These photographs were taken using a Philips 57.3 mm. camera of the Straumanis type, copper K a radiation at 32 kv. and 10 ma. Powder patterns were also obtained using a Philips x-ray Geigercounter spectrometer.
Experimental Results Metallographic. The polished catalyst before reaction and etching contains
TOLUENE HYDROGENATION
Figure 1.
Photomicrographs of Raney nickel catalyst mounted in Bakelite show a two-phase alloy
A. Before reaction, polished ( S O O X ) . 6. Before reaction, polished and etched with 1:l mixture of nitric and acetic acids in methanol Annealed a t 1200' C., polished, and etched with 1:l mixture o f nitric and acetic acids in methanol ( l O O O X )
two types of materials: light and dark (Figure 1, A ) . Some particles consist of only one material; others are composites of both. The size and shape of the particles give the appearance of a ground macerial with a wide range of particle size. I n a composite etched particle (Figure 1, B ) : the relatively complex structure appears to consist of two phases, one light and one dark, which differ in etching characteristics and exist in approximately equal amounts. The catalyst after removal of the hydrogen at 400' C. was structurally unchanged, having the same micrograph as shown in Figure 1, B, for the catalyst with hydrogen. O n annealing at 1200' C. in a vacuum, the appearance of the catalyst is altered considerably (Figure 1, C). The specimen has the appearance of a single phase, though an unusually large number of voids may be seen. Rehydrogenation causes no changes in the micrographic appearance of the catalyst. X-Ray. Precision back-reflection photographs (Figure 2) compare the catalyst untreated and annealed with pure nickel sheet. Nickel sheet annealed at 1200 ' C. in a vacuum shows the K, doublets of the (311) and (222) lines clearly resolved (Figure 2, A ) . The spottiness of the lines is probably due to the large grain size produced during the anneal. The dry untreated catalyst has a low maximum line intensity and considerable broadening is apparent (Figure 2> B ) . The K,, and K,, lines are not resolved. An x-ray taken after the catalyst had been annealed to 400' C. in a vacuum to remove all the hydrogen showed no change in line intensity or degree of broadening; it was identical with Figure 2, B. The 1200' C. anneal of the catalyst sharpens the lines considerably and the K , doublet is clearly resolved (Figure 2, C). In position and sharpness of the lines this photograph compared favorably with that for pure nickel sheet. Thus the cause of the line broadening is not removed with the removal of hydro-
gen from the catalyst but requires a prolonged anneal at high temperatures for complete restoration of sharp lines. The pressure gage on the vacuum furnace showed emission of gas only when the catalyst is at about 300' C.; no further emission of gas is noticed up to 1200' C. I t is thus reasonably certain that the hydrogen content of the catalyst after the 1200' C. anneal does not differ from that after the 400" C . anneal compared to the original amount of gas in the catalyst. T o make sure that the line broadening is not due to some effect of the original presence of hydrogen but retained after removal of hydrogen, rehydrogenation was performed. No line broadening has taken place (Figure 2, D),offering conclusive evidence that the line broadening is in no way connected with the presence of hydrogen. Samples of the catalyst taken one month apart (Figure 3, A and B ) show no change in structure with age. This is in agreement with the fact that hydrogenation runs were duplicated when made 6 months and longer apart. The catalyst after being used for hydrogenation (Figure 3. C) is the same as those before hydrogenation (Figure 3, A and B ) , except that the 26 = 45' line (where .$ is the angle which the rays make \vith the plane of the crystal) has sharpened slightly; the line is still broad, hoivever. T o be sure that this small change did not take place during the run. catalyst from
(lOOOX).C.
the same run taken just after the catalyst and toluene had been heated to the reaction temperature and pressure was x-rayed. The photograph is identical to that in Figure 3, C. The x-ray examination does not indicate the presence of two phases, as does the metallographic examination, but only one phase with a face-centered cubic structure and a lattice parameter of about 3.53 -4. Because the lines on the photographs are very broad, the samples were x-rayed using a Geigercounter spectrometer. The lines could not be resolved. An x-ray powder photograph of the blued molybdenum trioxide and Raney nickel as taken from the autoclave at the conclusion of a hydrogenation run (Figure 3, E:)was compared with photographs of yellow molybdenum trioxide (Figure 3>D)and Raney nickel taken at the end of a hydrogenation run which did not have molybdenum trioxide (Figure 3. C). Figure 3, E, is a composite of 3, C and D; hence it is concluded that the structures of the yellow and blued oxides are identical and that the structure of Raney nickel catalyst is unaffected by the presence of the oxides.
Discussion Metallographic and x-ray examinations of the catalyst appear to be in conflict as to the number of phases
Figure 2. X-ray precision back-reflection powder photographs show a single face-centered cubic phase A.
B. C. D.
Pure nickel sheet, annealed a t 1200' C. Raney nickel catalyst as received Catalyst, annealed a t 1200° C. Catalyst, annealed a t 1200' C. and rehydrogenated
VOL. 51, NO. 5
M A Y 1959
663
Figure 3. X-ray photographs of Raney nickel catalyst and molybdenum trioxide show no change in structure with age A.
Raney nickel catalyst before reaction Raney nickel catalyst befare reaction, sample from same batch as used in A taken 1 month later Raney nickel catalyst after reaction (1 3 1.3’ C., 1 0 0 9 p.s.i.a.) Yellow molybdenum trioxide Mixture of Raney nickel catalyst and molybdenum trioxide after reaction (131.3’ C., 1 0 0 9 p.s.i.a.)
B.
C.
D.
E.
present in the catalyst. The micrographs indicate that the catalyst is a two-phase alloy, while the x-ray examination indicates only a single face-centered cubic phase of lattice parameter 3.53 A., though the lines are very broad. As the presence of 170 iron modifies the nickel-aluminum phase diagram only slightly ( I ) , this discussion is based on this diagram as given by Bradley and Taylor (2) (Figure 4). According to this diagram an alloy containing 2.7 weight 7 0 aluminum should consist of one face-centered cubic phase, the primary nickel solid solution, 01, with a lattice parameter of 3.53 A. Thls is in agreement with x-ray observations. If the micrographs were interpreted as showing two phases, the second phase must be the intermetallic compound Ni& cy’, which is face-centered cubic with a lattice parameter of 3.56 A. X-ray photographs by Bradley and Taylor of an alloy containing both a and a‘ show the two phases distinctly. The amounts of each phase are great enough to give strong x-ray diffraction lines characteristic of each phase. As nowhere can these lines be found on any x-ray patterns, it must be assumed that the alloy consists of a single phase. The micrographic appearance must be explained otherwise. As the micrographic appearance of the catalyst changes to a single phase concurrently with the formation of sharp x-ray lines on annealing at 1200’ C., both the x-ray line broadening and the two-phase appearance of the catalyst may be associated with the same cause. The line broadening was originally thought to result from the large amount of hvdrogen present in the catalyst. Figure 2 indicates that this is not the case. T o account for both the micrographic appearance and the x-ray line broadness, it is necessary to consider the method of preparing the catalyst. The catalyst is prepared from a 50-50 weight 70 664
nickel-aluminum alloy by caustic leaching. The original alloy prior to leaching should consist of two phases, the 6phase. NisAl3. which is trigonal, and the ephase, which is orthorhombic. Presumably as the aluminum is leached out of the alloy by the caustic, collapse of the lattice and diffusion toward equilibrium occur. This has been observed by Taylor and LYeiss ( 6 ) during the leaching of NiZA13 with sodium hydroxide solution. The apparent two phases in the micrograph of the as-received catalyst result from the original t\vo phases prior to leaching. As the rates of attack of the caustic on the two original phases would not be expected to leave identical remains, leaching the two original phases would result in tM.0 types of areas in the alloy with somelvhat different aluminum concentratioqs. As the etching characteristics of these areas will be
0-
20
1
1
1600
I
40 1
1
,
WEIGHT % N I C K E L 60 80
100
I
different, the micrographic appearance is accounted for. The amount of attack might also differ throughout any one of the original phases, depending on the location in the material, regions near the surface of the material suffering greater attack. As the lattice collapses and diffuses toward equilibrium, some unit cells will have approached equilibrium more clo5ely than others. Thus the resultant catalyst will consist of two primary nickel solid solution, cy, areas of differing aluminum concentration, and within each area small variations of aluminum concentration and distorted unit cells. This will give rise to a spectrum of lattice parameters causing broad x-ray lines. The high temperature smooths out all gradients, giving sharp x-ray lines and a single-phase structure. The reintroduction of hydrogen into the lattice Jsould not be expected to change the microstructure or x-ray lines.
Conclusions The catalyst does not age in storage. X-ray analysis shows that no structural changes occur in storage; duplication of kinetic runs at intervals as great as 6 months indicates no activity changes. The structure of Raney nickel catalyst does not change during hydrogenation; however, during hydrogenation it is slightly different from the stored catalyst. The changes apparently occur when the catalyst and reactants are heated to the reaction temperature. The Raney nickel catalyst is a singlephase alloy, a primary nickel facecentered cubic a! phase with a lattice parameter of 3.53 A. The broadness of its x-ray lines does not result from the hydrogen but from concentration variations and distorted unit cells. The blued oxide appears to be structurally the same as the yellow molybdenum trioxide. Any differences must be in the surface structure.
I
-
1400
I
-
J 60
ATOMIC % NICKEL
Figure 4. Nickel-aluminum phase diagram (2) shows the alloy to consist of one face-centered cubic phase
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Acknowledgment Helpful discussions \sit11 FV. D. Robertson are gratefully ackno\vledged. Much credit is due to Robert Bakish, who carried out the original micrographic \sork. literature Cited (1) Am. SOC. Metals, Cleveland, Ohio “Metals Handbook,” p. 1245, 1948. (2) Bradley, A. J., Taylor, A., Proc. Roy. SOC.(London) Ai59, 56 (1937). (3) Dupont, G., Piganiol, P., Bull. sod. chim. 6 , 322 (1939). (4) Melville, H. W., Robb, J. C., Proc. Roy.SOC. (London) A196, 445 (1949). (5) Smith, D. P., “Hydrogen in Metals,” p. 247, University of Chicago Press, Chicago, Ill., 1948. (G) Taylor, A., Weiss, J., Nature 141, 1055 (1938). RECEIVED for review April 16, 1958 ACCEPTEDDecember 15, 1958
