Ruthenium and Osmium As Hydrogenation Catalysts

Ruthenium and Osmium as Hydrogenation Catalysts

By G. C. Bond, Pm., and G. Webb, B.s~. Department of Chemistry, University of Hull

Little attention has been given to the use of ruthenium and osmium as catalysts for vapour-phase reactions. This paper shows that both these metals when supported on alumina possess signijcant activity for the hydrogenation of unsaturated hydrocarbons. In the hydrogenation of butene, butadiene and acetylene their activity is comparable with that shown by the other platinum group metals, although ruthenium is generally more active than osmium.

Although much attention has been paid in oximes and hydroxylamines to the corre- recent years to the use of Group VIII metals sponding amines (6). When mixed with as hydrogenation catalysts, comparatively either palladium or platinum, ruthenium dis- little work has been done with ruthenium and plays a synergistic effect, the mixed catalysts even less with osmium. showing greater activity for the reduction of As early as 1925, Fischer and Tropsch (I) both aliphatic and aromatic nitro compounds, found that the order of activity of the Group ketones, pyridine, nitriles and butyne-diol VIII metals for the formation of methane than either metal alone. from carbon monoxide and hydrogen was Sheridan and Reid (7) found that both Ru > Ir > Rh > 0s > Pt > Pd, ruthenium and osmium possessed very little and later Pichler (2) discovered that, at high activity for the vapour phase hydrogenation pressures and temperatures, the Fischer- of acetylene and Sokol’skii (8) found that, in Tropsch synthesis yielded high molecular- alcoholic solution, ruthenium was completely weight paraffins when ruthenium was used as inactive for the hydrogenation of acetylenic catalyst. compounds, although when mixed with Most of the literature deals with the cata- platinum or palladium both ruthenium and lytic properties of ruthenium and osmium in osmium promoted the hydrogenation of the liquid phase. Supported ruthenium cata- ethylenic compounds. Acetylenes are non- lysts are specific for the hydrogenation of car- selectively reduced by ruthenium (9). Ruthen- bony1 groups and will preferentially reduce ium in the liquid phase is claimed (9) to carbonyl groups in the presence of ethylenic exhibit a high selectivity for the hydrogenation linkages (3). Recent work by Rylander et aE. of mono-substituted olefins in mixtures of (4) has shown that most of these reductions olefins, because the rate of double-bond are not possible unless water is used as solvent. migration is less than the rate of hydrogena- Ruthenium will reduce sugars (3), and, at tion. It is also claimed (9) that ruthenium high temperatures and pressures, poly- shows little tendency to promote cis-trans saccharides (9,to polyhydroxy alcohols. isomerisation. Ruthenium is also active for Ruthenium alone is completely inactive for the hydrogenation of aromatic molecules (10). the hydrogenation of nitro groups, although The catalysts studied in the course of the under suitable conditions it will readily reduce present investigation consisted of I per cent

Platinum Metals Rev., 1962, 6, (l), 12-19 12 metal supported on =-alumina and were prepared by evaporation of a solution of a salt of the metal on to the support. Ruth- enium trichloride and ammonium chloro- osmate were the salts chosen since both are water soluble. However, it was soon found that when an aqueous solution of either salt was heated, the salt decomposed the osmium salt and to evaporate off the with the formationof a black precipitate. Since water in the usual manner even though some acids attack the support, acidic solutions could decomposition does occur, which results in not be used, although they were stable to heat. the loss of a small amount of the metal. In 2N ammonium hydroxide solution, the The activation of the catalysts is described ruthenium trichloride does not decompose in the following section. and the ammonium chloro-osmate decom- poses to an appreciably smaller extent than in Experimental Results aqueous solution. To overcome the decom- Two types of system were used: one static, position of the osmium salt, the excess water the other dynamic. In the static system, the was removed, after the impregnation of the catalyst rested on the bottom of a cylindrical support, by evaporation under reduced reaction vessel, which was sealed into a con- pressure at room temperature. This method ventional high vacuum system. The reactants proved unsatisfactory, however, since in the were admitted separately and the reaction resulting catalyst, the metal was very un- progress followed manometrically. In the evenly distributed on the support. It was flow system, the catalyst was placed in the finally decided to use an alkaline solution of form of a thin bed, in a U-shaped reaction

1.00

0.90

:0.80 5 F U W A # 0.70

Fig. 1 Variation of selectivity 0.6C with conversion for ucetylene at 135" (hatched circles) and butadiene at 0" (open circles): ruthenium catulyst. Initial 0.50 hydrocarbon pressure, 50 mm: 0 20 40 60 ea 100 initial hydrogen pressure, PER CENT CONVERSION 100 mm

Platinum Metals Rev., 1962, 6, (1) 13 0 20 40 Fig. 2 Variation of selectivity 1.00 0 TEMPERATURE OC with temperature for acetylene (hatched circles: initial ff2/ C,H,=2) and butadiene (open 0.90 circles: initial H,/C,H, =3): ruthenium catalyst

-: 0.80 2 c Y 0.70

0.60

+ TEMPERATURE "C 0.50 0 Id0 I60 200 vessel and hydrogen was allowed to flow out using 0.3 g of catalyst, but this was active through it continually. Shots of hydrocarbon, only in the temperature range 170' to 200'C of known pressure, were admitted into the and was subsequently replaced by a 1.0 g hydrogen stream and after each had passed sample which was active at 90°C. Addition of through the catalyst bed the products were the hydrocarbon to the reaction vessel before condensed in a liquid air trap. On warming the hydrogen led to continuous deactivation the trap, the products were passed directly of the catalyst and most of this work was through a gas-liquid chromatography column. carried out using the reverse order of addition A 40 per cent W/W acetonyl-acetone on fire- of the reactants. This procedure led to con- brick (30-60 B.S.S. mesh) column was used stant activity and to slightly faster rates. The at room temperature for the analysis of C4 selectivity, defined as thc ratio of olefin pro- hydrocarbons and a silica-gel (40-60 B.S.S. duced to total olefm and paraffin, was found mesh) column, heated to 80°C, for C, hydro- to vary slightly from one sample of catalyst to carbons. another and also with use of the catalyst, but in all cases was found to be 0.80 & 0.05 at Ruthenium Catalysts 150°C. The variations of selectivity with con- In the flow system the catalysts were version, temperature and initial hydrogen activated by reduction of the supported salt pressure are shown in Figs. I ,z and 3 respec- at 20ooCJthe reduction being continued until tively; selectivity was found to be independ- the effluent gas contained no hydrogen chlor- ent of initial acetylene pressure and of the ide. In the static system, the catalysts were order of addition of reactants. The pressure- either obtained from samples previously fall against time curves showed a continuous reduced in the flow system, or were reduced decrease in rate and were found to be approxi- in situ at zooo for about five hours. The latter mately second order in hydrogen. In reac- tended to show a somewhat greater activity tions with an excess of hydrogen, the rate than the former. increased after a certain pressure fall, the The hydrogenation of acetylene, butadiene increase being due to the hydrog-snation of and the isomerisation and hydrogenation of ethylene which is faster than the hydrogena- butene-I were studied, and the kinetics of tion of acetylene. The point at which this these reactions are shown in Table I. acceleration occurs was found to be independ- Initial reactions with acetylene were carried ent of temperature and initial hydrogen

Platinum Metals Rev., 1962, 6, (1) 14 Fig. 3 Variation of selectivity with initial hydrogen pressure, for acetylene (hatched circles: 110") andbutadiene (open circles: 13.5"): 2. ruthenium catalyst. I- Initial hydrocarbon - I-t pressure, 50 mm V w throughout d

0 100 200 800 INITIAL HYDROGEN PRESSURE (mm) pressure, and in every case was equal to 1.40 shown in Figs. I, 2 and 3 respectively. The + 0.05 times the initial acetylene pressure. pressure-fall against time curves were linear Analysis of the products at IOO and 200 per until about 85 per cent conversion, when a cent conversion revealed the presence of slight acceleration was observed. It was about 5 per cent of C,and C4hydrocarbons, observed that up to IOO per cent conversion formed by the polymerisation of the acetylene. the distribution of the isomeric butenes For the hydrogenation of butadiene, 0.2 g remained constant and thence adjusted itself of catalyst were found to be active at o°C: until about 140 to 160 per cent conversion, butadiene was admitted before the hydrogen. depending upon the temperature, it reached The selectivity for mono-olefin formation was the equilibrium value obtained from thermal determined and its dependence on conversion, data. The initial distribution appears to be temperature and initial hydrogen pressure is independent of hydrogen pressure but varies

100

I- 5 " 60 0 Wn + U 2 40 nB

Fig. 4 The hydrogenation of butene-1 20 over ruthenium at 25°C: variation of hydrocarbon composition with conversion (initial HglC4H8=l). Filled circles, n-butane: open circles, bu- 0 tene-1 :jilled squares, trans-butene-2: 0 20 40 60 80 100 open squares, cis-butene-2: triangles, CONVERSION PER CENT total butene-2

Platinum Metals Rev., 1962, 6, (1) 15 of the catalyst varied from reaction to reaction and it was found impossible to estab- lish any dependence of rate on any of the variables in the system. With both acetylene and butadiene the selectivity was found to be much lower than in the static system. Thus for acetylene a maxi- slightly with temperature; at 0°C the distri- mum selectivity of about 0.5 was obtained bution of butene-1 : trans-butene-a : cis- and with butadiene it varied within the wide butene-2 is 69 : 20 : 11 per cent, while at limits of 0.35 to 0.75 depending primariIy 50°C it is 60 : 24 : 16 per cent. upon the age of the catalyst. By carrying out Fig. 4 shows the variation in the composi- reactions in the static system using very large tion of the products of the hydrogenation of excesses of hydrogen and adding the hydrogen butene-1 with conversion, using 0.3 g of first, it has been shown that neither of these catalyst at 25°C; it can be seen that isornerisa- factors is responsible for the low selectivities tion occurs quite rapidly. From the rate of found in the flow system. The most reaction and the analysis of products it has plausible explanation appears to be that, been possible to calculate the individual while the catalyst at the bottom of the velocity constants for hydrogenation and bed may behave normally with respect to isomerisation. In this way an activation energy selectivity, by the time the reaction products and an order of reaction have been calculated have passed completely through the bed some for the double bond migration (see Table I). of the olefin has been further hydrogenated. Both acetylene and butadiene were also With one sample of catalyst, a selectivity of hydrogenated in the flow system. The activity about 1.0 was obtained for butadiene, but

2. -I- I-t V w d

0.30- Fig. 5 Variation of selectivity with conversion for acetylene at 120" (hatched circles) and butadiene at 67" (open circles); 0.201 osmium catalyst, (initial 0 20 40 60 60 100

HJhydrocarbon ~ 5) CONVERSION PER CENT

Platinum Metals Rev., 1962, 6, (1) 16 Fig. 6 Variation of selectivity 3 4P 60 80 with temperature for acetylene 0.80 (hatched circles: initial H,/ 0 TEMPERATURE 'C C2H2=5)and butadiene (open circles: initial H,IC4H6=5): 0.70 osmium catalyst

>. -f 0.60 I- WV 4 0.50

0.40

+ TEMPERATURE 'C 0.30 I 0 160 180 200 this is abnormal and is presumably due to per cent of the total analysed product. The selective poisoning of the catalyst, the exact pressure-fall against time curves showed a nature of which has not yet been established. continuous decrease in rate although the pre- cise shape of the curves during the initial Results for Osmium Catalysts stages of reaction was dependent upon the The catalysts were activated in an analo- order of addition of reactants. The depend- gous manner to that described for ruthenium ence of selectivity on conversion, temperature and the same reactions were studied. Table 11 and initial hydrogen pressure is shown in summarises the results obtained. Figs. 5, 6 and 7 respectively. Using a fivefold For the hydrogenation of acetylene it was excess of hydrogen, an acceleration in rate necessary to use 2.0 g of catalyst, and this was was observed after a pressure fall of 1.75 times active at IZOTusing a fivefold excess of the initial acetylene pressure. The depend- hydrogen. Analysis of the reaction products ence of the acceleration point on temperature after zoo per cent conversion revealed that C, and initial hydrogen pressure has not been and C, hydrocarbons were present, although established. they were undetectable at IOO per cent con- For butadiene 0.3 g of catalyst was found version. The polymers constituted about 10 to be active at 25°C using a fivefold excess of

0.70

> k 0.60 2 I- V W

Fig. 7 Variation of selectivity with initial hydrogen pressure, for acetylene (hatched circles: 117") and butadiene (open circles: 67"). InitiuE acetylene 0.40L------0.3 0 0 50 100 I50 200 250 pressure 30 mm: initial butadiene pressure, 50 mm INITIAL HYDROGEN PRESSURE (mm)

Platinum Metals Rev., 1962, 6, (1) 17 100 Fig. 8 Hydrogenation of butene-I over osmium at 25°C: variation of hydrocarbon composition with conversion (initial H,/C,H, =5). Filled 60 circles, n-butane; open circles, butene-l; fclled squares, trans-butene-2; !- open squares, cis-butene-2; triangles, WZ total butene-2 lJ 60 a W a e- osmium is, however, less active than ruthenium. At high temperatures, both metals are also

20 active for the hydrogenation of acetylene, in disagreement with earlier work (7). The order of reactivity of the hydrocarbons is 20 40 60 'O loo acetylene < butadiene < butene-I, CONVERSION PER CENT a sequence which is common to the other platinum metals, hydrogen; the pressure-fall against time plots which are only of comparable activity to were linear. The dependence of selectivity on rutheniun and osmium. conversion, temperature and initial hydrogen Two points in particular require discussion. pressure is shown in Figs. 5, 6 and 7 respec- The first concerns the role played by the geo- tively. The distribution of the butenes, metric structure of a metal in determining its which was constant up to IOO per cent con- catalytic behaviour. Now ruthenium and version, was (at 55T) 60.0 : 20.5 : 19.5 per osmium crystallise in the hexagonal close- cent. The dependence of the distribution on packed form, while the other platinum metals temperature was as with ruthenium. have the face-centred cubic structure. The Fig. 8 shows the distribution of products previously-found (7) inactivity of ruthenium for the hydrogenation of butene-I; it can be and osmium in regard to the hydrogenation seen that much less isomerisation occurs over of acetylene was ascribed to the absence of osmium than over ruthenium and the maxi- any spacings in the exposed planes suitable mum isomerisation occurs at a much higher for the adsorption of acetylene. However, conversion. The activation energy and order even if we assume, as is commonly done, that have been calculated for the double bond acetylene prefers to adsorb on the longer migration and are shown in Table 11. interatomic spacings, this need not mean that The results obtained for osmium in the close-packed hexagonal metals should be in- flow system showed the same trend as for active, since such spacings are indeed avail- ruthenium. Both acetylene and butadiene able. Fig. 9 illustrates this: the grey spheres gave a lower selectivity than in the static sys- represent atoms forming part of a (3034) tem and the catalytic activity varied from plane, and this plane contains atoms suitably reaction to reaction. spaced. Particularly in large crystallites, such planes may not be commonly available, but Discussion in view of the relatively small difference in We have established that supported ruthen- activity between say ruthenium and rhodium ium and osmium are active catalysts at for acetylene hydrogenation, it is to say the moderate temperatures for the vapour phase least questionable whether such geometric hydrogenation of mono-olefins and a diolefin; considerations play any part at all. A final

Platinum Metals Rev., 1962, 6, (1) 18 Fig. 9 A model of’ a portion of a close- packed hexagonal crys- tal. The grey spheres represent part of’ a (30j4)plane,which contains spaces suitable for adsorbing acetylene

decision cannut yet be reached, but at least is readily understood in a very qualitative the question has been reopened. way, and will be discussed in a subsequent The second point requiring discussion con- article. The reasons underlying the differ- cerns the similarities which exist between, on ences between the second and third row the one hand, ruthenium, rhodium and pal- metals are, however, much more difficult to ladium, and, on the other, osmium, iridium understand, but this problem will be returned and platinum. The second row metals are to again. characterised by (i) a fairly high selectivity We may summarise the work reported here in the hydrogenation of acetylene and buta- by saying that the study of the catalytic diene, and (ii) a marked tendency to encour- behaviour of ruthenium and osmium is play- age double-bond migration, whereas the third ing a vital role in increasing our understand- row metals show (i) lower selectivities than ing of the behaviour of the other platinum the metal immediately above it, and (ii) metals. little tendency to promote double-bond The authors would like to thank Johnson, migration. The interconnection between Matthey & Co Ltd for giving this project selectivity and rate of double-bond migration their generous support.

References I F. Fischer, H. Tropsch and P. Dithey Brennstoff-Chem., 1925, 6, 265 2 H. Pichler ...... ibid., 1938, 19, 226 3 G. Gilman and G. Cohn . . .. Adv. Catalysis, 1957, 9, 733 4 E. Breitner, E. Roginski and I?. N. Rylander ...... J. Chem. SOC.,1959, 2918 5 A. A. Balandin, N. A. Vasyunina, G. S. Barysheva and S. V. Chepigo . , Izvest. Akad. Nauk S.S.S.R., Otdel. Khim. Nauk, I957J 3g2 6 P. N. Rylander and G. Cohn . . .. Second International Congress on Catalysis (Paris, July 19601, Paper 43 7 J. Sheridan and W. D. Reid , . .. 3. ChW. SOC., 1952, 2962 8 D. V. Sokol’skii ...... Zhur. obschei Khim., 1952,22, 1934 9 L. M. Berkowiz and P. N. Rylander . . J. Org. Chem., 1959, 24,708 10 A. Amano and G. Parravano .. Adu. Catalysis, 1957, 9, 716

Platinum Metals Rev., 1962, 6, (1) 19