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 I0 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.

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