Nature and Reactivity of Intermediates in Hydrogenation ofButa-l,3-Diene Catalyzed by Cobalt and Palladium-GoldAlloysBY B. J. JOICE, J. J. ROONEY,” P. B. WELLS AND G. R. WILSONDept. of Chemistry, The University, HullReceived 3 1st Jdnudry, I966The gas-phase hydrogenation of buta-1,3-diene has been studied in a static system using alumina-supported cobalt, a series of cobalt powders, and pumicesupported palladium-gold alloys as catalysts.But-1-ene and but-2-enes, which were formed by 1,2- and 1,4-addition of hydrogen respectively toadsorbed diene, were initial products under all conditions, and yields of n-butane were either very lowor zero. The distributions of butenes were very dependent on the physical nature of the cobaltcatalysts. Well-sintered metal, consisting of large particles, gave selective formation of trans-but-2-ene, whereas, cobalt prepared in a finely divided state at lower temperatures was more active per unitsurface area, but gave high yields of but-l-ene and much lower trans to cis ratios in the but-2-enes.These variations are attributed to differences in the catalytic properties of different crystal faces andthe change in reaction mechanism is believed to be mainly due to a change in the ease of formationand stability of 1-methyl-n-ally1 intermediates.Stable n-allylic complexes are formed readily on the surface of palladium and various amountsof gold in the alloys have a small effect on the mechanism of buta-1,3-diene hydrogenation.Thebut-1-ene yields and activation energies exhibited sharp maxima which coincided in the 60-75 % goldrange and an explanation in terms of variations in the nature of the palladium &orbitals is attempted.The temperature dependencies of the product distributions were influenced between 80 and 130°Cby the p- to a-phase change of the hydrides of palladium and palladium-rich alloys and the effect isdiscussed.The concept that the chemisorption bond may, in appropriate cases, have n-character has been introduced in recent years to interpret several features of hydro-carbon reactions with hydrogen or deuterium on transition metal catalysts.Themajor distinction between this and previous theories is that multi-centred bondingbetween hydrocarbon species and individual metal atoms in the surface is considered.Thus, reactions such as dehydrogenation and hydrogenation of paraffins, olefins,diolefins, acetylenes, aromatic hydrocarbons, etc., may occur by the interconversionof a variety of 6- and 7c-bonded complexes.*-4 The n-complexes may include boththe metal-olefin and metal-arene types.Besides providing mechanistic insights, this theory leads to important generalconclusions about surface reactions.First, catalytic activity and selectivity areconsidered to be functions of the chemical properties of individual surface atomsrather than functions of the bulk properties of the metals. Secondly, there may be acorrelation between metal-adsorbate bonding on the one hand, and metal-ligandbonding in organometallic compounds on the other ; a preliminary attempt at sucha correlation has shown some success.4 Thirdly, the role of the geometric factor incatalysis is given a new significance.Because of metal-metal bonding the arrangementand number of atoms which are nearest neighbours to a given surface atom influencethe energy levels and electron occupation of the valency-shell atomic orbitals of thelatter, and thus its bonding properties, as well as determining the number of co-ordinating positions available for the intermediates in a catalytic reaction. A view,* present address : Dept. of Chemistry, David Keir Building, Stranmillis Road, Belfast, 9.22224 HYDROGENATION OF BUTADIENEpreviously held, that metal-metal spacings and thus the type of crystal face exposedplay a vital role because only some of these match the geometrical requirements formultiple a-bonding of adsorbates may be of limited validity.The present theory also proposes that interpretation of catalytic behaviour mustultimately take into account, as does co-ordination chemistry, fundamental electronicproperties which influence the nature and strength of metal-ligand bonds.Theseproperties include, among others, the number of electrons in the valency shell of theisolated atom, the effective nuclear charge, d+s and d+p promotion energies andionization potentials. Such properties distinguish the co-ordination chemistry, andpossibly to some extent, the catalytic behaviour of one metal from that of another.Progress in heterogeneous catalysis is limited by uncertainties concerning thenature of metal-metal bonds in solids and by ignorance of the precise arrangements ofatoms which constitute active centres in catalysts.Therefore, a fruitful approach inthis field at the present time may be one in which comparisons are made of the forma-tion, stability and reactivity of the same ligands in organometallic complexes and inhomogeneously and heterogeneously catalyzed reactions, the comparisons being madefor as wide a range of metals as possible. By using co-ordination chemistry as aguide, a clearer picture of some of the fundamental factors governing heterogeneousreactions should emerge.The hydrogenation of buta-l,3-diene (subsequently referred to as butadiene) is aparticularly suitable reaction to test the validity of this approach.This is the simplestconjugated diene and it has been used to prepare a variety of organometallic compoundswhich exhibit several types of CT- and n-bonds. Furthermore, something is alreadyknown of the properties of all the group 8 metals as catalysts for the heterogeneoushydrogenation of this compound,S and homogeneous hydrogenation of butadiene andother dienes, catalyzed by some transition metal complexes is being studied byseveral workers.697There are several reports of the liquid phase hydrogenation of butadiene,s-11with nickel, palladium and platinum as catalysts and ethanol as solvent, but only onereport 11 contained a discussion of mechanism which was based on an ionic model.In 1963, Meyer and Burwell reported details of the gas-phase reaction of butadienewith deuterium using a palladium-alumina catalyst.This was followed by a moredetailed study by Bond, Wells et d 4 ~ 5 9 13-15 of the catalytic activities of each of thegroup 8 metals and copper €or the gas-phase reaction of butadiene with hydrogen;deuterium was also used when examining cobalt, nickel, copper, palladium andplatinum.14, 15Iron, cobalt, nickel, copper and palladium are the only metals which selectivelyhydrogenate butadiene to butenes.5,14 The yield of n-butane in the initial productsis zero for iron, cobalt and copper, 0-3 % for nickel and palladium, and considerablefor all the other group 8 metals.4, 5 , 13 The butene distributions obtained by selectivehydrogenation are far removed from the thermodynamic equilibrium compositionsand they remain constant, or nearly so, until the near removal of butadiene.Theyare also independent of the initial hydrogen pressure. These features suggest thatbutene desorption occurs in preference to isomerization, i.e., the butenes are formedentirely by 1,2- and 1,4-addition of hydrogen, a conclusion which is supported byisotopic studies. The reaction of butadiene with deuterium using cobalt, nickel andcopper,ls and palladium 14 has revealed a close similarity in the deuterium contentsand distributions in each of the three n-butenes.Some typical butene distributions for alumina-supported cobalt, nickel, copperand palladium catalysts are given in table 1, which shows two important features.First, the importance of 1,4-addition varies widely (from 15 % over copper to 70 % oveB.J. JOICE, J. J. ROONEY, P. B. WELLS AND G. R. WILSON 225cobalt), and secondly, the trans/& ratio in the but-2-enes is much higher for cobaltand palladium ( > equilibrium) than for nickel and copper ( < equilibrium).In the adsorbed state a butadiene molecule must take up one of two possibleconformations.CH2=CH CH-CH// \\CH2 CH2\CH=CH2I 11Conformational interconversion may or may not occur and the same is true of thehalf-hydrogenated state, i.e., adsorbed C4H7 species. For butadiene molecules in thegas phase, rotation about the central carbon-carbon bond occurs but is hindered bysteric repulsion of the vinyl groups in conformation 11. Smith and Massingill 16have estimated the ratio I : I1 to be 95 : 5 approximately at ambient temperatures.Thus, at the instant that adsorption occurs the population of I is expected to exceedthat of 11 on the surface.TABLE 1 .-TYPICAL INITIAL B UTENE DISTRIBUTIONS OBTAINED FROM BUTADIENE HYDROGENATIONIN A STATIC SYSTEM USING ALUMINA-SUPPORTED COBALT, NICKEL,COPPER AND PALLADIUM CATALYSTSInitial butadiene pressure = 50 mm ; initial hydrogen pressure = 150 mm ;initial butane yield = zero.temp.butene composition (%)("C) B-1 t-B-2 C-B-2 metalc o 125 29 65 6Ni 77 49 34 17c u 100 85 6 9Pd 21 60-2 3 7.0 2.8Pd 18 68-4 30.1 1.7Co and Ni reduced at 400" and 250" respectively.When 1,4- addition produces a high yield of trans-but-Zene compared to cis-but-2-ene we propose that the nature of chemisorption is such that (i) the conformationsof adsorbed C4H7 do not interconvert and (ii) for adsorbed diene either the conforma-tions are non-interconvertible, or they interconvert but maintain an equilibriumsimilar to that which is operative in the gas phase.The notation for the adsorbedspecies is shown in fig. 1 . Table 1 shows some results for palladium, obtained byA0.orb.t w I I - I - DaIhyI -TT- d l y l (- 5%)FIG. 1 .-Notation suggested for the adsorbed species in butadiene hydrogenation when conforma-tional interconversion of the half-hydrogenated state does not occur.Leszczynski and Wells where translcis ratios in the but-Zenes approach closely thevalue of 19, which would signify virtually complete non-interconvertibility of theconformations on this metal at room temperature226 HYDROGENATION OF BUTADIENEAt the other extreme a trans/cis ratio of about unity signifies that conformationalinterconversion of adsorbed C4H7 and perhaps C4H6 occurs readily.The mechanismshown in fig. 2 allows this possibility to the extent that the key a-bonded C4H7 species,CH3--CH--CH=CH2, is formed during reaction. The two 1 -methyl-n-ally1 com-plexes are believed to be less stable in this case and are either true intermediatesor represent a more transient state attained by C4H7 species before the addition ofthe second hydrogen atom.I8-1/CH./ n cn2 ,/ =;+.c;=,c*. =- cr' -ig'c"-c*, H, I-'-',,'! . * . cn,= C M c H;= c nw 1 \cu-c";,,:;cHf" -- 'FC", w_ C H , / a'" - c " 1 'C",' ,'''I * \\ 1 ).:2 !I ',%\? C H = C Y , ---+j <\ ' CY,C" c*=c*,-K- 8-111 :W - c*<' 4 Cv\cn, - <-'-' ..;. I: 1".... '.+. &U.-c** II - I C H I ~ 'cn, --- 1FIG. 2.-Notation suggested for the adsorbed species in butadiene hydrogenation when conforma-tional interconversion of the half-hydrogenated state occurs.Two points are noted. Addition of the first hydrogen atom to adsorbed butadieneis believed to occur exclusively at a terminal carbon atom in agreement with the generalbehaviour of this compound in addition reactions. Secondly, for those butenylspecies other than n-allylic complexes, addition of the second hydrogen atom takesplace to the carbon atom which is a-bonded to the surface.It follows that if l-methyl-n-allylic complexes are not formed during hydrogenation only 1 ,Zaddition of hydrogencan occur.Comparisons of fig. 1 and 2 with table 1 suggest strongly that x-allylic complexesare formed readily on cobalt and palladium surfaces but not on those of nickel andcopper. This concurs with the organometallic properties of these metals 17 and withthe previous finding that palladium has outstanding ability to form n-allylic complexesduring the catalyzed exchange of cycloalkanes with deuterium.192 Our contentionis therefore that the compositions of the product butenes yield information about themode of chemisorption of their precursors.We have successfully modified cobalt and palladium catalysts in that their stereo-selectivity for the formation of trans-but-2-ene has been diminished.Modificationhas been achieved (i) by varying the conditions of preparation of the cobalt catalystsand (ii) by alloying palladium with gold. These features and other accompanyingfeatures are discussed in terms of the mechanisms described above.EX PER1 M EN'T ALCATALYSTSA stock of alumina-supported cobalt catalysts containing 10 % (wt/wt) metal was pre-pared by depositing the required amount of A.R. cobalt nitrate by evaporation of an aqueoussolution in which the support (Peter §pence type A alumina) was immersed. The drymaterial was then calcined, followed by reduction of metal oxide under 300 mm of hydrogenin a static system at 329°C. The hydrogen was changed after 6 h and the reduction con-tinued for a further 14h.Samples from the stock were reduced again at 200°C under100 mm of hydrogen for 2 h before being used.Cobalt powder P1 was prepared by the reduction of an aqueous solution of cobalt sulphatB. J. JOICE, J. J. ROONEY, P. B. WELLS AND G. R. WILSON 227by hypophosporous acid. The procedure, developed by Holbrook and Marsh,l8 consistedof adding hypophosphorous acid to the metal ions in a boric acid+ sodium hydroxide buffersolution at pH9.0. Hydrogen was evolved after heating to 90°C for 20min and a blackprecipitate of cobalt deposited. Spectrophotometric analysis showed a phosporus contentof 2 %.Cobalt powders P2-PS were prepared by reduction of grey cobalt oxide (B.D.H. Ltd.)in a stream of hydrogen for 24 h at elevated temperatures.X-ray powder photographswere obtained for all the powders, surface areas of P2, P4 and P8 were measured by theB.E.T. method, and P2 and P8 were examined by electron microscopy.Stocks of pumice-supported palladium, gold, and palladium-gold alloys were preparedby deposition of palladium and gold chlorides (Johnson, Matthey and Co. Ltd.) from acidsolution on 18-30 B.S.S. mesh pumice (Hopkin and Williams Ltd.). After evaporation todryness, the salts were reduced to metal in a stream of hydrogen for 24 h at room temperature,then at 200°C for 2 h, followed by further reduction for 3 h under 100 mm of hydrogen at300°C in a static system. Finally, the preparations were heated in cacuo for 3 h at 450°C.Samples from the stock were reduced again at 200°C under 100 mm of hydrogen for 2 hbefore being used.These catalysts contained 5 % (wtlwt) metal.APPARATUS, MATERIALS AND EXPERIMENTAL METHODSThe apparatus consisted of a conventional high vacuum system. Each catalyst samplerested on the bottom of a cylindrical 100 ml Pyrex reaction vessel and pressure changes duringreaction were measured manometrically.Butadiene (Distillers Company Ltd.) contained no detectable impurities either by G.L.C.or mass-spectrometric analysis.Hydrogen was purified by passage over a Pt-silica catalyst at 300°C and then dried, or bydiffusion through a palladium thimble.Gas samples withdrawn from the reaction vessel were analyzed using a G.L.C. unitcontaining a 25 ft column of 40 % (wtlwt) hexane-2,5-dione supported on 30-60 B.S.S.meshfirebrick. The column was operated at room temperature with nitrogen as carrier gas,and the detector was a katharometer.Mass-spectrometric analyses were obtained using a modified A.E.I. M.S.3 spectrometer,with an electron beam energy of 12.0 eV.RESULTSCOBALT CATALYSTSThe distributions of isomeric n-butenes obtained from butadiene hydrogenationwere dependent on the previous temperature treatment of the catalyst. For example,when cobalt-alumina was prepared by reduction of the supported oxide by deuteriumat 329"C, subsequent hydrogenation of diene at 144°C gave but-1-ene as the mostabundant isorner, whereas, a catalyst sample which had received further treatmentat 414°C under 200 mm deuterium for 20 h, gave trans-but-2-ene as the major product.The following results are typical (reaction temp.= 144°C ; initial PD2/PC4H6 =I= 211 ;analyses after 20 % removal of diene).I________ ___ reduction products (%)temp. ("C) B-1 t-B-2 C-B-2 n-butanereaction A 329 51 31 17 1reaction B 414 28 56 16 0The yields of cis-but-2-me and n-butane were only slightly influenced by the moredrastic treatment. Both types of butene distribution were independent of conversionup to 60 % removal of butadiene. In reaction A the average number of deuteriumatoms present in each butene was 2-00+0*01 ; it was also observed that the '' hydroge228 HYDROGENATION OF BUTADIENEexchange ” reaction [Dz + H (ads) +€ID + D (ads) J and the butadiene exchangereaction [C4H6 + D (ads) +C4H5D + M (ads)] occurred at similar rates.In reaction Bthe average deuterium content of each butene was 1.39f:O-Ol but here the rate ofbutadiene exchange exceeded the rate of “ hydrogen ” exchange.Orders of reaction measured before and after the treatment at 414°C were similar,being zero or slightly negative in butadiene and 1-0-1-5 in hydrogen. Apparentactivation energies were 12-2* 1.2 kcal mole-1 (94-153°C) and 8.9 f 1.0 kcal mole-1(100-1 87°C) before and after this treatment respectively.These results suggest either that a change in the physical structure of the cobaltparticles accompanied the treatment at 414°C or that a change in the support hadoccurred which altered the properties of the metal. To decide between these alterna-tives the catalytic activity of unsupported cobalt, in powder form, was investigated asa function of preparation temperature.P 2 P3 P4PSPb p7 p 0I Typo B Bchoviou200 4 0 0 6 0 0reduction temp.(“C)FIG. 3.-Butene distributions and selectivities (S = C~HS/(C~H~+ C4H10)) for the hydrogenation ofbutadiene at 110°C over a series of cobalt powders Pl-Ps, prepared at various reduction temperatures.Initial butadiene pressure = 50 mm ; initial hydrogen pressure = 100 mm ; analyses after 12f 1 %removal of butadiene.S, 0 ; but-1-ene, 0 ; trans-but-2-ene, A ; cis-but-2-ene, 0.The effect of reduction temperature on the physical characteristics of a series ofcobalt powders is shown in table 2 and in plate 1, the products obtained from butadienehydrogenation at 110°C are shown in fig.3. The powders produced at lower tem-peratures were also more active per unit weight or per unit surface area than thoseproduced at higher temperatures.Powders Pl-P3, which catalyzed the preferential formation of but-1-ene (type Abehaviour), had relatively high surface areas and consisted of small particles of metalwhich were mostly or completely in the a-phase (c.p.h.) ; micrograph (a) in plate 1shows the presence of large numbers of particles below 50 A in size.Powders P5-P8, which catalyzed the preferential formation of trans-but-2-ene(type €3 behaviour), were sintered ; their surface areas were relatively low and electronmicrographs revealed an almost complete absence of small particles (micrograph (b)is typical). The metal was mostly or completely in the P-phase (f.c.c.).Powder P4PLATE 1.-Typical electron micrographs of the edges of cobalt particle clusters : (a) powder P2prepared at 298°C ; (b) powder P8 prepared at 580°C ; width of micrographs = 0.75 p.[To face page 228B. J. JOICE, J. J. ROONEY, P. B. WELLS AND G. R. WILSON 229reduced at intermediate temperatures, exhibited intermediate physical characteristicsand catalytic behaviour. Fig. 3 shows, as for cobalt-alumina, that the yields of cis-but-2-ene and n-butane are not greatly influenced by the reduction temperature.Behaviour similar to that shown in fig. 3 has also been observed using nickelpowders where there is no change of phase as the reduction temperature is increased.TABLE 2.-PHYSICAL CHARACTERISTICS OF COBALT POWDERSP1 p2 p3 P4 PS p6 P7 P8reductiontemperature(’C) 97 298 324 415 443 467 514 580weight used (g) 0.263 0.034 0.021 0.183 0.202 0.176 0.277 0.303surface area (m2 g-1) 144 4-8 0.9phase of cobalt pure a pure a pure *a mixture p p purep pure p* some cobalt oxide present.a+P (trace a) (trace a)PALLADIUM-GOLD ALLOYSThe initial mixture of reactants used throughout this work consisted of 100 mmof hydrogen and 50mm of butadiene unless otherwise stated.Although freshlyprepared catalysts were highly active they were somewhat unstable but after about5 runs at room temperature the rate of reaction had declined to a steady value and theproduct distributions became reproducible and virtually independent of conversionup to some 80 % removal of butadiene.The selectivity for butene formation wasvery high on all the catalysts and only decreased slightly with conversion. Somebutene distributions and selectivity values for low and high conversions on threecatalysts of widely different gold content are given in table 3.TABLE 3 .-DEPENDENCE OF BUTENE DISTRIBUTIONS AND SELECTIVES ONCONVERSION ON PALLADIUM-GOLD ALLOYStemp. Pd conversion butenes (%)(“0 ( %I ( %) B-1 t-B-2 C-B-2 S19 100 22.4 48.1 47.5 4.4 1 -00019 100 76.4 48.8 46.2 5.3 0.96720 50 17.9 47-6 46-9 6-3 1.00020 50 85.8 48.1 44.9 7.0 0.98 119 25 20.5 49.4 43.5 7.2 0.99119 25 74.0 49.0 43.3 7.6 0.989Orders of reaction were determined from initial rates, the initial pressure of onereactant being held constant at 50mm while the pressure of the other was variedbetween 50 and 200 mm.Orders in hydrogen were between 0-7 and 0-9 while theorder in butadiene was zero in all cases. During these estimations of order inhydrogen the products were analyzed at 50 % conversion and found to be virtuallyindependent of hydrogen pressure.Activation energies were evaluated from initial rates in the range 048°C but,because of its low activity, the value for the 95 % gold alloy (all compositions areexpressed as atomic percentages) was obtained for a slightly higher, though over-lapping temperature range. A 100 % gold catalyst was almost completely inactiveat 200°C. The activation energies and corresponding initial yields of but-1-ene inthe total butenes are given in table 4.While the activation energies did not vary within experimental error between 0 and60 % gold a sharp increase of -4.5 kcal mole-1 was noted when the gold content230 HYDROGENATION OF BUTADIENEreached 65 %.At 70 % gold the value was still high but on increasing to 75 % asharp drop of -5.0 kcal mole-1 was found and the lowest activation energy was thatfor the 95 % alloy. A corresponding maximum in the yields of but-1-ene was alsoobserved in the 60-70 % gold range.TABLE 4.-oRDERS OF REACTION, ACTIVATION ENERGIES, AND BUT-1 -ENE YIELDSAT ROOM TEMPERATURE FOR PALLADIUM-GOLD ALLOYSPd( %)1008050403530255order(H2)0.90.80.90.90.70.7--order(C4H6)0.00.00.00.0-0.10.0--E(kcal mole-1)10.510.211.010.314-914.69.48.9B- 1(%I48.555.050.059.561.060.050.051.0The temperature dependencies of the product distributions were then examined byanalyzing reaction mixtures after 50 % conversion at several temperatures between18 and 200°C.Yields of the individual butenes are plotted against temperature forthree catalysts in fig. 4. The major effect of increasing the reaction temperature,4 0 8 0 120 160 100T "CFIG. 4.-Temperature dependence of butene distributions in the hydrogenation of butadiene onpalladium-gold alloys.0 % gold, A ; 65 % gold, B; 75 % gold, C.but-I-ene, 0 ; trans-but-2-ene, @ ; cis-but-Zene, 0.using pure palladium, was an increase in the yield of cis-but-2-ene at the expense of thetrans-isomer, while the yield of but-1-ene remaified almost constant.A feature ofthese results was the sharp dip through a minimum and corresponding rise througha maximum in the yields of but-1-ene and trans-but-Zene respectively between 100and 130°C. Similar results were also obtained with the alloys but as the gold contentincreased the dip and rise in the yields of but-1-ene and trans-but-2-ene became lesB. J. JOICE, J. J. ROONEY, P. B. WELLS AND G. R. WILSON 231sharply defined and broadened over a much wider temperature range. The minimumand maximum in the appropriate curves also shifted to slightly lower temperatures,especially with alloys of higher gold content. The only alloy which did not exhibitthese features was that containing 95 % gold where the yields of but-1-ene and cis-but-2-ene fell and rose in a linear fashion respectively, while the yield of trans-but-2-eneremained virtually constant, as the temperature was raised.Yields of cis-but-2-eneoften rose more steeply in the same temperature range where the dip and rise in thepercentages of the other isomers occurred than at higher or lower temperatures.The yields of but-1-ene were also about 10 % higher over the whole temperaturerange for the 60-70 % gold alloys than for those of higher or lower gold content.Maxima in the yields of n-butane were also found between 90 and 130°C andthese are shown for some of the catalysts in fig. 5. While the maximum was highestfor 100 % palladium it decreased steadily and broadened as the gold content increaseduntil it was barely perceptable, within experimental error, at 75 % gold.4 0 8 0 I 2 0 I60 200T ("C)FIG. 5.-Temperature dependence of n-butane yields in the hydrogenation of butadiene on palladium-gold alloys.0 % gold, 0 ; 20 % gold, A ; 50 % gold, 9 ; 65 % gold, 0.Isothermal plots of the ratios of trans-but-2-ene to cis-but-2-ene as a function ofalloy composition are shown in fig.6. An increase in gold content at a fixed tem-perature resulted in a continuous decline in this ratio over the whole range of com-position. When the temperature was raised at a given composition there was also adecrease in the ratios, the effect being most marked for 100 % palladium, and becoiningless as the alloys became richer in gold.We believe that the peculiar changes in product yields in the temperature range80 to 130°C might be associated with the p- to a-phase change in palladium andpalladium-gold hydrides so hydrogen solubility in these catalysts was briefly studied.A weighed quantity of each catalyst (-2.5 g) was maintained at 200°C in 150 mm ofhydrogen for 2 h, cooled at room temperature, and allowed to equilibrate with freshhydrogen at a pressure of 75 mm.The temperature was then raised by 1°C every2-3 min and the pressures recorded up to 160°C; the sample was then cooled at thesame rate and again the pressures noted. Since the system was effectively one ofconstant volume these pressures, when corrected to s.t.p., were a measure of the hydro-gen evolved or absorbed by the catalysts.A slight evolution of hydrogen from 100 %palladium, 0.016 atoms per palladium atom, was found up to 89"C, but at this tem-perature rapid evolution commenced and finally ceased at 109°C ; the total hydroge232 HYDROGENATION OF BUTADIENEevolved in this range was 0.38 atoms per palladium atom. The alloys behavedsimilarly with slow evolution of a comparable quantity of hydrogen, but in the criticaltemperature range the amount of hydrogen released decreased continuously withincreasing gold content and at 70 % gold or more evolution could not be detected.The temperature range for rapid evolution of hydrogen from the alloys was between72 and 97°C and therefore slightly below that for 100 % palladium. The usualhysteresis effect in the evolution and absorption of hydrogen by palladium and thealloys was also observed.2 0 b 0 I00PdFIG.6.-Dependence of isothermal ratios of trans-but-2-ene to cis-but-2-ene on composition in thehydrogenation of butadiene on palladium-gold alloys.40°C, 0 ; 80°C, A ; 100°C, 0 ; 160°C, 0.DISCUSSIONCOBALT CATALYSTSCobalt-alumina reduced at 4 14°C behaved exactly as reported previously.1sThe present results again show that butenes are formed on this catalyst solely by 1,2-and 1,4- addition of hydrogen to butadiene and that the syn- and anti- conformationsof the C4H7 intermediates do not interconvert. However, the product distributionsobtained using cobalt-alumina reduced at 329°C are similar to those obtained usingnickel-alumina 15 (table 1) and we conclude that although the same modes of hydrogenaddition are operative conformational interconversion of the C4H7 intermediatesoccurs readily.These conclusions are supported by two further observations (i) thatthe kinetics before and after the treatment at 414°C are similar and (ii) that eachbutene from reaction A had the same deuterium content (D.N. = 2-00), as did eachbutene from reaction B (D.N. = 1.39). Had the but-2-enes produced in eitherreaction been formed in two stages, i.e., by the primary formation of but-1-enefollowed by its isomerization to but-2-ene before desorption, the deuterium numberof the but-Zenes would have differed from that of but-1-eneB. J. JOICE, J. J. ROONEY, P. B. WELLS A N D G . R. WILSON 233Powdered cobalt prepared at temperatures in the range 90-330°C behaved likecobalt-alumina prepared at 329°C (type A behaviour in fig.3), whereas, powdersprepared at temperatures in the range 440-580°C behaved like cobalt-alumina preparedat 414°C (type B behaviour in fig. 3). Thus conformational interconversion of theC4H7 intermediates occurs readily on powders P1-P3 which were mostly or completelya-cobalt in a finely divided state (micrograph (a)). On the other hand, conformationalinterconversion did not take place readily at the surfaces of powders p5-P~ whichwere mostly or completely ,&cobalt in a sintered state of relatively low surface area(table 2 and micrograph (b)).The change of mechanism provided a change in product distributions that is inthe sense expected. According to the mechanism in fig.1, the predominant C4H7intermediate is syn- 1 -methyl-n-ally1 which on addition of a hydrogen atom gives but- 1 -ene and trans-but-2-ene, although the latter will be somewhat favoured if the methylgroup sterically hinders hydrogen addition. However, if n-allylic complexes are notformed readily, as provided by the mechanism in fig. 2, addition of hydrogen to C4M7intermediates is expected to give mainly but-1-ene. Consequently, a gross change inthe butene distributions as the mode of chemisorption changes is both expected andobserved.The present results (fig. 3) also closely resemble those which have been obtainedfrom the homogeneous hydrogenation of butadiene by pentacyano-cobalt catalysts.6Here but-1-ene amounted to 80 % of the total butenes when the CN/Co ratio washigh but as this ratio decreased below a critical value of -6 trans-but-2-ene becamethe predominant product. Yields of cis-but-2-ene were also low in all cases andindependent of variations in the nature of the catalyst.The mechanisms suggestedfor these homogeneous hydrogenations 19 have also many features in common withthose which we have postulated.The change from type A to type B behaviour (fig. 3) is not a consequence of thea- to P-phase change in cobalt because a similar change in catalytic behaviour hasbeen observed for a series of nickel powders which show no phase change. Thus,the product distribution is apparently a function of the particle size of the metal,i.e., n-allylic bonded intermediates are more important at the surface of well-sinteredcobalt than on cobalt prepared below 330°C.This marked distinction in behaviourmay be due to considerable differences in the catalytic properties of different crystalplanes, which might also explain why the activities per unit surface area, activationenergies, and the results using deuterium also differed widely, as the physical charac-teristics of the metal altered. Well-sintered cobalt particles may expose low-indexplanes, whereas, the smaller particles formed below 330°C probably expose both highand low-index planes and may also possess a greater number of structural defects.There is therefore the possibility that the type of bonding of the intermediates variesfrom one plane to that of another and an examination of butadiene hydrogenationat particular planes of single crystals is being undertaken at present to test this.PALLADIUM-GOLD ALLOYSThe present results for pure palladium agree well with those previously re-ported5, 129 14 and again demonstrate the marked ability of this metal for formingn-allylic complexes in heterogeneous reactions of hydrocarbons.Thus the mechanismof butadiene hydrogenation is mainly that shown in fig. 1 and it is significant thatsuch ready conversion of dienes to n-allylic complexes is also an important featureof the homogeneous organometallic chemistry of palladium.20Since pumice-supported gold was virtually inactive, even at 200"C, the palladiumatoms are the active centres in the alloys and the results are a measure of the chang234 HYDROGENATION OF BUTADIENEin catalytic properties of this metal with progressive modification by gold.In theregion 0-60 % gold there was very little variation in the major features of the productdistributions and apart from the 65 and 70 % gold alloys the yields of but-1-ece didnot alter significantly over the whole range of composition. Although the trans/cis-ratio in the but-2-enes decreased at lowcr temperatures by a factor of -2 from 0 to95 % gold (fig. 6) this was due to a relatively minor increase in the yield of cis-but-2-eneat the expense of the trans-isomer. Thus, the relative proportions of 1,2- and 1,4-addition of hydrogen were virtually constant so that the presence of gcld has no majoreffect on the ability of palladium to form n-allylic complexes.The slight decreasein stereoselectivity for trans-but-2-ene formation indicates that the stability of then-allylic complexes was not greatly diminished by increasing gold content so that thefraction of the C4H7 intermediates which undergo conformational interconversionat room temperature is small. The stereoselectivity also decreases with temperatureat a given composition but this could also be partly due to a decrease in the relativeproportions of anti- and syn-butadiene in the gas-phase and on the surfaces.The activation energies (table 4) show several features which are similar to thoseobtained by Couper and Eley 21 for parahydrogen conversion on palladium-goldalloy wires.Thus, the activation energies fcr both reactions are independent of goldcontent up to 60 % and the average value of 10-5+0-5 kcal mole-1 for butadienehydrogenation in this range, obtained under conditions where both palladium andthe alloys form the p-phase liydride (see later), is close to the value of - 11 kcal mole-1for parahydrogen conversion on a palladium wire, which had been charged withhydrogen. Couper and Eley also noted a sharp increase in activation energy of some5 kcal mole-1 as the gold content increased from 60 to 70 % and we have found anincrease of the same magnitude from the 60 to 65 % gold alloy. In this region,60-70 % gold, both the paramagnetism and solubility of hydrogen drop to zero. Themajor difference between the two investigations is that while the activation energiesfor parahydrogen conversion remained high for alloys of gold content above 70 %,those for butadiene hydrogenation decreased sharply between the 70 and 75 %alloys.Consequently, the activation energies for butadiene hydrogenation exhibita peak of some 5 kcal mole-1 in height in a narrow range of composition.Couper and Eley suggested that the slow step in parahydrogen conversion is theformation of an activated complex between an adatom of hydrogen and a hydrogenmolecule. The above similarities indicate that the slow step in butadiene hydro-genation may also be activation of molecular hydrogen on surfaces which are coveredby adsorbed diene, and that the same orbitals of the palladium atoms are responsiblein both systems.These authors argued that the increase in activation energy, as theparamagnetism of the alloys decreased to zero at -60 % gold, showed that d-orbitalvacancies help to lower the energy of the activated complex. The present resultssupport this view since the maximum in but-1-ene yields, which paralleled the maxi-mum in activation energies in the 60-75 % gold range, reveals that 1,4-addition ofhydrogen and thus n-allylic bonding also occurs with maximum difficulty in thiscomposition region. Now n-allylic bonding should occur more readily if the pallad-ium atoms have appropriate dn-orbitals which are not fully occupied and the sameorbitals may also participate in activating molecular hydrogen.The sharp drop in both but-1-ene yields and in activation energies between 70 and75 % gold (table 4) is interesting because the palladium atoms at the latter compositionare apparently behaving like those in the 0-60 % range, in contrast with those of the65 and 70 % gold alloys, in spite of the fact that the gold-rich alloys are no longerparamagnetic.The siniple electronic theory of catalysis based on the presence orabsence of holes in the d-band is not completely adequate to account for these result€3. J. JOICE, J. J. ROONEY, P. B. WELLS AND G. R. WILSON 235because the theory makes no distinction between palladium and gold atoms when thereis 65 "/o or more of the latter. Also, a strongly adsorbing species such as butadienemay have as much influence on the d-orbitals of the surface palladium atoms asneighbouring gold atoms.Although the surface atoms in alloys containing 75 % ormore gold may have no d-vacancies some of the palladium atoms seem to be sufficientlymodified by adsorbed diene to behave in much the same way as atoms in alloys of lessthan 65 % gold. Apparently chemisorbed hydrogen cannot exert this influence, asthe activation energies for parahydrogen conversion remain high in the gold-richregion, which might suggest that donor bonding between dn-orbitals on the metal andempty anti-bonding x-orbitals on butadiene has an important influence.In order to account for the behaviour of the 65 and 70 % gold alloys we suggestthat there may be a tendency towards phase-separation in this narrow range where thecomposition is given by the formula PdAu2, or very close to it.Since gold canbehave as a quasi-halogen the palladium atoms may tend to adopt an ordered environ-ment and have essentially the electronic configuration associated with a square-planararrangement as in PdC12. If this were so the dn-orbitals would be fully occupiedand perhaps less readily influenced by adsorbed diene in a direction which wouldfavour easier activation of hydrogen or .n-allylic bonding.The product distributions, especially with palladium and palladium-rich alloys,are apparently influenced by the nature of dissolved hydrogen. Thus the minimain but-1-ene and maxima in trans-but-2-ene yields (fig. 4) and the maxima in n-butanepercentages (fig. 5) occurred in the same temperature range where the p- to a-phasechange of palladium and palladium-gold hydrides was found at roughly the samehydrogen pressures used in the hydrogenation experiments. Moreover, the decreasein size of the maximum in n-butane yields with increasing gold content of the alloysparalleled the smooth decrease in the quantity of liydrogen rapidly evolved when theP-phase hydrides became unstable. These changes in product distributions areunderstood if the surface concentration of hydrogen atoms goes through a maximumin the region where the p- and u-phase hydrides co-exist.Such an increase inhydrogen concentration would favour the chance of adsorbed butene hydrogenatingto butane via the formation of adsorbed C4H9 species before desorption occurred.The surface concentration of butyl groups must also have been at a maximum in thesame temperature region so adsorbed butenes would also have the highest chance ofisomerizing by the mechanism of alkyl reversal before leaving the surface.However,this may not be the complete explanation because these changes in product dis-tributions were observed over too broad a temperature range with alloys of higher goldcontent, and were still present with the 75 % alloy, even though rapid evolution ofhydrogen was not detected with alloys containing more than 65 % gold.Thanks are due to S.R.C. for grants to B. J. J. and G. R. W.NOTE ADDED IN PROOFThe following are the weights of the pumice-supported palladium-gold alloysrequired to maintain an initial rate of hydrogenation of butadiene of 2 % per minuteat 20°C (Pc,H, = 50 mm ; PH, = 100 mm).weight (g) 0.14 0.8 3.3 3.0 2-0 4.0 2.8 50If these weights are assumed to be a measure of surface areas the activation energies(table 4) show that the frequency factors for the 35 and 30 % alloys must be largerthan those of the 40 and 25 % palladium alloys by a factor of N 103.Pd (%> 100 80 50 40 35 30 25 236 HYDROGENATION OF BUTADIENE1 Gault, Rooney and Kemball, J. Catalysis, 1962, 1, 255.2 Rooney, J. Catalysis, 1963, 2, 53.3 Garnett and Sollich, J. Catalysis, 1963, 2, 350.4 Bond and Wells, Ado. Catalysis, 1964, 15, 92.5 Bond, Webb, Wells and Winterbottom, J. Chem. Soc., 1965, 3218.6 Kwiatek, Mador and Seyler, Reactions of Co-ordinated Ligands, A.C.S., (Adv. Chem. Series,7 Frankel, Enken, Peters, Davison and Butterfield, J. Org. Chem., 1964, 22, 3292.8 Paal, Ber., 1912, 45, 2221.9 Lebede’v and Yabubchik, J. Chem. Suc., 1928,2190.no. 37), 1963, p. 201.10 Young, Meier, Vinograd, Bollinger, Kaplan and Linden, J. Amer. Chem. SOC., 1947, 69, 2046.11 Reiche, G r i m and Albrecht, Brennstof-Chernie, 1961, 42, 5 .12 Meyer and Burwell, J. Amer. Chem. Soc., 1963, 85, 2881.13 Wells, Chem. and Ind., 1964, 1742.14 Leszczynski and Wells, Przenysl. Chem., 1964, 43, 508.1s Phillipson and Wells, two papers submitted to J. Chem. Soc.16 Smith and Massingil, J. Amer. Chem. Soc., 1961, 83, 4301.17 For pertinent reviews see Guy and Shaw, Adv. Inorg. Chem. Radiochem., 1962,4,78 ; Bennett,18 Holbrook and Marsh, personal communication.19 Kwiatek and Seyler, J. Organometal Chem., 1965, 3, 421.20 Churchill, Chem. Comm., 1965, 625.21 Couper and Eley, Disc. Faraday Soc., 1950, 8, 172.Chem. Rev., 1962, 62, 611