HYDROGENATION OF ETHYLENE THE MECHANISM OF THE CATALYTIC HYDROGENATION OF ETHYLENE BY G. H. TWIGG Received 1st May, I950 Ethylene has been hydrogenated on a nickel catalyst at -78" C with (a) a mixture of H, + D,, and (b) the equilibrium mixture of H, + HD + D,. Infra-red spectroscopic examination showed that the mixtures of ethanes pro- duced in the two experiments were identical, and were different from an equi- molar mixture of C,H, and C,H4D,. This is direct proof that in hydrogenation the hydrogen is first dissociated into atoms on the catalyst. This result and the existing knowledge concerning the hydrogenation and exchange reactions are reviewed, and a mechanism for hydrogenation is pro- posed which accounts for all the known facts. and in particular for the temperature dependence of the energy of activation.Hydrogen is not adsorbed directly on the catalyst, but only through reaction with a chemisorbed ethylene mole- cule t o form an adsorbed ethyl radical and an adsorbed hydrogen atom ; hydro- genation takes place through the addition of a further hydrogen atom t o the ethyl radical, while exchange is controlled by the reverse of the first step. A further fast reversible reaction occurs in exchange between adsorbed ethylene and a hydrogen atom to form an adsorbed ethyl radical. The mechanism of the hydrogenation of ethylenic double bonds on metal catalysts such as nickel has been the subject of much investigation. The determination of the kinetics alone has not thrown much light on this problem, but the discovery of the exchange reaction between ethylene and deuterium 1 appeared t o offer a fresh approach. Horiuti and Polanyi 2 proposed the following reaction schemes : CH, CH,-cH, + H + H 1 I L2 CH, - CH,--CH,; Ni Ni - H 1 Ni the first reaction and its reverse gives rise to exchange, whilst the addition of a second hydrogen atom to the " half-hydrogenated state " completes 1 Farkas, Farkas and RideaI, Proc. Roy.SOC. A , 1934, 146, 630. Horiuti and Polanyi, Trans. Faraday SOC., 1934, 30, 1164.G. H. TWIGG I 5 3 the hydrogenation. Farkas 1 9 proposed different mechanisms, and suggested that the first step in exchange was loss of a hydrogen atom from the ethylene, and that hydrogenation was independent of exchange and proceeded through the simultaneous addition of a pair of hydrogen atoms. and with higher olefines,6 and of the migration of the double bond in butene-1 which occurs simultaneously 7 with exchange, has provided a great deal of evidence that the exchange reaction did in fact proceed by the " associ- ative " mechanism of Horiuti and Polanyi.The difference in order of reaction between exchange and hydrogenation predicted by Horiuti and Polanyi did not, however, materialize, and the fact that both reactions were of the same kinetic order, and yet had different energies of activation, led Twigg and Rideal4 to suggest that in both reactions the rate-determin- ing step was reaction of a van der Waals' adsorbed hydrogen molecule with a chemisorbed ethylene molecule t o produce on the one hand an ad- sorbed ethyl radical and an adsorbed hydrogen atom (exchange), and on the other hand a non-adsorbed ethane molecule (hydrogenation). Farkas a pointed out that in many cases cis-addition of hydrogen took place, even though the product was not the more stable.Greenhalgh and P ~ l a n y i , ~ however, showed that this was not proof of the simultaneous addition of a pair of hydrogen atoms, since the attachment of the ethylene molecule to the catalyst prior to the formation of the half-hydrogenated state automatically means that addition of single atoms would occur in the cis-position. It was realized that it would be possible to distinguish between these two mechanisms by a direct experiment in which ethylene was hydrogen- ated with a mixture of H, and D,. The addition of single atoms would produce ethane of composition whereas the simultaneous addition of a pair of atoms would produce a mixture of A fuller investigation of the exchange reacticn with ethylene 4 7 $CH3-CH3 + $CH,--CH,D + $CH,D--CH,D, BCH3-CHa + 6CH2D-CH2D.These experiments were begun in 1938, but the analytical technique -infra-red spectroscopy-was not then sufficiently developed to dis- tinguish easily these two mixtures. The development of automatically recording spectrometers of good resolution has enabled this experiment t o be made. Experimental Hydrogenation must be carried out with a minimum of exchange, which means as low a temperature as possible. A supported catalyst was therefore used made by impregnating Kieselguhr pellets with nickel nitrate, igniting and i hen reducing in hydrogen a t 300' C.The rest of the apparatus used was similar to that described previously.* With 7 g. of this catalyst in a reaction vessel of 400 ml. volume, the half life of a typical hydrogenation was about 2 hr. at Cylinder ethylene was given three single-plate distillations to remove high and low boiling impurities. C,H, was prepared by reacting C,H, + H, at -78" C for 15 hr. C2H4D2 was prepared by reacting C2H4 + D, at -78" C. The relative importance of exchange with respect to hydrogenation declines as the temperature is lowered, and at -37" C on a similar catalyst it was shown that after the addition of deuterium to a sample of ethylene only 2 yo of the hydrogen of the ethylene had been exchanged. The C2H4D, obtained should thus have been of over 95 yo purity.- 78" c. Farkas and Farkas, J . Amer. Chem. SOC., 1938, 60, 22. * Twigg and Rideal, Proc. Roy. Soc. A , 1939, 171, 55. Conn and Twigg, ibid., 1939, 171, 70. Twigg, Trans. Faruday SOC., 1939, 35, 934. Twigg, PYOC. Roy. SOC. A , 1941, 178, 106. Farkas, Trans. Faraduy Soc.. 1939, 35, 906. Greenhalgh and Polanyi, ibid., 1939, 35, 520.I 5 1 HYDROGENATION O F ETHYLENE Two experiments were carried out : EXPT. 11. C2H4 + H, + D,. The catalyst was treated with H, at -78" C for 6 hr. and the H, pumped off. To prevent the reaction H, + D, + zHD occurring, the ethylene (107 mm.) was admitted first to the reaction vessel, and an additional 120 mm. of a 50-50 mixture of H, + D, added. Reaction was allowed t o proceed for 18 hr., and the ethane produced was separated from residual hydrogen by passage through a trap cooled in liquid N,.This experiment was carried out t o obtain a product similar to that anticipated if addition proceeds atom by atom. A 50-50 mixture of H, + D, (120 mm.) was admitted to the catalyst at -78" C after the previous experiment. The reaction vessel was allowed t o reach room temperature and kept there for 6 hr. before cooling to -78°C. Ethylene (107 mm.) was added and reaction allowed to proceed ior 16 hr. The product was separated as before. EXPT. 12. C,H, + H, + HD + D,. Analysis .-Infra-red spectra were recorded on a Perkin-Elmer Spectro- meter Model IZB, at medium resolving power in the region 650-4000 cm.-l and a t the highest resolving power over the range 670-890 and 1027-1450 cm.-l.The cell length was 10 cm., and the pressure 700 &- 5 mm. Fig. I and z are photographs of relevant parts of the records obtained for (a) a mixture of 50 yo C,HG + 50 yo C,H,D,, ( b ) products of Expt. 11, and (c) products of Expt. 12. i ;v frequency €xperimenl~~ FIG. I . - I O Z ~ - I ~ ~ O cm.-1. (a) 50 % CH3-CH3+50 % CH,D-CH,D ; (b) from Expt. 11 ; (c) from Expt. 12. Infra-red spectra of deuterated ethanes, Note: The arrows mark the same wavelength on the other two spectra. It is very clear that the ethanes from Expt. 11 and 12 are identical and different from the mixture of C,H, and C,H4D,. The identity was complete over the whole spectral range. In order t o check that the reaction H, + D, zHD was inhibited during the hydrogenation, the ortho-para hydrogen conversion was examined.Ethylene (48.9 mm.) was admitted first to the reaction vessel at - 78" C, and f i - H, (75.5 mm.) added. Samples were removed at various times, ethylene and ethane separated, and the hydrogen analyzed in a coiled-coil type microthermal con- ductivity gauge lo immersed in liquid N,. The results are shown in the table. No residual ethylene was observed (< 0.1 yo). Bolland and Melville, Trans. Faraday Soc., 1937, 33, 1316.G. H. TWIGG I 5 5 Time (min.) Residual Ethylene yo 73 60 38.5 5'8 0'0 Conversion Y O _- I 2 5 I3 92 The half-life of the conversion in the absence of ethylene was 13 min. FIG. 2.-670-890 cm. -'. Infra-red spectra of deuterated ethanes, (a) 50 % CH3-CH,+5o % CH,D-CH,D ; (b) from Expt. 11 ; (c) from Espt. 12. Note: The arrows mark the same wave- lengths on the other two spectra.Discussion The experiments show unequivocally that addition of hydrogen to the double bond does not take place in a single act, but that the hydrogen molecule is first split into atoms which then add one at a time. The previous work has shown that exchange proceeds through attach- ment of ethylene to the catalyst at two points and the formation of the half-hydrogenated state. In considering the mechanism of hydrogen- ation, the following salient facts will be taken into account : (a) The order of reaction is identical in exchange and hydrogenation, the rate being given by pEZH, x pE2 from - 78" C to + 150° C, where n is unity or slightly less.156 HYDROGENATION OF ETHYLENE (b) The energy of activation for exchange is greater than for hydro- genation, (G) The energy of activation for both exchange and hydrogenation decreases with increasing temperature above about 100’ C, although the order of reaction is unchanged and the reaction H, + D, + zHD is still inhibited.(d) The ortho-para conversion and the reaction H, + D, + 2HD are inhibited by ethylene except in so far as they proceed via exchange with the ethylene. (e) The hydrogen returned to the gas phase during exchange between ethylene and deuterium is very largely H, molecules. (f) Hydrogen is dissociated into atoms before hydrogenation. Fact (d) means that the available surface is almost entirely covered with ethylene. The fact that the reactions are of zero and not negative order with respect to ethylene then means11 either that the two gases do not compete for the same surface or that adsorption equilibrium by the Langmuir mechanism, is not achieved.The previous mechanism pro- posed for hydrogenation,4 and that of Beeck,’, take the former alternative. Beeck’s mechanism, that gaseous ethylene reacts with adsorbed hydrogen, does, with certain assumptions, fit the kinetics, but is unsatisfactory in that it does not explain the exchange reaction, and particularly the close connection between the tMJo reactions. We are left, therefore, with the second alternative, that adsorption equilibrium is not maintained, and it is now proposed that up to 1 5 0 O C the reaction H, -+ 2Ki-H is of no significance because the free surface available is too small ; in other words, the adsorption of the ethylene is much faster than that of the hydrogen on the available bare surface.Also, in accordance with (e) above, the interchange of ethylene between the gas and the catalvst is much faster than that of the hydrogen. The present experiments have shown that hydrogen therefore adsorbed, before hydrogenation, and proposed : (1) - C2H4 *- CH,--CH, (2) I I Ni Ni 4 H, CH3 ( 3 ) 1 7) must be dissociated, and the following scheme is ~ . I CH,-CH, + CH, H ---+ CH3-CH3 Li hl IT. Ni Ni (1) I I 4 93 9, Ethylene is adsorbed by opening of the double bond. The adsorption of the hydrogen takes place by reaction of a van der Waals’ adsorbed molecule with an adsorbed ethylene molecule. Since exchange and hydrogenation have different energies of activation, this step (3) cannot be the rate determining one.Since both reactions have identical kinetics, the rate-determining step must proceed from the same adsorbed state, and therefore reaction (4) controls exchange while reaction (7) controls hydrogenation. Because the hydrogen returned to the gas phase during exchange between ethylene and deuterium is largely H,, and reactions (3) and (4) would only give HD, we must add another and faster reaction CH3 (5) I CH,-CH, H kH, Ni Ni Ni I (6) i Ni I 1 l1 Twigg, l2 Beeck, this Discussion (comment on Prof. Laidler’s paper). Rev. M o d . Physics, 1945, 17, 61 ; this Discussion.G. H. TWIGG I 5 7 These steps can be shown to explain all the features of the exchange and hydrogenation reactions. If the fractions of the surface covered by the different species are as indicated, steady-state equations can be set up, the solutions to which give The expression for O1 is more complex, but is in agreement with the fact that 8, is close to unity for the temperature range considered. Then : Rate of hydrogenation Rate of exchange These exmessions show both reactions = k 4 8 2 8 3 , to be of the same kinetic order.i.e., rateLproportional to the first power of the hydrogen pressure, and independent of the ethylene przssure (since 8, = I). The term pH2 should strictly be replaced by the concentration of hydrogen in the van der Waals' layer, so that at low temperatures the order of reaction is reduced slightly below unity, The ratio, excha.nge/hydrogenation = k 4 / k 7 and therefore the differ- ence in energies of activation of the two reactions (E4 - E7) should be constant over the entire temperature range, as was found.4 The above expressions for the rates of reaction now provide an ex- planation of the long-known fact that the energy of activation for hydro- genation,13~ 14 and for exchange, decreases with increasing temperature above ca.goo C, and even becomes negative. Previous explanations I 4 have attributed the phenomenon to desorption of the ethylene, but it has been shown that this does not occur until temperatures above 150' C. There are two extreme cases : Hydrogenation faster than exchange, i.e. (a) Low TEMPERATURES. k7 > k4. Rate of hydrogenation = k3pH281, EH = E 3 ; Ex = E3 + E4 - E7. - P E 2 8 1 , k 3 k 4 Rate of exchange - k7 -- (b) HIGH TEMPERATURES : Exchange faster than hydrogenation, i.e.k4 > k , . Rate of hydrogenation = -. pH2e,, E , = E, + E, - E , ; k4 Rate of exchange = k 3 p & e 1 , = E 3 * On proceeding from low to high temperatures, both energies will decline by the term E, - E,. 13 Rideal, J . Cham. Soc., 1922, 121, 309. Tucholski and Rideal, ibid., 1935, Maxted and Moon, ibid., 1935, 1190. l4 Schwab, 2. physik. Chenz. A , 1934, 171, 421. zur Strassen, ibid., 1934, 1701. 169, 81.HYDROGENATION OF ETHYLENE Taking the value previously determined 6 for Ex - EH = 9.0 kcal. = E, - E,, and taking E , = 11 kcal., and noting that rate of exchange = rate of hydrogenation at goo C, a curve was constructed of log rate of hydrogen- ation against I/T. It showed the typical behaviour associated with this reaction and the apparent energies of activation at different temperatures were as tabulated. 10-4 9.8 7'3 3'4 2'5 At temperatures above about 15ooC, it has been shown that de- sorption of ethylene begins to set in, and the apparent energy of activation will decline further, and become negative.There is, however, a partial compensation as the uncovering of the surface now permits direct entry of the hydrogen (H, --f zNi-H). Numerical values for the individual energies of activation can be estimated. Making the not unreasonable assumption that the heats evolved (x) in the following two reactions are equal : CH, I CH, - CH, + H, -+ CH, Ni Ni H + x kcal. I I I I FH3 Ni Ni I CH, + H, --+ H + C,H, + x kcal. Ni Ni I I and taking the heats of adsorption for hydrogen (17 kcal.) and ethylene (36 kcal.) calculated by Eley,15 one finds x = E, - E , = 7 kcal., and E , - E, = 10 kcal. Using Beeck's values for heats of adsorption on covered surfaces, x = 11 kcal. and E, - E, = I kcal. Assuming E , = 11 kcal. from low-temperature hydrogenation, E , = 18 kcal. (Eley) or zz kcal. (Beeck), and E , = 9 kcal. (Eley) or 13 kcal. (Beeck). It should be possible to estimate E, and E, from data on double bond migration, since this reaction is mainly effected by the faster processes (5) and (6) above. On this scheme, Rate of double bond migration = k,8, However, experiments showed that all three reactions were of the same order (Pi, . ptutene) and that the olefine did not completely cover the surface.16 The above equations cannot therefore be applied, since it is probable that direct adsorption of the hydrogen was taking place. 15 Eley, this Discussion. l6 Twigg and Rideal, Trans. Faraday SOL, 1940, 36, 533.G. H. TWIGG I59 I am pleased to record my indebtedness to Messrs. A. R. Philpotts and W. Thain who carried out the infra-red spectroscopic analyses. My thanks are also due to Mr. R. D. Richards for assistance with the experi- ments, and to the Directors of the Distillers’ Company Limited for per- mission to publish this work. The Distillers’ Company Limited, Research and Development Department, Great Burgh, Epsorn, Surrey.