Effect of Toluene-d8 on the Hydrogenation of 1,3-Hexadiene over a Pd

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Langmuir 2000, 16, 6519-6526

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Effect of Toluene-d8 on the Hydrogenation of 1,3-Hexadiene over a Pd/Silica CatalystsPromoter and Poison S. D. Jackson,*,† S. Munro, and P. Colman Synetix, P.O. Box 1, Belasis Avenue, Billingham, Cleveland TS23 1LB, U.K.

D. Lennon*,‡ Department of Chemistry, University of Glasgow, Glasgow G12 8QQ, U.K. Received December 9, 1999. In Final Form: April 3, 2000 The hydrogenation of 1,3-hexadiene over a Pd/silica catalyst has been investigated using in situ FTIR spectroscopy. This study shows that the presence of an organic species on the surface of the catalyst can have a marked effect on the rate of hydrogenation of 1,3-hexadiene. Toluene-d8 was coadsorbed on the surface of the catalyst and, depending on the order of introduction of the reactants to the system, has been shown to act as either a promoter or a poison for the hydrogenation of the diene. Introduction of the toluene-d8 prior to the diene has a promoting effect with the toluene-d8 acting as a H-transfer agent. When the toluene-d8 is introduced after the diene it acts as a poison, blocking active sites and covering the surface. The ability of toluene-d8 to act as a promoter or poison appears to be dependent on the availability of active sites at the time of its introduction to the reaction system. To act as a promoter, or H-transfer agent, the toluene-d8 must first be able to form σ bonds to the surface by dissociation of the D atoms of the methyl group. When the site availability is poor, the toluene-d8 retains its molecular identity upon adsorption, thus blocking active sites and reducing the rate of hydrogenation.

Introduction Since the early 1900s a large number of studies on catalytic hydrogenation of unsaturated hydrocarbons have been reported (see review by G. Webb in ref 1 and references therein). Within this general scope, an area of particular interest is that of selective hydrogenation of multiply unsaturated hydrocarbons.2-4 This is an important area both academically and industrially; for example, selective hydrogenation of ethyne (acetylene) or other polyenes to a simple alkene on palladium catalysts is a key unit within a refinery complex. This process is used industrially to remove alkynes and dienes from gas streams consisting mainly of alkenes.5 Owing, in part, to the large body of published material in this area, over the past decade there have been fewer studies concerning selective hydrogenation of alkynes and polyenes. However, in a number of recent studies, we have found that the current state of knowledge is considerably less complete, and the basic process less well understood, than would be expected.6 Therefore in this work we have investigated the hydrogenation of 1,3-hexadiene in the presence and absence of toluene-d8 on a silica-supported palladium catalyst. The aim was to study the effect of the presence of another organic species on the hydrogenation of the diene. It has been observed in the literature that the addition of a large organic molecule to a catalyst can have * To whom correspondence may be addressed. † E-mail: [email protected]. ‡ ICI Lecturer in Heterogeneous Catalysis. (1) Webb, G. In Comprehensive Chemical Kinetics; Bamford, C. H., Tipper, C. E. H., Eds.; Elsevier: Amsterdam, 1978; Vol. 20, p 1. (2) Guo, X.-C.; Madix, R. J. J. Catal. 1995, 155, 336. (3) Vasquez, N.; Madix, R. J. J. Catal. 1998, 178, 234. (4) Bond, G. C.; Dowden, D. A.; Mackenzie, N. Trans. Faraday Soc. 1954, 54, 1537. (5) Bond, G. C.; Webb, G.; Wells, P. B.; Winterbottom, J. M. J. Catal. 1962, , 74. (6) Jackson, S. D.; Kelly, G. J. Curr. Top. Catal. 1997, 1, 47.

a marked effect on the rate of reaction.7-11 These studies suggest that factors such as hydrogen-bonding interaction7 and hydrogen-transfer mechanisms8-11 induced by the presence of coadsorbates are important factors in understanding and controlling catalyst activity and selectivity. The effect of toluene on the hydrogenation of 1,3hexadiene over a Pd/silica catalyst has been investigated using in situ FTIR spectroscopy. The reaction was followed by monitoring the IR absorption bands due to vibrational modes associated with the unsaturation of the 1,3hexadiene molecule, for example, the stretching vibrations of the unsaturated C-H and CdC bonds that occur in the 3100-3000 and 1700-1600 cm-1 regions of the spectrum, respectively. As the hydrogenation reaction proceeds, the intensity of these bands decreases. Fully deuterated toluene was used throughout this work because of the advantages it affords when using in situ FTIR to monitor the reaction process. Significant reductions in the vibrational frequencies of IR absorption bands occur when protons are replaced by deutero atoms, thus aiding spectral identification. By using deuterated toluene, the IR spectrum of the multicomponent reaction system can be greatly simplified because the absorption bands due to the toluene-d8 occur in a lower frequency region of the spectrum than those of the diene. Second, because IR (7) Bond, G.; Meheux, P. A.; Ibbotson, A.; Wells, P. B. Catal. Today 1991, 10, 371. (8) Siegel, S. In Heterogeneous Catalysis and Fine Chemicals II; Studies in Surface Science and Catalysis; Guisnet, M., et al., Eds.; Elsevier: Amsterdam, 1991; Vol. 59, p 21. (9) Siegel, S.; Forman, G. M.; Johnson, D. J. Org. Chem. 1975, 40, 3589. (10) Jackson, S. D.; Shaw, L. A. React. Kinet. Catal. Lett. 1996, 58, 3-6. (11) Jackson, S. D.; Hardy, H.; Kelly, G. J.; Shaw, L. A. In Heterogeneous Catalysis and Fine Chemicals IV; Blaser, H. U., et al., Eds.; Elsevier Science B.V.: Amsterdam 1997; p 303.

10.1021/la991614o CCC: $19.00 © 2000 American Chemical Society Published on Web 07/06/2000

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Table 1. Time Required for the Hydrogenation of 1,3-Hexadiene upon Sequential Injection of the Reactants into an Evacuated IR Cell Containing Pd/Silica Catalyst at 323 K reaction

1st

order of introduction to catalyst 2nd

1 2 3 4 5 6 7 8

hydrogen hydrogen hydrogen toluene-d8 1,3-hexadiene 1,3-hexadiene 1,3-hexadiene toluene-d8

1,3-hexadiene toluene-d8 1,3-hexadiene hydrogen hydrogen hydrogen toluene-d8 1,3-hexadiene

is very sensitive to proton/deuterium replacement, information regarding any role that the toluene might play in the hydrogenation process can be examined along with any proton/deuterium exchange reactions. Besides information about reaction rates, selectivity, deuteration, and exchange, it was also possible to gain information about hydrocarbon lay down. This was done by using IR to study the species adsorbed on the surface of the catalyst, postreaction. As predicted by the literature, the experimental data has shown that the presence of toluene-d8 effects the rate of reaction. The order in which the reactants are introduced to the palladium catalyst also greatly influences the reaction, with toluene capable of both promoting and poisoning the hydrogenation reaction of 1,3-hexadiene.

3rd

approximate time required for hydrogenation of 1,3-hexadiene (min) 32 3 15 5 67 130 260 30

1,3-hexadiene toluene-d8 1,3-hexadiene toluene-d8 hydrogen hydrogen

silica support (3M Cab-o-sil, S. A. 203 m2g-1). The resulting mixture was evaporated to dryness at 343 K, giving a weight loading of 0.83% w/w Pd/silica. The dispersion was measured using a standard carbon monoxide adsorption. Assuming a ratio of 1:1 for CO:Pd, the dispersion was calculated to be 17%.

Results

This study was carried out using an in situ FTIR environmental cell,12 where pressure, temperature, and reactant composition could be controlled. The catalyst sample (typically 0.02-0.03 g) was mounted within the cell in the form of a self-supporting pressed disk and the reaction process studied by transmitting the infrared beam through the sample. The catalyst was reduced in situ in flowing 3% H2/N2 (18 cm3 min-1) at 323 K for 20 min. After reduction the environmental cell was evacuated for 10 min, to a pressure of less than 10-3 mbar, while maintaining the temperature at 323 K. Following evacuation, reactants were introduced sequentially to the cell via a septum by injecting known volumes. The time taken to introduce each reactant was kept to a minimum, typically less than 0.5 min. The temperature of the cell was maintained at 323 K for all adsorption and IR measurements. The infrared spectra were collected using a commercial FTIR spectrometer (Nicolet 5DXC). All spectra were collected as single beams at 4 cm-1 resolution with the coaddition of 100 scans, using a TGS detector. The spectra are ratioed with respect to the spectrum of a clean, evacuated catalyst. The cell has a path length of 17 cm, and, unless mentioned otherwise, spectra were recorded in the presence of the reacting gas. In this way, the cell behaved as a batch reactor, with the reacting gases making a contribution to the spectrum. The reactants were toluene-d8 (isotopic purity >99.95 atom % D, Aldrich), 1,3-hexadiene, (99% purity, Aldrich), and hydrogen (99.999% Linde). It is generally accepted that for alkene hydrogenation the rate of reaction is first order with respect to hydrogen atom concentration.1 Therefore, to promote reasonable rates of reaction, the concentration of hydrogen used was in large excess, with quantities used as follows: 10 cm3 hydrogen gas at atmospheric pressure (∼4.2 × 10-4 mol H2), 9 µL of 1,3-hexadiene (∼7.8 × 10-5 mol), and 1 µL of toluene-d8 (∼1.0 × 10-5 mol). The times recorded for the hydrogenation of the 1,3-hexadiene were recorded from the moment that the diene and hydrogen were both present in the cell. The catalyst used throughout this study was prepared by impregnation. Palladium chloride (PGP Industries, Ireland) was dissolved in sufficient dilute hydrochloric acid to fully wet the

Rates of Hydrogenation. The effect of toluene on the hydrogenation of 1,3-hexadiene was investigated by carrying out a series of reactions where the order in which reactants were introduced to the catalyst was varied, as shown in Table 1. The decrease in intensity of the olefinic C-H stretching features at 3100 cm-1 and the CdC stretch at 1660 and 1600 cm-1, along with the overtone of the dCH2 wag at 1800 cm-1,13 was used as an indication of the extent of hydrogenation. Hydrogenation was judged to be complete on loss of these features from the spectrum. In this way, the time required to hydrogenate the diene, in the presence and absence of a coadsorbate, can be used as a measure of the relative hydrogenation rates for the adsorption systems. It was observed that the rate of hydrogenation of the diene depended upon the order in which the reactants were introduced. In the absence of toluene-d8, the order of injection of the diene and hydrogen had a significant effect on the rate of hydrogenation. The reactions 1-8, shown in Table 1, can be divided into two groups depending on the order of injection of 1,3-hexadiene relative to the introduction of hydrogen. In reactions 1-4 the 1,3-hexadiene is introduced after the introduction of hydrogen, whereas in reactions 5-8 the 1,3-hexadiene is introduced before hydrogen. In reaction 1, where hydrogen is introduced prior to the diene, the rate of hydrogenation is twice as fast as that exhibited for reaction 5, where the diene is introduced first. Therefore, in the latter case, the diene molecule acts as a poison slowing down the rate of hydrogenation. Figures 1 and 2 show the infrared spectra for reactions 1 and 5, respectively. Figure 1 shows the spectra collected for reaction 1 where hydrogen was introduced to the catalyst before the diene. The hydrogenation of the diene can be easily followed by monitoring the intensity of the olefinic C-H stretching bands at 3100 and 3050 cm-1, the overtone of the dCH2 wag at 1800 cm-1, and the CdC features at 1660 and 1605 cm-1. Spectrum a was collected immediately after the introduction of the diene to the reaction system at time zero, denoting the time at which data collection was initiated. Data collection for spectrum f was initiated after 32 min and shows that the diene has been fully hydrogenated as characterized by loss of the features at 3100, 3050, 1800, 1660, and 1605 cm-1. Considering the relative reduction in intensity of the bands noted above, broadly speaking, they all progres-

(12) Jackson, S. D.; Glanville, B. M.; Willis, J.; McLellan, G.; Webb, G.; Moyes, R. B.; Simpson, S.; Wells, P. B.; Whyman, R. J. Catal. 1993, 139, 207.

(13) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 2nd ed.; Academic Press: New York, 1975; p 247.

Experimental Section

Toluene-d8 as Promoter and Poison

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Figure 1. Infrared spectra for reaction 1 (hydrogen and 1,3-hexadiene). Times, in minutes, for the initiation of data collection after the introduction of 1,3-hexadiene are as follows: (a) 0, (b) 3, (c) 7, (d) 15, (e) 20, and (f) 32.

Figure 2. Infrared spectra for reaction 5 (1,3-hexadiene and hydrogen). Times, in minutes, for the initiation of data collection after the introduction of hydrogen are as follows: (a) 0, (b) 10, (c) 32, (d) 52, and (e) 67.

sively decrease in intensity at a comparable rate upon injection of the diene. However, the broad feature ca. 3010 cm-1, present as a high-frequency shoulder on the asymmetric C-H stretch at 2975 cm-1, appears to decrease only after ca. 15 min reaction time. The 3010 cm-1 band is tentatively assigned to the symmetric dC-H stretch.14 Hence the decrease in intensity of the 3010 cm-1 band represents hydrogenation of the second double bond, to (14) Compton, D. A. C.; George, W. O.; Maddams, W. F. J. Chem. Soc., Perkin Trans. 2 1977, 1311.

form the fully hydrogenated species. The hexane thus formed is expected to desorb from the catalyst surface into the gas phase, where it continues to contribute to the spectrum. A clue as to the first stage of the hydrogenation process is provided by the profile of the overtone of the dCH2 wag at 1800 cm-1, which Figure 1b shows to have disappeared after a reaction time of 15 min. This mode, in particular, is representative of the terminal double bond in the 1,3-hexadiene, and the early loss of this mode suggests this position is hydrogenated first, followed by the central olefinic functionality.

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It is noted that there is residual intensity at 1620 cm-1 at t g 20 min, which does not correlate with the vibrational modes of the diene, hexene, or hexane.15 This band is most probably due to adsorbed water on the surface of the catalyst, which was formed after the completion of the hydrogenation. The spectra collected for reaction 5 (1,3-hexadiene + H2) are shown in Figure 2. As previously, spectrum a was collected immediately after the two reactants were present in the cell. A similar loss of absorption band intensity occurs with time, but the time taken for full hydrogenation is significantly longer at ca. 67 min, taking over twice as long as when hydrogen was introduced into the cell first. The trend of first losing the overtone of the dCH2 wag before loss of the symmetric dC-H stretch is repeated with reaction 5 and shows that sequential hydrogenation occurs independent of the order in which the hydrogen and diene are introduced. Introduction of Hydrogen prior to the Diene (Reactions 1-4). Reaction 1 provides a base rate of 32 min for the hydrogenation of 1,3-hexadiene. It is evident from the hydrogenation times indicated for reactions 2-4 (Table 1) that the presence of toluene-d8 in these reaction systems has the effect of promoting the hydrogenation reaction, as indicated by relatively faster hydrogenation rates. The greatest promoting effect was observed when the toluene was introduced prior to 1,3-hexadiene (reactions 2 and 4). Figure 3i shows the spectra collected for reaction 2 where the order of reactants were hydrogen, toluene-d8, and then 1,3-hexadiene. The spectra highlight the effect that toluene has on the rate of the hydrogenation reaction with full hydrogenation occurring in ca. 3 min as indicated by the 3100 cm-1 band. Upon the introduction of toluene-d8 and hydrogen, in either order, to the reduced catalyst, some dissociation of the methyl group of toluene occurs. This is observed spectroscopically by the loss of intensity of the -CD3 bands between 2150 and 2050 cm-1 as shown in spectrum b of Figure 3ii. Subsequent introduction of the diene was accompanied by the immediate appearance of a broad absorption band in the region 2200-2100 cm-1, which indicates the presence of a saturated hydrocarbon containing deuterium atoms. When toluene is introduced after 1,3-hexadiene (reaction 3), the time taken for hydrogenation is approximately 15 min. Therefore it lies between the base rate of 32 min (reaction 1) and the 3 min required when toluene is introduced before the diene (reactions 2 and 4) as shown in Table 1. Introduction of Diene prior to Hydrogen (Reactions 5-8). The base rate for hydrogenation where the 1,3-hexadiene is introduced to the reaction system before hydrogen is 67 min (reaction 5). The time taken for hydrogenation is significantly longer than the time observed in reaction 1 (32 min). In reaction 8, where toluene is introduced prior to 1,3-hexadiene, the effect of the toluene is to promote the hydrogenation reaction, reducing the time required to 30 min. This result is in agreement with the observations in reactions 2-4 (Table 1). However, in reactions 6 and 7, where the 1,3-hexadiene is introduced prior to toluene, the time required for hydrogenation is greatly increased relative to the base rate of 67 min (reaction 5). The times required for hydrogenation in reactions 6 and 7 are 130 and 260 min, respectively, indicating that the toluene is acting as a (15) Pouchert, C. J. The Aldrich Library of FT-IR Spectra, 1st ed.; Aldrich Chemical Co.: Milwaukee, WI, 1985; Vol. 1.

Jackson et al.

Figure 3. (i, top) Infrared spectra for reaction 2 (hydrogen, toluene-d8, and 1,3-hexadiene). Times, in minutes, for the initiation of data collection after the introduction of 1,3hexadiene are as follows: (a) 0, (b) 3, (c) 6, and (d) 9. (ii, bottom) Spectrum of gas-phase toluene-d8 and spectra from reaction 2 (hydrogen, toluene-d8, and 1,3-hexadiene) where (a) is gas-phase toluene-d8, (b) reaction 2, prior to the introduction of the diene, and (c) reaction 2, immediately after the introduction of the diene.

poison. Parts i and ii of Figure 4 show the spectra collected for reaction 7 where the order of introduction of reactants was 1,3-hexadiene, toluene-d8, and then hydrogen. Figure 4ii shows the spectra of toluene-d8 during reaction 7. The three absorption bands associated with the methyl group of the toluene-d8 are evident in the 2150-2050 cm-1 region. When the diene is introduced to the surface prior to the toluene-d8, the toluene adsorbs

Toluene-d8 as Promoter and Poison

Figure 4. (i, top) Infrared spectra for reaction 7 (1,3-hexadiene, toluene-d8, and hydrogen). Times, in minutes, for the initiation of data collection after the introduction of hydrogen are as follows: (a) 0, (b) 40, (c) 100, (d) 170, (e) 230, and (f) 260. (ii, bottom) Spectra from reaction 7 (1,3-hexadiene, toluene-d8, and hydrogen) showing that toluene-d8 retains its molecular identity when introduced after 1,3-hexadiene. Times, in minutes, after the introduction of the hydrogen are as follows: (a) 0, (b) 40, (c) 100, (d) 170, (e) 230, and (f) 260.

such that it retains its molecular identity, i.e., there is no dissociation of the methyl group in the toluene-d8 as observed for reaction 2, Figure 3ii. Only when full hydrogenation of the 1,3-hexadiene occurs, do these bands totally disappear, being replaced by a broad feature at 2200-2100 cm-1 in spectrum f, indicating the presence of a saturated hydrocarbon containing D atoms.

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Figure 4ii (reaction 7) reproduces the trend observed in Figure 3ii (reaction 2), where the spectra are characterized by an initial loss of the three peaks in the 2050-2150 cm-1 region (assigned to the C-D stretch of the toluene methyl group) before a broad feature at 2100-2200 cm-1 (assigned to the C-D stretch of hexane) appears. Thus, it is deduced that in both reactions 2 and 7 the dissociative adsorption of the toluene yields surface D atoms, which are then incorporated into the saturated product. The fact that this process is active in reaction 2, where hydrogen is added prior to toluene (and hexadiene), reveals the toluene (the modifier) to be closely associated with the hexadiene adsorption complex, even in the presence of prior adsorbed hydrogen. The relative concentrations of H and D atoms available to the adsorption system, and the resulting implications on the reaction mechanism, are considered in the Discussion section. Hydrocarbon Lay Down. On completion of the hydrogenation reaction, the gas-phase species are evacuated from the IR cell containing the catalyst. This facilitated the use of IR to investigate the species adsorbed on the surface of the catalyst. The spectra of the surface species retained on the catalyst after evacuation of the cell are shown in Figure 5i. Somewhat surprisingly, the spectra are essentially the same in all cases, characterized by bands at 2961, 2926, and 2874 cm-1. These are indistinguishable from the C-H stretching bands for hexane. However, as the experimental procedure described will, on the basis of the low heat of adsorption of the alkane on the catalyst, remove all of the hexane from the IR cell, Figure 5 is interpreted as arising from a saturated hydrocarbonaceous layer retained at the catalyst surface. Examination of these spectra reveals that all the reactions resulted in the lay down of saturated hydrocarbon species in varying amounts, as shown: reaction 7 > reaction 5 > reactions 1 and 6 . reactions 3 and 8 . reactions 2 and 4. Thus lay down of hydrocarbon was greatest for reaction 7, where 1,3-hexadiene was introduced first, followed by toluene and then hydrogen. Reaction 7 also had the slowest hydrogenation rate of all the reactions studied. This suggests that the decomposition of 1,3-hexadiene on the surface of the catalyst leads to hydrocarbon lay down that has a deleterious effect on the rate of hydrogenation; i.e., the active sites are being poisoned. In the reactions where toluene and hydrogen were introduced prior to the diene (reaction 2 and 4), there was very little hydrocarbon lay down with a small amount of deuterated aromatic remaining adsorbed on the surface of the catalyst after evacuation of the cell as shown in Figure 5ii. This surface species is not molecular toluene, as the only absorption bands observed are those associated with the aromatic ring at 2285 and 2276 cm-1; there is no evidence of the methyl group.16 As well as the deuterated aromatic absorption bands there is an absorption band at 2756 cm-1, which can be attributed to the presence of surface OD groups,17 indicating a degree of exchange with the support material. Discussion The mechanism for alkene hydrogenation catalyzed by transition metals was first proposed by Horiuti and Polanyi in 1934,18 setting out a process that consisted of four steps: (i) the dissociation of the hydrogen to form adsorbed H atoms, (ii) the coordination of the alkene with the surface (16) Orozco, J. M.; Webb, G. Appl. Catal. 1983, 6, 67. (17) Kiss, J. T.; Palinko, I. J. Mol. Catal. A: Chem. 1997, 115, 297. (18) Horuti, I.; Polanyi, M. Trans. Faraday Soc. 1934, 30, 164.

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Figure 5. (i, top) Infrared spectra of the hydrocarbon surface species retained by the catalyst after evacuation of gas-phase species: (a) reaction 2, (b) reaction 4, (c) reaction 8, (d) reaction 3, (e) reaction 1, (f) reaction 6, (g) reaction 5, and (h) reaction 7. (ii, bottom) Infrared spectra of the hydrocarbon surface species retained by the catalyst after evacuation of gas-phase species showing a small amount of deuterated aromatic adsorbed on the surface of the catalyst: (a) reaction 2, (b) reaction 4.

of the metal, (iii) the migratory insertion reaction between adsorbed H atoms and the coordinated alkene to form an alkyl species adsorbed on the metal, and (iv) the reductive elimination reaction between adsorbed H atoms and the alkyl species to form the alkane. In the absence of toluene the most likely reaction mechanism for 1,3-hexadiene hydrogenation is one suggested by Guo and Madix,2 whereby during hydrogenation

Jackson et al.

of a conjugated diene a half-hydrogenated intermediate adsorbed species is formed with the diene gaining one proton; this species is stabilized by at least one C-Pd σ bond in addition to the π bonding resulting from the unreacted double bond. The half-hydrogenated intermediate is more strongly adsorbed than the monoene, which can form a π bond with the surface of the palladium. Therefore, the hydrogenation of a diene on a Pd surface is expected to be thermodynamically selective with the adsorption of the diene favored over the monoene.2,5,19 As hydrogenation proceeds, the half-hydrogenated species, adsorbed on the surface, accepts a second proton to form an adsorbed monoene species, which is displaced from the surface by further 1,3-hexadiene molecules. Once all the diene has been converted to hexene, hydrogenation of the monoene can proceed, forming hexane. In principle, it should be possible to confirm the presence of the hexene on the catalyst surface by the presence of the CH wag, seen at 965 cm-1 for 3-hexene.13,15 However, this is not possible in the current experimental arrangement owing to strong absorptions by the silica support below 1300 cm-1. Nevertheless, as described above, the intensity profiles of the overtone of the dCH2 wag (1800 cm-1) and the symmetric dC-H stretch are consistent with such a sequential hydrogenation process being active with the hexa-1,3-diene. In previous studies it has been observed that the presence of coadsorbates can play an important role in determining catalyst activity and selectivity.6-11 Bond et al.7 have shown that the addition of a “modifier”, such as cinchonidine, a large organic molecule, can significantly enhance the reaction rate. They attribute this increase to a hydrogen-bonding interaction that lowers the energy of the half-hydrogenated state, thus increasing its effective surface concentration. The concept of rate enhancement by improved hydrogen transfer from an adsorbed species than from the metal itself has been shown by Siegel et al.8,9 and by Jackson et al.10,11 for different systems. Siegel8 compares the rates of hydrogenation reactions catalyzed by metals and by diimide (HNdNH). In the case of diimide it was possible to enhance the rate by a factor of 20 or decrease the rate by an order of magnitude over that obtained when the metal catalyst was used. The controlling factors suggested were angle strain, torsional strain, and R-alkyl substituents.9 Jackson et al.10,11 observed that the rates of hydrogenation of styrene and phenyl acetylene were enhanced by the coadsorption of a “competitor” molecule, benzonitrile. The reaction mechanism postulated by Jackson et al. is such that the benzonitrile adsorbs to the surface with the phenyl acetylene and styrene molecules being adsorbed onto the benzonitrile. Hydrogenation of the phenyl acetylene and styrene occur by hydrogen transfer from the metal via the benzonitrile. Somorjai et al. have investigated the role of carbonaceous overlayers during hydrogenation reactions using Pt(111) single-crystal systems.20-22 They and others23,24 have proposed a model suggesting direct participation of a carbonaceous layer in the steady-state hydrogenation of ethene. The carbonaceous layer on Pt(111), at atmo(19) Sautet, P.; Paul, J.-F. Catal. Lett. 1991, 9, 245. (20) Somorjai, G. A.; Van Hove, M. A.; Bent, B. E. J. Phys. Chem. 1988, 92, 973. (21) Godbey, D.; Zaera, F.; Yeates, R.; Somorjai, G. A. Surf. Sci. 1986, 167, 150. (22) Koel, B. E.; Bent, B. E.; Somorjai, G. A. Surf. Sci. 1984, 146, 211. (23) Thomson, S. J.; Webb, G. J. Chem. Soc., Chem. Commun. 1976, 526. (24) Webb, G. Catal. Today 1990, 7, 139 and references therein.

Toluene-d8 as Promoter and Poison

spheric pressure and 298 K, consists of ethylidyne (C-CH3) and ethylidene (CH-CH3) species bonded to the metal surface. A mechanism has been proposed whereby the hydrogenation of the ethene takes place “on top” of the ethylidyne/ethylidene species rather than on the bare metal and that the ethylidene species supplies the protons for the hydrogenation reaction.20 1,3-Hexadiene Hydrogenation. The two base rates for the hydrogenation of 1,3-hexadiene are given by reactions 1 and 5 where the order in which the reactants are introduced to the catalyst are varied. Reaction 1 has the fastest reaction rate of the two reactions, with hydrogen being introduced first (Table 1). As the rate in most hydrogenation reactions is proportional to the effective surface hydrogen atom concentration,1 this behavior is expected. The adsorbing diene is presented with a surface that is rich in hydrogen and hence is rapidly hydrogenated. In reaction 5, however, when the diene is introduced first and no hydrogen is present on the catalyst surface, the diene will self-hydrogenate, fragmenting some of the diene molecules as hydrogenation proceeds in a manner similar to that during the self-hydrogenation of ethene.19,25 As has been observed with other systems,26,27 where the extent and nature of the carbonaceous deposit have been shown to be sensitive to the hydrogen surface concentration, it appears that once the diene adsorbs onto the catalyst in a hydrogen-deficient environment, the fragments produced are permanently retained. In this state they are not effective in H transfer and so inhibit hydrogenation. The order of reactant introduction will therefore lead to different forms and levels of hydrocarbon lay down, as shown in Figure 5i. Toluene-d8 as a “promoter”. In reactions 2-4, where toluene-d8 and hydrogen are introduced before the diene, dissociation of the methyl group of the toluene-d8 was observed (Figure 3ii), confirming the suggestion by Orozco and Webb16 that toluene can adsorb onto the surface of palladium via the methyl group yielding a surface species of the type C6H5-CtM. This adsorbed species results in both H and D atoms being present on the surface of the catalyst. The presence of D atoms also results in the H-D exchange on surface OH groups, which is evidenced in the infrared spectra by the presence of an absorption band in the region 2600-2800 cm-1, Figure 3ii.17 The deuterium atoms also exchange with the diene to produce a saturated C-D stretch at ca. 2200 cm-1 (Figure 3ii). Further support for the concept that the toluene methyl group is acting as the source of deuterium comes from the work of Guo and Madix,30 who demonstrated using temperature-programmed reaction spectroscopy on Pd single-crystal surfaces that the methyl group in toluene exhibited efficient H-D exchange characteristics. Horrex, Moyes, and Squire28 observed that on a palladium catalyst reacting D2 and toluene resulted in H-D exchange on the methyl group and that this exchange was favored over hydrogenation of the toluene molecule. From this study there may be both exchange and dissociation of the methyl (25) Ouchaib, T.; Massardier, J.; Renouprez, A. J. Catal. 1989, 119, 517. (26) Al-Ammar, A. S.; Webb, G. J. Chem. Soc., Faraday Trans. 1 1978, 74, 195. (27) Lennon, D.; Kennedy, D. R.; Webb, G.; Jackson, S. D. Catalyst Deactivations1999; Studies in Surface Science and Catalysis; Froment, G. F., Waugh, K. C., Eds.; Elsevier: Amsterdam, 1999; Vol. 126, p 341. (28) Horrex, C.; Moyes, R. B.; Squire, R. C. Proceedings of the Fourth International Congress on Catalysis, Moscow, 1968; Akademiai Kiado, 1971; Vol. 1, p 332. (29) Dobson, N. A.; Eglinton, G.; Krishnamurti, M.; Raphael, R. A.; Willis, R. G. Tetrahedron 1961, 16, 16. (30) Guo, X.-C.; Madix, R. J. J. Chem. Soc., Faraday Trans. 1995, 91, 3685.

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group on the toluene. Hence our results are in keeping with the studies of Webb,16 Madix30 and Moyes,28 all of whom support a dissociated adsorption of toluene through the methyl group giving a species of the type C6H5-CtM. Introduction of the 1,3-hexadiene, after toluene-d8, resulted in the very rapid hydrogenation/deuteration of the diene to the hexane species as shown in Figure 3ii, as evidenced by the broad band at 2200 cm-1. During the hydrogenation of the diene the presence of toluene-d8 has a significant effect on the rate of reaction. The amount of hydrogen atoms present during each reaction is in excess of that required for complete hydrogenation, the ratio of H atoms:diene being 10.8:1. If all the D atoms from the methyl group were to dissociate and adsorb onto the surface of the catalyst, the concentration of H/D:diene would only increase to 11.2:1. Thus, toluene adsorption has little affect on the overall reservoir of hydrogen atoms in the system. This suggests that the extra deuterium atoms released to the surface from the toluene should not have a significant effect on the rate, even although they do occur in the product. Therefore given that a substantial number of D atoms are commuted into OD groups on the support (Figure 5ii, 2756 cm-1) and incorporated into the hydrogenated product (Figure 3ii, 2200 cm-1), the observed promotional affect of this molecule, by analogy with the work of Siegel et al.8,9 and Jackson et al.,10,11 can be ascribed to the toluene acting as a hydrogen-transfer agent. There are similarities between the structure of the surface toluene species, suggested by Orozco and Webb,16 and the structures of ethylidyne and ethylidene.20,21 A similar reaction mechanism to that described by Somorjai et al.20 could be applied to the hydrogenation reaction of 1,3hexadiene in the presence of surface toluene. Recent temperature-programmed reaction spectroscopy studies of 1,3-hexadiene hydrogenation on a Pd single crystal by Vasquez and Madix3 suggest the central double bond to be hydrogenated before the terminal double bond. Figures 1 and 2 provide evidence that, on the finely divided supported metal catalyst studied here, the terminal double bond is attacked first. Our understanding is entirely consistent with pioneering work undertaken by Dobson et al.29 who identified hydrogenation to occur on multifunctional molecules in clear-cut stages, with hydrogenation over a Pd/C catalyst occurring first at the terminal double bond and then at the internal double bond. If the adsorbed toluene species alternates between forming two and three σ bonds to the metal surface, the method of H transfer appears to be quite simple. This type of mechanism also helps to explain why so much deuterium appears in the saturated product despite its relatively low concentration with respect to hydrogen. As the toluene-d8 dissociates to form σ bonds with the surface, some D may remain attached to the toluene and the other D atoms will populate the active sites closest to the adsorbed toluene species. It would therefore, follow that the first atoms to be transferred, via the toluene to the diene, during hydrogenation would be the D atoms. Toluene as a “Poison”. For reactions 6 and 7 (Table 1) the rate of hydrogenation was slowed considerably by the presence of toluene-d8. In both cases, toluene-d8 was introduced to the reaction system after the diene. The presence of the diene on the surface of the catalyst has a deleterious effect on the hydrogenation reaction, blocking the active sites for hydrogen and toluene adsorption and dissociation. When toluene and hydrogen were introduced prior to the diene (reactions 2-4) and were therefore able to adsorb and dissociate, the rate of hydrogenation was dramatically increased. When ad-

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sorbed on the surface of the reduced palladium catalyst, in a hydrogen, deficient environment, 1,3-hexadiene will slowly self-hydrogenate.3 In doing this, hydrocarbon fragments are left on the catalyst, poisoning the surface to further reaction. This is evidenced by the difference in the rate of reaction observed for reactions 1 and 5, in the absence of toluene-d8. The slowest rate of hydrogenation was observed for reaction 7 where the diene, and then toluene-d8 were introduced before the hydrogen. As well as blocking sites, toluene-d8 adopts a different mode of adsorption as shown by the spectra in Figure 4ii, which show the toluene to be molecularly adsorbed. The adsorbed diene and dehydrogenated residue on the surface of the catalyst prevent toluene from dissociating easily through the methyl group. Some dissociation does occur, however, as evidenced by the presence of deuterium in the product, Figure 4ii, which shows a broad band at about 2170 cm-1 that is assigned to the C-D stretch of the saturated hydrocarbon. The toluene-d8 also appears to block the active sites for hydrogen dissociation. In reaction 6, the diene followed by hydrogen is introduced prior to the toluene-d8. Again, the toluene-d8 has the effect of slowing the rate of reaction relative to reaction 5, but the rate is faster than that observed for reaction 7. In reaction 6, more hydrogen dissociation may be able to occur relative to reaction 7, thus allowing the hydrogenation reaction to occur slightly faster. However, the toluene-d8 is still having a overall effect of poisoning the hydrogenation reaction. This slowing of the hydrogenation rate may be due to blocking of hydrogen dissociation sites or failing to act as an efficient hydrogen-transfer agent.

Jackson et al.

Conclusions This study has shown that the presence of an organic species on the surface of the catalyst can have a marked effect on the rate of hydrogenation of 1,3-hexadiene. Toluene-d8 has been shown to act as either a promoter or a poison depending on the order of introduction of the reactants to the system. Introduction of the toluene-d8 prior to the diene has a promoting effect with the toluene-d8 acting as a H-transfer agent. When diene is introduced prior to toluene-d8, the mode of toluene adsorption is changed, and it then acts as a poison blocking active sites. The ability of toluene-d8 to act as a promoter or poison appears to be dependent on the availability of active sites at the time of its introduction to the system. To act as a promoter, or H-transfer agent, the toluene-d8 must first be able to form σ bonds to the surface by dissociation of the methyl group. When the site availability is poor, i.e., the surface is populated by diene molecules, the toluene-d8 retains its molecular identity upon adsorption, thus blocking active sites and reducing the rate of hydrogenation. Finally it is clear that further work is needed to obtain a fuller understanding of the operating envelope of such a process. A full kinetic analysis, understanding the effect of temperature and gas ratios on the rates of the competitive reactions, e.g., hydrogen desorption vs hydrogen transfer, will be important in determining the full scope of the mechanism. Acknowledgment. D.L. thanks ICI for the award of an ICI Lectureship in Heterogeneous Catalysis. LA991614O