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he role of carbon deposits in the hydrogenation of C5 hydrocarbons

ames McGregor *, Lynn F. Gladden

niversity of Cambridge, Department of Chemical Engineering, New Museums Site, Pembroke Street, Cambridge CB2 3RA, United Kingdom

Applied Catalysis A: General 345 (2008) 51–57

R T I C L E I N F O

rticle history:

eceived 12 February 2008

eceived in revised form 3 April 2008

ccepted 16 April 2008

vailable online 22 April 2008

eywords:

dsorption

ydrogenation

icrobalance

lkenes

lkynes

A B S T R A C T

The hydrogenation of pentenes, pentenenitriles and 1-pentyne has been studied over Al2O3-supported Ni

and Pd catalysts. In particular the role of hydrocarbonaceous residues in such reactions has been probed.

This has been achieved through the use of a tapered element oscillating microbalance to record mass

changes in situ during the catalytic reactions with simultaneous analysis by gas chromatography.

Sequential adsorption and reaction studies demonstrate that the influence exerted by hydrocarbonac-

eous deposits is critically dependent upon the compound from which they are derived, with deposits

from different isomers of the same compound exhibiting different effects. In particular, in the case of 2-

pentenes, a hydrocarbonaceous overlayer resulting from the hydrogenation of cis-2-pentene significantly

activates Ni/Al2O3 towards hydrogenation of trans-2-pentene and vice versa. In the case of pentenenitrile

hydrogenation however, trans-3-pentenenitrile deposits an overlayer which renders inactive the sites

responsible for the selective hydrogenation of cis-2-pentenenitrile to pentanenitrile. Results from 1-

pentyne hydrogenation studies over Pd/Al2O3 support previous conclusions regarding the development

of a carbon-rich active phase. This work demonstrates the importance of the consideration of

heterogeneous catalysts as dynamic materials which evolve during reaction and adds to the increasing

awareness of the potential positive roles played by carbonaceous deposits in such reactions.

� 2008 Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

Applied Catalysis A: General

journal homepage: www.elsevier.com/locate/apcata

1. Introduction

Catalytic hydrogenation reactions, and in particular thereduction of carbon–carbon multiple-bonds, are of great signifi-cance and interest. In addition to the catalytic hydrogenation ofunsaturated fats to saturated fats required for the food industry,alkene hydrogenation is an important process in, for example, thepotential production of high octane number fuels from trimethyl-pentenes [1–3]. The hydrogenation of alkynes also receiveswidespread attention, not least due to the industrial significanceof acetylene hydrogenation [4–7]. Additionally, the selectivereduction of a carbon–carbon double bond in the presence ofother reducible functionalities receives significant attention. Forinstance, C C reduction in enamides, enol acetates, itaconate acidderivatives and alkenenitriles is widely studied [8–17]. Despite theimportance of these reactions, and significant research into them,the mechanism of double-bond reduction is not yet fully under-stood. However, it is widely accepted that (hydro)carbonaceousmaterial deposited on the catalyst plays a significant, positive, rolein addition to the accepted negative contribution through

* Corresponding author. Tel.: +44 1223 330134; fax: +44 1223 334796.

E-mail address: jm405@cam.ac.uk (J. McGregor).

926-860X/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

oi:10.1016/j.apcata.2008.04.018

deactivation by coking of active sites [18,19]. We can thereforebegin to consider the deliberate formation of catalytically activecoke to enhance activity or selectivity.

The majority of the work in the area of carbon–carbon multiplebond reduction has focused on the hydrogenation of C2 molecules,such as ethene in the case of alkene hydrogenation. However, thewealth of information which exists on ethene hydrogenationcannot necessarily be extended to higher alkenes. Extrapolationsfrom studies of C2 molecules have also been shown to be fraughtwith difficulties in alkyne hydrogenation studies [20]. Historically,relatively little work has been carried out on higher alkenes, with ayet smaller number of studies considering sequential andcompetitive reactions. Recent studies have attempted to redressthis balance. For instance, Dobrovolna and co-workers havestudied the competitive hydrogenation of heptene isomers overpalladium based catalysts [21], while more recently Canning andco-workers have studied the liquid-phase, competitive hydro-genation of pentene isomers over Pd/Al2O3 [22]. The same grouphave also considered the hydrogenation of pentenenitriles [16,17].The present study investigates the gas-phase hydrogenation of C5

molecules (pentene, pentenenitrile and 1-pentyne) over a varietyof catalysts with a particular focus on the role of hydrocarbonac-eous residues, specifically those deposited by isomers of thereactant molecule, on subsequent catalyst performance.

J. McGregor, L.F. Gladden / Applied Catalysis A: General 345 (2008) 51–5752

To this end, a tapered element oscillating microbalance (TEOM),coupled to an on-line gas chromatograph (GC) providing productanalysis, is employed. The TEOM is ideally suited to studying smallmass changes, of the order of micrograms, and has a temporalresolution of 0.1 s. This ability to characterise the kinetics ofadsorption and desorption with respect to carbon laydown hasbeen exploited by a number of workers [23–26]. Adsorption,diffusion, reaction and deactivation in zeolites has also been apopular area of investigation [27–32]. In the present work, we haveexploited the ability of TEOM to measure the small equilibriummasses retained on the surface of a catalyst during and post-reaction, to explore not only single-component adsorptioncharacteristics of C5 hydrocarbons, but also the importance ofcompetitive and sequential adsorption processes on catalystactivity and selectivity.

The TEOM records mass changes by monitoring the change inresonant frequency of a quartz element containing the sample inpowder form packed within a small cylindrical container [33]. Thecap to the container has small holes through it such that thereactant gas stream is caused to pass through the catalyst,therefore overcoming problems of sample by-pass associatedwith other sample-holder configurations. During measurement,the element is made to oscillate, in a clamped-free mode as acantilever beam, by a mechanical drive. As the mass of the sampleholder increases (i.e. as molecules adsorb on to the catalyst) thefrequency of the oscillation changes, according to the behaviour ofa single harmonic oscillator. Thus the frequency of theseoscillations is converted to a mass change through the relation-ship f2 = k/m, where f is the frequency of oscillation, m is the totaloscillating mass and k is the experimentally determined springconstant; k is a measure of the stiffness of the quartz element andis unique for each different element. It therefore follows that anincrease in catalyst mass will be associated with a decrease in theoscillation frequency of the TEOM. In particular, this method ofmeasurement allows for highly sensitive measurement of masschange (of the order of mg) with high temporal resolution (up to0.1 s). A more detailed explanation of the principles of massmeasurement using TEOM has been reported by Zhu et al. [32] andby Chen et al. [34] who provide full details on the operation of suchmicrobalances.

2. Experimental

In this study the hydrogenation of different C5 molecules overtwo alumina-supported catalysts has been examined. The materi-als employed are: (i) Ni/u-Al2O3 (Johnson Matthey, characterisedby a BET surface area of 106 m2 g�1, a pore volume of 0.39 ml g�1, ametal loading of 16 wt.% and a metal dispersion of 18%); and (ii) Pd/u-Al2O3 (Johnson Matthey, characterised by a BET area of97.6 m2 g�1, a pore volume of 0.49 ml g�1, a metal loading of1 wt.% and a metal dispersion of 32.5%). For comparison the puresupport has a BET surface are of 101 m2 g�1 and a pore volume of0.42 m2 g�1.

The catalysts, supplied in the form of trilobes of nominal size1.2 mm, were ground and sieved to a particle size of 75–90 mm.Approximately 35 mg of material was then loaded into the TEOM(Rupprecht and Patashick, TEOM 1500 PMA) and constrainedbetween two plugs of quartz wool. A purge flow of helium (100%,Messer) through the TEOM housing was maintained at a flow rateof 50 cm3 min�1 at a pressure of 1.2 bar throughout the experi-ments to flush the volume immediately outside the taperedelement, thereby removing unwanted material and maintainingthe pressure balance between the interior and exterior of theelement. In this work mass change data were recorded at timeintervals of 0.8 s.

Pre-treatment was carried out in situ. A flow of hydrogen (100%,Messer) was established through the sample bed at a flow rate andpressure of 50 cm3 min�1 and 1.1 bar, respectively. The catalystbed was then heated to an appropriate temperature (723 K for Ni/u-Al2O3, 393 K for Pd/Al2O3) at a rate of 7 K min�1. Thistemperature was then maintained for 120 min. At the end of thistime, the hydrogen flow was switch to a helium flow (100%,Messer) at a flow rate of 50 cm3 min�1 and a pressure of 1.2 barwhile the reactor was cooled to the reaction temperature.

Following the above pre-treatment procedure, the reactant wasintroduced to the TEOM in either a hydrogen or helium carrier gasvia a saturator. The reagents used were 1-pentene (Aldrich, 95%),cis-2-pentene (Sigma, >98.0%), trans-2-pentene (Fluka, >99%), cis-2-pentenenitrile (Aldrich, 98%), trans-3-pentenenitrile (Aldrich,95%) and 1-pentyne (Fluka,�99.9%). In all cases the pressure in theTEOM during reaction was 1.1 bar, with hydrocarbon partialpressures of 0.1 bar (pentenes and pentynes) or 0.08 bar (pente-nenitriles). The catalysts were subjected to periods, each 1 h inlength, of reagent flow. Each period was separated by 1 h ofstripping with helium. The TEOM continuously recorded the masschange occurring in the sample bed, while a gas chromatogram (HP6890GC) was used to determine the product distribution at regularintervals. Selectivity and conversion data were calculated from theresultant chromatograms. Additionally, reaction rates areexpressed as the quotient of the fractional conversion (productarea/total chromatogram area) and W/F, where W is the mass ofcatalyst and F is the molar flow rate of reactant (assuming ideal gasbehaviour).

For completeness it is noted that it is necessary to correct masschange measurements for pressure differences between the pre-treat and carrier gases [35]. This is due to the nature of themeasurement being an oscillation rather than a gravimetricmeasurement. As such, changes in gas density will cause changesin oscillation frequency which might mistakingly be interpreted aschanges in mass. The magnitude of this effect was established, andhence corrected for, by conducting identical experiments to thosedescribed above on a material which shows negligible adsorption—thereby enabling the effect of change in gas density, as well as anyadsorption onto the quartz element itself, to be accounted for. Inthis study, the correction was made using low-surface area a-alumina (Fluka, BET surface area 0.10 m2 g�1) as the adsorbent.

3. Results and discussion

3.1. Pentene adsorption

3.1.1. Adsorption of single components

Fig. 1 shows the TEOM data for the adsorption of the threepentene isomers over Ni/Al2O3 in a helium carrier gas; significantdifferences in mass laydown behaviour are observed. Exposure totrans-2-pentene results in 32.7 mg/mgcat of hydrocarbonaceousmaterial being adsorbed on the catalyst during reaction, with22.3 mg/mgcat of this material remaining 30 min after the end ofthe pulse. The corresponding values for cis-2-pentene and 1-pentene are 21.3 and 2.7 mg/mgcat, and 22.0 and 13.7 mg/mgcat,respectively. Thus we see that the not only do the isomers rank inthe order trans-2-pentene > 1-pentene > cis-2-pentene, withrespect to the amount of carbon laydown during adsorption, butalso with respect to the fraction of the adsorbed material depositedduring reaction that is retained post-reaction. In particular, trans-2-pentene retains 68% of the hydrocarbon layer adsorbed duringreaction, whilst 62% and 13% of the 1-pentene and cis-2-penteneadsorbed layers are retained.

These adsorption characteristics can be related to the tendencyof the isomers to undergo isomerisation and exchange reactions,

Fig. 1. TEOM data showing mass adsorbed 1 h into the reaction (hatched bars) and

30 min after the end of the reaction (solid bars) during the adsorption of pentene

isomers in He over Ni/Al2O3.

J. McGregor, L.F. Gladden / Applied Catalysis A: General 345 (2008) 51–57 53

and hydrogenation. Bond et al. [36] have considered the tendencyof alkene (butene) isomers to undergo isomerisation and exchangereactions, and shown them to follow the trend cis-2-butene > 1-butene > trans-2-butene. Assuming the same trend for pentenes,the relative strength of interaction of adsorbed species with thesurface, as suggested by the amount of hydrocarbon retained onthe surface determined by the TEOM studies, is therefore inverselyrelated to the tendency to undergo exchange reactions. Thissuggests that only a relatively weak interaction with the catalystsurface is required to facilitate these reactions. If the reactantmolecules are too strongly bound no reaction can take place. Thisconclusion is supported by GC data on the hydrogenation of each ofthese isomers over Ni/Al2O3 where cis-2-pentene does showconsiderably greater isomerisation activity than trans-2-pentene.Specifically, trans-2-pentene makes up 7.3% of the materialdetected by GC during the reaction of cis-2-pentene and hydrogen;however the corresponding value for cis-2-pentene productionfrom the trans isomer is only 1.7%. The isomerisation of cis-2-pentene to trans-2-pentene represents a move towards thethermodynamic equilibrium for pentene isomers.

Fig. 2 shows the catalytic behaviour, expressed as rates, of thedifferent isomers under hydrogenation conditions after 40 min on-stream at 303 K. Negligible hydrogenation of the internal penteneisomers is observed under steady-state conditions, in starkcontrast to behaviour of 1-pentene. For the terminal isomer,98.9% of all material detected by GC is n-pentane, while for cis- and

Fig. 2. Reaction rates for the hydrogenation of pentene isomers over Ni/Al2O3 at a

time-on-stream of 40 min.

trans-2-pentene this value is 1.0% and 0.6%, respectively. Thesecorrespond to reaction rates of 9.3 � 10�5 s�1, 3.9 � 10�7 s�1 and9.5 � 10�8 s�1 for the three isomers. Some isomerisation of cis-2-pentene to trans-2-pentene is also observed. The reaction ratesremain approximately constant for the entire period studied. Thesedata are consistent with the dependence of hydrogenation activityon adsorption strength following a typical volcano plot wherebyintermediates can be neither too tightly (as in the case of trans-2-pentene), nor too loosely bound (as in the case of cis-2-pentene) ifreaction is to occur.

3.1.2. Sequential adsorption

Fig. 3a shows the catalytic activity of trans-2-pentene in asequential reaction system in which hydrogenation of the cis

isomer is followed by purging with helium for 1 h, followed byhydrogenation of the trans isomer. The reverse case of trans-2-pentene hydrogenation followed by cis-2-pentene hydrogenationis shown in Fig. 3b. The reaction temperature is 303 K. It is clearlyseen that the hydrogenation activity of the 2-pentenes is nowcomparable to that of the terminal isomer (see Fig. 2), with>95% ofthe detected material being n-pentane when trans-2-pentene isemployed as the second reagent, increasing to >99% for cis-2-pentene. No such effect is seen if the same isomer is adsorbedduring both the first and second pulses, in such cases there is nochange in the observed product distribution.

Fig. 4 shows the mass retained both during reaction and 30 minafter the end of each pulse of hydrocarbon for both the sequentialreactions of cis-2-pentene followed by trans-2-pentene (top) andtrans-2-pentene followed by cis-2-pentene (bottom). In each casethe upper bar shows the mass retained 30 min after the end ofreaction, and the lower bar shows the data for the mass retainedduring reaction. The first hydrogenation occurs over the freshcatalyst, whilst the second hydrogenation occurs over the catalystand adsorbed overlayer remaining, following the first hydrogena-tion and the subsequent helium purge.

Considering, in detail, the data for the hydrogenation of cis-2-pentene followed by trans-2-pentene, we see that the reaction ofthe cis isomer is associated with an adsorbed mass of 21.7 mg/mgcat

during the hydrogenation over the fresh catalyst, with 2.2 mg/mgcat retained post-reaction. The second hydrogenation pulse (i.e.reaction of the trans isomer) gives a total mass of adsorbedmaterial during reaction of 39.1 mg/mgcat, with the total masspost-reaction of 4.5 mg/mgcat; thus, only an additional 2.3 mg/mgcat has been retained on the catalyst surface by the trans-2-pentene hydrogenation—an amount very much smaller than isretained following hydrogenation of trans-2-pentene on freshcatalyst (Figs. 2 and 4 (bottom)).

Reaction of a different isomer therefore reduces the amount ofmaterial retained during reaction as compared to adsorption of theisomer onto a fresh catalyst. Furthermore, the percentage of themass retained post-reaction is modified. In particular, far less trans

isomer is retained post-reaction if the catalyst has previously beenexposed to the cis isomer than if it had been adsorbed onto freshcatalyst. In contrast, a higher percentage of mass is retained in thecase of the cis isomer when it is hydrogenated over a catalystpreviously exposed to trans-2-pentene hydrogenation comparedwith cis-2-pentene hydrogenation over fresh catalyst. These datademonstrate that even relatively small amounts of carbon on thecatalyst surface can effect a significant change in the subsequenthydrogenation of other species. For completeness we note that anestimate of the number of surface Ni atoms covered by adsorbedhydrocarbon might be as high as 53%, if it is assumed that allretained material is associated with metal sites (i.e., no spillover ofretained material onto the alumina support) and that the retainedmass is in the form of pentyl groups with a ratio of one pentyl

Fig. 3. (a) Product distribution arising from hydrogenation of trans-2-pentene over a Ni/Al2O3 catalyst first exposed to cis-2-pentene in H2. (b) Product distribution arising

from hydrogenation of cis-2-pentene over a Ni/Al2O3 catalyst first exposed to trans-2-pentene in H2 Species observed are n-pentane (*) trans-2-pentene (&) and cis-2-

pentene (�).

J. McGregor, L.F. Gladden / Applied Catalysis A: General 345 (2008) 51–5754

group to one Ni surface site. The total number of surface Ni atomsas calculated from CO chemisorption measurements is 2.96 � 1020

per gram of catalyst. This estimate is obtained from the data for theamount of trans-2-pentene retained post-reaction on freshlyreduced Ni/u-Al2O3.

Changes in catalytic hydrogenation activity due to thepresence of hydrocarbonaceous material have previously beenobserved in other systems. For instance, Jackson et al. haveobserved an enhancement in the rate of ethene hydrogenationover Pt/Al2O3 through first employing the catalyst in a 2-butynehydrogenation reaction [37]. While this effect was ascribed to theloss of a metal-support interaction, a residue was left on thecatalyst from 2-butyne hydrogenation which may have played arole in the rate enhancement. Similar results have also beenobserved in competitive hydrogenation systems. In the compe-titive hydrogenation of 1- and 2-pentyne over Pd/C the rate ofreaction of both components has been seen to increase, with theoverall activity of the catalyst doubling [19,38]. Two scenarioswere proposed to explain this; either a change in the adsorptionstrength/mode of the pentynes had occurred or the 1-pentyneresidue acted as a hydrogen-transfer agent for 2-pentynehydrogenation and vice versa. A kinetic study revealed the latterof these to be the most likely [38]. A similar effect has beenobserved in the competitive hydrogenation of 1-pentyne and 1-pentene, and the adsorption of cis-2-pentene has also beenreported to alter the order of reaction in 2-pentyne hydrogenation[19]. It should be noted however that these reactions were

Fig. 4. TEOM data corresponding to the hydrogenation of cis-2-pentene followed by

trans-2-pentene (upper) and trans-2-pentene followed by cis-2-pentene (lower).

The top bar in each pair shows the mass adsorbed 30 min after the end of the

reaction and the bottom bar the mass adsorbed 1 h into the reaction. White bars

show data for cis-2-pentene, shaded bars for trans-2-pentene.

conducted in the liquid phase and therefore the mechanism of rateenhancement may be different to that reported for the gas phasestudies reported in the present work.

3.2. Pentenenitriles

The hydrogenation of cis-2-pentenenitrile over both freshlyreduced Ni/u-Al2O3 and Ni/u-Al2O3 previously employed in thehydrogenation of trans-3-pentenenitrile has been studied. Allexperiments were performed at 373 K.

Fig. 5 shows the composition of species detected by GC as amole fraction, and mass laydown data during seven 1 h pulses ofcis-2-pentenenitrile in H2. The intermediate pulses of He, whichseparated the pentenenitrile pulses, are not shown. Consideringfirst the trend in carbon laydown (Fig. 5a), we see that over the first�2 h, a relatively rapid mass increase is observed; thereaftercarbon laydown continues to increase approximately linearly withtime, but at a much slower rate. This change in carbon laydowncharacteristic correlates strongly with changes in product selec-tivity (Fig. 5b). In particular, initially pentylamine is the dominantreaction product however, as time-on-stream increases selectivityincreases towards pentanenitrile—the product of selective hydro-genation of the olefin moiety. This change in product selectivityoccurs at the same time as the change in rate of carbon laydown. Asthe reaction proceeds beyond 2 h, the catalyst loses activity with amonotonic decrease in pentanenitrile production.

In contrast, Fig. 6 shows that quite different behaviour in masslaydown and product selectivity is observed during cis-2-pentenenitrile hydrogenation after exposure to trans-3-pentene-nitrile in H2 for 1 h. The mass deposited on the catalyst during thesingle 1 h pulse of trans-3-pentenenitrile is 52 mg/mgcat; thesedata are not shown. In this sequential hydrogenation experimentthe catalyst is observed to be much less reactive than at thecorresponding stage during cis-2-pentenenitrile hydrogenationover a fresh catalyst. The main species observed by GC is the initialreactant, however significant quantities (18% of the detectedmaterial at 90 min), of pentylamine are produced. As is seen fromFig. 6, production of pentylamine increases with the introductionof the second and third pulses of cis-2-pentenenitrile. During thesethree pulses, mass rapidly accumulates on the catalyst surface.From the fourth pulse onwards the mass of the sample beddecreases with increasing exposure to cis-2-pentenenitrile, andthis coincides with a rise in activity, resulting in an increase inproduction of pentylamine and the observed quantity of cis-2-pentenenitrile decreasing as its consumption increases. Despitethis, at no stage is the selective hydrogenation product, pentane-nitrile, formed in preference to the total hydrogenation product,pentylamine.

Fig. 5. (a) TEOM data for the hydrogenation of cis-2-pentenenitrile over Ni/Al2O3 showing mass build-up during reaction. (b) GC data acquired simultaneously with TEOM

data. Species observed are cis-2-pentenenitrile (*), pentanenitrile (^) and pentylamine (&).

Fig. 6. (a) TEOM data for the hydrogenation of cis-2-pentenenitrile over Ni/Al2O3 previously employed in the hydrogenation of trans-3-pentenenitrile. (b) GC data acquired

simultaneously with TEOM data. Species observed are: cis-2-pentenenitrile (*), pentanenitrile (^), pentylamine (&) and a species assigned to cis-3-pentenenitrile (�).

J. McGregor, L.F. Gladden / Applied Catalysis A: General 345 (2008) 51–57 55

These data show that the hydrocarbonaceous overlayerdeposited as a result of trans-3-pentenenitrile hydrogenationprevents, with a degree of selectivity, the transformation of cis-2-pentenenitrile to pentanenitrile. The above observations canbe rationalised by considering the different adsorption beha-viour of trans-3- and cis-2-pentenenitrile and the differentadsorption sites that have been shown to be present on thecatalyst. A more detailed description of the available sites willbe discussed elsewhere, however it has previously been shownthat among the active sites present are a high energy sitecapable of hydrogenating the nitrile group, and a low energy sitewhich can carry out only the more facile olefin hydrogenation[17]. We have shown here that activity towards pentylamineformation is closely linked to the observed mass laydown of thesystem. As the total mass adsorbed by the catalyst increasesthrough a pulse, activity towards pentylamine decreases. Therecovery of activity at the start of the second and third pulsessuggests that at least some of the material removed duringstripping with helium between pulses was located on sitescapable of the less facile nitrile hydrogenation. Overall, theinitial pulse of trans-3-pentenenitrile is seen to render thecatalyst inactive to selective carbon–carbon double-bondreduction. This indicates that the nature of the retainedoverlayer differs depending on the pentenenitrile isomer fromwhich it originates. This may be due to the mode of adsorptionof trans-3-pentenenitrile. The remaining hydrogenation activity,resulting in pentylamine production, is critically dependentupon the quantity and, most likely, location of adsorbedmaterial.

3.2.1. 1-Pentyne

Previous TEOM studies on the hydrogenation of 1-pentyne havehelped to establish the mechanism of alkyne hydrogenation overPd catalysts [4]. In that work an exceptionally high increase inmass associated with the catalyst during selective hydrogenationof pentyne to pentene, absent under total hydrogenation condi-tions, indicated the formation of a Pd–C surface phase. The precisenature of this phase was revealed by XPS and HRTEM studies inwhich it was unequivocally demonstrated that carbon can dissolveinto the palladium lattice. A full description of the structure of theresulting material is provided by Teschner et al. [4]. It wassuggested that it is this mixed metal-carbon phase which is theactive site for selective hydrogenation and not the palladiummetal. The present study extends this work to different reactionconditions and investigates the role of trans-2-pentene on theactivity of this system in both sequential and competitivehydrogenation. In all cases reaction was carried out at 358 K withthe reactant being introduced in a hydrogen carrier gas. Fig. 7shows TEOM data for two pulses of 1-pentyne separated by 1 h ofstripping with helium, and also for the sequential reaction of trans-2-pentene and 1-pentyne. Prior to introduction of the hydrocarbon(at time = 0 h) a flow of He was established through the TEOM.

As can be seen from Fig. 7, the hydrogenation of 1-pentynewithout pre-adsorption and reaction of trans-2-pentene results insignificant carbon laydown on the catalyst. After 1 h of reaction themass of the catalyst bed has increased by 192 mg/mgcat, rising to313 mg/mgcat after a second 1 h pulse. Considering now the case inwhich hydrogenation of trans-2-pentene proceeds for 1 h followedby a He purge, followed by 1-pentyne hydrogenation, we see that

Fig. 7. TEOM data showing mass changes occurring within Pd/Al2O3 catalyst bed

during the hydrogenation of trans-2-pentene followed by 1-pentyne, and for two

1 h pulses of 1-pentyne in hydrogen.

J. McGregor, L.F. Gladden / Applied Catalysis A: General 345 (2008) 51–5756

very little carbon laydown occurs during the hydrogenation oftrans-2-pentene. The retained mass deposited during alkenehydrogenation does not inhibit a significant increase in massduring the subsequent 1-pentyne hydrogenation, with nearidentical catalytic behaviour to that over the fresh catalyst beingobserved after a short induction period (Fig. 8). In both cases, therate of mass increase proceeds at an approximately linear rate afteran initial rapid adsorption period. This rate of mass increase during1-pentyne adsorption is �1.66 mg/mgcat per minute in thesequential hydrogenation system as compared to �2.76 mg/mgcat

per minute over the fresh catalyst. This small reduction in rate maybe ascribed to the reduction in accessible Pd sites due to thedeposition of carbonaceous material from trans-2-pentene. Forcomparison, the reduction in rate between the first and second

Fig. 8. Mole fraction of species evolved during sequential and competitive hydrogenation

Al2O3. (b) 1-Pentyne hydrogenation over fresh Pd/Al2O3. (c) 1-Pentyne hydrogenatio

hydrogenation of 1-pentyne and trans-2-pentene over Pd/Al2O3. Species observed are: n

(*) and 1-pentyne (4).

pulses of 1-pentyne (in which there is no exposure of the catalystto trans-2-pentene), where the carbonaceous deposits are in theform of a Pd–C phase, is only from 2.76 to 2.2 mg/mgcat per minutedespite the much larger quantity of carbon associated with thecatalyst in this case.

Fig. 8 shows the product distribution, as the mole fraction of allspecies detected by GC, during sequential and competitivehydrogenation of 1-pentyne and trans-2-pentene, and comparesthese data to hydrogenation of pure, single-component, trans-2-pentene (Fig. 8a) and 1-pentyne (Fig. 8b) pulses. As is seen fromFig. 8a, hydrogenation of trans-2-pentene produces a mixture ofisomerisation and hydrogenation products. Hydrogenation of 1-pentyne (Fig. 8b) produces the primary alkene in approaching100% selectivity. The data for the hydrogenation of 1-pentyne forthe catalyst first exposed to trans-2-pentene is shown in Fig. 8c. Insummary, the reactivity is of the system is barely influenced by theretained mass from the initial exposure to trans-2-pentenehydrogenation, with very similar product distributions beingobserved after an initial induction period as were seen for pure,single-component 1-pentyne hydrogenation. In the case of the 1-pentyne hydrogenation following exposure to the trans-2-pentenehydrogenation, a slight increase in 1-pentene production isobserved with time. Overall, the 1-pentyne hydrogenation appearsto proceed in a similar way regardless of catalyst history,consistent with the suggestion that the mechanism by whichthe active surface Pd–C phase forms is not inhibited by retainedcarbonaceous material, presumably so long as the alkyne hasaccess to surface Pd sites. The interpretation of these data using theconcept of the Pd–C phase previously reported elsewhere [4], issupported by calculations of the ratio of the number of pentylgroups to surface Pd atoms, based on the amount of 1-pentyneadsorbed (Fig. 7) and the dispersion of Pd, which is 32.5% based onCO chemisorption measurements. During the first and secondpulses of 1-pentyne, the ratio of pentyl groups to surface Pd atomswould be 89:1 and 151:1, respectively. If the carbonaceous

of 1-pentyne and trans-2-pentene. (a) trans-2-Pentene hydrogenation over fresh Pd/

n over Pd/Al2O3 first used for trans-2-pentene hydrogenation. (d) Competitive

-pentane (^); 1-pentene (&); cis-2-pentene (–); trans-2-pentene (�); iso-pentane

J. McGregor, L.F. Gladden / Applied Catalysis A: General 345 (2008) 51–57 57

material is assumed to be entirely dehydrogenated these valueschange to 524 and 857 carbons per surface Pd. These data areconsistent with both the formation of a Pd–C phase as previouslydiscussed [4], and the retention of significant amounts ofcarbonaceous material on the support.

In addition to studying the sequential reaction of trans-2-pentene and 1-pentyne a competitive reaction between the twospecies has also been investigated. This experiment was conductedin the same manner as the previous studies, however the saturator,at the commencement of the experiment, contained a 50/50 (v/v)mixture of both compounds. The product distribution in this case isshown in Fig. 8d. As can be seen, no products of trans-2-pentenehydrogenation are observed, and, ignoring trans-2-pentene itself,the product distribution is (again, after a short induction period)almost identical to that of the hydrogenation of 1-pentyne in theabsence of other hydrocarbonaceous species. Similarly, the onlydifference in mass laydown data is that the rate of mass build-up isslower in the competitive system compared to single-component1-pentyne hydrogenation, a fact that can be ascribed to the lowertotal amount of 1-pentyne present in the feedstream. Overall,when both the alkene and the alkyne are introduced to the catalystsimultaneously a competitive adsorption scenario develops whereadsorption of the alkyne, and surface species derived from it, isstrongly favoured over that of the alkene thus preventing reactionof the alkene. Similar results for competitive hydrogenationsystems involving alkenes and alkynes are widely reported inthe literature [21,22].

4. Conclusions

TEOM studies have provided insight into the crucial role playedby deposited hydrocarbonaceous material in the hydrogenation ofcarbon–carbon multiple bonds. In particular, it has been shownthat even material derived from different isomers of the samecompound can have dramatically different effects on thesubsequent hydrogenation. In the hydrogenation of 2-pentenesretained mass from the hydrogenation of one isomer significantlyactivated the hydrogenation of the other. This effect was notachieved by hydrocarbonaceous overlayers derived from thereactant molecule. For pentenenitriles however, the effect ofhydrocarbonaceous material derived from trans-3-pentenenitrileon the hydrogenation of cis-2-petenenitrile is to restrict theselective hydrogenation pathway. In the case of 1-pentynecarbonaceous material plays a very different role, forming a Pd–C surface phase which is active in the selective production ofpentenes. The ability of the catalyst to form this phase is notrestricted by the presence of either trans-2-pentene or residuesderived from it. In studying the behaviour of single-componentadsorption of pentenes it was also demonstrated that the ability ofpentenes to undergo isomerisation and exchange reactions isinversely related to their strength of interaction with the catalystsurface.

The ability to relate in situ mass laydown measurements tosimultaneously acquired activity measurements has been key inprobing the role of carbonaceous materials in catalyst perfor-mance. Such results demonstrate the opportunity to improve

catalytic performance through the deliberate deposition ofappropriate carbonaceous material under suitable reactionconditions.

Acknowledgements

The authors would like to acknowledge the ATHENA project(EPSRC grant GR/R47523/01), which is funded by EPSRC andJohnson Matthey plc. We also wish to thank Prof. S.D. Jackson andDr. A.S. Canning, University of Glasgow, for providing thechemisorption analysis and valuable discussions.

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