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Applied Catalysis A: General 384 (2010) 192–200

Contents lists available at ScienceDirect

Applied Catalysis A: General

journa l homepage: www.e lsev ier .com/ locate /apcata

he influence of carbon laydown on selectivity in the hydrogenation ofentenenitriles over supported-nickel catalysts

ames McGregora,∗, Arran S. Canningb, Scott Mitchellb, S. David Jacksonb, Lynn F. Gladdena

University of Cambridge, Department of Chemical Engineering and Biotechnology, New Museums Site, Pembroke Street, Cambridge CB2 3RA, UKCentre for Catalysis Research, WestCHEM, Department of Chemistry, University of Glasgow, Glasgow G12 8QQ, Scotland, UK

r t i c l e i n f o

rticle history:eceived 9 April 2010eceived in revised form 16 June 2010ccepted 17 June 2010vailable online 25 June 2010

eywords:atalytic hydrogenationis-2-Pentenenitrile

a b s t r a c t

Pentenenitriles contain two-reducible functionalities: a carbon–carbon double bond and a nitrile group,either of which may undergo hydrogenation during reaction. In this work we show how the depositionof hydrocarbonaceous material on the catalyst surface during pentenenitrile hydrogenation over 16 wt.%Ni/Al2O3 and 10 wt.% Ni/SiO2 catalysts has a significant impact on the observed catalytic activity andselectivity. The role of carbon laydown in controlling catalytic performance in this system has been eval-uated through activity measurements and mechanistic studies employing a Tapered Element OscillatingMicrobalance (TEOM) and a conventional flow-through reactor. TEOM data indicating the deposition ofcarbonaceous material during reaction are correlated with kinetic analysis which provides a description

rans-3-Pentenenitrileinetic analysisEOMluminaokeeactivation

of catalyst deactivation in terms of the deactivation of groups of active sites. Specifically five distinctactive sites are shown to exist on Ni/Al2O3 including a hydrogenation site on the support, which is notpresent in the case of Ni/SiO2. The nature and strength of these sites are discussed. Furthermore, deutera-tion studies provide mechanistic insights suggesting that the hydrogenation reaction proceeds via a cyclicintermediate. The reported data identify a correlation between mass laydown on specific active sites anddeactivation, thereby demonstrating the influence of hydrocarbonaceous deposits on selectivity. Both

re of

the location and the natu

. Introduction

Selectivity is one of the key drivers in catalysis research: improv-ng selectivity has direct economic and environmental benefitsssociated with less waste and reduced material and energy inputs.he single largest application of catalysis in chemical manufactur-ng is catalytic hydrogenation, and hence the selective reduction ofne-functionality within a multi-functional unsaturated molecules a subject that receives much attention [1–4]. Hydrogenation reac-ions over supported-metal catalysts are, in general, accompaniedy the deposition of carbonaceous material on the catalyst surface5]. This deposited coke can impart significant influence on cat-lytic activity and selectivity most commonly through deactivationf active sites [6–8], but also through action as a hydrogen-transfergent, or direct involvement in catalytically active or selective sites9–11]. In addition to simple chemical conversion data, kinetic anal-

sis can greatly assist in elucidating the processes occurring onhe catalyst surface during reaction [12–14]. For instance, kinetictudies on propenenitrile and cyclohexane hydrogenation havehown that the alkene functionality adsorbs to a Raney nickel cat-

∗ Corresponding author. Tel.: +44 0 1223 330134; fax: +44 0 1223 334796.E-mail address: jm405@cam.ac.uk (J. McGregor).

926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apcata.2010.06.036

such deposits are crucial in determining its influence on reaction.© 2010 Elsevier B.V. All rights reserved.

alyst less strongly than the nitrile moiety [14]. Despite this, underhydrogenation conditions the carbon–carbon double bond is morereactive than the carbon–nitrogen bond.

Following from the observation that nitrile functionalities aremore strongly adsorbed than olefinic functionalities, it might beexpected that in longer chained alkenenitrile molecules, wherethe two moieties are a significant distance apart, high selectivi-ties towards hydrogenation of the nitrile group could be obtained.This result has indeed been observed experimentally [14–16].In contrast, in shorter-chained molecules such as propenenitrileand butenenitriles selectivity is directed towards reduction of thecarbon–carbon double bond [14,17]. Recent studies have focusedon the hydrogenation of five-carbon chain species: penteneni-triles. Kukula and Koprivova investigated the hydrogenation ofcis-2-pentenenitrile (C2PN) and trans-3-pentenenitrile (T3PN) inthe liquid-phase over a variety of catalysts including Raney nickel[15]. In the case of the conjugated isomer (C2PN) the primary prod-uct was n-pentanenitrile (PN) while for the non-conjugated isomer,trans-3-pentenenitrile (T3PN), the major primary product is trans-

3-pentenamine. Elsewhere, the formation of PN from both C2PNand T3PN over Ni/Al2O3 has also been observed [18].

We have previously conducted independent investigationsinto gas-phase pentenenitrile hydrogenation over an alumina-supported-nickel catalyst employing a Tapered Element Oscillating

J. McGregor et al. / Applied Catalysis A: General 384 (2010) 192–200 193

Table 1Summary of our previous studies on the hydrogenation of pentenenitriles.

Reactant Catalyst Technique Parameters investigated Key conclusions Ref.

C2PN Ni/Al2O3 TEOM Mass laydown during reactionand variation in catalyticactivity with time-on-stream

Selectivity shift fromPA to PN withincreasing carbonlaydown

[18]

C2PN Ni/Al2O3 Flow reactor Catalytic performance withtime-on-stream

Selectivity shift fromPA to PN

[7]

C2PN Ni/Al2O3 Flow reactor Selective poisoning study usingCO and NH3

At least 3 distinctactive sites exist

[7]

C2PN �-Al2O3 Flow reactor Catalytic performance withtime-on-stream

Catalytic activity ofsupport; formation ofammonia

[7]

Masandacti

MtsiNusctiaamwwcltcsaotasthtotd

wawtaTr

tTittasio

T3PN followed by C2PN Ni/Al2O3 TEOM

icrobalance (TEOM) [18] and an atmospheric flow-through reac-or [7]. Although separate, these two investigations reported veryimilar results. For example, a significant variation in selectiv-ty with time-on-stream during C2PN hydrogenation over 16 wt.%i/Al2O3 was observed in both cases: initially the dominant prod-ct is n-pentylamine (PA); as time-on-stream increases selectivityhifts towards the product of selective hydrogenation of thearbon–carbon double bond, PN. As the reaction proceeds furtherhe catalyst loses activity with PN production decreasing. Stud-es on the pure support, �-Al2O3, have shown that even in thebsence of Ni initial activity is observed towards PA formation. Inddition to catalytic selectivity and activity data, measurements ofass change due to adsorption and carbon laydown over Ni/Al2O3ere also recorded in situ. The measured mass laydown profileas correlated with changes in selectivity over time [18]. Specifi-

ally, during C2PN hydrogenation an initial rapid period of carbonaydown was associated with a decrease in the production of theotal hydrogenation product, PA, while a second, slower, period ofarbon deposition was associated with deactivation towards theelective hydrogenation product, PN. Our preliminary hypothesisssociated these two periods of mass laydown with two groupingsf active site: high energy sites which undergo rapid deactiva-ion and lower energy sites which deactivate at a slower rate. Thedsorption of T3PN in H2 was also shown to deactivate certainurface sites [18]. Exposing the catalyst for a short time (1 h) tohe non-conjugated isomer subsequently prevented the selectiveydrogenation of C2PN to PN. PA formation however was not deac-ivated, the total hydrogenation product being the major productver the cumulative 7 h timescale of reaction. Increasing produc-ion of PA was observed from 3 h onwards, corresponding to aecrease in the mass of the retained carbonaceous overlayer.

A more detailed description of the catalytically active sitesas provided through selective poisoning studies [7]. In summary,

dsorption of CO on metal sites prevented the formation of PN,hile adsorption of NH3 (which will adsorb on acid sites) poisons

he isomerisation reaction. As a result of these poisoning studiesmodel describing three-distinct active sites was developed [7].

his however is insufficient to explain subsequent observationseported in the present work.

A summary of our previous studies indicating the reaction sys-ems investigated and the principle conclusions drawn is shown inable 1. The aim of the present study is to extend these earlier stud-es and to investigate the parameters which influence selectivity inhe hydrogenation of C2PN and T3PN. In particular, understanding

he extent and role of carbon laydown and the nature of the cat-lytic active site(s) are key parameters in this regard. The role of theupport in determining selectivity and deactivation has also beennvestigated. Furthermore, in order to fully understand the origin ofbserved selectivity and the origin and rate of catalyst deactivation,

s laydown during reactionvariation in catalytic

vity with time-on-stream

T3PN deactivatescatalyst towards PNformation

[18]

experiments have been conducted to elucidate the mechanism ofreaction.

2. Experimental

2.1. Materials and catalyst characterisation

Al2O3- and SiO2-supported-nickel catalysts, in addition to thepure support materials have been investigated. Ni/Al2O3 (HTC400, Johnson Matthey) is characterised by a BET surface area of106 m2 g−1, a pore volume of 0.39 cm3 g−1 and a metal loading of16 wt.%). The support is �-Al2O3 (Johnson Matthey, BET surface area101 m2 g−1, pore volume 0.42 cm3 g−1). Prior to use both the cat-alyst and support were activated in situ by reduction in flowingH2 at ≥523 K for 2 h. Previous TPR studies have demonstrated thatthis temperature is sufficient to reduce the catalyst [19]. Ni/SiO2 ischaracterised by a BET surface area of 132 m2 g−1, a pore volumeof 0.64 cm3 g−1 and a metal loading of 10 wt.%. The SiO2 supportis commercially available Aerolyst 3039 (Degussa) with a surfacearea of 250 m2 g−1 and a pore volume of 0.9 cm3 g−1.

2.2. Reaction studies

Catalytic reaction studies have been carried out in two differ-ent reactors: an atmospheric flow-through reactor and a TEOM. Inboth cases the as-received catalyst was ground and sieved priorto use. For the atmospheric flow-through reactor a particle sizeof 200–400 �m and a total of mass of 0.25 g was used, while thecorresponding values for the TEOM were 75–90 �m and 35 mg.Both experimental set-ups have previously been described in detail[7,18], while the operating principles and applications of the TEOMhave been reported by Zhu et al. [20] and Chen et al. [21]. Areas inwhich the TEOM has previously been employed include adsorption,diffusion, redox kinetics, reaction and deactivation due to carbondeposition [21–25]. In the present study the TEOM was operated at1.1 bar throughout catalyst pre-treatment and reaction. Correctionsfor pressure differences between the pre-treatment and reactiongas streams have been corrected for by conducting identical exper-iments over a low-surface area �-Al2O3 (Fluka, BET surface area0.10 m2 g−1) upon which only negligible adsorption occurs [18].A key advantage of the TEOM is the ability to monitor mass lay-down with high resolution (�g) as a function of time-on-stream(with time resolution ∼0.1 s), while simultaneously monitoring thecomposition of the outlet stream, in this case using gas chromatog-

raphy (GC) (Hewlett-Packard 6890, FID detector). By reverting fromthe reactant stream to an inert flow (He) the mass retained onthe catalyst surface, in the form of hydrocarbonaceous deposits orcoke, can be determined. In order to monitor the build-up of suchdeposits with time-on-stream during TEOM studies the reactant

1 lysis A: General 384 (2010) 192–200

flrr1abatotoHts

3

smanhtatoliTTcwjai

smshpto

3

s

Fig. 2. Relative quantity of species detected by GC (FID) during hydrogenation of

F

e

94 J. McGregor et al. / Applied Cata

ow (50 cm3 min−1, H2:pentenenitrile ratio 12.5:1) was, after 1 h,eplaced by a flow of He (50 cm3 min−1) for 1 h, before reverting toeactant flow. Seven such cycles were conducted, providing sevenh periods of pentenenitrile/H2 flow through the catalyst, each sep-rated by 1 h of He flow. In both reactors the catalyst was reducedy hydrogen in situ prior to use (Section 2.1) before being cooled toreaction temperature of 373 K. Pentenenitriles were introduced to

he catalyst via a vapouriser containing either C2PN (98%, Aldrich)r T3PN (95%, Aldrich). In the atmospheric flow-through reactorhe reaction was monitored via GC (Varian 3300, TCD) for a periodf 3 h on-stream. The reactant flow rate was 50 cm3 min−1, with a2:pentenenitrile ratio 3.6:1. Samples were then cooled to room

emperature prior to temperature programmed oxidation (TPO)tudies.

. Results

In order to more fully understand the factors that influenceelectivity and deactivation in the hydrogenation of C2PN, and ofulti-functional molecules in general, experiments were designed

s follows. Firstly, in order to further determine the number andature of the catalytically active sites present on Ni/Al2O3, theydrogenation of C2PN over the pure �-Al2O3 support is inves-igated. The roles played by the metal and support sites aredditionally probed by changing the support material from aluminao silica, hence C2PN hydrogenation studies have been conductedver Ni/SiO2 and SiO2. Carbon laydown on catalytic sites is corre-ated with changes in selectivity and catalyst deactivation, therebyndicating a dynamic catalytic system under reaction conditions.he hydrogenation of T3PN and the sequential hydrogenation of3PN and C2PN provide insights into the nature of the deactivatingarbonaceous deposits and their interaction with catalytic sites asell as illustrating differences between the behaviour of the con-

ugated and non-conjugated pentenenitrile isomers. The locationnd character of carbonaceous material deposited during reactions further characterised through TPO measurements.

In addition to probing the nature of the catalytically activeites, studies were undertaken in order to determine the reactionechanism. These experiments employed D2 in place of H2. By con-

idering the results of these deuteration studies with regard to theydrogenation experiments described above it is possible to pro-ose a detailed reaction mechanism. Together these studies identifyhe influence of carbon laydown on different active sites and hencen catalytic selectivity and activity.

.1. Nature of the catalytically active sites

In order to investigate the nature of the catalytically activeites present on Ni/Al2O3 the hydrogenation of C2PN over �-Al2O3,

ig. 1. Quantity of mass adsorbed on catalyst during reaction at 373 K, 1.1 bar, 50 cm3 min

nd of each 1 h pulse of reactant + H2. (a) Data for C2PN hydrogenation over Ni/Al2O3 ( ) a

C2PN over Al2O3 at 373 K, 1.1 bar, 50 cm3 min−1 flow rate and H2:pentenenitrileratio 12.5:1. Species detected are C2PN ( ), cis-2-pentene ( ), T3PN ( ) and trans-2-pentene ( ). Curves are fitted only as a guide to the reader.

Ni/SiO2 and SiO2 has been investigated. Additionally the hydro-genation of T3PN over Ni/Al2O3 and �-Al2O3 and the sequentialreaction of T3PN followed by C2PN have been carried out in orderto investigate the influence of deposited carbonaceous material onspecific active sites and therefore on catalytic performance. FinallyTPO studies provide an indication as to the number and strength ofthe adsorption sites present.

3.1.1. C2PN + H2 over Ni/Al2O3 and �-Al2O3Hydrogenation studies, complementary to those carried out

over Ni/Al2O3, have been conducted over the pure support in orderto deconvolve the roles of the Ni and Al2O3. TEOM data from thereaction of C2PN with H2 over �-Al2O3 shown in Fig. 1a reveal thatthe levels of mass laydown observed are significantly lower thanthose observed over Ni/Al2O3. During the first 1 h pulse the levelof adsorbed material reaches a maximum of 45.2 �g/mgcat overNi/Al2O3 but only 18.3 �g/mgcat over the support. Note that thisdoes not necessarily imply that the additional carbonaceous mate-rial formed over Ni/Al2O3 resides solely on Ni sites as spill-overof this material onto the support may occur. The reaction pro-file, for C2PN hydrogenation over �-Al2O3 as a function of time,is shown in Fig. 2. This profile demonstrates surprising activity of

the �-Al2O3 support. Initially C2PN and cis-2-pentene are detectedin approximately equal amounts. At increasing times-on-streamactivity towards formation of the olefin begins to decrease. Afterfour 1 h pulses, cis-2-pentene represents only ∼20% of the outlet

−1 flow rate and H2:pentenenitrile ratio 12.5:1. Mass measurement is taken at the

nd Al2O3 ( ). (b) Data for T3PN hydrogenation over Ni/Al2O3 ( ) and Al2O3 ( ).

J. McGregor et al. / Applied Catalysis A: General 384 (2010) 192–200 195

Ffl3

n

syhaicdflt

3

itdStfoadtPtdpa

3

ctcbhoropwtos

ig. 3. Reaction profile for C2PN hydrogenation over Ni/SiO2 in the atmosphericow-through reactor at 373 K, 50 cm3 min−1 flow rate and H2:pentenenitrile ratio.6:1, showing conversion ( ) and fractional composition of the gas-phase products,

amely PN ( ), PA ( ), T2PN (+) and T3PN ( ).

tream. The formation of cis-2-pentene suggests that a hydrogenol-sis or similar reaction is occurring leading to the scission andydrogenation of C2PN producing the alkene and ammonia. Indeed,mmonia was detected in the outlet gas stream from the TEOM dur-ng hydrogenation studies both over the pure support and over theatalyst in the present study. Ammonia has also previously beenetected during the reaction of C2PN and H2 in the atmosphericow-through reactor, although no cis-2-pentene was observed inhat work [7].

.1.2. C2PN + H2 over Ni/SiO2 and SiO2Considering the hydrogenation of C2PN over Ni/Al2O3, further

nsights may be gained into the roles played by the metal and byhe �-Al2O3, by varying the nature of the support. To this end, weescribe studies of C2PN hydrogenation over Ni/SiO2, and over theiO2 support, in an atmospheric flow-through reactor. In contrasto �-Al2O3, SiO2 is apparently inert and shows no evidence of PAormation. The main products formed, with a combined selectivityf ∼90%, are isomerisation products, namely trans-3-pentenenitrilend trans-2-pentenenitrile. In contrast, Ni/SiO2, as with Ni/Al2O3,oes produce PA in the initial stages of reaction. The overall reac-ion profile for C2PN hydrogenation over Ni/SiO2 is shown in Fig. 3.A is produced initially, however the catalyst then rapidly deac-ivates towards this reaction with PN and isomerisation productsominating from 40 min onwards. This is a very similar reactionrofile to that observed when Ni/Al2O3 is employed as the catalyst,s will be discussed in Section 4.3.

.1.3. T3PN + H2 over Ni/Al2O3 and �-Al2O3TEOM and atmospheric flow-through reactor experiments were

arried out for T3PN in the same manner as for C2PN in ordero investigate the influence of adsorbate structure on reactivity,arbon laydown and deactivation. TEOM studies demonstrate aroadly similar pattern of mass laydown over Ni/Al2O3 as for C2PN,owever as shown in Fig. 1b, significantly greater mass laydown isbtained with T3PN. After seven 1 h pulses T3PN hydrogenationesults in a retained carbonaceous overlayer of 75.4 �g/mgcat aspposed to 51.1 �g/mgcat for C2PN hydrogenation. The reaction

rofile, shown in Fig. 4, is quite different for the trans isomer thanas observed for the conjugated compound. Initially, as with C2PN,

he non-selective hydrogenation product, i.e. PA, is the main speciesbserved, alongside smaller amounts of the products of isomeri-ation reactions. After ∼20 min time-on-stream however, activity

Fig. 4. Reaction profile T3PN hydrogenation (373 K. 1.1 bar, 50 cm3 min−1 flow rateand H2:pentenenitrile ratio 12.5:1) over Ni/Al2O3 in the TEOM. Species detectedare: PA ( ), T3PN ( ) and species assigned to cis-3-pentenenitrile ( ) and trans-3-pentenamine ( ).

towards both the total hydrogenation and isomerisation productsdecreases rapidly, with >90% of the detected exit gas comprisingthe initial reactant. The remaining activity is towards the formationof small amounts of a species believed to be trans-3-pentenamine.The selective hydrogenation of the nitrile functionality can be ratio-nalised by considering the high mass laydown that occurs in thisreaction, which may sterically hinder the formation of an appropri-ate intermediate to facilitate C C hydrogenation. As a result, onlythe nitrile functionality, adsorption of which is favoured kinetically,undergoes reaction. Atmospheric flow-through reactor studies pro-vide complementary data to TEOM measurements, confirming thatPA is the major product produced in the early stages of reaction andthat no PN, the product of selective C C hydrogenation, is observed.

TEOM data for the reaction of T3PN and H2 with �-Al2O3 areshown in Fig. 1b. As with C2PN, a significantly lower mass uptakeoccurs than is observed over Ni/Al2O3. The decrease in mass lay-down is greater than twofold, the retained overlayer totalling31.3 �g/mgcat after seven 1 h pulses as compared to 75.4 �g/mgcat

over Ni/Al2O3. Unlike C2PN however, T3PN is largely unreactiveover the support. Greater than 95% of the detected material from thesecond pulse onwards comprises T3PN. During the first pulse smallamounts of the material assigned above as trans-3-pentenamineare observed, reaching a maximum of 25% of the exit gas compo-sition after 5 min. After 30 min however this value has fallen to<5%.

3.1.4. Sequential hydrogenation of T3PN followed by C2PNThe role of the carbonaceous overlayer deposited by T3PN, as

described in Section 3.1.3, on the hydrogenation of C2PN can beprobed through sequential hydrogenation studies. This in turn pro-vides insights into the interaction of carbonaceous material withcatalytically active sites, which plays a key role in directing catalyticselectivity. To this end, studies of the sequential hydrogenationof T3PN followed by C2PN have been carried in our atmosphericflow-through reactor over Ni/Al2O3 and over �-Al2O3. After 30 minof T3PN hydrogenation, the flow was switched to helium to flushthe reactor and then, when no material was detected in the efflu-

ent stream, the flow was switched to C2PN in 5% H2/N2. OverNi/Al2O3, as with the catalyst which was not first exposed to T3PN,PA was the initial product of C2PN hydrogenation. In contrast tothe untreated catalyst however, instead of decaying rapidly PA wasa major product for over 1 h. Considering �-Al2O3, C2PN hydro-

196 J. McGregor et al. / Applied Catalysis A: General 384 (2010) 192–200

Fig. 5. Hydrogenation of C2PN over Al2O3 first exposed to T3PN in the atmosphericfl 3 −1

3fs

gcCfsbwrnc

3

asthatowcvflar5s5clor

3

iadTr

ow-through reactor at 373 K, 50 cm min flow rate and H2:pentenenitrile ratio.6:1 as a function of time. Conversion ( ) is shown on the left-hand axis and theractional composition of the gas-phase products, namely PA ( ) and C2PN ( )hown on the right-hand axis.

enation over the T3PN treated alumina resulted in a maximumonversion of ∼25%, achieved in the early stages of reaction (Fig. 5).arbonaceous deposits however, represented the major product

ormed under these conditions with ∼68% of the reactant con-umed being converted to such deposits based on a simple massalance. The remaining conversion was to PA. No isomerisationas detected. After ∼25 min production of PA ceased, with a cor-

esponding decrease in conversion observed. The conversion didot drop to zero for another 2 h, indicating that carbon laydownontinued during this time.

.1.5. TPO studiesIn order to more clearly understand carbon deposition and to

ssist in identifying the number and nature of catalytically activeites TPO studies on catalysts after reaction have been under-aken. Kinetic analysis of the rate of catalyst deactivation in C2PNydrogenation has previously identified two deactivation stages,nd these are discussed in detail in Section 4.3. Briefly, an ini-ial, rapid, deactivation period is observed before isomer formationccurs. This is followed by a second, slower, period of deactivationhere isomerisation of C2PN occurs. TPO analysis was therefore

onducted on two catalyst samples, one from each stage of deacti-ation. Both of these samples were removed from the atmosphericow-through reactor. The TPO experiments were run between 298nd 873 K and are shown in Fig. 6. The catalyst sample from theapid deactivation phase showed CO2 desorption peaks at 519,41, 557, 571 585 and 591 K, while the catalyst sample from thelower deactivation phase displayed desorption peaks at 531, 549,70, 586, 591, 602 and 618 K. In general the TPO profile for theatalyst sample from the second, slower, deactivation period hasarger peaks. Furthermore, more peaks at higher temperatures arebserved for this sample than for that characterising the first, moreapid, deactivation phase.

.2. Probing the catalytic mechanism

The use of D2 in place of H2 is a well established technique

n probing hydrogenation reaction mechanisms [26]. Therefore, inddition to the hydrogenation of pentenenitriles, the reaction ofeuterium with C2PN over Ni/Al2O3 has also been investigated.hese studies were conducted in the atmospheric flow-througheactor described earlier under identical conditions as per hydro-

Fig. 6. Mass spectrometry data from TPO experiments performed on Ni/Al2O3 afteruse for C2PN hydrogenation: (a) sample from initial, rapid, deactivation period; (b)sample from second, slower, deactivation period.

genation reactions, bar the substitution of D2 for H2. ReactantC2PN was not deuterated. The species evolved from the reactorin these experiments comprised unreacted cis-2-pentenenitrile,n-pentylamine, PN, T3PN and trans-2-pentenenitrile in the sameproportions as for the hydrogenation experiments. The unreactedC2PN evolved was not deuterated in any position: that is, noH/D exchange took place. In contrast all reaction products, ofboth deuteration and isomerisation processes, were observed tobe deuterated in all positions. These results indicate all hydrogenatoms undergo H/D exchange, thereby implying that all five-carbonatoms must interact with the catalyst surface during the hydro-genation reaction thereby providing direct insight in the catalyticreaction mechanism, as discussed in Section 4.1.

4. Discussion

Experimental studies have been carried in order to probe thenumber and nature of the catalytically active sites, including theinfluence of carbon deposition upon those sites, and to elucidatethe reaction mechanism. In order to understand the requirementsfor an active site it is first necessary to determine the reactions thatoccur on them. To this end the elucidated reaction mechanism isreported in Section 4.1, while the active sites present are describedin Section 4.2. Finally, the role of carbonaceous material depositedon the catalyst surface during reaction is discussed in Section 4.3.

4.1. Hydrogenation reaction mechanism

Deuteration studies reported herein have provided evidence insupport of a reaction mechanism for the hydrogenation of C2PNover Ni/Al2O3. The reaction of C2PN with D2 over Ni/Al2O3 yieldedreaction products deuterated in all positions, with unreacted C2PNbeing fully protiated. The observed production of fully deuteratedproducts would not be possible if the molecule reacted in a flat,linear geometry as the C–H bonds in the methyl group wouldnot interact with the surface and hence no deuteration of thesewould occur. This therefore implies that the reaction proceeds via

a cyclic intermediate such as that shown in Fig. 7. This mecha-nism is consistent with previous hypotheses based on the reactionof H2 (as opposed to D2) with C2PN over Ni/Al2O3 [7] and of H2with butenenitrile over Raney-Ni [14,27]. In the proposed mecha-nism illustrated in Fig. 7, pentenenitrile co-ordinates to the surface

J. McGregor et al. / Applied Catalysis A

Foh

ttnHTcttwswsistmm4

baowPavrthgCihcci

4

c

ig. 7. Proposed interaction of C2PN with the catalyst surface leading to formationf a �-allyl species which allows hydrogen exchange and is an intermediate in theydrogenation process.

hrough the nitrile functionality, as is favoured kinetically. In ordero selectively hydrogenate the carbon–carbon double bond, it isecessary for the reaction to proceed via a cyclic intermediate.ydrogenation then takes place by a concerted hydrogen transfer.hat the alkene moiety does not adsorb directly to an active nickelentre is supported by the observation that no skeletal isomerisa-ion of the carbon backbone is observed. Following rearrangemento a cyclic geometry the adsorbed nitrile forms a �-allyl specieshich allows for the interaction of carbons 1–4 with the catalyst

urface. This in turn is in equilibrium with a second �-allyl specieshich permits interaction of the final carbon atom, C5. The final

tep is necessary in order to produce reaction products deuteratedn all positions, as is observed experimentally. Catalytically activeites are also present on the �-Al2O3 (Section 4.2) and contributeo the activity the Ni/Al2O3 catalyst. The products from these sites

ust also be fully deuterated. A potential structure of the active alu-ina site and a hydrogenation mechanism are discussed in Section

.2.Hydrogenation of non-conjugated alkenenitriles, namely 3-

utenenitrile and T3PN has also been proposed to proceed viacyclic intermediate [7,14]. This is consistent with the results

btained from the atmospheric flow-through reactor in the presentork, however the lack of the selective hydrogenation product,

N, in TEOM experiments is surprising. This however could bessigned to a combination of the lower reactivity of T3PN andariations in experimental conditions such as the higher gas flowate-to-catalyst mass ratio employed in TEOM studies. As a resulthe residence time may be too low for the less reactive T3PN toydrogenate on the low energy sites responsible for C C hydro-enation, as discussed in Section 4.2. Lower activity of T3PN cf.2PN has also been observed over ruthenium-complex catalysts

n water [28]. One reason for the differing activity towards C Cydrogenation between the two isomers may be the influence ofonjugation. Conjugation to nitrile groups, such as in the case ofis-2-pentenenitrile is known to activate olefinic groups resultingn increased hydrogenation rates [15,27,28].

.2. Nature and number of catalytically active sites

That a number of distinct active sites, of differing strength andonsequently differing activity, are present on Ni/Al2O3 has been

: General 384 (2010) 192–200 197

proposed previously [7,18]. On the basis of selective poisoning ofmetal sites with carbon monoxide and acid sites with ammonia, athree-site model was suggested to describe the surface sites activein the hydrogenation of C2PN. Those sites comprised: a nickel sitecapable of forming PN; a lower energy isomerisation site on the�-Al2O3 support and finally, a high energy support site which catal-yses the total hydrogenation reaction, producing PA. The presenceof a support site with such high hydrogenation activity is surprising.However, this same �-Al2O3 support has previously demonstratedactivity for the hydrogenation of trans-2-pentene to n-pentane [29].In that work 13C-DEPT NMR spectroscopy provided a direct, in situprobe of the activity of �-Al2O3. While this three-site hypothesisprovided an adequate description of results previously described,it is unable to account for the observations reported in the presentstudy. Specifically, TPO studies identify either six or seven dis-tinct carbon species depending on the stage at which the reactionis stopped. While there evidently need not be a 1:1 relationshipbetween TPO peaks and active sites this does perhaps hint at theneed to expand upon the previous three-site model. Additionally,the previous three-site model is unable to explain either the for-mation of PA over Ni/SiO2 or the poisoning of activity towardsPN production through pre-treatment with T3PN. As such, a five-site model is proposed with regard to hydrogenation of C2PN overNi/Al2O3.

The five sites determined are: (i) moderate energy Ni site for PNformation; (ii) moderate energy Al2O3 site for C2PN isomerisation;iii) high energy Al2O3 site for PA formation; (iv) high energy Ni sitefor PA formation; and (v) high activity Ni site for PN formation. Thefive sites have been determined as follows:

© Site (i) is identified through the selective poisoning of metal sitesby CO which has previously been shown [7] to prevent formationof PN formation thereby confirming that this reaction takes placeon metal sites. Through reaction studies these sites are shownto deactivate slowly thereby identifying site (i) as a moderateenergy site.

© Site (ii) is identified through NH3 adsorption studies whichresult in the poisoning of acid sites on the support surfaceand the cessation of the isomerisation reaction [7]. This iso-merisation site is thereby confirmed as an acid site. Elsewhere,studies of the liquid-phase hydrogenation of pentenenitrilesover Raney-Ni did not observe the formation of isomerisationproducts, lending weight to the concept that this reaction takesplace only over support sites [15].

© Site (iii) is confirmed through the observation of the formation ofPA over the pure alumina support, in the absence of nickel. Nei-ther adsorption of CO nor NH3 disrupts the activity of this site,hence it can be concluded that it is neither metallic nor acidicin nature. A tentative reaction mechanism and correspondingreaction site are shown in Fig. 8. This mechanism once againinvokes the cyclic intermediates discussed in Section 4.1; inthis case adsorption takes place at a bridging hydroxyl groupon the support surface. Further work is required to determinewhether this mechanism is correct and the nature of the inter-mediate steps. Any mechanism must also take into account thatall the deuteration products over Ni/Al2O3 are deuterated in allpositions.

© Hydrogenation experiments over Ni/SiO2 and SiO2 have eluci-dated the presence of site (iv). While �-Al2O3 clearly catalysesthe total hydrogenation of pentenenitriles, as reported in Sec-tion 3.1, SiO2 does not. Despite this, PA is produced in the early

stages of C2PN hydrogenation over Ni/SiO2, before rapid deacti-vation. This indicates that Ni is also capable of catalysing the totalhydrogenation reaction, identifying site (iv). Similarly, Kukulaand Koprivova [15] have previously observed the formation ofPA over Raney-Ni, starting from C2PN. That this is a high energy

198 J. McGregor et al. / Applied Catalysis A: General 384 (2010) 192–200

of C2

©

te

4

Nwtmcrgtat(tCssiTitstouc[

l

Fig. 8. Proposed structure of active site for the hydrogenation

site is confirmed by its rapid deactivation: if deactivation of thissite did not occur it is likely that PA would be formed fromthe hydrogenation of PN, as has been reported previously forthe liquid-phase reaction of PN employing 1-propanol as theH-transfer agent [30].The final active site, site (v), is a high activity site capable of thehydrogenation of cis-2-pentenenitrile to PN. It is these higheractivity sites which are deactivated through exposure of the cat-alyst to T3PN in hydrogen prior to reaction of cis-2-pentnenitrile(Section 3.2). The lower energy active sites, (i), remain unaf-fected.

A more detailed description of the kinetics of deactivation ofhese sites, and hence their assignment as “high” or “moderate”nergy is provided in Section 4.3.

.3. Deactivation and influence of carbon laydown

In light of the identification of five distinct active sites oni/Al2O3 (Section 4.2) it is important to consider that each of theseill likely show differing deactivation kinetics based on the adsorp-

ion and reaction processes occurring upon on them. That adsorbedaterial is retained on the catalyst surface in the form of carbona-

eous material is confirmed through TPO studies of Ni/Al2O3 aftereaction. The extent of this carbon laydown during C2PN hydro-enation has been quantified through TEOM analysis indicatinghat 51.1 �g/mgcat of material is retained on the catalyst surfacefter seven 1 h reaction periods (Fig. 1a). Additionally, the rela-ively high level of mass laydown observed on the pure support13.6 �g/mgcat at the end of the reaction) indicates that much ofhe retained carbon on Ni/Al2O3 may be present on alumina sites.arbon laydown is in turn associated with a change in productelectivity and deactivation of the catalyst. That different surfaceites exhibit different behaviour with respect to carbon laydowns evidenced by the apparent 2-component behaviour observed inEOM data (Fig. 9a), where an initial, rapid, period of mass increases followed by a second, slower, period beginning after ∼2 h (inotal) of reaction. The extent of C2PN loss to the catalyst in the atmo-pheric flow reactor is in agreement with the TEOM results: initiallyhere is significant laydown with the extent decreasing with time-n-stream. The deactivation profile is here examined in more detailsing a deactivation model derived for the analysis of data for a

onstant flow/mixed/batch reactor with independent deactivation31]:

n[(

CA0

CA

)− 1

]= ln(ktw) − kdt,

PN to PA over Al2O3. The reaction mechanism is also shown.

where CA0 represents the initial reactant concentration, CA is theconcentration of the reactant with time, k is the reaction rate,tw represents weight time (gcat

−1 hlfluid), kd is the rate of deacti-vation and t is time. From a plot of ln[(CA0/CA) − 1] vs time therate of deactivation can be evaluated. Fig. 9b shows the profilefor the deactivation relationship. From this model it is clear thatthe point where there is a dramatic change in the deactivationrate coincides with the formation of trans-2-pentenenitrile andT3PN isomers. This two-stage deactivation model is in excellentagreement with the TEOM mass change analysis. The initial rapid(0.0516 min−1) deactivation stage corresponds to an initial rapidbuild-up of carbonaceous material. During this time the catalystdeactivates towards PA formation providing a strong indicationthat the sites responsible for this reaction (sites (iii) and (iv) dis-cussed in Section 4.2) are deactivated directly through carbonlaydown. The second, slower (0.0005 min−1), deactivation phasecorresponds to the slower period of carbon accumulation on thesurface and is associated with the deactivation of the catalysttoward PN production, described in Section 4.3. Again, this deac-tivation is at least in part a result of carbon laydown. This secondperiod corresponds with the observation of isomerisation products.

In addition to quantifying the rate of mass laydown and its cor-relation with the deactivation of groups of active sites, there isevidence as to the nature of the carbonaceous material deposited.Ammonia has been clearly demonstrated to poison acid sites on thesupport thereby inhibiting the isomerisation reaction. It is likelythat PA, produced in the early stages of reaction, behaves in a sim-ilar way. The surface affinity of the amine will be comparable tothat of ammonia: this strong surface affinity arises from the inter-action of the lone pair present in the sp3 hybridised orbital withthe catalyst [32]. This is consistent with the observation that iso-merisation products are not formed simultaneously with PA. Theswitch in selectivity of the alumina support from PA production toisomerisation can therefore be explained in terms of the desorp-tion of PA retained on active acid sites, making them available forreaction. A similar phenomenon has previously been observed inTEOM studies where a decrease in the mass of a retained hydro-carbonaceous overlayer was correlated with product desorption,as observed by GC [18]. Further to the poisoning role likely playedby PA, ammonia is also produced directly during reaction. Studieson the pure �-Al2O3 support material demonstrated the formationof ammonia and cis-2-pentene from C2PN, indicating that the sup-port is active for this reaction in addition to the total hydrogenation

of C2PN. While cis-2-pentene is not observed in the reaction prod-ucts over Ni/Al2O3 it is likely that Al2O3 continues to catalyse thisreaction, forming ammonia. The hydrocarbon fragment, which des-orbs as the alkene in the absence of Ni, may instead be retained asa tightly bound hydrocarbon fragment on the Ni surface, further

J. McGregor et al. / Applied Catalysis A: General 384 (2010) 192–200 199

Fig. 9. (a) TEOM data showing mass laydown (blue) and species detected by GC during TEOM studies of the hydrogenation of C2PN over Ni/Al2O3 at 373 K, 1.1 bar, 50 cm3 min−1

fl ) anh tudiess he reft

csagatcppu

acTsotot4TrsptcC

Fabc

ow rate and H2:pentenenitrile ratio 12.5:1. Species detected are PA ( ), C2PN (ydrogenation over Ni/Al2O3 as derived from atmospheric flow-through reactor stages mirror the two mass laydown stages observed in (a). (For interpretation of the article.)

ontributing to catalyst deactivation. This mechanism is indicatedchematically in Fig. 10. A closely related category of compounds,lkylamines, are well known to behave in a similar manner, under-oing hydrogenolysis at the C–N bond in the presence of hydrogennd an appropriate catalyst [32–35]. As with C2PN hydrogenation,hese reactions produce ammonia and either a gas-phase hydro-arbon species, or a strongly adsorbed surface residue which canoison catalytically active sites [32,33,36,37]. Nickel is known toromote such reactions through the weakening of the C–N bondpon adsorption [38,39].

Studies of the sequential reaction of T3PN followed by C2PNlso reveal a greater understanding of the nature of the carbona-eous residues implicated in catalyst deactivation. Data from bothEOM and atmospheric flow-through reactor studies over Ni/Al2O3how that the hydrocarbonaceous overlayer deposited as a resultf T3PN hydrogenation prevents, with a degree of selectivity, theransformation of C2PN to PN. Correspondingly, no PN formation isbserved during T3PN hydrogenation. Specifically it can be deducedhat T3PN deactivates both the high energy Ni site (site (v), Section.2) and the Al2O3-based isomerisation site (site (ii), Section 4.2).his latter point is also evidenced from studies of the sequentialeaction over the pure support where, in contrast to the untreated

upport, no isomerisation products were observed. As such, therinciple reaction over both the pure support and Ni/Al2O3 is theotal hydrogenation of C2PN to PA. The greater mass of retainedarbon formed during T3PN hydrogenation compared to during2PN hydrogenation is reflected in the differing activities of the

ig. 10. Possible routes to the formation of ammonia from C2PN. The top route isresult of hydrogenolysis and is the mechanism proposed to occur on Al2O3. Theottom route involves the decomposition of PA and the formation of a retainedarbonaceous deposit and is the mechanism proposed to occur on Ni/Al2O3.

d PN ( ). Adapted from Fig. 6(a) and (b) [18]. (b) Deactivation kinetics of C2PN; 50 cm3 min−1 flow rate and H2:pentenenitrile ratio 3.6:1. The two deactivationerences to colour in this figure legend, the reader is referred to the web version of

isomer: mass laydown is greatest where the reactant (or a speciesderived from it) is not hydrogenated to a less tightly adsorbedspecies. The description of active surface sites provided herein canalso explain previous experimental observations. In particular, dur-ing T3PN hydrogenation the formation of PA has been shown todirectly correlate with mass laydown on the catalyst surface [18].An increase in the adsorbed mass was accompanied by a decreasein PA formation, however the removal of adsorbed species throughflushing with He resulted in an increase in activity towards PA for-mation. As a result of the studies reported in the present work thiscan now be assigned to the removal of hydrocarbonaceous mate-rial from site (v) described above. Concerning the hydrogenationof C2PN over Ni/Al2O3 it is likely that the reactant molecule con-tributes to deactivation, in particular of sites (ii) and (v) in a similarmanner to T3PN.

A key point to note is that the nature of the retained overlayerdiffers depending on the pentenenitrile isomer from which it orig-inates. As such, it necessary to think beyond generic concepts suchas “coke” and instead consider the structural nature and poten-tial catalytic role of any carbonaceous residue. The importanceof this has been shown in studies of alkyne hydrogenation overPd catalysts where retained carbon, in the form of Pd–C surfacephase, forms the catalytically selective sites for alkene formation[9]; in xylene isomerisation where “selectivation” through carbonlaydown is commonly employed [40,41]; and in the dehydrogena-tion of n-butane over VOx/Al2O3 where retained carbon directlyprovides the catalytically active centres [42].

Studies of the hydrogenation of C2PN over Ni/Al2O3, includingstudies on related systems such as Ni/SiO2, have therefore allowedus to develop a model of deactivation in this reaction, caused inlarge part by carbon laydown, in terms of the deactivation of groupsof active sites on the catalyst surface.

5. Conclusions

Understanding the factors which influence selectivity in thehydrogenation of multi-functional molecules is a key driver incatalysis research. In this study we have correlated carbon lay-down with catalyst deactivation and changes in selectivity in thehydrogenation of cis-2-pentenenitrile over Ni/Al2O3. In particular,five distinct active sites have been identified which are subject to

different deactivation processes and rates. Deactivation of thesesites therefore directly controls selectivity in this reaction. Thesites identified are: (i) moderate energy Ni site for PN formation;(ii) moderate energy Al2O3 site for C2PN isomerisation; (iii) highenergy Al2O3 site for PA formation; (iv) high energy Ni site for PA

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ormation; and (v) high activity Ni site for PN formation. Rapid massaydown in the early stages of reaction, quantified by TEOM mea-urements, can be directly implicated in the deactivation of sitesesponsible for PA formation, therefore changing reaction selectiv-ty from PA to PN. The rates of deactivation have been quantifiedhrough kinetic analysis. Of particular note is site (ii): an aluminaite capable of the total hydrogenation of C2PN. The identificationf such a site is surprising as alumina is not generally known ashighly active hydrogenation catalyst. Finally, a reaction mech-

nism, involving a cyclic intermediate has been proposed. Theseesults indicate the key role carbon laydown plays in controllingatalytic activity and selectivity in reactions of multi-functionalolecules, with the nature and the location of the deposited car-

onaceous material directly influencing the formation of specificeaction products.

cknowledgements

This work was supported by the ATHENA project (EPSRCR/R47523/01) funded by EPSRC and Johnson Matthey.

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