Catalyst support effects: gas-phase hydrogenation of phenol over palladium

ed with thecase butsicroscopy.

ohexanoneselectivity.raction(s).s exhibited

electronic

Journal of Colloid and Interface Science 266 (2003) 183–194www.elsevier.com/locate/jcis

Catalyst support effects: gas-phase hydrogenationof phenol over palladium

Colin Park1 and Mark A. Keane∗

Department of Chemical and Materials Engineering, University of Kentucky, Lexington, KY 40506-0046, USA

Received 12 November 2002; accepted 6 February 2003

Abstract

The catalytic action of 10% w/w Pd supported on two forms of graphitic carbon nanofibers (GCN) has been assessed and comparperformance of 10% w/w Pd on SiO2, Ta2O5, activated carbon (AC), and graphite. Palladium nitrate served as metal precursor in eachthe role of the starting metal salt was also considered by examining the action of palladium acetate impregnated SiO2. The activated catalysthave been characterized by hydrogen chemisorption, high-resolution transmission electron microscopy, and scanning electron mPhenol hydrogenation served as the test reaction, which proceeds in a stepwise fashion involving the partially hydrogenated cyclas a reactive intermediate. The occurrence and ramifications of Pd/support interaction(s) are related to hydrogenation activity andThe effects of contact time and reaction temperature (398–448 K) are reported and discussed in terms of phenol/catalyst inteHydrogenation kinetics have been adequately represented by a standard pseudo-first-order approximation. The specific activitiethe following sequence of increasing values: Pd/AC< Pd/GCN< Pd/SiO2 ∼ Pd/graphite< Pd/Ta2O5. A diversity of product compositionresponses to variations in reaction conditions points to the involvement of Pd particle size distribution, Pd particle geometry, andcharacter in determining overall catalytic behavior. 2003 Elsevier Inc. All rights reserved.

Keywords: Graphitic carbon nanofibers; Phenol hydrogenation; Palladium; Catalyst support; Selective hydrogenation

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1. Introduction

An overview of the recent literature (from 1996 oward) on selective catalytic hydrogenation reveals, if noavalanche, certainly a proliferation of papers that descthe selective hydrogenation of phenol to cyclohexan[1–16]. The reason for this upsurge in interest lies principin the fact that cyclohexanone is of commercial significaas a key raw material in the production of both caproltam for nylon 6 and adipic acid for nylon 66 [17]. Phenolon the other hand, an established environmental toxinarising from a variety of industrial sources associated wpetrochemicals and polymer manufacture [19]. A catalhydrogen treatment of such waste to generate reusablematerial is certainly preferable to the standard destrucroute by incineration [20]. A move from incineration to h

* Corresponding author.E-mail address: [email protected] (M.A. Keane).

1 Current address: Synetix, P.O. Box 1, Belasis Avenue, BillinghCleveland, TS23 1LB, UK.

0021-9797/$ – see front matter 2003 Elsevier Inc. All rights reserved.doi:10.1016/S0021-9797(03)00171-1

drogen treatment represents immediate savings in termfuel and/or chemical recovery. Phenol as a pollutant tycally arises in high concentrations in aqueous media [21and, for this reason, we have examined the catalytic trformation of an aqueous phenol feedstock.

The hydrogenation of phenol in gas [2,3,6,8,13,23–and liquid [30–32] phases has been reported for a raof palladium- [2–4,6,8,10,24,25,27,30–32], platinum- [226,29] and nickel- [5,7,10,33] based catalysts. Reactionlectivity, in terms of cyclohexanone production, is strondependent on reaction conditions [4,5,8,25,27] and, undnarrow range of conditions, selectivities in excess of 9have been quoted in the literature [3,9,27,31]. Both actiand selectivity are sensitive to changes in catalyst strucproperties where metal loading [2,4,9,23,24,26,28], suracidity/basicity [5,9,24,25], and the inclusion of promot(alkali metals, alkaline earth metals, and lanthanides)10,24,32] have all been found to impact on catalyst pformance. In order to optimize partial hydrogenation aachieve high cyclohexanone selectivity, it has been typicnecessary to sacrifice phenol consumption rates as highe

http://www.elsevier.com/locate/jcis

184 C. Park, M.A. Keane / Journal of Colloid and Interface Science 266 (2003) 183–194

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tivities favor complete hydrogenation to cyclohexanol. Tattainment of a high cyclohexanone selectivity (>95%) at el-evated conversions (>80%) remains a challenging catalysproblem.

The use a support to disperse a catalytically acmetal is not only of economic benefit (lower preparatcosts/longer productive lifetime) but the substrate caninfluence catalyst performance through electronic intetions [34–36]. Indeed, the occurrence of “strong metal/sport interactions” (SMSI) and associated catalytic implitions has been the subject of three widely cited revi[37–39]. The role of the catalyst support in phenol hydgenation has not yet been the subject of a comprehestudy. In this paper, we focus on supported Pd as thealytic agent and consider the use of catalytically genergraphitic carbon nanofibers (GCN) as novel Pd supportterial. The use of structured carbon (nanotubes/nanofibto modify the catalytic behavior of supported metal is nbeginning to attract the interest of the catalysis resecommunity [40,41]. In this study, we have also examinthe action of SiO2, Ta2O5, activated carbon, and graphas support materials. These supports present a range oface areas and energetics (refractory insulating oxideselectrically conducting graphitic carbon) that can facilitan assessment of the influence of Pd/support interactioon phenol hydrogenation activity/selectivity. In addition,have considered the possible effect(s) of varying the naof the catalyst precursor by examining the action of Pd/S2prepared from palladium nitrate and palladium acetate.

2. Experimental

2.1. Catalyst preparation, activation, and characterization

The SiO2 (fumed) and Ta2O5 substrates were supplieby Sigma-Aldrich and used as received. The activatedbon (G-60, 100 mesh) was obtained from NORIT (Uand the graphite (synthetic 1–2 µm powder) from SigmAldrich. Two novel graphitic carbon nanofiber suppowere synthesized by the catalytic decomposition of etene, a synthetic route that we have described in some delsewhere [42–44]. The catalytic GCN growth was pmoted using unsupported Ni and Ni/Cu catalysts, prepby standard precipitation/deposition where the precipiwas thoroughly washed with deionized water, oven drie383 K overnight, calcined in air at 673 K for 4 h, and threduced at 723 K in 20% v/v H2/He for 20 h. The fibersgenerated, using the unsupported Ni catalyst, from a 1/1C2H4/H2 reactant feed at 823 K and a gas hourly spacelocity (GHSV) of 11,300 h−1 are denoted here as GCN-The second set of fibers (GCN-2) were grown frombimetallic Ni/Cu at 848 K employing a 4/1 v/v C2H4/H2 feedat the same GHSV. The catalytically generated GNF puct was contacted with dilute mineral acid (1 M HNO3) for7 days to remove the parent catalyst particles. The latter

)

r-

)

il

Fig. 1. SEM image showing structural features associated with GCN

necessary to avoid any contribution to phenol hydrogetion from residual Ni and/or Cu; the commercial activacarbon and graphite samples were also subjected to aineralization. The carbonaceous supports were thorouwashed with deionized water (until pH approached 7)oven dried at 383 K for 12 h, and the GCN samples werejected to a partial oxidation (in a 5% v/v O2/He mixture) at673 K for 2 h to remove the amorphous carbon content.carbon nanofibers were characterized by scanning elemicroscopy (SEM), employing a Hitachi S900 field emsion SEM, operated at an accelerating voltage of 25the sample was deposited on a standard aluminum Sholder and coated with gold. The fibrous nature of the cargrowth (with aspect ratios of up to 106) is immediately evi-dent from the representative SEM image presented in Fi

The Pd loaded (10± 0.2% w/w) samples were prepared by standard incipient wetness impregnation whe2-butanolic Pd(NO3)2 (or additionally Pd(C2H3O2)2 in thecase of SiO2) solution was added dropwise at 353 Kthe substrate with constant agitation (500 rpm). Aquesolutions were not employed in this study as carbon sport materials are known to possess hydrophobic propeleading to difficulties with surface wetting that may aversely affect the ultimate Pd dispersion. The Pd conwas determined (to within±2%) by atomic absorption spetrophotometry (VarianSpectra AA-10); samples for analywere digested in HF (37% concn.) overnight at ambient tperature. The catalyst precursors were subsequently

C. Park, M.A. Keane / Journal of Colloid and Interface Science 266 (2003) 183–194 185

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overnight at 383 K and activated by heating (10 K min−1,controlled using a Eurotherm 91e temperature programmin 60 cm3 min−1 (monitored using a Humonics Model 52digital flow meter) dry H2 (99.999% v/v) to 523 K, andthis temperature was maintained for 2 h. Nitrogen Bsurface area measurements were carried out at 77 Kcromeritics TriStar) on freshly reduced samples. Samfor off-line TEM analysis were cooled (in He) and pasivated in a 2% v/v O2/He mixture at room temperaturThe latter step provided a protective oxide layer overPd surface that prevented bulk oxidation upon exposurthe atmosphere. High-resolution transmission electroncroscopy (HRTEM) analysis was carried out using a PhiCM200 FEGTEM microscope equipped with a UTW enedispersive X-ray (EDX) detector (Oxford Instruments) aoperated at an accelerating voltage of 200 kV. The specimwere prepared by ultrasonic dispersion in 2-butanol, evarating a drop of the resultant suspension onto a holey casupport grid. Selected area electron diffraction (SAED) cfirmed that the Pd distributed over each support was prein the metallic form and not as an oxide. The Pd partsize distribution profiles (and mean Pd sizes) presentethis study are based on a measurement of over 500 indual particles. The specific Pd surface area (SPd, m2 g−1

Pd) wascalculated from the relationship [45]

(1)SPd = 6

ρdPd,

wheredPd is the mean particle size andρ is the Pd specificmass (11.97 g cm−3). Palladium dispersion was also mesured by pulse H2 chemisorption, employing the commercQuantachrome ChemBET 3000 unit; dispersion values wreproducible to better than±5%.

2.2. Catalytic reactor system

Each catalyst precursor was activated as above in abed glass reactor: ratio of catalyst particle to reactor diam= 90. The catalytic reactor approximated plug flow behior and has been fully described previously [5,7] but sodetails, pertinent to this study, are given below. A Mo100 (kd Scientific) microprocessor-controlled infusion puwas used to deliver aqueous solutions of phenol at a ficalibrated flow rate, carried through the catalyst bed istream of dry H2. The phenol molar feed rate was varifrom 5.2×10−4 to 9.4×10−3 h−1 in order to test adherencto pseudo-first-order kinetics where H2 was maintained aleast 12 times in excess of stoichiometric quantities; thespace velocity was kept constant at 3620 h−1. The catalystwas supported on a glass frit and a layer of glass beads athe catalyst bed served as a preheating zone to ensurthe reactants were vaporized and reached the reactionperature (398–448 K) before contacting the catalyst;T wasconstant to within±1 K. The reactor was operated, using tcriteria outlined elsewhere [46], with negligible internalexternal diffusion retardation of reaction rate (effectiven

t

et-

factor>0.99). Heat transport effects can also be disregawhen applying the criteria set down by Mears [47]; the teperature differential between the catalyst particles andfluid phase was<1 K. In a series of blank experimentpassage of aqueous solutions of phenol in a stream of hygen through the empty reactor or over each of the supmaterials did not result in any detectable conversion.catalytic data quoted herein represent steady-state valuetained in the absence of any significant long-term catadeactivation; repeated catalytic runs generated resultswere reproducible to within±7%. The reactor effluent wafrozen in a liquid nitrogen trap for subsequent analysis whwas made using an AI Cambridge GC94 chromatogrequipped with a flame ionization detector and employinDB-1 50 m×0.20 mm i.d., 0.33-µm capillary column (J&WScientific), as described previously [3]. Quantitative anasis was based on relative peak area with acetone as sowhere analytical repeatability was better than±0.1% and thedetection limit typically corresponded to a feedstock convsion less than 0.1 mol%. The overall fractional conversiophenol (xphenol) is given by

(2)xphenol= [phenol]in − [phenol]out

[phenol]inwhile reaction selectivity (as a percentage) in terms ofclohexanone formation (SC=O) can be represented by

(3)SC=O = [C=O]out

[phenol]in − [phenol]out× 100

and percentage cyclohexanone yield (YC=O%) is given by

(4)YC=O = [C=O]out

[phenol]in × 100,

where [ ] denotes concentration and in and out refer toreactant/product(s) entering and exiting the reactor. Allreactants were Analar grade and were used without furpurification.

3. Results and discussion

3.1. Catalyst characterization

The Pd-based catalysts considered in this study are lin Table 1, which includes TEM and H2 chemisorption-derived average Pd particle diameters and BET surareas. Representative Pd particle size distributions, bon TEM analysis, are presented in the histograms gin Fig. 2. The activated Pd/SiO2 catalysts derived fromthe nitrate and acetate precursors, denoted Pd/SiO2-A andPd/SiO2-B, respectively, exhibit similar size distributionalbeit the average Pd diameter delivered by the nitratecursor is slightly larger. The low-resolution TEM imagprovided in Fig. 3 serve to illustrate the nature of tmetal dispersion. The Pd phase supported on SiO2 exhibitsa spherical morphology (Fig. 3a), suggestive of relativ


ted
Table 1Nitrogen BET surface areas, surface-weighted average Pd particle sizes based on H2 chemisorption (dH) and TEM (dTEM) measurements and the associaspecific phenol hydrogenation pseudo-first-order rate constants (k): T = 398 K
Catalyst N2 BET surface area dTEM (nm) dH (nm) 104k

(m2 g−1) (mol h−1 (m2 gPd−1)−1)

Pd/SiO2-Aa 148 8.3 9.7 1.7Pd/SiO2-Bb 156 7.8 9.3 1.6Pd/Ta2O5 15 11.6 12.9 2.5Pd/AC 865 16.7 15.9 0.4Pd/graphite 18 16.0 17.3 1.8Pd/GCN-1 102 6.9 8.5 0.5Pd/GCN-2 133 6.4 8.8 1.1

a Pd(NO3)2 precursor.b Pd(C2H3O2)2 precursor.

ated

andture[48].

xist-ationion(to

onbyav-iONi

,arti-

-:Pd

faceing

um-ionsde-

t is,tool

sts.ingfol-

andtrib-sticc-

fewetalallyofiallydedthatnge.try

itedallyeriescon-2].andield

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Fig. 2. Palladium particle size distributions associated with freshly activPd/SiO2-A (a), Pd/AC (b), Pd/GCN-1 (c), and Pd/GCN-2 (d).

weak metal/support interaction. The size distributionshape(s) of the supported Pd particles are inherent feaof the interfacial energies associated with each systemTo date, the reported use of the highly refractive Ta2O5

oxide as a metal support has been limited and the eing catalysis-related research has focused on its applicin selective oxidation as applied to pollution remediat[49,50]. Tantalum oxide is, however, known to possesssome degree) many of the qualities of TiO2 [51,52] withevidence of SMSI [53,54]. The Pd phase supportedthe low surface area Ta2O5 substrate is characterizeda symmetrical size distribution and a surface weightederage size that is greater than that recorded for Pd/S2.In two related studies [55,56], it was shown that thephase supported on TiO2 was in the form of significantlylarger crystallites than that with SiO2. It should, howeverbe noted that there is a decided mismatch in the Pd pcle sizes obtained from hydrogen chemisorption (dH) withthose derived from TEM analysis (dTEM), as revealed inTable 1; dTEM is typically lower thandH. Such discrepancies suggest some deviation from an exclusive 2:1 Hadsorption stoichiometry, as is normally applied. The surstoichiometry is known to be dependent on metal load

s

and catalyst composition [57] and there have been a nber of reported instances [9,58,59] where the dimensof supported metal particles that are measured differ,pending on the analytical technique that is employed. Ihowever, usually the case that each characterizationdelivers the same trends for a family of metal catalyThe latter holds in this study where the order of increasparticle size was the same for both techniques andlows the sequence Pd/GCN-2∼ Pd/GCN-1< Pd/SiO2-B ∼Pd/SiO2-A < Pd/Ta2O5 < Pd/graphite∼ Pd/AC.

The Pd phase supported on the conventional graphiteactivated carbon substrates exhibited a wider size disution when compared with the other supports, diagnoof significant Pd mobility/agglomeration during the redution step. Graphite has a low (BET) surface area withedge positions available for depositing any supported mwith the result that the supported metal phase is typicpresent in the form of larger particles [60]. Reductionthe Pd-impregnated activated carbon (AC), an essentamorphous material with a high (BET) surface area, yiela metal phase of comparable dimensions (Table 1) toassociated with Pd/graphite but with a broader size raThe Pd particles exhibit an indistinct or globular geome(Fig. 3b) that has been shown to be diagnostic of limmetal/support interaction [61]. Carbon fibers are generclassified as graphitic structures, characterized by a sof ordered parallel graphene layers arranged in specificformations with an interlayer distance of ca. 0.34 nm [6Carbon fiber synthesis is possible by arc dischargeplasma decomposition but such methodologies also ypolyhedron carbon nanoparticles (low aspect ratio) anappreciable amorphous carbon component [63,64]. Thethesis of carbon nanofibers via catalytic vapor deposidelivers, on the whole, a more uniform carbon product wa more feasible scale-up. The carbon product can be tamade to desired specifications, in terms of lattice orietion and mechanical/chemical/electrical properties, throa judicious choice of both catalyst and reaction contions [65–67]. Representative TEM images that illustratestructural characteristics of GCN-1 and GCN-2 are shoin Fig. 4 along with simple schematic representations of


(a) (b)

(c)

Fig. 3. Representative low-resolution TEM images of (a) Pd/SiO2-A, (b) Pd/AC, and (c) Pd/GCN-1.

was-

ne65,ne”-2,ngleomlllyllo-(s).

tantitionfu-atn-

nedf theetalof

ar-theys-tact

idealized structures as a visual aid. The GCN-1 productpredominantly of the “ribbon” form (Fig. 4a) with diameters in the range 50–125 nm (mean diameter= 85 nm) andlengths of up to 100 mm. In the ribbon form, the graphelayers are oriented parallel to the growing fiber axis [67]. An alternative lattice arrangement, the “herringboor “fishbone” [65] structure (Fig. 4b) characterizes GCNwhere the parallel carbon platelets are oriented at an ato the fiber axis. The diameter of GCN-2 extended fr50 to 150 nm (mean diameter= 105 nm) with an overallength of up to 200 mm. The structure of the catalyticagenerated carbon fiber is largely governed by the crystagraphic orientation of the exposed catalytic metal face

The commonly accepted mechanism [68] involves reac(C2H4 as the carbon source in this case) decomposon the top surface of a metal particle followed by a difsion of carbon atoms into the metal with precipitationother facets of the particle to yield the fiber, which cotinues to grow until the metal particle becomes poisoor completely encapsulated by carbon. The diameter onanofiber is governed by the dimensions of the seed mparticle while the length depends largely on the durationreaction [42,44]. The degree of crystalline order of the cbon product is controlled by various factors includingwetting properties of the metal with graphite and the crtallographic orientation of the metal faces that are in con


sent

(a)

(b)

Fig. 4. Schematic diagrams illustrating an idealized rendering of (a) “ribbon” (GCN-1) and (b) “herringbone” (GCN-2) configurations with repreativeTEM images.

ianttedthe

res-es iscat-tal)yerthis

henrbonerallsiblorts

icles

laneare

tedevi-PdPal-

rized

tersalsotionial.ivelyur-ndho-

with the carbon deposit, features that are ultimately relupon the choice of catalyst [66,69,70]. It should be nothat the TEM images presented in Fig. 4 correspond toGCN product taken directly from the reactor and the pence of an amorphous carbon layer on the fiber edgan artifact of the cooling stage, upon completion of thealytic step. A careful oxidation treatment (see Experimenwas employed to remove this amorphous carbon overlasample weight losses of ca. 5% were recorded duringmild oxidative step. The GCN fibers exhibited N2 BET sur-face areas that are intermediate in magnitude (Table 1) wcompared with the standard amorphous and graphitic casupports. The “herringbone” GCN-2 possessed an ovhigher surface area due to the greater number of accesedge sites in this more open structure. The GCN suppprovide exposed edge sites for anchoring the Pd part

;

e

as opposed to conventional graphite where the basal pis available. Representative high-resolution TEM imagesgiven in Fig. 5 that illustrate the lattice structure of isolaPd particles supported on GCN-1 (lattice fringes alsodent). Routine TEM analysis (Fig. 3c) revealed that theparticles were largely located along the GCN edge sites.ladium supported on both GCN substrates is characte(Fig. 2) by a narrow size distribution (>80% of the parti-cles <10 nm) and the smallest average particle diameof all the supported Pd catalysts. Pham-Huu et al. [71]noted a narrow (centered around 3–5 nm) size distribufor a 5% w/w Pd loading on comparable GCN materThis enhanced dispersion has been attributed to relatstrong metal/GCN interactions that limit particle growth ding activation [71–73]. The highly crystalline faceted arelatively thin Pd structures shown in Fig. 5 are morp


on

ofsup-resf dif-osedfor-

rvedpheera-cant

ol to

inhe-ed

udyva-3 Keseation

d fortion,7,8,rallelas a

rod-genble

st

id-oth

ody-tial

un-

K.qui-gesac-

ticsantss of

d Pd

tivee togo

antsanddis-,Mac-

The

Fig. 5. High-resolution TEM images of isolated Pd particles supportedPd/GCN-1.

logical features that are consistent with the existencestrong interaction between the metal particles and theport medium [74]. The diversity of surface structural featuassociated with these catalysts suggests the possibility oferent (preponderance of) Pd crystallographic faces expto the reactant that can influence overall catalyst permance.

3.2. Phenol hydrogenation

Cyclohexanone and cyclohexanol were the only obsereaction products in the gas-phase reaction of aqueousnol solutions over each supported Pd catalyst. At temptures in excess of ca. 473 K there was evidence of signifi

-

Fig. 6. Series/parallel reaction scheme for the hydrogenation of phencyclohexanol.

benzene formation (hydrodehydroxylation). Moreover,preliminary studies the transformation of methanolic pnol solutions yielded anisole as a major product, formthrough a catalyzed methylation step. The catalytic stwas accordingly limited to an aqueous phenol feed (withporization prior to reaction) at temperatures less than 47in order to asses the intrinsic hydrogenation activity of thcatalysts. Furthermore, there was no detectable condensreactions to form dicyclohexylether as has been reportePd on acidic supports [25,75]. A sequential hydrogenaof phenol has been noted in a number of instances [4,526] and the reaction can proceed through the series/pascheme shown in Fig. 6 where cyclohexanone servesreactive intermediate and cyclohexanol is the ultimate puct. Plug flow operation, under steady state, where hydrowas maintained far in excess yields the following applicareactor/kinetic expression

(5)ln(1− xphenol)−1 = k

(W

Fphenol

),

where Fphenol is the inlet molar feed rate (mol h−1) andxphenol is the fractional conversion for a given catalyweight (W ); the parameterW/Fphenol(units, g h mol−1) hasthe physical significance of contact time. From a conseration of gas-phase reaction equilibrium constants, b(partial and complete) hydrogenation steps are thermnamically favored [5]. Over the range of inlet phenol parpressures considered in this study, complete conversionder equilibrium conditions is achieved atT � 443 K with acontinual drop thereafter to a residual conversion at 573The catalytic data were far removed from gas-phase elibrium conversions and the response of activity to chanin temperature can be attributed positively to surface retion phenomena. Application of pseudo-first-order kineis a practical approach to extract meaningful rate constfrom conversion data and is employed here as a meanassessing the hydrogenation activity of each supportecatalyst. The linear relationships between ln(1 − xphenol)

−1

andW/Fphenol, generated for reactions over representacatalysts, shown in Fig. 7, are diagnostic of adherencpseudo-first-order kinetics. A least-squares fit, forced tothrough the origin, was used to determine rate constwhich were converted to specific values using Eq. (1)are recorded in Table 1. We have adopted the Pdpersion/size values derived from H2 chemisorption thatalthough differ from the values extracted from the TEanalysis, better represent the nature of the catalyticallytive surface in terms of adsorption/reactant activation.


n of

talrastticu-cificnd

her hasereif-Theba-

d ansizeer,ereTheCNenc. Itsup

ationin-porlainally

ns incts

nm)withMSIhery

hy-igh

ac-red

n-

rtedithif-atedre-um

rtedeamsentities

ions

pera-

yclo-be-y-ould

y

cia-y

t-ilena-

Fig. 7. Pseudo-first-order kinetic relationships for the hydrogenatiophenol over Pd/SiO2-A (Q), Pd/AC ("), and Pd/Ta2O5 (F): T = 423 K.

use of TEM (even at high resolution) to determine meparticle sizes requires judgment where insufficient contin the image can hamper an accurate analysis, parlarly for diameters of 1 nm and less. The resultant spephenol consumption activities exhibit the following treof increasing values: Pd/AC∼ Pd/GCN-1< Pd/GCN-2<

Pd/SiO2-B ∼ Pd/SiO2-A ∼ Pd/Graphite< Pd/Ta2O5. ThePd/SiO2-A and Pd/SiO2-B catalysts delivered essentially tsame specific activity, suggesting that the Pd precursolittle influence on the ultimate metal site efficiency. This, however, a decided support effect with a sixfold dference between the most and least active catalysts.activity sequence cannot be explained merely on thesis of a particle size effect as earlier work has revealeinvariant specific rate where the average Pd particle(on MgO and SiO2) exceeded ca. 2 nm [9,28]. Howevwhere the Pd particle size was lower than ca. 2 nm thwas an appreciable decrease in reactant turnover [8].lower phenol conversions delivered by Pd/AC and Pd/Gmay be attributable (to some degree) to a greater presof smaller Pd particles (0–2.5 nm), as revealed in Fig. 2has been established [9,10,32] that the d-character ofported Pd governs, to a great extent, phenol hydrogenactivity where electron-deficient Pd sites have a lowerherent turnover rate. Electron transfer between the supmedium and the Pd crystallites can be invoked to expthe observed catalytic phenomena where the electricconductive substrates can induce electronic perturbatiothe metal particles. While support-induced electronic effeare considered to be negligible for larger (ca. 10–40metallic particles [55], the Pd dispersions associatedthe catalysts considered in this study are such that Scan contribute directly to the metal site activity. The higactivities associated with Pd/Ta2O5 and Pd/graphite mathen be ascribed to significant SMSI that enhances thedrogenation efficiency of the supported Pd sites. The hsurface area AC exhibits little or no metal/support intertion and it is significant that the Pd/AC sample delive

e

-

t

Table 2Cyclohexanone selectivities (SC=O) at comparable phenol fractional coversions (xphenol) over each catalyst:T = 423 K

Catalyst W/Fphenol(g mol−1 h) xphenol SC=O (%)

Pd/SiO2-Aa 32 0.12 66Pd/SiO2-Bb 32 0.10 74Pd/Ta2O5 16 0.16 92Pd/AC 64 0.13 96Pd/graphite 24 0.07 74Pd/GCN-1 16 0.07 26Pd/GCN-2 24 0.11 56




the lowest specific rate. Bessel et al. have recently repo[74] quite distinct electrocatalytic activities associated w“ribbon” and “herring bone” GCN forms, suggesting dferent electron-donating/withdrawing properties associwith the two GCN systems. From a consideration of oursults, the “ribbon” fiber is the less effective support mediin terms of specific phenol consumption rate.

The product compositions generated from each suppoPd catalyst were essentially independent of time-on-str(up to 6 h) and the values reported in this paper represteady-state conversions. The cyclohexanone selectiv(SC=O) at three representative phenol fractional convers(xphenol= 0.12± 0.05, 0.29± 0.04, and 0.51± 0.06) arecompared for each catalyst at the same reaction temture in Table 2. In every instance an increase inxphenolwasaccompanied by a decrease (to varying degrees) in chexanone selectivity. The antisympathetic relationshiptweenxphenolandSC=O is to be expected in a sequential hdrogenation scheme where complete hydrogenation shbe enhanced at higherxphenol. Similar values ofxphenolwereachieved for each catalyst by varying theW/Fphenol para-meter. The Pd/AC and Pd/Ta2O5 catalysts delivered verhighSC=O values that were largely insensitive toxphenol. Atthe other extreme, Pd/GCN-1 is characterized by apprebly lower SC=O that was likewise relatively unaffected bvariations inxphenol. The two Pd/SiO2 and Pd/graphite caalysts exhibited very similar activity/selectivity trends whPd/GCN-2 was less selective in terms of partial hydroge


Table 3Rate constants for the partial (k1 andk2) and complete (k3) hydrogenation (see Fig. 6) of phenol over each supported Pd catalyst:T = 423 K

Catalyst k1 (mol g−1 h−1) k2 (mol g−1 h−1) k3 (mol g−1 h−1) k1/k3

Pd/SiO2-Aa 2.5× 10−3 3.1× 10−3 1.1× 10−3 2.1Pd/SiO2-Bb 2.5× 10−3 2.8× 10−3 9.7× 10−4 2.5Pd/Ta2O5 5.3× 10−3 1.4× 10−3 6.9× 10−4 7.7Pd/AC 2.1× 10−3 6.0× 10−5 6.1× 10−5 36.5Pd/graphite 2.4× 10−3 2.0× 10−3 6.9× 10−4 3.5Pd/GCN-1 1.1× 10−3 2.3× 10−3 3.3× 10−3 0.3Pd/GCN-2 2.7× 10−3 7.1× 10−3 2.4× 10−3 1.1


se

dytial

ing

nsnts.

ig. 8rod-rmly

e-

tionhighac-

werel hy-andnd-ithxyl

se-

m-n ofosedivi-eus,toreero-n toor-heaterol.ar-r, toenthowuressup-ty.pe-

r-

tion but the associatedSC=O values were greater than thorecorded for Pd/GCN-1.

Applying pseudo-first-order conditions at quasi-steastate for irreversible hydrogenation steps, three differenequations describe the system

(6)Fphenol

(d[phenol]

dW

)= −(k1 + k3)[phenol],

(7)Fphenol

(d[C=O]

dW

)= k1[phenol] − k2[C=O],

(8)Fphenol

(d[C–OH]

dW

)= k3[phenol] + k2[C=O].

Integration of these rate equations provides the followdependence of product mol fraction (α) on space time

(9)αphenol= exp

{−(k1 + k3)

W

Fphenol

},

(10)

αC=O =(

k1

k2 − (k1 + k3)

)(exp

{−(k1 + k3)

W

Fphenol

}

− exp

{−k2

W

Fphenol

}),

(11)αC–OH = 1− αphenol− αC=O.

Taking the experimentally determinedα values at variouscontact times (W/Fphenol), the resultant nonlinear equatiowere solved to yield values for the stepwise rate constaThe applicability of this scheme can be assessed in Fwherein representative experimental and calculated puct compositions are presented: the agreement is unifogood. The rate constants for the partial (k1 and k2) asopposed to complete (k3) hydrogenation of phenol are prsented in Table 3. The value of thek1/k3 ratio signifiesthe relative importance of the two possible hydrogenapathways, stepwise and concerted, respectively. Thevalue of this rate constant ratio recorded for the lesstive Pd/AC is to be expected as are the significantly lovalues associated with both Pd/SiO2 catalysts. The surfacrequirement(s) for partial as opposed to complete phenodrogenation has not been established in the literaturethe reaction mechanism is not fully understood. Depeing on the nature of the catalyst, phenol can interact wthe surface through the aromatic ring and/or the hydro

Fig. 8. Experimental (symbols) and calculated (lines: based on aries/parallel scheme) product molar compositions (α) resulting from thehydrogenation of phenol over Pd/SiO2-A (Q), Pd/Ta2O5 (F), Pd/AC ("),Pd/GCN-1 (a), and Pd/GCN-2 (e): T = 423 K.

substituent [24]; the former is more likely to result in coplete hydrogenation [7]. A one site dissociative adsorptioboth phenol and hydrogen on palladium has been propby Chen et al. [4] to favor high cyclohexanone selectties. In the case of interaction through the aromatic nuclthe ring may be coplanar with [76] or perpendicular[77] the surface. Cyclohexanone production can follow throutes: (a) direct hydrogenation or addition of two hydgen molecules/four hydrogen atoms; (b) hydrogenatiocyclohexanol followed by a dehydrogenation step; (c) fmation of cyclohexene-1-ol followed by a tautomerism. Tstronger the interaction of phenol with the surface the grethe likelihood of complete hydrogenation to cyclohexanIt is not possible, from the combination of catalyst chacterization and catalysis data presented in this papeconclusively identify the predominant surface arrangemthat leads to the observed cyclohexanone selectivity andthis is dependent on reaction conditions. Certain featshould, however, be flagged that are suggestive of aport effect in modifying phenol hydrogenation selectiviWhile Pd/Ta2O5 and Pd/graphite delivered the highest scific phenol consumption rate constants (k1+k3, see Table 1)the associatedk1/k3 ratios indicate a greater relative impo


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ofe re-l.t thar Nistal

he N

is il-iththepernuslylim-

deered

e

. 8,phe-ich

ced.N-1ucedathe

uctThisichorp-

per-rentac-eredes-

this

t wetionrac-lystrt in-

t ispar-iver

usatesticex-

Fig. 9. Relationship between cyclohexanone selectivity (SC=O) and frac-tional phenol conversion (xphenol) over Pd/SiO2-A (Q), Pd/SiO2-B (P),Pd/Ta2O5 (F), Pd/AC ("), Pd/graphite (2), Pd/GCN-1 (a), and Pd/GCN-2 (e) where temperature was varied:W/Fphenol= 128 g mol−1 h.

tance of the partial hydrogenation step when comparedthe less active Pd/SiO2. Moreover, both Pd/GCN catalystwhile less active than Pd/Ta2O5, Pd/graphite, and Pd/SiO2,exhibit a significantly greater tendency to promote comphydrogenation to cyclohexanol (lowerk1/k3), behavior thatis quite distinct from Pd/AC. It has been shown elsewh[78–80] that transition metals supported on structuredbon deliver quite distinct activities and/or selectivities whcompared with conventional amorphous supports. The ufibrous supports also has decided benefits in terms of grmechanical strength and overall greater applicability irange of reactor configurations [81]. Given the diversityobserved activity/selectivity values associated with the raof structural features particular to this family of Pd catalyit seems likely that the Pd particle morphology, expocrystallographic orientations, and electronic characterinfluence the ultimate product composition. The Pd struc(geometric/electronic) is ultimately dependent on the chof support [82], which can, in turn, influence the naturethe chemisorbed intermediate(s) and the intrinsic surfacactivity [83,84]. It is worth noting that Molchanov et a[85] have provided some persuasive evidence to suggesunsaturated hydrocarbon hydrogenation selectivity ovesupported on structured carbon is dependent on the crylographic planes accessible to the reactants as well as telectronic state that they link with surface morphology.

The relationship betweenSC=O and xphenol where thecontact time was kept constant and temperature variedlustrated in Fig. 9: a variety of responses is in evidence. Wthe exception of the less active Pd/AC and Pd/GCN-1,steady-state phenol conversion decreased over the temture interval 398� T � 448 K. A drop in phenol conversiowith increasing temperature has been reported previo[7,24,25,27,28] and, in the absence of thermodynamicitations, has been attributed to a temperature-inducedcrease of the fraction of the catalyst surface that is cov

fr

t

-i

a-

-

Fig. 10. Cyclohexanone yield (YC=O) as a function of temperature in thconversion of phenol over Pd/SiO2-A (Q), Pd/SiO2-B (P), Pd/Ta2O5 (F),Pd/AC ("), Pd/graphite (2), Pd/GCN-1 (a), and Pd/GCN-2 (e):W/Fphenol= 128 g mol−1 h.

by reactant [7,28]. In contrast to the trends shown in Figthe reduction in temperature that resulted in increasednol conversion served to enhance the selectivity with whthe partially hydrogenated cyclohexanone was produThe exceptions to this response are Pd/AC and Pd/GCwhere the dependence was slight. The temperature-indenhancement ofSC=O can be considered to result fromdesorption of reactant and reactive intermediate fromsurface, the net effect of which is to enrich the prodstream with the partially hydrogenated cyclohexanone.effect is particularly marked in the case of Pd/GCN-2 whsuggests the involvement of quite different surface adstion energetics. A plot of cyclohexanone yield (YC=O) vstemperature (Fig. 10) reveals the diversity in catalystformance associated with Pd dispersed on the diffesupports. The observed drop in yield is due to lower frtional phenol conversions. The exception is Pd/AC whconversion was enhanced at higher temperatures andorption phenomena do not appear to play a role overtemperature interval. The highestYC=O is associated withPd/Ta2O5 at the lowest reaction temperature, a result thaascribe to a high phenol conversion due to electron donafrom the substrate combined with a phenol/surface intetion that favors stepwise hydrogenation. The Pd/AC catarepresents the other extreme where weaker metal/suppoteraction appears to result in lower intrinsic activity thaelevated somewhat at higher temperatures without apently impacting on reactant/surface interactions to delthe highestYC=O at 448 K.

4. Conclusions

Impregnation and activation of (10% w/w) Pd on variooxide and carbon- (including novel GCN) based substrhave led to significant variations in the intrinsic catalyphenol hydrogenation activity/selectivity. The Pd phase


Pdfirst-cifictureseing

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99)

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hibited a morphological diversity where the averageparticle sizes spanned the range 6–17 nm. A pseudo-order kinetic analysis has been used to deliver sperate constants that are indicative of a degree of strucsensitivity in that the intrinsic rate was higher for thosystems exhibiting SMSI and increased in the followsequence: Pd/AC∼ Pd/GCN-1< Pd/GCN-2< Pd/SiO2 ∼Pd/graphite< Pd/Ta2O5. In the case of Pd/SiO2, the use ofpalladium acetate or nitrate as the metal precursor hatle effect on catalyst productivity. Variations in selectivcan be ascribed to differences in Pd/support interactiothat impact on the supported Pd microstructure wherethe case of Pd dispersed on AC and Ta2O5, the metal canbe considered to adopt a crystallographic orientation thavors partial hydrogenation to cyclohexanone whereas PGCN promotes complete hydrogenation to the alcohol. Acrease in the effective contact time (at constantT ) loweredfractional conversion with a preferential formation of cychexanone. Where the contact time was maintained conan increase in reaction temperature lowered both conveand cyclohexanone selectivity (with the exception of Pd/and Pd-GCN-1), a result that is ascribed to desorption pnomena.

Acknowledgments

We are grateful to Gonzalo Pina and Alan Dozierassistance with the catalysis and catalyst characterizmeasurements.

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Catalyst support effects: gas-phase hydrogenation of phenol over palladium

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