Applied Catalysis A: General 349 (2008) 100–109

Catalytic and kinetic study of the liquid-phase hydrogenation of acetophenoneover Cu/SiO2 catalyst

Nicolas M. Bertero, Carlos R. Apesteguıa, Alberto J. Marchi *

Catalysis Science and Engineering Research Group (GICIC), Instituto de Investigaciones en Catalisis y Petroquımica (INCAPE), UNL-CONICET, Santiago del Estero 2654,

3000 Santa Fe, Argentina

A R T I C L E I N F O

Article history:

Received 2 June 2008

Received in revised form 10 July 2008

Accepted 15 July 2008

Available online 23 July 2008

Keywords:

Selective hydrogenation

Acetophenone

Copper-based catalyst

Kinetic modeling

A B S T R A C T

The liquid-phase hydrogenation of acetophenone (AP) to 1-phenylethanol (PhE) was studied on Cu(6.8%)/

SiO2 catalyst. Catalytic tests were carried out in a batch reactor by varying temperature, total pressure

and AP initial concentration between 353–373 K, 5–20 bar, and 0.038–0.251 M, respectively, and using

four different solvents: isopropylic alcohol (IPA), cyclohexane, toluene and benzene. The selectivity to

PhE was about 100% irrespective of the solvent used, but the initial AP conversion rate followed the order

IPA > cyclohexane > toluene > benzene. The differences in catalyst activity when changing the solvent

were interpreted by considering the effect of the solvent–metal interaction on the relative coverage of

adsorbed reactant species. Experimental data were well interpreted by kinetic modeling only when

assuming that: (i) the adsorption of AP and H2 is competitive; (ii) AP adsorption is strong; (iii) copper

surface is saturated in AP; (iv) the PhE coverage on the catalyst is negligible. These assumptions were

consistent with the fact that the reaction was negative order with respect to AP and first order in H2. The

highly selective AP hydrogenation to PhE was explained by considering that the strong electrostatic

repulsion between metallic Cu and the phenyl group tilts the AP molecule thereby favoring its adsorption

via the carbonyl group and the formation of the unsaturated alcohol. Also, PhE was not consecutively

converted via hydrogenolysis or other acid catalyzed reactions since the support was inert.

� 2008 Elsevier B.V. All rights reserved.

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1. Introduction

The selective hydrogenation of aromatic ketones is of greatimportance in Fine Chemistry because the resulting phenylalcohols find application as flavors, fragrances and intermediatesfor the synthesis of more valuable organic molecules [1]. However,the selective hydrogenation of the C O group of the ketonemolecule is frequently not easily achieved due to the formation ofunwanted products via side reactions such as the aromatic ringhydrogenation. In addition, the hydrogenolysis of the unsaturatedalcohol to the corresponding aromatic hydrocarbon is otherpossible and unwanted reaction. The selective hydrogenation ofacetophenone (AP) to 1-phenylethanol (PhE) has been investigatedon different metals because PhE is widely employed in pharma-ceutical and fragrance industries [1,2]. For example, in pharma-cology PhE is an intermediate of analgesic and anti-inflammatorydrugs, while in food additives it is widely used in chewing gumsand yogurts because of its characteristic strawberry scent.

* Corresponding author. Fax: +54 342 4531068.

E-mail address: amarchi@fiq.unl.edu.ar (A.J. Marchi).

0926-860X/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2008.07.014

A simplified reaction network for the AP conversion is shown inFig. 1. PhE and cyclohexylmethylketone (CHMK) are primaryproducts formed by hydrogenation of the carbonyl group and thearomatic ring of AP molecule, respectively. Both primary productscan be hydrogenated to 1-cyclohexylethanol (CHE). On the otherhand, the hydrogenolysis of the C–OH bond in PhE producesethylbenzene (EB) that is not a valuable product in this reaction.

The AP hydrogenation has been studied on noble metals such asPt, Ru and Pd [3–9]. Platinum hydrogenates the carbonyl andphenyl groups of the AP molecule at similar rates producingcomparable amounts of PhE and CHMK. Both products are thenconsecutively hydrogenated to CHE [3,4]. Ruthenium exhibits ahigh ability for reducing the aromatic ring of AP and is thereforepoorly selective for synthesizing PhE [5,6]. Palladium promotesinitially the selective hydrogenation of C O bond of AP formingPhE but is also active for consecutively converting PhE to EB byhydrogenolysis [7–9]. The AP hydrogenation has been alsoinvestigated on non-noble metals, especially on Ni-based catalysts[10–16]. These studies agree in that nickel promotes the selectiveAP hydrogenation to PhE, but the concomitant formation ofvariables quantities of CHMK and CHE is also observed. Moreover,nickel is active for producing EB by PhE hydrogenolysis. Veryfew papers have studied the use of Cu-based catalysts for AP

Fig. 1. Reaction network for acetophenone hydrogenation over metal catalysts.

N.M. Bertero et al. / Applied Catalysis A: General 349 (2008) 100–109 101

hydrogenation. Zaccheria et al. [17] reported that a Cu(8%)/Al2O3

catalyst converts aryl ketones, including AP, into the correspondingalcohols with selectivities higher than 90%. In contrast, theyobserved the formation of significant amounts of hydrogenolysisproducts and ethers when using Cu/SiO2, probably because of thecatalyst acidity. Two patents from Sumimoto Chemical Co. claimedfor the use of Cu/SiO2 catalysts to selectively hydrogenate AP toPhE [18,19]. High PhE yields from AP would be obtained whenusing Cu/SiO2 catalysts with Cu loadings higher than 65% [18] ormodified by the addition of alkaline or alkaline earth metals [19].

From the above survey on the liquid-phase AP hydrogenationliterature it seems that metallic copper is the best candidate forselectively obtaining PhE. However, the causes for this superiorPhE selectivity on Cu have not been established yet. Studies on theAP hydrogenation reaction kinetics and mechanism on coppercatalysts are also lacking. On the other hand, it has to be noted thatthe type of solvent used in the AP hydrogenation reaction maysignificantly modify the catalyst activity. For example, Aramendıaet al. [7] studied the AP hydrogenation over Pd-supported catalystsusing different alcohols as solvent and observed that the catalyticactivity decreases when increasing the alcohol dielectric constant.Bejblova et al. [9] observed large differences in AP conversionactivity on Pd when a high-polar solvent (methanol) was replacedby a non-polar one (n-hexane). Similarly, it was reported that theAP hydrogenation activity on Ni Raney was larger in cyclohexane(non-polar) than in methanol, ethanol or n-propanol [13].

In this paper, we present a detailed study of the liquid-phasehydrogenation of acetophenone over a Cu(6.8%)/SiO2 catalyst.Experimental data were obtained by varying the operativeconditions (temperature, H2 pressure, AP concentration) andusing four different solvents (isopropyl alcohol, cyclohexane,benzene and toluene). Results were interpreted by kineticmodeling employing heterogeneous Langmuir–Hinshelwood–Hougen–Watson (LHHW) and non-stationary models. A reactionmechanism based on the interaction between reactants and Cu0 isproposed to explain the selective AP hydrogenation to PhE. Theeffect of solvent on catalyst activity is interpreted by consideringsolvent–reactant and solvent–metal interactions.

2. Experimental

2.1. Catalyst preparation

A Cu/SiO2 catalyst with 6.8 wt.% of copper was prepared byincipient wetness impregnation method. Copper was deposited on

commercial silica (Grace G62, 99.7%, Na: 0.1%, SO42�: 0.1%, others:

0.1%) by adding dropwise a 0.57 M aqueous solution of Cu(NO3)2

(Merck, 98%). The solid was dried in an oven at 373 K for 12 h andthen calcined in air flow at 673 K for 3 h. Prior to catalytic tests, thecatalyst was activated ex situ in H2 flow at 543 K for 2 h and thenquickly transferred to the reactor in an inert atmosphere of N2 inorder to avoid metal reoxidation.

2.2. Characterization of the catalyst

The crystalline structure of the sample was determined by X-ray diffraction (XRD) using a Shimadzu XD-1 diffractometer andNi-filtered Cu Ka radiation in the range of 2u = 10–808 at a scanspeed of 28/min. Crystallites size were calculated from the CuO(1 1 1) diffraction lines using the Debye–Scherrer equation.

BET surface area (Sg), pore volume (VP) and mean pore diameter(dP) were measured by N2 physisorption at 77 K in a QuantochromeCorporation NOVA-1000 sorptometer. Copper content was deter-mined by atomic absorption spectroscopy (AAS).

The reducibility of the calcined sample was determined bytemperature-programmed reduction (TPR) using a MicromeriticsAutoChem II 2920 V2.00 equipment. The TPR profile was obtainedusing a H2(5%)/Ar gaseous mixture at 60 cm3/min STP and a samplesize of about 100 mg. The sample was heated from 298 to 773 K at10 K/min. H2 uptake was measured using a TCD detector. Becausewater was formed during sample reduction, the gas exiting fromthe reactor was passed through a cold trap before entering thethermal conductivity detector.

Sample acidity was determined by temperature-programmeddesorption (TPD) of NH3 preadsorbed at 373 K. Samples (100 mg)were treated in He (60 cm3/min) at 723 K for 2 h and then exposedto a 1% NH3/He stream for 40 min at 373 K. Weakly adsorbed NH3

was removed by flushing with He at 373 K during 2 h. Temperaturewas then increased at a rate of 10 K/min and the NH3 concentrationin the effluent was measured by mass spectrometry in a BaltzersOmnistar unit.

The metal copper dispersion was determined by titration withN2O at 363 K using a stoichiometry of (Cu0)s/N2O = 2, where (Cu0)s

implies a Cu0 atom on surface [20,21]. Pre-reduced samples wereexposed to pulses of N2O(10%)/Ar. The number of chemisorbedoxygen atoms was calculated from the consumption of N2Omeasured by mass spectrometry (MS) in a Baltzers Omnistar unit.

2.3. Catalytic tests

The liquid-phase hydrogenation of AP (Aldrich, 99%) wascarried out in a 600 ml autoclave (Parr 4843) equipped with amechanic stirrer. The temperature, total pressure and AP initialconcentration were varied between 353 and 373 K, 5-20 bar and0.0838–0.2514 M, respectively. The solvents used were isopropylicalcohol (Aldrich, 98%), cyclohexane (Merck, 99%), toluene (Aldrich,99%) and benzene (Merck, 99%). The autoclave was loaded with150 ml of solvent and 1 g of catalyst in a N2 inert atmosphere. Thereaction system was stirred and heated to reaction temperature at2 K/min. Then, 1.5–4.5 ml of AP were injected to the reactor andthe pressure was rapidly increased to 3.7, 8.7 or 18.7 bar withhydrogen. The batch reactor was assumed to be perfectly mixed. Itwas verified that the stirring speed (600 rpm) and the catalystparticle size (<100 mm) used insured the kinetic control of thereaction; i.e., diffusional limitations were negligible.

The concentrations of unreacted AP and reaction products werefollowed during the reaction by ex situ gas chromatography usingan Agilent 6850 chromatograph equipped with flame ionizationdetector (heated at 523 K), temperature programmer, and a 30 mInnowax capillary column with a 0.25 mm coating. Liquid samples

Fig. 2. Characterization of calcined Cu/SiO2 sample: (a) X-ray diffraction pattern; (b) TPR profile.

Fig. 3. Solvent effect on AP conversion over Cu/SiO2 [T = 363 K, pH2¼ 8:7 bar;

WCAT = 1 g, C0AP ¼ 0:168 M; VSOLV = 150 ml].

N.M. Bertero et al. / Applied Catalysis A: General 349 (2008) 100–109102

were withdrawn from the reactor by using a loop under pressure inorder to avoid flushing. Data were collected every 5–15 min at thebeginning of the reaction and then every 30–60 min. In all thecases, the only reaction product detected was PhE. AP conversion(XAP, mol of AP reacted/mol of AP fed) was calculated as XAP ¼ðC0

AP � CAPÞ=C0AP; where C0

AP is the initial concentration of acet-ophenone and CAP is the acetophenone concentration at reactiontime t.

3. Results

3.1. Catalyst characterization

The specific surface area (Sg = 220 m2/g) and pore volume(VP = 0.42 cm3/g) determined for the Cu/SiO2 sample were similarto those corresponding to SiO2 (Sg = 230 m2/g, VP = 0.49 cm3/g),thereby indicating that the addition of copper did not modifysignificantly the textural properties of the support. On the otherhand, the TPD profile of NH3 on Cu/SiO2 (not shown here) did notexhibit any NH3 desorption peak, thereby showing that the sampledoes not contain acid surface sites able to retain ammonia at 373 K.

The XRD pattern of the silica-supported copper sample afterdecomposition in air at 673 K is shown in Fig. 2a. The sampleexhibited a single crystalline phase of tenorite-like CuO (ASTMStandard 5-0661) with a large average crystallite size (19 nm).

The TPR profile of the calcined Cu/SiO2 sample exhibited only abroad peak, with a maximum at 536 K, arising from the reductionof CuO (Fig. 2b). The wide peak observed here for CuO reductionwould indicate a heterogeneous size distribution of the CuOparticles, as it has been suggested in the literatures [22,23]. Themetallic Cu dispersion on SiO2, determined by the dissociativeadsorption of N2O at 363 K, was 1.2% that corresponds to a meanCu0 particle size of 175 nm. This large value of the metallic Cucrystallites is consistent with the large CuO crystallite sizedetermined by XRD and probably reflects a weak interactionbetween the copper crystallites and the support.

3.2. Catalytic results

3.2.1. Solvent selection

The solvent effect on catalyst activity and selectivity for APhydrogenation was studied using a polar protic solvent (isopropylicalcohol), a naphtenic non-polar solvent (cyclohexane) and twoaromatic non-polar solvents (toluene and benzene). Fig. 3 comparesthe evolution of AP conversion as a function of time when using theabove-mentioned solvents. It is inferred that the catalyst activitypattern follows the order: IPA> cyclohexane> toluene> benzene.In contrast, the solvent had no effect on catalyst selectivity. In fact,

the selectivity to PhE was higher than 99% during the entire catalyticruns, irrespective of the solvent used.

The results in Fig. 3 clearly show that the Cu/SiO2 activity for APhydrogenation depends on the solvent used. The modification ofthe catalyst activity when changing the solvent is frequentlyobserved in liquid-phase catalyzed reactions, but it is hardlyexplained in terms of simple reaction parameters. Previous work[24–30] recognizes the complex role of the solvent on solid-catalyzed reactions because the solvent influence may be relatedwith reactant–solvent, product–solvent and/or catalyst–solventinteractions. While it is not the intent of this work to ascertain thefundamentals of the solvent effect on the reaction mechanism, weobserve in Fig. 3 that the Cu/SiO2 activity is hampered when usingaromatic non-polar solvents, such as benzene or toluene. The APconversion rate on Cu/SiO2, in fact, is clearly higher in cyclohexaneor IPA. Because it is expected that cyclohexane had a lowerinteraction either with reactant and product molecules or withcatalyst surface in comparison to isopropylic alcohol, we selectedcyclohexane as the solvent to perform our catalytic tests.

3.2.2. Acetophenone hydrogenation

The effect of the hydrogen pressure on AP hydrogenation overCu/SiO2 was studied in cyclohexane at 363 K and C0

AP ¼ 0:168 M:

The catalytic tests were conducted at total pressures of 5, 10 and20 bar, which implies hydrogen partial pressures of 3.7, 8.7 and

Fig. 4. Effect of (a) H2 partial pressure [T = 363 K, C0AP ¼ 0:168 M]; (b) AP initial concentration [T = 363 K, pH2

¼ 8:7 bar]; (c) temperature ½ pH2¼ 8:7 bar; C0

AP ¼ 0:168 M� on the

catalyst activity. WCAT = 1 g, VSOLV = 150 ml (cyclohexane).

N.M. Bertero et al. / Applied Catalysis A: General 349 (2008) 100–109 103

18.7 bar, respectively, by considering that at 363 K the solventvapor pressure is 1.3 bar (calculated with Antoine equation), andthe vapor pressures of AP and products are negligible. The obtainedXAP vs. time plots are presented in Fig. 4a. It is observed that thecatalytic activity increases with the hydrogen partial pressure. Thelocal slopes of the curves in Fig. 4a give the AP conversion rate at aspecific value of AP conversion and reaction time. The initial APconversion rates ðr0

AP; mol AP h�1 g�1Þ were calculated by poly-nomial differentiation of the curves at zero time. The reaction orderwith respect to hydrogen was calculated by considering for r0

AP apower-law rate equation:

r0AP ¼ kðPH2

ÞaðC0APÞ

b(1)

Reaction order a, determined by both linear and non-linearregression from Eq. (1) with C0

AP constant, was close to one.The effect of the AP initial concentration on catalytic activity

was studied at 363 K and PH2¼ 8:7 bar: The initial concentration of

AP was varied between 0.084 and 0.251 M. The results are given inFig. 4b and show that the catalyst activity decreases by increasingthe initial AP concentration. The reaction order with respect to APwas determined by calculating r0

AP from data represented in Fig. 4band applying both linear and non-linear regression with Eq. (1).The value calculated for b was negative and about �0.33.

The influence of temperature on catalytic activity wasinvestigated between 353 and 373 K, at PH2

¼ 8:7 bar and C0AP ¼

0:168 M: Fig. 4c shows the increase of catalytic activity withtemperature. The apparent activation energy (EA) was determinedby numerical regression using an Arrhenius-type function. A valueof EA = 54.9 KJ mol�1 was obtained.

Data in Fig. 4 show effects that changes in H2 partial pressure,initial concentration of AP and temperature have on catalystactivity. Regarding the catalyst selectivity, in all the catalytic runsof Fig. 4 the selectivity to PhE was higher than 99%, which showsthat the Cu/SiO2 catalyst is highly selective for hydrogenating AP toPhE in this range of experimental conditions.

In summary, our catalytic results show that the reaction orderwith respect to AP is negative for AP hydrogenation on Cu/SiO2,thereby indicating that the interaction of AP with surface-activesites is very strong. Moreover, taken into account that SiO2 is a non-reactive support and that the interaction between the large Cu0

crystallites and SiO2 is weak, it would be expected that APhydrogenation over Cu/SiO2 takes place essentially via a mono-functional mechanism on metallic copper. On these bases, wedeveloped several kinetic models in order to interpret and explainthe patterns of selectivity and activity experimentally determinedon Cu/SiO2 for AP hydrogenation.

3.3. Kinetic modeling

3.3.1. LHHW models

Based on the previously discussed results, we considered herethe following hypothesis for the formulation of LHHW hetero-geneous models:

(1) T

he adsorption of AP and H2 may be either competitive (modelswith only one active site) or non-competitive (models with twodifferent metal catalytic sites).

(2) T

he H2 adsorption is dissociative [31–33]. (3) P hE adsorption/desorption steps are reversible quasi-equili-

brium steps.

(4) T he hydrogenation surface reaction is irreversible (total

conversion of AP to PhE was obtained in the experimentalruns).

(5) H

ydrogen concentration in the liquid phase is constant,because of the constant hydrogen partial pressure during theentire experiment, the high solvent volume, and efficientmixing.

Considering the former hypothesis with active sites S1 and S2,and non-competitive H2 chemisorption (hydrogen adsorbed onmetallic site S1), the elementary steps shown in Eqs. (2)–(5)represent the general reaction mechanism:

H2 þ 2S1,2HS1; r1 ¼ kH2pH2ðCS1Þ2 � k�1

H2ðCHS1

Þ2 (2)

APþ S2,APS2; r2 ¼ kAPCAPCS2� k�1

AP CAPS2(3)

2HS1 þ APS2)PhES2 þ 2S1; r3 ¼ kSðCHS1Þ2CAPS2

(4)

PhES2,PhEþ S2;1

KPhE¼ CPhECS2

CPhES2

(5)

If the adsorptions of H2 and AP are competitive, then S1 = S2 = S.The general system of differential equations to be solved, based

on Eqs. (2)–(5), is:

dC�AP

dt¼ 1

C0AP

� dCAP

dt¼ � 1

C0AP

� r (6)

dC�PhE

dt¼ 1

C0AP

� dCPhE

dt¼ 1

C0AP

� r (7)

where r is the rate of the limiting step in the reaction mechanismand C�i ¼ Ci=C0

AP the relative concentration of the i component.By assuming different rate-limiting steps (r.l.s.), (adsorption

of H2, adsorption of AP or surface chemical reaction) and two

Table 1LHHW kinetic models used in this work

Model Rate Simplified rate

Model 1

r.l.s.: adsorption of H2, non-competitive H2–AP adsorption r ¼ ðC2S1

kH2ÞpH2

r = P1

Model 2

r.l.s.: adsorption of AP, non-competitive H2–AP adsorption r ¼ðCS2

kAPÞCAP

1þKPhECPhEr ¼ P2CAP

1þP3CPhE

Model 3

r.l.s.: surface chemical reaction, non-competitive H2–AP adsorption r ¼ðC2

S1CS2

kSKH2KAP ÞpH2

CAP

ð1þKAP CAPþKPhECPhEÞð1þffiffiffiffiffiffiffiffiffiffiffiffiffiffiKH2

pH2

pÞ2

r ¼ P4CAP1þP5CAPþP3CPhE

Model 4

r.l.s.: adsorption of H2, competitive H2–AP adsorption r ¼ðCSkH2

ÞpH2

ð1þKAP CAPþKPhECPhEÞ2 r ¼ P6

ð1þP5CAPþP3CPhEÞ2

Model 5

r.l.s.: adsorption of AP, competitive H2–AP adsorption r ¼ ðCSkAPÞCAP

1þffiffiffiffiffiffiffiffiffiffiffiffiffiffiKH2

pH2

pþKPhECPhE

r ¼ P7CAP1þP8þP3CPhE

Model 6

r.l.s.: surface chemical reaction, competitive H2–AP adsorption r ¼ðCSkSKH2

KAPÞpH2CAP

ð1þffiffiffiffiffiffiffiffiffiffiffiffiffiffiKH2

pH2

pþKAP CAPþKPhECPhEÞ

3 r ¼ P9CAP

ð1þP8þP5CAPþP3CPhEÞ3

Model 7

r.l.s.: surface chemical reaction, competitive H2–AP adsorption, saturated copper surface r ¼ðCS kSKH2

KAPÞpH2CAP

ðffiffiffiffiffiffiffiffiffiffiffiffiffiffiKH2

pH2

pþKAPCAPþKPhECPhEÞ

3 r ¼ P10CAP

ð1þP11CAPþP12CPhEÞ3

N.M. Bertero et al. / Applied Catalysis A: General 349 (2008) 100–109104

possibilities for the adsorption of H2 and AP (competitive and non-competitive), we analyzed seven different LHHW models. Detailsof these models are given in Table 1. The obtained expressions forthe reaction rate r in Table 1 were simplified by grouping kineticand equilibrium constants as parameters Pj, where j identifies thegroup of constants.

3.3.2. Non-stationary model

We also use non-stationary kinetic models for interpretingour experimental data. In contrast with LHHW kinetics, the non-stationary models do not assume the existence of rate-limiting steps, so that they consider all the surface reaction andadsorption/desorption rates involved in the reaction mechanism[34].

The catalytic surface coverage for the i-th component (AP orPhE) is given by:

dui

dt¼ kiCiuF � k0iui þ

Xj

ni jr j (8)

where ui is the coverage of species i, ki and k0i are the rate constantsof adsorption and desorption, respectively, uF, the fraction ofuncovered catalyst surface, rj the rate of j-th surface reaction, andnij the stoichometric coefficient of component i in j-th surfacereaction. Adsorbate conversion surface reactions are consideredelementary and thus expressed as first-order reactions (in our casei = AP and j = 1):

r j ¼ kS � ui (9)

The overall component concentration change is:

dCi

dt¼ CS

Xj

ni jr j (10)

where Ci is the overall concentration of i-component in themixture, disregarding whether it is adsorbed or not on the catalyst,and CS is the total concentration of surface-active sites. Theconcentration of the free i-component in the fluid phase (Ci) is thengiven by:

Ci ¼ Ci � CSui (11)

The balance of surface-active sites on the catalyst is representedby the following equation:

uF þX

i

ui ¼ 1 (12)

All the equations of this model were written here in terms of therelative concentration of the i-component ðC�i ¼ Ci=C0

APÞ. The activesites concentration, Cs, was determined from the metallic Cudispersion value and the catalyst concentration.

3.3.3. Numerical resolution and statisticals

The systems of differential equations (Eqs. (6) and (7) and (8)–(10)) were solved numerically using the Runge–Kutta–Mersonalgorithm. The model parameter estimation was performed bynon-linear regression, using a Levenberg–Marquardt algorithm,which minimizes the objective function (Eq. (13)):

S ¼X

i

ðC�i;t � C�CALCi;t Þ2 (13)

where C�i;t and C�CALCi;t are the experimental and calculated concen-

trations, respectively, i is the chemical compound, and t is thereaction time.

The coefficient of determination (r2) gives the fitting quality(i.e., the percentage of explanation of the total data variationaround the average observed value) and was calculated using thefollowing equation:

r2 ¼Pn

i¼1 ðC�CALCi � C

�Þ2Pni¼1 ðC�i � C

�Þ2(14)

The model adequacy and the discrimination between models weredetermined using the model selection criterion (MSC), according tothe following equation.

MSC ¼ ln

Pni¼1 ðC�i � C

�Þ2Pni¼1 ðC�i � C�CALC

i Þ2

" #� 2 p

n

� �(15)

where n is the number of experimental data, p is the amount ofparameters fitted, C

�is the average relative concentration and

C�CALCi;t and C�i;t are the predicted and the experimental values,

respectively. When various different models are compared, themost significant is that which leads to the highest MSC value.

3.3.4. Kinetic modeling results

3.3.4.1. LHHW models. The mathematical expressions obtained forAP conversion rate following different LHHW models are presentedin Table 1. Models 1, 2, 3 and 5 of Table 1 were not consideredbecause they cannot explain the negative order with respect to APand/or the first order in H2 determined from our experimental

Table 2Kinetic modeling results using LHHW models

Model Estimated parameters S r2 MSC

4.a P3 = 2.29 � 10�1 6.06 2.53 � 10�2 0.993 4.69

P5 = �2.15 � 10�1 3.89

P6 = 3.73 � 10�3 3.71 � 10�2

4.b P3 = 0 2.53 � 10�2 0.993 4.76

P5 = �3.61 � 10�1 6.96 � 10�2

P6 = 2.47 � 10�3 2.60 � 10�4

6.a P3 = 7.51 � 10�3 1.90 � 10�1 3.73 � 10�3 0.999 6.53

P5 = 8.37 � 10�1 2.42 � 10�1

P8 = 3.20 � 10�3 9.04 � 10�2

P9 = 2.71 � 10�2 1.25 � 10�2

6.b P3 = 0 3.74 � 10�3 0.999 6.60

P5 = 8.35 � 10�1 1.96 � 10�1

P8 = 1.72 � 10�3 8.95 � 10�2

P9 = 2.68 � 10�2 1.11 � 10�2

6.c P3 = 0 3.73 � 10�3 0.999 6.68

P5 = 8.21 � 10�1 8.68 � 10�2

P8 = 0

P9 = 2.62 � 10�2 2.89 � 10�3

7.a P10 = 6.69 � 10�3 2.01 � 10�2 3.73 � 10�3 0.999 6.61

P11 = 1.55 � 10�1 1.18

P12 = �3.66 � 10�1 6.34 � 10�1

7.b P12 = 0 3.73 � 10�3 0.999 6.68

P10 = 2.62 � 10�2 2.89 � 10�3

P11 = 8.21 � 10�1 8.68 � 10�2

Reaction conditions: T = 363 K, pH2 = 8.7 bar, WCAT = 1 g, C0AP ¼ 0:168 M,

VSOLV = 150 ml (cyclohexane).

Table 3Kinetic modeling results using non-stationary models

Model Estimated parameters S r2 MSC

8.a kAP = 459.87 3061.50 1.56 � 10�3 0.999 7.47

k0AP ¼ �14:87 324:92

kS = 62.82 2.78

8.b kAP = 602.91 105.84 1.56 � 10�3 0.999 7.55

k0AP ¼ 0

kS = 62.82 2.17

Reaction conditions: T = 363 K, pH2¼ 8:7 bar; WCAT = 1 g, C0

AP ¼ 0:168 M;

VSOLV = 150 ml (cyclohexane).

N.M. Bertero et al. / Applied Catalysis A: General 349 (2008) 100–109 105

data. Thus, calculations were performed using only Models 4, 6 and7 which fulfil these requirements; results are presented in Table 2.

A reasonable fitting was observed when applying Model 4(Table 2, Model 4.a) but the estimate for P5 was negative. Then, werecalculated by assuming P3 = 0, which implies to consider that PhEis not adsorbed on the metal copper surface. However the estimatefor P5 was again negative (Table 2, Model 4.b) and Model 4 wastherefore rejected.

Estimates of P3 and P8 not significantly different from zero wereobtained when applying Model 6 (Table 2, Model 6.a). This modelgave a satisfactory fit by removing P3 (Table 2, Model 6.b), but theestimate for P8 was again not significantly different from zero.Then, and taken into account that the estimate for P8 wassignificantly lower than the estimate for P5, we recalculated byassuming ðKH2

� pH2Þ0:5�KAP � CAP: A good fitting was obtained,

with positive estimates for P9 and P5, and high values of r2 and MSC,as shown in Table 2 (Model 6.c). Furthermore, calculations usingModel 6.c resulted in a good fitting of the experimental dataobtained in different operative conditions. For example, the resultsobtained for three reaction temperatures gave values of MSChigher than 6 in all cases whereas the sums of squared deviationswere in the order of 10�3. Fig. 5a shows a good agreement betweenthe experimental data and the Model 6.c predictions when therelative concentrations of AP and PhE are represented as functionsof time. Besides, the distribution of residuals followed anacceptable random trend when represented as a function of time(Fig. 5b). This is consistent with the hypothesis of random errorincluded in non-linear regression used for parameter estimationsand gives additional support to the model adequacy.

Model 7 gave also a good fitting of experimental data (Table 2,

Model 7.a). However, estimate for P12 ¼ KPhE=ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiKH2

pH2

qwas

negative. Besides, estimates for P11 ¼ KAP=ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiKH2

pH2

qand P10 ¼

ksCsKAP=ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiKH2

pH2

qnot significantly different from zero were

obtained. When it is assumed that PhE is not adsorbed on themetal copper surface, P12 = 0, Model 7 turns into Model 7.b, whichis identical to Model 6.c (see simplified rate, Table 1). The lastmeans that estimates for P10 and P11 (Model 7.b) equal to thosecalculated for P9 and P5 (Model 6.c), respectively, should beobtained. Thus, no difference between Model 6.c and 7.b frommathematical and statistical point of view can be concluded.

In order to find more information that permits discriminatebetween Models 6.c and 7.b, the variation of ksCs and P11 (or P5)with temperature was plotted (Fig. 6). As it can be seen fromFig. 6a, while ksCs is increasing with temperature, P11 (or P5) ispractically constant between 353 and 373 K (Fig. 6b). If weconsider that P5 = KAP (Model 6.c), this would mean that the APadsorption heat on metal copper is almost zero. This is contra-dicting the experimental results which show that AP adsorption onmetal copper is very strong. Instead, as P11 is a ratio of KAP and KH2

(Model 7.b), it is likely that variation with temperature of KH2

compensates the variation of KAP, giving as result an almostconstant value for the parameter estimate in the temperaturerange considered. Then, Model 7.b seems to be more consistentthan Model 6.c from a physical point of view.

In summary, the best LHHW model to interpret the APhydrogenation results on Cu/SiO2 catalyst considers that theadsorption of AP and H2 is competitive and the rate-limiting step isthe surface chemical reaction. Furthermore, the model assumesnegligible PhE coverage, strong adsorption of AP on copper andtotal coverage of metal copper surface.

3.3.4.2. Non-stationary model. Taken into account the resultsobtained with LHHW models, the fitting of the experimental datausing the non-stationary model was performed assuming that PhEdoes not adsorb on the metal copper surface. A very goodapproximation of the experimental data was observed when non-stationary model was applied (Table 3, Model 8.a), but the estimatefor k�1

AP was negative. Thus, we recalculated by assuming thatkAP� k�1

AP , i.e., the AP adsorption rate constant would be muchhigher than that of desorption which is in agreement with thenegative order (strong adsorption) with respect to AP determinedfrom our experimental data. A good fitting was obtained (Table 3,Model 8.b) and estimates of kAP and k were significantly differentfrom zero. Moreover, calculations using Model 8.b gave a goodfitting of all the experimental data obtained in different operativeconditions. In fact, the results obtained for three temperaturesshow that in all cases the values of the statistical S were lower than1.56 � 10�3 and of MSC higher than 7.5.

Fig. 7a shows that a very good agreement is verified betweenexperimental data and Model 8.b predictions. The differencesðC�OBS � C�CALCÞ were always <2 � 10�2, which corresponds to arelative residual of 5% or less, and were lower than those obtainedusing the LHHW Model 6.c. Furthermore, these residuals showed aclear random behavior (see Fig. 7b). Finally, by comparing Tables 2and 3 it can be inferred that the non-stationary model led to MSC

Fig. 5. Acetophenone hydrogenation on Cu/SiO2: (a) experimental (symbols) and modeling results (full lines) using LHHW Model 6.c or 7.b; (b) Evolution of residuals for AP

and PhE [pH2 = 8.7 bar, WCAT = 1 g, C0AP ¼ 0:168 M; VSOLV = 150 ml (cyclohexane)].

N.M. Bertero et al. / Applied Catalysis A: General 349 (2008) 100–109106

values higher than those resulting when applying the LHHWModel 6.c or Model 7.b.

On the other hand, from the values of kS (surface reactionconstant) determined at different temperatures (Fig. 6c) wecalculated, by numerical regression, the apparent activationenergy and obtained EA = 55.3 kJ mol�1 that is in good agreementwith the value determined directly from experimental data(54.9 kJ mol�1). As well, considering the kAP estimates at differenttemperatures, an apparent activation energy of 53.8 kJ mol�1 forthe AP adsorption on copper was calculated. This value is of thesame order to the one obtained for the surface chemical reac-tion, suggesting an important interaction between AP and metalsurface.

In summary, calculations using the non-stationary modelresulted in an acceptable fitting of the experimental results byassuming kAP� k�1

AP . This assumption is in agreement with the factthat the best fitting of experimental data using LHHW models wasobtained with Model 7.b that considers a strong adsorption of APon copper. Both models require, therefore, the existence of a stronginteraction between AP and the metallic copper surface to properlypredict the catalytic data.

4. Discussion

Our catalytic results showed that AP is selectively hydro-genated on Cu/SiO2 yielding almost 100% PhE under the operative

Fig. 6. Variation of parameters with temperature for AP hydrogenation over Cu/SiO2

catalyst. (a) ks�Cs from LHHW-type Model 7.b; (b) P11 ¼ KAP=ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiKH2

pH2

qfrom LHHW-

type Model 7.b; (c) ks and kAP from non-stationary Model 8.b.

N.M. Bertero et al. / Applied Catalysis A: General 349 (2008) 100–109 107

conditions used in this work. This selective conversion of AP to PhEdemonstrates that the promotion of parallel or consecutivereaction pathways such as the aromatic ring hydrogenation orthe hydrogenolysis of C–OH bond in the PhE molecule is negligibleon Cu/SiO2 in the conditions used in this work. Characterization ofCu/SiO2 by TPD of NH3 proved that it does not contain surface acidsites and this can easily explain the absence of by-products formedvia hydrogenolysis or cracking pathways. In contrast, a moredetailed discussion is required to justify the selective hydrogena-tion of the C O bond of the AP molecule on copper.

Our experimental data on Cu/SiO2 were properly predicted bykinetic modeling only when it was considered that AP was stronglyadsorbed on copper. Consistently, results obtained by varying theinitial AP concentration showed that the reaction was negativeorder with respect to AP. The AP–Cu interaction may be interpretedby taking into account the attractive and repulsive forces existingbetween the emergent d-orbitals of the metal and the adsorbate. Infact, the factors governing intrinsic metal selectivities have beenqualitatively related to the electronic filling of d-orbitals usingextended Huckel calculations [35]. In particular, the repulsiveforces existing between metal d-orbitals and the phenyl group ofAP would increase with the filling of d-orbitals. The d-orbitals inmetallic copper are completely filled and it is expected then thatthe repulsion with the phenyl group will be higher for Cu than forother non-noble metals having partially filled d-orbitals such as Ni.The strong electrostatic repulsion between metallic Cu and thephenyl group would tilt the AP molecule thereby favoring itsadsorption via the carbonyl group and the formation of theunsaturated alcohol (Fig. 8). It must be noted that the repulsionbetween copper and the phenyl group is also consistent with thefact that the best LHHW model to interpret the AP hydrogenationresults (Model 7.b) assumes that the PhE adsorption on copper isnegligible. The rapid desorption of PhE from Cu drasticallydecreases the possibility of the consecutive hydrogenation ofPhE to CHE that easily occurs on Ru [5,6] or the PhE hydrogenolysisto EB that is observed on Pd [7–9].

AP molecules may interact through its C O bond with thecopper surface via on-top, di-sCO or pCO adsorption modes, but it isexpected that di-sCO or pCO adsorptions led to the formation ofmore stable adsorbed species [35]. Fig. 8 represents the surfacecopper atoms covered by AP adsorbed via di-sCO and pCO

adsorptions and by atomic hydrogen formed by the dissociativeH2 adsorption. From the estimates obtained with LHHW-typeModel 7.b, it can be approximately inferred that KAP is about oneorder of magnitude higher than KH2

. Then, Model 7.b assumes thatthe adsorption of AP and hydrogen is competitive and that APadsorbs on copper much stronger than hydrogen. These kineticmodeling assumptions imply that the AP hydrogenation rate maydecrease with increasing initial AP concentration because thedensity of Cu-active sites available for H2 adsorption will bedrastically diminished by the AP covering. We effectivelydetermined that the AP hydrogenation rate is negative order withrespect to AP. Additionally; the adsorption modes of AP over metalcopper surface may play a role in the hydrogenation rate. It is likelythat the ratio of molecules adsorbed in the di-sCO mode respect topCO mode varies with AP concentration and reaction conditions. Ifthe most stable and less reactive mode of adsorption prevails, thenthe hydrogenation rate will diminish.

The interaction between copper and reactants described abovegives also insight on the effect that changing the solvent has oncatalyst activity. The AP hydrogenation rate was lower in aromaticsolvents in comparison to cyclohexane or IPA. It is known that thearomatic solvents may interact with the metallic surface of non-noble metals such as Cu or Ni [24–27]. In our case, this solvent–metal interaction would not hamper the strong adsorption of AP oncopper, but can decrease the concentration of Cu surface sitesavailable for the dissociative chemisorption of hydrogen. For agiven H2 partial pressure, the diminution of the surface metalfraction covered by adsorbed hydrogen should decrease the APhydrogenation rate because the reaction is positive order inhydrogen. In contrast, the interaction of cyclohexane with copperis very weak and would not interfere with the H2 chemisorption.Thus, differences in the solvent–metal interaction strength canexplain the higher catalyst activity for AP hydrogenation observedhere when the reaction is carried out in cyclohexane in comparison

Fig. 7. Acetophenone hydrogenation on Cu/SiO2: (a) experimental (symbols) and modeling results (full lines) using non-stationary Model 8.b; (b) evolution of residuals for AP

and PhE [ pH2¼ 8:7 bar; WCAT = 1 g, C0

AP ¼ 0:168 M, VSOLV = 150 ml (cyclohexane)].

N.M. Bertero et al. / Applied Catalysis A: General 349 (2008) 100–109108

to benzene or toluene. The highest catalyst activity for APhydrogenation was determined when IPA was used as solvent.Previous work has showed that IPA is adsorbed dissociatively onmetallic copper forming isopropoxide and atomic hydrogen assurface species [28–30]. IPA may therefore act as an extra source of

Fig. 8. Representation of hydrogen and acetophenone adsorbed

surface atomic hydrogen thereby increasing the AP hydrogenationrate. It has also to be noted that IPA probably interact with AP in theliquid phase via the formation of hydrogen-bridge bonds [8]. Thisinteraction may cause the activation of the carbonyl group via thepolarization of the C O bond and consequently increase the

species on the metallic copper surface of Cu/SiO2 catalyst.

N.M. Bertero et al. / Applied Catalysis A: General 349 (2008) 100–109 109

catalyst activity for AP hydrogenation. Thus, in the case of IPA boththe solvent–reactant and the solvent–catalyst interactions maycontribute to increase the AP hydrogenation rate. Finally, weremark that Cu/SiO2 was completely selective to PhE, irrespectiveof the solvent used in the catalytic test. This show that the strongadsorption of AP on copper via its C O bond, which is the essentialrequisite for selectively obtaining PhE, is not modified by thenature of the solvent employed in the reaction.

5. Conclusions

1-Phenylethanol yields of about 100% are obtained by liquid-phase acetophenone hydrogenation on a Cu(6.8%)/SiO2 catalyst.Several factors contribute to achieve the complete conversion ofacetophenone to 1-phenylethanol: (i) the strong electrostaticrepulsion between metallic Cu and the phenyl group tilts theacetophenone molecule thereby favoring its adsorption via thecarbonyl group and the formation of the unsaturated alcohol; (ii) theadsorption of 1-phenylethanol on copper is negligible; (iii) 1-phenylethanol is not converted to unwanted products via hydro-genolysis or cracking pathways because silica is an inert support.

The acetophenone conversion reaction is negative order withrespect to acetophenone and first order in hydrogen. Consistently,the experimental data are well interpreted by using heterogeneousLHHW and non-stationary models that assume that acetophenonecompetes with hydrogen for surface copper-active sites butadsorbs on the metal much stronger than hydrogen.

The Cu/SiO2 activity for acetophenone hydrogenation is higherin cyclohexane than in benzene or toluene. This is because thesolvent–copper interaction is stronger using aromatic solvents andthereby decreases the concentration of Cu surface sites availablefor the dissociative chemisorption of hydrogen. But the highest Cu/SiO2 activity is obtained in isopropyl alcohol, probably becauseisopropyl alcohol adsorbs dissociatively on metallic copper andforms extra atomic hydrogen that increases the acetophenonehydrogenation rate.

Acknowledgements

We thank the Universidad Nacional del Litoral (UNL), ConsejoNacional de Investigaciones Cientıficas y Tecnicas (CONICET), and

Agencia Nacional de Promocion Cientıfica y Tecnologica (ANPCyT),Argentina, for the financial support of this work.

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