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Catalytic and kinetic study of the liquid-phase hydrogenation of acetophenone over CuSiO2 catalyst.pdf

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Catalytic and kinetic study of the liquid-phase hydrogenation of acetophenone over Cu/SiO 2 catalyst Nicola´ s M. Bertero, Carlos R. Apesteguı´a, Alberto J. Marchi * Catalysis Science and Engineering Research Group (GICIC), Instituto de Investigaciones en Cata´lisis y Petroquı´mica (INCAPE), UNL-CONICET, Santiago del Estero 2654, 3000 Santa Fe, Argentina 1. Introduction The selective hydrogenation of aromatic ketones is of great importance
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  Catalytic and kinetic study of the liquid-phase hydrogenation of acetophenoneover Cu/SiO 2  catalyst Nicola´s M. Bertero, Carlos R. Apesteguı´a, Alberto J. Marchi* Catalysis Science and Engineering Research Group (GICIC), Instituto de Investigaciones en Cata´ lisis y Petroquı´ mica (INCAPE), UNL-CONICET, Santiago del Estero 2654, 3000 Santa Fe, Argentina 1. Introduction The selective hydrogenation of aromatic ketones is of greatimportance in Fine Chemistry because the resulting phenylalcohols find application as flavors, fragrances and intermediatesforthesynthesisofmorevaluable organicmolecules[1]. However,the selective hydrogenation of the C O group of the ketonemolecule is frequently not easily achieved due to the formation of unwanted 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 of acetophenone(AP)to1-phenylethanol(PhE)hasbeeninvestigatedon 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.AsimplifiedreactionnetworkfortheAPconversionisshowninFig. 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.TheAPhydrogenationhasbeenstudiedonnoblemetalssuchasPt, 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 onNi-based catalysts[10–16]. These studies agree in that nickel promotes the selectiveAP hydrogenation to PhE, but the concomitant formation of variables 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 Applied Catalysis A: General 349 (2008) 100–109 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 hydrogenationAcetophenoneCopper-based catalystKinetic modeling A B S T R A C T Theliquid-phasehydrogenationofacetophenone(AP)to1-phenylethanol(PhE)wasstudiedonCu(6.8%)/SiO 2  catalyst. Catalytic tests were carried out in a batch reactor by varying temperature, total pressureand AP initial concentration between 353–373 K, 5–20 bar, and 0.038–0.251 M, respectively, and usingfour different solvents: isopropylic alcohol (IPA), cyclohexane, toluene and benzene. The selectivity toPhEwasabout100%irrespectiveofthesolventused,buttheinitialAPconversionratefollowedtheorderIPA  >  cyclohexane  >  toluene  >  benzene. The differences in catalyst activity when changing the solventwere 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 whenassuming that: (i) the adsorption of AP and H 2  is competitive; (ii) AP adsorption is strong; (iii) coppersurface is saturated in AP; (iv) the PhE coverage on the catalyst is negligible. These assumptions wereconsistent with the fact that the reaction was negative order with respect to AP and first order in H 2 . Thehighly selective AP hydrogenation to PhE was explained by considering that the strong electrostaticrepulsionbetweenmetallicCuandthephenylgrouptiltstheAPmoleculetherebyfavoringitsadsorptionvia the carbonyl group and the formation of the unsaturated alcohol. Also, PhE was not consecutivelyconverted via hydrogenolysis or other acid catalyzed reactions since the support was inert.   2008 Elsevier B.V. All rights reserved. * Corresponding author. Fax: +54 342 4531068. E-mail address:  amarchi@fiq.unl.edu.ar (A.J. Marchi). Contents lists available at ScienceDirect Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata 0926-860X/$ – see front matter    2008 Elsevier B.V. All rights reserved.doi:10.1016/j.apcata.2008.07.014  hydrogenation. Zaccheria et al. [17] reported that a Cu(8%)/Al 2 O 3 catalystconvertsarylketones,includingAP,intothecorrespondingalcohols with selectivities higher than 90%. In contrast, theyobserved the formation of significant amounts of hydrogenolysisproducts and ethers when using Cu/SiO 2 , probably because of thecatalystacidity.TwopatentsfromSumimotoChemicalCo.claimedfor the use of Cu/SiO 2  catalysts to selectively hydrogenate AP toPhE [18,19]. High PhE yields from AP would be obtained whenusing Cu/SiO 2  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 coppercatalystsarealsolacking.Ontheotherhand,ithastobenotedthatthe type of solvent used in the AP hydrogenation reaction maysignificantly modify the catalyst activity. For example, Aramendı´aetal.[7]studiedtheAPhydrogenationoverPd-supportedcatalystsusing 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%)/SiO 2  catalyst.Experimental data were obtained by varying the operativeconditions (temperature, H 2  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 reactionmechanismbasedontheinteractionbetweenreactantsandCu 0 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/SiO 2  catalyst with 6.8 wt.% of copper was prepared byincipient wetness impregnation method.Copper was deposited oncommercial silica (Grace G62, 99.7%, Na: 0.1%, SO 42  : 0.1%, others:0.1%) by adding dropwise a 0.57 M aqueous solution of Cu(NO 3 ) 2 (Merck, 98%). The solid was dried in an oven at 373 K for 12 h andthencalcinedinairflowat673 Kfor3 h.Priortocatalytictests,thecatalyst was activated ex situ in H 2  flow at 543 K for 2 h and thenquickly transferred to the reactor in an inert atmosphere of N 2  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 K a  radiation in the range of 2 u   = 10–80 8  at a scanspeed of 2 8 /min. Crystallites size were calculated from the CuO(1 1 1) diffraction lines using the Debye–Scherrer equation.BETsurfacearea( S  g ),porevolume( V  P )andmeanporediameter( d P )weremeasuredbyN 2 physisorptionat77 KinaQuantochromeCorporation 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 obtainedusingaH 2 (5%)/Argaseousmixtureat60 cm 3 /minSTPandasamplesize of about 100 mg. The sample was heated from 298 to 773 K at10 K/min. H 2  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 NH 3  preadsorbed at 373 K. Samples (100 mg)weretreated inHe (60 cm 3 /min)at 723 Kfor 2 hand then exposedto a 1% NH 3 /He stream for 40 min at 373 K. Weakly adsorbed NH 3 wasremovedbyflushingwithHeat373 Kduring2 h.Temperaturewasthenincreasedatarateof10 K/minandtheNH 3 concentrationin the effluent was measured by mass spectrometry in a BaltzersOmnistar unit.The metal copper dispersion was determined by titration withN 2 O at 363 K using a stoichiometry of (Cu 0 ) s /N 2 O = 2, where (Cu 0 ) s implies a Cu 0 atom on surface [20,21]. Pre-reduced samples wereexposed to pulses of N 2 O(10%)/Ar. The number of chemisorbedoxygen atoms was calculated from the consumption of N 2 Omeasured 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.Thesolventsusedwereisopropylicalcohol (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 N 2  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.TheconcentrationsofunreactedAPandreactionproductswerefollowed 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 m m coating. Liquid samples Fig. 1.  Reaction network for acetophenone hydrogenation over metal catalysts. N.M. Bertero et al./Applied Catalysis A: General 349 (2008) 100–109  101  werewithdrawnfromthereactorbyusingaloopunderpressureinorder 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(  X  AP , mol of AP reacted/mol of AP fed) was calculated as  X  AP  ¼ð C  0AP  C  AP Þ = C  0AP ;  where  C  0AP  is the initial concentration of acet-ophenone and  C  AP  is the acetophenone concentration at reactiontime  t  . 3. Results  3.1. Catalyst characterization The specific surface area ( S  g  = 220 m 2 /g) and pore volume( V  P  = 0.42 cm 3 /g) determined for the Cu/SiO 2  sample were similarto those corresponding to SiO 2  ( S  g  = 230 m 2 /g,  V  P  = 0.49 cm 3 /g),thereby indicating that the addition of copper did not modifysignificantly the textural properties of the support. On the otherhand, the TPD profile of NH 3  on Cu/SiO 2  (not shown here) did notexhibitanyNH 3 desorptionpeak,therebyshowingthatthesampledoesnotcontainacidsurfacesitesabletoretainammoniaat373 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/SiO 2  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 SiO 2 , determined by the dissociativeadsorption of N 2 O at 363 K, was 1.2% that corresponds to a meanCu 0 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 APhydrogenationwasstudiedusingapolarproticsolvent(isopropylicalcohol), a naphtenic non-polar solvent (cyclohexane) and twoaromaticnon-polarsolvents(tolueneandbenzene).Fig.3comparesthe evolutionofAPconversionasafunctionoftime whenusingtheabove-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,theselectivitytoPhEwashigherthan99%duringtheentirecatalyticruns, irrespective of the solvent used.TheresultsinFig.3clearlyshowthattheCu/SiO 2 activityforAPhydrogenation depends on the solvent used. The modification of the 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/SiO 2  activity is hampered when usingaromatic non-polar solvents, such as benzene or toluene. The APconversion rateon Cu/SiO 2 , in fact, isclearly higherin 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/SiO 2  was studied in cyclohexane at 363 K and  C  0AP  ¼ 0 : 168M : 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. 2.  Characterization of calcined Cu/SiO 2  sample: (a) X-ray diffraction pattern; (b) TPR profile. Fig. 3.  Solvent effect on AP conversion over Cu/SiO 2  [ T   = 363 K,  p H 2  ¼ 8 : 7bar ; W  CAT  = 1 g,  C  0AP  ¼ 0 : 168M ;  V  SOLV  = 150 ml]. N.M. Bertero et al./Applied Catalysis A: General 349 (2008) 100–109 102  18.7 bar, respectively, by considering that at 363 K the solventvapor pressure is 1.3 bar (calculated with Antoine equation), andthevaporpressuresofAPandproductsarenegligible.Theobtained  X  AP  vs. time plots are presented in Fig. 4a. It is observed that thecatalyticactivity increaseswiththehydrogen partialpressure.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  ð r  0AP ;  molAPh  1 g  1 Þ  were calculated by poly-nomialdifferentiationofthecurvesatzerotime.Thereactionorderwith respect to hydrogen was calculated by considering for  r  0AP  apower-law rate equation: r  0AP  ¼ k ð P  H 2 Þ a ð C  0AP Þ b (1)Reaction order  a , determined by both linear and non-linearregression from Eq. (1) with  C  0AP  constant, was close to one.The effect of the AP initial concentration on catalytic activitywasstudiedat363 Kand P  H 2  ¼ 8 : 7bar : Theinitialconcentrationof AP was varied between0.084 and 0.251 M. The resultsare given inFig. 4b and show that the catalyst activity decreases by increasingthe initial AP concentration. The reaction order with respect to APwasdeterminedby calculating r  0AP  from datarepresentedin 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  P  H 2  ¼ 8 : 7bar and  C  0AP  ¼ 0 : 168M :  Fig. 4c shows the increase of catalytic activity withtemperature. The apparent activation energy ( E  A ) was determinedby numerical regression using an Arrhenius-type function. A valueof   E  A  = 54.9 KJ mol  1 was obtained.Data in Fig. 4 show effects that changes in H 2  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 showsthattheCu/SiO 2 catalystishighlyselectiveforhydrogenatingAPtoPhE 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/SiO 2 ,thereby indicating that the interaction of AP with surface-activesitesisverystrong.Moreover,takenintoaccountthatSiO 2 isanon-reactive support and that the interaction between the large Cu 0 crystallites and SiO 2  is weak, it would be expected that APhydrogenation over Cu/SiO 2  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/SiO 2  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)  TheadsorptionofAPandH 2 maybeeithercompetitive(modelswithonlyoneactivesite)ornon-competitive(modelswithtwodifferent metal catalytic sites). (2)  The H 2  adsorption is dissociative [31–33]. (3)  PhE adsorption/desorption steps are reversible quasi-equili-brium steps. (4)  The hydrogenation surface reaction is irreversible (totalconversion of AP to PhE was obtained in the experimentalruns). (5)  Hydrogen 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 S 1  and S 2 ,and non-competitive H 2  chemisorption (hydrogen adsorbed onmetallic site S 1 ), the elementary steps shown in Eqs. (2)–(5)represent the general reaction mechanism:H 2 þ 2S 1 , 2HS 1 ;  r  1  ¼ k H 2  p H 2 ð C  S 1 Þ 2  k  1H 2 ð C  HS 1 Þ 2 (2)AP þ S 2 , APS 2 ;  r  2  ¼ k AP C  AP C  S 2   k  1AP C  APS 2  (3)2HS 1 þ APS 2 ) PhES 2 þ 2S 1 ;  r  3  ¼ k S ð C  HS 1 Þ 2 C  APS 2  (4)PhES 2 , PhE þ S 2 ;  1 K  PhE ¼ C  PhE C  S 2 C  PhES 2 (5)If the adsorptions of H 2  and AP are competitive, then S 1  = S 2  = S.The general system of differentialequations to be solved, basedon Eqs. (2)–(5), is:d C   AP d t   ¼  1 C  0AP  d C  AP d t   ¼  1 C  0AP  r   (6)d C   PhE d t   ¼  1 C  0AP  d C  PhE d t   ¼  1 C  0AP  r   (7)where  r   is the rate of the limiting step in the reaction mechanismand  C   i  ¼ C  i = C  0AP  the relative concentration of the  i  component.By assuming different rate-limiting steps (r.l.s.), (adsorptionof H 2 , adsorption of AP or surface chemical reaction) and two Fig.4. Effectof(a)H 2 partialpressure[ T   = 363 K, C  0AP  ¼ 0 : 168M];(b)APinitialconcentration[ T   = 363 K,  p H 2  ¼ 8 : 7bar];(c)temperature ½  p H 2  ¼ 8 : 7bar ;  C  0AP  ¼ 0 : 168M  onthecatalyst activity.  W  CAT  = 1 g,  V  SOLV  = 150 ml (cyclohexane). N.M. Bertero et al./Applied Catalysis A: General 349 (2008) 100–109  103
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