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HUGO ROJAS, JOS J. MARTNEZ, PATRICIO REYESKINETIC BEHAVIOR IN THE HYDROGENATION OF FURFURAL OVER Ir CATALYSTS SUPPORTED ON

TiO2

Dyna, vol. 77, nm. 163, septiembre, 2010, pp. 151-159,

Universidad Nacional de Colombia

Colombia

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2/10Dyna, year 77, Nro. 163, pp. 151-159. Medellin, September, 2010. ISSN 0012-735

KINETIC BEHAVIOR IN THE HYDROGENATION OFFURFURAL OVER Ir CATALYSTS SUPPORTED ON TiO2COMPORTAMIENTO CINTICO DE LA HIDROGENACIN DEFURFURAL SOBRE CATALIZADORES DE Ir SOPORTADOS ENTiO2

HUGO ROJASEscuela de Ciencias Qumicas, Facultad de Ciencias, Grupo de Catlisis (GC-UPTC), Universidad Pedaggica y Tecnolgica d

Colombia, Tunja, Colombia, hurojas@udec.cl

JOS J. MARTNEZEscuela de Ciencias Qumicas, Facultad de Ciencias, Grupo de Catlisis (GC-UPTC), Universidad Pedaggica y Tecnolgica

de Colombia, Tunja, Colombia

PATRICIO REYESFacultad de Ciencias Qumicas, Universidad de Concepcin, Chile

Received for review November 4 th, 2009, accepted December 15 th, 2009, final version January, 18 th, 2010

ABSTRACT: The kinetics of the liquid-phase hydrogenation of furfuraldehyde to furfuryl alcohol over Ir catalyssupported over TiO2 was studied in the temperature range of 323 to 373 K. The effect of furfural concentratiohydrogen pressure and the solvent effect were also studied. A high selectivity towards furfuryl alcohol wademonstrated. Initial rates describes the order global of the reaction. The experimental data could also be explaineusing the Langmuir-Hinshelwood model with of a single-site with dissociative adsorption of hydrogen and thsurface reaction as the rate-controlling step provided the best fit of the experimental data. KEYWORDS:Furfural, hydrogenation, SMSI effect, kinetic study.RESUMEN:La cintica de la hidrogenacin en fase lquida de furfural a alcohol furfurilico sobre catalizadores dIr/TiO2 se estudio en el rango de temperaturas de 323 a 373 K, tambin se estudio el efecto de la concentracin dfurfural, presin de hidrogeno y del solvente empleado. Se obtuvo una alta selectividad haca el alcohol furfurilicoCon las velocidades inciales de reaccin se determino el orden de reaccin global. Los datos experimentales puedetambin explicarse usando un modelo Langmuir-Hinshelwood considerando un solo tipo de sitio activo coadsorcin disociativa de hidrogeno, siendo la reaccin superficial la etapa limitante de la reaccin, este modelo sajusta a los datos experimentales.PALABRAS CLAVE: Furfural, hidrogenacin, efecto SMSI, estudio cintico.1 INTRODUCTIONHydrogenation reactions of , -unsaturatedaldehydes to their corresponding unsaturated alcoholare an interesting type of reaction in fine chemistry.They posses a C=O bond conjugated with a C=C

bond. The aim is to hydrogenate the carbonyl group,keeping intact the olefinic function, in spite of theC=C double bond is easily hydrogenated over most

conventional catalysts to give saturatealdehydes as the primary products [1-2].

One type of these reactions is thhydrogenation of furfural to obtain furfuryalcohol which find a variety of applicationin chemical industry, as starting material fothe manufacture of resins, tetrahydrofurfury

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alcohol (THFA) and as intermediate to obtain lysine,

vitamin C, lubricant, dispersing agent and plastisizer[3-4]. Furfuryl alcohol is industrially prepared bythe catalytic reduction of furfuraldehyde using Niand Cu/CrO catalysts. The disadvantage of this typeof catalysts is their high toxicity, which causessevere environmental pollution. To design catalystswithout chromium for the hydrogenation of furfuralwith high activity and selectivity is a hard butimportant task [4].

Liquid phase selective hydrogenation of furfural toproduce furfuryl alcohol over gas phase Raneynickel catalysts modified by the impregnation of

salts of heteropolyacids have been reported by Liuetal. [5]. Amorphous Ni alloys as Cu/C catalysts havealso been used [6] as well as Cu/MgO catalysts [4,7]and mixed CuZn oxides doped with Al, Mn and Fe[4]. Ag/SiO2 catalysts prepared by sol-gel methodand Rh-Sn/SiO2 showed selectivities to furfurylalcohol close to 79% and 93%, respectively [8].

LangmuirHinshelwood models involving abimolecular surface reaction as the rate-determiningstep have been used to describe the kinetics offurfural hydrogenation over Cu supported ondifferent carbon supports and the results indicated

that the catalytic activity are not significantlyaffected by the nature of the support. However, inthose copper catalysts, the metallic component existas a mixture of Cu0 and Cu(I), and both type of sitesmay be involved in the catalytic cycle. Reactionorder respect to furfural was close to zero on thesecatalysts whereas the dependence respect tohydrogen was in the range 0.6 to 0.8 [6].

Liquid phase furfural hydrogenation has beenstudied by Merat et al., [9] over Pt supportedcatalysts under mild operating condition and theyreported that no reaction takes place at 50 C. Onlya few studies on Pt-based catalysts have been

published [9-10], and especially Ru and Pd, alsoseem to be promising [6]. However, fewer results oniridium catalysts has been reported, even though thismetal displays interesting hydrogenation ability.Thus, Reyes, et al [11-12] has showed that Ircatalysts displayed a high activity and selectivity inthe hydrogenation of C=O group of , -unsaturatedaldehydes. Recently our group reported the kineticof the furfural over Ir/Nb2O5 catalysts [12]. In the

present work is studied the kinetics of the liquid

hydrogenation of furfural over Ir supporte

on TiO2 catalysts. Additionally, a possiblreaction mechanism is proposed.2 EXPERIMENTALTiO2 (Degussa P-25 SBET= 50 m

2g-1) waimpregnated with H2IrCl6 to obtain Ir/TiOcatalysts with Ir loading of 1 wt %. Thimpregnated solid was dried at 343 K for 6 hcalcined in air at 673 K for 4 h and reduced a773 K (HTR: High temperature reductionfor 2 h.

The BET-surface area was evaluated fromnitrogen adsorption isotherms at 77 K and thnumber of active sites of each catalyst werobtained by hydrogen chemisorption at 29K, both performed in a Micromeritic ASA2010. The surface acidity was determined btemperature programmed desorption, TPD, o

NH3. Transmission electronic microscop(TEM) studies were carried out in a JEOModel JEM-1200 EXII microscope.

Furfuraldehyde and furfuryl alcohol used iall experiments, both of analytical reagengrade were supplied by Aldrich. Catalytireactions were carried out in a stainless stee

batch reactor at a constant stirring rate (100rpm). To carry out the kinetic study over thcatalysts only one variable was modified ieach experiment, keeping constant all thothers. The effect of furfural concentratiowas studied in the concentration range 0.02to 0.1 M. The hydrogen partial pressure wastudied in the range 0.48 MPa to 0.84 MPThe temperature was varied in the range 32

to 363 K and the catalyst weight, ranged from0.1 to 0.3 g.Prior the experiment, all catalysts wertreated in situ under hydrogen flow of 2cm3min-1 at atmospheric pressure antemperature of 363 K to remove possiblsurface oxide species generated durinhanding. The avoid the presence of oxygenonce the reactor was loaded with the reactanmixture and catalyst, the system was flushewith He at atmospheric pressure during 3min.

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Analysis of the reaction mixture and products was

carried out using a gas chromatograph Varian 3400furnished with a HP Wax column of 30 m lengthand 0.53 mm ID. The GC analysis was performedusing a flame ionization detector, using He ascarrier. The products of reaction were analyzed in aVarian 3800-Saturn 2000 GC-MS provided with aionic trap and using the same separation conditions.

Starting with the fresh catalyst, the same catalystwas reused to study possible catalyst deactivationduring catalytic test. In all experiments, reactant and

product concentrations were measured at differenttime intervals. From these data, initial rates were

calculated allowing obtaining the reaction orderswith respect to furfural concentration and H2

pressure.3 RESULT S AND DISCUSSION3 1 Catalyst char acter izationThe results of catalysts characterization have been

provided previously [11], however some relevantaspects are considered here. These solids when theyare reduced at high temperature in H2, lead to a

partial reduction of the support, TiO2-x which coversthe metal surface, by the so called SMSI effect(strong metal support interaction). This behavior isassociated with a decrease of H/Ir ratio inchemisorption measurements (table 1) but withoutany effect in the metal particle size because there areno changes in the metal particle size in catalysts ofIr/TiO2 reduced at different temperatures ofreduction. This phenomenon has been widelystudied by TEM [1,11]. Surface aciditymeasurements were carried out by DTP of ammonia,

previously adsorbed at 393 K. Ir/TiO2 exhibits a

very low acidity compared with others supports[12].Table 1 Characterization results of Ir/TiO2 catalystParameter valuedTEM (nm) 4.0

H/Ir Quim 0.09

NH3 mmolg-1 2.56

3 2 Mass tr ansferIn the liquid phase hydrogenation ofurfuraldehyde, mass-transfer processes (gasliquid, liquid-solid, and intraparticldiffusion) can influence the rates of reactio[13]. For the kinetic study these diffusionaresistances should be absent. The liquid-sidmass-transfer coefficient and liquid-solimass-transfer coefficient depend on thintensity of turbulence in the liquid phasFor this reason highest stirring speed werused (1000 rpm). For prevent intraparticlmass-transfer resistance, it was used smalle

catalyst particles sizes (100 m). Aexperimental approach following thguidelines of Satterfield and Sherwood (eq1) [14] for corroborate the absence of gasliquid or liquidsolid mass transfer resistancwas applied. The equation 1 describes thathe reciprocal of the conversion rate as function of the reciprocal of catalyst masshould give a straight line indicating absencof external mass-transfer limitation.

++=

hkakmakr

C

ccbb

i 11111 (1)

Where the intercept 1/kbab represents resistance to gas absorption. The term 1/krepresents a resistance associated with thsurface reaction, while the term 1/kcacassociated with the transport of hydrogethrough the bulk liquid.

The figure 1 displays the results obtaineapplying this equation. It can be observed thabsence of external mass-transfer limitationTherefore, the mass-transfer rates thu

obtained were considerably higher than thmaximum reaction rates observed in thkinetic experiments, consequently, it waconcluded that external mass transfer did nohave an effect on the observed rates in thhydrogenation experiments.

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0

0,01

0,02

0,03

0,04

0 2 4 6 8 10 12

1/m, 1/gca

1/r,

m

in

gcat

/

m

ol

Figure 1 Effect of catalysts weight on the initial rates inthe furfural hydrogenation at 363 K, 0.1 M and 0.62 Mpa 3 3 Effect of solventSome solvents were used for furfural hydrogenation,such as, ethanol, heptane, ethanol-heptane (1:1).Table 2 summarizes the solvents used, theconversion level at 60 min of reaction time as wellas the initial activity expressed as TOF (turnoverfrequency) for Ir/TiO2 catalysts.

Table 2 Catalytic activity in furfural hydrogenation at363 K and 0.62 MPa, expressed as conversion, TOF and

selectivity at 60 min, SFOL=Selectivity to furfuryl alcohol,SFDA to 2-furaldehyde-diethylacetal

Solvent Conv. % TOF, s-1 SFOL SFDA

n-heptane 10.2 1.4 100 -

Ethanol 24 3.2 96 4

n-ethanol -

heptane28.7 3.8 > 99 < 1

As polarity increases also conversion as well as in

the TOF is observed at higher solvent, but theselectivity to the unsaturated alcohol decreases athigher polarity. Similar trends have been reportedfor other hydrogenation reactions [15]. It isinteresting note the synergestic effect of mixedsolvents which increases both activity andselectivity.

The results in table 2 indicate clearly that the mainproduct is furfuryl alcohol produced byhydrogenation of C=O bond in the furfural.

The production of furfuryl alcohol

associated with new actives sites in which thmetallic component exhibits a partiadecoration, with the creation of Ird+ speciewhich are more active in the polarization othe C=O bond [11,12]. With n-heptane asolvent the selectivity to furfuryl alcohol 100 %, but acetals are produced witalcoholic solvents, in this case, 2furaldehyde-diethylacetal (m/z= 39, 97, 12m/z).

The acetalization takes place over acid sitespecially Brnsted acid sites on the cataly

surface, these sites are placed near to iridiumcrystals. However, it has been suggested thathe acid sites in the catalysts did not modifthe active metal properties [16] and favorthe electron transfer from acid sites in thsupport to the active metal sites, bsuppressing the hydrogenation of C=C bondThe rate of formation of acetals is fasteduring the initial period of reaction but thselectivity of acetal decreased with thconsumption of reactant and became lowethan 1 % at high conversion (figure 2)

Figure 2 Conversion vs selectivity, effect ofsolvent in furfural hydrogenation for Ir/TiO2 at

363 K and 0.62 MPa with different solvents used() heptane, () ethanol, () ethanol-heptane 1:

The Figure 3 shows the hydrogenatioproducts of furfural over Ir/TiO2, arisinfrom the reduction of the C=O group and thformation of 2-furaldehyde-diethylacetal. Ithis catalyst was not observed thhydrogenation of the furan ring, or othecompounds derived from secondarreactions, such as hydrogenolysis of the CO

0

25

50

75

100

0 25 50 75 100

Conversion, %

Se

lectiv

ity,

%

Furfuryl alcohol

2-furaldehyde-diethylaceta

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bond, decarbonylation, hydrogenation and furan ring

opening that may appear [17].

OO

OO

O

O

O

CH3

CH3

Furfural

Furfuryl alcohol 2-furfuraldehyde-diethylacetal

H2+

Figure 3 Reaction pathway for furfuralhydrogenation over Ir/TiO2

3 4 Analysis of initial r ate dataThe effect of initial concentration of furfural and H2

pressure on the initial rate data of furfuralhydrogenation was studied. Experiments wereconducted at various initial concentrations offurfural in the range of 0.025 M to 0.1 M, at 363 Kand 0.62 MPa hydrogen partial pressure. The initialrates were expressed as TOF. The order calculatedtaking out the plot of -log initial concentration vs. log TOF for Ir/TiO2 was of 0.5 (figure 4).

0,0

0,4

0,8

1,2

0,8 1,0 1,2 1,4 1,6 1,8

Log [Furfural]

logTOF0

Figure 4. Effect of initial concentration on hydrogenationof furfural over Ir/TiO2 as a plot of log C0 furfural vs

log TOF.

An explanation for this result can be the adsorptionmode of furfural on the catalysts surface. Furfural

possibly adsorbs preferentially towards C=C withplane geometry competitive with atop geometrysimilar in cinnamaldehyde [18]. This could beattributed to the fact that C=C on the ring of furfuralare conjugated. At first time reactions the coverageof surface of molecules of furfural with atopgeometry is larger, but whiles the reaction progressthe surface is totally coverage of furfural in planegeometry.

The effect of the hydrogen partial pressure o

the initial rates was studied in the range o0.48 MPa to 0.84 MPa at constant initiaconcentration (0.1 M) and temperature (36K). The results are displayed in figure 5. Thorder calculated for initial rate was -1,0 usinthe initial rates expressed as TOF.

22

23

24

0 0,1 0,2 0,3 0,4Log pH2

LogTOF0

Figure 5 Effect of hydrogen partial pressure infurfural hydrogenation over Ir/TiO2 catalysts as

plot of log PH2 vs log TOF3 5 Kinetic modelTwo types of kinetic equations weremployed in the quantitative description othe experimental results: first, empirica

power-law equations based on initial rateand a second, equations based on thLangmuir-Hinshelwood mechanism. Powerlaw equations for the hydrogenation ofurfural were written as

q

H

p

FALiHpCkr

2=- (2)

where i can be Ir/TiO2, pand q are the ordeobtained at initial rate with respect to furfurconcentration and partial pressure of HThus for Ir/TiO2 the global order is -0,5Because the order with respect t

concentration of furfural is fractional, it idesirable to obtain insight into the kineticvia Langmuir-Hinshelwood models.

Considering that the hydrogenation oaldehydes ,-unsaturated is potentially complex combination of series and parallereactions, can be expected a parallel reactionthat involve the hydrogenation of botfunctional groups of the furfural (C=CC=O), however the results showed fo

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Ir/TiO2 in furfural hydrogenation produces a highly

selectivity towards furfuryl alcohol, being the resultof the presence of SMSI effect on this type ofsupport. For this catalyst studied the selectivitytowards unsaturated alcohol is greater of 98% whichcan be interpreted as the presence of a single site

present (i.e. Ir+ species are preferential).

A typical LangmuirHinshelwood model (LH) of asingle site can be developed to describe thehydrogenation of furfural towards furfuryl alcohol

based on the following assumptions:

1. The adsorption of hydrogen occurs in form

dissociative and competitive with the organicmolecules, due to high H2partial pressure employedand the hydrogenation surface reaction isirreversible.

2. Surface reaction between dissociatively adsorbedhydrogen and adsorbed liquid phase components onthe same site is considered as the rate determiningstep (RDS) while that the adsorption and desorptionare in quasi-equilibrium (QE).

3. The diffusionals steps not are taken in account.

Considering these observations and assuming onbasis of the experimental observation the sequenceof reaction is expressed as:

H2 + 2* = 2H QEFAL + * = FAL QEFAL + 2HFOL + 2* RDSFOL = FOL + * QE

Where is the active site, FAL is furfural, H, FAL,FOL, * are respectively the adsorbed hydrogen,furfural adsorbed, furfuryl alcohol adsorbed and

vacant sites. Based on the basis asumptionsmentioned above, the reaction rate can be written as:

21 HFALr

t

FAL

c

FALk

d

dC

W

Vr qq=-= (3)

Where V/Wc is the ratio of he reaction volume to themass of catalyst CFAL is the concentration offurfural, kr1 is the rate constant for the reaction.

The quasi-equilibrium adsorption /desorptio

process (steps 1, 2 and 4) provide:

vHHH pK qq2/12/1

22= (4)

vFALFALFAL CK qq = (5)

vFOLFOLFOL CK qq = (6)

KFAL, KFOL and KH2 are equilibriumadsorption constants. These expressions ca

be substituted into the RDS step.

22 v

FAL

c

FALpKCKk

d

dC

W

V (7)

Considering that the expression for thfractional surface coverage of sites is:

FOLFALHv qqqq +++=1 (8)

In agreement of the results of hydrogenatioreactions at 363 K with addition of thunsaturated alcohol (furfuryl alcohol) to threaction mixture in the beginning had neffect on the rate of reaction the product termFOL was omitted.

12/12/1 )1(22

KCKq (9)

Finally the reaction rate for model 1 expressed as:

2/1

1

22

22FALr

t

FALdC

W

V=-=

(1

A second LH model was proposed using thassumptions presented for model I, except foassumption 2, which assumes that addition ohydrogen takes place in two steps and iconsidered that the addition of first hydrogeis the RDS. Thus the sequence LH can bexpressed as:

pKCKk1/ 2L

K C K p1 )FA

+ +

1-

v = + FA

FAL L H H

rc d (

L pH-

F

ALFAL F H H

t

=- =r

qr

AL H H

FO

L+ HFO

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Rojas et al1584 CONCLUSIONSFurfural hydrogenation over Ir/TiO2 catalystsreduced to 773 K does not present limitations ofmass transfer to describe a kinetic inadequate

behavior. The solvent affects the selectivity of thereaction due to reactions of acetalization producing2-furaldehyde-diethylacetal. Initial rates describesthat Ir/TiO2 posses a -0,5 order global. A kineticmodel Langmuir-Hinshelwood of a single-site thatinvolves dissociative adsorption of first hydrogen onthe organic molecule as the rate-controlling step

provided the best fit of the experimental data. Thusthermodynamic parameters obtained for this model

demonstrates that the mechanism proposed isconsistent physically and it describes the highselectivity towards the unsaturated alcohol due

principally to SMSI effect.NOTATIONab interfacial area at gasliquid

interfaceac specific surface area of catalystkb mass transfer coefficient for

gas absorption across gas

liquid interfacekc mass transfer coefficient for

hydrogen transport through thebulk liquid

active site.v vacant active site. catalyst effectiveness factor

CFAL bulk liquid phase concentrationof furfural

CFAL-H half-hydrogenated intermediatek apparent rate constantKFAL adsorption equilibrium constant

for single site adsorption offurfural

KH2 adsorption equilibrium constantfor hydrogen

KFOL adsorption equilibrium constantfor furfuryl alcohol

m catalyst loading (density), gcat/L solution

p, q Orders reactionr rate of furfural hydrogenation,

mol/L min

pH2 gas phase partial pressure of

hydrogenrFAL specific rate of furfural

hydrogenation, mol g-1s-1

wc catalyst massRSS Residual sum of squaresACKNOWLEDGEMENTSWe thank to COLCIENCIAS-SENA anDIN-UPTC for the financial support undethe project N 110948925094.REFERENCES[1] GALLEZOT, P., RICHARD, R. Selectivhydrogenation of ,-unsaturated aldehydeCatal. Rev. Sci. Eng, 40, 81-126, 1998.

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