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Catalysis Communications 8 (2007) 2032–2036

On the structure-sensitivity of 2-butanol dehydrogenationover Cu/SiO2 cogelled xerogel catalysts

Stephanie Lambert a,*, Caroline Cellier b, Fabrice Ferauche a,Eric M. Gaigneaux b, Benoıt Heinrichs a

a Laboratoire de Genie Chimique, B6a, Universite de Liege, B-4000 Liege, Belgiumb Unite de Catalyse et Chimie des Materiaux divises, Universite Catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium

Received 9 February 2007; received in revised form 3 April 2007; accepted 4 April 2007Available online 22 April 2007

Abstract

Cu/SiO2 cogelled xerogel catalysts were synthesized by cogelation of tetraethoxysilane (TEOS) and chelates of Cu with 3-(2-amino-ethylamino)-propyltrimethoxysilane (EDAS). The resulting catalysts are composed of metallic crystallites with a diameter of about 3 nmlocated inside microporous silica particles and larger metallic particles with a diameter of 8–30 nm located outside silica network. Cu/SiO2 catalysts were tested for 2-butanol dehydrogenation. This reaction over Cu/SiO2 cogelled xerogel catalysts is a structure-sensitivereaction: large metallic particles located outside silica particles are responsible for the dehydrogenation reaction, while small metallicparticles located inside silica particles do not participate to the reaction.� 2007 Elsevier B.V. All rights reserved.

Keywords: Sol–gel process; Cogelification; Copper catalysts; 2-Butanol dehydrogenation; Structure-sensitive reaction

1. Introduction

The sol–gel cogelation process can be used to synthe-size Pd/SiO2 [1], Ag/SiO2, Cu/SiO2 [2–4], Pd–Ag/SiO2

[5,6] and Pd–Cu/SiO2 [7,8] xerogel catalysts. This sol–gelprocess consists in the use of 3-(2-aminoethylamino)pro-pyltrimethoxysilane (EDAS) to complex Pd, Ag and Cuin an ethanolic solution containing TEOS and aqueousammonia allowing to obtain mono- or bimetallic cogelledcatalysts. As shown previously by 3-D electron tomogra-phy [9], rotating TEM [10] and nitrogen adsorption[3,8], these xerogel catalysts are composed of mono- orbimetallic crystallites with a diameter of about 2–3 nmlocated inside microporous silica particles whose diame-ters are between 10 and 50 nm. Silica particles exhibit amonodisperse microporous distribution centered on apore width of about 0.8 nm. The continuous meso- and

1566-7367/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.catcom.2007.04.004

* Corresponding author. Tel.: +32 4366 4771; fax: +32 4366 3545.E-mail address: stephanie.lambert@ulg.ac.be (S. Lambert).

macroporous distribution of the catalysts is located invoids between particles and between aggregates consti-tuted of those particles. Silica particles are thus basicblocks which constitute aggregates which themselves con-stitute the catalyst pellet. All these highly dispersedcogelled xerogel catalysts have remarkable catalytic char-acteristics, in particular for benzene oxidation over Pd/SiO2 [4], Ag/SiO2 and Cu/SiO2 [2,4] catalysts and for1,2-dichloroethane hydrodechlorination over Pd/SiO2 [2],Pd–Ag/SiO2 [5] and Pd–Cu/SiO2 [7] catalysts. In all previ-ous studies, it was shown that catalytic activity increaseswith metal dispersion and that hydrodechlorination andoxidation reactions are structure-insensitive reactions overmetallic cogelled xerogel catalysts.

Dehydrogenation of methanol [11] and 2-propanol [12]over Cu/SiO2 catalysts is a known structure-sensitive reac-tion example. The aim of this work is to study the behav-iour of Cu/SiO2 cogelled xerogel catalysts with variousmetal dispersions in the case of 2-butanol dehydrogenationand to check if 2-butanol dehydrogenation over thoseunique catalysts is a structure-sensitive reaction or not.

Fig. 1. TEM micrographs of (a) Cu1.0 sample and (b) Cu2.1 sample(350,000·).

S. Lambert et al. / Catalysis Communications 8 (2007) 2032–2036 2033

2. Experimental and methods

Four Cu/SiO2 cogelled xerogel catalysts were examined:Cu1.0, Cu1.5, Cu2.1 and Cu4.5 samples, where Cu is fol-lowed by the wt.% of metal in the sample.

Nitrogen adsorption–desorption isotherms were mea-sured at 77 K on a Fisons Sorptomatic 1990 after outgas-sing for 24 h at ambient temperature. After a 2-houtgassing at ambient temperature, mercury porosimetrymeasurements were performed with sample monolithsusing a manual porosimeter from 0.01 to 0.1 MPa and aCarlo Erba Porosimeter 2000 from 0.1 to 200 MPa. Thesize of copper particles was examined by transmission elec-tron microscopy (TEM) with a Philips CM100 microscope.The size of the copper particles was also examined by X-raydiffraction (XRD) on a Siemens D5000 goniometer usingthe Cu Ka line. Copper dispersion in Cu/SiO2 catalystswas determined from O2 chemisorption at 30 �C on aMicrometrics device. Some details for the catalyst prepara-tion and for the characterization techniques are presentedin [4].

The four Cu/SiO2 (Cu1.0, Cu1.5, Cu2.1 and Cu4.5 sam-ples) catalysts were tested for 2-butanol dehydrogenation,which was carried out at atmospheric pressure in a fixedbed flow-microreactor with 0.25 g of catalyst pellets sievedbetween 0.315 and 0.500 mm. The reactor feeding consistedin 0.05 mmol/s of helium and 0.005 mmol/s of 2-butanol.Reaction temperatures were raised from 100 �C to 300 �Cby steps of 20 �C. Concentrations of reactant (2-butanol)and products (2-butanone, butene) were measured at thereactor outlet by on-line gas chromatography (HewlettPackard 5880A) with a ParaPLOT Q column(25 m · 0.32 mm; 150 �C). The reproductibility of theresults was found to be better than 10%.

3. Results and discussion

Only metallic copper particles (Cu0) and no copperoxide particles are observed by XRD in all Cu/SiO2

cogelled xerogel catalysts. Table 1 gives copper particle sizedetermined by TEM, by chemisorption of O2 and by XRD.The results from these three characterization techniques

Table 1Copper particle size and copper dispersion

Catalyst Copperloading (wt%)

rbut (240 �C)(mol/s kgCu)

rbut (260 �C)(mol/s kgCu)

TEM

dTEM1 (10�9

±0.01 ±0.01 ±0.1

Cu1.0 1.00 0.005a 0.06 3.2Cu1.5 1.53 0.03 0.18 3.1Cu2.1 2.14 0.32 0.56 2.8Cu4.5 4.52 1.02 1.21 2.4

rbut: consumption rate of 2-butanol; dTEM1, dTEM2: mean diameters of small anmeasured by chemisorption; dchem: mean diameter derived from dispersion D;

a Not significantly different of zero.b Not observed.c Not measurable.

clearly show that the greatest dispersions, and so the small-est copper particles are obtained for low copper loadings.Furthermore, the number of large copper particlesincreases with the copper loading. Finally, TEM resultsin Table 1 and Fig. 1a and b indicate that beyond a valueof copper loading of 1 wt.%, copper samples exhibit metalparticles distributed in two families of different sizes: small

O2 chemisorption XRD

m) dTEM2 (10�9 m) dchem (10�9 m) D (%) dXRD (10�9 m)±20% ±1 ± 2 ±20%

–b 4 28 –c

8 5 20 1215 6 16 1830 9 12 24

d large metal particles, respectively measured by TEM; D: metal dispersiondXRD: mean size of metal particles estimated from X-ray line broadening.

0.001

0.01

0.1

1

10

0.1 1 10 100 1000

pore size (nm)

cum

ulat

ive

volu

me

(cm

3 /g)

Fig. 2. Pore size distribution of samples Cu1.0 (n), Cu1.5 (e), Cu2.1 (m)and Cu4.5 (s).

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8

Time (h)

Con

vers

ion

(%)

Cu 4.50

Cu 2.15

Cu 1.50

Cu 1.00

160˚C 180˚C 200˚C 220˚C 240˚C 260˚C 280˚C 300˚C

Fig. 3. Conversion of 2-butanol as a function of time for (n) Cu1.0sample, (e) Cu1.5 sample, (m) Cu2.1 sample, and (s) Cu4.5 sample.

2034 S. Lambert et al. / Catalysis Communications 8 (2007) 2032–2036

crystallites between 2.4 and 3.2 nm and large crystallitesbetween 8 and 30 nm.

Fig. 2 shows the evolution of the cumulative volumedistributions over the entire pore size range for the fourCu/SiO2 cogelled xerogel catalysts. These curves wereobtained from nitrogen adsorption–desorption isothermsand mercury porosimetry measurements by applying acombination of various methods to their respective valid-ity domains and by adding the porous volume distribu-tions corresponding to each domain as proposed by Alieet al. [13,14]. All Cu/SiO2 catalysts are characterized bya steep volume increase around 0.8 nm followed by a pla-teau. In the range of meso and macropores, one observesthat all samples exhibit a broad distribution. These xero-gels then contain micropores (width <2 nm), mesopores(2 nm < width < 50 nm) and macropores (width > 50 nm).It has been shown in previous papers [15,16] that this veryparticular structure of those catalysts allows to avoid dif-fusional limitations. Indeed, in Cu/SiO2 cogelled xerogelcatalysts, in order to reach active sites, reactants must firstdiffuse through large pores located between aggregates ofSiO2 particles and then through smaller pores betweenthose elementary particles inside the aggregates. Finally,they diffuse through micropores inside the silica particles.It was shown that there is no problem of mass transfer ateach of the three levels. The Weisz modulus, which com-pares the observed reaction rate to the diffusion rate [17],has a value much smaller than one at the three discretelevels (macroscopic pellet, aggregate of silica particles, ele-mentary silica particle containing an active metallic crys-tallite), even in the strongest case (Cu4.5 sample and260 �C). So there are no pore diffusion limitations andthe observed rate rbut is equal to the intrinsic rate of thechemical reaction.

Fig. 3 shows conversion of 2-butanol as a function oftime at each temperature for the four Cu/SiO2 catalysts.With these experimental conditions, 2-butanol is selectivelydehydrogenated in 2-butanone and an insignificant amountof butene is produced. For all catalysts, the conversionincreases with temperature and with metal loading. How-ever, for Cu2.1 and Cu4.5 samples, it is observed that a

maximum in activity is obtained when the temperaturereached 260 �C. Moreover, Cu4.5 sample deactivatesstrongly above this temperature. To show the stability ofcatalytic activity over time, 2-butanol conversion is mea-sured over Cu4.5 sample at the temperature of 260 �C dur-ing 10 h. After 10 h, 2-butanol conversion decreases from90% to 85%. Furthermore, for Cu2.1 sample, when thetemperature decreases from 260 �C to 200 �C, and thenincreases again from 200 �C to 260 �C, 2-butanol conver-sion preserves the same value at each temperature andthe selectivity towards 2-butanone is always equal to 100%.

In order to explain the results presented from conversionvalues in Fig. 3, it is necessary to calculate intrinsic ratesfor 2-butanol dehydrogenation. The consumption rate of2-butanol, rbut, is calculated from the plug flow reactormodel, after checking that rbut = kbut [Butanol]. So rbut iswritten as follows:

rbut ¼ � F but

W

� �lnð1� f Þ; ð1Þ

where rbut is the consumption rate of 2-butanolðmmol kg�1

Cu s�1Þ, Fbut is the molar flowrate of 2-butanolat the reactor inlet (mmol s�1), W is the copper mass insidethe reactor (kgCu) and f is the 2-butanol conversion (-). Forall Cu/SiO2 cogelled xerogel catalysts, rbut is calculatedfrom Eq. (1) at temperatures of 240 �C and 260 �C (Table1) because above 260 �C, Cu4.5 sample deactivatesstrongly. In Fig. 4a, rbut is presented as a function of cop-per dispersion. It is observed that rbut increases when thecopper dispersion values decreases.

From all the catalytic results, it can be observed thathighly dispersed copper cogelled xerogel catalysts, asCu1.0 sample in which small copper particles only areobserved by TEM (Table 1 and Fig. 1a), are almost inac-tive for 2-butanol dehydrogenation, while poor dispersedcopper cogelled xerogel catalysts, as Cu2.1 and Cu4.5 sam-ples in which small and large copper particles are observed(Table 1 and Fig. 1b) are very active for this reaction. So asto better understand this observation, the total surface area

0.001

0.01

0.1

1

10

0 10 20 30

Copper dispersion (%)

rbu

t (m

ol/s

kg C

u)

0.01

0.1

1

10

0 10000 20000 30000 40000 50000 60000

S2 (m2/kgCu)

rbu

t (m

ol/s

kg C

u) Cu4.5Cu2.1

Cu1.5

a

b

Fig. 4. Consumption rate of 2-butanol at T = 240 �C (m) and T = 260 �C(D) as a function of copper dispersion for all Cu/SiO2 cogelled xerogelcatalysts. (b) Consumption rate of 2-butanol at T = 240 �C (m) andT = 260 �C (D) as a function of the total surface area developed by largeparticles by total mass of copper, S2, for Cu1.5 sample, Cu2.1 sample, andCu4.5 sample.

S. Lambert et al. / Catalysis Communications 8 (2007) 2032–2036 2035

developed by large particles by total mass of copper, S2

(m2/kgCu), has been calculated for Cu1.5 sample, Cu2.1sample, and Cu4.5 sample

S2 ¼ 4pdTEM2

2

� �2 n2

m1 þ m2

� �

¼ 4p.Cuv2

dTEM2

2

� �2 m2

m1 þ m2

� �

¼ 4p.Cuv2

dTEM2

2

� �2 1m1

m2þ 1

!

¼ 4p.Cuv2

dTEM2

2

� �21

n1v1

n2v2þ 1

!

¼ 4p

.Cu43p dTEM2

2

� �3� � dTEM2

2

� �21

n1d3TEM1

n2d3TEM2

þ 1

0B@

1CA

¼ 6d2TEM2

.Cun1

n2d3

TEM1 þ d3TEM2

� � ; ð2Þ

where dTEM1 and dTEM2 are the mean diameters of smalland large metal particles, respectively measured by TEM(m) (Table 1); m1 and m2 are the masses per kg of catalystof small and large copper particles present in Cu/SiO2 cat-alysts, respectively (kgCu/kgcata); .Cu is the copper density(8920 kgCu/m3) [18]; v1 and v2 are the volumes of one smalland one large copper particles, respectively (m3); n1 and n2

are the numbers per kg of catalyst of small and large par-

ticles present in Cu/SiO2 catalysts respectively ðkg�1cataÞ. The

ratio n1/n2 is calculated from [18,19]

n1

n2

¼ d3TEM2 � dchemd2

TEM2

dchemd2TEM1 � d3

TEM1

ð3Þ

In Fig. 4b, rbut is represented as a function of S2 forCu1.5 sample, Cu2.1 sample, and Cu4.5 sample. It isobserved that S2 decreases when the copper loadingincreases because the size of large particles, dTEM2,increases strongly (Table 1). Although S2 decreases whenthe copper loading increases, rbut always increases withthe copper loading. As a consequence, 2-butanol dehydro-genation is a structure-sensitive reaction: the largest copperparticles are the most active ones. Those results are inagreement with previous studies mentionning differencesof alcohol conversion related to the state and distributionof CuO/Cu species [11,12,20].

4. Conclusions

The sol–gel cogelation process applied to the synthesisof Cu/SiO2 xerogel catalysts allows to stabilize copper spe-cies by the silica network and to obtain very highly dis-persed Cu/SiO2 catalyst when the copper loading is lowerthan 2 wt.%. In many chemical reactions such as chlori-nated wastes hydrodechlorination, volatile organic com-pounds oxidation, very highly dispersed Cu/SiO2 cogelledxerogel catalysts have shown a great activity. Furthermore,these reactions over Cu/SiO2 cogelled xerogel catalysts arestructure-insensitive reactions with regard to the ensemblesize concept. In the present study, the cogelation methodhas led both highly and poorly dispersed Cu/SiO2 catalystsallowing to clearly show the structure-sensitive character of2-butanol dehydrogenation on such catalysts: while largecopper particles (>8 nm) catalyze the reaction, small ones(2–3 nm) are completely inactive.

Acknowledgements

S.L. is grateful F.R.S.-F.N.R.S. for a postdoctoral re-searcher position. The authors also thank the Fonds deBay, the Fonds de Recherche Fondamentale et Collective,the Ministere de la Region Wallonne and the Interuniver-sity Attraction Pole (IAP) for financial support. Theinvolvement of both laboratories in the Network of Excel-lence FAME of the European Union sixth framework pro-gram is also acknowledged.

References

[1] B. Heinrichs, F. Noville, J.-P. Pirard, J. Catal. 170 (1997) 366.[2] S. Lambert, C. Cellier, P. Grange, J.-P. Pirard, B. Heinrichs, J. Catal.

221 (2004) 335.[3] S. Lambert, C. Alie, J.-P. Pirard, B. Heinrichs, J. Non-Cryst. Solids

342 (2004) 70.[4] S. Lambert, C. Cellier, E.M. Gaigneaux, J.-P. Pirard, B. Heinrichs,

Catal. Commun. 8 (2007) 1244.

2036 S. Lambert et al. / Catalysis Communications 8 (2007) 2032–2036

[5] B. Heinrichs, P. Delhez, J.-P. Schoebrechts, J.-P. Pirard, J. Catal. 172(1997) 322.

[6] S. Lambert, F. Ferauche, B. Heinrichs, N. Tcherkassova, J.-P. Pirard,C. Alie, J. Non-Cryst. Solids 352 (2006) 2751.

[7] S. Lambert, F. Ferauche, A. Brasseur, J.-P. Pirard, B. Heinrichs,Catal. Today 100 (2005) 283.

[8] S. Lambert, C. Gommes, C. Alie, N. Tcherkassova, J.-P. Pirard, B.Heinrichs, J. Non-Cryst. Solids 351 (2005) 3839.

[9] C.J. Gommes, K. de Jong, J.-P. Pirard, S. Blacher, Langmuir 21(2005) 12378.

[10] B. Heinrichs, J.W. Geus, S. Lambert, J.-P. Pirard, J. Catal. 241 (2006)229.

[11] E.D. Guerreiro, O.F. Gorriz, J.B. Rivarola, L.A. Arrua, Appl. Catal.A 165 (1997) 259.

[12] E. Agoretta, M. Salvador, J. Santamaria, A. Monzon, in: C.H.Bartholomew, J.B. Butt (Eds.), Studies in Surface Science andCatalysis: Catalyst Deactivation, vol. 68, Elsevier Science PublishersB.V., Amsterdam, 1991, p. 391.

[13] C. Alie, R. Pirard, A.J. Lecloux, J.-P. Pirard, J. Non-Cryst. Solids 246(1999) 216.

[14] C. Alie, R. Pirard, A.J. Lecloux, J.-P. Pirard, J. Non-Cryst. Solids 285(2001) 135.

[15] Bi. Heinrichs, J.-P. Schoebrechts, J.-P. Pirard, AIChE J. 47 (2001)1866.

[16] C. Alie, F. Ferauche, A. Leonard, N. Tcherkassova, B. Heinrichs, M.Crine, P. Marchot, E. Loukine, J.-P. Pirard, Chem. Eng. J. 117 (2006)13.

[17] G.F. Froment, K.B. Bischoff, Chemical Reactor Analysis and Design,Wiley, New-York, 1990.

[18] G. Bergeret, P. Gallezot, in: G. Ertl, H. Knozinger, J. Weitkamp(Eds.), Handbook of Heterogeneous Catalysis, vol. 2, Wiley-VCH,Weinheim, 1997, p. 439.

[19] S. Lambert, J.-F. Polard, J.-P. Pirard, B. Heinrichs, Appl. Catal. B 50(2004) 127.

[20] J. Cunnigham, G.H. Al-Sayyed, J.A. Cronin, J.L.G. Fierro, C. Healy,W. Hirschwald, M. Ilyas, J.P. Tobin, J. Catal. 102 (1986) 160.