Influence of the active phase loading in carbon supported molybdenum–cobalt catalysts for hydrodeoxygenation reactions

Microporous and Mesoporous Materials 56 (2002) 279–290 www.elsevier.com/locate/micromeso

Influence of the active phase loading in carbon supported molybdenum–cobalt catalysts for hydrodeoxygenation reactions Maria Ferrari, Bernard Delmon, Paul Grange

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Unit e de catalyse et chimie des mat eriaux divis es, Universit e catholique de Louvain, Croix du Sud, 2 bte 17, B-1348 Louvain-la-Neuve, Belgium Received 3 January 2002; received in revised form 14 June 2002; accepted 5 August 2002

Abstract The influence of the active phase loading on the physico-chemical properties and on the hydrodeoxygenation (HDO) activity of carbon supported catalysts was studied. Four molybdenum catalysts (Mo), with a MoO3 loading ranging from 6 to 15 wt.% and four cobalt promoted molybdenum catalysts (CoMo), with a total oxide content (MoO3 þ CoO) ranging from 7.2 to 18 wt.% (with constant Co/(Co þ Mo) ratio), were prepared and characterized. Both series of samples exhibit a non-uniform distribution of the active phase between the inside and outside of the carbon particles. The dispersion decreases with increasing metal loading. Cobalt seems to be mainly impregnated on the external particle surface and to be responsible for a partial remobilization of molybdenum which migrates towards the external part of the particles where cobalt is impregnated. Bulk cobalt oxide or bulk molybdenum oxide were never detected. For MoO3 contents higher than 6 wt.%, micropore blocking takes place. For a MoO3 content lower than 6 wt.%, most of the molybdenum is located in the microporous structure. It is fairly well dispersed and relatively strongly bound to the support. The CoMo catalysts were tested in HDO reactions. Due to the decrease of the dispersion, the catalytic activity does not increase proportionally with the amount of active phase. The most important parameter for the reaction of ketonic and ester groups is the dispersion of the active phase. Ó 2002 Elsevier Science Inc. All rights reserved. Keywords: Activated carbons; Active phase loading; Molybdenum; Cobalt; Hydrodeoxygenation

1. Introduction Carbon supported sulfided molybdenum-based catalysts seem attractive for HDS and hydrotreating, especially for slurry processes. The ab-

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Corresponding author. Tel.: +32-10-47-36-48; fax: +32-1047-36-49. E-mail address: [email protected] (P. Grange).

sence of acidity of the support makes them potentially useful for special hydrotreating processes [1,2], especially in the case of oils obtained by pyrolysis of biomass. Reports indicated that such catalysts were as active, and even more active, than similar ones supported on alumina and silica [3]. In particular, smaller amounts of molybdenum gave better catalysts with activated carbons than with silica or alumina supports. But the performances of

1387-1811/02/$ - see front matter Ó 2002 Elsevier Science Inc. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 2 ) 0 0 4 9 2 - 4

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carbon-, silica- and alumina-supported catalysts tended to the same limit when the amount of molybdenum was increased. These results are explained by the authors in terms of interactions between active phase and support, dispersion of molybdenum and an easy sulfidation process during activation. The high surface area (SA) of activated carbons allows to obtain a well dispersed and completely sulfided active phase at low metal loading, in spite of the weakness of the interactions between molybdenum and the carrier; sintering and agglomeration are favored with increasing metal content. With alumina, strong interactions occur with molybdenum oxide. The transformation of the oxide species to the sulfide phase is difficult at low metal loading, the fraction of sulfidable molybdenum increases at higher loading, and a good dispersion is maintained. In a previous work [4] we have shown that molybdenum is not homogeneously distributed between the inside and outside of the carbon particles in catalysts supported on activated carbons prepared with 15 wt.% of MoO3 and 3 wt.% of CoO. Moreover, like on silica supports, the addition of cobalt seems to be responsible for a partial remobilization of molybdenum. This leads to an accumulation of molybdenum at the external surface of the carbon particles. But bulk MoO3 and CoOx oxides were never detected. In spite of their better performances compared to silica- or alumina-supported catalysts at low active phase loading, those supported on carbon are still not sufficiently active for applications. To take full advantage of the positive characteristics of activated carbon, like weak acidity and low coking tendency, it is crucial to obtain high activity per metal atom, also at high active phase loading. To explore the potential of activated carbon as catalyst support, we have studied a series of four molybdenum catalysts (Mo) with a MoO3 loading ranging from 6 to 15 wt.% and four cobalt-promoted molybdenum catalysts (CoMo), with a total oxide content (MoO3 þ CoO) ranging from 7.2 to 18 wt.% (with constant Co/(Co þ Mo) ratio). The textural and structural properties were characterized. A special objective was to better understand the interactions existing between the two metals when impregnated in different amounts

and how these interactions affected the dispersion and the distribution. After sulfidation, the bimetallic catalysts were tested in hydrodeoxygenation (HDO) reactions. The context was the upgrading of the liquids produced from the pyrolysis of biomass [5].

2. Experimental 2.1. Catalyst preparation The support was a commercial activated carbon, BKK-100 (particle size diameter between 0.3 and 0.5 mm, specific SA 1200 m2 g1 , total pore volume (Tot PV) 0.9 cm3 g1 ). This carbon is obtained by thermal treatment of bituminous coal. This explains the presence of inorganic impurities, mainly calcium, iron and magnesium (0.5, 0.8 and 0.5 wt.%, respectively). The catalysts were prepared following the incipient wetness method. Prior to impregnation, the activated carbon was dried under argon flow at 130 °C overnight, in order to eliminate water physisorbed at the surface. Aqueous solutions of the precursors, namely ammonium heptamolybdate ((NH4 )6 Mo7 O24  4H2 O) and cobalt nitrate (Co(NO3 )2  6H2 O), both from Merck were used. Molybdenum was impregnated first and cobalt second. Different amounts were used to reach the desired active phase loading, expressed as oxide weight percentage; the atomic ratio Co/(Co þ Mo) was kept constant and equal to 0.28. After each impregnation, the sample was dried under argon flow at 130 °C overnight and calcined at 400 °C for 3 h. The catalysts are denoted as Mox (samples containing only molybdenum) and Coy Mox (samples containing molybdenum and cobalt); x and y are the molybdenum and cobalt content expressed as weight percentage of MoO3 and CoO, respectively. The real composition of the catalysts was checked by ICP analysis. The experimental values, expressed as metal weight percentage, are close to the calculated ones, reported in parentheses (Table 1). As regards the drying step after impregnation, it is known that carbon inertness to oxygen is restricted to temperatures below 150–200 °C [6]. Therefore a thermal treatment at 400–600 °C in an

M. Ferrari et al. / Microporous and Mesoporous Materials 56 (2002) 279–290 Table 1 Chemical composition of the catalyst: calculated active phase loading (oxide weight percentage (wt.%), molybdenum and cobalt content (metal weight percentage (wt.%)) determined by ICP analysis and calculated (in parenthesis) Catalyst

Calculated oxide loading (wt.%)

Mo (wt.%)

Co (wt.%)

Mo6 Mo9 Mo12 Mo15

6.0 9.0 12.0 15.0

3.9 5.8 7.7 10.4

– – – –

Co1.2Mo6 Co1.8Mo9 Co2.4Mo12 Co3Mo15

7.2 10.8 14.4 18.0

3.9 5.5 7.6 8.8

1.0 1.3 1.8 2.2

(4) (6) (8) (10)

(1) (1.4) (1.9) (2.4)

inert gas (nitrogen or argon) has to be carried out to decompose the precursor salts [7]. This allows a better dispersion through the pores, even if a partial reduction of the oxides can constitute a potential drawback [8]. In preliminary experiments we have checked that reduced molybdenum species are formed only at temperatures higher than 500 °C. This justifies the procedure described above. The catalysts in the oxide state were characterized by ICP chemical analysis, N2 physisorption, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). 2.2. Nitrogen physisorption Nitrogen physisorption measurements were performed at 77 K ()196 °C) using a Micromeritics ASAP 2000M apparatus. The sample (100 mg) was previously outgassed overnight at 200 °C at a pressure of 0.1 Pa. The adsorption isotherms were recorded in the 106 < P =P0 < 0:99 pressure range, in order to characterize the micropore and mesopore region. Specific total SAs were calculated using the Langmuir equation for monolayer adsorption (0:01 < P =P0 < 0:05). The Tot PV was estimated by the amount of nitrogen adsorbed at P =P0 ¼ 0:985  0:99. The micropore volume (micro PV) and the micropore size distribution were calculated form the Horvath–Kawazoe model assuming a slit-like geometry [9] or by the Dubinin– Astakov method [10]. The difference between the

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micropore volume (micro PV) and the Tot PV will be referred to as mesopore volume. 2.3. X-ray photoelectron spectroscopy The analyses were performed on a SSI X-probe (SSX 100/206) spectrometer from Surface Science Instruments equipped with a monochromatized micro-focused AlKa X-ray source (1486.6 eV), operating at 10 kV and 12 mA. The analysis pass energy was set at 150 eV and the analyzed area 0.5 mm2 . In these conditions the energy resolution determined by the Au4f7=2 full width at half maximum of gold was 1.4 eV. The general spectrum and the spectra of the peaks of C1s , O1s , Mo3d and Co2p were recorded. Binding energies were calculated by reference to the binding energy of the C–(C–H) component of C1s fixed at 284.2 eV. Intensity ratios were converted into atomic concentration ratios (Mo=C  100 and Co=C  100) using the sensitivity factors given by the manufacturer. All the peaks were decomposed using an iterative least-square computer program and a non-linear baseline [11], the curve being taken as 85% Gaussian and 15% Lorenzian. The interval between the Mo3d5=2 and Mo3d3=2 peaks was fixed at )3.15 eV. The signal to noise ratio for Co2p3=2 is relatively weak. Two kinds of information were deduced from the XPS results, namely: (i) whether the supported oxides are equally distributed between the internal and external surface of the support particles; (ii) and whether the active phase is well dispersed in the form of a monolayer, or forms aggregates. To evaluate the first aspect the analyses were made with both non-ground (particle diameter between 0.3 and 0.5 mm, (named ‘‘not ground’’ afterward) and finely ground samples (‘‘ground’’). The atomic ratios obtained for the non-ground samples are representative of the active phase concentration on the external grain surface, whereas for the ground samples it assumes values intermediate between this concentration and that in the inside of the particles. The difference between the atomic ratios for the non-ground and ground samples indicates whether a preferential deposition of the active phase at the external surface of the grains occurs. It should be noted

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that when the non-ground samples were analyzed, some accidental particle breaking or ‘‘crushing’’ could not be avoided, due to the use of a press to prepare the troughs on which the samples were held. However, significant differences are observed, and the results can be considered, at least qualitatively, as representative of the difference in metal concentration in the external and internal regions of the particles. To evaluate the second aspect, namely the dispersion of the active phase oxides, it is necessary to compare the experimental XPS intensities of the different elements to the ICP analyses and to use theoretical models corresponding to the geometry of the active phase––support system. The model used was that of Kerkhof and Moulijn [12]. The support is represented by sheets (particularly adequate for carbons), and the active phase as cubic crystallites of dimension c. If the supported metal phase is present as isolated or polymerized monolayer species (c ! 0), the XPS metal-to-support intensity ratio (Im =Is ) will be proportional to the bulk metal-to-support atomic ratio (m/s), as long as the full monolayer coverage is not reached. The ratio Im =Is can be expressed in the following form: Im =Is ¼ ðm=sÞðDm =Ds Þðrm =rs Þðbs =2Þ  ½1 þ expðbm Þ =½1  expðbm Þ

where bs ¼ t=ks and bm ¼ t=km ðks and km are the support and the metal photoelectron escape depths, as reported by Penn [13]; t is the thickness of support ‘‘sheets’’). The thickness of the sheets can be estimated from the density (qs ) and the SA (S) of the support: t ¼ 2=qs S. Ds and Dm are the detector efficiencies calculated as reported by Weng et al. [14]: D ¼ Ea ðEa =Ek Þ

n

n ¼ 0:5594 þ 0:6072 logðEa =Ek Þ þ 0:3309 log2 ðEa =Ek Þ where Ek is the kinetic energy of the element and Ea is the energy of the analyzer. rs and rm are the cross sections for photoelectron emission of the support and metal, respectively, as reported by Scofield [15].

2.4. X-ray diffraction XRD analyses were performed on a Siemens D500 diffractometer using the Ka1;2 emission of Cu ) for 2H angles varying from 5 to 60°. (k ¼ 1:54 A The scan rate was 0.03° min1 corresponding to a step size of 0.05° and a step time of 100 s (in some cases a step time of 30 s was also used). Samples were not ground before analysis, they were analyzed in the form of small grains (diameter size between 0.3 and 0.5 mm). 2.5. Catalytic tests The catalytic tests were performed in a batch reactor, using the non-ground catalysts. We report here after a summary of the procedure for the sulfidation of the catalyst, the reaction conditions, the composition of the mixture of the oxygenated model molecules and the reaction pathways, which were fully described in a previous paper [4]. Before the test, 1.5 g of catalyst were activated at atmospheric pressure, with a mixture of 15 vol.% of H2 S in H2 at 400 °C, for 3 h. The reaction tests were performed in a 570 ml stainless steel batch reactor, at 280 °C and 7 MPa pressure of hydrogen, under vigorous agitation. Ten samples were withdrawn at 0, 10, 20, 30, 40, 50, 70, 90, 120 and 150 min after the moment when the reaction conditions were reached. The catalytic test consists of the simultaneous reaction of a mixture of model compounds selected on the basis of bio-oils characterization. The composition was as follows: 4-methylacetophenone (MA) (0.22 mol l1 ), ethyldecanoate (ED) (0.15 mol l1 ), guaiacol (GUA) (0.24 mol l1 ); pxylene was used as solvent. Carbon disulfide (0.025 mol l1 ) was added as a precursor of hydrogen sulfide. The HDO pathways of the model compounds are recalled in Fig. 1. MA can be hydrogenated to 4-ethylmethylbenzene; the intermediates a,4-dimethylbenzylalcohol and 4-methylstyrene were never detected in our reaction conditions. Ethyldecanoate can react following three pathways. The first (1) is the hydrogenation to decanol and ethanol, followed by decanol dehydration to give olefins (decene) that are hydrogenated to de-

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283

Fig. 1. HDO pathways of 4-methylacetophenone, ethyldecanoate and GUA.

cane. The second (2) is the direct decarboxylation to produce nonane. The carboxylic acid can also be formed via a de-esterification reaction (3). This acid is an intermediate product, it can follow both the hydrogenation (1) and/or the decarboxylation (2) pathways. The decarboxylation selectivity is calculated as the ratio between the concentration of nonane (further indicated as C 9) and the sum of the concentration of all the hydrogenated and decarboxylated products (nonane þ decane þ decene þ decanol): Sdecarb ¼

Cnonane  100 Cnonane þ Cdecane þ Cdecene þ Cdecanol

Guaiacol can be demethylated to give catechol (breaking of the O-methyl bond) which can be transformed into phenol with the hydrogenolysis of the CAromatic –O bond (dehydroxylation). Benzene, cyclohexene and cyclohexane can be obtained from phenol (dehydroxylation reaction). Phenol can also be directly formed from GUA by a demethoxylation reaction (hydrogenolysis of the CAromatic – O bond). The ratio between the concentration of the products which undergo the hydrogenolysis of the CAromatic –O bond (Phenol ¼ phenol þ

benzene þ cyclohexene; cyclohexaneÞ and catechol is calculated as follows: Phenol Cphenol þ Cbenzene þ Ccyclohexene þ Ccyclohexane ¼ Catechol Ccatechol

For the evaluation of the catalytic activity in the reaction of the different molecules, pseudo firstorder kinetics are assumed as an approximation [16]. Actually, apparent first-order is generally valid for most hydrotreating reactions, including those with model compounds [17]. The rate constants were calculated according to the kinetic equations used by Gevert et al. [18]:  ln

Ci ¼ kWf ðt=V Þ C0

where f ðt=V Þ is n X ti  ti1 f ðt=V Þ ¼ Vi i¼1 Ci and C0 are the reactant concentration in the sample at time i and at the initial time 0; k is the pseudo first-order rate constant (ml min1 g1 ); W is the catalyst weight (g); n corresponds to the numbering of the sample taken; t is the time (min);

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Vi is the solution volume (ml) left after the ith sample. The function (t=V ) takes into account that the reactant volume decreases after each sampling. The experimental values follow apparent firstorder kinetics at moderate conversion and they fit the equation used. The results reported correspond to those obtained from the initial reaction rate. The error affecting the experimental values of the pseudo first-order rate constant can be estimated at 10%.

3. Results 3.1. Characterization of fresh catalysts 3.1.1. Textural characteristics The results of the textural characterization of the activated carbon used as support, Mox and Coy Mox samples are reported in Table 2. The same results are presented in Fig. 2. The SA (right-hand scale), the Tot PV, the micro PV (calculated with the Dubinin–Astakov method) and the difference between Tot PV and micro PV, namely meso PV (left-hand scale) have been plotted as a function of the amount of molybdenum oxide. The textural characteristics of the mono- and bimetallic samples do not significantly differ. This is logical as cobalt oxide represents only 20 wt.% of the total oxide amount. The micro- and nonmicroporous volumes equally contribute to the

Table 2 SA, Tot PV, micro PV (calculated with the Dubinin–Astakov method) and the difference between Tot PV and micro PV, namely meso PV, for the activated carbon used as support, Mox and Coy Mox samples SA (m2 g1 )

Tot PV (cm3 g1 )

micro PV (cm3 g1 )

D volume (cm3 g1 )

Support Mo6 Mo9 Mo12 Mo15

1220 1220 1120 1090 980

0.92 0.89 0.82 0.82 0.74

0.47 0.46 0.44 0.42 0.39

0.45 0.43 0.38 0.40 0.35

Co1.2Mo6 Co1.8Mo9 Co2.4Mo12 Co3Mo15

1180 1110 1020 900

0.88 0.85 0.78 0.70

0.44 0.42 0.39 0.34

0.44 0.43 0.39 0.36

Fig. 2. Textural characteristics of Mox and Coy Mox samples. Left-hand scale: total pore volume, Tot PV (cm3 g1 ). Micropore volume, micro PV (cm3 g1 ). Mesopore volume, meso PV (cm3 g1 ). Right-hand scale: surface area, SA (m2 g1 ).

total porosity of the support and of the impregnated samples, both Mox and Coy Mox . The SA and the pore volume decrease with increasing active phase loading. On the basis of 2 structural data (SA of MoO3 unit cell ¼ 15–25 A  and thickness of MoO3 monolayer ¼ 5 A [19]), the volume of molybdenum oxide can be estimated between 0.02 and 0.03 cm3 g1 , for a sample impregnated with 6 wt.% of MoO3 . The experimental results agree with this value, account taken of the accuracy. For the samples prepared with metal loadings higher than 6 wt.%, the decrease in the pore volume (mainly of micropores) is always higher than the one that can be theoretically calculated. These results indicate that molybdenum oxide is mainly located at the mouth of the micropores and in the mesopores, causing partial blocking of the microporous structure. 3.1.2. X-ray photoelectron spectroscopy The binding energies values of Mo3d5=2 and Co2p3=2 levels recorded for Mox and Coy Mox samples are reported in Table 3. It can be observed that the BE values for Mo and Co levels for the samples with different active phase loadings are very similar.

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Table 3 Binding energy values for Mo3d5=2 and Co2p3=2 levels recorded for the support and for Mox and Coy Mox samples Catalyst

Mo3d5=2 (eV)

Co2p3=2 (eV)

Support Mo6 Mo9 Mo12 Mo15

– 232.6 232.5 232.6 232.3

– – – – –

Co1.2Mo6 Co1.8Mo9 Co2.4Mo12 Co3Mo15

232.3 232.6 232.3 232.3

781.3 782.0 781.4 781.5

The C1s peak was taken as reference at 284.2 eV. The interval between Mo3d5=2 and Mo3d3=2 peaks was fixed at )3.15 eV. The signal to noise ratio for Co2p3=2 is relatively weak.

The first set of results concerns possible differences in composition between the external and internal surface of carbon grains, as detected by comparison of the non-ground and ground samples. Whatever the metal loading, the metalto-carbon atomic ratios for Mo and MoCo for the non-ground samples are higher than for the ground samples (Figs. 3 and 4). As previously reported [4], this shows an unequal distribution of the active phase between the inside and the outside of the carbon grains. This is observed even at low molybdenum and cobalt contents. The Mo/C ratios increase with the metal loading for both Mo and CoMo series (Fig. 3) but less

Fig. 3. Mo/C atomic ratio, for non-ground and ground samples: Mo6, Mo9, Mo12, Mo15 and Co1.2Mo6, Co1.8Mo9, Co2.4Mo12, Co3Mo15.

Fig. 4. Co/C atomic ratio, for non-ground and ground samples: Co1.2Mo6, Co1.8Mo9, Co2.4Mo12 and Co3Mo15.

than proportionally for the non-ground samples. Except for Mo6 and Co1.2Mo6 the Mo/C ratios for the non-ground samples are higher for the CoMo than for Mo series. The Mo/C ratios for the ground samples in both series are very similar. The increase of the Co/C ratios with the metal loading is more important for the non-ground than for the ground samples with the consequence that the difference between the Co/C ratios for the non-ground and the ground samples increases with the metal loading (Fig. 4). The results concerning the dispersion of molybdenum are reported in Fig. 5. The experimental

Fig. 5. Experimental molybdenum-to-carbon XPS intensity ratios (IMo =IC ) versus catalyst composition (Mo=C  1000).

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XPS intensity ratios (IMo =IC ), measured for the ground and non-ground Mox samples, are plotted against the catalyst composition determined by ICP analyses (Mo=C  1000). The theoretical line ending at IMo =C ¼ 25 and Mo=C  100 ¼ 20 was calculated according to the method described in the experimental section. As expected, as a consequence of the enrichment in Mo on the external part of the particles, the Mo signal for the nonground samples is larger than predicted by theory. This effect has been reported in the literature [20]. The measurement on the ground samples indicates a dispersion very close to the theoretical monolayer thickness, with the normal tendency to fall below the theoretical line at higher loadings. This is due to the formation of MoO3 agglomerates of a size larger than the mean free path of the photoelectrons.

3.1.3. X-ray diffraction The two broad peaks at 2H ¼ 26° and 2H ¼ 43°, observed on all samples (Fig. 6), correspond to carbon. Peaks at 2H ¼ 30:1–35:5–57:0°( ) are present even for the non-impregnated material: they correspond to FeFe2 O4 (magnetite, JCPDS standard file 19-0629), or Fe2 O3 (JCPDS standard file 39-1346), or MgFe2 O4 (JCPDS standard file 360398) [21]; Mo15 also contains CaMoO4 (JCPDS standard file 29-0351 [21]). The presence of molybdenum oxide or sub-oxide phases was not observed. Coy Mox samples present similar patterns of increasing amounts of cobalt and molybdenum oxides, characterized by the presence of CaMoO4 (in addition to iron oxides). In the region around 2H ¼ 26°, a thinner peak at 2H ¼ 26:4° appears, superimposed to the carbon peak. Although slightly shifted (DH ¼ 0:1°), it could be attributed to CoMoO4 (JCPDS standard file 21-0868) or FeMoO4 (JCPDS standard file 28-0488) [21]. The crystallinity of CaMoO4 is more important in the sample with lower amounts of active phase. A possible explanation could be that the higher concentration of ammonium heptamolybdate used for high active phase loading brings about a dissolution and dispersion of the calcium impurities.

Fig. 6. XRD patterns of carbon BKK, Mo15 and Coy Mox samples with different active phase loading. ¼ CoMoO4 ; ¼ CaMoO4 ; ¼ Fe2 O3 (or FeFe2 O4 or MgFe2 O4 ).

3.2. Catalytic activity Fig. 7 shows that the activity (pseudo-first-order rate constants) for the conversion of MA, ED and GUA increases less than proportionally with the active phase loading and not in a similar way for the three molecules. As regards MA, the same activity is observed with Co2.4Mo12 and Co3Mo15 catalyst, however the rate constant values (5.1 and 5.0, respectively) are higher than those theoretically calculated (3.6 and 4.5, respectively) on the basis of the results obtained with the Co1.2Mo6 catalyst (1.8). The rate constant for ED increases proportionally with the active phase loading up to a metal content of 10.8 wt.%, and then levels off. The selectivity decarboxylation/hydrogenation of ED increases with the conversion, for conversions lower than 4%, but

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Fig. 7. Pseudo-first-order rate constant (k, ml min1 g1 ) for the HDO of MA, ED, GUA obtained with Co1.2Mo6, Co1.8Mo9, Co2.4Mo12 and Co3Mo15 catalysts.

does not change significantly afterwards (0:25 0:05) account taken of the accuracy of the measurements. The experimental values for the GUA rate constant are always lower than those expected on the basis of the results obtained with the Co1.2Mo6 catalyst. Previous results indicated a default in the mass balance in the reaction of GUA [22], due to the formation of a coke precursor. The same effect is observed with the Coy Mox series, with a default decreasing from 50–40% to 19 þ 1% for GUA conversion higher than 15%. The default seems substantially higher for Co1.2Mo6 (31%). Fig. 8 is a plot of the yield in the product fraction including phenol and completely deoxygenated products like benzene, cyclohexene and cyclohexane that we call phenol , against the yield in catechol. The phenol -to-catechol ratio (Ph / Cat), which reflects the HDO activity concerning GUA, increases with the reaction time. Higher Ph /Cat ratios are obtained after the time (about 1 h) that corresponds to the stabilization of the balance default mentioned above. To compare the HDO activity of the four catalysts towards GUA, we have reported the phenol -to-catechol ratio obtained at the same conversion value (Fig. 9) as a function of the active phase content. The highest phenol -to-catechol ratios were obtained with the catalysts containing intermediate amounts of active phase, namely

287

Fig. 8. Influence of the active phase loading (7.2, 10.8, 14.4 and 18.0 wt.%) on the phenol -to-catechol ratio for the conversion of GUA: yields in phenol versus yields in catechol. We characterize by an asterisk (i.e. phenol or Ph ) the product fraction containing phenol and completely deoxygenated products like benzene, cyclohexene and cyclohexane.

Fig. 9. Phenol -to-catechol ratio versus the active phase loading (7.2, 10.8, 14.4 and 18 wt.%) for different values of GUA conversion.

Co1.8Mo9 and Co2.4Mo12. This result, however, is based on results obtained at different reaction times (because of the different activities of the catalysts), and might actually be influenced by both the catalyst composition and activity.

4. Discussion 4.1. Physico-chemical characterization Considering first the case of molybdenum, its introduction in the support causes a decrease of porosity higher than expected. As there is no detectable change in the pore diameter, this effect

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must be explained by pore blocking. The dispersion, as measured by XPS, remains very good. The amount of molybdenum involved in pore blocking is therefore small, compared to that forming a molecular thickness layer. It is presumably limited to the formation of a thin ‘‘top’’ in the narrower mouths. There is always an enrichment in active elements in the near-surface parts of the support. Similar results have been reported in the literature [23] in the case of 3 wt.% copper and zinc supported on carbon, while a quite uniform distribution was observed with molybdenum. Other authors [24] found that for molybdenum contents between 1.5 and 7.1 wt.%, the molybdenum oxide and sulfide phases were homogeneously distributed on the support. However, these results are not directly comparable with those of our experiments because extremely fine carbon black particles (diameter between 20 and 30 nm) were used as supports. Nevertheless, all these results show that MoO3 tends to be very well dispersed on carbon. The difference between the Co/C ratio for the non-ground and the ground samples increases with the active phase loading. This result confirms that cobalt is preferentially impregnated at the exterior of carbon grains [4]. Quite logically, the higher the active phase loading the more cobalt is preferentially deposited near the particle surface. Except for Mo6 and Co1.2Mo6, the difference between Mo/C atomic ratios for the non-ground and ground samples are higher for CoMo than for Mo. The interpretation is that the interactions between molybdenum and carbon are weaker than between molybdenum and cobalt. When cobalt is added after molybdenum, it is preferentially impregnated on the external particle surface as in the case that it is deposited first [4]. The enrichment in molybdenum of the outer part of the particles is likely due to a remobilization process (partial re-dissolution and migration). In addition, small amounts of cobalt-molybdenum mixed oxides can be formed due to the interaction between the two oxides [25,26]. But the major part of molybdenum and cobalt is highly dispersed. Molybdenum oxide or cobalt oxide phases have never been detected by XRD analyses in samples treated at 400 °C. MoO2

and Co2 Mo3 O8 are only formed at temperatures higher than 500 °C [4]. The BE values for the Mo and Co levels for the samples with different active phase loading are very similar. These results are a reasonable consequence of the weak interactions existing between Mo and the carbon support. The catalysts prepared with the lower amounts of active phase, namely Mo6 and Co1.2Mo6 do not behave as the other samples. The differences between the metal to carbon atomic ratios for the ground and non-ground catalysts or between Mo and CoMo are less marked. This suggests that the re-dissolution of molybdenum remains very limited in that case. The interpretation could be that carbon supports possess a certain amount of active sites on which molybdenum oxide can be anchored and which are probably saturated at molybdenum contents near 6 wt.%. This speculative conclusion can account for the differences observed with Mo6 and Co1.2Mo6 samples. Our results partially differ from those reported by Visser et al. [27], who observed deviations of the experimental values from the theoretical line for Mo/C bulk ratios higher than 0.004. The deviations were more important, and thus the dispersion decreased, after sulfidation due to the weakness of the interactions between the carbon support and molybdenum. Conversely, Rondon et al. [28] obtained a linear increase of the molybdenum-to-carbon intensity ratio in the same range of compositions as the one examined in our work. The discrepancies could be explained by differences in the carbon supports or impregnation procedure. The results of the latter study can be explained by the very small particle size (57 lm), but also by the high SA (3100 m2 g1 ) of the carbon used and the fact that the carbon was oxidized prior to use, thus increasing the number of ‘‘anchoring sites’’. These anchoring sites are phenolic and/or carboxylic groups with acidic character that could attach molybdenum in the oxide form, as the c- or g-alumina acidic group do. 4.2. Hydrodeoxygenation activity The MA and ED conversions tend to reach a constant level with an active phase loading higher

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than 14 wt.%, while GUA conversion does not follow a proportional increase with the active phase content. As far as the reaction products are concerned, only the phenol /catechol ratio is affected: higher phenol /catechol ratios are obtained with the catalysts containing 10.8 and 14.4 wt.% of oxides. The decrease of the dispersion could be responsible for the constant activity obtained with the Co2.4Mo12 and Co3Mo15 catalysts in the conversion of MA and ED. It could also partially account for the non-proportional increase of the GUA conversion. The decrease of the dispersion corresponds to a decrease of activity per molybdenum atom. A compensation would occur between this decrease and the increase of the molybdenum amount. Concerning the special behavior of GUA, the difference of the results obtained with Co1.2Mo6 compared to the other catalysts may suggest an interpretation. The distribution of molybdenum for the Co1.2Mo6 catalysts is more homogeneous. A large proportion of the active phase lies deep in the pores. The access to the active phase by the reactant would be more difficult than for the other catalysts. If diffusion limits the reaction rate, the conversion should be negatively affected. In the case of molecules like GUA, non-selective reactions leading to coke formation would be increased by the difficulty of back-diffusion of GUA and the activity would consequently decrease. Actually, a higher molar balance default for GUA was observed with the Co2.4Mo12 catalyst (31% against 19%). As for the higher phenol /catechol ratios observed with the Co1.8Mo9 and Co2.4Mo12 catalysts, a possible explanation could be found in the variation of the Co/(Co þ Mo) atomic ratio with increasing active phase loading (Fig. 10). In the Co1.2Mo6 catalyst, the external carbon surface would be enriched with cobalt, while molybdenum would remain ‘‘anchored’’ in the interior of the support. Values appreciably higher than the theoretical Co/(Co þ Mo) atomic ratio (0.28), as measured for the Co1.2Mo6 catalyst, or a significant decrease of the dispersion, as evidenced for the Co3Mo15, both negatively affect the dehydroxylation of GUA to phenol. The only study concerning the influence of the Co/(Co þ Mo) ratio

289

Fig. 10. Co/(Co þ Mo) atomic ratio, for not ground and ground samples: Co1.2Mo6, Co1.8Mo9, Co2.4Mo12 and Co3Mo15.

in HDO reactions is due to Centeno [29]. It was shown that the GUA conversion and the Phenol / Catechol ratio decreased with increasing Co/(Co þ Mo) atomic ratio. A good inter-dispersion between cobalt and molybdenum, in addition to a good dispersion and accessibility of the active phase, would be crucial elements for the conversion of GUA in partially or totally deoxygenated products.

5. Conclusions XPS characterization has shown a non-uniform distribution of the active elements distributed between the inside and outside of the carbon particles. The dispersion decreases with increasing metal loading. Cobalt seems to be mainly impregnated on the external particle surface and to be responsible for a partial dissolution and migration of molybdenum to the external parts. The formation of bulk cobalt or molybdenum oxides was never detected. For a MoO3 content higher than 6 wt.%, micropore blocking takes place. For a MoO3 content lower than 6 wt.%, most of the molybdenum is fairly well dispersed, relatively strongly bound to the support and mainly located in the micropores. The catalytic activity does not proportionally increase with the amount of active phase. The most important parameter for the reaction of MA

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and ED seems to be the dispersion of the active phase. However, other factors like the accessibility of the active phase and the Co/(Co þ Mo) atomic ratio probably play a crucial role in the conversion of GUA.

Acknowledgement The financial support of UE (project JOR3CT95-0025) is gratefully acknowledged.

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