Oxidative dehydrogenation of propane over cobalt-containing mixed oxides obtained from LDH precursors

Oxidative dehydrogenation of propane over cobalt-containing mixed oxides obtained from LDH precursors

Applied Catalysis A: General 417–418 (2012) 153–162 Contents lists available at SciVerse ScienceDirect Applied Catalysis A: General journal homepage...

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Applied Catalysis A: General 417–418 (2012) 153–162

Contents lists available at SciVerse ScienceDirect

Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata

Oxidative dehydrogenation of propane over cobalt-containing mixed oxides obtained from LDH precursors Gheorghit¸a Mitran a , Thomas Cacciaguerra b , Stéphane Loridant c , Didier Tichit b , Ioan-Cezar Marcu a,∗ a Laboratory of Chemical Technology & Catalysis, Department of Organic Chemistry, Biochemistry & Catalysis, Faculty of Chemistry, University of Bucharest, 4-12, Blv. Regina Elisabeta, 030018, Bucharest, Romania b Institut Charles Gerhardt, UMR 5253 CNRS/ENSCM/UM2/UM1, Matériaux Avancés pour la Catalyse et la Santé (MACS), Ecole Nationale Supérieure de Chimie, 8, rue de l’Ecole Normale, 34296, Montpellier Cedex 5, France c Institut de Recherches sur la Catalyse et l’Environnement de Lyon (IRCELYON), CNRS-Université Claude Bernard Lyon 1, 2, av. A. Einstein, F-69626, Villeurbanne Cedex, France

a r t i c l e

i n f o

Article history: Received 19 September 2011 Received in revised form 6 December 2011 Accepted 22 December 2011 Available online 31 December 2011 Keywords: Co-containing oxides Layered double hydroxides Propane Oxidative dehydrogenation

a b s t r a c t Co(x)MgAlO mixed oxide catalysts with cobalt content in the range from 1 to 20 at.%, were prepared by calcination of layered double hydroxide (LDH) precursors at 1023 K. Their characterization was performed using X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), N2 adsorption, Raman and photoluminescence spectroscopies, TG–DTG and H2 -TPR techniques. The catalytic activities were evaluated in the oxidative dehydrogenation of propane in the temperature range from 723 to 873 K. For all the catalysts the conversion increased when the reaction temperature increased, while the propene selectivity decreased continuously to the benefit of COx for Co(20)MgAlO and Co(10)MgAlO catalysts and of cracking products for the other cobalt-containing catalysts. In all the temperature range studied, the catalytic activity increased with increasing the cobalt content in the catalyst in line with the increase of reducibility. The best yields in propene of about 10% were obtained with Co(5)MgAlO and Co(7)MgAlO catalysts for the reaction performed at 873 K. Besides, a comparison of the behavior of Co(3)MgAlO and Co(7)MgAlO catalysts at 823 K showed that, at isoconversion, the propene selectivity was higher for the sample with lower cobalt content. The well-dispersed cobalt species with tetrahedral coordination played a main role in the oxidative dehydrogenation reaction of propane into propene. When the propaneto-oxygen molar ratio was increased from 1 to 4, the conversion decreased while propene selectivity increased continuously and the best propene yields were obtained for a propane-to-oxygen molar ratio of 1. © 2011 Elsevier B.V. All rights reserved.

1. Introduction A high market demand for propene has been observed in the recent years. The catalytic oxidative dehydrogenation (ODH) of propane into propene has been widely investigated as an alternative to the most largely used pyrolysis processes. A wide variety of catalytic systems have been proposed for this reaction, the most studied being based on vanadium [1–12] or molybdenum [13–21]. Though scarcely studied, other transition metal-based catalytic materials could exhibit comparable catalytic efficiencies [22–31]. Thus, Al-Zahrani et al. [22] showed that Cr, Mn, Zr or Ni oxides supported on ␥-Al2 O3 were reactive in this reaction. The Cr–Al–O mixed oxide led to the higher propene yield (∼9%) at 723 K and was more active and selective than a Mo–Al–O mixed oxide catalyst studied as a reference. Jibril et al. [23] studied Kieselguhr-supported

∗ Corresponding author. Tel.: +40 214103178x138; fax: +40 213159249. E-mail addresses: [email protected], ioancezar [email protected], [email protected] (I.-C. Marcu). 0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2011.12.038

transition metal-oxides (metal cations being Cr, Mn, Co, Ni, V and Mo) and showed that Mn- and Co-based catalysts exhibited the maximum propene yields: ∼5% and ∼4%, respectively, for the reaction at 773 K. Ge et al. [24] investigated lithium salts-promoted Portland cement-supported MnOx catalysts and showed that they give more than 60% alkane conversion and 80% olefins selectivity at 923 K for ODH of ethane and propane to ethylene and propylene, respectively. Jalowiecki-Duhamel et al. [25] studied Ce–M–O mixed oxides (M = Ni, Cu, Co, Cr, or Zn) and showed that Ce–Ni–O was the most efficient catalyst yielding 5.4% of propene at 648 K. Wang et al. [26] studied the ODH of ethane and propane over LiCl-promoted NiO/sulfated zirconia catalysts. They were very active and selective in both reactions leading to 73.5% ethylene yield at 923 K and 10.4% propylene yield at 873 K, respectively. More recently, Wu et al. [27] showed that TiO2 -doped nickel oxide yielded 12% propene from propane at 573 K. Cobalt supported catalysts also shown high efficiency in ODH of light alkanes. A Co(7.6 wt.%)/TiO2 catalyst has been found active in the ODH of ethane with an ethylene yield of 13.2% at 823 K [28].

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CoH-containing BEA, MFI, MOR and FER zeolites have been found active for the ODH of ethane and propane, the best alkene yields of ∼3% being obtained in both cases with Co–BEA catalyst. Their activity has been influenced by the zeolite topology and decreased following the order: Co–BEA > Co–MFI  Co–MOR > Co–FER [29]. Cobalt incorporated into the framework of MCM-41-like silicas has been reported to be active and selective for the ODH of isobutane into isobutene, even though methacrolein and other oxygenates were also formed in noticeable quantities [30]. The best isobutene yield obtained with Co–MCM-41 was 3.3% for the reaction at 798 K. It has been shown that the active and selective sites in Co–MCM-41 were Co2+ species with a unique tetrahedral coordination. We recently reported for the first time that MMgAlO mixed oxides (M = Mn, Fe, Co, Ni, Cu and Zn) obtained from layered double hydroxide (LDH) precursors were active and selective for ODH of propane, the best propene yields being obtained with CoMgAlO catalyst [31]. It is well known [32] that the mixed oxides obtained by thermal decomposition of LDHs show high surface area and metal dispersion, small crystallite size and good stability against sintering, properties which are usually required for high-temperature oxidation catalysts. Moreover, as the adsorbed alkenes are subjected to further transformation into carbon oxides negatively affecting the ODH selectivity and the LDH-derived Mgcontaining mixed oxides exhibit a high amount of strong basic O2− sites [32], their use as catalysts in the ODH reaction is expected to favor the desorption of alkenes, species with an electron-donating character, and, consequently, to improve the ODH selectivity. It must be pointed out that mixed oxides derived from LDH precursors and containing vanadium have already been studied as catalysts for the oxidative dehydrogenation of light alkanes [33–38]. VMgAlO mixed oxides [37,38] were indeed characterized by catalytic performances comparable or better than that of reference VMgO catalysts [39,40] in the oxidative dehydrogenation of propane. Here we report on a study on the preparation, characterization and activity of CoMgAlO mixed oxides catalysts obtained from LDH precursors for ODH of propane. To the best of our knowledge, these catalysts have not yet been investigated in the ODH of light alkanes. 2. Experimental 2.1. Catalysts preparation Co/Mg/Al LDHs precursors were prepared by coprecipitation of mixed metal nitrate solutions with an aqueous solution of NaOH (2 M) at a constant pH of 10 at room temperature. Thus, an aqueous solution containing the appropriate amounts of magnesium nitrate (Mg(NO3 )2 ·6H2 O) and aluminium nitrate (Al(NO3 )3 ·9H2 O) was contacted with the NaOH solution by dropwise addition of both solutions into a well-stirred beaker containing 200 cm3 of suitable amount of cobalt nitrate solution (Co(NO3 )2 ·6H2 O). The cobalt content, as atomic percent with respect to the cations, was varied between 1 and 20% (0.01 ≤ Co/(Co + Mg + Al) ≤ 0.2) and the Mg/Al atomic ratio was 3 for all preparations. The addition rate of the alkaline solution and the pH were controlled by pH-STAT Titrino (Metrohm). The precipitates formed were aged in their mother liquor overnight at 353 K under stirring, separated by centrifugation, washed with deionized water until a pH of 7 and dried at 353 K overnight. The dried LDH samples were hereafter noted Co(x)MgAl–LDH, where x is the cobalt content as atomic percent with respect to cations. These LDH precursors were calcined in air at 1023 K during 8 h in order to obtain the corresponding mixed metal oxides catalysts. They were noted Co(x)MgAlO. A cobalt-free Mg–Al mixed oxide obtained following the protocol already reported was noted MgAlO.

2.2. Catalysts characterization Powder X-ray diffraction (XRD) patterns were recorded using a Siemens D5000 Diffractometer and the Cu-K␣1 radiation. They were recorded with 0.02◦ (2) steps over the 3–70◦ 2 angular range with 1 s counting time per step. The chemical composition of the mixed oxide samples was determined by EDX microprobe on a Cambridge Stereoscan 260 apparatus. The textural characterization was achieved using conventional nitrogen adsorption/desorption method, with a Micromeritics ASAP 2010 automatic analyzer. Specific surface areas were calculated using the BET method. Prior to nitrogen adsorption, the samples were outgassed for 8 h at 523 K. Raman and photoluminescence spectra were recorded at room temperature using a LabRam HR (Jobin Yvon-Horiba) spectrometer equipped with a CCD detector cooled at 198 K. Measurements were carried out with a long working distance ×50 microscope. The exciting line at 514.53 nm of a 2018RM Ar+ –Kr+ laser (Spectra Physics) was used with a limited power of 250 ␮W. It was previously checked that the laser heating of the samples was negligible with such a power. A low dispersion grating of 300 grooves/mm was chosen to maximize the signal. Using such grating, the positions of Raman bands were accurate to within 4 cm−1 . Raman and photoluminescence mappings were achieved with a motorized stage on ca. 50 areas to control the homogeneity of the samples at the micrometer scale. The spectra were recorded and treated using the Labspec software (Jobin Yvon). Decompositions into individual components of the Raman spectra of Co(5)MgAlO, Co(7)MgAlO and Co(10)MgAlO were also achieved using this software. The thermal analysis (TG and DTG) was carried out using a Netzsch TG 209 device, in the following conditions: linear heating rate 10 K min−1 from room temperature to 1173 K, dynamic air atmosphere, Al2 O3 crucible, sample weight approximately 20 mg. Hydrogen temperature-programmed reduction (H2 -TPR) studies were achieved using a Micromeritics Autochem model 2910 instrument. Fresh calcined samples, placed in a U-shaped quartz reactor, were pretreated in air at 1023 K before reduction. After cooling down to room temperature and introducing the reduction gas of 3% H2 /Ar, the sample was heated at a rate of 10 K min−1 from room temperature to 1073 K. The hydrogen consumption was estimated from the area under the peak after taking the thermal conductivity detector response into consideration. Calibration of TCD signal has been done with an Ag2 O standard (Merck, reagent grade). Deconvolution of the peaks into Gaussian-shaped components was performed using OriginPro 7.5 program. Agreement factor was in all cases higher than 0.9950. 2.3. Catalytic oxidative dehydrogenation The catalytic oxidative dehydrogenation of propane was carried out in a fixed bed quartz tube down-flow reactor operated at atmospheric pressure. The internal diameter of the reactor tube was 15 mm. 1 cm3 of catalyst was used. The catalyst was supported by quartz wool. The axial temperature profile was measured using an electronic thermometer placed in a thermowell centered in the catalyst bed. The reactor temperature was controlled using a chromel–alumel thermocouple attached to the exterior of the reactor. Quartz chips were used to fill the dead volumes before and after the catalyst bed to minimize potential gas-phase reactions at higher reaction temperatures. The gas mixture consisting of propane and air was fed into the reactor at a volume hourly space velocity (VHSV) in the range of 3000–12,000 h−1 . The reaction temperature was varied between 723 and 873 K, the propane-to-oxygen molar ratio, between 1 and 4, and the catalyst bed volume was always kept to 1 cm3 . Before each activity test, the catalyst in the reactor was

G. Mitran et al. / Applied Catalysis A: General 417–418 (2012) 153–162

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#*

(003)

#

(g) (006)

(g)

(012)

(015) (018)

*

#

# #

* #

(110) (113)

(f)

Intensity (a. u.)

Intensity (a. u.)

(f) (e)

(d)

(c)

(e)

(d)

(c)

(b) (b)

(a)

(a) 0

10

20

30

40

50

60

70

0

activated under air at 873 K for about 30 min for cleaning its surface as suggested by the TG–DTG analysis (see below) then the reactor was set to the desired temperature in the flow of reactants. The system was allowed to stabilize for about 1 h at the reaction temperature before the first product analysis was made. Each run was carried out over a period of 2–3 h. The reaction products were analyzed in a Thermo Finnigan Gas-Chromatograph equipped with a thermal conductivity detector (TCD) using an alumina column and a flame ionization detector (FID) using a CTR I column. Propene, CO, CO2 and cracking products (methane and ethylene) were the major products formed under the reaction conditions. Conversions of propane and products selectivities were expressed as mol% on a carbon atom basis. The carbon balance was satisfactory in all runs with an error margin of ±5%. 3. Results and discussion 3.1. Catalysts characterization The XRD patterns of the dried as-synthesized LDH precursors and of the corresponding mixed oxide catalysts are displayed in Figs. 1 and 2, respectively. All Co(x)MgAl–LDH samples exhibited the XRD pattern characteristic of the LDH structure (JCPDS 370630). Except for Co(5)MgAl–LDH, intense and narrow diffraction peaks at ∼11◦ and 22◦ (2), ascribed to (0 0 3) and (0 0 6) planes, respectively, were noted. Wide and asymmetric (0 k l) reflections were obtained above 30◦ 2 values, as usually observed, while the (1 1 0) and the very broad (1 1 3) reflections were noted around 61◦ . These reflections corresponded to a hexagonal lattice with R-3m rhomboedral symmetry. As the nominal Co content was increased the crystallinity decreased but no Co-containing phase was detected. These features suggested that multicationic brucitelike layers containing Co, Mg and Al cations were formed. In all

20

30

40

50

60

70

o

2 theta ( )

2-theta (°) Fig. 1. XRD patterns of the LDH precursors: MgAl–LDH (a), Co(1)MgAl–LDH (b), Co(3)MgAl–LDH (c), Co(5)MgAl–LDH (d), Co(7)MgAl–LDH (e), Co(10)MgAl–LDH (f), Co(20)MgAl–LDH (g).

10

Fig. 2. XRD patterns of the mixed-oxide catalysts after calcination at 1023 K (* – Mg(Al)O mixed oxide phase; # – CoAl2 O4 and Co3 O4 phases): MgAlO (a), Co(1)MgAlO (b), Co(3)MgAlO (c), Co(5)MgAlO (d), Co(7)MgAlO (e), Co(10)MgAlO (f), Co(20)MgAlO (g).

samples the d0 0 3 basal spacing of the LDH phase was ca. 0.80 nm, in agreement with the value generally reported for the intercalation of nitrate species [41]. Nitrates were provided by the precursor metal salts. The positions of the (0 0 3) and (1 1 0) reflections allowed to calculate, considering the hexagonal lattice, the c (c = 3 × d0 0 3 ) and a (a = 2 × d1 1 0 ) parameters reported in Table 1. Their values were almost similar in all Co(x)MgAl–LDH samples, although variations were expected due to substitution of Co2+ for Mg2+ . That accounted for probable variations in the nitrate and carbonate contents of the sample and to uncertainties in the determination of the position of the poorly intense and broad (1 1 0) peaks. Presence of carbonates was due to contamination by CO2 dissolved in water. All the Co(x)MgAlO samples calcined at 750 ◦ C exhibited the reflections at 2 ∼37.5◦ , 43.5◦ and 63◦ characteristic of the well known Mg(Al)O mixed oxide phase with the periclase-like structure (JCPDS-ICDD 4-0829). Except for Co(5)MgAlO sample, the peak intensity of the cobalt-containing samples decreased with increasing the cobalt content. Moreover, for Co loadings higher than 7%, diffraction peaks ascribed to CoAl2 O4 (PDF 70-0753) and/or Co3 O4 (PDF 09-0418) spinel phases were observed. Table 1 Unit cell parameters of the Co(x)MgAl–LDH precursors. LDH precursor

MgAl–LDH Co(1)MgAl–LDH Co(3)MgAl–LDH Co(5)MgAl–LDH Co(7)MgAl–LDH Co(10)MgAl–LDH Co(20)MgAl–LDH

Unit cell parameters (nm) a

c

0.305 0.306 0.306 0.305 0.306 0.304 0.305

2.494 2.436 2.458 2.387 2.393 2.389 2.419

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Table 2 Specific surface areas, chemical compositions and TPR data obtained with the Co(x)MgAlO catalysts. SSAa (m2 g−1 )

Catalyst

188 122 81 167 109 114 114

MgAlO Co(1)MgAlO Co(3)MgAlO Co(5)MgAlO Co(7)MgAlO Co(10)MgAlO Co(20)MgAlO a b

Chemical composition (at.%)b

Co content (at.% with respect to cations)

Total H2 consumption

Theoretical

EDX

Co

Mg

Al

␮mol g−1

mol H2 /mol Co

– 1.0 3.0 5.0 7.0 10.0 20.0

– 1.3 3.5 7.5 9.1 12.7 22.4

– 0.44 1.21 1.97 3.13 4.16 7.66

22.23 25.72 26.53 17.22 23.75 21.35 18.71

6.71 7.28 6.92 7.03 7.48 7.32 7.74

– 16.7 – 82.8 – 267.0 633.3

– 0.07 – 0.08 – 0.13 0.18

Specific surface area. Oxygen in balance.

The chemical compositions of the samples, determined by EDX analysis, are reported in Table 2. They showed that the cobalt content is slightly higher than the nominal value for all samples. The Mg/Al molar ratios were, within the margin of error, around the value of 3 in solution. The specific surface areas decreased significantly upon introduction of Co in the MgAlO mixed oxide (Table 2). They indeed reached ∼190 m2 g−1 for MgAlO and, except for Co(5)MgAlO, decreased to values ranging from 81 to 122 m2 g−1 for the Co(x)MgAlO catalysts and varied irregularly with the cobalt content. Such decrease was larger than that previously reported upon introduction of closer contents of Cu2+ in MgAlO [42]. All the catalysts displayed type IV nitrogen adsorption/desorption isotherms, according to IUPAC classification, and H3-type hysteresis loops characteristic of mesoporous materials [43] with a broad distribution of pore sizes (Fig. 3). It must be noted that the adsorption isotherms do not present a plateau at high P/P0 values showing that N2 physisorption was taking place between the aggregates of platelets particles and accounted for the lamellar morphology of the materials.

200

3

Volume adsorbed (cm /g)

150

Fig. 4 shows the Raman spectra of the Co(x)MgAlO catalysts. The Raman spectrum of Co(1)MgAlO exhibited a band at 1090 cm−1 that was due to the 1 vibration of residual carbonate species [44,45]. The band at 565 cm−1 was assigned to Al–O–Mg stretching vibrations in the mixed oxide with the periclase-like structure. The Raman spectrum of the Co(3)MgAlO compound contained bands at 695, 612 and 467 cm−1 that revealed the presence of CoO crystalline phase [46,47]. The band at 303 cm−1 was assumed to arise also from this phase. Above x value of 3, the five Raman active modes of Co3 O4 were systematically observed around 195, 490, 540, 620 and 695 cm−1 [48,49]. The slight shift of these bands from one sample to the other could reveal different crystallinity of the spinel structure as it was shown that they are blue-shifted with an increase in the crystallite size [50]. The presence of Cox Al(3−x) O4 seemed to be ruled out since rather intense bands near 200 and 620 cm−1 should have been observed [51]. The bands expected for MgAl2 O4 at 770, 670, 410 and 310 cm−1 were not observed [52]. The band of carbonate species at 1071 cm−1 was particularly intense for Co(5)MgAlO probably because of its higher specific surface area. However, above x value of 3, two bands were observed around 587–594 cm−1 and 635–643 cm−1 in addition to those of Co3 O4 . Decompositions of Raman spectra into individual components were achieved using Lorentzian functions for Co3 O4 and Gaussian functions for these two bands (Fig. 4). It appeared that the two bands were due to the same species since the ratios of their areas was almost constant for Co(5)MgAlO, Co(7)MgAlO and Co(10)MgAlO. The band around 590 cm−1 was previously ascribed to Co–O stretching vibrations of well dispersed cobalt species [53,54]. In fact, this band and the other one around 640 cm−1 could correspond respectively to the s symmetric and as

100

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0 0

0.2

0.4

0.6

0.8

1

Relative pressure (P/P0) Fig. 3. Nitrogen adsorption/desorption isotherms of the mixed-oxides after calcination at 1023 K: MgAlO (black), Co(1)MgAlO (blue), Co(3)MgAlO (magenta), Co(5)MgAlO (yellow), Co(7)MgAlO (sea green), Co(10)MgAlO (green), Co(20)MgAlO (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)

Fig. 4. Raman spectra of the mixed-oxide catalysts after calcination at 1023 K: Co(1)MgAlO (a), Co(3)MgAlO (b), Co(5)MgAlO (c), Co(7)MgAlO (d), Co(10)MgAlO (e).

G. Mitran et al. / Applied Catalysis A: General 417–418 (2012) 153–162

Fig. 5. Photoluminescence spectra of the mixed-oxide catalysts after calcination at 1023 K: Co(3)MgAlO (a), Co(5)MgAlO (b), Co(7)MgAlO (c), Co(10)MgAlO (d).

anti-symmetric stretching vibrations of Co O Al bonds; the symmetric vibrations being more intense in Raman spectroscopy. Interestingly, the areas of the two bands compared to those of Co3 O4 was maximal for x equal to 7 suggesting that the relative amount of the corresponding species reached then a maximum. The intensity of the two bands was rather constant from one analyzed point to the other in the Raman mappings for the Co(5)MgAlO and Co(7)MgAlO compounds but significantly varied for Co(10)MgAlO revealing heterogeneities at the micron scale. Fig. 5 compares the photoluminescence spectra of the Co(x)MgAlO catalysts in the visible range. The small bands below 560 nm corresponded to the Raman bands of the samples. Above x value of 5, an intense band was observed at 665 nm that was assigned to Co2+ cations in tetrahedral environment [55]. A similar band was reported at 665 nm for Co doped MgAl2 O4 [56,57] and at

a

100

695 nm for magnesium aluminosilicate glass-ceramics doped with Co2+ [58]. In Co(x)MgAlO catalysts, Co2+ cations could occupy tetrahedra present as structural defects of Al2 O3 leading to formation of Co O Al bonds. The photoluminescence band raised up to x equal to 7 and then was damped. This quenching phenomenon was probably due to a lower dispersion of tetrahedral Co2+ species [55] above x value of 7. The maximum intensity of the fluorescence band coincided with the maximum relative intensity of the two Raman bands near 590 and 640 cm−1 suggesting all these bands are mainly due to the same species namely well-dispersed CoO4 tetrahedra in interaction with Al2 O3 . The TG–DTG curves of the Co(3)MgAl–LDH precursor and of the corresponding Co(3)MgAlO mixed oxide, representative of all samples, are presented in Fig. 6. As classically reported, the first weight loss in the TG–DTG curves of the LDH precursor (∼13 wt.%) which gives a broad DTG peak extending from room temperature to 473 K was ascribed to the elimination of weakly bounded and interlayer water molecules, the second weight loss above 473 K (∼36 wt.%) giving rise to a DTG peak at 673 K, was due to the dehydroxylation of the layers and the decomposition of the compensating anions (Fig. 6a). In the case of the calcined Co(3)MgAlO mixed oxide an almost continuous weight loss of ca. 15 wt.% was observed between room temperature and 873 K with two peaks at 378 K and 548 K in the DTG profile (Fig. 6b). We suggested that the former corresponded to the removal of water molecules coming from the rehydration. The latter was assigned to the removal of water and CO2 provided by the partial rehydroxylation and carbonation of the mixed oxide. This showed that an activation of the catalyst in the reactor at temperatures higher than 873 K before reaction was necessary to remove the adsorbed species, as previously reported for CuMgAlO mixed oxide catalysts [42]. The reducibility of an oxide catalyst is one of the main parameters which can govern its catalytic performances. The H2 -TPR patterns of the Co(x)MgAlO samples are shown in Fig. 7. It was checked that MgAlO sample does not give rise to H2 consumption. Three reduction peaks can be observed in the TPR profiles. The first one, below 450 K, can be assigned to highly reducible CoO and Co3 O4 species weakly interacting with the support evidenced by Raman spectroscopy. The second one, at intermediate temperature (450–750 K), showing different components at high cobalt contents, can be attributed to cobalt oxide particles interacting more strongly with the support. The peaks at high temperature which showed different components likely accounted for the reduction of several aluminate mixed oxide phases [59], in agreement with the composition of the samples evidenced by XRD. These phases involving Co, Mg and Al cations exhibit different reducibilities due to large variations of stoichiometries. The reduction peaks shifted

b

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80 273

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Fig. 6. TG–DTG profiles of uncalcined Co(3)MgAl–LDH precursor (a) and of the resulting Co(3)MgAlO mixed-oxide (b).

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G. Mitran et al. / Applied Catalysis A: General 417–418 (2012) 153–162

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-1

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6

8

(d)

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4 2

0 100

0

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200

300

400

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-1

Total H2-TPR consumption (μmol.g ) Fig. 8. Specific rate of propane transformation at 873 K, VHSV = 9000 h−1 and propane-to-oxygen molar ratio = 2, versus total hydrogen consumption in TPR experiments.

(b) (a)

with MgAlO mixed oxide species migrating from the support which decreased the accessibility of H2 molecules [31,42,60].

300 400 500 600 700 800 900 1000 1100

3.2. Catalytic oxidative dehydrogenation of propane

Temperature (K) Fig. 7. H2 -TPR profiles for Co(1)MgAlO (a), Co(5)MgAlO (b), Co(10)MgAlO (c) and Co(20)MgAlO (d) samples.

to higher temperatures and the corresponding H2 consumption as well as the total H2 consumption decreased with the decrease of Co content. The lower reducibility of the samples as the cobalt content decreased may be attributed to the higher dispersion of Co species in the aluminate mixed oxide phases which can possess pre-spinel structures. This inhibits the hydrogen approach to Co2+ cations and thus delays their reduction [59]. The total H2 consumptions per g of catalyst as well as per mol of Co are reported in Table 2. The observed decrease of the H2 /Co molar ratio as the cobalt content decreased confirms that at lower Co content the well dispersed Co species are less easily reduced. On the other hand, the H2 /Co molar ratios were by far lower than 1 for all the Co(x)MgAlO samples studied, indicating that cobalt cations were not quantitatively reduced. As previously suggested for supported transition metal oxide catalysts, the cobalt-containing particles could be decorated 90

b 10

80 70

Propene yield (%)

Conversion & Selectivity (%)

a

The conversion of propane and the product selectivities as a function of the reaction temperature obtained with the different Co(x)MgAlO catalysts were presented in Table 3 for a total VHSV of 9000 h−1 and a propane-to-oxygen molar ratio of 2. We have previously shown that the contribution of the homogeneous reaction is negligible in these reaction conditions: the propane conversion in the absence of a catalyst was lower than 2% for the reaction at 873 K [61]. For all the catalysts the conversion increased with the reaction temperature. We noted that MgAlO was not active below 773 K. Propene selectivity decreased continuously in the temperature range from 723 K to 873 K for all the cobalt-containing catalysts, probably due to the increase in the conversion, a well known fact in the ODH of light alkanes. At the same time, the selectivity for cracking products (ethene and methane) increased in all cases, as expected, with the reaction temperature. It is noteworthy that the observed decrease of selectivity for propene with the reaction temperature occurred mainly at the benefit of cracking products for Co(1)MgAlO, Co(3)MgAlO, Co(5)MgAlO and Co(7)MgAlO samples and of COx for Co(10)MgAlO and Co(20)MgAlO samples.

60 50 40 30 20

8

6

4

2

10 0

0 0

2

4

6

8

10

12

14

16

18

20

22

Co content in catalyst (% at. with respect to cations)

0

2

4

6

8

10

12

14

16

18

20

22

Co content in catalyst (% at. with respect to cations)

Fig. 9. The effect of the Co content in catalyst (a) on the propane conversion (, ) and propene selectivity (♦, ) for the reaction at 823 (, ♦) and 873 K (, ) and (b) on the propene yield for the reaction at 823 (䊉) and 873 K () (VHSV = 9000 h−1 and propane-to-oxygen molar ratio = 2).

G. Mitran et al. / Applied Catalysis A: General 417–418 (2012) 153–162

159

Table 3 Conversions and selectivities obtained in the ODH of propane with the Co(x)MgAlO catalysts.a

Propene

Cracking products

Co(1)MgAlO

723 773 823 873

1.8 3.2 5.1 9.7

96.4 90.8 79.8 70.5

2.1 8.3 18.2 27.9

1.5 0.9 2.0 1.6

0.62 1.10 1.75 3.33

57.7

Co(3)MgAlO

723 773 823 873

1.9 3.7 6.6 10.3

96.6 93.1 81.9 70.0

0.9 2.5 13.3 25.5

2.5 4.4 4.8 4.5

0.79 1.54 2.75 4.30

59.5

Co(5)MgAlO

723 773 823 873

1.5 3.1 7.2 15.4

92.5 90.3 82.0 67.5

3.0 5.7 14.4 29.5

4.5 4.0 3.6 3.0

0.54 1.12 2.60 5.56

82.0

Co(7)MgAlO

723 773 823 873

3.0 5.2 9.3 15.8

92.6 86.4 72.1 52.2

5.1 10.3 20.5 35.7

2.3 3.3 7.4 12.1

1.17 2.03 3.62 6.16

58.3

Co(10)MgAlO

723 773 823 873

3.1 6.9 14.6 18.0

68.4 59.5 47.4 35.8

1.3 5.2 14.7 25.0

30.3 35.3 37.9 39.2

1.21 2.69 5.69 7.02

76.6/25.0b

Co(20)MgAlO

723 773 823 873

6.4 17.5 21.7 27.0

63.4 50.3 36.3 24.1

1.5 4.4 10.7 17.5

35.1 45.3 53.0 58.4

2.49 6.82 8.46 10.52

93.5/24.3b

MgAlO

773 823 873

0.8 2.9 7.7

10.8 20.5 25.6

2.9 12.8 28.3

86.3 66.7 46.1

0.34 1.24 3.31

a

Selectivities (%)

Eact (kJ mol−1 )

Reaction temperature (K)

b

Propane conversion (%)

Specific rate (106 mol g−1 s−1 )

Catalyst

Carbon oxides

127.2

Reaction conditions: total VHSV = 9000 h−1 ; propane-to-oxygen molar ratio = 2. For the reaction at high temperatures.

Moreover, the Co(10)MgAlO and Co(20)MgAlO samples gave much larger quantities of COx within all the temperature range studied. This is not surprising if one considers that, on one hand, a higher number of active centers will increase the probability of further oxidation of propene [30] and, on the other hand, the Co3 O4 spinel phase active for short-chain alkanes combustion [62,63], has been

-11

-12

ln v

-13

-14

-15

-16 1.1

1.2

1.3 3

1.4

-1

10 /T (K ) Fig. 10. Arrhenius plots for the propane conversion over the different catalysts: MgAlO (*), Co(1)MgAlO (♦), Co(3)MgAlO (), Co(5)MgAlO (), Co(7)MgAlO (), Co(10)MgAlO (×) and Co(20)MgAlO (+) (temperature range from 723 to 873 K, VHSV = 9000 h−1 and propane-to-oxygen molar ratio = 2).

clearly evidenced by XRD and Raman spectroscopy in Co(10)MgAlO and Co(20)MgAlO samples. In all the temperature range studied, the activity decreased by decreasing the cobalt content in the catalyst in line with the decreased reducibility already suggested in the H2 TPR experiments. We have indeed shown that both the maximum reduction peak was shifted toward higher temperatures and that the H2 /Co molar ratio decreased as the cobalt content decreased. Moreover, a correlation between the specific rate of propane transformation at 873 K over the different Co-containing catalysts and the corresponding total hydrogen consumption in TPR experiments (Fig. 8) has been observed. This suggests that the same CoOx species are involved in both the catalytic reaction and the reduction. MgAlO less active than the cobalt-containing catalysts, showed an increase of selectivity toward propene and cracking products at the expense of the selectivity toward COx that decreased as the reaction temperature increased. The observed increase of selectivity toward propene with the reaction temperature for the MgAlO support is an unusual behavior for the ODH reaction performed over oxide-based catalyst, although it has been reported in some studies but at low conversions [22,64,65]. The highest propene yields were obtained in all cases above 823 K. The evolution of the propane conversion and propene selectivity as well as of the propene yield as a function of the cobalt content in the catalyst for the reaction temperatures of 823 and 873 K was depicted in Fig. 9. For both reaction temperatures considered, the propane conversion continuously increased with the cobalt content in the catalyst while the propene selectivity remained almost constant for cobalt contents up to 7.5 at.% (Co(5)MgAlO sample) then it decreased for higher cobalt contents. For the reaction at 873 K the propene yield passed through a maximum of ∼10% for cobalt contents between 7.5 and 9.1 at.%, that is for Co(5)MgAlO and Co(7)MgAlO samples. For the reaction at 823 K the propene yield increased continuously up to ∼7% for 9.1 at.%

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b

100

Cracking & COx Selectivities (%)

a

Propene selectivity (%)

90

80

70

60

50

40

30

20

10

50

0 0

5

10

15

0

5

Propane conversion (%)

10

15

Propane conversion (%)

Fig. 11. Effect of conversion on the propene selectivity (a) and cracking (filled symbols) and COx (open symbols) selectivities (b) in the oxidative dehydrogenation of propane over Co(3)MgAlO (, ♦) and Co(7)MgAlO (, ) catalysts at 823 K (propane-to-oxygen molar ratio = 2).

cobalt content (Co(7)MgAlO sample) and remained almost constant for higher contents. It can be underlined that the highest propene yield was reached at higher temperature (873 K) below 9.1 at.% Co content (Co(7)MgAlO sample) and at lower temperature (823 K) above this Co content. Interestingly, as observed by Raman and photoluminescence spectroscopies, the amount of well-dispersed cobalt species with tetrahedral coordination reached a maximum for Co(7)MgAlO, suggesting that these species play a main role in the ODH of propane. This type of cobalt species with tetrahedral coordination has already been found to be active and selective for the ODH of isobutane on Co–MCM-41 catalysts [30]. It is noteworthy that compared with other cobalt-containing catalysts reported in the open literature [23,29], the CoMgAlO catalysts studied in this work gave much higher propene yields. It must also be pointed out that in all cases the selectivity for COx and CH4 was higher than that for ethylene in the temperature range studied, the highest values being observed for Co(10)MgAlO

b

100

100

10

90

9

80

8

70

7

60

6

50

5

40

4

30

3

30

20

2

20

10

1

10

0

0

0 0

1

2

3

4

Propane-to-oxygen molar ratio

5

15

90

70

10

60 50 40 5

Conversion (%)

Selectivities (%)

80

Conversion (%)

Selectivities (%)

a

and Co(20)MgAlO. This suggests that the total oxidation products (COx ) were formed, not only from C1 species resulting from the cracking of propane, but also by the direct oxidation of propane or by further oxidation of propene. This is in line with the already reported combustion activity of cobalt-based catalysts in an excess of air [59,62]. The apparent activation energies (Eact ) of reaction for the different catalysts have been determined (Table 3) from the Arrhenius plots presented in Fig. 10. The values obtained for the activation energies fall within the usual range measured for propane ODH over oxide-based catalysts [61,66]. A change in the activation energy was observed at 823 and 773 K for Co(10)MgAlO and Co(20)MgAlO samples, respectively, obviously due to diffusional limitations. The influence of the propane conversion on the propene selectivity has been studied over Co(3)MgAlO and Co(7)MgAlO catalysts at 823 K and a propane-to-oxygen molar ratio of 2, by varying the VHSV in the range from 3000 to 12,000 h−1 (Fig. 11). As

0 0

1

2

3

4

5

Propane-to-oxygen molar ratio

Fig. 12. Effect of propane-to-oxygen molar ratio on the oxidative dehydrogenation of propane over Co(3)MgAlO (a) and Co(7)MgAlO (b) catalysts at 823 K: propene (), cracking (*), COx (×) (total VHSV = 9000 h−1 ).

G. Mitran et al. / Applied Catalysis A: General 417–418 (2012) 153–162

expected, the selectivity to propene decreased in both cases when the conversion increased. At isoconversion, the propene selectivity was higher for Co(3)MgAlO sample than for Co(7)MgAlO. The extrapolation to zero conversion, for Co(7)MgAlO led to non-zero selectivity for cracking products indicating that they were also primary products. For Co(3)MgAlO, the same extrapolation revealed that propene is the only primary product. These results suggested that, for both catalysts, total oxidation products (COx ) were formed rather by oxidation of propene than by direct oxidation of propane. The effect of the propane-to-air molar ratio on the oxidative dehydrogenation of propane over Co(3)MgAlO and Co(7)MgAlO catalysts at 823 K and a VHSV kept to 9000 h−1 is presented in Fig. 12. The propane conversion decreased from 8.3 to 4.1% and from 11.7 to 5.1% for Co(3)MgAlO and Co(7)MgAlO, respectively, when the propane-to-oxygen molar ratio increased from 1 to 4. At the same time, as usual for the ODH reactions, the selectivity to propene increased from 78.8 to 88.6% and from 60.4 to 81.0% for Co(3)MgAlO and Co(7)MgAlO, respectively, at the expense of cracking products and carbon oxides. The observed lower propene selectivity at lower propane-to-oxygen molar ratio is attributed mostly to the higher reactivity of propene for its further oxidative transformation into carbon oxides. It must be noted that the effect of the propane-toair molar ratio on the ODH of propane was more pronounced for the sample with higher Co content. The best propene yields were obtained, in both cases, for a propane-to-oxygen molar ratio equal to 1, i.e., the lowest value in the range studied. These results could be explained by the decrease of the oxygen availability when the propane-to-oxygen molar ratio increased. Moreover, the observed decrease in the selectivity for cracking products when the propaneto-oxygen molar ratio increased at constant VHSV, can be explained by the increase of the propane partial pressure in the reaction mixture.

4. Conclusion CoMgAlO mixed oxide catalysts with different cobalt contents (1–20 at.%) were prepared by calcination of LDH precursors at 1023 K. Beside a CoMgAlO mixed oxide phase exhibiting the periclase-like structure, Co-containing phases with the spinel structure were also observed for higher Co contents. One of these highly dispersed spinel-like phase reached a maximum amount in the Co(7)MgAlO sample. The catalytic activity in the ODH reaction of propane increased with the cobalt content in the catalyst in agreement with their increase of reducibility, and the highest propene selectivities were obtained at lower cobalt content. For all the CoMgAlO catalysts, the conversion increased with the reaction temperature, while the propene selectivity decreased continuously to the benefit of COx for the catalysts with higher cobalt contents and of the cracking products for the catalysts with lower cobalt contents. Besides the temperature, the space velocity and the propane-to-oxygen molar ratio strongly influence the catalytic performances. The well-dispersed cobalt species with tetrahedral coordination played a main role in the ODH reaction of propane into propene while the spinel Co3 O4 phase appeared responsible for the large quantities of COx observed at higher cobalt contents. The highest propene yields (∼10%) were obtained with Co(5)MgAlO and Co(7)MgAlO catalysts at 873 K.

Acknowledgement This research was supported by the Romanian National University Research Council (CNCSIS) under the project “IDEI” no. 1906/2009.

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