Decavanadate-intercalated Ni–Al hydrotalcites as precursors of mixed oxides for the oxidative dehydrogenation of propane

Catalysis Today 192 (2012) 36–43

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Decavanadate-intercalated Ni–Al hydrotalcites as precursors of mixed oxides for the oxidative dehydrogenation of propane Julian Antonio Valverde a , Adriana Echavarría a , Maria Filipa Ribeiro b , Luz Amparo Palacio a,c , Jean-Guillaume Eon d,∗ Grupo Catalizadores y Adsorbentes, Bloque 1-317, Universidad de Antioquia, Calle 67 N◦ 53-108, A. A. 1226, Medellín, Colombia Instituto Superior Técnico, Departamento de Engenharia Química e Biológica, Av. Rovisco Pais, 1049-001 Lisboa, Portugal c Instituto de Química, Universidade do Estado do Rio de Janeiro, Rua São Francisco Xavier 524, Rio de Janeiro, Brazil d Instituto de Química, Universidade Federal do Rio de Janeiro, Ilha do Fundão, CT bloco A, Rio de Janeiro, Brazil a

b

a r t i c l e

i n f o

Article history: Received 30 October 2011 Received in revised form 28 March 2012 Accepted 14 April 2012 Available online 1 June 2012 Keywords: Nickel Hydrotalcite Oxidative dehydrogenation of propane EXAFS

a b s t r a c t Two nickel–aluminum hydrotalcite type precursors were synthesized with molar ratio Ni/Al = 3, using hydroxyl and carbonate as compensation anions. These precursors were intercalated by 30% (theoretical) decavanadate anion. The catalysts were obtained after calcination at 550 ◦ C and their performance was evaluated in propane oxidative dehydrogenation at 400 ◦ C using space velocities of 100, 150, 200 and 400 ml g−1 min−1 under conditions of oxygen deficiency and oxygen excess. Catalysts derived from nickel–aluminum-hydroxyl precursors attained an excellent performance under oxygen deficiency conditions promoting high propene selectivity and conversion. Under oxygen excess conditions the catalyst was more active but its selectivity was lower. Vanadium-containing catalysts showed improved selectivity under this operation condition. The formation of the hydrotalcite phase in precursors and mixed oxides in the catalysts was evidenced by X-ray diffraction (XRD). Analyses of textural and reduction properties were carried-out with nitrogen adsorption and temperature programmed reduction techniques respectively. Structural studies were performed by X-ray absorption spectroscopy for materials that showed best performance in the oxidative dehydrogenation reaction. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Propene is the basic raw material for several industries such as textile, detergents and especially plastics industry, consuming 60% of world production of propylene [1]. An unsatisfied demand for this olefin is projected in the short term; for this reason alternative processes for alkenes production are under study. Among exclusive technologies for their production processes are catalytic dehydrogenations of paraffins such as CATOFIN, OLEFLEX, STAR, and PDH (ASF-Linde-Statoil) [2], which are endothermic processes and require reaction temperatures above 600 ◦ C. In propane oxidative dehydrogenation oxygen and propane react to form propylene and water, so that the overall balance reaction is exothermic and operation temperatures lower than 600 ◦ C are required. This energy factor opens prospects for a future industrial implementation [3]; however the main disadvantage of this reaction is the high propensity toward carbon oxides formation, due to facile oxidation of propene. The development of new catalysts is needed that promote high selectivity toward propene at

∗ Corresponding author. E-mail address: [email protected] (J.-G. Eon). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2012.04.043

moderate propane conversion. For this purpose, a variety of studies of supported and bulk catalysts have been performed, as reported by Cavani et al. [3]. However, the properties of a catalyst are strongly influenced by the characteristics of its precursor. Hydrotalcites have been used as precursors of mixed oxides because of structural and textural features with high industrial applicability [4] and with potentiality in the oxidative dehydrogenation of propane (ODHP) reaction. Hydrotalcite is a layered double hydroxide structurally derived from brucite [Mg(OH)2 ] after exchange of divalent magnesium by some trivalent metal, generating positively charged sheets which are compensated by anions such as carbonate, hydroxyl, chloride, etc., localized in the interlayer space. The general formula is [M1−x 2+ M x 3+ (OH)2 ]x+ Ax/m m− ·nH2 O, where M and M correspond to di- and tri-valent elements respectively and A to the interlayer anion; the x value varies normally between 0.17 and 0.33[5]. Nickel hydrotalcites have been little studied as precursors of mixed oxides for ODHP, however Ni has been used in a variety of reactions due to its hydrogenating/dehydrogenating properties. In ODHP, nickel may be associated with other metals such as molybdenum and cobalt, as reported by Thyrion and Barsan [6]. These authors made kinetic studies in nickel–cobalt molybdate catalysts and evaluated the impact of operation conditions on catalytic

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performances. Trimetallic wolframites belonging to the system Co–Mo–W and Ni–Mo–W were tested in ODHP and a much higher activity and selectivity was reached with Ni catalysts [7]. Kunzru et al. [8] studied the addition of Cr and Ni to V2 O5 /ZrO2 oxides and obtained an increase in selectivity with Ni incorporation. Recently, Li et al. [9] synthesized oxide catalysts with nanometric particle size in the system Ce–Nb–Ni by a sol–gel method and found good yields to propene at low reaction temperature. Mixed NiO–MOx (M = Bi, Sb, P) oxides in different proportions were also applied, supported on MgO, Al2 O3 , SiO2 , TiO2 and ZrO2 . The best performance was obtained for NiO-Bi2 O3 /ZrO2 system [10]. In the majority of cases nickel improved the catalytic behavior in oxidative dehydrogenation reaction. Nickel catalysts have also been successfully tested in the oxidative dehydrogenation of ethane. Recent works of Lemonidou et al. [11] and Millet et al. [12] showed that a modification of NiO by Nb, Li, Mg, Al, Ga, Ti or Ta enhanced the selectivity to ethene, being Li and Mg the more effective dopants. These results were associated to non-stoichiometric oxides formed by dopant incorporation that decrease O− surface species concentration, thus avoiding total oxidation reactions. Some studies deal with properties of Ni–V [13] or Ni–V–(Si, Zr) oxides [8,14] applied to propane ODH and obtained from different preparation techniques, such as impregnation, evaporation, sol–gel and mechanical mixture. To the best of our knowledge, however, catalysts based on Ni–Al and Ni–V–Al systems have not been studied in propane ODH. In this paper we report a study of the performance of catalysts derived from nickel–vanadium–aluminum hydrotalcites in ODHP reaction under conditions of oxygen deficiency and oxygen excess. Hydrotalcites were chosen as precursors since their calcination yields solids with a very homogeneous distribution of metallic elements. Two precursors were prepared using respectively hydroxyl and carbonate as compensation anions which were further exchanged by decavanadate ions. Textural, redox and structural properties of the catalysts were compared in order to interpret the catalytic results.

2. Experimental 2.1. Synthesis of the materials The nickel–aluminum hydrotalcite with hydroxyl and carbonate anions were synthesized by a hydrothermal method. Two aqueous solutions were prepared, the first with the di- and tri-valent elements using NiSO4 ·6H2 O (Vetec) and Al2 (SO4 )3 ·18H2 O (Merck) and the second with the carbonate anion and hydroxyl sources using Na2 CO3 (Carlo Erba) and NaOH (Merck). The metal solution was added slowly to the anion compensation one with constant stirring forming a green precipitate. The molar ratio used for Ni:Al was 3:1 and Ni:A was 6:1 (A being the compensation anion). The mixture was poured into a Teflon-lined autoclave, which was heated into a convection oven at 100 ◦ C for 24 h. The solid product was recovered by filtration, washed to neutral pH and dried at 100 ◦ C for 12 h. The vanadium isopolyanion was obtained in solution at room temperature by dissolving V2 O5 (Sigma, 96%) in methyl amine (Aldrich) and acidifying with 0.5 M HNO3 (Merck, 85%) to pH 4.5, in order to favor the formation of the decavanadate species [15]. Hydrotalcite were intercalated with the vanadium isopolyanion by ion exchange at room temperature, following a similar procedure to that established by Gardner et al. [16]. The vanadium solution was slowly added to a dispersion of hydrotalcite in water, previously acidified to pH 4.5 with 0.5 M HNO3 ; then the pH was controlled with the same acid for 2 h at this same pH value. The system was then kept under stirring for 20 h, and the final pH was 6. The solid was washed several times with de-ionized water up to neutral pH, separated by filtration and finally dried at 100 ◦ C for 12 h.

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The added vanadium amount was calculated from the theoretical formulae of the hydrotalcites [Ni6 Al2 (OH)16 ]CO3 ·4H2 O and [Ni6 Al2 (OH)16 ](OH)2 ·4H2 O, for a 30% replacement of the compensation anion by the decavanadate species (V10 O28 6− ), and using an additional of 10% of excess relative to the stoichiometric value. The solids obtained were named NiAlH, NiAlC, NiAlHV and NiAlCV, where H and C letters stand respectively for hydroxyl and carbonate compensation anions. The catalysts were obtained by calcination of the hydrotalcites at 550 ◦ C for 3 h with a 5 ◦ C min−1 heating rate. For the designation of the calcined solids, a “ Cal” suffix was added to the hydrotalcite precursor name. 2.2. Characterization of the materials X-ray diffraction (XRD) powder patterns of the precursors were obtained using a Synchrotron light source, at the D10B-XPD line of LNLS (Laboratório Nacional de Luz Sincrotron, Campinas, Brazil), with Si (1 1 1) monochromator. The analysis was performed in the range 5–70◦ in 2, with step size of 0.02◦ and wavelength  = 0.137762 nm. XRD powder patterns of the catalysts were obtained on a Rigaku Miniflex instrument operated at 40 kV and 30 mA with a Cu source ( = 0.15418 nm) and scanning from 3◦ to 70◦ in 2. The elemental composition of the catalysts was determined using a Rigaku, Rix3100 X-ray fluorescence spectrometer, with a rhodium tube operated at 4 kW. Temperature-programmed reduction analyses were carried-out in a Zeton Altamira AMI-70 equipment. Samples were pre-treated by heating from room temperature to 300 ◦ C under argon atmosphere. The reduction of the materials was carried-out with a flow of 30 mL min−1 H2 /Ar (10%, v/v) and heating from room temperature to 1000 ◦ C at 10 ◦ C min−1 . Surface areas were obtained from N2 adsorption isotherms using the BET method and a Micromeritics ASAP 2010 apparatus, after the samples were outgassed at 200 ◦ C. XAS measurements were performed at the XAS beam line of the LNLS storage ring, operated at 1.37 GeV with a maximum beam current of 250 mA. Monochromatization of the incident beam was made using channel cut Si (1 1 1) crystals, spectra were obtained at room temperature. Energy calibration of the monochromator was performed with a V-metal foil and Ni-metal foil. The scans were recorded at the V K edge for each sample with incremental energy steps of 2 eV from 5355 to 5455 eV, 0.3 eV from 5455 to 5510 eV and 1 eV from 5510 to 6800 eV. The scans for the Ni K edge were recorded for each sample with incremental energy steps of 2 eV from 8230 to 9400 eV. The EXAFS spectra were analyzed by a standard procedure of data reduction and least squares fitting using the IFEFFIT code [17] with phase and amplitude functions calculated using the FEFF 8.20 code [18,19]. Simulations were made in R-space using k ≈ 2–14 A˚ −1 , dk = 2 and kweight = 3 and known model structures to determine structural parameters in each case: mean bond distances (R), Debye–Waller factors ( 2 ) and coordination numbers (N). All fitting parameters (Rfactor , E) along with associated errors are given as they are provided by the IFEFFIT code. 2.3. Catalytic test The solids were tested in oxidative dehydrogenation of propane using a conventional flow system. The catalyst (200 mg) was deposited on a porous fixed bed in a Pyrex reactor in U tube form operating under atmospheric pressure and two different feed conditions; oxygen deficiency with molar ratios C3 H8 :O2 :N2 of 3:1:96 and oxygen excess with molar ratios of C3 H8 :O2 :N2 of 1:20:79, both with nitrogen as an inert diluent. The space velocity was 100, 150, 200 and 400 ml g−1 min−1 . The catalytic zone was isothermal at 400 ◦ C. Complete, on line analysis of reactants and products (C3 H8 and CO, CO2 , C3 H6 ) was done in a single step using a Shimadzu GC-17AAF chromatograph with FID detection. The gas sample ran

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Table 1 Crystallographic parameters of hydrotalcites precursors, composition and surface area of the corresponding catalysts. Hydrotalcite

a (Å)

c (Å)

NiO (% w/w)

Al2 O3 (% w/w)

V2 O5 (% w/w)

Surface area (m2 g−1 )

NiAlC NiAlH NiAlCV

3.0405 3.0428 3.0433 3.0439 3.0428

23.139 23.252 23.252 35.288 23.252

80.4 80.5 64.8

19.6 19.5 17.5

– – 16.2

97 138 118

68.3

16.0

15.3

112

NiAlHV

through a MTN-1 methanizer after elution on a Porapak Q column and before detection. In all cases, carbon balances attained 100%, within the experimental precision of the system. Lack of external and internal diffusion limitations was confirmed. As usual [20], experiments to check interphase transport effects were carried out varying the velocity flow while maintaining space velocity constant. The Weisz–Prater Criterion [21] was calculated with observable quantities in order to check for absence of internal diffusion. Initial reaction rates were obtained using fractional propane conversion vs space time data at 400 ◦ C and finding the tangent of the curve at space time equal to zero. In order to find the tangent a polynomial fitted to conversion-space time data was differentiated, following proposed techniques [21].

3. Results and discussion 3.1. Characterization The precursors before vanadate intercalation showed a higher crystallinity than intercalated materials (see Supplementary material). The hydroxyl anion materials are more crystalline than those containing carbonate. Indexing of the pattern lines of NiAlH confirms that hydrotalcite-like material was obtained. Taking this as a reference it is possible to say that the NiAlC material possesses also the hydrotalcite structure. However, broadening of the peaks attributed to (0 1 2), (0 1 5) and (0 1 8) planes indicates the presence of structural defects due to polytype stacking of manasseite and hydrotalcite phases, according to studies by De la Calle et al. [22]. Miller indexes were attributed to different XRD peaks of the precursors before intercalation and the lattice parameters were calculated and refined with the CHEKCELL program in the hexagonal crystal system with space group R-3m; the results are shown in Table 1. The samples with vanadium showed, besides the peaks attributed to hydrotalcite, two additional peaks. In the case of the carbonate precursor, these additional lines are in the 2 position at 6.7 and 13.4 and correspond to the families (0 0 3) and (0 0 6) planes ˚ of hydrotalcites with interlayer distance of approximately 11.7 A. It is precisely the size that is expected when the decavanadate is intercalated between the layers [16]. In the diffraction pattern there also appears reflections of the precursor without vanadium, then, it may be said that the precursor NiAlCV is a mixture of two hydrotalcite-type phases, one with carbonate only and the other with decavanadate anion only in the interlayer space. Table 1 shows the cell parameters for the two phases, where a is almost the same for the two phases (a represents the hexagonal parameter of the layer that appears to be independent of the intercalated species) and c is 23.2 A˚ for the carbonate-intercalated phase and 35.3 A˚ for the vanadate-intercalated phase. In the case of the hydroxylintercalated precursor, additional peaks are centered at 8.0 and ˚ Indexing of those 12.0 giving inter space distances of 9.8 and 6.6 A. lines was attempted in the same crystal system assuming an inter˚ but a good fit was not layer distance of the hydrotalcite of 9.8 A, found. V4 O12 4− would be the intercalated species that would best fit our results, corresponding to (0 0 3) and (0 0 6) families at 9.6 ˚ respectively [23], but the second reflection (0 0 6) did and 4.8 A,

not appear in NiAlHV. Therefore, in the case of precursor NiAlHV intercalation with the decavanadate anion was not confirmed, suggesting that a pure vanadium phase might precipitate. Taking this result into account we may conclude that NiAlHV is a mixture of a hydrotalcite-type compound and some unidentified vanadium compound. The crystalline phases in the catalysts were identified by XRD (see Fig. 1) and compared with the Inorganic Crystal Structure Database (ICSD). It can be seen that after calcination the hydrotalcite-type precursor collapses to a NiO structure (ICSD 92127). In the catalyst NiAlC Cal there appears a series of low intensity peaks in the region between 20 and 35◦ which were attributed to 5Al2 O3 ·NiO from the Powder Diffraction File database (PDF 00037-1294). Crystalline vanadium phases were not observed. On the other hand, it was observed that the two materials derived from hydrotalcite-type precursors with carbonate compensation anion have similar textural properties (adsorption isotherms can be found in supplementary material), i.e. hysteresis curves and adsorbed volumes are similar. In the case of the catalysts derived from precursors with hydroxyl anion, the hysteresis curve is similar, but the adsorbed volume is higher in the catalyst without vanadium. Adsorption isotherms according to IUPAC are of type IV, indicating possibly mesoporous solids [24]. The hysteresis loop of NiAlHV Cal and NiAlH Cal materials could be classified as H3 type, which characterizes rigid plate-shaped particles aggregates, creating pores in slots. The other two catalysts could be classified as H1 type hysteresis, associated with openat-the-two-extremities cylinder type mesoporous materials. The specific area data shown in Table 1 indicates that the solid with the largest area was obtained after calcination of the precursor with hydroxyl ion as the compensation anion, and the lowest area was from the carbonate-intercalated precursor. After vanadium intercalation, intermediate values are observed. It is clear that the nature of the compensation anion influences the textural properties of the final solid.

Fig. 1. X-ray diffraction patterns of the catalysts.

J.A. Valverde et al. / Catalysis Today 192 (2012) 36–43

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Table 2 Ni-K edge EXAFS simulation for NiAlH and NiAlH Cal. NiAlH Shell Ni–O Ni–O Ni–Ni Ni–Al

Na 3 (3) 3 (3) 4.94 (3) 1.06 (3)

NiAlH Cal Shell Na Ni–O 6 (6) Ni–Ni 12 (12) Ni–Ni 6 (6)

 2 (Å2 ) × 103 1.5 (1) 1.8 (1) 6.5 (5) 6.5 (5)

R (Å)a 2.10 (1.97) 2.10 (2.13) 3.06 (3.05) 3.12 (3.06)

Rfactor

E0

5.11 × 10−3

−0.8132

 2 (Å2 ) × 10−3 6.4 (1) 8.7 (5) 10.7 (2)

R (Å)a 2.11 (2.11) 2.95 (2.95) 4.18 (4.18)

Rfactor

E0

1.67 × 10−4

−1.29

a The value between parentheses is the theoretical value obtained from the model structure.

Fig. 2. H2 -TPR of the catalysts.

The results of XRF measurements are also shown in Table 1. The Ni and Al amounts for samples from the same anion are similar. Atomic ratios were calculated for Ni/Al and V/Al giving approximately 3 and 0.5, respectively in all samples, in agreement with theoretical values. Fig. 2 shows the hydrogen reduction profiles of the catalysts. The samples without vanadium were reduced almost in a single step with maximum at 665 ◦ C and 711 ◦ C for NiAlC Cal and NiAlH Cal, respectively. The vanadium-containing catalysts showed two reduction events, the first with maximum at 380 ◦ C for NiAlCV Cal and 378 ◦ C for NiAlHV Cal, and the second event overlapping with the first reduction event at 561 and 596 ◦ C, respectively, close to the reduction temperature of materials without vanadium. Hydrogen consumption was calculated, giving very similar values for compounds with vanadium (10.8 mmol g−1 for NiAlHV Cal and 10.3 mmol g−1 for NiAlCV Cal) and without vanadium (12.9 mmol g−1 for NiAlH Cal and 13.7 mmol g−1 for NiAlC Cal). Considering complete reduction of nickel to Ni0 and partial reduction of vanadium V5+ to V3+ [25] in the temperature range studied, the total theoretical hydrogen consumption would be 10.8 for NiAlHV Cal and 10.5 mmol g−1 for NiAlCV Cal, in agreement with experimental measurements. In the case of precursors without vanadium, only nickel can be reduced and the theoretical uptake assuming the reduction of Ni+2 to Ni0 for the two samples would be 10.8 mmol g−1 . However, higher experimental uptakes might indicate the presence of nickel species with higher oxidation state such as Ni+3 . Similar conclusions were reached by Dziembaj et al. in oxides derived from Ni–Mg–Al hydrotalcite type with carbonate as compensation anion [26]. Significant amounts of Ni3+ were also observed by XPS at the surface of NiO–ZrO2 catalysts [27]. 3.2. Structural studies X-ray absorption spectra were obtained for hydroxyl precursor and the corresponding catalyst, because this material showed interesting catalytic properties (shown in the next section). For the EXAFS simulation of the precursor NiAlH, atomic positions of a magnesium–aluminum hydrotalcite with Mg:Al ratio of 2 and carbonate compensation anion were used [28]. In this case the presence of the compensation anion has no influence on the analysis, since only atoms from the same brucite-type layer contribute to

the EXAFS spectrum. The results for simulations are in Table 2 (the respective Figure is supplied as Supplementary material). The fitting parameters in Table 2 show that the fit of nickel as part of a brucite-type layer is very good, however, analysis of the third shell around 3 A˚ gave a ratio Ni/Al of 4.7, a quite different value from that determined by XRF (2.8). This EXAFS ratio may suggest that part of aluminum was not inserted in the brucite-type layer. The results of structural studies of the calcined material (NiAlH Cal) are shown in Fig. 3 and Table 2. EXAFS simulation was based on the NiO structure. The fitting parameters are quite good, however for coordination shells farther than 3 A˚ the spectrum may be affected by the presence of other species, probably aluminum. Simulations using NiAl2 O4 spinel as model structure were attempted, but this structure was dismissed due to low correlation values and lack of physical significance for Debye–Waller parameters. For this reason aluminum is expected to be found as isolated species, maybe as amorphous Al2 O3 oxide. Table 3 shows the EXAFS simulations at the absorption edges of nickel and vanadium for NiAlHV precursor (the respective figure is included as Supplementary material). Good fit of the experimental results was found at Ni-K edge, corroborating that nickel is coordinated within a brucite-type layer similar to NiAlH. On the other hand, Table 3 shows a similar result to that reported in Table 2, that is, the Ni/Al ratio calculated by EXAFS is 4.2, suggesting again that part of aluminum species does not belong to the hydrotalcite phase. On the other hand, the results of X-ray diffraction showed that vanadium was not intercalated in the hydrotalcite structure, suggesting that some vanadate phase formed as a precipitate. Simulation of the EXAFS spectrum at the absorption K-edge of vanadium using the decavanadate V10 O28 6− as a model structure was carried out. Taking into account XRD data, the good quality of this EXAFS fitting indicates that decavanadate precipitated as a separate phase and did not exchange with the hydroxyl group inside the hydrotalcite layer. Table 3 Ni K and V K of EXAFS simulation for NiAlHV. Edge

Shell

Na

 2 (Å2 ) × 103

R (Å)a

Rfactor

E0

Ni

Ni–O Ni–O Ni–Ni Ni–Al

3 (3) 3 (3) 4.84 (3) 1.16 (3)

1.9 (2) 1.2 (9) 6.5 (5) 6.5 (5)

1.99 (1.97) 2.10 (2.13) 3.06 (3.05) 3.13 (3.06)

2.95 × 10−3

−1.12

V–O V–O V–O V–O V–V

0.8 (1) 2.8 (3) 1.2 (1) 1.2 (1) 4.8 (5)

1.0 (1) 11.6 (4) 5.8 (7) 3.2 (3) 9.5 (1)

1.64 (1.61) 1.86 (1.82) 2.08 (2.00) 2.30 (2.22) 3.14 (3.05)

1.49 × 10−2

3.22

V

a The value between parentheses is the theoretical one obtained from model structure.

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Fig. 3. Ni-K edge EXAFS oscillations in R and k spaces of the NiAlH Cal. – observed spectrum, -o- simulated spectrum.

The Ni-K and V-K EXAFS spectra of NiAlHV Cal are displayed in Fig. 4. Different simulations were performed of the V K-edge spectrum, comparing the structure of four compounds that could possibly form at a temperature of 550 ◦ C, namely Ni2 V2 O7 , Ni3 V2 O8 , V2 O5 and AlVO4 . Simulations using the last two structures were quite unsuccessful. In contrast, simulation using both nickel orthovanadate and nickel pyrovanadate provided good results, as shown in Fig. 4a and b and Table 4, the best fit was obtained with

a proportion Ni3 V2 O8 :Ni2 V2 O7 of 8:2. These two compounds have been shown to be quite selective in ODHP [29]. In agreement with XRD data, the Ni-K EXAFS spectrum could be satisfactorily fitted using NiO as a model structure (Table 4 and Fig. 4c). However, spectral features between 3 and 4 A˚ could only be interpreted by considering a contribution from two Ni–V paths at 3.19 and 3.43 A˚ corresponding to nickel pyrovanadate and/or orthovanadate (Fig. 4d). The spectrum was simulated using the structure

Fig. 4. (a) V-K edges EXAFS oscillations in R space, (b) V-K edges EXAFS oscillations in k spaces, (c) Ni-K edges EXAFS oscillations in R space, NiO contributions and (d) Ni-K ˚ of NiAlHV Cal. – observed spectrum, -o- simulated spectrum. edges EXAFS oscillations in R space, Ni3 V2 O8 contributions (V1 1: 3.43, V1 2: 3.44 and V1 3: 4.41 A)

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Table 4 Ni-K and V-K EXAFS simulation for NiAlHV Cal. Edge

Shell

Na

 2 (Å2 ) × 103

R (Å)a

Rfactor

E0

Ni

Ni–O Ni–Ni Ni–V Ni–V Ni–Ni Ni–Ni

6 (6) 12 (12) 5 (5) 4 (4) 6 (6) 24 (24)

6.97 10.64 8.20 8.20 15.16 13.70

2.06 (2.11) 2.96 (2.95) 3.19 (3.17) 3.43 (3.45) 4.16 (4.18) 5.11 (5.12)

1.67 × 10−3

4.89

V–O V–V V–Ni V–Ni

4 (4) 1 (1) 6 (6) 9 (9)

7.28 6.38 6.41 10.96

1.72 (1.63) 2.89 (3.14) 3.10 (3.17) 3.37 (3.35)

8.06 × 10−2

−4.10

V

a

The value between parentheses is the theoretical one obtained from model structure.

Table 5 Activity of the catalysts.

a

Operation conditions

Catalysts

r0  (␮mol min−1 g−1 )

r0 (␮mol min−1 m2 )

Activitya (mmol min−1 atom Ni−1 )

Oxygen deficiency

NiAlC Cal NiAlH Cal NiAlCV Cal

16 27 22

0.16 0.20 0.18

1.1 2.3 1.5

Oxygen excess

NiAlC Cal NiAlH Cal NiAlCV Cal NiAlHV Cal

63 128 31 43

0.65 0.92 0.26 0.38

4.6 9.3 2.8 3.7

Ni bulk content.

of nickel orthovanadate only, for the sake of simplicity leading to a proportion of vanadium to nickel of about 0.21; this value is close to that reported in Table 2, where the molar ratio V/Ni is 0.18. 3.3. Catalytic tests Catalytic performances of materials derived from nickel–aluminum hydrotalcite-type precursor under conditions of oxygen deficiency and oxygen excess are shown in Fig. 5. It can be observed that operating conditions have a strong influence

on the behavior of these materials; the catalyst NiAlH Cal reached the highest selectivity under oxygen deficiency, but the conversion increased almost five times and its selectivity decreased under oxygen excess, which agrees with the findings of Barsan and Thyrion [6]. Kinetic studies carried out with a Ni–Co–Mo catalyst by these authors showed that increasing oxygen partial pressure in ODHP favored propane conversion and selectivity toward CO2 , negatively affecting selectivity toward propene. Fig. 6 displays the conversion profile with space time (inverse of space velocity, 1/Ve) for the two operation conditions. An increase

Fig. 5. Catalytic performance under (a) oxygen deficiency and (b) excess oxygen conditions. Dotted lines represent iso-yield curves. Temperature 400 ◦ C.

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Fig. 6. Conversion vs. space time (1/Ve) at 400 ◦ C under (a) oxygen deficiency and (b) oxygen excess conditions: (1) NiAlH Cal, (2) NiAlHV Cal, (3) NiAlC Cal, (4) NiAlCV Cal. (*) represents oxygen conversion.

of the conversion is observed with space time for almost all catalysts, but NiAlHV Cal under oxygen deficiency has a roughly constant behavior with high conversion, indicating that under that reaction oxygen consumption was complete; since the reaction is limited by oxygen, activity values could not be calculated, however, this is the catalyst with the highest conversion up to space time of 0.0067 min g ml−1 . The most representative material under any conditions is NiAlH Cal. Making comparisons with its homologous NiAlC Cal, the two catalysts show good performances, but NiAlH Cal promotes higher conversion and selectivity. Both have the same Ni/Al ratio, however, the main difference is in their textural properties since, NiAlH Cal has a higher surface area. The initial reaction rates were calculated from data in Fig. 6 and are showed in Table 5. Intrinsic activity calculated per Ni atom shows (last column in Table 5) that NiAlH Cal is the most active catalyst in deficiency and excess of oxygen. Our catalysts exhibited initial reaction rates higher than other reported in the literature for the same temperature (reported r0  values are 3.7 ␮mol min−1 g−1 for NiMoO4 [30], 28.2 ␮mol min−1 g−1 for NiW0.5 Mo0.5 O4 [7] and 1.04 ␮mol min−1 g−1 for NiAl2 O4 [31]). In general, all nickel catalysts exhibited good performance, promoting moderate conversion and selectivity above 40%; however, for vanadium-containing catalysts, selectivity improved and conversion doubled under oxygen excess. It is expected that the local oxygen/propane ratio in the feed should contribute to inducing the reaction route, favoring dehydrogenation pathways at low ratio and high temperature and oxidative dehydrogenation pathways at high ratio and moderate temperature. In the present study the temperature reaction was fixed at 400 ◦ C (moderate temperature). From this point of view, our results indicate that Ni catalysts are selective when reaction conditions are not ideal for oxidative dehydrogenation and V is selective when oxidative dehydrogenation is expected from the reaction conditions. The nature of the compensation anion in the interlayer space of the hydrotalcite-type precursor has a strong influence on the properties and catalytic behavior of the final material; all calcined

catalysts contain basically NiO as a crystalline phase, but starting from hydroxyl anion provides more active catalysts. The way vanadium is incorporated in the catalyst, that is: inside or outside the hydrotalcite layer, may have a strong influence too. When decavanadate ion did not enter the interlayer space and formed an isolated phase, nickel pyrovanadate and nickel orthovanadate were found in the final catalyst and a higher activity was observed. Xu et al. tested the Ni–V–O catalyst in ODHP and found a synergic effect between NiO and Ni3 V2 O8 , with Ni/V ratio approximately equal to 9. They also determined that traces of NiV2 O7 and V2 O5 lead to a decrease of selectivity toward propene [13]. Other works with Ni/V ratio of approximately 0.2 showed lower selectivities under similar conditions [8,14]. Our data are consistent with these observations and confirm that high Ni/V ration (5.4) leads to preferential crystallization of Ni3 V2 O8 in a material, showing high selectivity to propene. On the other hand, activity and selectivity toward propene are strongly correlated to redox properties of the material. We observed that catalyst activity for ODHP (Table 5) approximately followed the same order as the maximum reduction temperature: NiAlH Cal > NiAlC Cal > NiAlHV Cal > NiAlCV Cal, under both oxygen deficiency and oxygen excess conditions, although activities are about half an order of magnitude higher under excess of oxygen for NiAl catalyst. Under oxygen deficiency conditions (Fig. 5a), selectivity toward propene followed this same sequence in the 8–9% propane conversion range, when oxygen conversion is higher than 80%. The first (activity) correlation indicates that the surface of more easily reduced catalysts (lower maximum reduction temperature), which is probably more depleted in oxygen promotes propane oxidative dehydrogenation with more difficultly. Some very reducible oxides can indeed have low activity in oxidative dehydrogenation, because they have more capacity to reduction than to re-oxidation. In this case chemisorption is the determining step in the Mars–van Krevelen redox mechanism. This behavior can be depicted by a volcano-type plot with a maximum rate at moderate reducibility [32]. Analogously, high maximum reduction temperatures (low reducibility) can limit complete oxidation of

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propane and propene and improve the selectivity to propene [9]. This may explain why we observed an increase of selectivity with maximum reduction temperature under oxygen deficiency conditions. In contrast vanadium-containing catalysts displayed the highest selectivity toward propene in this same propane conversion range under oxygen excess conditions (Fig. 5b). Interestingly, due to these differences in propene selectivity under oxygen excess conditions, the two vanadium-containing materials provide almost equal propene yield as the more active NiAlH Cal catalyst (the three catalysts lay over the same selectivity–conversion curve). In such conditions where oxygen conversion is low, the vanadiumcontaining phase is probably present in its oxidized form. These results combined with EXAFS data suggest that reduced vanadium phases should be less selective toward propene or less active than oxidized nickel orthovanadate and pyrovanadate phases. 4. Conclusions Nickel–aluminum hydrotalcite type precursors with carbonate and hydroxyl as compensation anions were synthesized. Decavanadate anion was successfully intercalated into the layers in the carbonated precursor, but not in the hydroxylated one. Hydroxylated precursors, however, promoted structural and textural properties that are suitable to promote the ODHP reaction. Analyses of EXAFS spectra confirmed isomorphic substitution of Al for Ni in the NiAlH (hydroxylated) hydrotalcite-type precursor and the presence of decavanadate species in the sample after contact with vanadate solution. The catalyst NiAlHV Cal presented nickel pyrovanadate and nickel orthovanadate besides NiO, the only phase identified by XRD. Properties such as specific area and reducibility are clearly related to the catalytic behavior of the samples. Materials derived from NiAlH hydrotalcite were the most representative catalysts with activities of 2.3 and 9.3 mmol min−1 atom Ni−1 for NiAlH Cal under oxygen deficiency and excess conditions respectively and 3.7 mmol min−1 atom Ni−1 for NiAlHV Cal under excess oxygen condition; this last catalyst showed the higher conversion under oxygen deficiency, but it was not possible to calculate activity reaction rate, due to complete oxygen conversion. Lower activity values were observed for the other catalysts. Operating conditions had a strong influence on catalytic performances; under conditions of oxygen deficiency, the catalyst NiAlH Cal still displayed selectivity to propene around 57% at 10% propane conversion while the other materials needed lower conversion, typically between 3 and 7%, to achieve this selectivity. Under excess oxygen condition the vanadium catalysts were more selective than the other materials. The catalyst NiAlH Cal showed almost the same selectivity but higher activity with an initial reaction rate of 0.13 mmol min−1 g−1 , being two or three times more active than the other catalysts. Both catalysts provided approximately equal propene yield of 6%. Acknowledgements This work was carried out with financial support from the Colombian institutions SENA-COLCIENCIAS, CODI-UdeA and

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Sostenibilidad 2011-2012-UdeA. We acknowledge NUCAT (COPPE, Universidade Federal do Rio de Janeiro - Brazil) for XRF measurements and LNLS (Laboratório Nacional Luz Síncrotron, Campinas–Brazil) for XAS (XAFS1-9126) and XRD (XPD-9183) measurements. J.-G.E. thanks CNPq (Conselho Nacional de Pesquisa e Desenvolvimento of Brazil) for support during this work. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.cattod.2012.04.043. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]

[21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]

M. T. Devanney, Propylene, CEH Report, July, 2009. D. Sanfilippo, I. Miracca, Catalysis Today 111 (2006) 133–139. F. Cavani, N. Ballarini, A. Cericola, Catalysis Today 127 (2007) 113–131. F. Cavani, F. Trifirò, A. Vaccari, Catalysis Today 11 (1991) 173–301. S.M. Auerback, K.A. Carrado, P.K. Dutta, Handbook of Layered Materials, Marcel Dekker, New York, 2004. M.M. Barsan, F.C. Thyrion, Catalysis Today 81 (2003) 159–170. M. Salamanca, Y. Licea, A. Echavarría, A. Faro, L. Palacio, Physical Chemistry Chemical Physics 11 (2009) 9583–9591. M. De, D. Kunzru, Reaction Kinetics and Catalysis Letters 91 (2007) 263–271. J. Li, C. Wang, C. Huang, W. Weng, H. Wan, Catalysis Letters 137 (1–2) (2010) 81–87. K. Fukudome, Catalysis Letters 141 (2010) 68–77. E. Heracleous, A.A. Lemonidou, Journal of Catalysis 270 (2010) 67–75. B. Savova, S. Loridant, D. Filkova, J.M.M. Millet, Applied Catalysis A-General 390 (2010) 148–157. B. Zhaorigetu, W. Li, R. Kieffer, H. Xu, Reaction Kinetics and Catalysis Letters 75 (2002) 275–287. ´ A. Klisinska, A. Haras, K. Samson, M. Witko, B. Grzybowska, Journal of Molecular Catalysis A: Chemical 210 (2004) 87–92. J.W. Larson, Journal of Chemical and Engineering Data 40 (1995) 1276–1280. E.A. Gardner, S.K. Yun, T. Kwon, T.J. Pinnavaia, Applied Clay Science 13 (1998) 479–494. M. Newville, Journal of Synchrotron Radiation 8 (2001) 322–324. J.J. Rehr, R.C. Albers, Reviews on Modern Physics 72 (2000) 621–654. A.L. Ankudinov, B. Ravel, J.J. Rehr, S.D. Conradson, Physical Review B 58 (1998) 7565. G. Emig, R. Dittmeyer, Simultaneous heat and mass transfer and chemical reaction, in: G. Ertl, H. Knozinger, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, vol. 3, VCH, 1997, pp. 1209–1242. H.S. Fogler, Elements of Chemical Reaction Engineering, 4th ed., Prentice Hall, New York, 2005. C. De la Calle, C.H. Pons, J. Rou, V. Rives, Clays and Clay Minerals 51 (2003) 121–132. F. Kooli, V. Rives, M.A. Ulibarri, Inorganic Chemistry 34 (1995) 5114–5121. J.L. Figueiredo, F.R. Ribeiro, Catálise Heterogênea, 2nd ed., Fundac¸ão Calouste Gulbenkian, Lisboa, 2007. I.E. Wachs, Y. Chen, J. Jehng, L.E. Briand, T. Tanaka, Catalysis Today 78 (2003) 13–24. P. Lucjan Chmielarz, A. Ku´strowski, R. Rafalska-Łasocha, Dziembaj, Thermochimica Acta 395 (2003) 225–236. K. Fukudome, A. Kanno, N. Ikenaga, T. Miyake, T. Suzuki, Catalysis Letters 141 (2011) 68–77. A.V. Arakcheeva, D.Yu. Pushcharovskii, R.K. Rastsvetaeva, D. Atencio, G.U. Lubman, Kristallograhie 41 (1996) 1024–1034. Y. Wu, Y. He, T. Chen, W. Weng, H. Wan, Applied Surface Science 252 (2006) 5220–5226. Y.-S. Yoon, N. Fujikawa, W. Ueda, Y. Moro-oka, K.-W. Lee, Catalysis Today 24 (1995) 327–333. ´ J. Słoczynski, J. Ziókowski, B. Grzybowska, R. Grabowski, D. Jachewicz, K. Wcisło, L. Gengembre, Journal of Catalysis 187 (1999) 410–418. K. Chen, A.T. Bell, E. Iglesia, Journal of Physical Chemistry B 104 (2000) 1292–1299.