Article pubs.acs.org/IECR
Propane Oxidative Dehydrogenation on ZnCoMo and NiCoMo Catalysts Obtained from ϕy and ϕx Precursors Juliana Velasquez,† Adriana Echavarria,† Arnaldo Faro,‡ and Luz A. Palacio†,§,* †
Grupo Catalizadores y Adsorbentes, Universidad de Antioquia, Calle 67 No. 53-108, A.A. 1226 Medellín, Colombia Instituto de Química, Universidade Federal do Rio de Janeiro, Avenida Athos da Silveira Ramos, 149 Bloco A. CEP: 21941-909 Rio de Janeiro, Brazil § Instituto de Química, Universidade do Estado do Rio de Janeiro, Rua São Francisco Xavier, 524, CEP: 20550-900 Rio de Janeiro, Brazil ‡
S Supporting Information *
ABSTRACT: The synthesis and characterization of trimetallic oxides of the type MCo−Mo (with M = Ni or Zn) in both ϕyand ϕx-type phases (depending on the compensation ion) is described. The materials were calcined at 673 K to obtain mixed oxides with surface areas between 27 and 32 m2 g−1 that were found to be nonstoichiometric. The catalysts were tested in the oxidative dehydrogenation of propane at moderate temperature (673 K). It was found that precursors with a ϕy-type phase provide higher catalytic performance than those obtained as ϕx-type materials, which could be related to the higher suface area and better reducibility properties of the former. The influence of Ni on the catalytic performance was found to be positive in the two types of structures compared to the undoped CoMo catalysts. On the other hand, Zn introduction had a negative effect on the intrinsic reaction rate for both ϕy- and ϕx-derived catalysts. The NiCoMoϕy400 catalyst exhibited the best performance; thus, further tests were performed with this catalyst at a lower space velocity, and a higher propene yield was obtained. The NiCoMoϕy400 catalyst exhibited good stability after being used for 24 h. The yields achieved were around 5%, which is a good result compared with similar systems described in the literature. described by Pezerat8 in the mid-1960s. Their structure was resolved later by Clearfield et al.9 (ϕx) and Ying et al.10 (ϕy), both in 1996. These materials can be broadly described with the ideal formula AOH(TMoO4)2·H2O, where T is a divalent cation, such as Ni2+, Co2+, Zn2+, or Mn2+, and A is a chargecompensating cation, such as NH4+, Na+, or K+. In both structures, the divalent metal is in octahedral coordination, and the hexavalent metal is in tetrahedral coordination, forming sheets that are attached to one another by van der Waals forces. In the ϕy structure, the octahedra share an edge to form the layer, and the tetrahedra are attached to this layer by Mo−O−T bonds. On the other hand, in the ϕx structure, the layers are made of parallel chains of vertex-sharing octahedra connected by tetrahedral molybdate groups. In this work, layered precursors with specific metal ratios were synthesized and then calcined to obtain the mixed-oxide catalysts. The materials were tested as catalysts in the oxidative dehydrogenation of propane, and the effects of dopants, such as nickel and zinc, were studied.
1. INTRODUCTION Oxidative dehydrogenation (ODH) of light alkanes is a potentially interesting method of producing ethylene, propylene, and butylene because the demand for these important primary petrochemicals is growing rapidly and production by traditional paths, such as cracking and direct dehydrogenation, is inefficient and has thermodynamic limitations.1,2 Nevertheless, ODH has some disadvantages that currently make it unfeasible for commercial implementation, the main one being the large amount of carbon oxides produced at the temperatures normally needed to activate the alkanes.3 This problem could be solved by developing more selective catalysts. Among the catalysts tested in propane ODH, those with V and Mo in their composition were found to be highly active,4,5 but there is little information on how the synthesis route of the transition metal oxides influences their performance, as most of them are obtained by calcination of a precursor that is usually not characterized. Supported cobalt molybdate is one of the most investigated and potentially useful catalysts for this reaction.6,7 However, there is little information in the literature concerning unsupported catalysts of this kind, their precursors, or the influence of a third metal in the catalyst composition, which are the topics explored in this article. The method used to obtain such unsupported catalysts is the precipitation of the metals by either NaOH or NH4OH addition, which leads to two different structures known as ϕx and ϕy phases. When calcined, these layered precursors produce mixed oxides that are potentially useful for catalysis, because they normally have a good distribution of the metals. The ϕx and ϕy phases were first © 2013 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Synthesis. To obtain the ϕy phase, we followed our previously published procedure.11 Two solutions were prepared, one with 0.01 mol of Mo (as ammonium Received: Revised: Accepted: Published: 5582
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heptamolybdate) in 15 mL of hot distilled water (353 K) and the other with 0.009 mol of Co and 0.001 mol of metal M (as nickel or zinc nitrate) in 15 mL of distilled water at room temperature. The second solution (red colored) was added to the first. Then, 1.4 mL of ammonium hydroxide (28%) was added, and an intense violet suspension was formed. The final pH reached was 8.5. The system was stirred for 6 h at 80 °C. After this time, the solid was filtered, washed, and dried. To obtain ϕx precursors, a similar method was used, but in this case, the Mo source was sodium molybdate, and the precipitating agent was NaOH (3.0 mL, 4.0 M). The final pH reached was around 7.5. Finally, catalysts were obtained by thermal annealing of the precursors at 673 K (heating rate of 10 K min−1) for 6 h under static air. 2.2. Characterization. Powder X-ray diffraction patterns were obtained with a Rigaku Ultima IV diffractometer using Cu Kα radiation, operated at 40 kV, 20 mA, and 2° min−1. The precursors were analyzed by thermogravimetry in a TA Instruments Hi-Res TGA 2950 instrument under an air flow of 30 mL min−1 and a heating rate of 10 K min−1 to 1073 K. The metal content analysis was carried out in a S4 Thermo AA spectrometer. N2 adsorption−desorption isotherms were obtained in a Micromeritics ASAP 2010 instrument after the samples had been outgassed at 423−473 K. The surface areas were calculated from the N2 adsorption isotherms using the Brunauer−Emmett−Teller (BET) equation. Infrared spectra of the samples diluted to 1 wt % in KBr were obtained in a PerkinElmer Spectrum One spectrometer. Temperature-programmed reduction (TPR) analyses were carried out in a Zeton Altamira AMI-70 instrument, with a flow of 30 mL min−1 H2/Ar (10 vol %) and heating from room temperature to 1173 at 10 K min−1. Finally, NH3 temperature-programmed desorption (TPD) analyses were carried out in a Zeton Altamira AMI-70 instrument. Samples were pretreated by heating from 423 to 673 K under helium and then cooling at 423 K under the same atmosphere; the samples were subsequently saturated with NH3. The ammonia desorption was carried out with a He flow of 30 mL min−1 and with heating from 423 to 673 at 10 K min−1 and holding at 673 K for 30 min. 2.3. Catalytic Tests. Propane ODH experiments were carried out in a fixed-bed quartz reactor. The reactor was heated electrically and operated isothermally at 673 K. The samples were activated in a flow of air (50 mL/min) at 673 K for 45 min. The composition of the reaction mixture used for the experiments was C3H8/O2 = 2:1 with a total flow rate of 20 mL min−1 and 200 mg of catalyst (space velocity of 100 mL min−1 g−1). The reaction products were analyzed online with a Shimadzu GC-9A gas chromatograph equipped with a thermal conductivity detector (TCD). Two columns were used in the analysis: Porapak Q and MS 5A. Negligible amounts of oxygenates were observed. The alkane conversion and the selectivity to the reaction products were calculated by area normalization using response factor. No reaction products were obtained without catalyst.
Figure 1. Diffraction patterns for the precursors MCo−Mo in the (a) ϕx and (b) ϕy phases. The calculated X-ray pattern is also provided as a reference.
(NH4)OH(NiMoO4)2·H2O. The differences between calculated and experimental results for the same type of structure are due to differences in the ionic radii of the metals and in the amounts incorporated. Highly crystalline samples were obtained in both sample sets, but in ϕx materials the peaks were narrower and more intense, indicating higher crystallinity. No other phases were observed in either the undoped or doped cobalt materials, which indicates that pure phases were obtained and that Ni and Zn were incorporated into the structures, forming solid solutions. The infrared analysis (Figure S1 in the Supporting Information) confirmed the presence of water and ammonium ion in the ϕy structure, with the stretching modes for N−H and O−H in the range of 3500−3000 cm−1 and bending modes for H−N−H around 1410 cm−1 and H−O−H at 1618 cm−1. For both series, the bands at 920 and 750−800 cm−1 correspond to symmetric and asymmetric O−Mo−O stretching, respectively, and are typical of metallic molybdates where Mo is in tetrahedral coordination.13 Between 700 and 400 cm−1, only the ϕy structure presents some bands that can be attributed to M−O−Mo (for M = Co, Ni, Zn) and that are typical of this phase, but not of the ϕx phase. Thermal analysis of the materials (Figure S2 in the Supporting Information) showed that both types of phases decomposed around 673 K; thus, 673 K was chosen as the calcination temperature to obtain the catalysts. The total weight loss (from 373 to 673 K) is reported in Table 1 (fourth column). A higher amount of volatiles evolved from ϕy phases, as expected, and in general, a larger weight loss was observed for the doped precursors, as compared the undoped cobalt ones, in both the ϕx and ϕy phases. Table 1. Chemical Analysis of the Catalysts, Amounts of Volatile Decomposition Products in the Catalyst Precursors, and Surface Areas of the Catalysts
3. RESULTS AND DISCUSSION 3.1. Characterization of the Precursors. Figure 1 shows the XRD patterns for all precursors. It can be seen that either ϕy or ϕx phases were obtained, depending on whether the compensation anion selected was NH4+ or Na+, respectively. The experimental patterns were compared with calculated ones using the atomic parameters of the ϕx phase12 with formula (NH4)OH(MnMoO4)2·H2O and the ϕy phase10 with formula
catalyst
M/ Moa
(M + Co)/ Moa
volatilesb (%)
surface area (m2/g)
CoMoϕx400 NiCoMoϕx400 ZnCoMoϕx400 CoMoϕy400 NiCoMoϕy400 ZnCoMoϕy400
− 0.10 0.12 − 0.17 0.14
0.90 1.10 1.22 1.70 1.37 1.34
6.7 6.7 7.4 8.7 10.0 10.9
1 4 14 27 32 27
a
Molar ratio, where M = Ni or Zn. bAmount of water and ammonia evolved during calcination, relative to the fresh materials. 5583
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3.2. Characterization of the Catalysts. Figure 2 shows the diffraction patterns for the calcined catalysts. The ϕy
Figure 3. TPR profile of catalysts from MCo−Mo in the (a) ϕx and (b) ϕy phases.
ϕx-derived phase, whereas it seems not to have a pronounced effect on the ϕy-derived one: The profile is very similar to that of the undoped sample, and the H2 uptake is essentially the same (4.6 versus 4.7 mol of H2/mol of catalyst). Theoretical H2 uptakes were calculated using the proposed formulas obtained from chemical analysis (see Table S1 in the Supporting Information). Good agreement was obtained between the observed and calculated hydrogen uptakes, except in the case of CoMoϕx400, for which the experimental value was significantly lower than the theoretical value. Comparing the TPR profiles for catalysts obtained from the two types of phases, it can be seen that ϕx-derived catalysts have a more complex reduction path, because a larger number of peaks appeared. This is related to the different environments of the metals in the several phases formed during calcination of this phase. In the case of ϕy-derived catalysts, a simpler process seems to occur. In general, three peaks are convoluted: The first, which is highest and narrowest, occurs between 773 and 819 K; the second, shorter and broader, occurs between 933 and 981 K; and the last occurs around 1043−1103 K. Studies carried out by Brito and Barbosa17 on β-CoMoO4 established that the reduction in the first step, at 838 K, leads to an equimolar mixture of Co2Mo3O8 and Co2MoO4 and that reduction to the metals occurs at higher temperatures. The ϕy catalyst profiles were deconvoluted, and agreement with the results of Brito and Barbosa was found. For NiMoϕy400, a small peak appeared at 523 K, as a result of the reduction of dispersed Ni. TPR studies18 have revealed that pure nickel oxide has only one reduction peak with a maximum at 473 K, but when nickel oxide is impregnated on alumina in small amounts, the peak is displaced to higher temperature (523 K). The same study showed that incorporation of nickel by coprecipitation leads to higher nickel reduction temperatures (>723 K).18 The NH3 TPD profiles are shown in Figure 4, from which it is possible to confirm that the ϕx-derived catalysts are not acidic. In contrast, the ϕy catalysts present desorption of ammonia in the temperature range 423−673 K, with the catalyst containing Zn showing the highest amount of desorbed ammonia, indicating a higher acidity. This behavior could be expected, owing to the fact that calcined ϕy comes from an NH4+-containing material. This ion is known to generate protonic sites upon calcination in some catalysts, such as zeolites. On the other hand, the sodium in calcined ϕx materials is expected to prevent the appearance of acidity. 3.3. Catalytic Tests. Table 2 presents the results for the catalytic screening of the materials in the oxidative dehydrogenation of propane. In terms of conversion, selectivity, and yield, the performance was generally found to be better for ϕy-derived
Figure 2. Diffraction patterns for the catalysts from MCo−Mo in the (a) ϕx and (b) ϕy phases. beta, β-CoMoO4; NaCoMo, NaCo2.31(MoO4)3 calculated X-ray pattern provided as a reference.
calcined samples essentially correspond to β-CoMoO414 (identified as beta in the figure), whereas the calcined ϕx materials contained a mixture of β-CoMoO 4 , NaCo2.31(MoO4)315 (identified as NaCoMo in the figure), and the original ϕx phase. Qualitatively, it can be said that the undoped catalyst derived from ϕx consisted basically of the NaCoMo phase; that containing Ni had similar proportions of both beta and NaCoMo phases and a small amount of the ϕx phase; and the Zn-doped catalyst had a small amount of the beta phase and the ϕx phase remaining after calcination, without any considerable amount of the NaCoMo. Amorphous phases must be present for the mass balance to be complete in ϕx catalysts. To explain the TGA results, thermal decomposition is proposed to involve water production as the volatile species for the ϕx phase and water and ammonia for the ϕy phase. With the proposed formula for the ϕ x ph ase of NaOH(M0.1Co0.9MoO4)2·H2O, the theoretical weight-loss percentage was calculated to be 5.4%, which is slightly lower than the experimental values obtained for CoMoϕx, ZnCoMoϕx, and NiCoMoϕx. This could be related to the presence of more water of hydration than expected in these materials. In the case of ϕy, the x value in the formula (NH4)H2x(NiyCo1−y)3−xO(OH)(MoO4)2 was calculated from the chemical analysis, yielding 0.0 for CoMoϕy, 0.26 for NiCoMoϕy, and 0.32 for ZnCoMoϕy; this would correspond to (M + Co)/Mo ratios of 1.5, 1.37, and 1.34 and y values of 0, 0.12, and 0.10, respectively. The calculated weight-loss percentages for the proposed formula are 6.4% for CoMoϕy, 7.5% for NiCoMoϕy, and 7.7% for ZnCoMoϕy. These values are lower than the experimental values measured by TGA, possibly because of the presence of more water of hydration than expected, as in the case of the ϕx phases. Surface areas are also listed in Table 1. It is clear that the materials that originate from the ϕy structures have higher surface areas than those derived from the ϕx phases, possibly because of the evolution of ammonia, as well as water, during their decomposition. Results of temperature-programmed reduction (TPR) are shown in Figure 3, and the hydrogen uptakes are summarized in Table S1 (Supporting Information). In general, the reduction occurs at higher temperature for the ϕx-derived phases. For the two structures, the addition of Ni increases the reducibility (higher H2 uptake and lower-temperature onset of reduction). In the case of Zn, the reducibility is somewhat improved for the 5584
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On the other hand, the last column of Table 2 lists the initial reaction rates per unit catalyst surface area (r′0). As can be observed, the values are of the same order of magnitude for the undoped and Ni-doped catalysts, but the rate is certainly lower when Zn is present, which could indicate that, regardless of the structure, sodium content, and acidic properties, the presence of the dopant has a significant influence on the intrinsic activity of the catalysts. Because of its better yield, the NiCoMoϕy400 catalyst was chosen to be tested at a lower space velocity (50 mL g−1 min−1). The yield increased to 6.3% because of a higher propane conversion but a small decrease in selectivity to propene. Furthermore, the catalyst was regenerated in an air flow and used again, and in this case, a 12% conversion and a 52.7% selectivity to propene were obtained. After 24-h time on stream, these value changed only to 9.5% and 59.5%, respectively, probably because of some coke deposition, which inhibits the conversion but especially the total oxidation. The reused catalyst was characterized by XRD, but no structural changes were noticed. The results obtained with this catalyst are better than those reported in the literature using other catalyst systems such as Cs-substituted polyoxometalates, VOx/MgO aerogel, and Kcontaining chromia/alumina catalyst.21−23 Yoon et al. tested several metal molybdates in the oxidative dehydrogenation of propane at 733 K.19 Their best results in terms of activity and selectivity were obtained with cobalt molybdate. The highest activity was observed with nickel molybdate, but its selectivity was very poor. In comparison, our catalyst displayed better yields, even at lower temperature. In more recent research,6,23 bulk Ni−Co−Mo oxides and bimetallic Mo−Ni (supported)24 were tested in propane ODH. In the former case, results similar to ours were obtained, but the catalyst stability and the influence of the precursor were not investigated, whereas for the alumina-supported Mo−Ni catalyst, conversions ranging from 91.34% to 83.2% and selectivities ranging from 7.1% to 16.3% were obtained, but using much more severe conditions, such as a propane partial pressure of 50 kPa at 923 K. Maione and Devillers25 compared some bulk and silicasupported Ni−Co molybdates prepared by different methods and showed a generally lower intrinsic activity even at higher temperatures. For instance, a bulk NiCoMo catalyst prepared by coprecipitation at pH 6 and evaluated in the ODH of propane at 723 K presented a 58.8 μmol m−2 h−1 areal reaction rate, which is lower than our results with NiCoMo or CoMo catalysts, indicating that our preparation method using a layered precursor is promising.
Figure 4. NH3 TPD profiles of catalysts from MCo−Mo in the (a) ϕx and (b) ϕy phases.
Table 2. Performance of the Catalysts in the Oxidative Propane Dehydrogenation at 673 K and a Space Velocity of 100 mL g−1 min−1
a
catalyst
XC3H8a (%)
XO2a (%)
SC3H6a (%)
YC3H6a (%)
r0′ a (μmol m−2 h−1)
CoMoϕx400 NiCoMoϕx400 ZnCoMoϕx400 CoMoϕy400 NiCoMoϕy400 ZnCoMoϕy400
0.3 0.9 1.9 8.8 9.0 1.1
1.7 5.3 9.1 50.0 42.0 2.1
0.0 28.4 48.1 46.4 57.7 62.8
0.0 0.3 0.9 4.3 5.2 0.7
96.5 72.4 43.7 104.9 90.5 13.1
X, conversion; S, selectivity; Y, yield; r0′ , initial areal reaction rate.
catalysts. This could be due to the low surface areas and the presence of sodium for the ϕx-derived catalysts. Yoon et al.19 observed a loss of activity in the ODH of propane when sodium was present in cobalt molybdenum catalysts. For the ϕy-derived materials, only the Ni-modified catalyst improved both the conversion of propane and the selectivity to propene, resulting in a higher propene yield. It is clear that, for the ϕy-derived catalysts, Zn doping severely impaired the performance in the ODH of propane. It was observed that Zn doping had no significant effect on the textural properties (surface area) or the reducibility of the catalyst, but it increased the catalyst acidity, as is clear from the NH3 TPD profiles, so this might be an effect of acidity. It could also be related to the ability of the metals to switch between oxidation states. Oxidative dehydrogenation is thought to proceed according to a Mars−van Krevelen mechanism20 that, in the case of a cobalt molybdate, for example, can be represented by the simplified mechanism shown in Scheme 1. In this scheme, □l represents a lattice Scheme 1
4. CONCLUSIONS Layered metal molybdates are promising precursors for mixedoxide catalysts, which can be used, for example, in the oxidative dehydrogenation of propane, and their properties and activity depend not only on their composition, but also on the nature of the precursor itself. The so-called ϕy phase allows the preparation of mixed oxides with higher surface areas than those obtained from the ϕx structure. With NiCoMoϕy400, yields up to 6.3% were achieved under the conditions explored in this work. This catalyst was found to have the highest surface area and lowest reduction temperature. The intrinsic activities of the NiCoMo and CoMo catalysts were better than those of Zn-doped catalysts, as well as those of other related catalysts reported in the literature. The catalyst NiCoMoϕy400 was
anion vacancy, and [O2−]l represents a lattice oxide anion. It is possible to envisage a similar cycle for the reduction of Mo to the 5+ state. The activity and selectivity in this catalytic cycle depend on several factors, among them the ability of the metal ions to switch between oxidation states.20 Both cobalt and nickel can switch easily between oxidation states 2+ and 3+ and thus participate efficiently in the cycle, but that is not the case with zinc, which might explain its negative effect on the activity. 5585
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reused and found to be fairly stable, at least during the 24-h stability tests performed here.
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ASSOCIATED CONTENT
S Supporting Information *
FTIR and TGA results of precursors (Figures S1 and S2, respectively). H2 consumption in TPR and proposed formulas of catalysts (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel./fax: +574 2195667. E-mail: luzamparopalacio@gmail. com. Author Contributions
This manuscript was written through the contributions of all authors. All authors gave approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge Colciencias for their program “Jóvenes Investigadores e innovadores - Virginia Gutierrez de Pineda”. We also acknowledge Laboratório Multiusuário de Análise de Raios-X, IQ/UFRJ (Brazil).
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REFERENCES
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