Oxidative Dehydrogenation of Ethane to Ethylene over V2

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Oxidative Dehydrogenation of Ethane to Ethylene over V2O5/Al2O3 Catalysts: Effect of Source of Alumina on the Catalytic Performance A. Qiao,† V. N. Kalevaru,*,† J. Radnik,† A. Düvel,‡ P. Heitjans,‡ A. Sri Hari Kumar,§ P. S. Sai Prasad,§ N. Lingaiah,§ and A. Martin† †

Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Str. 29a, D-18059 Rostock, Germany Leibniz Universität Hannover, Institut für Physikalische Chemie und Elektrochemie and ZFM - Zentrum für Festkörperchemie und Neue Materialien, Callinstr. 3, D-30167 Hannover, Germany § Indian Institute of Chemical Technology, Hyderabad - 500 007, India ‡

ABSTRACT: A series of V2O5/Al2O3 catalysts with fixed V2O5 content (10 wt %) but varying Al2O3 supports were prepared by impregnation technique and tested for the oxidative dehydrogenation of ethane (ODHE) to ethylene in a fixed bed quartz reactor in the temperature range from 500 to 600 °C. The nature of alumina support applied had a significant influence on the catalytic performance. Among all alumina supports investigated, the high surface area γ-Al2O3 (200−300 m2/g) supported V2O5 catalyst showed the best performance in ODHE with selectivity to ethylene of ca. 45−50%. This result is mainly due to the high dispersion of vanadia species on the support surface. On the other hand, low surface area α-Al2O3 (5 m2/g) and γ-Al2O3 ( V/Al-4-C > V/Al-5-C > V/Al-1-C. With the exception of V/Al-1-C, the decreasing tendency was found to be in the direction opposite to that of the BET-SA value. The unique vanadia dispersion of V/Al-1-C might be attributed to the modified structure of γ-Al2O3 support containing unsaturated penta-coordinated Al ions, which might act as efficient anchoring sites for active species such as VOx in this study, preventing the formation of larger vanadia units. 3.1.5. X-ray Photoelectron Spectroscopy. The XPS analyses of the fresh and spent solids were performed to gain more information about the oxidation state and the chemical environment of the elements present in the near-surface region as well as their atomic ratios.29,30 Figure 4a,b summarizes these results. In addition, the Δ-value (i.e., the energy difference between the binding energy of the O 1s level and that of V 2p3/2) is also given. The results show the presence of vanadium in the +5 oxidation state in all fresh solids with evidence of V 2p3/2 peaks located at 517.0 eV (Figure 4a).7,31 The spent samples reveal a different behavior. It was reported by Mendialdua et al.31 that the decreased degree of oxidation could be correlated to larger values of the binding energy difference Δ. For the V/Al-4 and V/Al-5 samples, no significant differences between the fresh and spent samples were found; however, for the V/Al-1 and V/Al-3 catalysts, a slight reduction of the vanadium ion could be deduced from the XPS results. It must be noted that this observation is more pronounced for the V/Al-1 catalyst, leading to values which are typical for VO231,32 showing the specific character of the interaction of the vanadium ions to the penta-coordinated Al sites. As expected, the behavior of the V/Al-2 with α-Al2O3 as support significantly differs. Additionally, this catalyst (V/Al-2-C) revealed an increased intensity of the V 2p3/2 peak. This result also points to a redispersion of larger V2O5 crystallites into smaller units, reflecting a higher near-surface concentration (cf. Table 1). In contrast, the intensity of the peak corresponding to V 2p3/2 particularly decreased in the spent γ-Al2O3 supported solids compared to that of their corresponding fresh samples. 3.1.6. Py-FTIR spectra. Figure 5a displays the results of the Py-FTIR investigations. In general, the typical bands from pyridine adsorption related to Lewis acid sites (L-Py, LS) can

dramatically increased V/Al ratio (cf. Table 1). When the XRD patterns of the four fresh γ-alumina supported catalysts are compared with those of the spent ones, no considerable differences could be noticed in terms of phase composition and crystallinity. 3.1.4. UV−Vis Diffuse Reflectance Spectroscopy. UV−vis was used to obtain some deeper information on the nature of VOx species formed. After calcination (600 °C), the vanadium oxide species could be present in these catalysts in several VOx structures, such as isolated monomeric tetrahedral VO4 species (O = V-(O-sup)3), one-dimensional polymeric surface species connected by V−O−V bonds in distorted tetrahedral coordination, two-dimensional polymeric species in octahedral coordination, and bulk V2O5.25 Figure 3a shows the UV−vis diffuse reflectance spectra of V/ Al-x fresh samples plus that of pure bulk V2O5 for better

Figure 3. (a) UV−vis diffuse reflectance spectra of the fresh vanadia catalysts supported on different aluminas along with pure V2O5. (b) Deconvoluted UV−vis spectra of the same samples (measured, solid lines; deconvoluted, dashed lines; vertical line at 400 nm points to oligomeric species peak maximum region).

comparison. It should be noted that the UV−vis spectra of all these samples were taken under ambient conditions. It appears that V/Al-1-C revealed a high proportion of monomeric VOx species. Compared to the V/Al-1-C sample, a red shift was observed in the case of the other four catalysts, which is more pronounced in the case of the V/Al-2-C and V/Al-3-C samples compared to that of V/Al-4-C and V/Al-5-C solids. This shift in the band position (oxygen-to-vanadium charge-transfer bands) to lower energy (longer wavelength) indicated the E

dx.doi.org/10.1021/ie5008344 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 5. (a) Py-FTIR spectra of fresh alumina supported vanadia catalysts. (b) Comparison of the number of Brønsted and Lewis acid sites of the fresh vanadia catalysts supported on different types of alumina.

Figure 4. (a) XP spectra of the fresh alumina supported vanadia catalysts. (b) XP spectra of the spent alumina supported vanadia catalysts.

concentration of LS and BS in turn shows a clear influence on the performance of the solids in the ODHE reaction. 3.1.7. Transmission Electron Microscopy. Figure 6 shows some selected micrographs of three typical alumina supported vanadia catalysts. The TEM analyses showed that the nature of the alumina support determined the morphology, composition, and the size of vanadium species and their distribution over the support. The vanadium oxides formed were uniformly distributed over the surface of the support in the case of the V/Al-1-C sample (Figure 6a1,a2). Obviously, no larger VOx particles could be seen even at the 10 nm scale. Moreover, the V/Al atomic ratio was mostly uniform and remained at 1:1 as revealed by an EDX analysis. Otherwise, it could be clearly seen from Figure 6b1,b2 (300 and 100 nm scale) that highly aggregated vanadium oxide particles were formed on the sample V/Al-2-C. The VOx particle morphology and its composition were observed to be completely different from those of the V/Al-1-C sample. Here, we found a plate-like morphology for VOx particles; in addition, the composition was sometimes V-rich (i.e., V/Al 1:0.01; area marked in b2) and sometimes Al-rich (i.e., V/Al 1:11; area marked in b1), pointing to a nonhomogeneous distribution of VOx species. This result is mainly due to the small BET-SA leading to some V-rich regions containing bigger vanadia crystallites. These seem to be responsible for the high activity in

be observed at around 1450 cm−1 and in the region 1625−1600 cm−1, while the presence of Brønsted acid sites (PyH+, BS) can be seen from the appearance of a band at 1540 cm−1.33 As expected, there was almost no adsorption of pyridine over the V/Al-2-C sample, indicating the absence of both BS and LS. However, the Py-FTIR spectra of all four other γ-alumina supported catalysts revealed the presence of both LS and BS. Interestingly, the LS dominate in all the samples irrespective of the type of support used. Among all, from the intensity of the band at 1450 cm−1, it appears that the V/Al-5-C sample is characterized by the highest amount of Lewis sites, while the V/ Al-2-C sample exhibits the lowest proportion. Figure 5b depicts the integral intensity values of BS and LS as well as the LS-to-BS ratio. The intensities of the characteristic bands at 1538 cm−1 for BS and 1448 cm−1 for LS were normalized to the BET surface area of the corresponding samples. The V/Al-1-C and V/Al-3-C catalysts showed comparable Brønsted acidity higher than that of the V/Al-4-C and V/Al-5-C samples, which possessed the same Brønsted acidity. The number of LS of all samples is much higher than the number of BS sites. In terms of the integral intensity ratio of LS/BS, the following increasing order was found: V/Al-3-C < V/Al-1-C < V/Al-5-C < V/Al-4-C. This ranking in the F



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Figure 6. TEM images of fresh V/Al-1-C (a1, a2), V/Al-2-C (b1, b2), and V/Al-5-C (c1, c2) samples (the given V/Al ratios belong to the marked square areas).

Figure 7. HAADF-STEM image (left-hand panel) and V−K, Al−K, and O−K EDXS elemental maps of V/Al-5 catalyst.

1:8 was found; on some regions, the V/Al ratio is somewhat lower (down to 1:16). High-angle annular dark field (HAADF)-STEM image and V−K, Al−K, and O−K EDXS elemental maps of the best V/Al5 catalyst are illustrated in Figure 7. It is obvious from the images that neither free VOx particles nor Al-rich regions could be observed and the vanadium is located always together with the support. In other words, the VOx species are finely

ethane conversion and low selectivity to ethylene, as discussed below. Figure 6c1,c2 depicts the morphology and composition of the V/Al-5-C catalyst (30 and 50 nm scale). It is obvious from the image that the deposited vanadia is evenly distributed on the surface of the support. In most regions, the composition of V and Al remained unchanged and uniform, i.e., a V/Al ratio of G



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dispersed over the surface of the support because of its very high surface area among the series. 27 Al MAS NMR spectra of pure alumina supports (Al-1 to Al-5, without V2O5) and alumina supported VOx catalysts (V/ Al-1 to V/Al-5, with V2O5) are compared in Figure 8a,b. It can

Figure 9. Concentration of penta-coordinated Al sites as a function of temperature in Al-1 and V/Al-1 samples. (a) Pure Al-1 support at different temperatures (without V2O5) and (b) V/Al-1 catalyst at different temperatures (with V2O5); 27Al MAS NMR spectra were recorded at ν0 = 156.42 MHz).

the figure that the formation and the concentration of PC-Al sites are very sensitive to the heat treatment. A rise from room temperature to 600 °C resulted in the amount of PC-Al sites decreasing considerably, from 13 to 6% in the case of pure Al-1 support (Figure 9a). A similar decreasing tendency with increasing temperature can also be observed in the case of NH4VO3 (precursor of V2O5) impregnated Al-1 samples (Figure 9b). However, the decrease is more pronounced in these V2O5-containing catalysts. Consequently, PC-Al decreased from 13% to a negligibly low amount (

Oxidative Dehydrogenation of Ethane to Ethylene over V2

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