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Oxidation of C2–C4 hydrocarbons over MoO3 and V2O5 supported on a YSZ-aided membrane reactor
Applied Catalysis A: General 273 (2004) 133–141
Oxidation of C2 –C4 hydrocarbons over MoO3 and V2 O5 supported on a YSZ-aided membrane reactor Katsuomi Takehira a,∗ , Takuya Komatsu a , Nobuyuki Sakai a , Hideshi Kajioka b , Satoshi Hamakawa c , Tetsuya Shishido d , Tomonori Kawabata a , Ken Takaki a a
c
Department of Chemistry and Chemical Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan b Western Hiroshima Prefecture Industrial Research Institute, 2-10-1 Aga-minami, Kure 737-0004, Japan Laboratory for Membrane Chemistry, AIST Tohoku, 4-2-1 Nigatake, Miyagino-ku, Sendai, Miyagi 983-8551, Japan d Department of Chemistry, Tokyo Gakugei University, Nukui-kita 4-1-1, Koganei, Tokyo 184-8501, Japan Received in revised form 31 May 2004; accepted 14 June 2004 Available online 24 July 2004
Abstract An electrochemical reactor, using yttria-stabilized zirconia (YSZ) as a solid electrolyte and gold and silver as the anode and cathode, respectively, has been employed for the selective partial oxidation of C2 –C4 hydrocarbons at 775 K. MoO3 or V2 O5 was deposited as the catalyst film on the Au anode of the electrochemical cell. The reaction was carried out over the catalyst film under oxygen pumping through the cell; the results are compared with those obtained by the reaction with gaseous oxygen. Under oxygen pumping, no alkane was oxidized over MoO3 , while isobutane and propane were partially oxidized to methacrolein and propene, respectively, and ethane was oxidized a little over V2 O5 . It is supposed that C–H cleavage is the rate determining step of the alkane oxidation over V2 O5 . Alkenes were partially oxidized over both MoO3 and V2 O5 , and the selectivities to the oxygenated products, i.e., acetaldehyde, acrolein and methacrolein from ethene, propene, and isobutene, respectively, were higher on the former than on the latter. Over both MoO3 and V2 O5 , the selectivity to aldehyde was higher in the reaction under oxygen pumping than in that by gaseous oxygen. Over MoO3 , the other oxygenated compounds than aldehyde were observed in the reaction by gaseous oxygen, while this is not the case under oxygen pumping. Under oxygen pumping, the highest selectivity was observed with methacrolein, followed by acrolein and acetaldehyde, among which acetaldehyde was further oxidized to carbon oxides. Over V2 O5 , alkenes were far more rapidly oxidized by gaseous oxygen than under oxygen pumping, and the selectivity to the oxygenated products were still higher under oxygen pumping than by gaseous oxygen. The highest selectivity was observed with methacrolein, and those of both acrolein and acetaldehyde were low. No further oxidation of acetaldehyde was observed over V2 O5 . © 2004 Elsevier B.V. All rights reserved. Keywords: Partial oxidation; C2 –C4 hydrocarbons; YSZ; Electrochemical cell; MoO3 ; V2 O5
1. Introduction Since Mars and Van Krevelen [1] first suggested the important role of lattice oxide ions in the selective oxidation of hydrocarbons over metal-oxide catalysts, many studies have been made in this field. A typical catalyst is the molybdate system, which displays a high selectivity in alkene oxidation [2]; lattice oxide ions are the key reaction participants at temperatures >600 K, while gaseous oxygen reoxidizes ∗ Corresponding author. Tel.: +81-824-24-7744; fax: +81-824-24-7744. E-mail address: [email protected] (K. Takehira).
0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.06.022
the catalyst [3–6]. The oxide ions resulting from chemisorption and dissociation of the gaseous oxygen on the catalyst surface diffuse through the bulk of the catalyst to the active sites at which adsorption and oxidation of the alkene occur. In the alkene oxidation, an MoO3 –Bi2 O3 catalyst behaves as a solid electrolyte possessing a multifunctional active site. ␣-H abstraction, followed by allylic intermediate formation and oxygen insertion into allylic intermediate takes place by oxide ions [5,6]. Moreover, O2 is chemisorbed, reduced and dissociated to form oxide ions on the surface [6]. YSZ is a well-known solid electrolyte that can transport oxide ions through its lattice anion vacancies. A cell system using YSZ as a solid electrolyte, e.g. P(O2 ), M|YSZ|M ,
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P(O2 ) (M and M are electrodes), can serve as an ‘oxygen pump’ [7], in which the oxygen flux transferred across the YSZ can be controlled by the electric potential externally applied between the two electrodes. We have reported the oxidation of alkenes over Au|YSZ|Ag [8], Mo–Bi mixed oxide/Au|YSZ|Ag [9] and (Pb, Bi)MoO4 /Au|YSZ|Ag [10] and we have discussed the role of lattice oxygen in the molybdate catalysts. The rate of formation of oxygenated compounds from alkenes on both Mo–Bi mixed oxide and (Pb, Bi)MoO4 catalysts was substantially enhanced by oxygen pumping through the YSZ, suggesting that the bulk diffusion of lattice oxide ions in the catalyst plays important roles in both activity and selectivity. Moreover, use of MoO3 /Au|YSZ|Ag resulted in a significant enhancement in the formation of acrolein from propene with increasing oxygen pumping, in which strong structure sensitivity, i.e., dependency of the activity on the orientation of crystal growth of MoO3 , was observed [11]. MoO3 film alone did not show activity for propene oxidation in the presence of gaseous oxygen. Vanadium oxide catalyst has been frequently used in the oxidation of alkanes [12–21], even though molybdates catalyst is sometimes active for the oxidative dehydrogenation of alkanes [22–25]. The vanadium catalysts are effective not only for oxidative dehydrogenation of alkanes [12–15,20,21] but also for production of maleic anhydride from butane [16,18]. Among these catalysts, supported vanadium catalysts on TiO2 [17], MCM-41 [13,15], Al2 O3 [19] and MgO [20,21] are tested in the oxidative dehydrogenation of alkanes, indicating that vanadium species is effective for the C–H activation of inert alkanes. We have reported that C2 –C4 alkanes were partially oxidized to the oxygenated compounds over Au|YSZ|Ag under oxygen pumping [26], among which ethane was oxidized to acetaldehyde with high selectivity of 45% [27]. It is expected that use of V2 O5 or MoO3 on the oxygen pump will enhance the selective oxidation activity. In the present paper, we report the results of the oxidations of both C2 –C4 alkanes and alkenes over MoO3 or V2 O5 supported on the oxygen pump.
ing of each metal target (99.95 and 99.9%, respectively) under 0.667 Pa of O2 /Ar(1/1) atmosphere at 573 K. Apparent rate of sputtering evaluated by the film thickness was 0.9 and 0.2 m h−1 for MoO3 and V2 O5 , respectively. MoO3 formed a thick and porous film composed of many small crystallites, while V2 O5 formed a thin and dense film. The catalyst films as deposited were apparently in the form of MoO3 and V2 O5 , respectively, even though the crystallinity was not high. The films were annealed at 823 K in air for 1 h, during which process the films were crystallized to each metal oxide. The crystal structure of each catalyst film was confirmed by X-ray diffraction using a Mac Science M18XHF-SRA diffractometer with Cu K␣ radiation. The thicknesses of the metal electrode and the catalyst films were controlled by the amount of source materials and were finally evaluated by scanning electron micrograph (SEM) observation of the cross-sections of the film. SEM results were recorded on a JEOL JSM-6340F instrument after coating the sample with Au–Pd film by using Plasma Mult Coater. Surface morphology of the MoO3 or V2 O5 catalyst and the layered structure were also evaluated by the SEM observations of the surface of the catalyst films and the cross-section of the cell systems, respectively.
2. Experimental
3. Results and discussion
2.1. Preparation of the cell system
3.1. Structure of MoO3 and V2 O5 on the Au anode
An electrochemical reactor was constructed from an 8 mol.% YSZ disk, 32 mm in diameter and 1 mm in thickness. Precise details were reported in the previous papers [8,9]. First, Au (1 m thick) was deposited as the anode metal onto a face of the YSZ disk; then, the MoO3 (∼4 m thick) or V2 O5 (∼1.0 m thick) catalyst was deposited onto the Au anode; finally, Ag (∼2 m thick) was deposited as the cathode metal onto the other face of the YSZ disk. Both Au and Ag were deposited by vacuum evaporation using a glass bell-jar-type apparatus equipped with a diffusion pump and rotary pump evacuation system. Both MoO3 and V2 O5 were deposited on the Au anode by reactive sputter-
X-ray diffraction patterns of MoO3 (A) and V2 O5 (B) films on the Au anode are shown in Fig. 1. MoO3 film showed weak lines of MoO3 [JCPDS: 5-0508] just after the deposition (Fig. 1Aa), while the intensities were significantly strengthened after the annealing (Fig. 1Ab). After the propene oxidation for 12 h at 773 K, the line intensities of MoO3 were substantially weakened, together with those of Au and YSZ, but no line of the reduced molybdenum oxide species was observed (Fig. 1Ac). V2 O5 film after the deposition (Fig. 1Ba) showed the lines of V2 O5 [JCPDS: 41-1426]; the pattern did not substantially change after the annealing (Fig. 1Bb). After the propene oxidation for 12 h
2.2. Oxidation of C2 –C4 hydrocarbons A gaseous mixture of hydrocarbon (5%), nitrogen (5%) and helium (90%) was passed (1.4 l h−1 ) over the catalyst film at 773 K for testing the activity of the catalyst. To check the effect of oxygen from the gaseous phase, we also passed a gaseous mixture of hydrocarbon (5%), oxygen (5%), nitrogen (5%) and helium (85%) over the catalyst film at the same rate. Oxygen gas (0.6 l h−1 ) was passed through the cathode space. The applied potential (V) for controlling the oxygen flux in these experiments was 0–3.5 V, corresponding to 0–10 mA in electric current and 0–93 mol h−1 of O2 flux. The products in the effluent gas were determined by gas chromatography.
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Fig. 1. X-ray diffraction patterns of MoO3 (A) and V2 O5 (B) films on the Au anode. (a) After the deposition; (b) after the annealing; (c) after the reaction: (䊉) MoO3 ; (䊏) V2 O5 ; (䉱) V6 O13 ; (䉲) V2 O4 ; (䊊) Au; (䊐) YSZ.
at 773 K, the reduction to V6 O13 [JCPDS: 43-1050] and V2 O4 [JCPDS: 43-1051] was clearly observed and the lines of V2 O5 disappeared, suggesting that V2 O5 was easily reduced during the reaction (Fig. 1Bc). SEM images of the surface of MoO3 (a, b and c) and V2 O5 (a , b and c ) are shown in Fig. 2. After the deposition, MoO3 showed a porous film composed of a mixture of various sizes of the crystallites together with powder-like products (Fig. 2a). After the annealing, the size of crystallites became uniform (Fig. 2b). After the propene oxidation for 24 h at 773 K, the surface of the film became rather flat. The crystallites were destroyed and sintered with each other to form aggregates of small-sized particles (Fig. 2c). After the deposition, V2 O5 showed a dense film composed of aggregates of small-sized crystallites sticking to each other tightly (Fig. 2a ). After the annealing, the crystallites grew slightly but no significant change was observed in the size and the form of the V2 O5 crystallites (Fig. 2b ). After the reaction for 24 h, the surface morphology of the film significantly changed and a flat and sintered surface appeared (Fig. 2c ). All the data of the oxidation reactions were corrected on the fresh films of both MoO3 and V2 O5 deposited on the Au anode. SEM images of the cross-section of MoO3 /and V2 O5 / Au|YSZ just after the deposition are shown in Fig. 3. In both cases, layered structures of the catalyst film and gold anode film on the YSZ disc were clearly observed. MoO3 formed a thick and rather porous film composed of various sizes of MoO3 crystallites (Fig. 3A), while V2 O5 formed a thin and dense film, probably composed of small-sized crystallites (Fig. 3B). The results are consistent with those observed in the surface morphology in Fig. 2.
3.2. Oxidation of alkanes over V2 O5 Oxidations of C2 –C4 alkanes, i.e., ethane, propane and isobutane, were tested over the MoO3 and V2 O5 films under oxygen pumping. No oxidation of alkane took place over the MoO3 film, while alkanes were oxidized over the V2 O5 film (Fig. 4). Over the V2 O5 film, propane produced mainly propene, together with small amounts of acrolein and acetaldehyde, and isobutane gave methacrolein, even though the selectivity was not so high. Some ethane was converted to carbon dioxide, but no partial oxidation product was observed. If the reaction starts from the H-abstraction from alkane by active oxygen formed on V2 O5 through the YSZ, the rate of reaction can be correlated with the C–H bond energy of alkane [28]. The rates of oxygen consumption calculated from either electric current for oxygen pumping or total amount of oxygen in the products coincided well and could be logarithmically plotted against the bond energy of the weakest C–H bond of each alkane (Fig. 5). A good correlation was observed, suggesting that the rate-determining step is the H-abstraction from alkane by oxygen on V2 O5 pumped through the YSZ. Oxidation of alkane on the V2 O5 film by oxygen from gaseous phase produced a large amount of carbon oxides, resulting in a lowering in the selectivity to oxygenated products. Vanadium oxide catalysts have frequently been used in the oxidative dehydrogenation of alkanes [12–15,20,21]. Magnesium vanadate is active for oxidative dehydrogenation of propane; both V5+ and V4+ catalyze the oxidative dehydrogenation of propane; however, V4+ seems to be more selective, though less active than V5+ [12]. We have tested vanadium-containing MCM-41 for oxidative dehydrogenation of ethane, propane and isobutane, where the catalysts
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Fig. 2. SEM images of the surfaces of MoO3 (a, b and c) and V2 O5 films (a , b and c ). (a and a ) After the deposition; (b and b ) after the annealing; (c and c ) after the reaction.
Fig. 3. SEM images of the cross-section of MoO3 (A) and V2 O5 (B) films on the An anode and the YSZ disc.
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Fig. 4. Oxidation of alkanes over V2 O5 films under oxygen pumping. Anode gas: alkane/N2 /He (5/5/90), 23.5 ml min−1 ; cathode gas: O2 , 20.0 ml min−1 ; current: 10 mA (93.3 mol O2 h−1 ).
prepared by direct hydrothermal methods with vanadium content exceeding 1 wt.% were effective for the reaction [13]. Solsona et al. [15] also reported similar results by using V-MCM-41 catalyst for the oxidative dehydrogenation of propane. In the cases of both magnesium vanadate [12] and V-MCM-41 [13,15], isolated and tetrahedrally coordinated VO4 seems to be selective for the oxidative dehydrogenation. V4+ was also effective for the dehydrogenation of butane over VOx /Al2 O3 catalyst, while V3+ was less active than V4+ , even though both species can be produced during the reaction [14]. Even over V2 O5 /Al2 O3 catalyst, in situ Raman spectroscopy study showed that surface polymeric vanadium oxide species were more reducible than isolated surface vanadium species, and that the active site for propane oxidative dehydrogenation on alumina-supported vanadia must be a single VO4 site [19]. Moreover, it was
Fig. 5. Relationship between the rate of oxygen consumption and C–H bond energy.
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also found that the V–O–V bond might not be critical for this reaction [19]. However, an important catalytic effect of acid/base properties was observed in the oxidative dehydrogenation of alkanes over V/MgO catalyst [21] as well as over V-MCM-41 [13]; this effect is still a cause of controversy in the discussions on the active species. Reduction–oxidation of vanadium sites may also produce acid/base properties on the catalyst surface and may affect the catalytic behavior. In the present work, V2 O5 crystals of rather large size were put on the Au anode and vanadium species are not isolated but are all combined with each other by V–O–V bonding. Over the V2 O5 film, propane was mainly dehydrogenated to propene, while isobutane was converted to methacrolein under oxygen pumping. During the reaction, V2 O5 was reduced to V6 O13 and V2 O4 , on which V4+ was formed and works as the active species for the oxidative dehydrogenation of alkanes. Among the alkanes, ethane was not markedly oxidized probably due to the strong C–H bond energy, while propane was dehydrogenated to propene due to secondary C–H bonds. Propene is stable due to resonance structure of allylic intermediate; it therefore desorbed and appeared in the effluent gas. Due to the weakest C–H bond at the tertiary carbon atom, isobutane was easily dehydrogenated to isobutene, which is not as stable as propene due to the presence of another methyl group and was in situ oxidized to methacrolein (vide infra). The further oxidation of isobutene may be due to the presence of neighboring active vanadium sites on V2 O5 crystal compared to a single VO4 site on supported vanadia catalyst, on which high selectivity was obtained in the oxidative dehydrogenation of isobutane [13]. 3.3. Oxidation of alkenes over MoO3 Alkene oxidation over the MoO3 film under oxygen pumping (A) or by gaseous oxygen (B) is shown in Fig. 6. Interestingly, over the MoO3 film, the partial oxidation of alkene was rather enhanced under oxygen pumping compared with the results obtained by gaseous oxygen, which coincided well with our previous results obtained in the oxidation of propene over MoO3 /Au|YSZ|Ag [11]. Not only the rate of oxidation but also the selectivity to the oxygenated products was higher in the reaction under oxygen pumping than in that by gaseous oxygen. The highest selectivity to partial oxidation product was observed with methacrolein, followed by acrolein and acetaldehyde in both cases: under oxygen pumping and by gaseous oxygen. Aldehyde alone was observed as the partial oxidation product under oxygen pumping, while the other oxygenated compounds were produced in the reaction by gaseous oxygen. The selectivity of 80% was obtained with methacrolein from isobutene under oxygen pumping; this value was far higher than that of 60% obtained by gaseous oxygen over the MoO3 film. The effect of oxygen flux was tested in the alkene oxidation over the MoO3 film by increasing the electric current through the YSZ (Fig. 7). The rate of formation of aldehyde
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Fig. 6. Oxidation of alkenes over MoO3 film under oxygen pumping (A) and by gaseous oxygen (B). Anode gas: alkene/N2 /He (5/5/90), 23.5 ml min−1 ; cathode gas: O2 , 20.0 ml min−1 ; current: 10 mA (93.3 mol O2 h−1 ); (B) mixed gas: alkene/O2 /N2 /He (5/5/5/85), 23.5 ml min−1 .
increased, while the selectivity gradually decreased with increasing the oxygen flux in all cases of methacrolein, acrolein and acetaldehyde, suggesting that further oxidation of the aldehyde took place on the MoO3 film. Under the presence of an excess amount of active oxygen species, the aldehyde must be further oxidized to carbon oxides, resulting in a decrease in the selectivity. The selectivity of aldehyde is plotted against the reciprocal flow rate of alkene, corresponding to contact time in the gas-flow reactor using a fixed-bed catalyst (Fig. 8). When the flow rate of alkene increased, i.e., the contact time decreased, the selectivity gradually increased for methacrolein and acrolein. In the case of acetaldehyde, a drastic increase was observed in the selec-
Fig. 7. Effect of oxygen flux in the oxidation of alkenes over MoO3 film under oxygen pumping. Anode gas: alkene/N2 /He (5/5/90), 23.5 ml min−1 ; cathode gas: O2 , 20.0 ml min−1 . Rate of formation of methacrolein (䊉), acrolein (䊏) and acetaldehyde (䉱). Selectivity to methacrolein (䊊), acrolein (䊐) and acetaldehyde ().
tivity with increasing the flow rate of ethene, suggesting that acetaldehyde once formed on the MoO3 film was rapidly oxidized to carbon oxides. 3.4. Oxidation of alkenes over V2 O5 Over the V2 O5 film, ethene was most quickly oxidized, followed by propene and isobutene in both cases: under oxygen pumping and by gaseous oxygen (Fig. 9). In these reactions, many products formed by fission of carbon–carbon bond were observed, i.e., C2 and C2 –C3 compounds in the oxidation of propene and isobutene, respectively. In the presence of gaseous oxygen, the rate of oxidation was drastically enhanced, but the selectivity to aldehyde decreased, compared to that under oxygen pumping. This is probably due
Fig. 8. Effect of flow rate of alkenes in the oxidation of alkenes over MoO3 film under oxygen pumping. Anode gas: alkene/N2 /He = 5/5/90 (vol. ratio); cathode gas: O2 , 20.0 ml min−1 ; current: 10 mA (93.3 mol O2 h−1 ).
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Fig. 9. Oxidation of alkenes over V2 O5 film under oxygen pumping (A) and by gaseous oxygen (B). (A) Anode gas: alkene/N2 /He (5/5/90), 23.5 ml min−1 ; cathode gas: O2 , 20.0 ml min−1 ; current: 10 mA (93.3 mol O2 h−1 ); (B) mixed gas: alkene/O2 /N2 /He (5/5/5/85), 23.5 ml min−1 .
to gaseous phase oxidation via radical intermediate formation; vanadium catalyst probably produces radical species by homolytic cleavage of C–H bond, followed by radical chain-reaction with molecular oxygen in gaseous phase. This mechanism also leads to the formation of C–C bond cleavage products such as C2 and C2 –C3 from propene and isobutene, respectively. The highest selectivity was still obtained for methacrolein, but followed by that for acetaldehyde and then by that for acrolein over the V2 O5 film. In the oxidation of both isobutene and propene, significant amounts of products with lower carbon numbers, i.e., acetone, propene, propionaldehyde, acetaldehyde and ethene were observed. This can be clearly seen in the decreasing selectivity to the aldehyde with increasing the oxygen flux (Fig. 10). It is likely that methacrolein and acrolein were further oxidized under the presence of excess amounts of oxygen. The fact that the selectivity of acrolein is lower than that of acetaldehyde suggests that the rate of acrolein oxidation is higher than that of acetaldehyde on the V2 O5 film (Fig. 10), oppositely to the case observed on the MoO3 film (Fig. 7). The selectivities to both acrolein and acetaldehyde were extremely low (20%) compared to that to methacrolein when the flow rate of alkene increased over the V2 O5 film (Fig. 11). Isobutene was also oxidized to acetone, propene and acetaldehyde, but the selectivity to methacrolein still remained at the highest value of 60% among the three aldehydes.
in industrial applications and in academic research [2–6]. One of the “model reactions” investigated in detail for clarifying the catalytic mechanism of molybdenum-containing systems is the partial oxidation of propene to acrolein over MoO3 [29,30]. The mechanism consisting of alternating oxidation and reduction of the metal oxide catalyst surface and/or bulk has been suggested and is referred to as the redox mechanism (or Mars–Van Krevelen mechanism [1]). Varying degrees of participation of bulk oxygen in partial oxidation reactions have been reported in the literature. For the two oxides V2 O5 (674–823 K) and MoO3 (873–1023 K), it was found that practically all of the oxygen from the bulk could be exchanged with 18 O2 and, hence, could participate in oxidation reactions proceeding on the surface of the catalysts [31–34]. The difference between the two temperature
3.5. Comparison of the activity between MoO3 and V2 O5 V2 O5 was reduced to V6 O13 and V2 O4 after the oxidation reaction of propene at 773 K, while the reduction of MoO3 was not clearly observed (Fig. 1). It was concluded that vanadium catalysts were effective for oxidative dehydrogenation of alkanes to alkenes (Fig. 4) [12–15,20,21]. On the other hand, molybdenum oxide-based catalysts are extensively employed for the partial oxidation of alkenes both
Fig. 10. Effect of oxygen flux in the oxidation of alkenes over V2 O5 film under oxygen pumping. Anode gas: alkene/N2 /He (5/5/90), 23.5 ml min−1 ; cathode gas: O2 , 20.0 ml min−1 . Rate of formation of methacrolein (䊉), acrolein (䊏) and acetaldehyde (䉱). Selectivity to methacrolein (䊊), acrolein (䊐) and acetaldehyde ().
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steps of the small (1 0 0) planes were formed on the (0 1 0) plane, suggesting the formation of high concentration of the active sites on the surface of the MoO3 crystal [11]. These results are consistent with the mechanism proposed by Ressler et al. [34]. Though XRD patterns of MoO3 film did not show clear evidence of oriented crystal growth, the MoO3 film was also prepared by reactive sputtering in the present work. It is therefore possible that a similar redox mechanism works on the surface of the MoO3 film and catalyzes the oxidation reaction of propene. Oxygen pumping system can efficiently deliver atomic oxygen species to the reduced active site on the catalyst surface, resulting in the selective oxidation of alkenes. V2 O5 can work as the catalyst for alkane oxidation, and it is therefore expected that a combination of V2 O5 and MoO3 can catalyze the selective oxidation of alkane. Fig. 11. Effect of flow rate of alkenes in the oxidation of alkenes over V2 O5 film under oxygen pumping. Anode gas: alkene/N2 /He = 5/5/90 (vol. ratio); cathode gas: O2 , 20.0 ml min−1 ; current: 10 mA (93.3 mol O2 h−1 ).
ranges well explains apparently higher stability of MoO3 compared with that of V2 O5 observed under the present reaction conditions (773 K as the reaction temperature). Alkenes were effectively oxidized both under oxygen pumping and by gaseous oxygen over the MoO3 film (Fig. 6). The reduction of MoO3 is a crucial step in the redox mechanism of partial oxidation reactions on this material; thus, the reduction of MoO3 with propene and/or hydrogen is inevitably suggested. Formation of partially reduced molybdenum “sub-oxides” during reduction has been reported as a result of the redox mechanism and of the difference in rates of catalyst reduction and reoxidation [35]. In the present work, weak peaks of Mo9 O26 [JCPDS: 5-0441] as a “sub-oxide” were observed in the XRD patterns of the samples after the propene oxidation reaction by gaseous oxygen. Ressler et al. [34] reported that a “Mo18 O52 ”-type shear structure formed as intermediate during reduction of MoO3 in propene and during oxidation of MoO2 in oxygen by using in situ XAS technique. A schematic reaction mechanism for the reduction of MoO3 in propene and reoxidation in oxygen is proposed that consists of (i) generation of oxygen vacancies at (1 0 0) or (0 0 1) plane by reaction with propene, (ii) vacancy diffusion in the MoO3 bulk, (iii) formation of Mo18 O52 -type shear structure in the lattice, and (iv) formation and growth of MoO2 nuclei. The mechanism is in agreement with previous reports indicating that the oxidation reaction of propene on MoO3 is structure-sensitive [36]. We have reported that MoO3 film prepared on the Au anode by the reactive sputtering method showed high activity for the oxidation of propene [11]. This film was composed of many leaf-like MoO3 crystals each of which grew perpendicularly to the Au surface and had fine structure, resulting in many small sub-surfaces of (1 0 0) and (0 0 1) planes on the outer surface. Moreover, many atomic
4. Conclusion Oxidation of C2 –C4 hydrocarbons was carried out over MoO3 or V2 O5 catalyst film deposited on an electrochemical cell, Au|YSZ|Ag, at 775 K under oxygen pumping through the cell. The results were compared with those obtained by gaseous oxygen over the catalyst film. Propane and isobutane were oxidized to propene and methacrolein, respectively, over V2 O5 , while neither alkane was oxidized over MoO3 , under oxygen pumping. Ethane was not remarkably oxidized even over V2 O5 . It is concluded that homolytic C–H cleavage is the rate-determining step of the alkane oxidation on V2 O5 . Alkenes were partially oxidized over both MoO3 and V2 O5 . The selectivity to acetaldehyde, acrolein and methacrolein from ethene, propene, and isobutene, respectively, were higher on the former than on the latter. Alkenes were oxidized more efficiently under oxygen pumping than by gaseous oxygen over MoO3 . The highest selectivity was obtained with methacrolein, followed by acrolein and acetaldehyde; among these, acetaldehyde was further oxidized to carbon oxides. On the other hand, alkenes were far more rapidly oxidized by gaseous oxygen than under oxygen pumping over V2 O5 , while the selectivity to aldehyde was still higher under oxygen pumping than the value obtained by gaseous oxygen. The highest selectivity was observed with methacrolein, and the values of both acrolein and acetaldehyde were low. No further oxidation of acetaldehyde was observed over V2 O5 .
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Oxidation of C2 –C4 hydrocarbons over MoO3 and V2 O5 supported on a YSZ-aided membrane reactor Katsuomi Takehira a,∗ , Takuya Komatsu a , Nobuyuki Sakai a , Hideshi Kajioka b , Satoshi Hamakawa c , Tetsuya Shishido d , Tomonori Kawabata a , Ken Takaki a a
c
Department of Chemistry and Chemical Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan b Western Hiroshima Prefecture Industrial Research Institute, 2-10-1 Aga-minami, Kure 737-0004, Japan Laboratory for Membrane Chemistry, AIST Tohoku, 4-2-1 Nigatake, Miyagino-ku, Sendai, Miyagi 983-8551, Japan d Department of Chemistry, Tokyo Gakugei University, Nukui-kita 4-1-1, Koganei, Tokyo 184-8501, Japan Received in revised form 31 May 2004; accepted 14 June 2004 Available online 24 July 2004
Abstract An electrochemical reactor, using yttria-stabilized zirconia (YSZ) as a solid electrolyte and gold and silver as the anode and cathode, respectively, has been employed for the selective partial oxidation of C2 –C4 hydrocarbons at 775 K. MoO3 or V2 O5 was deposited as the catalyst film on the Au anode of the electrochemical cell. The reaction was carried out over the catalyst film under oxygen pumping through the cell; the results are compared with those obtained by the reaction with gaseous oxygen. Under oxygen pumping, no alkane was oxidized over MoO3 , while isobutane and propane were partially oxidized to methacrolein and propene, respectively, and ethane was oxidized a little over V2 O5 . It is supposed that C–H cleavage is the rate determining step of the alkane oxidation over V2 O5 . Alkenes were partially oxidized over both MoO3 and V2 O5 , and the selectivities to the oxygenated products, i.e., acetaldehyde, acrolein and methacrolein from ethene, propene, and isobutene, respectively, were higher on the former than on the latter. Over both MoO3 and V2 O5 , the selectivity to aldehyde was higher in the reaction under oxygen pumping than in that by gaseous oxygen. Over MoO3 , the other oxygenated compounds than aldehyde were observed in the reaction by gaseous oxygen, while this is not the case under oxygen pumping. Under oxygen pumping, the highest selectivity was observed with methacrolein, followed by acrolein and acetaldehyde, among which acetaldehyde was further oxidized to carbon oxides. Over V2 O5 , alkenes were far more rapidly oxidized by gaseous oxygen than under oxygen pumping, and the selectivity to the oxygenated products were still higher under oxygen pumping than by gaseous oxygen. The highest selectivity was observed with methacrolein, and those of both acrolein and acetaldehyde were low. No further oxidation of acetaldehyde was observed over V2 O5 . © 2004 Elsevier B.V. All rights reserved. Keywords: Partial oxidation; C2 –C4 hydrocarbons; YSZ; Electrochemical cell; MoO3 ; V2 O5
1. Introduction Since Mars and Van Krevelen [1] first suggested the important role of lattice oxide ions in the selective oxidation of hydrocarbons over metal-oxide catalysts, many studies have been made in this field. A typical catalyst is the molybdate system, which displays a high selectivity in alkene oxidation [2]; lattice oxide ions are the key reaction participants at temperatures >600 K, while gaseous oxygen reoxidizes ∗ Corresponding author. Tel.: +81-824-24-7744; fax: +81-824-24-7744. E-mail address: [email protected] (K. Takehira).
0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.06.022
the catalyst [3–6]. The oxide ions resulting from chemisorption and dissociation of the gaseous oxygen on the catalyst surface diffuse through the bulk of the catalyst to the active sites at which adsorption and oxidation of the alkene occur. In the alkene oxidation, an MoO3 –Bi2 O3 catalyst behaves as a solid electrolyte possessing a multifunctional active site. ␣-H abstraction, followed by allylic intermediate formation and oxygen insertion into allylic intermediate takes place by oxide ions [5,6]. Moreover, O2 is chemisorbed, reduced and dissociated to form oxide ions on the surface [6]. YSZ is a well-known solid electrolyte that can transport oxide ions through its lattice anion vacancies. A cell system using YSZ as a solid electrolyte, e.g. P(O2 ), M|YSZ|M ,
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P(O2 ) (M and M are electrodes), can serve as an ‘oxygen pump’ [7], in which the oxygen flux transferred across the YSZ can be controlled by the electric potential externally applied between the two electrodes. We have reported the oxidation of alkenes over Au|YSZ|Ag [8], Mo–Bi mixed oxide/Au|YSZ|Ag [9] and (Pb, Bi)MoO4 /Au|YSZ|Ag [10] and we have discussed the role of lattice oxygen in the molybdate catalysts. The rate of formation of oxygenated compounds from alkenes on both Mo–Bi mixed oxide and (Pb, Bi)MoO4 catalysts was substantially enhanced by oxygen pumping through the YSZ, suggesting that the bulk diffusion of lattice oxide ions in the catalyst plays important roles in both activity and selectivity. Moreover, use of MoO3 /Au|YSZ|Ag resulted in a significant enhancement in the formation of acrolein from propene with increasing oxygen pumping, in which strong structure sensitivity, i.e., dependency of the activity on the orientation of crystal growth of MoO3 , was observed [11]. MoO3 film alone did not show activity for propene oxidation in the presence of gaseous oxygen. Vanadium oxide catalyst has been frequently used in the oxidation of alkanes [12–21], even though molybdates catalyst is sometimes active for the oxidative dehydrogenation of alkanes [22–25]. The vanadium catalysts are effective not only for oxidative dehydrogenation of alkanes [12–15,20,21] but also for production of maleic anhydride from butane [16,18]. Among these catalysts, supported vanadium catalysts on TiO2 [17], MCM-41 [13,15], Al2 O3 [19] and MgO [20,21] are tested in the oxidative dehydrogenation of alkanes, indicating that vanadium species is effective for the C–H activation of inert alkanes. We have reported that C2 –C4 alkanes were partially oxidized to the oxygenated compounds over Au|YSZ|Ag under oxygen pumping [26], among which ethane was oxidized to acetaldehyde with high selectivity of 45% [27]. It is expected that use of V2 O5 or MoO3 on the oxygen pump will enhance the selective oxidation activity. In the present paper, we report the results of the oxidations of both C2 –C4 alkanes and alkenes over MoO3 or V2 O5 supported on the oxygen pump.
ing of each metal target (99.95 and 99.9%, respectively) under 0.667 Pa of O2 /Ar(1/1) atmosphere at 573 K. Apparent rate of sputtering evaluated by the film thickness was 0.9 and 0.2 m h−1 for MoO3 and V2 O5 , respectively. MoO3 formed a thick and porous film composed of many small crystallites, while V2 O5 formed a thin and dense film. The catalyst films as deposited were apparently in the form of MoO3 and V2 O5 , respectively, even though the crystallinity was not high. The films were annealed at 823 K in air for 1 h, during which process the films were crystallized to each metal oxide. The crystal structure of each catalyst film was confirmed by X-ray diffraction using a Mac Science M18XHF-SRA diffractometer with Cu K␣ radiation. The thicknesses of the metal electrode and the catalyst films were controlled by the amount of source materials and were finally evaluated by scanning electron micrograph (SEM) observation of the cross-sections of the film. SEM results were recorded on a JEOL JSM-6340F instrument after coating the sample with Au–Pd film by using Plasma Mult Coater. Surface morphology of the MoO3 or V2 O5 catalyst and the layered structure were also evaluated by the SEM observations of the surface of the catalyst films and the cross-section of the cell systems, respectively.
2. Experimental
3. Results and discussion
2.1. Preparation of the cell system
3.1. Structure of MoO3 and V2 O5 on the Au anode
An electrochemical reactor was constructed from an 8 mol.% YSZ disk, 32 mm in diameter and 1 mm in thickness. Precise details were reported in the previous papers [8,9]. First, Au (1 m thick) was deposited as the anode metal onto a face of the YSZ disk; then, the MoO3 (∼4 m thick) or V2 O5 (∼1.0 m thick) catalyst was deposited onto the Au anode; finally, Ag (∼2 m thick) was deposited as the cathode metal onto the other face of the YSZ disk. Both Au and Ag were deposited by vacuum evaporation using a glass bell-jar-type apparatus equipped with a diffusion pump and rotary pump evacuation system. Both MoO3 and V2 O5 were deposited on the Au anode by reactive sputter-
X-ray diffraction patterns of MoO3 (A) and V2 O5 (B) films on the Au anode are shown in Fig. 1. MoO3 film showed weak lines of MoO3 [JCPDS: 5-0508] just after the deposition (Fig. 1Aa), while the intensities were significantly strengthened after the annealing (Fig. 1Ab). After the propene oxidation for 12 h at 773 K, the line intensities of MoO3 were substantially weakened, together with those of Au and YSZ, but no line of the reduced molybdenum oxide species was observed (Fig. 1Ac). V2 O5 film after the deposition (Fig. 1Ba) showed the lines of V2 O5 [JCPDS: 41-1426]; the pattern did not substantially change after the annealing (Fig. 1Bb). After the propene oxidation for 12 h
2.2. Oxidation of C2 –C4 hydrocarbons A gaseous mixture of hydrocarbon (5%), nitrogen (5%) and helium (90%) was passed (1.4 l h−1 ) over the catalyst film at 773 K for testing the activity of the catalyst. To check the effect of oxygen from the gaseous phase, we also passed a gaseous mixture of hydrocarbon (5%), oxygen (5%), nitrogen (5%) and helium (85%) over the catalyst film at the same rate. Oxygen gas (0.6 l h−1 ) was passed through the cathode space. The applied potential (V) for controlling the oxygen flux in these experiments was 0–3.5 V, corresponding to 0–10 mA in electric current and 0–93 mol h−1 of O2 flux. The products in the effluent gas were determined by gas chromatography.
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Fig. 1. X-ray diffraction patterns of MoO3 (A) and V2 O5 (B) films on the Au anode. (a) After the deposition; (b) after the annealing; (c) after the reaction: (䊉) MoO3 ; (䊏) V2 O5 ; (䉱) V6 O13 ; (䉲) V2 O4 ; (䊊) Au; (䊐) YSZ.
at 773 K, the reduction to V6 O13 [JCPDS: 43-1050] and V2 O4 [JCPDS: 43-1051] was clearly observed and the lines of V2 O5 disappeared, suggesting that V2 O5 was easily reduced during the reaction (Fig. 1Bc). SEM images of the surface of MoO3 (a, b and c) and V2 O5 (a , b and c ) are shown in Fig. 2. After the deposition, MoO3 showed a porous film composed of a mixture of various sizes of the crystallites together with powder-like products (Fig. 2a). After the annealing, the size of crystallites became uniform (Fig. 2b). After the propene oxidation for 24 h at 773 K, the surface of the film became rather flat. The crystallites were destroyed and sintered with each other to form aggregates of small-sized particles (Fig. 2c). After the deposition, V2 O5 showed a dense film composed of aggregates of small-sized crystallites sticking to each other tightly (Fig. 2a ). After the annealing, the crystallites grew slightly but no significant change was observed in the size and the form of the V2 O5 crystallites (Fig. 2b ). After the reaction for 24 h, the surface morphology of the film significantly changed and a flat and sintered surface appeared (Fig. 2c ). All the data of the oxidation reactions were corrected on the fresh films of both MoO3 and V2 O5 deposited on the Au anode. SEM images of the cross-section of MoO3 /and V2 O5 / Au|YSZ just after the deposition are shown in Fig. 3. In both cases, layered structures of the catalyst film and gold anode film on the YSZ disc were clearly observed. MoO3 formed a thick and rather porous film composed of various sizes of MoO3 crystallites (Fig. 3A), while V2 O5 formed a thin and dense film, probably composed of small-sized crystallites (Fig. 3B). The results are consistent with those observed in the surface morphology in Fig. 2.
3.2. Oxidation of alkanes over V2 O5 Oxidations of C2 –C4 alkanes, i.e., ethane, propane and isobutane, were tested over the MoO3 and V2 O5 films under oxygen pumping. No oxidation of alkane took place over the MoO3 film, while alkanes were oxidized over the V2 O5 film (Fig. 4). Over the V2 O5 film, propane produced mainly propene, together with small amounts of acrolein and acetaldehyde, and isobutane gave methacrolein, even though the selectivity was not so high. Some ethane was converted to carbon dioxide, but no partial oxidation product was observed. If the reaction starts from the H-abstraction from alkane by active oxygen formed on V2 O5 through the YSZ, the rate of reaction can be correlated with the C–H bond energy of alkane [28]. The rates of oxygen consumption calculated from either electric current for oxygen pumping or total amount of oxygen in the products coincided well and could be logarithmically plotted against the bond energy of the weakest C–H bond of each alkane (Fig. 5). A good correlation was observed, suggesting that the rate-determining step is the H-abstraction from alkane by oxygen on V2 O5 pumped through the YSZ. Oxidation of alkane on the V2 O5 film by oxygen from gaseous phase produced a large amount of carbon oxides, resulting in a lowering in the selectivity to oxygenated products. Vanadium oxide catalysts have frequently been used in the oxidative dehydrogenation of alkanes [12–15,20,21]. Magnesium vanadate is active for oxidative dehydrogenation of propane; both V5+ and V4+ catalyze the oxidative dehydrogenation of propane; however, V4+ seems to be more selective, though less active than V5+ [12]. We have tested vanadium-containing MCM-41 for oxidative dehydrogenation of ethane, propane and isobutane, where the catalysts
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Fig. 2. SEM images of the surfaces of MoO3 (a, b and c) and V2 O5 films (a , b and c ). (a and a ) After the deposition; (b and b ) after the annealing; (c and c ) after the reaction.
Fig. 3. SEM images of the cross-section of MoO3 (A) and V2 O5 (B) films on the An anode and the YSZ disc.
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Fig. 4. Oxidation of alkanes over V2 O5 films under oxygen pumping. Anode gas: alkane/N2 /He (5/5/90), 23.5 ml min−1 ; cathode gas: O2 , 20.0 ml min−1 ; current: 10 mA (93.3 mol O2 h−1 ).
prepared by direct hydrothermal methods with vanadium content exceeding 1 wt.% were effective for the reaction [13]. Solsona et al. [15] also reported similar results by using V-MCM-41 catalyst for the oxidative dehydrogenation of propane. In the cases of both magnesium vanadate [12] and V-MCM-41 [13,15], isolated and tetrahedrally coordinated VO4 seems to be selective for the oxidative dehydrogenation. V4+ was also effective for the dehydrogenation of butane over VOx /Al2 O3 catalyst, while V3+ was less active than V4+ , even though both species can be produced during the reaction [14]. Even over V2 O5 /Al2 O3 catalyst, in situ Raman spectroscopy study showed that surface polymeric vanadium oxide species were more reducible than isolated surface vanadium species, and that the active site for propane oxidative dehydrogenation on alumina-supported vanadia must be a single VO4 site [19]. Moreover, it was
Fig. 5. Relationship between the rate of oxygen consumption and C–H bond energy.
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also found that the V–O–V bond might not be critical for this reaction [19]. However, an important catalytic effect of acid/base properties was observed in the oxidative dehydrogenation of alkanes over V/MgO catalyst [21] as well as over V-MCM-41 [13]; this effect is still a cause of controversy in the discussions on the active species. Reduction–oxidation of vanadium sites may also produce acid/base properties on the catalyst surface and may affect the catalytic behavior. In the present work, V2 O5 crystals of rather large size were put on the Au anode and vanadium species are not isolated but are all combined with each other by V–O–V bonding. Over the V2 O5 film, propane was mainly dehydrogenated to propene, while isobutane was converted to methacrolein under oxygen pumping. During the reaction, V2 O5 was reduced to V6 O13 and V2 O4 , on which V4+ was formed and works as the active species for the oxidative dehydrogenation of alkanes. Among the alkanes, ethane was not markedly oxidized probably due to the strong C–H bond energy, while propane was dehydrogenated to propene due to secondary C–H bonds. Propene is stable due to resonance structure of allylic intermediate; it therefore desorbed and appeared in the effluent gas. Due to the weakest C–H bond at the tertiary carbon atom, isobutane was easily dehydrogenated to isobutene, which is not as stable as propene due to the presence of another methyl group and was in situ oxidized to methacrolein (vide infra). The further oxidation of isobutene may be due to the presence of neighboring active vanadium sites on V2 O5 crystal compared to a single VO4 site on supported vanadia catalyst, on which high selectivity was obtained in the oxidative dehydrogenation of isobutane [13]. 3.3. Oxidation of alkenes over MoO3 Alkene oxidation over the MoO3 film under oxygen pumping (A) or by gaseous oxygen (B) is shown in Fig. 6. Interestingly, over the MoO3 film, the partial oxidation of alkene was rather enhanced under oxygen pumping compared with the results obtained by gaseous oxygen, which coincided well with our previous results obtained in the oxidation of propene over MoO3 /Au|YSZ|Ag [11]. Not only the rate of oxidation but also the selectivity to the oxygenated products was higher in the reaction under oxygen pumping than in that by gaseous oxygen. The highest selectivity to partial oxidation product was observed with methacrolein, followed by acrolein and acetaldehyde in both cases: under oxygen pumping and by gaseous oxygen. Aldehyde alone was observed as the partial oxidation product under oxygen pumping, while the other oxygenated compounds were produced in the reaction by gaseous oxygen. The selectivity of 80% was obtained with methacrolein from isobutene under oxygen pumping; this value was far higher than that of 60% obtained by gaseous oxygen over the MoO3 film. The effect of oxygen flux was tested in the alkene oxidation over the MoO3 film by increasing the electric current through the YSZ (Fig. 7). The rate of formation of aldehyde
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Fig. 6. Oxidation of alkenes over MoO3 film under oxygen pumping (A) and by gaseous oxygen (B). Anode gas: alkene/N2 /He (5/5/90), 23.5 ml min−1 ; cathode gas: O2 , 20.0 ml min−1 ; current: 10 mA (93.3 mol O2 h−1 ); (B) mixed gas: alkene/O2 /N2 /He (5/5/5/85), 23.5 ml min−1 .
increased, while the selectivity gradually decreased with increasing the oxygen flux in all cases of methacrolein, acrolein and acetaldehyde, suggesting that further oxidation of the aldehyde took place on the MoO3 film. Under the presence of an excess amount of active oxygen species, the aldehyde must be further oxidized to carbon oxides, resulting in a decrease in the selectivity. The selectivity of aldehyde is plotted against the reciprocal flow rate of alkene, corresponding to contact time in the gas-flow reactor using a fixed-bed catalyst (Fig. 8). When the flow rate of alkene increased, i.e., the contact time decreased, the selectivity gradually increased for methacrolein and acrolein. In the case of acetaldehyde, a drastic increase was observed in the selec-
Fig. 7. Effect of oxygen flux in the oxidation of alkenes over MoO3 film under oxygen pumping. Anode gas: alkene/N2 /He (5/5/90), 23.5 ml min−1 ; cathode gas: O2 , 20.0 ml min−1 . Rate of formation of methacrolein (䊉), acrolein (䊏) and acetaldehyde (䉱). Selectivity to methacrolein (䊊), acrolein (䊐) and acetaldehyde ().
tivity with increasing the flow rate of ethene, suggesting that acetaldehyde once formed on the MoO3 film was rapidly oxidized to carbon oxides. 3.4. Oxidation of alkenes over V2 O5 Over the V2 O5 film, ethene was most quickly oxidized, followed by propene and isobutene in both cases: under oxygen pumping and by gaseous oxygen (Fig. 9). In these reactions, many products formed by fission of carbon–carbon bond were observed, i.e., C2 and C2 –C3 compounds in the oxidation of propene and isobutene, respectively. In the presence of gaseous oxygen, the rate of oxidation was drastically enhanced, but the selectivity to aldehyde decreased, compared to that under oxygen pumping. This is probably due
Fig. 8. Effect of flow rate of alkenes in the oxidation of alkenes over MoO3 film under oxygen pumping. Anode gas: alkene/N2 /He = 5/5/90 (vol. ratio); cathode gas: O2 , 20.0 ml min−1 ; current: 10 mA (93.3 mol O2 h−1 ).
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Fig. 9. Oxidation of alkenes over V2 O5 film under oxygen pumping (A) and by gaseous oxygen (B). (A) Anode gas: alkene/N2 /He (5/5/90), 23.5 ml min−1 ; cathode gas: O2 , 20.0 ml min−1 ; current: 10 mA (93.3 mol O2 h−1 ); (B) mixed gas: alkene/O2 /N2 /He (5/5/5/85), 23.5 ml min−1 .
to gaseous phase oxidation via radical intermediate formation; vanadium catalyst probably produces radical species by homolytic cleavage of C–H bond, followed by radical chain-reaction with molecular oxygen in gaseous phase. This mechanism also leads to the formation of C–C bond cleavage products such as C2 and C2 –C3 from propene and isobutene, respectively. The highest selectivity was still obtained for methacrolein, but followed by that for acetaldehyde and then by that for acrolein over the V2 O5 film. In the oxidation of both isobutene and propene, significant amounts of products with lower carbon numbers, i.e., acetone, propene, propionaldehyde, acetaldehyde and ethene were observed. This can be clearly seen in the decreasing selectivity to the aldehyde with increasing the oxygen flux (Fig. 10). It is likely that methacrolein and acrolein were further oxidized under the presence of excess amounts of oxygen. The fact that the selectivity of acrolein is lower than that of acetaldehyde suggests that the rate of acrolein oxidation is higher than that of acetaldehyde on the V2 O5 film (Fig. 10), oppositely to the case observed on the MoO3 film (Fig. 7). The selectivities to both acrolein and acetaldehyde were extremely low (20%) compared to that to methacrolein when the flow rate of alkene increased over the V2 O5 film (Fig. 11). Isobutene was also oxidized to acetone, propene and acetaldehyde, but the selectivity to methacrolein still remained at the highest value of 60% among the three aldehydes.
in industrial applications and in academic research [2–6]. One of the “model reactions” investigated in detail for clarifying the catalytic mechanism of molybdenum-containing systems is the partial oxidation of propene to acrolein over MoO3 [29,30]. The mechanism consisting of alternating oxidation and reduction of the metal oxide catalyst surface and/or bulk has been suggested and is referred to as the redox mechanism (or Mars–Van Krevelen mechanism [1]). Varying degrees of participation of bulk oxygen in partial oxidation reactions have been reported in the literature. For the two oxides V2 O5 (674–823 K) and MoO3 (873–1023 K), it was found that practically all of the oxygen from the bulk could be exchanged with 18 O2 and, hence, could participate in oxidation reactions proceeding on the surface of the catalysts [31–34]. The difference between the two temperature
3.5. Comparison of the activity between MoO3 and V2 O5 V2 O5 was reduced to V6 O13 and V2 O4 after the oxidation reaction of propene at 773 K, while the reduction of MoO3 was not clearly observed (Fig. 1). It was concluded that vanadium catalysts were effective for oxidative dehydrogenation of alkanes to alkenes (Fig. 4) [12–15,20,21]. On the other hand, molybdenum oxide-based catalysts are extensively employed for the partial oxidation of alkenes both
Fig. 10. Effect of oxygen flux in the oxidation of alkenes over V2 O5 film under oxygen pumping. Anode gas: alkene/N2 /He (5/5/90), 23.5 ml min−1 ; cathode gas: O2 , 20.0 ml min−1 . Rate of formation of methacrolein (䊉), acrolein (䊏) and acetaldehyde (䉱). Selectivity to methacrolein (䊊), acrolein (䊐) and acetaldehyde ().
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steps of the small (1 0 0) planes were formed on the (0 1 0) plane, suggesting the formation of high concentration of the active sites on the surface of the MoO3 crystal [11]. These results are consistent with the mechanism proposed by Ressler et al. [34]. Though XRD patterns of MoO3 film did not show clear evidence of oriented crystal growth, the MoO3 film was also prepared by reactive sputtering in the present work. It is therefore possible that a similar redox mechanism works on the surface of the MoO3 film and catalyzes the oxidation reaction of propene. Oxygen pumping system can efficiently deliver atomic oxygen species to the reduced active site on the catalyst surface, resulting in the selective oxidation of alkenes. V2 O5 can work as the catalyst for alkane oxidation, and it is therefore expected that a combination of V2 O5 and MoO3 can catalyze the selective oxidation of alkane. Fig. 11. Effect of flow rate of alkenes in the oxidation of alkenes over V2 O5 film under oxygen pumping. Anode gas: alkene/N2 /He = 5/5/90 (vol. ratio); cathode gas: O2 , 20.0 ml min−1 ; current: 10 mA (93.3 mol O2 h−1 ).
ranges well explains apparently higher stability of MoO3 compared with that of V2 O5 observed under the present reaction conditions (773 K as the reaction temperature). Alkenes were effectively oxidized both under oxygen pumping and by gaseous oxygen over the MoO3 film (Fig. 6). The reduction of MoO3 is a crucial step in the redox mechanism of partial oxidation reactions on this material; thus, the reduction of MoO3 with propene and/or hydrogen is inevitably suggested. Formation of partially reduced molybdenum “sub-oxides” during reduction has been reported as a result of the redox mechanism and of the difference in rates of catalyst reduction and reoxidation [35]. In the present work, weak peaks of Mo9 O26 [JCPDS: 5-0441] as a “sub-oxide” were observed in the XRD patterns of the samples after the propene oxidation reaction by gaseous oxygen. Ressler et al. [34] reported that a “Mo18 O52 ”-type shear structure formed as intermediate during reduction of MoO3 in propene and during oxidation of MoO2 in oxygen by using in situ XAS technique. A schematic reaction mechanism for the reduction of MoO3 in propene and reoxidation in oxygen is proposed that consists of (i) generation of oxygen vacancies at (1 0 0) or (0 0 1) plane by reaction with propene, (ii) vacancy diffusion in the MoO3 bulk, (iii) formation of Mo18 O52 -type shear structure in the lattice, and (iv) formation and growth of MoO2 nuclei. The mechanism is in agreement with previous reports indicating that the oxidation reaction of propene on MoO3 is structure-sensitive [36]. We have reported that MoO3 film prepared on the Au anode by the reactive sputtering method showed high activity for the oxidation of propene [11]. This film was composed of many leaf-like MoO3 crystals each of which grew perpendicularly to the Au surface and had fine structure, resulting in many small sub-surfaces of (1 0 0) and (0 0 1) planes on the outer surface. Moreover, many atomic
4. Conclusion Oxidation of C2 –C4 hydrocarbons was carried out over MoO3 or V2 O5 catalyst film deposited on an electrochemical cell, Au|YSZ|Ag, at 775 K under oxygen pumping through the cell. The results were compared with those obtained by gaseous oxygen over the catalyst film. Propane and isobutane were oxidized to propene and methacrolein, respectively, over V2 O5 , while neither alkane was oxidized over MoO3 , under oxygen pumping. Ethane was not remarkably oxidized even over V2 O5 . It is concluded that homolytic C–H cleavage is the rate-determining step of the alkane oxidation on V2 O5 . Alkenes were partially oxidized over both MoO3 and V2 O5 . The selectivity to acetaldehyde, acrolein and methacrolein from ethene, propene, and isobutene, respectively, were higher on the former than on the latter. Alkenes were oxidized more efficiently under oxygen pumping than by gaseous oxygen over MoO3 . The highest selectivity was obtained with methacrolein, followed by acrolein and acetaldehyde; among these, acetaldehyde was further oxidized to carbon oxides. On the other hand, alkenes were far more rapidly oxidized by gaseous oxygen than under oxygen pumping over V2 O5 , while the selectivity to aldehyde was still higher under oxygen pumping than the value obtained by gaseous oxygen. The highest selectivity was observed with methacrolein, and the values of both acrolein and acetaldehyde were low. No further oxidation of acetaldehyde was observed over V2 O5 .
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