Communication pubs.acs.org/EF
Controlling Selectivity in Direct Conversion of Methane into Formaldehyde/Methanol over Iron Molybdate via Periodic Operation Conditions P.-A. Carlsson,* D. Jing, and M. Skoglundh Department of Chemical and Biological Engineering and Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden
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be used to affect the product selectivity. As an example, we revisit oxidation of methane over alumina-supported Pd and Pt catalysts,19 focusing on the selectivity toward partial oxidation products. In Figures 1 and 2, the oxidation of a continuous feed
he production of carbon-neutral fuels from biological feedstocks, so-called biofuels, is often considered as a potential contributor to sustainable transports. At present, the transformation of biomass into, e.g., biogas, biodiesel, alcohols, and ethers, is studied intensively. For an overview see, e.g., the reviews by Huber et al.1 and Briens et al.2 and, more recently, Serrano-Ruiz et al.3 and Bulushev et al.4 and references therein. In the case of biogas, the dominating constituent is methane (CH4). Although methane is often considered to be a relatively clean fuel as such, it is sometimes more attractive to convert CH4 into other energy carriers that can be more readily liquified than methane. This is relevant also for more convenient use of natural gas resources located in remote areas as fuel transportation becomes more facile. The conventional routes for production of liquid fuels from methane usually involve, as an intermediate step, the formation of syngas,5,6 a CO/H2 gas mixture, which, in turn, is often produced via steam reforming, carbon dioxide (or dry) reforming, partial oxidation processes, or combinations thereof.7 However, as an example, the demanding process conditions, i.e., high temperatures and pressures, required for the steam reforming of methane are a strong driving force to explore less energy-intensive alternatives. Among the possible routes, a heterogeneous catalytic process for direct conversion of methane into desired fuels would be preferred. We mention that other types of processes have been considered as well; for example, homogeneous catalytic8,9 and plasma processes10 have been proposed for direct production of methanol. For heterogeneous catalytic processes, numerous catalyst formulations have been evaluated. Consider again, as an example, the direct conversion of methane to methanol, which, for several reasons, is a challenging reaction.5,6,11,12 For this reaction, ZSM5 ion exchanged with copper and iron has been reported selective in the presence of N2O.13 Similarly, the production of methanol through coupling of formaldehyde and carbon dioxide over a ZnO catalyst doped with Cu and Fe has been found during methane oxidation conditions.14 Another composite material, which will be considered in more detail here, that can convert methane directly into formaldehyde and methanol is iron molybdate [Fe2(MoO4)3].11,12 We have previously employed periodic operation of the gasphase composition, i.e., controlled pulsing of reactant supply, for supported noble metal catalysts to induce catalytic ignition of CO oxidation,15−17 increase total oxidation of both unsaturated15 and saturated hydrocarbons,18−20 and conceptualize the fundamentals of catalytic processes involved in oxidation of both CO21 and CH4.22,23 Although periodic operation in these cases was used for activity studies, it can also © 2012 American Chemical Society
Figure 1. Oxidation of 500 vol ppm CH4 during periodic operation, i.e., pulsing of 1250 vol ppm O2, over a 5% Pd/Al2O3 catalyst at 623 K. The dashed and dotted lines indicate the oxygen level corresponding to (stoichiometric) total and partial methane oxidation, respectively.
of 500 vol ppm CH4 during oxygen-pulsing conditions over alumina-supported Pd and Pt is shown, respectively. The oxygen pulsing refers here to cycling of 1250 vol ppm O2 for 5 min, followed by an oxygen-free period also for 5 min. This cycle was repeated 10 times. Using the formation of CO and H2 as evidence for partial oxidation, it is clear for both catalysts that, during periodic operation, the catalytic process alternates between total and partial methane oxidation. Generally, the total oxidation dominates when oxygen is present in the feed, and the activity is particularly high for short time periods after the gas switches, even in the case where the oxygen supply is switched off. This is especially clear for the Pt catalyst. Contrarily, during the oxygen-free periods (t = 32−37 min), partial oxidation occurs. The transient behavior of the CO and H2 formation is qualitatively different for the two catalysts; i.e., more CO is produced over the Pt catalyst, which also shows a Received: September 13, 2011 Revised: February 23, 2012 Published: February 24, 2012 1984
dx.doi.org/10.1021/ef300007n | Energy Fuels 2012, 26, 1984−1987
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Figure 3. X-ray diffractograms for the dried as-prepared and used Fe2(MoO3)4 catalyst.
Fe2(MoO4)3 sample as well as for the used sample reveal a dominating monoclinic structure at room temperature, which is anticipated below 786 K.28 The evaluation of activity and selectivity for partial methane oxidation as a function of the temperature and reactant stoichiometry during periodic operation was performed using a continuous gas-flow reactor system equipped with a mass spectrometer described in detail elsewhere.29 In the present study, the reactor setup was modified essentially by replacing the conventional horizontal quartz reactor tube designed for structured catalysts by a vertical quartz tube (inner diameter of 8 mm) with a fixed inner porous quartz plate holding the powder sample. The gas flow was passed through the bed from above; thus, a fluidizing bed with possible severe bypass flow was avoided. About 0.5 g of the Fe2(MoO4)3 sample was used. Different experiments where the reactant feed was periodically changed, i.e, constant methane concentration and varying oxygen concentrations, were performed. Argon was used as balance, and the total flow was kept constant at 100 mL/min. The mass/charge ratios (m/z) of 15 (CH4), 28 (CO), 30 (HCHO), 32 (O2), 40 (Ar), and 44 (CO2) were followed. During this study, an appropriate conversion of the m/z signals into concentration units could unfortunately not be made, and thus, the mass spectrometer signals are displayed as such. Despite this, however, the conversion of methane could be estimated from the m/z 15 signal. It turned out that the degree of methane conversion is low in all experiments, less than 2%, indicating that diffusion limitations that can often obscure observed reaction kinetics are likely absent in this study. Moreover, the present study is focused on the qualitative changes of activity and selectivity imposed by periodic operation, and thus, determination of absolute reaction rates is not crucial. The influence of the temperature on the partial oxidation of methane over Fe2(MoO4)3 during periodic operation is shown in Figure 4. The top panel shows the signals corresponding to nonconverted methane and oxygen in the outlet stream, while the reaction products, i.e., formaldehyde, carbon monoxide, and carbon dioxide, are shown in the panels below. It is clear that an increased temperature leads to an increase in the product formation. At the lowest temperature, only CO can be observed, while at the higher temperatures, also HCHO and CO2 are formed. This may be due to a too low oxygen dissociation rate and/or low activation of oxygen species on the surface or in the surface region, limiting the formation of
Figure 2. Oxidation of 500 vol ppm CH4 during periodic operation, i.e., pulsing of 1250 vol ppm O2, over a 5% Pt/Al2O3 catalyst at 673 K. The dashed and dotted lines indicate the oxygen level corresponding to (stoichiometric) total and partial methane oxidation, respectively.
more rapidly declining hydrogen formation probably because of water formation. The partial oxidation of methane during the oxygen-free periods occurs by the use of oxygen that has been stored in the catalysts, in the form of oxides and/or oxygencontaining species on the support,22−24 during the preceding periods with oxygen excess. Although the Pd catalyst can likely store a larger amount of oxygen because of the higher ability to form bulk oxides, the stability of palladium oxides is higher than that of platinum oxides25,26 and, thus, less prone to react with methane. This explains the lower methane conversion over the Pd catalyst during the oxygen-free periods. Bearing the described results in mind, we address in the present study the partial oxidation of methane over a Fe2(MoO3)4 catalyst under periodic operation conditions. Special attention is paid to the influence of gas composition changes on the methane conversion and product selectivity. The preparation of the Fe2(MoO4)3 catalyst was performed using a hydrothermal synthesis route.27 Analytical-grade Fe(NO3)3·9H2O (2.73 g, Sigma Aldrich) and (NH4)6Mo7O24·7H2O (1.79 g, Acros) were dissolved separately in distilled water (88 mL each). The molybdate solution was then added dropwise to the iron nitrate solution under continuous stirring to form a homogeneous solution, which was pH-adjusted to 3 by adding NH3·H2O (25 wt %) and HNO3 (1 mol/L). The solution was transferred to a Teflonlined autoclave, sealed, and kept at 140 °C for 12 h. The formed precipitate was then washed several times with distilled water and ethanol. Finally, the sample was dried in vacuum at 60 °C for 4 h. The Brunauer−Emmett−Teller (BET) surface area (Micromeritics, Tristar) of the dried sample was measured to be 16.3 m2/g. The sample was further characterized by X-ray diffraction using a powder diffractometer (Siemens AXS, D5000) with Cu Kα radiation (45 kV, 40 mA, and 1.540 56 Å) in grazing incidence geometry (incidence angle fixed at 5°) using a curved multilayer Göbel mirror on the primary side and long Soller slits and the energy-dispersive solid-state (Sol-X) detector on the secondary side. The measurement was made using the detector scan in the 10−60° region in 2θ with a step of 0.05° and measurement time of 2 s in each step. The X-ray diffractograms (cf. Figure 3) for the dried as-prepared 1985
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Figure 4. Influence of the temperature on catalytic activity and selectivity during periodic operation, i.e., pulsing of oxygen (0.25 vol %) to a constant concentration of CH4 (1.0 vol %), of an Fe2(MoO3)4 catalyst at a constant total gas flow.
Figure 5. Influence of the reactant ratio on catalytic activity and selectivity during periodic operation of an Fe2(MoO3)4 catalyst at 873 K using a constant total gas flow. The methane and oxygen concentrations were 1.0 and 0.25 vol % and 0.9 and 0.3 vol % for the CH4/O2 ratios of 4 and 3, respectively.
HCHO and CO2 at low temperatures. The different responses in the oxygen signal at the different temperatures indicate not only that more oxygen is consumed in the reaction but also that oxygen is stored at the introduction of oxygen excess conditions and released under the oxygen-free periods. For example, the slower decay of the oxygen signal after the oxygen supply has been switched off at the higher temperatures supports the release of oxygen via desorption. Also, this supports that the oxygen dissociation rate increases with an increasing temperature. Another scenario could be that other reactions that involve oxygen to form products not measured here can occur at the lower temperature, which would lead to a more rapid decline of the measured oxygen signal. Figure 5 displays the results where the influence of the reactant ratio on the partial oxidation of methane under periodic operation was specifically addressed. As seen, both the methane and oxygen responses are similar for the different reactant feeds, respectively, indicating similar consumption of reactants. Despite this, the product spectra are different for the two reactant ratios studied. In both cases, HCHO and CO show similar responses; however, the CO2 response is different. In the case of more oxygen, i.e., CH4/O2 = 3, less CO2 is formed. In principle, the underlying reason for this may be that the higher oxygen concentration leads to a detrimental high oxygen coverage, i.e., oxygen selfpoisoning analogous for Pt/Al2O3 systems,19,22−24 which can then lead to a lower activity for CO2 formation. However, this scenario seems somewhat counterintuitive because the change in the oxygen concentration is rather small. Another, more interesting, explanation is that other products than could be detected in the present experiments might be formed, i.e., methanol. This is supported by the fact that both similar amounts of reactants are consumed and similar formation of HCHO and CO is observed, while the formed CO2 differs. This means that, in the case of CH4/O2 = 3, other carbon-
containing species must be formed or accumulated on the catalyst surface, which is less likely because, here, the oxygen concentration is higher and also the oxygen coverage is higher. Here, we comment on the fact that methanol may decompose at the temperature of the experiment, which, in turn, may be the reason for the fact that no methanol could be detected. Nevertheless, the experimental results show that periodic operation affects both activity and selectivity for the catalytic oxidation of methane over Fe2(MoO3)4. We primarily communicate the potential benefits in terms of selectivity that can be achieved using the concept of periodic operation. It is, however, appropriate to make some comments also on the stability of the used catalysts. In the case of the Pd/ Al2O3 and Pt/Al2O3 catalysts, no significant changes in activity or selectivity related to stability issues could be observed during the experiments. This is much due to the supporting alumina matrix, which stabilizes the nanometer-sized noble metal crystallites sufficiently well at the present experimental conditions. One reason for this is that the catalyst preparation was carried out at temperatures well exceeding the temperatures used in the methane oxidation experiments. In the case of the iron molybdate catalyst, the situation is different. During the methane oxidation experiments, the BET surface area decreased significantly at the highest temperatures, especially in the presence of hydrogen (not shown). In applications, it is likely that iron molybdate needs to be stabilized, for example, via distribution onto a supporting matrix analogous to the alumina-supported noble metal catalysts. It is however beyond our present aim to explore this further here. In summary, the partial oxidation of methane over a Fe2(MoO3)4 powder catalyst has been studied under transient inlet gas conditions, i.e., oxygen pulsing. The results show that, besides reaction temperature and gas stoichiometry, transient 1986
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(27) Ding, Y.; Yu, S.-H.; Liu, C.; Zang, Z.-A. Chem.Eur. J. 2007, 13, 746. (28) Massarotti, V.; Flor, G.; Marini, A. J. Appl. Crystallogr. 1981, 14, 64. (29) Lundgren, S.; Keck, K.-E.; Kasemo, B. H. Rev. Sci. Instrum. 1994, 65 (8), 2696.
operation of the gas mixture can be a viable method to control product selectivity. This may be used in new processes for chemical transformations/fuel production, e.g., direct catalytic conversion of methane into methanol.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work has been performed by the Competence Centre for Catalysis, which is hosted by Chalmers University of Technology and financially supported by the Swedish Energy Agency and the member companies AB Volvo, Volvo Car Corporation, Scania CV AB, Saab Automobile Powertrain AB, Haldor Topsøe A/S, and ECAPS AB.
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REFERENCES
(1) Huber, G. W.; Iborra, S.; Corma, A. Chem. Rev. 2006, 106, 4044. (2) Briens, C.; Piskorz, J.; Berruti, F. Int. J. Chem. React. Eng. 2008, 6, 1. (3) Serrano-Ruiz, J. C.; West, R. M.; Dumesic, J. A. Annu. Rev. Chem. Biomol. Eng. 2010, 1, 79. (4) Bulushev, D. A.; Ross, J. R. H. Catal. Today 2011, 171, 1. (5) Rostrup-Nielsen, J. R. Cat. Rev. 2004, 46, 247. (6) Holmen, A. Catal. Today 2009, 142, 2. (7) Lunsford, J. H. Catal. Today 2000, 63, 165. (8) Periana, R. A.; Taube, D. J.; Evitt, E. R.; Löffler, D. G.; Wentrcek, P. R.; Voss, G.; Masuda, T. Science 1993, 259, 340. (9) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii, H. Science 1998, 280, 560. (10) Indarto, A.; Yang, D. R.; Palgunadi, J.; Choi, J.-W.; Lee, H.; Song, H. K. Chem. Eng. Proc. 2008, 47 (5), 786. (11) Hall, T. J.; Hargreaves, J. S. J.; Hutchings, G. J.; Joyner, R. W.; Taylor, S. H. Fuel Proc. Technol. 1995, 42, 151. (12) Otsuka, K.; Wang, Y. Appl. Catal., A 2001, 222, 145. (13) Aderson, J. R.; Tsai, P. J. Chem. Soc., Chem. Commun. 1987, 1435. (14) Sojka, Z.; Herman, R. G.; Klier, K. J. Chem. Soc., Chem. Commun. 1991, 185. (15) Carlsson, P.-A.; Thormählen, P.; Skoglundh, M.; Persson, H.; Fridell, E.; Jobson, E.; Andersson, B. Top. Catal. 2001, 16/17 (1−4), 343. (16) Carlsson, P.-A.; Skoglundh, M.; Fridell, E.; Jobson, E.; Andersson, B. Catal. Today. 2002, 73, 307. (17) Carlsson, P.-A.; Thormählen, P.; Skoglundh, M.; Andersson, B. Top. Catal. 2004, 30/31, 375. (18) Carlsson, P.-A.; Mollner, S.; Arnby, K.; Skoglundh, M. Chem. Eng. Sci. 2004, 59, 4313. (19) Carlsson, P.-A.; Fridell, E.; Skoglundh, M. Catal. Lett. 2007, 115 (1−2), 1. (20) Becker, E.; Carlsson, P.-A.; Skoglundh, M. Top. Catal. 2009, 52, 1957. (21) Carlsson, P.-A.; Ö sterlund, L.; Thormählen, P.; Palmqvist, A.; Fridell, E.; Jansson, J.; Skoglundh, M. J. Catal. 2004, 226, 422. (22) Becker, E.; Carlsson, P.-A.; Grönbeck, H.; Skoglundh, M. J. Catal. 2007, 252, 11. (23) Becker, E.; Carlsson, P.-A.; Kyhammar, L.; Newton, M. A.; Skoglundh, M. J. Phys. Chem. C 2011, 115, 944. (24) Carlsson, P.-A.; Nordström, M.; Skoglundh, M. Top. Catal. 2009, 52, 1962. (25) Ho, Y.-S.; Wang, C.-B.; Yeh, C.-T. J. Mol. Catal. A: Chem. 1996, 112, 287. (26) Wang, C.-B.; Yeh, C.-T. J. Catal. 1998, 177, 450. 1987
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