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CO2 Reforming of Methane by Thermal Diffusion Column Reactor with Ni/Carbon-Coated Alumina Tube Pyrogen Kenichi Suzuki, Verina J. Wargadalam, Kaoru Onoe, and Tatsuaki Yamaguchi* Chiba Institute of Technology, 2-17-1, Tsudanuma, Narashino, Chiba 275-0016, Japan Received June 22, 2000. Revised Manuscript Received January 4, 2001
The CO2 reforming of methane with a thermal diffusion column (TDC) reactor by using an alumina tube coated with nickel-loaded carbon as the pyrogen, acting as a heated catalyst, was studied. Large surface area of carbon-coat support (400-500 m2/g) was obtained by carbonization of a novolak-type phenolic resin. Although the coated carbon showed a minor effect, Ni loading increased the activity of the pyrogen for the reaction. For example, by using 10 wt % Ni-loaded pyrogen at the surface temperature 1260 K, each conversion of methane and carbon dioxide exceeded 65% with upward feeding of 0.44 mmol/min (CO2/CH4 ) 1). The downward feeding of reactant gas showed lower conversions but better stability of the catalytic action of the pyrogen. Performance of TDC reactor for CO2 reforming of methane can be improved by the better preparation of the pyrogen to increase the activity and with downward feeding to reduce the carbon formation.
Introduction The reforming of methane with CO2 as an oxidizer becomes a more attractive route to produce synthesis gas (CO and H2) from the standpoints of both the production of mixed gases, in which gas ratio (H2/CO) meets the requirement as a key material for an efficient dimethyl ether (DME) manufacture,1 and the challenging target for the chemical utilization of greenhouse gas as well. Furthermore, due to its strong endothermic reaction 1, CO2 reforming of methane also may apply as a thermochemical heat-pipe for the recovery, storage, and transmission of solar and other renewable energy sources.2
CH4 + CO2 f 2CO + 2H2 ∆H° ) 247 kJ mol-1 (1) Application of a thermal diffusion column (TDC) reactor for CO2 reforming of methane has been investigated.3-5 Using various metal wires (W, Mo, Chromel), high conversion of reactants was obtained.3,4 There was no carbon deposit observed; however, some metal oxides of the wire were found. In a recent work,5 modification of pyrogen by loading some active metals (Ni, Pt, and Ir) onto a carbon-rod pyrogen and its effect on the activity were investigated. It consequently showed that an Ir-loaded pyrogen gave the thermodynamical equilibrium value of the conversion to syngas at 1212 K. * Author to whom correspondence should be addressed. Tel: +8147478-0420. Fax: +81-47478-0439. E-mail:
[email protected]. (1) Adachi, Y.; Komoto, M.; Watanabe, I.; Ohno, Y.; Fujimoto, K. Fuel 2000, 79, 229-234. (2) Edwards, J. H.; Maitra, A. M. Proceedings of Natural Gas Conversion II 1994, 291-296. (3) Gesser, H. D.; Hunter, N.; Shigapov, N. R.; Januati, V. Energy Fuels 1994, 8, 1123-1125. (4) Wargadalam, V. J.; Hunter, N.; Gesser, H. D. Fuel Process. Technol. 1999, 59, 201-206. (5) Gesser, H. D.; Takahashi, R.; Suzuki, K.; Onoe, K.; Yamaguchi, T. Fuel Process. Technol., submitted.
However, some technical problems still remain in its applications, i.e., carbon deposition and catalyst sintering, which cause catalyst deactivation and reactor plugging. Supported metal catalysts such as Ni, Ru, Rh, Pd, Ir, and Pt are known as active catalysts for CO2 reforming of methane.6,7 Noble metal catalysts show good resistance to carbon formation, whereas nickelbased catalysts rapidly deactivated due to carbon deposits. For industrial scale application, nickel would be favorable in cost, therefore many investigations have been reported focusing on the mechanism of CO2 reforming and carbon antiformation using nickel-supported catalysts.8-15 Nickel catalysts on ZrO2,10 MgO,11 and La2O312 were reported to have fine activity and good stability, but are more expensive than those on SiO2. On the other hand, Nickel on SiO213 should give some solutions, i.e., to the deactivation in the initial stage of reaction or filamentous carbon deposition on catalyst. These prompt us to study with TDC reactor focused on Ni-loaded pyrogen, using a carbon-coated alumina tube as subject to the planning of scale-up in further work. (6) Solymosi, F.; Kutsan, Gy.; Erdohelyi, A. Catal. Lett. 1991, 11, 149-156. (7) Ashcroft, A. T.; Cheetham, A. K.; Green, M. L. H.; Vernon, P. D. F. Nature 1991, 352-353. (8) Rostrup-Nielsen, J. R.; Bak Hansen, J. H. J. Catal. 1993, 144, 38-49. (9) Erdohelyi, A.; Cserenyi, J.; Papp, E.; Solymosi, F. Appl. Catal. A: General 1994, 108, 205-219. (10) Turlier, P.; Brum Pereira, E.; Martin, G. A. Proceedings of the International Conference on CO2 Utilization; Bari: Italy, 1993; pp 119126. (11) Seshan, K.; Ten Barge, H. M.; Hally, W.; van Keulen, A. N. J.; Ross, J. R. H. Stud. Surf. Sci. Catal. 1994, 81, 285-290. (12) Hu, Y. H.; Ruckenstein, E. Catal. Lett. 1996, 36, 145-149. (13) Zhang, Z. L.; Verykios, X. E. Appl. Catal. 1996, 138, 589-595. (14) Bradford, M. C. J.; Vannice, M. A. Appl. Catal. 1996, 142A, 73-96. (15) Wang, S.; Lu, G. Q. Ind. Eng. Chem. Res. 1999, 38, 2615-2625.
10.1021/ef000134y CCC: $20.00 © 2001 American Chemical Society Published on Web 03/27/2001
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Suzuki et al. Table 1. Characteristics of Ni-Loaded and Unloaded Carbon Coat surface area moles of Ni content of C-coat Ni loaded [wt %] [m2/g] [µmol/m2] 0 10 15 20
413 483 499 464
3.0 4.4 5.8
Ni average crystallite size [nm] before after reaction reaction 24 24 24
47 47 47
(CH4/CO2 ) 1) was introduced into the TDC reactor upwardly or downwardly at a constant flow rate of 0.44 mmol/min. The inlet- and outlet-flow rates were measured by bubble flow meters. The hourly space velocity at 1150 K (surface temperature of Ni/C tube) was 21/h. Furthermore, thermal analysis of carbon antiformation on the surface of Ni/C-A tube was carried out by putting a piece of the tube (10 mm length) in thermogravimeter (Shimadzu, TGC-31).
Results
Figure 1. TDC reactor using a Ni/carbon-coated alumina tube as pyrogen.
Experimental Section The simple scheme of TDC reactor with dimension was illustrated in Figure 1. Ni/carbon-coated alumina (Ni/C-A) tube was mounted vertically at the center of the TDC reactor and heated from inside of the Ni/C-A tube by carbon-rod pyrogen, connected electrically to a variable transformer. An alumina tube was used with two purposes: for guarding of carbon-rod pyrogen and support of coated carbon. There is a little clearance (0.35 mm) between the alumina tube (3.7 mm i.d.) and carbon rod (3.0 mm o.d.). Carbon coating on the surface of the alumina tube was carried out by the following procedures. A novolak-type phenol resin (2 g, Dainippon Ink & Chemicals Co.) was diluted with 20 mL of 5 M NaOH solution. The prepared phenol resin solution is swept on the outer surface of alumina tube, followed by drying, then left the tube at ambient temperature for 0.5 h. The resin-coated alumina tube is positioned vertically in the quartz tube set in furnace for carbonization at 973 K for 2 h under atmospheric nitrogen of 1.79 mmol/min, and the carboncoated alumina (C-A) tube is obtained. Prior to the carbonization, it is necessary to substitute nitrogen for air in the quartz tube for ca. 3 h. The C-A tube is washed to remove the sodium by soaking in ion-exchange water then dried at ambient temperatures. A 0.064 M nickel solution prepared by dissolution of Ni(NO3)2‚6H2O was loaded onto the C-A tube. Prior to reactions, a Ni-loaded C-A tube is in-situ reduced under hydrogen flow of 1.79 mmol/min for 2 h at 973 K. Physical property measurement of Ni-loaded or -unloaded carbon coat was carried out for the sample torn off from alumina tube. The surface area of each sample was measured by liquid nitrogen adsorption (BET) method (Nikkiso, Model 4200). Before and after reaction, Ni-loaded carbon coat was submitted to surface analysis by XRD measurement (RIGAKU, RINT2100). This experimental reaction system was similar to that illustrated in the preceding paper.5 The surface temperature of the Ni/C-A tube was controlled by the current setting and measured by a pyrometer (Minolta IR-630) from outside of the acrylic outer tube. The gas compositions were analyzed by a Shimadzu 14B TCD gas chromatograph with a column packed with Shincarbon T60-80 (2 mm φ × 6 m). The experiments were carried out under atmospheric condition. The mixed gas
Characterization of Pyrogen. Table 1 shows the comparison of physical properties of carbon coat prepared by changing of Ni content. Consequently, a large surface area of carbon coat (400-500 m2/g) was obtained by treatment with novolak-type resin in NaOH solution, while only 3.5 m2/g was shown by treatment with THF solution.5,16 Hence, in this report, only the activity of an alumina tube treated with the resin as support for nickel was investigated. Loading of 10-20 wt % of nickel based on the weight of carbon coat were obtained by putting the corresponding volume of 0.064 M nickel solution onto the surface of carbon coat and drying for several times. As a result, rather low nickel concentrations (3.0-5.8 µmol/m2, which means the nickel moles per BET surface area of reduced Ni-loaded carbon coat) were obtained. The average size of nickel crystallite was measured by XRD line-broadening. The freshly reduced nickel crystallite size was about 24 nm and the estimated percentage dispersion, or percentage of the exposed nickel particles, was ca. 4%. The growth of the reduced nickel crystallite size was observed to 47 nm after reactions, but the grown crystallite size was independent of nickel content. Neither graphite nor a nickel carbide peak was detected in all analysis. Effect of Nickel Loading. Figures 2 and 3 show conversions and yields, respectively, of the various nickel contents of pyrogen as a function of the surface temperature. The activity of pyrogen increased by increasing nickel content. The Ni-unloaded pyrogen shows the least activity compared to 10 wt % Ni and 15 wt % Ni. The conversions of CH4 and CO2 using Ni-unloaded pyrogen were about 15% and 10%, respectively, at temperatures below 1200 K. Both conversions of reactants increased to ca. 20% at temperature of 1250 K. By using 10 wt % Ni pyrogen, about 66% conversions of CH4 and CO2 were obtained at 1260 K, with 66% yield of H2 and 52% of CO. In the temperature range from 1130 to 1260 K, the activity of 10 wt % Ni pyrogen increased with temperature. A 15 wt % Ni pyrogen shows the best performance over the others at lower temperatures. The conversions of CH4 and CO2 at 1200 (16) Yan, Z. F.; Ding, R. G.; Song, L. H.; Qian, L. Energy Fuels 1998, 12, 1114-1120.
CO2 Reforming of Methane with TDC Reactor
Figure 2. Temperature dependence of CH4 and CO2 conversions by varying nickel content on carbon coat.
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Figure 4. Time change of CH4 conversion under different conditions (1160 K).
Effect of Feeding Direction. Performance of pyrogens as a function of time was tested for 6 to 8 h at 1160 K and for different flow directions (Figure 4). It shows that the activity of 15 wt % Ni pyrogen with upward-flow decreased from 55 to 40% of CH4 conversion after 3 h, then it remains almost constant at about 40% for the rest of 5 h. In the case of 20 wt % Ni pyrogen with the upward-flow, the conversion of CH4 decreased from 66% to 47% after 5 h. Whereas, with the downward flow, 20 wt % Ni pyrogen kept the most stable activity for 7 h with the initial CH4 conversion of around 48%. Thermal Analysis. The effect of feed composition on carbon antiformation has been examined using thermogravimeter. Figure 5 shows the time change of weight increase owing to carbon formation during methane dehydrogenation over Ni-unloaded, 15 wt % Ni and CO2 reforming of methane over 15 wt % Ni, where the atmosphere temperature is constant at 1173 K. The progress of each reaction was confirmed by the analysis of output gases by gas chromatograph. It was cleared that carbon formation was suppressed when carbon dioxide was made to coexist in methane, while a remarkable carbon formation occurred in the case of a pure methane supply. Discussion Figure 3. Temperature dependence of H2 and CO yields by varying nickel content on carbon coat.
K were 76% and 69%, respectively, with 73% yields of H2 and 53% of CO. At 1240 K, about 62% conversion of CH4 and CO2 were achieved; however, deactivation of pyrogen occurred and a significant amount of oil product was observed.
The Ni-unloaded pyrogen gave the lowest conversion of reactants compared to other pyrogens and a trace of oily product was observed at 1250 K. However, in comparison to previous works,5,16 carbon coat treated with novolak-resin in NaOH solution showed better performance; this indicates that both surface and gasphase decomposition of methane occurred especially at higher temperatures. The coated carbon may have catalytic effect though its role will be minor in the
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Figure 5. Confirmation of carbon antiformation in CH4/CO2 system as a time function under different conditions using a thermogravimeter (1173 K).
presence of nickel. In pyrolysis of methane over Ni/Ca/ carbon, it was found that the external surface of carbon played an important role in the reaction at lower temperature.17 The observation of oily product at higher temperature suggested the dehydrogenative coupling of methane into C2 hydrocarbons (C2H4, C2H6) which further oligomerized in the cold area as reported previously.18 However, in the presence of CO2, most of the C2 hydrocarbons may be rapidly consumed in the gas phase by its oxidative action.18 This fact explains the absence of C2 hydrocarbons in the product gases. Nickel loading on carbon coat increased the conversion of reactants, and this effect became more remarkable at higher temperatures. However, at high nickel loading, the carbon formation also rapidly increased and caused deactivation of 15 wt % Ni pyrogen at 1240 K (Figure 2). The same explanation as mentioned above may apply for the significant oily product observed in this case. In CO2 reforming of CH4, there are two chemical formula which may be responsible for carbon formation:
CH4 f C + 2H2 2CO f C + CO2
∆H° ) 75 kJ mol-1
(2)
∆H° ) -171 kJ mol-1
(3)
Decomposition of methane, reaction 2, is an endothermic one, which will be enhanced at high temperatures, while disproportionate reaction of CO, the reaction 3, is exothermic and favorable at lower temperatures. In this work, although the surface temperature of the pyrogen was at high temperatures (>1100 K), temperatures within the TDC reactor decreased in radius from the (17) Suzuki, K.; Takahashi, R.; Onoe, K.; Yamaguchi, T. Energy Fuels 1999, 13, 482-484. (18) Murata, K.; Fujita, K.; Uchida, K. Sekiyu Gakkaishi 1997, 40, 129-133.
hot pyrogen surface to the cold wall. It means that reaction 2 may have importance in the formation of carbon in the hot zone around the pyrogen while reaction 3 may play a role in the temperature-gradient zone between the pyrogen and cold wall. The result of TG analysis on the 15 wt % Ni pyrogen under CH4/CO2 atmosphere suggests that the presence of CO2 prevents the carbon formation in gas phase. Although the CO molecule is known to adsorb strongly onto most of metal surface, the contribution of CO disproportionation to carbon deposit (reaction 3) through surface reactions is likely to be minor in this case. This might be explained by thermal diffusion effect, i.e., H2 molecules accumulated in the hot zone around the pyrogen and CO molecules moved toward the cold wall. Whereas disproportionate reaction 3 might be occurring in the moderate temperatures zone and followed by physical adsorption of the carbon product onto the nickel surface. It is likely that under these conditions, carbon deposit through disproportionate reaction of CO is more dominantly occurred than that from dehydrogenation of methane on the surface of the pyrogen. Furthermore, CO disproportionation in temperature-gradient zone may also be taken into account. Since CO is a heavier molecule compared to H2, the possibility of radial diffusion of CO will be higher whereas hydrogen molecules concentrated around the hot surface of pyrogen, which may result less carbon formation through disproportionate reaction 3. Further investigations are still needed to confirm these possibilities. In comparison with the upward flow, it is found that the downward feeding of the mixed gas decreased both the conversion of CH4 and carbon formations (Figure 4 and Figure 5). The explanation might be deduced from our former results on methane dehydrogenation.20 In downward feeding, hydrogen molecules concentrated around the hot surface of pyrogen by thermal diffusion effect, resulting in the less carbon formation on the surface through reaction 3. It was also found that CH4 molecules were concentrated in the lower part of the TDC reactor while H2 molecules were concentrated in the upper part, either the feed gas was introduced upwardly or downwardly. This was confirmed by RTD measurement for CH4 molecules in TDC reactor,21 and it is in consistence with principle of thermal diffusion effect. In upward feeding, heavier molecules should have slower velocity compared with downward feeding, giving a longer contact time to the pyrogen. This explains the higher conversion of methane with upward feeding in the case of dehydrogenative coupling of methane with TDC reactor.22 However, in the present reaction, the coexistence of CO2, which is a heavier molecule, means that surface carbon is easily oxidized into CO by its longer contact with pyrogen. Acknowledgment. The authors are grateful to Professor H. D. Gesser, the University of Manitoba, for his helpful suggestions for this work. EF000134Y (19) Nishiyama, T.; Aika, K. J. Catal. 1990, 122, 346-351. (20) Yamaguchi, T.; Cui, X.; Gao, Z. Sekiyu Gakkaishi 1992, 35, 292-295. (21) Suzuki, K.; Onoe, K.; Yamaguchi, T. Sekiyu Gakkaishi, submitted. (22) Yamaguchi, T.; Kadota, A.; Saito, C. Chem. Lett. 1988, 681682.
